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Precipitation Kinetics and Partitioningof Rare Earth Elements (REE) bet\yeen

Calcite and Seawater

by

Shaojun Zhong

A thesis submitted to the Faculty of Graduate Studies and Researchin partial fuifilment of the requirements for

the Degree of Doctor of Philosophy.

Earth and Planetary SciencesMcGiII UniversityMontreal, Canada

March 1993

© Shaojun Zhong 1993

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ISBN 0-315-91658-3

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SHORT TITLE

PRECIPITATION AND REE PARTITIONINGBETWEEN CALCITE AND SEAWATER

Shaoj un ZhongINRS-Oceanologie310 Ursulines

-, Rimouski. PQCanada G5L 3A 1

Tel: 814-723-1834

May 4, 1993

MS. Anna Cecile JungerCopyright DepartmentPergamon PressHeadington Hill HallOxford, OX3 OBWUnited Kingdom

Dear MS. Junger:

1 would like to requst an Official Copyright Waiver(s) from your of'ice for including anarticle 1 co-authored fOl)jGeochimica et Cosmochimica Acta (MS 8222; S. Zhong and A.Mucci, Calcite precipitation in seawater using a constant addition technique: a new overallreaction kinetic expression. GCA, Vo1.57: 1409-1417) in my thesis (Thesis tille: PrecipitationKinetics and Partitioning of Rare Earth Elements (REE) between Calcite and Seawater),submitted to McGiIl U'liversity, Montreal, Canada in partial fui filment of the requirementsof the degree of Ph.D.

Thank you very much for your corporation.

Sincerely,,

S'IC?:;' '\....., Shaojun Zltong.

/1

.. '). r ....,1~v /~

r--;.,.·'" -'

PERMISSION REQUEST

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SHORT TITLE

PRECIPITATION AND REE PARTITIONING

BETWEEN CALCITE AND SEAWATER

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ABSTRACT

A novel and simple "constant-addition" technique was used to study calcite

precipitation kinetics and the partitioning of REE between calcite overgrowths and

their parent seawater solutions under steady state conditions.

As a consequence of solute interactions in solution and at the growing mineraI

surface, the calcite precipitation mechanism in seawater is complex. It is dominated

by the following reversible overall reaction:

A kinetic expression is proposed which describes the precipitation rate according

to this reaction. A partial reaction order of 3 with respect to CO/ is obtained.

.REE have a strong affinity for calcite and substitute for Ca2+. REE partition

'~oe;~~i~n~i~~âI8ît~overgrowths were calculated from their concentrations in the

overgrov.ihs and their par~nt solutions using a non-thermodynamic homogeneous

mode!. The concentrations were determined by chelation and gradient ion

chromatography (CGIC) us.:ng a revised procedure. REE partition coefficients

decrease gradually with incr.~asing REE atomic number. They are sensitive to

changes in [REE]:[Ca2+] and th~ presence of O2 in solution, but unaffected by the

precipitation rate, [CO/] or Pc02 .0f the solution. The partitioning behaviour of

REE is negatively correlated to the s01ubility of their respective carbonates and

influenced by speciation, adsorption, and subsequent surface reactions (e.g.,

dehydration).

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RESUMÉ

Une nouvelle technique d'addition constant~ fut utilisée pour étudier la cinétique de

précipitation de la calcite et le partage des terres-rares (TR) entre les précipités et leurs

solutions mères d'eau de mer dans des conditions stationnaires.

Le mécanisme de précipitation de la calcite dans l'eau de mer est complexe et résulte

d'interactions des électrolytes en solution et à la surface du minéral en croissance. La

précipitation est dominée par la réaction réversible suivante:

Une expression cinétique est proposée qui décrit la vitesse de précipitation selon celte

réaction. l':l ordre de réaction partiel de 3 par rapport à l'ion CO,'- a été obtenu.

Les TR ont une grande affinité pour la calcite et se substituent pour l'ion Ca" dans

le réseau cristallin. Les coefficients de partage des TR dans la calcite ont été calculés à

partir des concentrations dans les précipités et les solutions mères et d'un modèle non­

thermodynamique appliqué à un solide homogène. Les concentrations ont été déterminées

par chromatographie ionique chélatante à gradient en utilisant une procédure revisée. Les

coefficients de partage des TR diminuent progressivement avec une augmentation du

chiffre atomique. Leurs valeurs absolues sont influencées par des changements du rapport

[TR]:[Ca'+] et la présence d'a, en solution mais sont indépendantes de la vitesse de

précipitation, de la [CO/-J, et de la PCO, en solution. Les coefficients de partage des TR

varient selon la solubilité de leurs carbonates respectifs et sont déterminés par la

spéciation en solution ainsi que l'adsorption et d'autres réactions à la surface du solide (par

ex: déshydratation).

11

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MANUSCRIPTS AND AUTHORSHIP

This thesis is prepared following the "Guidelines Concerning Thesis

Preparation", Faculty of Graduate Studies and Research, McGill University:

"Candidates have the option, subject to the approval oftheir Departmellt,

of including, as part of their thesis, copies of the text of a paper(s)

submitted for publication, or the clearly-duplicated text of a published

paper(s) , provided that these copies are bound as an integral part of the

thesis.

-If this option is chosen, c01l1lectillg texts, providillg logical bridges

betwee/l the differellt papers, are malldatory.

- The thesis must still conform to ail other requirements ofthe "Guidelines

Concerning Thesis Preparation" and should be in a literary form that is

more than a mere collection ofmanuscripts published or to be published.

The thesis must illclude, as separate cllapters or sectiolls: (1) a Table of

Contents, (2) a general abstract in English and French, (3) an introduction

which clearly states the rationale and objectives of the study, (4) a

comprehensive general review of the background literature to the subject

of the thesis, when this review is appropriate, and (5) a final overall

conclusion and/or summary.

- Additional material (procedural and design data, as weil as descriptions

111

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ofequipment used) must be provided where appropriate and ill su;tliciellt

detail (eg. in appendices) to allow a clear alld precise judgemem to be

made of the importance and originality of the research reported ill the

î'l:?sis.

- ln the case ofmanuscripts co-authored by the candidate and others. tlze

calldidate is required to make ail explicit statemellt ill tlze tlzesis ofw/III

cOlltributed to sucll work and to wllat extellt; supervisors must aUest to

the accuracy ofsuch claims at the Ph.D. Oral Defence. Since the task of

the examiners is made more difficult in these cases, il is in the candidate's

interest to make perfectly clear the responsibilities of the diflerent authors

of co-authored papers. "

The following papers, ail of which have been, or will be, submitted for

publication in scientific journals, are included in this dissertation:

1, Calcite precipitation in seawater using a constant addition technique:

a new overall reaction kinetic expression.

S. Zhong and A. Mucci,

Geochim. Cosmochim. Acta (Volume 57, /409-1417)

2, Quantitative determination of REE III seawater by chelation and

gradient ion chromatography.

S. Zhong and A. Mucci,

Submitted to Analy. Chim. Acta

IV

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3, Partitioning of rare earth elements (REE) between calcite and seawater

solutions at 25°C and 1 atm.

S. Zhong and A. Mucci,

To be submitted to Geochim. Cosmochim. Acta

Ali research work presented in these papers was performed by the author.

Professor A. Mucci, the research director and co-author, contributed

significantly through instruction, consultation, and editing (sometimes very

extensive).

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ACKNOWLEDGEMENTS

Above ail, 1 would like to express my sincere gratitude and appreciation

to my research director, Professor Alfonso Mucci, for suggesting the research

topic and having confidence in me to take on such a challenging and

interesting research project. 1 don't think 1 could have made it through

without his excellent supervision and guidance, constant encouragement and

motivation. 1 am especially grateful to him for showing great concem and

empathy during "difficult" times.

1 thank Drs. T. Barrett, E. Mountjoy, P. Pan, and S. Wood at McGill for

valuable discussions, enthusiastic support and encouragement.

Technical assistance contributed by T. Ahmedali, A. Bono, C. Colassin,

C. Guignard, L. Hendelman, G. Keating, G. Kopp, S. Lalli, X. Wu, and A.

Yannakis at McGill during various stages of this study are greatly

appreciated. 1 am also grateful to S. Boyajian, P. Chang, J. Grant, and K. Lin

from DionexQ!) Canada and G. Keating for showing me tlle art of ion

chromatography.

Special thanks to G. Hartley for proofreading rough drafts of the text and

numerous valuable discussions; to Dr. D. Baker for many constructive

comments and suggestions; to Dr. P. Zuddas for having faith in the

"constant-addition" system and various helpful discussions; to Drs. J. Zullig

(Exxon Long-Range Research Centre, Houston, Texas) and M. Harrold

VI

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(Dionexlll Corp., Sunnyvale, Califomia) for insightful criticism and comments;

to Dr. E. Burton and !Wo anonymous reviewers affiliated wit.h Geochimica

et Cosmochimca Acta and !Wo anonymous reviewers associated with

Analytica Chimica Acta for their constructive criticism.

1 am deeply indebted to my lovely wife, Xiaoxing, and my parents for

their immense love and support..

Financial support for this study was provided by the National Sciences

and Engineering Research Council of Canada (NSERC) to Dr. Mucci. 1 am

also grateful to the Department ofEarth and Planetary Sciences at McGill for

awarding me the Davison, LeRoy, Lynch, Reinhardt (x5), William

scholarships, and to GEOTOPIUQAM for providing graduate scholarships

through FCAR-Centre and Team grants.

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TABLE OF CONTENTS

Abstract

Resumé Il

Manuscript and Authorship 111

Acknowledgements VI

Table of Content V1I1

List of Figures XII

List of Tables XVI

Chapter 0 Introduction 1

0.1 Rationale and Objectives 1

0.2 Experimental System 3

0.3 Calcite Precipitation Kinetics 4

0.4 Analysis of Rare Earth Elements 4

0.5 Rare Earth Elements Partitioning 5

0.6 Conclusion 6

0.7 Reference 7

Chapter 1 Calcite Precipitation in Seawater Using a Constant

Addition Technique: a New Overall Reaction Kinetic

Expression 9

1.1 Introduction Il

1.2 Experimental System 14

1.3 Steady State Condition 21

1.4 Calcite Precipitation Kinetics 27

1.5 Conclusions 42

1.6 Acknowledgements 44

Vlll

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3.3.3.2 The Influence of Precipitation Rate or [CO):-]

3.3.3.3 The Influence of Solution Pco:

3.3.3.4 The Influence of [REE] or [REE]:[Ca:·] Ratio

3.3.3.5 The Influence of Redox Potential

3.3.3.6 The Role of Adsorption

3.3.3.7 Partitioning and the Solubility of

Individual REE Carbonates

3.3.3.8 Comparison of Laboratory Studies

3.3.3.9 Comparison of Laboratory and Field Results

3.4 Conclusions

3.5 Acknowledgements

3.6 References

Chapter 4 Concluding Remarks

4.1 Contributions to Original Knowledge

4.2 Suggestions for Future Work

4.2.1 Calcite Precipitation Kinetics

4.2.2 Analysis of REE Using CGIC

4.2.3 REE Partitioning

Appendix 1. Raw Experimental Data on Calcite Precipitation

from REE-free Seawater Solutions

Appendix II. Raw Data on Calcite Precipitation for

the "5-g" Type Experiments.

Appendix III. Composition of Calcite Overgrowths Precipitated

from the "5-g" Type Experiments.

x

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116

116

126

130

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-'-

134

136

138

140

141

148

148

149

149

150

151

152

154

156

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Appendix IV. Raw Data on Calcite Precipitation for

the "0.6-g" Type Experiments.

Appendix V. Steady State REE Concentrations (nglg) in Parent

Solutions for the "0.6-g" Type Experiments

Appendix VI. REE" Concentrations (mglg) in Calcite

Overgrowths Precipitated from the "0.6-g" Type Experiments.

Appendix VII. REE Partition Coefficients (i.e., Log(D))

for the "0.6-g" Type Experiments.

Appendix VIII. REE Adsorption by Calcite: Variations

of [REE] (nglg) in Calcite-equilibrated Seawater Solutions

with Reaction Time (hr). (Solid to Solution Ratio = 1:3000)

Xl

158

161

164

167

170

• LIST OF FIGURES

Page Figure Title

15 1.1 Schematic diagram of the constant addition system.

25 1.2 (a) Change in total calcium ion concentration (t.), carbonate

alkalinity (0), and (b) pH in the reacting seawater solution

with time during calcite precipitation by the constant

addition system.

28 1.3 Log(Rate) vs. Log(Q-1) for calcite obtained by the constant

addition system in phosphate-free seawater at 25°C and

Pco2=0.0031 atm.

30 lA CompaÎison of the available empirical calcite precipitation

rate equations obtained in phosphate-free seawater solutions

at 25°C and Pco2=0.0031-0.01 atm.

37 1.5 Log(Rate) vs. Log«(C03D for calcite in phosphate-free

seawater at 25°C and Pco2=0.0031 atm.

39 1.6 Log(Rate+0.29) vs. Log«(C03D for calcite in phosphate-free

seawater at 25°C and Pco2=0.0031 atm.

56 2.1 Schematic diagram of the chelation and gradient ion

chromatographie system.

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• 59 2.2 Schematic diagram of the analytical procedure (Modified

after Harrold ct al. [16]).

64 2.3 Typical REE chromatograms of (a) deionized water and (b)

artificial seawater samples (concentrations of individual

REE: 5 ppb) after REE instrumental extraction (AU:

Absorbance Unit).

68 2.4 Typical standard calibration curves for REE.

70 2.5 Typical REE chromatogram of artificial seawater samples

(concentrations of individual REE: 5 ppb) containing 40

ppm Fe3+.

87 3.1 Schematic diagram of the "constant addition" experimental

system.

88 3.2 Variability of individual REE concentrations in the reacting

solution throughout an experirnental mn by the "constant

addition" system. Concentrations of individual REE in the

input solutions were 100 ng/g.

94 3.3 Solubility products of REEiC03)3 in dilute aqueous

solutions. Data from Smith and Martel! (1976).

100 3.4 The sorption behaviour of sorne REE by calcite in calcite-

equilibrated seawater solutions.

• X111

• 103 3.5 The inhibitory effect of REE on the calcite precipitation rate

in seawater solutions.

106 3.6 Constancy of the Mgl +partition coefficient in calcite

precipitated from seawater as a function of (a) calcite

precipitation rate and (b) the total REE content of calcite

overgrowths on "5-g" type experiments.

109 3.7 Valencies and ionic radii (coordination number: 6) for

cations of interest. Data from Shannon (1976).

110 3.8 Na+ partition coefficients as a function of the total REE

content in calcite overgrowths precipitated from seawater

solutions.

113 3.9 REE partition coefficients as a function of calcite

precipitation rate.

115 3.10 REE speciation as a function of the total C03l- ion

concentration in seawater solutions.

118 3.11 REE partition coefficients as a function of the solution Pcol .

119 3.12 REE partition coefficients as a function of their steady state

concentrations in solution.

128 3.13 The influence of Eh or the presence of H2S and O2 iil

seawater on REE partition coefficients in calcite

overgrowths.

• XIV

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131 3.14 REE partition coefficients and REE adsorption coefficients

between calcite and seawater solutions as a function of

atomic number.

133 3.15 Relationship between REE partition coefficients and the

thermodynamic solubility products of REE carbonates. The

solubility data are from Smith and Martell (1976)

135 3.16 Comparison of field and experimentally derived REE

partition coefficients between calcite and its parent

solutions. Field data are from Parekh et al (1977), Scherer

and Seitz (1980), and Palmer (1985), experimental

measurements are those of Terakado and Masuda (1988)

and this study.

xv

• LIST OF TABLES

Page Table Title

18 1.1 Constants and equations used (seawater at S=35, 25°C,

atm.).

31 1.2 Summary of empirical rate equations for calcite-seawater

precipitation (r is the linear correlation coefficient).

57 2.1 Compositions of the eluent solutions and post-column

reagent.

65 2.2 Example of "Timed Events File" for (a) GPMI and (b)

GPM2.

71 2.3 REE concentration of samples analyzed by chelation and

gradient ion chromatography.

102 3.1 REE adsorption coefficients (# of measurements: 3; Ali data

are within ±0.1 of the given values).

112 3.2 Average REE partition coefficients versus calcite

precipitation rate (Ali data are within ±D.2 of the given

values).

117 3.3 Av~rage REE partition coefficients as a function of solution

Pco2 (Ali data are within ±D.2 of ,he given :values).

• XVI

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127 3.4 REE partition coefficiems versus solution Eh (Ail data are

within ±O.2 of the given values.

xvii

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CHAPTER 0

INTRODUCTION

0.1 RATIONALE AND OBJECTIVES:

in recent years, there has been increased interest, and expectations, for the

potential applications ofREE partitioning in carbopate mineraIs to diagenetic,

paleoceanographic, and environmental studies (e.g., Banner et al., 1988;

Dorobek and Filby, 1988). However, very limited research efforts have been

directed at understanding the systematics of REE incorporation in these

mineraIs. In fact, REE partition coefficients between carbonate mineraIs and

their parent solutions have not been accurately Jetennined. Many essential

and important questions such as what factors control the incorporation of

REE in calcite remain unanswered.

Mucci and Morse (1990) have reviewed much of the literature on

experimental studies of coprecipitation reactions of "foreign" elements in

calcite. They pointed out that these reactions are often affected by kinetic

factors such as specifie solution components (i.e., inhibitors), calcite

precipitation rate, and reaction pathways. In otller words, "foreign" element

partition or distribution coefficients in calcite reported in tlle literature are

phenomenological measurements of kinetic partition coefficients rather than

thennodynamic distribution coefficients (Morse and Bender, 1990).

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The study of kinetics is inherently more difficult and complex tllan that

of thermodynamics, because kinetic processes are time dependent and thus

path dependent (Lasaga, 1981). However, if the composition of the reacting

sol"·;on, including tlle concentrations of ail the participating species, is kept

constant throughout an experiment despite the ongoing reaction or reactions

(i.e., steady state conditions), we expect reactions to proceed at constant rates

and follow identical reaction paths. Under this condition, the time and path

dependent nature of a kinetic process will, therefore, be eliminated or

controlled.

Terakado and Masuda (1988) conducted the tirst and only experimental

study on the partitioning of REE in carbonate mineraIs at room temperature.

They noted that REE partition coefficients in calcite varied with tlleir

solution concentration, indicating tllat the partitioning of REE in calcite was

a kinetic process or was affected by kinetic factors. The intcrpretation oftheir

experimental data, however, is ambiguous and their applicability to natural

environments is profoundly limited due to tlleir failure to maintain steady

state conditions during the precipitations. Factors which may have affected

the partitioning process were not adequate!y controlled. Consequently, we

decided to launch an experimental study to make quantitative measurements

of REE partition coefficients between calcite precipitates and tlleir parent

scawater solutions as weil as investigate factors which may influence the

partitioning process at 25°C. We conducted our experiments in seawater

solutions for the simple reason that the fonnation and diagenesis of most

carbonate:minerals and rocks occur in seawater or seawater related solutions.

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0.2 EXPERIMENTAL SYSTEM:

Various experimental techniques have been appl ied to achieve and

maintain steady state conditions during calcite precipitation and "foreign"

element coprecipitation studies (see chapter ] for an extensive review). In the

case of REE, solution concentrations must be maintained at extremely low

levels (i.e., at the nglg leveI) to confonn with natural conditions and to avoid

the precipitation of discrete REE carbonate mineraIs. Furthennore, based on

the results of field studies, REE partition coefficients in calcite are expected

to be extremely high (_102 to 103; Parekh et al., 1977; Scherer and Seitz,

1980; and Palmer, 1985). Under these conditions and using existing

experimental teclmiques, it would have been extremely difticult, if not

impossible, to maintain REE concentrations constant during their partitioning

from solutions. Consequently, a new experimental design was required,

A simple "constant addition" experimental technique was designed based

on the working principle of the fluidized bed reactor. Before il could be

tested for conducting REE partitioning experiments, its ability to maintain

steady state conditions during calcite precipitation had to be verified. This led

to t11e first paper or the first chapter of this dissertation.

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0.3 CALCITE PRECIPITATION KINETICS:

The kinetics of calcite precipitation m seawater solutions has been

extensively reviewed by Morse (1983) and more recently by Morse and

Mackenzie (1990). Until now, calcite precipitation rates in complex solutions

such as seawater have most frequently and successfully been described by an

empirical rate mode!. Although of use for predictive purposes, tlle empirical

model gives very little insight into the precipitation mechanisms.

Consequently, in addition to confirm tlle suitability oftlle "constant addition"

technique to calcite precipitation study, we also strived to derive a detailed

mechanistic model that would adequately describe bOtll the calcite

precipitation rate and mechanism in complex electrolyte solutions such as

seawater under near-equilibrium conditions.

0.4 ANALYSIS OF RARE EARTH ELEMENTS:

The second paper or chapter 2 of this dissertation was bom as a result of

the recent acquisition of a chelation and gradient ion chromatograph (COlC)

in the Department of Earth and Planetary Sciences at McGiIl University and

our limited access to altemative ana1ytical instrumentation. In addition,

modem analytical techniques such as inductively coupled plasma mass

spectrometry (lCP-MS) and instrumental neutron activation analysis (INAA)

are only applicable to the detennination of REE in sampIes with simple

matrices. Samples with complex matrices such as most geological materials

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and our seawater solutions are subject to tedious and undcsirablc malrix

elimination procedures before they can be analyzed. Coincidentally, one

advantage the CGlC does offer is its capability of handling sampi es of

complex matrices. Using preliminai)' results provided by Dionex~', a revised

procedure was developed for the quantitative separation and delennination

ofREE in salI:ples with diverse matrices using CGIC. This method was used

to determine the REE concentrations in calcite overgrowths and seawatcr

solutions for the subsequent REE partitioning study (chapter 3).

0.5 RARE EARTH ELEMENTS PARTITlONING:

FinaIly, the main goal of this thesis, a study of REE partitioning in calcite

from seawater could only be undertaken after the experimental design and

analytical obstacles had been resolved. In chapter 3, the partitioning of REE

between calcite precipitates and their parent seawater solutions was studied

under steady state conditions using the "constant addition" technique. REE

partition coefficients were obtained and the influence of calcite precipitation

kinetics and a number of solution variables on the partitioning process were

examined. The REE partition coefficients were compared with results of

previous laboratory and field studies.

5 -,

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0.6 CONCLUSION:

The fourth chapter is a general conclusion to the dissertation. It contains

11 detailed description of the thesis contributions to original knowledge and

suggestions for future studies. Our raw experimentaJ data are presented in the

appendices.

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0.7 Reference:

Banner J.L, Hanson G.N. and Meyers W.J. (1988) Rare earth c1cmcnt

and Nd isotopic variations in regionally extensive dolomites fi'0111 thc

Burlington-Keokuk Fonnation (Mississippian): Implications for REE

mobility during carbonate diagenesis. J. Sediment. Petrol., 58, 415-432.

Dorobek S.L. and Filby R.H. (1988) Ongin of dolomites in a downslope

biostrome, Jefferson Fonnation (Devonian), Central Idaho: Evidence [Tom

REE patterns, stable isotopes, and petrography. Bull. Canad. Petrol. Geol.,

36,202-215.

Lasaga A.C. (1981) Rate laws of chemical reactions. In: Kinetics of

Geochemical Processes (ed. A.C. Lasaga and R.J. Kirkpatrick), pp.I-68.

Mineral. Soc. Amer., Reviews in Mineralogy, VoL8, Washington D.C.

Morse J.W. (1983) The kinetics of calcium carbonate dissolution and

precipitation. In: Carbonates: Mineralogy and Chemistry (ed. R.J. Reeder),

pp.227-264, Mineral. Soc. Amer., Reviews in Mineralogy, VoU 1,

Washington D.C.

Morse J.W. and Bender M.L. (1990) Partition coefficients in calcite:

Examination offactors influencing the validity of experimental results and

their application to natural systems. Chem. Geol., 82, 265-277.

Morse J.W. and Mackenzie F.T. (1990) Geochemistry of Sedimentary

Carbonates. Elsevier Sci. Publ., chapter 2, Amsterdam.

Mucci A. and Morse J.W. (1990) Chemistry of low-temperature abiotic

calcites: Experimental studies on coprecipitation, stability, and

fractionation. Reviews in Aquatic Sciences 3, 217-254.

7

•

•

Palmer M.R. (1985) Rare earth elements II1 foraminifera tests. Earth

Planet. Sei. Lett. 73, 285-298.

Parekh P.P., Müller P., Dukski P. and Bausch W.M. (1977) Distribution

of trace elements between carbonate and non-carbonate phases of

limestone. Earth Planet. Sei. Let!. 34, 39-50.

Scherer M. and Seitz H. (1980) Rare-earth e1ement distribution in

Holocene and Pleistocene corals and their redistribution during diagenesis.

Chem. Geol. 28, 279-289.

8

•

•

CIIAPTER 1

Calcite Precipitation in Scawatcr Using a Constant Addition

Techniquc: a New Ovcrall Rcaction Kinctic Exprcssion

S. Zhong and A. Mucci

Earth and PlanetaI)' Sei., McGill Univ., Montreal, PQ, Canada

ABSTRACT

A simple "constant addition" system was developed to stlldy calcite

precipitation reaction kinetics in seawater under steady state conditions. lt can

be applied to carbonate-trace element coprecipitation stlldies and may also

provide an interesting altemative for kinetic stlldies of calcite dissolution

reactions and other mineral-solution interactions.

Calcite precipitation 111 seawater can be represented by a reversible

overall reaction:

The measured precipitation rate, Ri is adequately described by a c1assic

kinetic model of the fonn:

where Rf, Rb and kf , kb are the forward and backward reaction rates and rate

9

•

•

constants for the overall reaction, respectively; (i], Yi, and ni are the total

concentration, activity coefficient, and reaction rate order, respectively, for

each species involved in the reaction.

If (Ca2+] is held constant throughout the precipitation experiments, the

above equation reduces to:

R=K [CO 2-]":-kf 3 b

The equation was used to fit calcite precipitation rate data measured over

a wide range of saturation states and extending to near saturation conditions.

The least-squares fit to the above expression yields values ofK f=103.s Jlmol

kg-sw3m-2hr-1mmol-3, n2=3, and kb=0.29 Jlmol m-2hr-1 with a correlation

coefficient of 0.99 at 25°C, when Pc02=0.0031 atm. and (Ca2+]"'10.5

mmollkg-sw.

The partial reaction order for the carbonate ion suggests that calcite

precipitation in seawater proceeds through a complex mechanism as

suggested by previous calcite-seawater interaction studies. The calcite

dissolution rate constant derived from this study is significantly lower than

values obtained in dill1te solutions. This observation is in agreement Witll

results of prev:ous studies which indicate tllat calcite dissolution is much

faster in diIute solutions tllan in seawater under identical saturation

conditions.

10

•

•

1.1 INTRODUCTION

Natural fluids are not always in equilibrium with solid carbonate phases

with which thty are in temporary or pennanent contact. The deviation fTom

equilibrium is usually small and the reaction kinetics under this condition are

often very sensitive to environmental factors and solution composition.

Therefore, it is desirable to obtain accurate reaction rate data of individual

carbonate minerais in various environmen',ally-relevant solutions and

conditions. Ideally, laboratory kinetic experiments should be conducted when

the system under study is at steady state so that reactions suclr as

precipitation, dissolution, and trace element coprecipitation occur at a

constant rate, in an invariant enviranment, and following the same reactional

pathway. Under these conditions, any measurable thennodynamic and kinetic

property can be reasonably obtained in the time frame required by the

measurements without having to take into consideration changes of the

reaction parameters with time. Factors which may directly or indirectly

influence the reactions can be studied by conducting a set of experiments

while varying a particular parameter and keeping others constant. A detailed

kinetic description of the reaction mechanism can be derived from a series

of investigations of individual parameters.

Experimental techniques have always played an important raie in the

evolution of our understanding of calcite-solution reaction kinetics. Various

experimental techniques have been applied to achieve and maintain steady

state conditions. Examples of such techniques include: (1) the "free-drift"

11

•

•

method utilizing a single calcite crystal in a large volume of solution Ce.g.,

Nancollas et al., 1981; Busenberg and Plummer, 1986); (2) the "pH-stat"

technique (e.g., Morse, 1974; Inskeep and Bloom, 1985); (3) the "chemo-stat"

system (e.g., Mucci and Morse, 1983; Zhong and Mucci, 1989) or "constant

composition" system (e.g., Kazmlerczak et al., 1982); and more recently, (4)

the "fluidized bed" reactor (Chou et al., 1989). Of these experimental

techniques, only the "chemo-stat" or "constant composition" system and t11e

"fluidized bed" reactor provided actual steady state conditions for the calcite­

solution reaction. However, to apply the "chemo-stat" system, preliminary

knowledge of the rates of the reactions under study is essential for the

preparation of "titrant" solutions. It often requires trial-and-error

experimentation before a successful ron can be conducted. More importantly,

a non-steady state period exists at the beginning ofany experimental ron. The

duration of this period is a function of t11e reaction rate and the diftèrence

between initial and steady state conditions. Difficulties were also encountered

when the "chemo-stat" system was used to conduct calcite precipitation or

dissolution experiments near calcite saturation, a situation more representative

of natural conditions. Under these conditions, the reaction rate is extremely

slow and observed rates are highly sensitive to t11e presence of reaction

inhibitors (Mucci, 1986). The "chemo-stat" system is not weil suited for

conducting trace element partitioning experiments where maintaining a

constant trace element solution concentration is essential (Morse and Bender,

1990). The "fluidized bed" reactor has been used successfully in carbonate

(Chou et al., 1989) and albite (Chou and Wollast, 198.4) dissolution studies,

but its applicability to the precipitation of carbonate mineraIs still remains to

12

•

•

be examined.

This paper introduces a novel yet simple expcrimental systcm which

proveà to be excellent for kinetic studies of carbonate precipitation and

coprecipitation reactions. Il may also provide a suitable altemative for thc

investigation of calcite dissolution reactions and other mineral-solution

interactions. In addition, calcite precipitation rate data obtained in this study

were used to derive a kinetic expression which adequately describes the

reaction in seawater. Results of the application of this system to the study of

trace element partitioning, more specifically rare earth elements, between

calcite and seawater under various conditions will be presented elsewhere

(Le., Chapter 3).

13

•

•

1.2 EXPERIMENTAL SYSTEM

The experimental system is schematically illustrated in Fig. 1.1. It was

inspired by the working principle of the "fluidized bed" reactor described by

Chou and coworkers (Chou and Wollast, 1984; Chou et al., 1989). In the

reactor, a steady state can be reached if a constant input of the reactants is

maintained and the characteristics of the solid-solution interface remain

unchanged (see Chou and Wollast, 1984 for details). However, in our early

attempts to apply the "fluidized bed" system to calcite precipitation kinetic

studies, variations of the carbon dioxide partial pressure (Pc02) in the solution

\Vere significant enough that steady state conditions would not be maintained.

Pc02 variations affect the pH of the solution and the relative concentrations

of carbonic acid species which, in tum, influences the saturation state and

calcite precipitation kinetics.

Two fi.ll1dameI;(al changes were made to t:1e "fluidized bed" system: agas

phase was added and the reacting solution was no longer circulated and

pumped ou,. of the reactor. The introduction of a gas phase to the system

served two purposes. Il kept the solid and liquid phases weil mixed and more

importantly, it maintained the Pco! of the solution at a constant and fixed

value. Furthennore, pumping solution out of the reactor at a constant speed

while keepll1g ail the fine calcite seed material in the reaetor was technically

difficult to achieve. This procedure is theoretically unnecessaI)' ifmaintaining

a steady state is the primaI)' purpose (see next section). Using the new

system, a steady state is reached and maintained if the reactant solution is

14

•

~

<J,

pH electrode

Gas Inlet

Stirrer 1 Gas OUlletSol'n Inlet

PeristalticPump

G;J

.---_."'-----_____ a- _

-----------------------_)PP'!t§.s>l'!l __

•

Constant Temperature Bath

Fig. 1.1 Schematic diagram of the constant addition system.

•

•

added to the reactor at a constant rate and the total reactive surface area of

the solid does not change significantly with precipitation. For this reason, we

dubbed the experimental design the "constant addition" system.

Baker"" "lnstra-analyzed flux reagent" grade calcite, treated by the

procedure described by Mucci (1986), was used as seed material for the

calcite precipitation experiments. The material has a weil restricted size

range, 3 to 7 Ilm, as observed by scanning electron microscopy (SEM) and

a specifie reactive surface area of 0.52 m~/g as detennined by the Kr-BET

method (deKanel and Morse, 1979). Aged artificial seawater with a salinity

of 35 was used for ail the experiments after being filtered through a

Millipore® 0.45 Ilm filter. This artificial seawater contained ail the major

constituents of natural seawater, including fluoride (F), while temporarily

excluding carbonic acid species. It also had a slightly higher calcium

concentration CI 1.0 mmollkg-sw) than natural seawater. Molybdate blue

spectrophotometric analysis (Koroleff, 1976) indicated that the seawater was

essentially phosphate-free, or contained Jess than 1.6 nmollkg-sw of soluble

reactive phosphate. The aged artificial seawater was further pre-treated by

suspending calcite powder in solution CI glkg-sw) for an average of 30

minutes to scavenge possible inhibitors before it was filtered. However,

calcite precipitation experiments conducted using tlle treated and untreated

seawater yielded identical results and the pre-treatment procedure was

abandoned. Prior to each experiment, weighed amounts of Na~C03 and

NaHCO, were added to a known volume of artificial seawater and

equilibrated with a water vapour saturated CO~-N~ gas phase of known Pco~

16

•

•

(i.e., 0.0031 atm.) to obtain a solution ofdesired initial calcite supersaturation

(n, defined in Table 1.1).

Calcite precipitation experiments were conducted by first introducing a

weighed amount of calcite seed material (-0.6 g) into the empty reactor (250

ml) while the C01-N1 gas was flowing through. The calcite supersaturated

artificial seawater, or input solution, was then pumped into the reactor at a

constant rate (0.03 to 1.0 g/min.) with a peristaltic pump. Once the input

solution was in contact with the seed material, a magnesian calcite; (-8 11101%

MgC03) precipitated on the seed (Hartley et al., 1992), as previously

observed (e.g., Mucci, 1986; Burton and Walter, 1987). No spontaneous

nucleation or precipitation took place from the calcite supersaturated input

solutions (I <n<15) before they were introduced into the reactor. The volume

of the solution in the reactor increased from zero to a maximum of 250 ml.

The solution to soIid ratio, therefore, changed dramatically during the course

of an experimental mn. However, we have no reason to believe that the

change in solid to solution ratio would affect the calcite precipitation reaction

kinetics once the solid material was immersed completely in the solution.

Suspension of the seed material was achieved within a very short period of

time relative to the total length of an experimental mIl. About 15 ml of

solution was generally required to suspend the solid completely. The solid

and Iiquid phases were weil mixed and the Pc01 of the solution was kept

constant by continuous bubbling of the gas mixture. An overhcad electric

motor with an one-bladed glass propeller can also be added to the system to

17

•Table LI. Constants and equations used (seawater at S=35, 25°C, 1 atm.):

Ct =s[H2C03°] = 2.839xlO-3 ( mol ); (Weiss, 1974)

Pc02 kg-swatm.

KI =1

1O-PH[HC03-]

[H2C03°]= 9.965x10-7 ( mOl); (Mil/ero, 1979)

kg-sw

10-pl/[CO)2-] ()ICzl = '--- = 7.772xlO-JD mol; (Millero, 1979)[HCO) -] kg-sw

KO = [Ca 2o] [CO 2-1. = 4.39xlü-7 ( mol )2; (Mucci, 1983)ST' ,q) q kg-sw

= 2.033xl0-9 ( mOl); (Millero, 1979)kg-sw

Ac =At - [B(OH). -] =At -[B(OH);] + [B(OH))]

lü-pH+ 1

KIB

2 Ac[CO) -] = ---­

lü-pH+ 2

•

.Q =[<7a20

] [<7()32-]

K"sp

18

•

•

msure sufficient solid-solution mlxmg and gas-solution interaction. The

reactor was partly immersed in a constant temperature bath maintained at

?-±O -oC_:>_.:> .

Aliquots of the input and reacting solutions were drawn fr0111 the

reservoir at the beginning of each experiment and from the reactor

throughout or at the end of the precipitation. The solutions were immediately

filtered through Milliporell\' 0.45 !lm filters. These samples were analyzed for

total calcium concentration ([Ca2+], hereafter referred to as [Ca)) and titration

alkalinity (At). The pH of the input solution and the steady state pH of the

reacting solution were measured directly in the reservoir and in the reactor,

respectively, at times corresponding to sampling intervals. The [Ca], At, pH,

and total boric acid concentration in solution were used to calculate the

solution carbonate alkalinity (Ac), total carbonate ion concentration ([CO/-],

simplified as [C03)), and calcite saturation state (n). Apparent and

stoichiometric constants and equations used in the calculations are listed in

Table 1.1. The steady state calcite precipitation rate (R, !lmol m-2hr- l) was

calculated by multiplying the solution addition rate (1, kg-sw/hr) by the

difference in Ac (meqlkg-sw) between the input solution (AcQ) and reacting

solution (Acs), The rate was normalized with respect to the initial reactive

surface area of the calcite seeds (specific reactive surface area, S, multiplied

by the weigh~ of seed introduced in the reactor, W,ccd):

19

• I(Aco - Ac.JR = xl00ü

2S W""d(U)

•

pH measurements were conducted using a combination electrode

(Radiometer'" GK2401C) connected to a pH/mV meter (Radiometer'" M84).

The electrode was calibrated against three NBS (now NIST) buffer solutions

(pH of 6.838, 7.382, 9.180 at 25°C). Reproducibility of pH calibrations

carried out before and after measurements of a single sample solution was

better than ±O.005 pH unit. In addition, a TRIS buffer solution in artificial

scawater (8.074 at 25°C and S=35, Hansson, 1973; or 8.067 according to

Millero, 1986) was used to evaluate liquid junction potential variations

(Dickson, 1984). pH measurements on the TRIS buffer scale, when used v.'ith

the appropriate constants (Hansson, 1973; Millero, 1979), give an

independent assessment of the concentrations of carbonic acid species.

Calcite saturation calculations using the two sets ofpH and constants agreed

to within ±5% or better. Results presented in this study were calculated from

pH measurements and carbonic acid apparent dissociation constants (Table

1.1) based on the NBS scale. The total calcium concentration and total

titration alkalinity detenninations were perfonned according to the procedures

described by Mucci (1986), with estimated precisions of better than ±O.5%

and ±O.4%, respectively.

20

• 1.3 STEADY STATE CONDITION

One important criteria to be met by the system is to provide a steady

state environment for the calcite precipitation reaction. In this section, we

demonstrate, tirst from a theoretical standpoint, that the constant addition

system will indeed create and maintain a steady state environment for calcite

precipitation reactions throughout an experiment. We then examine this

c0nclusion using experimental data obtained from the system. The change of

calcium concentration in the reacting solution ([Ca]y) with time (T) is chosen

as the principal variable in the foUowing discussion.

A weighed amount of calcite seed material was introduced into the empty

reactor. A calcite supersaturated seawater solution with a calcium

concentration [Calo was then pumped into the reactor at a constant rate, I. In

the reactor, calcium can only be removed from the solution by calcite

precipitation. On the basis of mass balance, at time T, the total amount of

calcium precipitated as calcite is given by the difference between the total

amount of calcium introduced into the system and that which remains in the

reacting solution:

•

T

JR dT = ([Calo-[Cah)ITo

R is the calcite precipitation rate. Differentiating Eqn. 1.2, we obtain:

21

(1.2)

•Differentiating again, yields:

dR =1 [_ 2 d[CaJT _ T d2[Cah]dT dT dT2

T dR =1 [_ 2T d[Cah _ T2

d2[CaJT]

dT dT dT2

This equation can be integrated ta:

~T dRlnT = _1T2

d[Cah

il dT f dT

According to the above equation:

(1.3)

(1.4)

(1.5)

(1.6)

(1.7)

d[C J TIf. a T >0 then: IfT dRlnT<o

dT il dT fthus: dR<O

dT(1.8)

•

If. d[CaJT <0dT

T

then: IfT dRlnT>oil dT f

22

thus: dR >0dT

(1.9)

• T

d[Cah QT dR~ dRIf. =0 then: -- T=O thus: - =0dT idT dT

(1.10)

On the other hand, based on basic kinetic principles, the precipitation rate

of calcite should be positively correlated with [Ca]T (or calcite saturation

state, Q) under our experimental conditions. In other words:

d[CahIf. >0

dT

d[Ca]TIf. < 0

dT

d[CahIf. =0

dT

chen: dR> 0dT

then: dR < 0dT

dRchen: - = 0

dT

(1.11 )

(1.12)

(1.13)

The only situation which can satis!}' both the mass balance and kinetic

considerations is:

d[Cah= 0

dTdR = 0dT

(1.14)

•

This means that both the calcium concentration in the reactor and the

precipitation rate of calcite are invariant with time. Similar conclusions can

be reached by choosing the carbonate alkalinity of the reacting solution as

the principal variable. The dissociation reactions of carbonic acid in solution

23

•

•

are quasi-instantaneous (Lasaga, 1981). Given a constant carbonate alkalinity

and Pc02 in solution, concentrations of ail the carbonic acid species and

solution pH should be constant throughout an e~;perimental mn. In view of

the stoichiometry of the possible calcite precipitation reactions (see next

section), it can be concluded that the system is held at steady state for the

calcite precipitation reaction.

Exp,~rimental results, presented in Fig. 1.2a, indicated that both the (Cah

and ACr of the reacting solution were, within the precision of the analytical

methods, constant throughout the whole precipitation experiment. Periodic

monitoring of the reacting solution pH throughout the mn also indicated that

after a very short period of time the pH also became very stable (Fig. 1.2b).

Two factors could be used to explain the lower solution pH at the

beginning of each mn. A faster calcite precipitation reaction may occur at the

initial stage caused by the presence of a greater density of high energy

surface sites, such as kinks, steps, and holes (Nancollas et al., 1981) on the

seed materiaI. Il is also possible that the sluggish CO2 hydration reaction

could not keep up with the calcite precipitation reaction (Bemer, 1975;

Plummer et al., 1978) and result in the drifting of solution pH. In other

words, degassing of CO2 from the reacting solution was too slow and

resulted in an supersaturation of the solution with respect to the gas phase.

24

12,0 12.0

...-..~ ,,-..-• '" ~ [Ca] of input solution ~1 ...-

0.0 '".... 10,0L 06

~ AM 10,0 1- e S 0.0...... .:::::~- ......0 0-

S ~

8, ~ Ac of input solution 8,0 ES '-''-'

13 6 0 999 0 C)........ 0 0 -<t:co;:

U 6,0 6,0~

0 500 1000 1500 2000 2500 3000

Time (minutes)

8,50,..-------...----------.....,

8,25

pH of input solution

1\ /\

•

7,75

7,50 0...--~500~--..lOOO....---15~OO....--2~OOO....--..25~OO--......3000

Time (minutes)

Fig. 1.2 (a) Change in total calcium ion concentration (.e.), carbonate

alkalinity (0), and (b) pH in the reacting seawater solution with

time during calcite precipitation by the constant addition system.

25

•

•

It appears that the constant addition design does provide a steady state

environment for the calcite precipitation reaction. The fact that it is a simple

and self-regulating system makes it an attractive choice for conducting

carbonate precipitation-dissolution expeziments at near equilibrium conditions.

ln a subsequent paper (i.e., Chapter 3), we will demonstrate that it is also

weIl suited for carbonate-trace element coprecipitation studies. It may also

provide an interesting alternative to researchers investigating other mineral­

water interactions. There is one important limitation to the application of this

system, however; when the rate of the reaction under investigation is either

too low or too high, an unrealistic solution injection rate may be required.

26

• 1.4 CALCITE PRECIPITATION KINETICS

Calcite precipitation rate data obtained in seawater solutions have 11l0st

frequently been fitted to an empirical rate law (Morse, 1983) of the l'Olin:

R = k(O - l)n

or its logarithmic expression:

Log(R) =nLog(O -1) + Log(k)

where n is the empirical reaction order and k is the rate constant.

(1.15)

(1.16)

•

This empirical rate law fits our data very weil over the whole range of

calcite supersaturations covered in this study (Fig. 1.3, Table 1.2). One of the

major advantages of the constant addition system is its ability to conduct

calcite precipitation experiments in solutions close to calcite saturation. A

possible change in the calcite precipitation reaction mechanism in the near

equilibrium region has frequently been suggested (e.g., Reddy et al., 1981;

Busenberg and Plummer, 1986). Yet, because of the difficulties involved in

obtaining accurate rate data in this region by existing experimental methods,

this hypothesis could not be confinned. From our limited data set, however,

we were unable to observe a deviation trom the empirical rate law in the

near equilibrium region. Comparison with previous studies shows that calcite

precipitation rate data obtained by the constant addition system agree

reasonably weil with those generated by the chemo-stat system under similar

27

•2.0

.-;--......-NS-. 1.0-0S::l..'-'

~....~es 0.0CIl0~

-1.0

-0.5 0.0

Log(ü-l)

0.5 1.0

•

Fig. 1.3_Log(Rate) vs. Log(O-l) for calcite obtained by the constant

addition system in phosphate-free seawater at 25°C and

Pco2=O.0031 atm.

28

• experimental conditions (Fig. 1.4. Table 1.2). confimling the suitability of the

new system for conducting experimental calcite precipitation kinetic studies.

The empirical model. although useful in relating calcite precipitation rates to

solution supersaturation states. offers little insight into the kinetic mechanisl11

of calcite precipitation and/or dissolution.

Plummer et al. (1978) made the tirst successful allempt to dcrive a

mechanistic expression for calcite dissolution kinetics in simple solutions.

Three parallel elementary reactions were combined to represent the ovcrail

reaction and their respective rate constants were detennined:

(U7)

(US)

(U9)

•

Difficulties were cncountered when this reaction control model was

applied to the crystal growth of calcite (Reddy et al., 1981; House, 1981 a,b;

Busenberg and Plummer, 1986). At Pc02<O.03 atm., the model failed to fit

experimental precipitation rate data. To explain the discrepancy, it was

proposed that the Pc02 at the calcite surface during initial phases of

precipitai:ion must be greater, or surface pH lower, than in the bulk solution

(Reddy et al., 1981). Unfortunately, these surface parameters cannot be

-.-"

29

• 4.0

(1) BUI"ton and Walter, 1987;(2) Mueci, 1986;(3) This Study.

~-... 2.0-N ...C........-0c-:::t.'-'

".....,~....

·ee~'-' 0.00Jl0~

-2.0 L..-_L-.-__....i-__--!-__---J.__-"

-0.5 0.0 0.5 1.0

Log(O-l)

Fig. 1.4 Comparison of the available empirical calcite precipitation rate

equations obtained in phosphate-free seawater solutions at 25°C

and Pco2=0.0031-0.01 atm.

30

•

Table 1.2. Summary of empirical rate equations for calcite-seawater precipitation

(r is the linear correlation coefficient).

•

Source Il n Log(k) r # points Pco,

vJ~

TIlis sludy 1.2 - 8 2.22±0.05 0.21±0.13 99% 37 0.0031

Burlon & Walter (1987) 4 - 17 1.9±0.1 0.59±0.08 90% 28 0.01

Mucci (1986) 2 - 14 2.83±0.04 -0.26±0.08 99% 28 0.0031

• measured directly at this time. More importantly, disequilibrium or the Jack

of a quantitative relationship between surface and bulk solution composition

(Busenberg and Plummer, 1986) compromises the applicability of the mode!.

lnskeep and Bloom (1985) conducted a senes of calcite precipitation

experiments using a pH-stat technique in solutions with ionic strengths of

less than O.lm at a pH greater than 8 and Pc02 les~ than 0.01 atm. Using

their own experimental data set, they examined a number ofkinetic models,

including the Plummer et al. (1978) mechanistic mode!. They concluded that

calcite precipitation kinetics under their experimental conditions was best

represented by a simple elementary reaction:

Ca 2+ + CO 2- "'" CaCO3 3(S)

(1.20)

or by the Nancollas and Reddy (1971) model, which can be derived from this

reversible elementary reaction:

R = ken - 1) (1.21)

•

Chou et a!. (1989) condl1cted calcite dissolution experiments in dilute

solutions using a fluidized-bed reactor technique. Applying the Plummer et

al. (1978) approach, they also proposed the use of three parallel elementary

reversible calcite dissolution-precipitation reactions (i.e., Eqns. 1.17, 1.18,

and 1.20) to describe the overall reaction. Like 1nskeep and Bloom (1985),

they concluded that at high pH (i.e., 7 to 10) and low Pc02 (i.e., less than

0.001 atm.) calcite precipitation is dominated by Eqn. 1.20. In other words,

•

•

the rate can be expressed sim ply in tenns of the activity product of carbonate

and calcium ions, or by the Nancollas and Reddy (] 97]) mechanistic mode!.

A companson of the Nancollas and Reddy (] 97]) model with the

empirical rate model reveals that the fonner can be regarded as a special case

of the latter, and more specifically as an empirical rate law with a reaction

order of one. As was observed for other processes such as calcite-solution

reaction inhibition by Mi+ and phosphate (Burton and Walter, ]990; Mucci,

1986) and foreign ion incorporation in calcite (Burton and Walter, ]987;

1990; Mucci et al., 1989), mechanistic descriptions of calcite precipitation

(Nancollas and Reddy, 1971; Inskeep and Blooll1, 1985) and dissolution

(Chou et al., 1989) in dilute solutions may not be directly applicable to

calcite precipitation in seawater. Indeed, results from Mucci (1986), Burton

and Walter (1987; 1990), as weIl as from this study indicate that the

empirical order of the calcite precipitation reaction is larger than one in

seawater solutions (Table 1.2). The higher reaction order suggests that calcite

precipitation from seawater solutions does not proceed through simple

elementary reactions. Furthennore, studies on the inhibition of the reaction

by Mg2\ a major seawater component, support the hypothesis that crystal

growth proceeds through a complex surface-controlled mechanism in

seawater solutions (Mucci and Morse, 1983).

The ,'ontribution of individual species (e.g., HC03', W, and H2C03") to

the calcite precipitation reaction in seawater can be eval uated by conducting

experiments at different Pc02• By changing. Pc02, it is possible to vary

33

•

•

[HC03-], [H'], and [H2C03'], while at the same time keeping [C03] constant.

Preliminary experiments were conducted using Pc02 ranging from 0.00033

(equivalent to present atmospheric CO2 partial pressure) to 0.3 atm. At a

calcite saturation state typical of average surface seawater, 0=5, and over

this range of Pc02, [HC03-] will vary from 1.6 to 48 mmol/kg-sw, pH from

8.32 to 6.76, and [H2C03'] from 0.001 to 0.85 mmol/kg-sw. We observed

that the calcite precipitation rate is not affected significantly by variations of

Pco,. This conclusion is supported by a more complete study carried out by

Burton and Walter (1990). Their results show that varying Pc02 from 0.0003

to 0.1 atm. had almost no influence on the calcite precipitation rate in

phosphate-free natural seawater solutions (see their Fig. la). Similar results

were also obtained by Mucci (1986, his Fig. 1). Thus, we conclude tllat Eqn.

1.20 is the dominant calcite precipitation reaction in seawater solutions.

Nevertheless, a more careful examination of our preliminary data and tllOse

of Burton and Walter (1990) reveals tlJat tlle calcite precipitation rate at a

given saturation state may be slightly faster wben a higber Pc02 is used. This

observation suggests that the other two parallel reactions (Eqns. 1.17 and

1.18) may also participate to the precipitation. Their contribution is so small

that it will be difficult to quantifY or to determine tlleir respective rate

constants, unless the precision of the experimental system is improved

significantly.

A kinetic rate equation for the overall calcite precipitation reaction in

seawater, as represented by Eqn. 1.20, can be written as tlle difference

between the forward and backward reaction rates (Lasaga, 1981; bis Eqns.

34

• 2 and 109):

(Lm

where R is the observed calcite precipitation rate; Rf' Rh and kt". kh are the

forward and backward reaction rates and rate constants for the overall

reaction, respectively, and ai and ni are the activity and partial reaction order

for species involved in the reaction.

Since the activity of a relatively pure sol id can be considered as one.

Eqn. 1.22 reduces to:

kn n.,R = "fa ) I(a ). - kJ' Ca C03 b

(1.23)

In this study, we were able to conduct a series of experiments for which

[Ca] was held constant and [C03] varied between 0.05 and 0.35 meqlkg-sw.

Othervariables include pH (7.44-7.84) and [HC03'] (2.4-6.0 meqlkg-sw); but

these have been shown not to influence the reaction rate significantly. The

concentration of aIl the other species in solution were maintained essentially

constant. Under such conditions, the activity of calcium and the activity

coefficient of the carbonate ion (yc03) can be considered as constants. Eqn.

1.23 can, therefore, be simplified to:

(1.24)

•where:

35

• (1.25)

Rearranging Eqn. 1.24 and taking the logarithm, we obtain:

When R » kh, the above equation reduces to:

Log(R) = nLog([C0;J) +Log(K)

(1.26)

(1.27)

and thus, a linear relationship should exist between Log(R) and Log([C03]).

Our experimental data were plotted accordingly in Fig. 1.5. It is apparent

that a linear relationship does exist in the far-from-equilibrium region under

our experimental conditions. In this region, where Log(R»O, a least-squares

linear regression gives a slope corresponding to a reaction order of 3.09±O.09

and an intercept corresponding to a rate constant of 103.57±O.o9 with a

correlation coefficient of 0.99. For the foIIowing discussion we adopted a

reaction order of 3.0 and a raie constant of 103.6 which were substiiuted into

Eqn. 1.26:

Log(R +k~ = 3.0Log([CO;J) + 3.6

At equilibrium, [C03]=[C03Lq and R=O, therefore:

Log(k~ = 3.0Log([C0;Jeq) +3.6

(1.28)

(1.29)

•The stoichiometric solubility of calcite in seawater under our experimental

conditions (i.e., seawater, S=35, t=25°C) is given as (Table LI):

36

•2.0 -

"C' 1.0­.::N

~oS::t

-2.0 ­6

1 1 1 1

•

-1.4 -1.2 -1.0 -0.8 -0.6 -004

Log([C03 ]) (mmolelkg sw)

Fig. 1.5 Log(Rate) vs. Log([C03D for calcite in phos'phate-free seawater

at 25°C and Pco2=O.0031 atm.

37

• (1.30)

Since an average calcium concentration of 10.5 rnmollkg-sw was maintained

at ail time throughout the experiments:

K' 7[CO] =---!!!.. = 4.39xlO- =OA18xlO-4 ( mol )

3 tq [Ca] 10.SxlO-3 kg-sw

Substituting into Eqn. 1.29, we obtain:

(1.31)

Log(kJ = -0.536

!ild Eqn. 1.26 becomes:

(1.32)

Log(R +0.29) =3.0Log([C03D+Log(W'·6) (1.33)

•

Our experirnent?l data set was fitted to the above equation (i.e.,

Log(R+0.29) vs. Log([C03])) (Fig. 1.6). AIl the data are distributed on a

straight line. A least-squares linear regression gives a partial carbonate ion

reaction order of 3.0±O.OS, a rate constant of 103.5±O.08, and a correlation

coefficient of 0.99. This kinetic expression (i.e., Eqn. 1.33) appears to

adequately characterize the calcite precipitation reaction in seawater under the

experimental conditions of this study, including those obtained near calcite

saturation.

A partial carbonate ion reaction rate order of 3 indicates that calcite

precipitation in seawater is a cornplex process which cannot be expressed by

38

•2.0

'l:'--N5;:::.05 1.0:::t'-"

,.......,0'\NC+Q,l.......~

~ 0.0'-"b.ll0~

•

-1.0 L--_---L.__....L-__.L--_--L__.....J

-1.4 -1.2 -1.0 -0.8 -0.6 -0.4

Log([C03 D (mmolelkg sw)

Fig. 1.6 Log(Rate+O.29) vs. Log((C03D for calcite in phosphate-free

seawater at 25°C and Pco2=O.0031 atm.

39

•

•

a simple elementary reactioll. il does not, however, necessarily reflect the

molecularity of the reaction. The difference between calcite precipitation

kinetics in distiIIed water and seawater can probably be explained by the high

ionic strength and the presence of other components such as Mg2+, SO/,

Na" and CI- in seawater. These ions are bound to interact with each other to

fonn complexes, or ion-pairs, which may influence the behavior ofindividual

ions in solution and at the solid-solution interface. Furthennore, a variety of

foreign ions can be cop,-ecipitated in the calcite lattice and may modify the

thennodynamic characteristics of calcite growth surface. In fact, Busenberg

and Plummer (1985; 1989) demonstrated that the incorporation of Na+ and

;:,0/ in the calcite lattice increases the unit eell size and decreases the

stability of the sol id. Mg2+ and SO/- interact strongly with each other in

solution (Millero and Schreiber, 1982) while both influence calcite

precipitation rate in seawater. Mucci et al. (1989) observed that calcite

precipitation is faster in sulphate-free seawater than in "nonnal" seawater at

identical calcite supersaturations. Furthennore, they observed that the amount

of Mg2+ incorporated in calcite following precipitation in seawater increased

from 8 to 10.5 mol% MgC03 when SO/" ions were withheld from the

artificial seawater preparation. On the other hand, calcite precipitation rate

is nearly independent of the seawater salinity between 5 and 44, whereas the

amount of MgC03 incorporated in calcite increases with decreasing salinity

(Zhong and Mucci, 1989). AlI these observations suggest that the calcite

precipitation reaction mechanism in seawater is complex and depends on both

solute interactions in solution and at the surface of the growing solid.

40

•

•

The calcite dissolution rate constant obtliined in seawater, kh=0.29 ~lmol

rn'2h(1, is significantly smaller than that of Chou et al. (1989; kJ=2324.4

Jlmol m'2hr· l) in dilute solutions. This is in agreement with observations that

calcite dissolution rate is significantly slower in seawater than in low ionic

strength CaCI2+MgCI2 solutions at identical undersaturations and close to

equilibriurn (Walter, 1986). The rate difference would be larger if the low

ionic strength solutions were Mg2+-free since Mg2

+ acts as an inhibitor on the

calcite dissolution reaction (Sjoberg, 1978).

Further resolution of the calcite precipitation rate constant, k.., and

deterrnination of the partial reaction order for the calcium ion will require

conducting experiments by varying the calcium ion and maintaining the

carbonate ion concentrations invariant. Furtherrnore, we believe that to fully

characterize the calcite precipitation-dissolution reaction in complex

electrolyte solutions such as seawater, a systematic investigation is required

to carefuIly study the effect of aIl major seawater components as weIl as

some of the more powerflll and naturaIly occllmng trace inhibitors sllch as

phosphate.

41

•

•

1.5 CONCLUSIONS

ln this study, we were able to demonstrate that a simple experimental

design is suitable for conducting carbonate-solution reaction kinetic studies.

It is self-regulating in tenns of achieving steady state conditions for

carbonate precipitation-dissolution reactions. It might also be applied equally

weil to other mineral-solution kinetic investigations.

Of ail the possible calcite parallel precipitation reactÏons, the fastest and

therefore the rate determining, involves interaction of the carbonate and

calcium ions (i.e., Eqn. 1.20) in simple dilute solutions (Inskeep and Bloom,

1985; Chou et al., 1989) as weil as in seawater solutions. However, while the

interaction is adequately described by an elementary reaction in simple dilute

solutions (Inskeep and Bloom, 1985; Chou et al., 1989), rate data trom this

study and numerous others show that it is a complex reaction in seawater

solutions. This observation suggests that major components of seawater play

an important raIe in calcite and even other carbonate mineraI precipitation

and dissolution reactions. Consequently, mechanistic kinetic expressions

derived to describe calcite precipitation in dilute solutions cannot be applied

directly to seawater.

. The calcite precipitation reaction in seawater can, however, be adequately

described by a complex (or overall) reaction rate model. Data trom this study

yield a partial reaction order of 3 with respect to the carbonate ion and a

backward (i.e., dissolution) reaction rate constant of 0.29 /lmol m·2hr·1• The

42

•

•

interaction of seawater solutes in solution as wel1 as at the surface of the

precipitating solid are most likely responsible for the diffcrence betwcen

reaction mechanisms in dilute and complex electrolyte solutions. The

backward reaction rate constant determined in this study is significantly

lower than that obtained by Chou et al. (1989) from dissolution experiments

in dilute solutions. However, this discrepancy is in accordance with

observations that calcite dissolution rates are much faster in dilute solutions

under identical saturation conditions. Further experimentation (i.e., varying

[Ca] while keeping [C03] and other species constant) is required to ful1y

develop the kinetic expression (i.e., reaction order with respect to [Ca],

forward reaction rate) which describes the precipitation of calcite trom

seawater. In addition, refinements to this model would also require deiailed

investigations on the influence of other major seawater components such as

Mg2+, HC03', H2C03, H+, SO}", Na+, as weIl as reaction catalysts and

inhibitors (e.g., phosphate).

43

•

•

1.6 ACKNOWLEDGEMENTS

The authors wish to express their gratitude to A. Bono, C. Guignard, L.

Hendelman, X. Wu, and A. Yannakis for their technical assistance during

various stages of this study. We also wish to acknowledge the insightfuI

criticism and comments of George Hartley and Dr. James Zullig on an earlier

version of this manuscript. We thank Dr. E. A. Burton and two anonymous

reviewers for their constructive criticism.

FinanciaI support was provided by the NaturaI Sciences and Engineering

Research Council ofCanada (NSERC) to AM. SZ would like to acknowledge

the financial assistance provided by the Lynch, Reinhardt, and William and

Reinhardt funds from the Department of Geological Sciences at McGiIl

University and graduate scholarships awarded by GEOTOPfUQAM through

FCAR-Centre and Team grants.

44

•

•

1.7 REFERENCES

Berner RA. (1975) The role of magnesium in the crystal growth of calcite

and aragonite from seawater. Geochim. Cosmochil11. Acta 39, 489-504.

Burton E.A. and Walter L.M. (1987) Relative precipitation rates of

aragonite and Mg calcite from seawater: tel11perature or carbonate ion

control? Geology 15, 111-114.

Burton E.A. and Walter L.M. (1990) The role of pH in phosphate

inhibition of calcite and aragonite precipitation rates in seawater.

Geochim. Cosmochim. Acta 54, 797-808.

8usenberg E. and Plummer L.N. (1985) Kinetic and thennodynal11ic

factors controlling the distribution of Sa/and Na+ in calcites and

aragonites. Geochim. Cosmochim. Acta 49, 713-725.

Busenberg E. and Plummer L.N. (1986) A comparative study of the

dissolution and crystal growth kinetics of calcite and aragonite. In:

Studies in Diagenesis (ed. Mumpton F.A.). pp.139-168. U.S. Geological

Survey Bulletin 1578.

Busenberg E. and Plummer L.N. (1989) Thennodynamics of magnesian

calcite solid-solutions at 25°C and 1 atm. total pressure. Geochim.

Cosmochim. Acta 53, 1189-1208.

Chou L. and Wollast R (1984) Study of the weathering of albite al room

temperature and pressure with a fluidized bed reactor. Geochim.

Cosmochim. Acta 48, 2205-2217.

Chou L., Garrels RM. and Wollast R (1989) Comparative study of the

kinelics and mechanisms of dissolution of carbonate minerais. Chem.

45

•

•

GeoI., 78, 269-282.

deKaneI J. and Morse J.W. (1979) A simple technique for surface area

determinations. J. Phys. E. Scî. Instr. 12, 272-273.

Dickson A.G. (1984) pH scales and proton-transfer reactions 111 saline

media such as sea water. Geochim. Cosmochim. Acta 48, 2299-2308.

Hansson I. (1973) A new set of acidity constants for carbonic acid and

boric acid in sea water. Deep-sea Res. 20,461-478.

Hartley G., Zhong S. and Mucci A. (1992) The influence of Pcoz on the

incorporation of magnesium in calcite overgrowths precipitated from

seawater at 25°C (abstr.). Eos 73, 168.

Bouse W.A. (1981a) Kinetics of crystallisation of calcite from calcium

bicarbonate solutions. J. Chem. Soc., Faraday Trans. 77,341-359.

House W.A. (1981b) An experimental investigation of carbon dioxide

adsorption during calcite precipitation. Colloids and Surfaces 2, 119-131.

Inskeep W.P. and Bloom P.R. (1985) An evaluation of rate equatiolls for

calcite precipitation kinetics at pCOz less than 0.01 atm and pH greater

than 8. Geochim. Cosmochim. Acta 49, 2165-2180.

Kazmlerczak T.F., Tomson M.B. and Nancollas G.H. (1982) Crystal

growth of calcium carbonate. A cOlltrolled composition kinetic study. J.

Phys. Chell1. 86, 103-107.

Koroleff F. (1976) Detenl1inatioll of phosphorus. In: Methods of Seawater

Analysis (ed. K. Grasshoff), pp. 117-126. Verlag-Chimie.

Lasaga A.C. (1981) Rate laws of chemical reactiollS. In: Kinetics of

Geochemical Processes (eds. A.C. Lasaga and R.J. Kirkpatrick). pp.1-68.

Milleralogical Society of America, Reviews in Mineralogy,Vol. 8,

46

•

•

Washington D.C.

Millero F.J. (1979) The thennodynamics of the carbonate system 111

seawater. Geochim. Cosmochim. Acta 43. 1651-1661.

Millero F.J. (1986) The pH of estuarine waters. Limno1. Oceanogr. 31.

839-847.

Millero F.J. and Schreiber D.R. (1982) Use of the ion pairing model to

estimate activity coefficients of the ionic components of natural water.

Amer. J. Sei. 282, 1508-1540.

Morse J.\V. (1974) Dissolution kinetics of calcium carbonate in sea water.

III: a new method for the study of carbonate reaction kinetics. Amer. J.

Sei. 274, 97-107.

Morse J.W. (1983) The kinetics of calcium carbonate dissolution and

precipitation. In: Carbonates: Mineralogy and Chemistl)' (ed. RJ.

Reeder), pp.227-264, Mineralogical Society of America, Review in

Mineralogy, Vol. 11, Washington D.C.

Morse J.W. and Bender M.L. (1990) Partition coefficients in calcite:

Examination of factors influencing the validity of experimental results

and their application to natural systems. Chem. Geol. 82, 265-277.

Mucci A. (1983) The solubility of calcite and aragonite in seawater at

various salinities, temperatures, and one atmosphere total pressure. Amer.

J. Sei. 283, 780-799.

Mucci A. (1986) G~owth kinetics and composition of magnesian calcite

overgrowths precipitated from seawater: Quantitative influence of

orthophosphate ions. Geochim. Cosmochim. Acta 50, 2255-2265.

Mucci A. and Morse J.W. (1983) The incorporation of Mg2- and Sr' into

47

•

•

calcite overgrowths: influences of growth rate and solution composition.

Geochim. Cosmochim. Acta 47, 2 J7-233.

Mucci A., Canuel R. and Zhong S. (1989) The solubility of calcite and

aragonite in sulphate-fTee seawater and the seeded growth kinetics and

composition of the precipitates at 25°C. Chem. Geo!. 74, 309-320.

Nancollas G.R. and Reddy M.M. (1971) The crystallization of calcium

carbonate. II. Calcite growth mechanism. J. Colloid Interface Sei. 37,

824-830.

Nancollas G.R., Kazmlerczak T.F., and Schuttringer E. (1981) A

ccntrolled composition study of calcium carbonate crystal growth: the

influence of scale inhibitors. Corrosion 37, 76-80.

Plummer L.N., Wigley T.M.L. and Parkhurst D.L. (1978) The kinetics

of calcite dissolution in CO2-water systems at 5°C to 60°C and 0.0 to 1.0

atm CO2• Amer. J. Sei. 278, 179-216.

Reddy M.M., Plummer L.N. and Busenberg E. (1981) Crystal growth of

calcite from calcium bicarbonate solutions at constant Pc02 and 25°C: a

test of a calcite dissolution mode!. Geochim. Cosmochim. Acta 45,

1281-1289.

Sjoberg E.L. CI 978) Kinetics and mechanism of calcite dissolution In

aqueous solutions at low temperatures. Stockholm Contributions to

Geology 32, 1-96.

Walter L.M. CI 986) Relative efficiency of carbonate ûissolution and

precipitation during diagenesis: a progress report on the raie of solution

chemistry. In: Roles of Organic Matter in Sediment Diagenesis (ed. D.L.

Gautier), pp.l-ll, SEPM Spec. Pub!. Vo!. 38.

48

•

•

Weiss R.F. (19ï4) Carbon dioxide in water and seawater: The solubility of

a non-ideal gas. Mar. Chem. 2, 203-215.

Zhong S. and Mucci A. (1989) Calcite and aragonite precipitation tram

seawater solutions ofvarious salinities: Precipitation rates and overgrowth

compositions. Chem. Geol. ïS, 283-299 .

49

•

•

CHAPTER 2

Quantitative Determination of REE in Seawater by Chelation apd

Gradient Ion Chromatography

S. Zhong and A. Mucci

Earth and Planetary Sei., McGill Univ., Montreal, PQ, Canada

ABSTRACT

A revised procedure is described for the quantitative detennination ofrare

earth elements (REE) in aqueous sampies of diverse matrices and high levels

of aluminium, iron, and other transition metals by means of chelation and

gradient ion chromatography. Matrix components such as anions and alkali

and alkali-earth cations were eliminated through selective trapping of REE

and transition metals on a chelating resin. The amounts of aluminium, iron,

and other transition metals in the sample were then reduced to a non­

interfering level following elution through a cation exchange column using

a mixture of HCI-ethanol as eluent. REE ions were separated individually by

a gradient mixture oftwo complexing eluents in an anion exchange analytical

column. Individual REE ions were then derivitized with a post-column

reagent and their concentrations detennined colorimetrically at 520 nm using

a variable wavelength detector. The procedure has direct applications to

various aquatic samples and dissolved solid Illa:erials with limits of detection

of 10-20 ng and precision of ±5% relative s~ji1dard devi:.>tion (1 cr). lt is also

50

•

•

flexible enough to allow a direct REE extraction ITom a large volume of

sarnple solution. This extraction capability is especially valuable for sampics

whose REE concentrations are low and close to the dctection lim its .

51

•

•

2.1 INTRODUCTION

Rare earth elements (REE, or lanthanides), because of their umque

chemical and geochemical properties, have been used as tracers to provide

conclusive infonnation about various high temperature (see, e.g. [1-2]) and

more recently, low temperature geological and geochemical [3-7] (e.g.

sedimentary and oceanographic) processes. One of the major impediments to

the advancement of such applications, however, is the lack of a convenient

and reliable technique to obtain accurate REE concentration data from

sampIes with complex matrices such as seawater, estuarine water, brine, and

geological materials. While modern analytical techniques such as inductively

coupled plasma mass spectrometry (lCP-MS), isotopic dilution lCP-MS,

inductively coupled plasma emission spectrometry (lCP-ES), and instrumental

neutron activation analysis (lNAA) are readily applicable to dilute solutions,

difficulties were experienced when they were applied to samples with

complex matrices [1]. The extremely high levels of alkaline and alkaline­

earth metals, halogens, silicates, carbonates, phosphates, transition metals,

and other elemental and molecular matrix constituents in such materials,

compared to the often extremely low levels of REE, present a fonnidable

obstacle to direct instrumenta! analysis. REE in complex matrices are most

frequently separated by open column ion-exchange procedures, which are

often tedious, expensive, and for which extreme care must always be taken

to avoid contamination [8-10].

An alternative method for the quantitative analysis of REE in geological

•

•

samples was introduced by le Roex and Watkins [II] using chelation and

gradient ion chromatography. This method was originally developcd by

Heberling and coworkers at Dionex"" Corp. [12-16]. It has been successflllly

applied to the direct detennination of REE and trace metals in samples of

diverse matrices. However, difficulties were encountered when it was uscd

to analyze samples with high levels of aluminum, iron, or other transition

metals [11]. An undesirable pre-instrumental open column clean up procedure

was required to eliminate those interfering constituents (e.g., aluminum, iron).

Unfortunately, most often than not, geological sampIes and natural fluids

contain much higher levels of aluminum and iron than REE. ln addition,

procedures for REE extraction and preconcentration, such as the ferric iron

coprecipitation method [10], which is routinely applied to the detennination

of REE in seawater and other low REE-containing natural fluids, invariably

1ead to the introduction of large quantities of iron and an increase in the

concentration of other interfering metals in the sample. It would be extremely

desirable, therefore, if a procedure could be developed that would allow

samples of such nature to be analyzed for their REE content following direct

injection onto an automated ion chromatographic system.

The separation ofmetals in a cation exchange column using a mixture of

hydrochloric acid (HCl) and an organic solvent as eluent lias been studied

extensively by Fritz and Rettig [17], Korkisch and Ahluwalia [18], Strelow

et al. [19], and many others. Preliminary results, using a mixture of HCI and

ethanol for the separation of aluminium and iron from REE in an iOIl

chromatographic system similar to the one used in the le Roex and Watkins

53

•

•

[II] study, are promising [20). Further studies are warranted.

In this paper, a revised chelation and gradient ion chromatographie

procedure is introduced for the quantitative analysis of REE in solutions of

diverse background matrices and with high levels of iron. This method was

used to detennine the REE concentration of artificial seawater solutions and

dissolved carbonate mineraIs prepared during the course of a study on the

REE partitioning between calcite and seawater solutions [21].

54

•

•

2.2 EXPERIMENTAL

2.2.1. Instrumentation:

A Dionex® 4000i chelation and gradient ion chromatograph, similar 10 Ihe

one described by le Roex and Watkins [Il], was used in this study. The

system was constructed around three slider-double-stack-four-way valves

(Fig. 2.1). These valves were controlled by (wo identical and programmable

gradient pump modules (GPM1 and GPM2) which also handled and

delivered ail the eluent solutions required for chelation concentration (GPM 1)

"nd separation (GPM2). Three columns were emp10yed in the system: a

chelating concentrator co1umn (MetPac CC-l), a high capacity cation

exchange concentrator co1umn (TMC-1), and a metal separation anion

exchange ana1ytical column (IonPac CSS). Sampie solutions were de!ivered

to the system by an autosampler or a sample pump, and derivitized REE ions

were measured using a variable wavelength detector module (YOM). The

whole system was controlled by a microcomputer through an advanced

computer interface (ACI).

2.2.2. Eluent and Standard Solutions:

Eight eluent solutions and one post-column reagent were required (Table

2.1). The 3.0 M HCi/SO V.% ethanol eluent (Eluent l, GPM1) was prepared

by mixing 6.0 M HCl (Baker Instra-Analyzed"") with an equal volume of

ethanol (HPLC Spectrograde Reagent). Other solutions were prepared by

55

• •

, ," , , , .

r ........ El2

~3

..... .L E4

Eluents ln

,2

s ,6

r ,\=EIE2E3

..... L E4IYS-GrMT'la- 7

E

Posl-columnreagenl in

,4 2) ... (

'3PLUO

*lr.y"S"O'G;;;P"M""2'11

(V6GPMj-,=<',4

1AUIO- 1" ' 3SNDP v'"!cr SAMPLE T METI'ACIN CC·)

V,0\

cssWASTE

v RDM

""1" • l , ••• " •••••• '" l '"' • Il •• "I •••• 1 •• 1 •••• 1. 1 ., •• 1 • "'1" •••••• t ••• 1 ••• 1 •• "" • "' 1 •••••• , •••••••••••••••

• Solution follows - - - - - ­Solution follows

when valve is ON.when valve is OFF.

Fig. 2.1 Schematic diagram of the chelation and gradient 1011

chromatographie system.

•Table 2.1. Cnmpositions of the eluent solutions and post-column rcagent.

GPMI GPM2

Eluent 1: 3.0 M HCl Eluent 1:. deionized watcr50 v.% ethanol

Eluent 2: 2.0 M ammonium acetate Eluent 2: 6.0 mM PDCApH=5.5±0.1 6.5 mM NaOH

40 mM sodium acctatc50 mM acetic acidpH=4.7±O.1

Eluent 3: 0.50 M HNO) Eluent 3: 0.1 0 M oxaiic acid0.19 M LiOHpH=4.95±O.05

Eluent 4: 0.10 M NH.NO) Eluent 4: 0.1 0 M diglycolic acidpH=3.5±O.2 0.19 M LiOH

pH=5.075±0.025

•

Post-column Reagent: 0040 mM PAR1.0 M 2-dimethylamino ethanol0.50 M NH.oH0.30 M : :aHCO)

57

• following the procedures and conditions outlined by Dionex''ll [13-14]. High

grade chemicals and Milli-Q deionized water (>17.9 Mn) were used in the

preparation of ail eluents and standard solutions. A REE standard stock

solution was prepared by mixing and diluting concentrated (1000 Ilg/ml, or

ppm) individual REE atomic absorption standard solutions (Aldrich<l!l Inc.).

Twelve REE were used: lantllanum, cerium, praseodymium, neodymium,

samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, and

ytterbium. Lutecium was not inc1uded because it is not possible to separate

it from ytterbium [11], while a thulium standard was not available in our

laboratory at the time. REE standard solutions were prepared from tlle

standard stock solution and were acidified to a pH of 1.5 to 2.0 using ultra­

pure 4.0 M nitric acid.

2.2.3. Procedures:

Most of the analytical procedure described below is also given by

Dionex<l!l [13-14]. The major contribution of tllis paper is tlle addition of a

step which serves to eliminate aluminum and iron from sample solutions. An

on-line buffering procedure was also inc1uded to make tlle system more

flexible. A complete and detailed description of tlle procedure is prcsented- .-

so tllat tlle working principle can be more c1early understOod. A simplified

overview of tlle analytical procedure is provided by Fig. 2.2.

•SampIe injection and matrix purification: Sample solutions were

acidified to a pH of 1.5 to 2.0 with ultra-pure 4.0 M nitric acid to prevent

58

• •

To

etector

PDCA Oxalic Acid3.0 M HCI~léiU:1 50% Ethanol PAH

1 '" Diglycolic Acid1

Chelating Resin Cation Exchanger Analytical Column,Ir Ir , Ir Ir Ir

~ CC-1 ~ TMC-1 ~ CS5 •~ -

,Ir 'v

2.0MAmmoniumAc

Sampleln

v.'0

Alkaline andAlkaline Earth Metals

(Waste)

Transition Metals(Waste) ,

Fig. 2.2 Schematic diagram of the analytical procedure (Modified aftcr

Harrold et al. [16]).

•

•

losses of REE due to precipitation and adsorption onto the walls of sample

containers. The acidified sampIe or standard was introduced into the system

by an autosampler or by a sample pump ifunconventional volumes of sample

(>5 ml) had to be loaded when REE cor.centrations were very low «25 ppb).

Sampie volumes ranging from several millilitres (ml) to severa! litres (1) can

be loaded onto the system by a sample pump. The pH of the sample or

standard was adjusted to a value of 5.5±O.2 before it was pushed through the

chelation concentrator column, MetPac CC-l. Optimum selectivity for

polyvalent metal, REE and transition metal ions, is achieved at this pH. The

selectivity of the resin used in the MetPac CC-l column is extremely pH

Jependent. At a pH less than 2, the resin is in the fully protonated fonn and

is not able to chelate cations from solution. The inability of the resin to

chelate at low pH allows chelated metals to be efficiently removed from the

resin. At an intennediate pH, ranging from 4 to 7, the resin displays an

excellent selectivity for REE and transition metals relative to alkali-earth

metals. Alkali metals are not retained within this pH range. At a higher pH

the resin is fully ionized and the selectivity for REE and transition metals

relative to alkali-earths is greatly diminished.

The pH adjustment was achieved through an on-line buffering step:

mixing equal volumes of the sample and the 2.0 M ammonium acetate

eluentJbuffer solution (Eluent 2, GPMl) via a tee connector (Fig. 1.1). The

mixture, having a pH of approximately 5.5, t11en passes t11rough the MetPac

CC-l column. Anions and alkali metals were essentially unretained and went

directly to waste. The weakly retained alkali-earth metal ions were then

60

•

•

selectively eluted to \Vaste by the 2.0 M ammonium acetate eluent (Eluent 2.

GPMl). The direct sample injection and on-line buffering greatly simplif)'

pre-instrumental sampIe manipulation and allow for the injection of a large

volume of sample solution if desired.

Elimination of transition mctals: The strongly retained REE and

transition metals were then eluted to the high capacity cation exchangc

concentrator column, TMC-l, by dilute (0.50 M) nitric acid (Eluent 3,

GPM1). The TMC-l column contains fully sulfonated cation exchange resin

which has sufficient capacity to retain the metal ions under elution conditions

fi'om the MetPac CC-l.

Aluminum and iron were eluted to waste by a HCI-ethanol mixed eluent

(Eluent l, GPM1). The selective removal of aluminum and iron trom the

cation exchange column is based upon the fonnation of metal-chloride

complexes (anions) induced by the water miscible organic solvent, ethanol

[19]. The presence of an organic solvent limits the hydration of metal ions

by water molecules and thus enhances the fonnation of metal-chloride

complexes. The degree of complexation is detennined by the concentration

of HCI, the proportion of organic solvent in the eluent, and the nature of the

cations. Because of the dissimilarities in their charge and charge dcnsities,

"c-0iff<:rent cations will fonn metal-chloride complexes with distinct stabilities.

Using the optimal mixture of HCI and ethanoL 3.0 M HCI/50 v.% ethanol

[19, 22], the relatively stable metal-chloride complexes of aluminium and

iron were selectively eluted to waste while REE, which fonned less stable

61

• metal-chloride complexes, were quantitatively retained on the TMC-l

column.

The TMC-l column was then converted from hydrogen to ammonium

form with 0.10 M ammonium nitrate (Eluent 4, GPM1) before REE and the

remaining transition metals were eluted to the analytical column, IonPac CS5.

This step was necessary to prevent the protonation of the next weak acid

eluent solutions (e.g., pyridine-2,6-dicarboxylic (PDCA), oxalic, and

diglycolic acids) and disruption of subsequent analytical separation.

Separation of transition metals and individua! REE ions: The

concentrated REE and remaining transition metals were eluted from the

TMC-I column as metal-PDCA anionic complexes to the IonPac CS5 column

by the PDCA eluent (Eluent 2, GPM2). PDCA is a strong complexing agent.

Transition metals fonn stable monovalent or divalent anionic complexes,

while REE ions fonn stable trivalent anionic complexes with PDCA [23].

Transition metals were chromatographed isocratically from the IonPac CS5

analytical column by the PDCA eluent through anion exchange. The

difference in ionic charge of the metal-PDCA complexes permitted separation

of the transition metals and allowed the REE to be quantitatively retained on

the CS5 separator coiumn.

The REE ions were then eluted separately using a gradient mixture of

oxalate (Eluent 3, GPM2), diglycolate (Eluent 4, GPM2), and deionized

water (Elu,:nt l, GPM2) through an anion exchange process. The separation

62

•

•

of the REE \Vas not possible with a single eluent due to the chemical

similarity of the ions in the series. The gradient mcthod givcs a high

resolution separation of the REE in the mixed solutions (scc. Fig. 2.3a).

Detection: In this study. the separated metal-acid complexes from the

analytical column were derivitized with 4-(2-pyridylazo) resorcinol

monosodium salt (PAR) and detected by measuring the absorbance at 520 nm

using a variable wavelength detector module (VDM). A membrane reactor

and a reaction coil. which maximized the efficiency of the derivitization

reaction, were used to improve the detection limits to -20 ng for light rare

earths (LREE) and -lOng for heavy rare earths (HREE).

System Control: The system is completely automated and controlled by

a microcomputer (PC 286). It manages the system and collects data from the

detector through an advanced computer interface (ACI) using the Dionex'"

Corp. AI-450 chromatography software. An example of the instrumental

controlling "Timed-events" program is given in Table 2.2. This software was

also used to process raw data into the final desired results and output.

63

,;"',

C>y~o

(a)1 1

• Tb

0.10 1

YbCd 1

Eul

sm l

AU f>ll:d 1

0.05Coll

lai

l 1 \..A.

- ""'"0.00 , , ~T TT

1 ~1 , , 1 1 1 • 1 • , , "

10 15 20 25 30 35Minutes

AU

0.10

0.05

(b)o,.t'

ni 1

1Yb

1

353020 25Mlnute$

150.00 \-T-,--r-r,-..-+Tï-,-.,..--,r-r-r-rT-,-,.....,--,-,.-,.-,-,

,10

•

Fig. 2.3 Typical REE chromatograms of (a) deionized water and (b)

artificial seawater samples (concentrations of individual REE: 5

ppb) after REE instrumental extraction (AU: Absorbance Unit).

64

Table 2.2a. Example of "Timed Events File" for GP~11.

• Step Time Description

1nit. VDM·2 Recorder Mark OFFInit. VDM-2 Recorder Range = 0.100 AUInit. VDM·2 Wavelength = 520 nmInit. GPM StartInit. GPM Hold Gradient ClockInit. GPM Reset ON

1 0.0 GPM Run Gradient Clock1 0.0 GPM Reset OFF2 5.0 ACI NS ON2 5.0 ACI START 2 ON3 15.9 ACI NS OFF~ 15.9 ACI START 2 OFF~

3 15.9 VDM-2 autoOffset ON4 16.I Start Sampling4 16.I VDM-2 AutoOffset OFF5 56.3 GPM Hold Gradient Clock5 56.3 GPM Reset ON

Elnent 1 - HCl/ethanol Eluent 2 - ammonium acctateElnent 3 - HNO, Eluent 4 - NH,NO,VS: OFF - 0 ON - 1 V6: OFF - 0 ON - 1

Time Flow El E2 E3 E4 VS V6 Comment

0.0 2.0 0 100 0 0 1 1 wait for GPM2 to complete5.5 2.0 0 100 0 0 1 0 load sample to CCI7.4 2.0 O. 100 0 0 1 1 elute Ca, Mg, etc. to waste9.5 3.0 0 100 0 0 1 19.6 3.0 0 0 100 . 0 1 0 condition tubing (4-8 of V6)

10.1 3.0 0 0 100 0 0 1 elute REE, TMs to TMC-I12.5 3.0 0 0 100 0 0 112.6 2.0 100 0 0 0 0 0 elute TMs to waste14.5 2.0 100 0 0 0 0 014.6 3.0 0 0 0 100 0 0 eonvert TMe-1 from HO to NH,o16.1 3.0 0 0 0 100 1 016.3 3.0 0 0 100 0 1 1 wash CC lIinjeelion position17.2 3.0 0 0 100 0 1 117.3 2.0 0 100 0 0 1 1 condition CC118.1 0.0 0 100 0 0 1 1 stop

• 65

•

•

Table 2.2b. Example of "Timed Events File" for GPM2.

Stcp Timc Description

. Init. GPM Star!Init. GPM Hold Gradient ClockInit. GPM Reset ON

1 0.0 GPM Run Gradient Clock1 0.0 GPM Reset OFF7- 50.5 Star! Sampling3 51.2 GPM Hold Gradient Clock3 51.2 GPM Reset ON

Eluent 1 - deionized water Elucnt 2 - PDCAEJuent 3 - oxalic acid Elucnt 4 - diglycolic acidVS: OFF - 0 ON - 1 V6: OFF - 0 ON - 1

Time Flow El E2 E3 E4 VS V6 Comment

0.0 1.0 0 100 0 0 1 00.1 1.0 0 100 0 0 0 0 set autosampler path2.4 1.0 0 100 0 0 1 0 close autosampcr pathlU 1.0 0 100 0 0 1 0 star! TMs elution23.1 1.0 0 100 0 0 1 0 stop TMs "lution23.2 1.0 100 0 0 0 1 02S.0· 1.0 100 0 0 0 1 02S.1 1.0 40 0 60 0 1 0 star! REE elution32.0 1.0 40 0 60 0 1 032.1 1.0 20 0 SO 0 1 041.0 1.0 SI 0 26 23 1 051.1 1.0 51 0 26 23 1 0 stop:REE elution51.2 1.0 0 100 0 0 1 0 stop

66

•

•

2.3. RESULTS AND DISCUSSIONS

Analyses were conducted on REE spiked artificial samples with various

background electrolyte matrices, transition rr.etal content, and known REE

concentrations. The REE stock solution used for spiking was the same as the

standard solution preparation. Calibrations using peak area and peak height

generally agreed very weIl with each other, with peak area giving slightly

more consistent results. Peak area was selected for the calibration in this

study. Excellent linearity in the calibration curves for individual REE \Vas

observed when the amount of element in the system was above 75 ng and

below 300 ng for the LREE and between 20 and 300 ng for the HREE (Fig.

2.4). Within the linear range, a precision of ±5% (la, relative standard

deviation) or better was obtained for individual REE based on repeated

analyses (i.e. 10 to 16 times) of the same sample solutions within a 48-hour

period.

Solutions of Simple vs. Complex Matrices: REE spiked distilled water

and artificial seawater [24] with a salinity of 35 were analyzed. Identical

results were obtained for the two groups of samples despite the difference in

background electrolytes (Fig. 2.3). The similarity is more striking than it

appears to be since these analyses used large volumes of sample solutions

(-60 ml).

Solutions with High Transition Metal Content: REE spiked artificial

seawater solutions containing as much as 40 ppm iron (Fe3') were also-

67

• 16000

12000

u~

"g,~

u 8000cr;

4000

o

150000

50000

100

100

200La (Dg)

200Yb (Dg).

300

300

e·

Fig. 2.4 TypicaI standard calibration curves for REE.

68

•

-'

•

analyzed. SampIes of such hich iron content could not be analyzed din;ctly- '.

by ion chromatography using the le Roex and Watkins [11] procedure. ln the

worst case, precipitation of iron following the post-column reagent reaction

resulted in obstruction of the membrane reactor and a system breakdown.

With the revised procedure, however. such sample solutions were analyzed

successfully without any pre-instrumental open column c1ean up tn:atmenL

The iron content was effectively reduced to a non-interference level and REE

were quantitatively retained (Fig. 2.5; Table 2.3). Measured concentrations

were aIl within ±5% of the expected values.

REE extraction experiments were conducted on one litre REE-spiked

artificial seawater sample~; using the ferric iron coprecipitat:ùn procedure

described by Greaves et al. [10]. The resulting solutions had a 25 ml final

volume and an iron concentration of approximately 40 ppm. They were

loaded directly into tlle ion chromatograpil. The results are presented in Table

2.3. In fuis particular case, the average standard deviation from the expected

concentration for aIl of the REE in the sample is ±3.5%.

Direct REE Extraction: The revised procedure is aiso flexible enough

to allow a direct REE extraction from sam pIe solutions by the ion

chromatographie system. This is achieved by pumping a large volume of

sample solution (up to several litres if desired) through the MetPac CC-I

column. At a pH of 5.5, REE and transition metals were quantitatively

retained while the other ions went directly to waste. This extraction method

is especially valuable for analyzing samples whose REE concentrations are

69

•

0.10 Fe

oWI,1EtTb 11

Yb

1

353025MInutes

20150.00 1r'r-r.....,--r..,-,r-r-nJ,-r-T.,.-,,-.,.-,-.,--,-,-,-,-,-,-..,....,.­

10

AU 0.05

Fig. 2.S Typicai REE chromatogram of artificial seawater samples

(concentrations of individual REE: 5 ppb) containing 40 ppm

Fe3+.

• 70

• •

Table 2.3. REE concentration of samples analyzed by chelation and gradient ion chromatography.

Sam pic VII) [REE)(') lLa) [Cc) [Pr) [Nd) [Sm) [Eu] [Gd) [Tb) (Dy) [Ho) (Er) [Yb} .

Fell) -3 75.0 76.3 77.6 74.2 72.0 74.3 78.9 71.7 76.4 77.3 78.1 75.8 75.2ptl!" -3 2.50 2.60 2.74 2.53 2.44 2.32 2.38 2.44 2.57 2.49 2.37 2.47 2.49wnler -30 5.00 4.53 4.83 4.78 4.85 4.89 4.85 4.97 4.89 4.87 4.80 4.76 4.80

-J5wl (5) -3 100 96.5 93.8 92.2 95.7 93.3 94.1 94.8 97.4 101 102 105 101

~ 5w2 -JO 6.00 6.05 6.28 6.03 6.23 6.37 6.14 6.18 5.77 5.74 5.76 5.72 5.625w3 -60 2.00 1.86 1.90 1.94 1.88 2.15 2.17 2.06 2.05 2.02 2.14 2.08 1.89sw4 -180 1.00 1.00 1.04 0.95 1.06 0.99 1.00 1.03 1.03 1.04 1.06 1.05 1.0651\'5 -180 0.50 0.55 0.52 0.49 0.56 0.47 0.45 0.56 0.47 0.49 0.45 0.54 0.55

(1) sampIe volume in ml;(2) concentrations of individual REE added to the artificial samples (J-lg/l);(3) seawater sample containing 40 mg/lof Fe2+;(4) seawater solutions after Fe(OH») coprecipitation extractions;(5) seawatcr samples.

•

•

close to the detection limits. Ir~ this study, samples with volumes ranging

from 3 ml to 180 ml were used and œsults indicated that quantitative

extractions were achieved (Table 2.3).

Future Efforts: The detection limit and allalytical preCISion of the

procedure were largely determined by tile detector. Solutions, after passing

through the ion chromatograph, were free from matrix interferences and REE

were isolated from each other or can be divided into sub-groups. These

sampIe solutions could be analyzed by other analytical techniques, such as

ICP-MS (or isotopic dilution ICP-MS) and INAA, that provide better

precision and 10wer detection limits for REE than the VDM used in this

study. Such combinations may provide more c;·,mvenient and reliable methods

for the routine analysis of REE in samples that traditionally require tedious

and undesirable manipulations such as metal extraction, matrix purification,

and instrumental matrix correction. Finally, it is worth no,i!1g that the

applicatiori of ion chromatography is not limited to the analysis of REE. In

some cases, it may provide a better alternative for the detennination of many

other anions and trace and ultra-trace metals.

72

•

•

2.4. ACK1'10WLEDGEMENTS

We are very grateful to Dionex" Corp. for providing us with unpublished

preliminary laborato;y results on the use of the HCl-ethanol eluent. W~ wish

to thank G. Keating at McGiIl University and S. Boyajian, P. Chang, J.

Grant, and K. Lin from Dionex" Corp., Canada for sharing th~ir expertise

and providing tech:1ical assistance. Special thanks are due to Dr. M. Harrold

ofDionex® Corp. at Sunnyvale, Califomia, for providing insightful comments

on an early draft of this manuscript. Financial support for this study was

provided by the National Sciences and Engineering Research Council of

Canada (NSERC) through equipment and operating grants to A.M., and by

the Ministère de l'Education du Québec through FCAR team and centre

(GEOTOP) grants.

73

•

•

2.5. REFERENCES

1 P. Henderson (Ed.), Rare Earth Element Geochcl1listr\', Elscvicr,

Amsterdam, 1984, 510p.

2 B.R. Lipin and G.A. McKay (Eds.) Geochel1listry and rVlineralogy ûf

Rare Earth Elements, Mincralogical Society of America, Reviews in

Mineraiogy, Vol.21, 1989, 348p.

3 J.L. Banner, G.N. Hanson and W.J. Meyers, J. Sediment. Petrol., 58

(1988) 415.

4 S.L. Dorobek and R.H. Filby, Bull. Cano Petrol. Geol.. 36 (1988) 202.

5 H.J.W. de Baar, C.R. German, H. Elderfield and P.V. Gaans, Geochil1l.

Cosmochim. Acta, 52 (1988) 1203.

6 Y.G. Liu, M.R.U. Miah and R.A. Schmitt, Geochim. Cosmochim. Acta,

52 (1988) 1361.

7 D.J. Piepgras and S.s. Jacobsen, Geochim. Cosmochim. Acta, 56 (1992)

1851.

8 J.G. Crock, F.E. Lichte and T.R. Wildeman, Chem. Geol., 45 (1984) 149.

9 1. Jarvis and K.E. Jarvis, Chem. Geol., 53 (1985) 335.

10 M.J. Greaves, H. Elderfield and G.P. Klinkhammer, Anal. Chim. Acta,

218 (1989) 265.

11 A.P. le Roex and R.T. Watkins, Chem. Geol., 88 (1990) 151.

12 S.S. Heberling, J.M. RivieJlo, M. Shifen and A.W. Ip, Res. Dev., 29

(1987) 74.

13 Dionexoo, Technical Note, 23 (1987) 4p.

14 Dionexoo, Technica1 Note, 25 (1990) 17p.

74

•

•

15 A. Siriraks, H.M. Kingston and J.M. Rivieilo, Anal. Chem., 62 (1990)

1185.

16 M.P. Harrold, A. Siriraks and J. Riviello, Pittsburgh Conference, New

Orleans, Louisiana, 1992.

17 J.S. Fritz and TA Rettig, Anal. Chem., 34 (1962) 1562.

18 J. Korkisch and S.S. Ahluwalia, Talanta, 14 (1967) 155.

19 F.W.E. Strelow, C.R. Van Zyl and C.J.C. Bothma, Anal. Chim. Acta,

45 (1969) 81.

20 Dionex"", unpublished work, 1991.

21 S. Zhong and A. Mucci, in preparation.

12 K.Lin, personal communication, 1991.

23 A.E. Martell and R.M. Smith, Critical Stability Constants. VoU: Amino

Acids, Plenum Press, NY., 1974, pp.367-370.

24 D.R. Kester, .LW. Duedall, D.N. Connors and R.M. Pytkowicz, Limnol.

Oceanogr., 12 (1967) 176.

75

•

•

CHAPT ER 3

Partitioning of Rare Earth Elements (REE) b(;tween Calcite and

Seawater Solutions at 25"C and 1 atm

S. Zhong and A. Mucci

Earth and Planetary Sei., McGill Univ. Montreal, PQ, Canada

ABSTRACT

The partitioning ofREE in calcite oV'~rgrowths precipitated from seawater

solutions under steady state conditions was investigated experimentally using

a constant addition technique. The steady state compositions of the

overgrowths and their parent solutions were detennined by chelation and

gradient ion chromatography (CGIC) and described using non-thennùdynamic

homogeneous partition coefficients. REE are strongly partitioned into calcite

and substitute for Caz+ in the crystal lattice. Their partition coefficients

decrease gradually with atomic number, from _103.4 for La3+ to ':""101.& for Yb3

+.

Under our experimental conditions, the partition coefficient of individual

. REE is not affected by the calcite precipitation rate, [Got] and Pcoz of the

solutions. Ce3+ was stable or metastable in our oxidizing solutions and was

believed to be incorporated in calcite as a trivalent ion since its partition

coef.:icient followed the general partitioning pattem of other REE. On the

other hand, we noticed that increases in the absolute concentrations of REE

or [REE]:[Caz+] ratio and the presence of Oz in solutionincrease REE

76

•

•

partition coefficients significnntly, especially for the light REE.

The partitioning behaviour of REE is closely related to the solubility of

their respective carbonate minerais. However, REE speciation in solution,

adsorption on the surface of calcite, and subsequent reactions Ce.g.,

dehydration) most likely participate in the partitioning process. The

compatibility of our experimental data with results obtained from field

studies suggests that partition coefficients derived from this study can serve

as a too: for the interpretation of environmental, diagenetic, and

paleoceanographic studies.

77

•

•

3.1 INTRODUCTION

Rare earth elements (REE) represent a spectnllll of elements lalOllIic

numbers: 57 to 71) which have the same number (3) and type of valence

electrons in ùleir outennost shell (GS"). They compensate for an increased

positive charge in the nucleus by filling the inne~ partia!ly occupied 4rsubshelI. It is because of this unique electronic configuration that REE

usually occur in nature as trivalent ions and have similar chemical properties.

They behave as a coherent group and appear together in most geological

environments and processes. On the other hand, their chemical properties are

not identical. There is a progressively stronger attraction between the growing

positive charge in the nucleus ~nd the increasing negative charge in the 4f

subshell across the REE series. Consequently, the size ofREE trivalent ions

shrinks smoothly with increasing atomic number; this is known as the

lantha,ide contraction. In tum, it is reflected in graduaI and systematic

variations in the fundamental chemical properties of REE with atomic

number. The unique systematics among REE allow them, as a group, to be

used as a particularly incisive tool in assessing relationships between their

geochemical behaviours and fundamental chcmical properties (e.g.,

Henderson, 1984; Taylor and McLennan, 1988; Lippin and McKay, 1989).

ln addition, two REE, Ce and Eu, can exhibit peculiar geochemical

behaviours due almost exclusively to their distinctive ability to adopt

different oxidation states in aqueous solutions. Under reducing conditions Eu

may assume a divalent state and Ce may exist as a highly insoluble

tetravalent ion in oxidizing environments. Distribution anomalies exhibited

78

•

•

by Eu and Ce in relation to o:i1er REE can, thereforc., be used to extract

infonnation conceming the redox and pH conditions of geological

environments (e.g., Brookins, 1983; 1989; Sverjensk)', 1984; Elderfield and

Sholkovitz, 1987; deBaar et al., 1988; Liu et al., 1988).

Carbonate mineraI~; are sorne of the most important components of

sediments and sedimentary rocks. They contain a variety of coprecipitates,

including REE, that refleçt the mode and environment oftheir formation and

subsequent alteration. Knowledge of the factors that control the incorporation

(Jf foreign ions in these minerais has widespread applications to the study of

paleo-environments and the pathways hy whieh diagenesis and lithification

occur (see Mucci and Morse, 1990 for a r~view). In 1ight of the unique

properties of REE, it is surprising that only very lirriite~ research efforts have-''':-,

been directed at understanding the incorporation of REr::-i~.carhon<!te .

mineraIs.

The incorporation of "foreign" metal ions 111 carbonate mineraIs

precipitated from a solution or melt can be described by a non­

t:.ermodynamic partition coefficient. When the composition of the

precipitating solution, including the concentration(s) of coprecipitating

components, is kept constant throughout the growth process (i.e., steady

state), a compositionaIly-homogeneous precipuate is expected. Under these

conditions, the composition of the precipitate can be described by the

Henderson-Kracek (1927) (or homogeneous) partition coefficient (D):

79

•D =

(XM,)

XCC solid

([Me])[Ca] solin

(3.1 )

where X is the molar fraction of the coprecipitating foreign metal (Me) or

calcium (Ca) ion in the precipitate; [Me] and [Ca] are molar concentrations

of Me and Ca in the reacting soIutior·, resp'ectively. On the other hand. when

the composition of the reacting solution varies with time during precipitation

(i.e., non-stf~ady state), a heterogeneclus solid may be precipitated and the

Doerner-Hoskins (1925) (or heterogeneous) partition coefficient is llsed to

describ':: its composition:

D =t

(dMe)dCa solid

([Me]t)[Ca]t 1

sol n

(3.2)

•

where dMe and dCa are infinitesimal increments of Me and Ca precipitated

in the soIid from a solution which had Me and Ca concentrations of [Me],

and [Ca], at a time t of the precipitation (i.e., instantaneous correlation).

Because of the practicaI difficulties in obtaining dMe and dCa and their

corresponding instantaneous solution concentrations, Eqn. 3.2 is usually

integrated to:

80

•D = (3.3)

•

where subscripts i and f denote, respectively, initial and final solution

compositions. Although more practical, Eqn. 3.3 is valid only if the partition

coefficient (0) remains invariant throughout the precipitation reaction and no

recrystallization of the r'ecipitate takes place (Mucci and Morse, 1990).

Parekh et al. (1977) examined the distribution ofREE in the calcite phase

ofmarine limestones. The close simiJarity between REE distribution patterns

in calcite and normal seawater was interpreted to indicate that the

coprecipitation of REE with calcite occurred directly from seawater with no

. subsequent diagenetic redistribution. REE partition coefficients were

estimated according to Eqn. 3.1, assuming that the calcite was

compositionally homogeneous and precipitated from a seawater solution of

average North Atlantic Oeep Water composition (Hogdahl et al., 1968). The

estimated REE partition coefficients decreased gradually from about 1400 for

the Iightest REE, La, down to about 460 for the heaviest, Lu. They

postu!ated that fractionation among the REE series during coprecipitation

with calcite was controlled by REE complexation in seawater and t1leir

sorption at t1le minerai surface.

Scherer and Seitz (1980) estimated the REE partition coefficients III

81

•

•

Holocene and Pleistocene corals (aragonite) and their œments (!\lg-calcitd.

They also applied the Henderson-Kracek (1927) trace metal partitioning

model to the interpretation of their data, assuming that "average" l'<orth

Atlantic Deep Water was the precipitating fluid. ln contrast to Parekh et al.

(1977), their data suggested a relative heavy REE (HREE) enrichment in thc

Mg-calcite phase. They proposed that the coprecipitation of REE in Mg­

calcite was n::>t an equilibrium process and that the re!a(ive HREE enrichmcnt

might be the result of oacterial activÎ1y.

Palmer (1985) investigated REE incorporation in foraminifera! calcite

.::ùJlected from Atlantic Ocean sediment core tops of variolls locations.

Similar to previous studies, the calcite lattice phase was assllmed

homogeneous and the Henderson-Kracek (1927) partition coefficient was

calculated between the lattice phase and a fluid with a composition

corresponding to average seawater collected from a depth of 100 m in the

Atlantic Ocean. They concluded that whereas the REE distribution pattem in

the surface authigenic Fe-Mn-rich coating was detennined by REF. speciation

in seawater, REE partition coefficients in the calcite lattice phase was

controlled by the similarity of the REE ionic radii to that of Ca" and the

solubility of REE carbonates. Their estimated REE partition coefficients in

calcite were about one order of magnitude lower than those reported by

Parekh et al. (1977) for calcite in marine limestones.

Unfortunately, natural carbonate mineraIs are rarely homogeneous.

Furtllennore, experimental investigations indicate that partitioning of foreign

82

•

•

metal ions in carbonate minerais during precipitation is often affected by

kinetic factors (see Mucci and Morse, ] 990 for review). In fact, Morse and

Bender (1990) pointed out that most of the trace element distribution

coefficients in calcite reported in the ]iterature are phenomenological

measurements of kinetic partition coefficients rather than thennodynamic

distri'Jution coefficients. In other words, panition coefficients genera1iy vary

with solution composition and/or reaction rate and pathways. Bt:'cause of the

uncertainties stated above and the complexity ofnatural environments, results

obtained from these field studies remained inconclusive. For these reasons,

we believed that an experimental investigation of REE partitioning in calcite

in a weil defined and controlled system was perhaps more likely to yield

c1early interpretable results and thus worthy of our efforts.

To the best of our knowledge, only one experimental st'Jdy has been

conducted to detennine the partitioning of REE in carbonate mineraIs and

identifY factors that influence this process. Terakado and Masucia (1988)

detennined the partition coefficients of REE between CaC03 (calcite and

aragonite) and simple aqueous solutions at room temperature (20 to 25°C)

using a fre:':-drift experimental technique. The composition of their reacting

solutions, including REE concentrations, and reaction rates changed

dramatically with time during precipitation. Partition coefficients were

calculated according to the Doemer and Hoskins (1925) model (Eqn. 3.3)

despite the fact that D was found to be depeIident on REE solution

concentrations (Terakado and Masuda, 1988). ln addition, carbonate

nucleation occurred in the eariy stages oftheir experiments and was followed

83

•

•

by crystal growth. It is most likely that REE partitioning was strongly

affected at different stages of a given precipitation experiment. Thercfore. the

interpretation of their experimental data is ambiguous and their applicability

to natural environments is profoundly limited. ln facto their results were one

to two oràers of magnitude lower than REE partition coefficients estimated

from field studies.

The major objective ofthis study is to determine the partition coet1icients

ofREE in calcite precipitated from seawater solutions under weil detïned and

strictly controlled laboratory conditions. While experimental conditions and

solution variables, inc1uding calcite precipitation rate (or calcite saturation

state and [Cot] in solution), redox potential (Eh), REE concentration, and

Pc02 (or pU and (BCO)"]) were kept constant throughout a given precipitation

reaction, they were systematically varied in order to detennine their influence

on REE partitioning in calcite.

84

•

•

3.2 EXPERIMENTAL METHODS

3.2.1 Material:

Artificial seawater with a salinity of 35 was prepared according to Kester

et al. (1967) with two modifications: it had a slightly higher calcium

concentration (10.55 instead of 10.28 mmol!kg-sw) and carbonic acid species

were temporarily withheld from the preparation. This seawater was stored in

a 20-litre NaIgenellO plastic container tor at least three months to reduce

soluble reactive phosphate concentrations. Phosphate ions are known to be

strong inhibitors of calcite precipitation reactions (Mucci, 1986; Burton and

Walter, 1990). Molybdate blue spectrophotometric analysis (Koroleff, 1976)

of the aged artificial seawater indicated that it was essentially phosphate-free

or contained less than 1.6 nmol!kg-sw of soluble reactive phosphate.

BakerOll "Instra-analyzed flux reagent" grade calcite, treated by the

procedure described by Mucci (1986), was used as seed material for the

calcite precipitation and REE coprecipitation experiments. The material has

a weil restricted size range (3-7 ).lm) as observed by scanning electron

microscopy (SEM) and a specifie reactive surface area of 0.52 m2/g as

detennined by the Kr-SET method (deKanel and Morse, 1979).

A REE spike solution was prepared by mixing and diluting concentrated

individual REE atomic absorption standard (1000 ).lg/ml) solutions (AldrichOll

lnc.). Twelve REE were included: lanthanum, cerium, praseodymium,

85

,:::-::---'-~-. ,

•

•

neodymium, samanum, europIUm, gadolinium, terbium, dysprosium.

holmium, erbium, and ytt~rbium. The spike solution contained 50.00 Ilg/m1

of each of the twelve individual REE.

Gases of known compositions were purchased commercially, slored in

high pressure gas cylinJers. Compositions used in this study inc1udcd:

0.033% CO:, 0.31 % CO:, 2.0% CO:, 30% CO:, 0.031 % CO: + 21 % 0:, and

0.031 % CO: + 0.000264% H:S. Ali gases were balanced by N:.

3.2.2 Calcite Precipitation and REE Coprecipitation:

A "constant-addition" experimental system, similar to the one described

by Zhong and Mucci (1993a) (or Chapter 1), was used to conduct calcite

precipitation and REE coprecipitatioll experiments (Fig. 3.1). It provided a

steady state environment for the calcite precipitation reactions (Zhong and

Mucci, 1993a). The system also kept the concentrations of coprecipitating

components (e.g., REE) constant in the reacting solution (Fig. 3.2) and

therefore, created and maintained a steady state environment for the

coprecipitation reactions. Under such conditions, the composition of the

precipitate, including the concentration ofthe coprecipitating components, are

expected to be invariant during the course of a given experimental run. Thus,

a homogeneous precipitate should be obtained.

Briefly, the system consisted of a reactor constructed from a modified

86

•

00-:J

pH e1ectrode

Gas Inlet

Stirrer 1 Gas Outlet

Sol'n Inlet.. 1

'J Glass FritGas Bubbler

PeristalticPump

G:;] __ IllPt.!1§9r!! __

•

Constant Temperature Bath

Fig. 3.1. Schematic diagram of the "constant addition" experimenlal

system.

•100

Eu

GdA ::::.A.,:..A A~-------_._------ 1::t ~1::t

50-...

~5 201:o.-......,~

.b 10=Cl,)u=o 5U

x·Yb

**x

2 oCe

08 0

5 10 15 20Time (hr)

25 30

•

Fig. 3.2. Variability of individual REE concentrations in the reacting

solution throughout an experimental run by the "constant- ~.~

addition" system. Concentrations of individual REE in the

input solutions were 100 nglg.

88

•

•

2-litre Pyrex"" glass separatory funnel into which the reacting solution is

introduced by a peristaluc pump from a 2-litre NalgenellC narrow-mouth

plastic reservoir bottie. The input solution was delivered from the reservoir

to the reactor at a selected, constant rate by the pump using Tygon"" tubing.

A water-saturated gas mixture of known composition was introduced to the

reactor at a controlled rate through a glass frit titted at the bottom of the

separatory funnei. Bubbling of the gas through the reactor served to keep the

Pco2 and redox potential (Eh) of the reacting solution at d.esired and constant

values as weIl as maintaining the calcite solid seed material in suspension.

Mixing of the soIid, liquid, and gas phases in the reactor was further

.:nhanced by a Pyrex"" glass propeller driven by an overhead electric motor.

The temperature of the system was maintained at 25±loC by partly

immersing the reactor in a constant temperature bath.

Before eacl1. experimental run, the input solution was prepared by tirst

introducing two litres oftiltered (0.45 Ilm MiIIiporellC) carbonate-free artiticial

seawater to the reactor. Weighed amounts of Na2C03 and NaHC03 were

added to the seawater to obtain the desired calcite supersaturation after

equilibration with the gas phase. A precise quantity (:>;6 ml) of REE spike

solution was then added to the solution, foIIowed by a smaII and pre-

."- detennined volume of 10 wt.% of NaOH solution to neutralize the nitric acid" '

introduced with the REE spike solution. The introduction of the REE spike

and NaOH solutions had no measurable effect on the carbonate alkalinity and

salinity of the seawater solution. This seawater solution was aIIowed to

equilibrate \Vith the gas phase for at least two hours. pH measurements at ,the

89

•

•

end of the procedure confinned anainment of equilibrium between the

solution and the gas phase. REE complexation reactiong are also t"xpectcd to

reach equilibrium within this period (Cantrell and Byme. 1987). When gases

whose composition included 0" or H"S were used. the input solution.s were

allowed to equilibrate with the gas phase for at least 48 hours to cnsure that

possible redox reactions were at or close to equilibrium. Eh mcasurcments

indicated that the redox potential of the solution stabilized within 48 hours.

The solution was then transferred to the 2-litre plastic reservoir bottle. Calcite

precipitation and REE coprecipitation experimcnts were':;;:';; conductcd

according to the procedure described by Zhong and Mucci (1993a).

pH measurements were conducted usmg a combina~ion electrode

(Radiometer<lO GK2401C) connected to a pH/mV meter (Radiometer<lO M84).

The electrode was calibrated against three NBS (now NIST) buffer solutions

(pH of 6.838, 7.382, 9.180 at 25°C). Reproducibility of pH calibrations

çarried out before and after measurements of a single sample solution was

better than ±0.005 pH unit. In addition, a TRIS buffer solution in artificial

seawater (pH of 8.074 at 25°C and S=35, Hansson, 1973; or 8.067 according

to Millero, 1986) was used to evaluate liquid junction potential variations. pH

measurements on the TRIS buffer scale, when used with the appropriate

constants (Hansson, 1973; Millero, 1979), give an independent assessment of

the concentrations of carbonic acid species. Calcite saturation calculations

using the (wo sets of pH and constants agreed to within ±5% or better.

Results presented in this study were calculated from pH measurements and

carbonic acid apparent dissociation constants based on the NBS scale. Eh

90

•

•

measurements were conducted usmg a combination redox electrode

(Radiometer<lO PK1401), a platinum sensing element a:ld a caloIr.el reference

cell, calibrated against two Eh buffer solutions (Eh(SCE)=196 mV; Eh(sCE)=430

mV, Light, 1972).

At the end of the experiment, the input and the reacting solutions were

collected directly from the reservoir and reactor, respectively, using c1ean 60­

ml B_D<IO syringes and immediately filtered through 0.45 !lm Millipore<lO

fiIters. Their total calcium concentrations and titration alkalinities were

determined by automated potentiometric titrations according to the procedures

described by Mucci (1986), with estimated precisions of better than ±O.5%

and ±O.4% (lcr), respectively. The rest of the reacting solution was vacuum

fiItered through a 0.45 !lm MilliporeClil filter. The filtrate was acidified with

4.0 M ultrapure HN03 to a pH between 1.5 and 2.0 and stored in 1-1itre

Nàlgene<lO plastic bottle. The REE concentrations in the filtered and acidified

reacting solutions were determined by chelation and gradient ion

chromatography (COle). A solution volume ranging from 3.0 ml to 200 ml

was injected directly into the COlC. The analytical procedure developed by

Zhong and Mucci (l993b) (or Chapter 2) has a detection limit of 10 to 20

ng and precision of ±5% (lcr). The filter residue (calcite seed plus

overgrowth) was rinsed with calcite-saturated water and air dried. A weighed

fraction of the solid was dissolved in dilute HN03 and REE concentrations

determined by COlC. [Sr·] and [Mg2.] in the reacting solution and in the

acid digested solid were analyzed by atomic absorption spectrophotometry

(AAS) whereas [Na·] was detennined by atomic emission spectrophotometry

91

• (AES), with estimated precisions of better than ±5%. +1%. and ±5% (1 cr).

respectively.

Calcite precipitation rates (R, /lm01 m·~hr·l) were calculated from the

difference in carbonate alkalinity (meq!kg-sw) between the input (Ac) al,d

reacting solutions (Ac,) and the addition rate of the input solution to the

reactor (1, kg-sw/hr). The rate was nonnalized to the initial reactive surface

area of the calcite seeds:

R = _I_(A_c_o _-.A_c_s_)x_S_OO_

S w:.ud(3.4)

where S is the specific reactive surface area and W,ccd is the weight of seed

introduced in the reactor. The amount of calcite overgrowth (Wovcrg" g) was

calculated according to:

(3.5)

where t is the duration (hr) of the experiment. The molar fractions (XMc) of

foreign metal ions in the overgrowths were calculate-d from their

concentrations in the weighed fraction of solid (seed plus overgrowth)

analyzed ([Me],olid' mollg), the amounts of calcite overgrowths, the seed

weights, and the molecular weight of calcite (M.Wc.lcil.):

[Me]solid M.Wcalclu (W....d + WOvtrg)

Wov.rg.

(3.6)

•In order to avoid discrete REE minerai precipitation, REE concentrations

92

• in solutions should be lower than the solubility of the least soluble REE

mineraI. In the absence of phosphate, the solubility of REE in our

experimental solutions is limited by the precipitation of carbonates (Byme

and Kim, 1993). The solubility products of REE carbonates, REElCÛ3)3,

increase gradually from 10-33.4 for La to 10-31.1 for Yb (Fig. 3.3) (Smith and

Martell, 1976). Since La;:(Cû3)3 is the least soluble among al! REE

carbonates, the equilibrium La3+ concentration in seawater will provide us

with an upper stability limit for the reacting solutions. Its value can be

estimated from the thennodynamic solubility product of LalC03)3 (KOsp=IO­

33.4, Smith and Martel!, 1976\, estimates of the C03;:- total ion activity

~üefficient (YT=0.037, Morse and Mackenzie, 1990), molar fraction

(XF=[La3+]~[La3+h, Millero, 1992; Lee and Byme, 1993) and actlVlty

coefficient of free La3+ (rF-0.10, Millero, 1992) in our seawater solutions:

(3.7)

•

A minimum value of about ISO ng/g for [La3+h is obtained for seawater

solutions within the range of compositions covered in this study (i.e.,

2:5:0.:5:15 orO.OS:5:[CO/-]:5:0.63 mmol/kg-sw). Individual REE concentrations

in our seawater solutions should ideally be kept below this value. On the

other hand, we are also constrained by the detection limits of our analytical

technique. REE concentrations exceeding 0.5 ng/g are required for precise

measurements by COIC (Zhong and Mucci, 1993b).

93

• -30 ~--------------.

-31 ~

0~~

,,,,,,

-34 1-

•

-35

Fig. 3.3.

III III 1 1 1 1 l , 1 l ,

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Solubility products of REE2(C0 3)3 in dilute aqueous solutions.

Data from Smith and Martel! (1976).

94

•

•

Results from previous field slUdies and our preliminary results indicated

that REE are strongly partitioned into calcite precipitates in seawater. Light

REE (LREE) partition coefficients in calcite exceed 10'. This implies that in

order to maintain the concentrations of individual REE in the precipitating

solution above the 0.5 nglg limit, the amount of calcite precipitated from a

unit volume of solution must be extremely smal!. On the other hand, the

amount of calcite precipitated ITom a unit volume of solution must be

quantifiabie (Eqns. 3.4 and 3.5) and thus requires that the difference in

alkalinity between the input and precipitating solutions (Aca-Ac,) be at least

one order of magEitude larger than the precision of the potentiometric

titration (±0.4%, 10'). The weight of calcite overgrowth is essential to

calculate the molar fractions of coprecipitated foreign metal ions (i.e., Eqn.

3.6).

ln summary, it is not possible to maintain the concentrations ofindividual

REE in the input solution below 180 nglg and those in the precipitating

solution above 0.5 ng/g, ifreliable estimates of the amount of overgrowth are

to be ~cquired. To overcome this dilemma, for each parameter investigated,

two ditferent sets of experiments were. conducted. In the tirst set, a large

mass of calcite seed material (5 grams) was used so that signiticant quantities

of calcite were precipitated from a unit volume of solution. Conseqllently,

aCCllrate detenninations of the amount of calcite overgrowth and precipitation

rate were obtained (hereafter reférred to as "5-g" type experiments). In the

second set, a smaller amollnt of seed material (0.6 gram) was introduced to

the reactor so that while accurate estimates of the amount of calcite

95

•

•

precipitated could not be obtained, individual REE in the final rcacting

solutions were not strongly depleted and accurate detenllinations of their

concentrations cOi.:ld be achieved (hereafter referred to as '0.6-g" type

experiments). Since absolute calcite precipitation rates are directly

proportional to the reactive surface areas and the two type of experiments

were condueted under nearly identieal conditions, rate data obtained from the

"5-g" type experiments were used to estimate calcite precipitation rates and

weights of overgrowths precipitated in the "0.6-g" type experiments at the

same degree of supersaturation. For both types of experiments, the input

solutions had equivalent individual REE concentratio'lS, which ranged iTom

40 to 150 ng/g.

Finally, because of uncertainties 111 the estimates of the solubility

pmducts of REE carbonates and the activity of REE ions in our solutions,

there was a possibility that sorne of our initial input solutions were

supersaturated. Ali input solutions, however, were indisputably stable.

Analyses of the solutions indicated that there was no CaC03 or REE

carbonate nucleation and precipitation in the input solutions stored in the

reservoir during the course of a ôven experiment. Apparently, even if the

input solutions were supersaturated with respect to REE carbonates, the

supersaturation states were not high enough to induce the nucleation of

solids. While minerai surface induced precipitation reactions occur in slightly

supersaturated solutions, nucleation requires much higher supersaturation

conditions to overcome the interfacial iTee energy barrier (Niel sen, 1964).

96

•

•

3.2.3 Sorption:

Parallel REE adsorption experiments were conducted in calcite­

equilibrated seawater solutions to establish the importance of adsorption

reactions in the incorporation of REE during calcite precipitation. These

experiments also provided estimates of the importance ofREE adsorption by

the inner walls of the reactor. The experiments were perforrned in the

precipitation reactor filled with two litres of carbonate-free artificial seawater.

Agas containing 0.31 % CO2 in N2 was bubbled through the solutions at

25±1°C. The overhead electric stirrer was tumed on to ensure efficient

ll11xing and interaction between the gas and liquid phases. A weighed amount

of NaHC03 was added to the solution to saturate it with respect to calcite at

the experimental Pc02• After one hour, about 0.2 g of calcite seed material

was added and allowed to equilibrate with the solution for an additiona1 four

hours. This step was necessary to make sure that the seawater solution was

at, or very close to, equilibrium with calcite. 4.0 ml of the REE spike

sclution was then introduced to yield a seawater concentration of about 100

ng/g for each of the twelve individual REE. A pre-detennined volume of 10

wt.% of NaOH solution was added to neutralize the nitric acid introduced

with the REE spike solution. This REE-containing seawater solution was

allowed to equilibrate with the calcite solid and the gas phase for an

additional 15 hours and then filtered through a 0.45 l!m Millipore"'îilter. An

aliquot of the solution was collected, acidified with a few drops of 4.0 M

ultrapure HN03 to a pH of less than 2.0, and stored in 15-ml screw-cap

plastic tubes (SarstedtOt') for later REE analysis. The separatory funnel reactor

97

•

•

was washed with a dilute He' solution and carefully rinsed with deionized

water. The prepared solution was transferred back into tne reactor wherc gas

bubbling and stirring were resumed. After 1.5 hours. another sample of

solution \Vas collected and acidified before 0.6 g of calcite seed matcrial was

added to the reactor, initiating the sorption experiment. Aliquots were

collected at regular inte!'Vals using c1ean 60-ml B-D"" plastic syringes and

immediately filtered through 0.45 !-un Mi!1ipore<l\l filters. acidificd. and ston~d

for later analysis.

98

•

•

3.3 RESIJLTS AND DISCUSSIONS

3.3.1 REE Adsorption on Calcite:

Aliquots of solutions collected before and after they were in contact with

the reactor for 1.5 hours had almost identical REE concentrations. REE

adsorption by the inner walls of the glass reactor was insigriificant and thus

neglected in ail caiculations. The adsorption of REE by the calcite seed

material from calcite saturated seawater solutions was relatively slow and,

followed a sorption behaviour that has typically been documented for

divalent cations (e.g., Davis et al., 1987; Zachara et al., 1988): an initial

relatively fast uptake ofREE, followed by a slow adsorption, and eventually

reaching steady state or equilibrium conditions (Fig. 3.4). The adsorption of

REE by other mineraI surfaces (e.g., hydroxyapatite, goethite, vernadite) in

seawater at higher REE concentrations (250 Ilg/g) also displayed a similar

behaviour (Koeppenkas:::-op and De Carlo, 1992). Again, should our input

solutions be supersaturatcd with respect to REE carbonates, even the presence

of calcite seeds in solution failed to induce the nucleation or precipitation of

discrete REE carbonate mineraIs.

An adsorption coefficient (K), modified after Koeppenkastrop and De

Carlo (1992), was calculated from the steady state or equilibrium solution

concentration for each REE according to:

99

•

•

100 ...-_0 ---..,

90 Yb

........ CV CID 0 0~ 0Olc

80---c0 0-co 6-.....-c 70 g~

GdIDüC ~0ü 0cYJ

60 []J 6-0

La 0 0

5060 80 100a 20 40

Time (hr)

Fig. 3.4. The sorption behaviour of sorne REE by calcite in calcite­

equilibrated seawater solutions.

100

• K=[REEJ.sw:rau<t.Lg m -2)

[REE1so/'n(llg g-l)(3.8)

•

where [REE]surracc is the concentration of individual REE at the calcite

surface. It is defined as the mass of individual REE adsorbed per unit of

calcite reactive surface area (m1). Results of REE adsorption coefficient

calculations are presented in Table 3.1. These results and the raIe of

adsorption on the partitioning of REE between calcite overgrowths and

seawater solutions will be discussed later in section 3.3.3.6.

3.3.2 Calcite Precipitation from Seawater:

3.3.2.1 REE Inhibition of Calcite Precipitation:

Terjesen et al. (1961) observed that some metal ions, inc1uding La3"

could significantly inhibit calcite precipitation reactions in aqueous solutions.

Their experimental data indicated that the effectiveness of metal ions as

inhibitors increases with decreasing. solubility of the metal carbonates. The

inhibitors were effective even at concentrations below 10-6 mollI. Data from

our"5-g" type experiments indicated that the calcite precipitation rate from

seawater was more sluggish in the presence of REE al identical calcite

saturation states. An empirical re1ationship was derived between the ratio of

the rates measured in the presence (RREE) and absence (R) of REE and the

total REE concentrations in solution Œ[REE]) (Fig. 3.5):

101

•

•

Table 3.1 REE Adsorption coefficients (# ofmeasurements: 3; Ail data are within±0.1 of the given values)

Log(K) Log(K) Log(K)

La 3.52 Sm 3.54 Dy 3.07

Ce 3.62 Eu 3.46 Ho 3.06

Pr 3.62 Gd 3.43 Er 2.91

Nd 3.58 Tb 3.25 Yb 2.90

102

•0.0 r--------------,

-0.2

-0.4

-0.6

/" .......

t>:l -0 8e::..~ e::.. .

" ~./ -1.0

~

-1.2

-1.4

2.52.0-1.6 ~_-'--_...J.__ _ __'__ ___'__ ____''__----J

-0.5 0.0 0.5 1.0 1.5

Log(E[REE))

in the precipitating solution (ng/g)

Fig. 3.5. The inhibitory effect of REE on the calcite precipitation rate

in seawater solutions.

• 103

• Lo~R;:) = -(0.54±O.037) Log(E[REE]) - (O.12±O.12) (3.9)

Data on calcite precipitation rate in REE-free seawater solutions (R) arc

presented elsewhere (Zhong and Mucci, 1993a). The inhibitory effect of REE

on calcite precipitation reactions is further evidence of REE coprecipitation

in the calcite structure.

3.3.2.2 Amount of Calcite Overgrowth Precipitated:

For the "0.6-g" type experiment, the difference in the carbonate alkalinity

between the input and the reacting solutions at the end of the run (Ac,,-Ac,)

was usually small and close to the analytical error of the method (± 0.4%,

la). Consequently, it was not possible to accurately detennine the calcite

precipitation rate and the amount of calcite overgrowth using Eqns. 3.4 and

3.5. Alternatively, two independent methods were used to estimate the

amount of precipitate and the precipitation rate.

Calcite precipitation rates in REE-free artificial seawater solutions can be

calculated from their calcite saturation state (.0) according to the following

empirical rate law (Morse, 1983):

Log(R) = nLog(O -1) + Log(k) (3.10)

•where n is the empirical reaction order and k is the rate constant.

Experimental data obtained in REE-free seawater using a similar "constant

104

•

•

addition" system gave the following results: n=2.22±O.05 and

Log(k)=0.21±O.13 (Zhong and Mucci, 1993a). Rate estimates using the above

equation yield values with a relative precision of ±15% (1 cr). By combining

Eqns. 3.9 and 3.10, it is possible to estimate the calcite precipitation rate in

a REE-containing seawater solution from its il and I[REE). The amount of

calcite overgrowth can then be calculated from the precipitation rate and the

duration of the experiment. Errors involved in using this approach, however,

are expected to be relatively high since besides being derived from a limited

set of data, Eqn. 3.9 had to be extrapolated to higher I[REE] in order to

cover the range of REE concentrations investigated in the "0.6-g" type

experiments.

The second alternative takes advantage of the rate (Mucci and Morse,

1983; Mucci, 1986) and solution Pc02 (Hartley et al. 1992) independence of

the Mg2+ partition coefficient in calcite precipitated from seawater. Results

of this study also indicate that REE and the presence of H2S and O2 in

solution do not affect the Mg2+ partition coefficient (Fig. 3.6). Calcite

overgrowths precipitated from our "5-g" type experiments contained

7.00±O.41 mol% of MgC03. This corresponds to a Mg2+ partition coefficient

value of 0.015±O.001. Magnesium calcites of identical composition should

also precipitate from the "0.6-g" type experiments. Based on this assumption,

it is possible to estimate the amount of calcite overgrowth precipitated in a

"0.6-g" type experiment from the Mg2+ concentration ([Mg].olid) in the

weighed fraction of solid analyzed and the average v~lue of the Mg2+

partition coefficient (i.e., 0.015) applying Eqns. 3.1 and 3.6. Calcite

105

:O.i

• C N2/C0 2/H 2So N2/CO"0,036N2/02/C02

:.t...""~ 0,02

~ c8~ @fla 0 0 DO

0,01

0'OOO~----~10::------2~0-----3"'0----...J40

Calcite Precipitation Ràte (/lmol m-2hr-1)

0,04

o N/CÛ/H2S

0,03 o N/CÛ2

l:::,. N/û/Cû2..:;~ 0,02

0 O~l:::,.° ~arb 00

0,01 f-

8,0o,oo'=-----~----_'_----.......-----J6,0 6,S 7,0 7,5

LogCE[REE])In Calcite Overgrowths (ng/g)

•

Fig. 3.6 Constancy of the Mg2+ partition coefficient in calcite precipitatcd

from seawater as a function of (a) calcite precipitation rate and

(b) the total REE content of calcite overgrowths on "5-g" type

experimtnts.

106

•

•

precipitation rate can then be calculated from the amount of calcite

overgrowth and the duration of the experiment according to Eqns. 3.4 and

3.5. Accuracy ofthis estimation method was Iimited mostly by the analytical

precision of the Mg2' determination by AAS (±3%, 1a) and the uncertainty

in the value of the Mg2' partition coefficient (±7%, la).

We believe that Wovcrg. estimates obtained from the latter method are

likely to be more accurate than those calculated from the extrapolation of the

rate data. lt was chosen to estimate the weight of overgrowth and the rate of

precipitation of calcite for aIl the "0.6-g" type experiments. Overgrowth

weight estimates from the two methods yielded results that agreed to within

±100% )r better for the "0.6-g" type experiments.

3.3.3 REE Partitioning in Calcite:

3.3.3.1 Mode of REE Partitioning:

Foreign metal ions cO.n be incorporated in calcite through two principal

modes. They can substitute for Ca2+in the calcite crystal structure and form

dilute solid solutions. Mg2' incorporation in calcite is a typical example of

this mode (e.g., Mackenzie et al., 1983). They can also enter the crystal

structure at defect sites and exist as occlusions. Na+ ions are believed to exist

in such sites in calcite (e.g., Busenberg and Plummer, 1985). In addition,

sorne cations (e.g., Sr') can be found in both lattice and crystal defect sites

(Lorens, 1981; Pingitore and Eastman, 1986; Pingitore et al., 1992). In both

107

•

•

cases, the Henderson-Kracek (1927) homogeneous partition mode! has becn

successfully applied to describe the incorporation(s) of l\1g~', Na', and Sr'

in calcite overgrawths precipitated under steady s.ale conditions.

It has long been recognized that ionic size has major efTects on elemcnt

substitution in crystals (Goldschmidt, 1937). The similarit)' in size betwccn

the carrier (Ca"') and foreign metal ions plays an important raIe in

detennining the extent and their mode of incorporation in calcite (Kretz,

1982; Zachara et al., 1991). Cations must have similar or smaller ionic radii

than Cal + in order to substitute into stmctural sites. In this respect, the

trivalent REE ions are compatible with Ca"+ (Fig. 3.7). In fact, REE3' are

found to be associated with calcium mineraIs, such as fluonte and calcite,

and readily substitute for Ca"" despite the charge difference (Muecke and

MoIler, 1988).

A strong positive correlation between the total concentration of REE

(L[REE]) and the Na+ partition coefficient in the calcite precipitates was

observed (Fig. 3.8). Busenberg and Plummer (1985) proposed that Na'

primarily occupies crystal defect sites in calcite and its abundance in calcite

is positively correlated with factors that affect the density ofstmcture defects,

such as precipitation rate and the amounts ofMg"+ and sa/" in the solid. The

positive correlation presented in Fig. 3.8 maysll~gest that the incorporation

of Na+ serves to balance the excess charge created by the coprecipitatiol~9f

108

•

1

0.80 1.00 1.20o

Ionie Radius (A)

K'"o

1.40 1.60

• "',-,-

Fig. 3.7. Valencies and ionic radii (coordination number: 6) for cations

of interest. Data from Shannon (1976).,

109

•-1.0

~zê -2.0eJlo~

-3.0

6.0 7.0

Log(E[REE])

8.0

•

in calcite overgrowths (ng/g)

Fig. 3.8. Na+ partition coefficients as a function of the total REE

content in calcite overgrowths precipitated from seawater

solutions.

110

•

•

REE in calcite. If REE substitute for Ca1- in the calcite structure, a cationic

site must be created which either remains vacant or is occupied by Na·. The

charge imbalance created by the incorporation of REE may also cause an

increase in the crystal defect density and thus an increase in the amount of

Na· incorporated.

3.3.3.2 The Influence of Precipitation Rate or [CO/-j:

REE partition coefficients obtained from calcite precipitation in seawater

solutions of constant Pc01 (0.031 atm) and a narrow range of individual REE

concentrations <I[REE]: 270 to 350 ng/g) appear to be independent of the

precipitation rate (Table 3.2; Fig. 3.9). However, it must be emphasized that

the range of precipitation rates investigated in this study was limited (1 to 20

).lmol m-1 hr-!). This corresponds to calcite saturation states from 3.5 to 15 or

cot concentrations from 0.15 to 0.63 mmollkg-sw. Further experimental

studies over a wider range of precipitation rates, perhaps by lowering REE

concentration in solution, will be required before a generalization of our

observation can be validated. Nevertheless, the lack of an apparent calcite

precipitation rate dependency reinforces the hypothesis that REE substitute

for Ca1• in calcite lattice sites. The partition coefficient of Sr· in calcite

shows a strong correlation with precipitation rate when the incorporation

occurs at crystal defect sites, but such a correlation becomes ambiguous when

the principal mode of coprecipitation is dominated by Ca1• -substitution at

lattice sites (Lorens, 1981; Pingitore and Eastman, 1986; Pingitore et al.,

1992).

111

• Table 3.2 Average REE partiticn coefiicicnts versuscalcite precipitation rate (Ali data are within±0.2 of the given values)

Rate 18.4-21.1

3

8.1-10.9 3.7-4.1

3

0.9-1.5

4

•

Log(D)

La 3.51 3.47 334 3.41

Ce 3.48 3.47 ~ 3" 3.45,). -Pr 3.43 3.42 ~ "8 3.42,).-

Nd 3.33 3.30 ~ "0 3.31,).-

Sm 3.22 3.14 3.0~ 3.17

Eu 3.22 3.01 2.96 3.09

Gd 2.95 3.00 2.80 3.00

Tb 2.66 " 6- 2.49 2.67_. :>

Dy " 4- 2.43 2.29 2.45-. :>

Ho 2.25 2.23 2.11 2.26

Er 2.08 2.07 1.94 2.12

Yb 1.78 1.81 1.67 1.91

(1) - number of m'~asurements.

112

• •

4.0 ri--------------------~

3.5 Itl ~ ~ ~

~~~'ï:l 3.0::!

B mcil 2.5

62~ 0 O=18.4~I.l~ ...:luJ o = 8.1~IO.9

6= 3.7~ 4.1. ~ ~2.0 1- *=O.9~ 1.5

~R: Jlmol m·2hr-1

1

1.5.La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Fig. 3.9. REE partition coefficients as a function of calcite precipitation

rate.

•

•

From another perspective. data presented in Table 3.2 also indicate that

REE partition coefficients are independent of the CO/, concentration in

seawater solutions. REE speciation calculations using available stability

constants (Cantrell and Byme, 1987; Wood, 1990; Millero, 1992; Lee and

Byrne, 1993) indicate that REE carbonates are the dominant ion pairs

(REE(C03f and REE(C03)2') for REE in our seawater solutions. They

account for more than 95% of the total REE in seawater solutions (Fig. 3.10).

Complexation of REE by other ligands in seawater are not important in the

absence of phosphate (Byme and Kim, 1993). The fraction of free REE ions

in OUi seawater solution was very small and decreased dramatically with

JI1creasing [Cot]. On the other hand, the molar fraction of REE(CO,f was

relatively stable over the range of [CO/] covered by this study. The

independent nature of REE partition coefficient on solution [CO/] seems to

suggest that the REE(C03f ion pair participated directly in the

coprecipitation reactions while the REE(C03)2' complex did not. From a

reaction mechanism standpoint, for the REE(C03)2' complex to precipitate

into the solid phase, a reconstruction of at least one of its two REE-CO,

bonds would be necessary. On the other hand, the REE(C03f ion pair can

probably be incorporated into the calcite structure without any major

modification of its bonding structure. Free REE ions may also participate in

the solution-surface interactions, but their contribution must be minor as a

result of their relatively low abundance in solution.

114

• 0.0 66 ~• La(C03);

G •

-1.0 ••• • • ••••La(C03

) +

~..-..c -2.0 ~0.-.....

i.(,,1

eo:l.o

.ii.r...'-'0Jl

0 .i0 -3.0..:l j,0

*La(s0,J+

00

La3+i.i.j,~

00-4.0 0

0La(HC03

)2+00 00

-5.0 L----I'----L_--'-_~_...L.__ _ _L__.J

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7[CotI (mmollkg-sw)

•

Fig. 3.10. REE speciation as a. function of the total C03

2• ion

concentration in seawater solutions.

115

•

•

3.3.3.3 The Influence of Solution Pco~:

The influence of solution Pc02 on REE partition coefficients \Vas also

investigated. This was achieved by equilibrating solutions with gases of

various PC02, from 0.00033 to 0.30 atm. The Pc02of the solution affects the

relative concentrations of the carbonic acid species and H+ in solutions. At

a calcite saturation state typical of natural surface seawater (Q=5), Pc02

variations over the range investigated would result in a change in [HCO)']

from 1.6 to 48 mmol/kg-sw and pH from 8.32 to 6.76. These changes have

only a limited effect on the calcite precipitation kinetics (Zhong and Mucci,

1993a) and the REE partition coefficients in calcite (Table 3.3; Fig. 3.11).

This probably reflects the fact that variations in [HCO]'] and pH over the

ranges covered by this study do not influence significant!ythe speciation of

REE in solution (Fig. 3.10) and thus the mechanism of REE coprecipitation

reactions. The relatively large data scatter, especially for LREE partition

coefficients, is not non-systematic and within the precision of our

measurements.

3.3.3.4 The Influence of [REE] or [REE]:[Ca2+] Ratio:

The partition coefficients of REE were plotted against their absolute

steady state concentrations in the reactiJ:g solutions (Fig. 3.12). LREE

partition coefficients showed positive correlations with their absolute

concentrations or the [LREE]:[Ca2'] ratios in solutions. (since the [Ca2+] were

kept constant and almost identical in ail our experiments). In other words, an

116

• Table 3.3 Average REE partition coefficients as a function of solution

Pcoz (AlI data are within ±0.2 of the given values)

PCOz 0.30

3

0.0208

6

0.0031

13

0.00033

4

•

Log(D)

La 3.23 ~ ~6 3.44 3.40.J •.J

Ce 3.21 ~ 3~ 3.44 ~ ~8.J. .J .J •.J

, Pr 3.17 ~ ?8 3.40 ~ ~5.J._ .J •.J

Nd 3.04 3.20 3.29 3.29

Sm 2.93 3.01 3.16 3.10

Eu 2.78 2.88 3.06 2.98

Gd 2.70 2.89 2.96 2.94

Tb 2.48 2.56 2.63 2.64

Dy 2.32 2.42 2.42 2.48

Ho 2.18 '2.28 2.22 2.32

Er 2.06 2.16 2.07 2.17

Yb 1.90 1.98 1.81 1.98

(1) - number of meàsurements.

117

• •

4.0 1 ---,

3.5~ § ~e,-...

3.0 a ~ ~ ~'"~Cl'-"

a PcorO.30 atm a ~ è ~O/J0

2.5...:l~

~

00 o Pco2=O.02D8 atm ~

2.0 6. Pco2=O·0031 atm

~*Pco2=O.00033 atm

1.5 '. 11 1 1 1 1 1 1 1 1 1 , 1 , ,

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er· Tm Yb Lu

Fig. 3.11. REE partition coefficients as a function of the solution Pco2•

•5.0,-------------------.

4.0 f-

2.0

l.~.c 0.2.

0.4,

0.6 0.8 1.0 1.2

Log((La)) (ng/g)

5.0,-----------------.

4.0

'"'Cl'ëO 3.0.5

2.0

D-

•

1 0 '---.,~_'___ _'__ __'___'____ _'___ __l

.D.O 0.2 0.4 0.6 0.8 1.0 1.2

Log([Ce))c (ng/g)

Fig. 3.12. REE partition coefficients as a function of their steady state

concentrations in solution.

119

• 5.0 ,------------------,

4.0

2.0

DQJ~ 00­

_120 --':Or-;:;;oo--

1.~.o 0.2 0.4 0.6 0.8

Log([pr]) (nglg)

1.0 1.2

5.0 ,.-----------------,

4.0

§et) 3.0j

2.0

1.~.o 0.2 0.4 0.6 0.8 1.0 1.2

•Fig. 3.12. Continued.

Log([Nd]) (nglg)

120

• 5.0.------------------,

4.0,-...Cl'ëli 3.0o

...J

2.0

o9:]0 ~e..-.C-c--

000 '@ db '"3&!J'B 0 0

0'" tb 0

l.~.o 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Log([Sm]) (nglg)

5.0r----------------....

4.0

§·co3.0j

2.0

1.~.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Log([EuJ) (nglg)

Fig. 3.12. Continued.

• 121

•5.0

4.0,......a003.00

.....l

2.0

1.~.4 0.6 0.8 1.0 1.2

Log([Gd]) (nglg)

1.4 1.6

•

4.0~-------------......,

3.0

§ClQ 2.0.3

1.0

0.0 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Log([Tb]) (nglg)

Fig. 3.12. Continued.

122

• 4.0.-------------------,

3.0~

Cl'Oli 2.0.3

1.0

~ o

o

\fbo o 0

0.0 0.6 0.8 1.0 1.2 1.4 1,6 1.8 2.0

Log([Dy]) (nglg)

4.0r-------------------,

3.0

•

Bco 2.0.3

1.0

Fig. 3.12. Continued.

1.2 1.4 1.6 1.8

Log([Ho]) (nglg)

l ?~_:>

2.0

• 4.0 r------------------,

3.0......Clèi!l2.0.3

1.0

a

lA 1.6 1.8 2.0

Log([Er)) (nglg)

2.2

4.0 r------------------,

3.0.......e002.0.3

1.0

Fig. 3.12. Continued.

1.4 1.6 1.8 2.0

Log([Yb)) (ngfg)

2.2

• 124

•

•

increase in the [LREE]:[Ca~'] ratio in solution results in a disproportional

lIlcrease in thc XLREE:XC. ratio in the calcite overgrowth (Eqn. 3.1). For

HREE, the relationships between partition coefficients and their

concentrations in solution were less c1ear. Terakado and Masuda (1988) also

observed that REE partition coefficients were affected by variations in their

solution concentrations.

The concentration dependence of LREE partition coefficients in calcite

my be explained by the response of the growing crystal surface to changes

in solution composition. LREE have extremely high partition çoefficients in

calcite, indicating that they have a much stronger affinity for the calcite

surface than Ca~+. LREE concentrations, however, were extremely 10w in our

seawater solutions (i.e., 1 to 15 ng/g) and thus the amount of LREE ions or

ions pairs available for sorption was 1imited. Under such conditions, a slight

increase in the LREE concentration in solution may significantly boost the

relative amount of LREE at the surface. Consequently, the XLREE:XCa ratio in

the overgrowth would increase dramatically. Furthennore, reactions occurring

after adsorption on the surface such as surface migration, reorientation,

dehydration would also participate in the partitioning process (Mucci and

Morse, 1983). The inhibitory behaviour of REE on calcite precipitation is

evidence supporting the existence of such crystal surface interactions. The

more ambiguous correlations between HREE partition coefficients and their

absolute concentrations in solution can probably be attributed to their relative

low partition coefficient values and higher steady state solution

concentrations.

125

•

•

Similar arguments have been proposed to explain the influcncc of

solution [Mg~+]:[Ca~+] ratio on the l'dg" partition cocflïcicnl (Lahann and

Siebert, 1982; Mucci and Morse. 1983) and the mechanism of Cd" sorplion

and subsequent solid solution fonnation in calcite (Davis d al.. 1987).

3.3.3.5 The Influence of Rcdox Potcntial:

The redox potential (Eh) of our seawater solution was maintaincd al thrcc

different levels by equilibrating the solutions with gases of thrce diffcrent

compositions. A N/CO/H~S (i.e., Pms=2.64x10.6 atm.; Pco~=0.0031 atm.) gas

ll11xture was used to simulate a slightly reducing environment. The seawaler

solutions had an estimated total H~S content of 10'" to 10.7 mol!kg-sw when

equilibrated-with the gas phase (Drummond, 198]) and a measured Eh,sCEl

of -140±20 mV. A relatively oxidizing environment was created by bubbling

the solutions with a N/O/CO, (i.e., Po~=0.21 atm.; Pco~=0.0031 atm.)

mixture. An Eh(sCE) of 190±10 mV was obtained. Eh(sCE) measuremenls of

solutions through whieh N/CO, (i.e., Pco,=0.0031 atm.) mixtures were

bubbled gave an average value of 150±10 mV.

The partition coefficients of Ce and Eu, the only two REE ions that are

sensitive to the redox conditions of aqueous solutions, were not specificaIly

influenced by the changes in Eh (Table 3.4; Fig. 3.13). In fact, both clements

adhere to the same REE partitioning pattem under aIl conditions of this

126

• Table 3.4 REE partition coefficients versussolution Eh (AIl data are within±O.2 of the given values)

Eh(sCF.) -140rnV +150rnV

3

+190rnV

7

• •

4.0

3.5 ~ 666

8 8 a 66 63.0 0 §

8""';;l §w 6<>:0

El 6'-"..... 002.5,'-' 0

0 ên00 ...:l

o N/COJHzS (Eh(sCE)=-140 rnV)2.0 1- a §aN/COz (Eh(SCE)=+l50 rnV)

6 N/O/COz (Eh(sCE)=+190 rnV) 01.5 ( 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

La Ce Pr Nd Pm Sm Eu Gd Th Dy Ho Er Tm Yb Lu

Fig. 3.13. The influence of Eh or the presence of H2S and O2 in scawalcr

on REE partition coefficients in caicite overgrowths.

•

•

study. Extrapolating from Eh-pH data for REE in simple aqueous solutions

(Brookins, 1983; 1989), the most stable oxidation state for Eu under aIl our

experimental conditions is probably Eu3+. Consequently, it is not expected

that Eu3- would show any irregular behaviour during partitioning. The

therrnodynamically stable oxidation state for Ce is Ct,'" in solutions with

Eh(sCE)~150 mV and pH~7 .6, or in our H~S-free seawater solutions. However,

Ce oxidation in solution is usuaIly extremely sluggish and it appears that Ce

remained in the trivalent state in aIl our solutions. This conclusion is

supported by results of adsorption studies in seawater solutions in which

positive Ce anomalies developed exdusively in the presence of specifie

mineraI surfaces (e.g., vemadite: 8-MnO~) (Goldberg, 1961; Koeppenkastrop

and De Carlo, 1992), indicating that oxidation of Ce3+ to Ce4

+ occurred at the

mineraI surfaces rather than in solution. Positive Ce anomalies are generaIly

caused by oxidation of Ce3+ to the insoluble Ce4

+. The surface of vernadite

is believed to have served as catalyst in the oxidation of Ce3+ to Ce4

+. When

such catalysts are not available or Eh is not high enough, as in our study,

Ce3+ is expected to remain as a metastable species in solution and in calcite

and thus to foIlow the general pattem of REE partitioning. This behaviour is

reminiscing of the metastability of Mn~+ and its incorporation in calcite in

oxidizing environments (Mucci, 1988).

LREE had" much higher partition coefficients when the solutions were

equilibrated with the N/O/CO~ mixture (Fig. 3.13). As indicated above, Eh

variations over the range investigated would not affect the oxidation state of

REE in our solutions. REE species are also not likely to interact with O~ in

129

•

•

solution. In other words, the presence of O~ in solution should not affect REE

speciation in solutions to the degree that is exhibited by the REE partition

coefficient variations. We suspect that 0: may participate in some of the

complex reactions occurring at the surface of the growing crystal and

consequentIy influel:ced tIle partitioning of LREE. However, there are no

data to substantiate this hypothesis.

3.3.3.6 The Role of Adsorption:

REE partition coefficients were compared with the distribution

coefficients for REE sorption on calcite. Botil coefficients decrease smoothly

witIl atomic number (Fig. 3.14). This similarity is in agreement with the

hypotIlesis tIlat adsorption of REE ions or ion pairs by the calcite surface is

tIle first step in a series of complex reactions leading to their incorporation

in tIle growing crystal. On tIle other hand, the general pattern for REE

sorption distribution coefficients is flatter than tIlat of the REE partition

coefficients in calcite. This observation reinforces the hypothesis that

coprecipitation of REE in calcite does not occur simply by incorporation of

aU ions adsorbed on the SUI Îace. Reactions occurring at the surface of the

solid must also lead to a further partitioning of REE between the adsorbed

layer and th~ bulk crystal. For example, Jess energy is required to dehydrate

tIle larger LREE ions or ion pairs, thus it is expected that these would be

more rapidly assimilated into the crystalline lattice (Muecke and Moller,

1988). This may explain, at least in part, the greater affinity of LREE for the

130

• •

14

.04.0 1

-13.5À-....~-Â----.....À_.. Log(J(•.J

"";;)

~~. '"'3.0 '"

3.5. "'-- ..._......... _____....

"~-

'-'0Jl

Log(DREJ

0

1 3.0

2.5 ...:l

Q

~ J2.0

'-'0Jl0

2.5...:l~

2.0 lVJ~

1.5 ' 1 t ' , , , , , 1 , , , l , 1 1 1.5La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Fig. 3.14. REE partition coefficients and adsorption coefficients between

calcite and seawater solutions as a function of atomic number.

• solid phase.

3.3.3.7 Partitioning and the Solubility of Individual REE Carbonates:

There is a negative correlation between REE partition coefficients and the

solubility products of their respective carbonates (Fig. 3.15). The correlation

suggests that the solubility of REE carbonates has significant control over

REE partitioning in calcite. It is weil established that under thennodynamic

equilibrium conditions, the distribution (thennodynamic) coefficients (DO) of

metal ions in calcite should be related to the ratio of the solubility product

! KO,p) of the metal carbonate to that of calcite (Mucci and Morse, 1990):

D~=JK°.sp(cacOJlt

KO.sp(REE,,(COJlJl

(3.11)

•

Although REE partition coefficients reported 111 this study are not

thermodynamic distribution coefficients and are of little thennodynamic

relevance, nonetheless and because of their similar chemical properties, ail

REE should follow the same reaction mechanism during coprecipitation.

Based on these assumptions, REE for which the carbonate minerais have the

lower solubility should have greater tendency of being incorporated into the

solid phase and therefore yield higher partition coefficients.

132

•4.0 r----------------,

3.5 -

1 3•0 ­

ebtI

:3 2.5 ~

2.0· ...

...·····6·Sm .,.0 LaGd q Ndo ..'

Dy •.....•····•·····•·

[J. .

.' .

34.0

•

1.5 l..-__.l...-'_--I.'__....J''---__.l...-'__...J......I_-J'__---J

30.5 31.0 31.5 32.0 32.5 33.0 33.5

-Log(Kesp)

Fig. 3.15. Relationship between REE partition coefficients and the

thennodynamic soIubility product of REE carbonates. The

soIubility data are from Smith and MarteII (1976)

•

•

3.3.3.8 Comparison of Laboratory Studies:

The REE partition coefficient data obtained in this study were c\early

different from those reported by Terakado and Masuda (1988) (Fig. 3.16).

The large discrepancies probably reflect the fact that the two studies were

carried out under very different conditions. Terakado and Masuda (1988)

conducted their experiments in a free-drift system in which spontaneous

nucleation of calcite and consequent crystal growth took place. Solution

compositions and calcite growth rates changed dramatically throughout the

precipitation. As was observed by Terakado and Masuda (1988), such

changes resulted in variations in the REE partitioning mechanism and

partition coefficient values during the course of a given experiment. In

contrast, our study was carried out under controlled experimental conditions

so that resulting REE partition coefficients should be representative of the

specifie geochemical conditions under which calcite was precipitated. We

suspect that because Terakado and Masuda (1988) canied out their

experiments at extremely high reaction rates and in solutions of low REE

concentrations, calcite precipitation and REE coprecipitation reactions were

probably controlled by the diffusion and adsorption rates of ions or ion pairs

from the bulk solution to the mineraI surface. As the diffusion and adsorption

rates of REE ions or ion pairs could not keep up with the precipitation rate,

only a limited amount of REE could be adsorbed on the surface of the

growing crystal before being included within the solid thus resulting in

anomalously low partition coefficients. This observation supports the idea put

forth by Morse and Bender (1990) that partition coefficient of foreign metals

134

• •4.0 i 1

--Â - - -- -~ - _.. - .....- -Â-Â- - - - -Â - - - - -Â- - - - - -Â--

/\ _....~ -L::!1-Â ~.B...o ...O' .. -o..·o........ "."':8 8 ....0

" _·e··_..e B - - - - - /\'0"

,,-" -'" . -' '-8- -t.=r.. ' "-..- e..'-.. - .. -r.Lo--..-~

0 0 ( .1:.:J - 1 ..

3.0

""";;ll:!B 2.0

b.O0~

.....1.0

u)v,

0.0~~~~~~~~Th~~&~n~

Fig. 3.16. Comparison of field and experimcntally derived REE partition

coefficients between calcite and ils parent solutions. Field data

are trom Parekh et al. (1977) 0, Scherer and Seitz (1980) 6.,

and Palmer (1985) 0, experimental measurements are those of

Terakado and Masuda (1988) Â and this study *.

•

•

In calcite are not equivalent to thermodynamic constants and gcnerally

represent phenomenological measurements under a given set of conditions.

3.3.3.9 Comparison of Laboratolj' and Field Results:

The fact that there are large discrepancies among the few available field

data sets makes it difficult. if not impossible. to compare our experimcntal

data with those obtained from field samples (Fig. 3.16). The discrepancics

among field data sets probably reflect the diversity of the environments under

which calcite was precipitated. Different sampIe treaUnent procedures and

ullcertainties inherent in partition coefficient detenninations and analytical

methods used in these earlier studies further contributed to the data scattering

(Parekh et al., 1977; Palmer, 1985). On the other hand, because of analytical

limitations, the REE concentrations investigated in this study were much

higher than those normally encounte,ed in seawater or other natura! solutions.

In addition, most natural calcites were biogenic. The uptake mechanism of

REE in biogenic calcites may be affected by "vital effects" which cannot be

reproduced under laboratory conditions. Other natural seawater componenls

such as reactive phosphate and dissolved organic matter which were not

investigated in this study form strong complexes with REE and/or influence

the surface properties of calcite as weil as its precipitation kinetics. Ali these

factors will certainly affect the compatibility of our experimental data to field

results. Nevertheless, despite of ail these uncertaintÎe::;::our REE partition

coefficient data are generally compatible with field observations.

136

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•

Finally, it should be noted that extrapolations oflaboratory results to field

studies are rarely straightforward. The discrepancy between Mg2' partition

coefficients in calcite detennined under controlled laboratory conditions and

field observations serves as a good illustration of the problem (see Morse and

Mackenzie, 1990 for summary). We acknowledge that it will certainly require

more than the present study and the study of Terakado and Masuda (1988)

to elucidate our understanding of the behaviour of REE during carbonate

mineral precipitation. Nevertheless, the compatibility ofour experimental data

with results obtained from field studies suggests that partition coefficients

derived from experimental study can be used for the interpretation of

environmental, diagenetic. and paleoceanographic studies.

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3.4 CONCLUSIONS

(1) Through this study, we successfully demonstrated that it is possible

to create and maintain a steady state environment for calcite precipitation and

foreign ion coprecipitation reactions using a simple "constant addition"

experimental technique. The reponed REE partition coefficients represent

quantitative measurements ofREE incorporation in calcite precipitated under

weil defined kinetic and strictly controlled conditions. The influence of

kinetic as weil as thennodynamic factors on the REE partitioning can be

individually and systematically resolved.

(2) REE are incorporated in calcite by substituting for Ca2+ in the crystal

lattiee, fonning solid solutions. Partitioning of REE in calcite will probably

reflect conditions under which the mineraI is fonned.

(3) The presence of REE in seawater solutions, even at low extremely

concentrations (i.e., L[REE]<10 ng/g), inhibit the precipitation of calcite.

(4) REE are strongly partitioned into calcite during precipitation !Tom

seawater. Their partition coefficients are as high as 103.5 and decrease

gradually with increasing atomic number. Based on this observation, we

predict that the general REE distribution pattern in calcite will exhibit strong

LREE enrichments when compared with its parent solution.

(5) Among the factors investigated, the partition coefficients of REE,

138

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•

especially LREE, in calcite are significantly affected by their absolute

concentrations or [REE]/[Ca~·] ratios in solutions. Calcite precipitation rate,

[CO/] or Pco~ in solution, however, have little effect on the REE partition

coefficients. Eh variations in the range covered by this study also do not

affect the oxidation state of Ce3• and Eu3

• in solution and their partition

coefficients, since both ions were stable or metastable under our experimental

conditions. On the other hand, the presence of O2 in solution influenced the

partition coefficients for LREE in calcite dramatically, but we have failed to

find an acceptable explanation for this behaviour.

(6) The non-thennodynamic REE partition coefficients are negatively

correlated to the solubility of their respective carbonate minerais. Individual

REE with lower carbonate solubility products have higher REE partition

coefficients. REE speciation in solutions, adsorption ofREE ions or ion pairs

(e.g., REE(C03n, and subsequent surface reactions such as dehydration also

influence the incorporation of REE in calcite.

(7) The compatibility of our data with results obtained from field studies

suggests that results of our experimental study could at least serve as a

general guide for the application of REE partItiOn coefficients to

environmental, diagenetic, and paleoceanographic studies.

139

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3.5 ACKNOWLEDG EM El\'TS:

We wish to thank A. Bono. L. Hendelman. S. Lalli. G. Keating. anô X.

Wu for their technical assistance in the laboratory. Financia! support \l'as

provided by the National Sciences and Engineering Research Council of

Canada (NSERC) to AM. SZ gratefully acknowledges the financia! assistance

provided by the Davison. LeRoy. Lynch. Reinhardt and William funds l'rom

the Department of Earth and Planetary Sciences at McGill University and

graduate scholarships awarded by GEOTOP/UQAM through FCAR-Centre

and l'eam grants.

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CH..\PTER 4

Concluding Remarks

4.1 CONTRIBUTION TO ORIGINAL KNOWLEDGE

There are four major contributions from this study which 1would like ta

emphasize:

(1) The development of the "constant addition" experimental technique

which has proven to be a superb technique for the study of precipitation

kinetics and "foreign" element partitioning in calcite. lt is a simple technique

which enabled us to res'rict operational eITors to a minimum. Its most

important advantage is its inherent ability to achieve and maintain steady

state conditions during calcite precipitation and "foreign" element

coprecipitation reactions. This feature aIIowed us to accurately measure

calcile precipitation rates in seawater at lower saturation states than could be

achieved before and to maintain a steady state environmcnt for the

partitioning of multi-elements present at extremely low solution

concentrations (e.g., REE). This technique may also provide aninteresting

altemative for other mineral-solution interaction kinetic studies.

(2) The establishment of a yet incomplete kinetic model for calcite

precipitation reactions in complex solutions such as seawater. The model is

based on the consideration of four paraIIel reactiOlls, which are believed to

participate in the global precipitation reaction.

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(3) The improvement of an analyticaI procedure for the separation and

determination of REE by chelation and gradient ion chromatography. It has

the ability of eliminating interfering matrix constituents and provides a

convenient and reliable method for the determination of REE in geological

materials and strong electrolyte solutions, with low detection limits and

relatively high precision.

(4) Most importantly, the accurate measurements of REE partition

coefficients between calcite overgrowths and their parent seawater solutions

and an investigation of factors which influence the partitioning process. This

is the tirst successful study on the subject and ail data and observations

presented can be considered contributions to original knowledge.

4.2. SUGGESTIONS FOR FUTURE STUDY

4.2.1. Calcite Precipitation Kinetics:

Further experimentation is required to fully develc;J the kinetic expression

which describes the precipitation of calcite from seawater solutions.

Retinements to this model will require systematic investigations on the

influence of major seawater constituents such as Mg!·, HC03-, H!C03, H+,

SO/", Na+, as weil as reaction catalysts and inhibitors (e.g., phosphate). As

a matter of fact, a study has already been initiated in our laboratory to

detennine the partial reaction order with respect to the Ca!· ion (Le., varying

[Ca!·) while keeping [CO/") and other species constant). Ultimately, the

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kinetic expression may take into account the participation of specifie

dissolved complexes such as ion pairs and the inhibitory efTect of other

naturaliy occurring substances (e.g., phosphate. dissolved organic carbon).

Finally, a similar study of dissolution kinetics would complete the mode\.

lt will be extremely beneficial if future studies could take ad-, antage of

the recent developments in atomic force microscopy (AFM), scanning

tunneliing microscopy (STM), and X-ray absorption spectroscopy (XAS) and

their potential application to the study of minerai surface reactions (e.g.•

growth, dissolution, partitioning, zoning).

4.2.2. Analysis of REE using CGIC:

The detection limit and analytical precision of the chelation and gradient

ion chromatographie method were largely detennined by the detector (i.e.,

variable wavelength detector) we used. Sample solutions, after passing

through the COIC, were matrïx-free and REE were isolated from each other.

These solutions could be analyzed by other analytical techniques, such as

ICP-MS and INAA, which provide better precision and lower detection

limits. Such combinations may provide more convenient and reliable methods

for the routine analysis of REE in samples that traditionally required tedious

and undesirable manipulations such as metal extraction, matrix purification,

and instrumental matrix correction (e.g., geological and seawater samples).

Il is also worth noting that the application of ion chromatography is not

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lirnited to the analysis of REE. In sorne cases, it rnay provide a better

alternative for the detennination of rnanv other anions, as weIl as trace and

ultra-trace rnetals.

4.2.3. REE Partitioning:

Future experirnental studies could consider lowering REE concentration

in the reacting solutions to a level that is more representative of natural

oceanic environrnents (i.e., pg/g), probably by using analytical techniques that

provide lower detection lirnits for REE analysis, such as cornbining CGIC

with isotope dilution ICP-MS. The effect of calcite precipitation rate on REE

partitioning should also be extended over a wider range of rates or saturation

states, which could probably also be achieved by lowering REE

concentrations in the reacting solutions. The effect of O2 on REE partitioning

certainly warrants further study. Partitioning reactions should be carried out

under more reducing conditions (by equilibrating seawater solutions with

gases of higher H2S partial pressures) so that the change in the oxidation

state of Eu3+ to Eu2

+ on its partitioning can be investigated.

151

•Appcndix J. Raw experimental data on calcite precipitation from REE-free seawater solutions.

EXil # (Cal At JIll "'(Ac)' (CO,I n \Vmd Timc' 1 Ralcmmollkg m('qfkg meq/kg mmollkg g hr glmln 'Imol/(mJhr)

57.-060 11.08 2.795 7.527 0.704 0.0688 1.74 1.0051 90.30 0.0220 0.85557.-061 11.17 3.153 7.582 0.350 0.0875 2.23 1.0010 27.38 0.1097 2.1357.-062 11.12 3.138 7.574 0.364 0.0856 2.17 1.0052 23.47 0.1097 2.2157.-063 11.21 3.136 7.562 0.365 0.0834 2.13 1.0012 96.47 0.1097 2.2257.-064 10.82 2.694 7.555 0.219 0.0704 1.74 1.0036 78.15 0.0672 0.81657.-065 10.83 2.700 7.556 0.213 0.0707 1.75 1.0000 64.68 0.0663 0.78657.-066 10.85 2.440 7.493 0.099 0.0557 1.38 1.0026 115.85 0.0336 0.18557.-067 10.84 2.440 7.494 0.099 0.0558 1.38 1.0060 98.87 0.0350 o.ln57.-068 10.84 2.440 7.496 0.100 0.0561 1.38 1.0043 169.67 0.0350 0.193- 57.-069 10.80 3.173 7.611 0.226 0.0937 2.31 1.0013 24.00 0.1949 2.45

VIIv 57.-070 10.70 3.131 7.576 0.266 0.0858 2.09 1.0000 32.15 0.0935 1.38

57.-071 10.69 3.120 7.583 0.277 0.0868 2.11 1.0018 41.00 0.0935 1.4457.-072 10.75 3.585 7.667 0.286 0.1195 2.93 1.0017 43.38 0.3610 5.7357.-073 10.73 3.438 7.635 0.352 0.1070 2.62 1.0019 24.93 0.2132 4.16

57.-074 10.73 3.420 7.649 0.371 0.1097 2.68 1.0000 49.00 0.1761 3.63

57.-075 10.72 3.443 7.646 0.347 0.1097 2.68 1.0066 47.23 0.1746 3.35

57.-076 10.75 3.456 7.646 0.334 0.1101 2.70 1.0025 48.53 0.1743 3.23

57.-100 10.30 5.119 7.703 0.740 0.1847 4.33 0.6003 10.13 0.3497 24.0

57.-101 10.Q3 5.234 7.732 0.772 0.2009 4.59 0.6002 12.07 0.3494 25.0

57.-102 10.30 5.136 7.714 0.874 0.1897 4.45 0.6001 10.82 0.3499 28.3

57.-103 10.88 3.997 7.630 0.437 0.1232 3.05 0.6000 35.83 0.1145 4.64

a - (Aco-Acs); b - Duration of experimenl.

•

•Appcndix I. Continucd

•

Exp # ICaf At pH tl(Ac) ICO,! il \V1tC'd Time 1 Ratemmol/kg mcqlkg mcq!kg mmol!kg g hr gtmln 'Imoll(m1hr)

sz-104 10.87 3.975 7.615 0.522 0.1187 2.94 0.6002 35.50 0.1145 5.54sz-105 10.91 3.992 7.621 0.503 0.1207 3.00 0.6008 37.45 0.1137 5.29sz-106 9.57 6.256 7.791 0.842 0.2721 5.93 0.6001 6.13 0.6614 51.60sz-107 9.54 6.105 7.790 1.119 0.2649 5.76 0.6000 6.58 0.6559 67.98sz-108 '9.63 6.200 7.806 0.976 0.2782 6.10 0.6003 6.02 0.6557 59.21sz-109 9.05 7.078 7.833 1.139 0.3362 6.93 0.6000 3.62 1.0092 106.4sz-IIO 8.70 7.167 7.814 1.087 0.3273 6.49 0.6002 3.62 0.9857 99.15

~ sz- 1II 8.28 7.088 7.817 1.163 0.3257 6.14 0.6003 3.77 1.0013 107.8V.vJ sz-128 10.36 5.269 7.724 1.023 0.1988 4.69 0.6021 11.53 0.3632 34.29

sz-129 10.20 5.232 7.728 1.051 0.1991 4.63 0.6023 12.00 0.3665 35.52sz-130 9.84 5.256 7.714 0.941 0.1942 4.35 0.6000 12.53 0.3657 31.88sz- 131 nIa 2.889 7.481 0.066 0.0644 nIa 1.0046 67.00 0.0678 0.246sz-132 10.71 2.819 7.458 0.125 0.0597 1.46 1.0053 52.67 0.0680 0.468sz-133 10.71 2.790 7.451 0.142 0.0582 1.42 1.0037 4R.35 0.0673 0.529sz-134 10.80 2.815 7.446 0.108 0.0581 1.43 1.0050 54.07 0.0667 0.398sz-135 10.76 2.576 7.421 0.038 0.0503 1.23 1.0040 87.25 0.0336 0.071sz-136 nIa 2.610 7.415 0.003 0.0503 nIa 1.0006 116.45 0.0332 0.005sz-137 10.77 2.579 7.428 0.035 0.051 1 1.~5 1.0001 121.90 0.0336 0.066sz-138 nIa 2.586 7.384 0.049 0.0465 nIa 1.0027 119.20 0.0336 0.092

•Appcndix Il. Raw data on calcite precipitation for the "5-g" type cxpcrimcnls.

•

Exp # " (C.) At pli ll(Ae) (CO,) n \Vmil Tlme 1 Rate Pco J \V....,'mmollk2 lI1l'q lkll: llIeqlk2 mmollkjt 2 hr t'min ,UIIOL'(III'hr} Lo~(alm) 2

sz-110. 10.03 1.990 8.206 1.034 0.1887 4.31 6.0000 45.17 0.6120 5.861 ·3,48 0'<)69sz·l72 9.96 5.930 7.874 1.411 0.3060 6.94 5.0057 24.00 1.15·10 18.07 ·2.60 0.1 17sz·173 9.89 5.929 7.862 1.514 0.2985 6.73 5.0000 23.50 12175 20,4 9 -2.59 0.130sz·174 10.17 7.110 7.939 1.304 0,4193 9.71 5.0015 12.12 2.3931 H.68 ·2.W 0.114sz·175 10.08 4.858 7.769 1.165 0.2014 4.62 5.0083 47.'15 0.6122 7.913 ·2.58 0.102sz-176 10.16 4.868 7.764 1.086 0.1997 4.62 5.0030 47,43 0.6126 7.385 ·257 0.095s7.·179 9.99 7.014 7.915 1.356 0.3940 8.96 5.0000 12.17 2.4186 36..13 -2.58 0.120

~ sz·180 10.57 4.210 7.721 0.185 0.1576 3.79 5.0056 95.00 0.3106 0638 -2.59 0.01(,<Jo 0,·181 10.08 4.899 7.759 1.116 0.1989 4.57 5.0054 48.67 0.6143 7611 ·2.56 0.100-l>o

0,·182 9.97 4.748 7.732 1.318 0.1817 4.13 5.0060 47.75 0.6191 9.058 ·2.55 0.117sz-183 9.56 11.84 7.395 1.895 0.2195 4.78 5.0008 47.22 0.6158 12.96 -1.79 0.164s7.·184 10.15 12.08 7.397 1.947 0.2250 5.20 5.~062 48.13 0.6169 13.33 -1.78 0173sz·186 10.59 4.298 7.7J.l 0.287 0.1654 3.99 5.0067 92.88 0.3098 0.986 ·2.59 0.025sz-187 10.32 5.332 7.818 0.718 0.2450 5.76 5.0021 47.15 0.6137 4.894 ·2.59 0'<J62sz-188 9.97 4.590 7.750 1.568 0.1828 4.15 5.0025 48.57 0.6137 10.68 ·2.58 0.140sz·190 10.31 1.993 8.204 0.701 0.1884 4,42 5.0015 47.85 0.6134 4.779 ·3,48 0.062sz·191 9.54 48,44 6.832 1.987 0.2529 5.50 5.0063 47.88 0.6124 13.50 ·0.60 o 175sz-l92 9.63 48.29 6.826 2.370 0.2487 5,45 5.0029 47.22 0.6078 15.99 ·0.60 0.2040,·195 9.89 4.823 7.759 1.313 0.1958 4,41 5.0038 47.50 0.6077 8.860 ·2.57 0.114sz·196 9.55 12.31 7,417 2.113 0.2396 5.21 5.0013 47.25 0.6051 14.20 ·1.80 0.181,,·197 10.12 5.385 7.752 0.859 0.2156 4.97 5.0039 48.70 0.6053 5.771 ·2.51 00765z-198 9.74 4.636 7.700 1.504 0.1661 3.69 5.0052 47.95 0.6059 10.11 -252 o 131

•Appendix II. Continued.

•

Exp /1 (CRI At pli 6(Ae) [CO,! n \Vurd Tlme 1 RRte PCO J 'Vo....mmoVk~ ml'qlk-= n1tqlkg mmolJ1cg g hr g/mln 'lmol!(m'hr) l.og(alm) g

sz-201 10.23 4.271 7.630 0.220 0.1317 3.07 5.0050 96.53 0.3072 0.750 -2.48 0.020hs-203 9.76 4.931 7.688 1.159 0.1723 3.83 5.0000 48.38 0.6698 8.628 -2.48 0.113hs-204 9.67 4.652 7.668 1.422 0.1557 3.43 5.0012 47.65 0.6490 10.25 -2.49 0.132hs-207 9.61 4.766 7.657 1.434 0.1559 3.41 5.0032 49.33 0.6231 9.924 -2.46 0.\32hs-209 9.76 4.685 7.712 \.493 0.\722 3.83 5.0034 50.30 0.5873 9.737 -2.53 0.\320,-210 9.69 4.719 7.719 1.486 0.\76\ 3.89 5.0051 49.05 0.6093 10.05 -2.53 0.133s,,-212 9.74 4.655 7.729 1.929 0.1774 3.93 5.0039 47.75 0.606\ \5.00 -2.55 0.194

......v,v.

• •Aflflcndix III. Composition of calcite overgrowths precipitatcd from thc "5-g" type cxpcrimcnts.

Exp # Mg DMR UNI Ds• E[REEI..,·. E[REE]..". Log(RRF.t:) Log(R)0101% ,100 ,100 ,100 (nglg) (ntglg)

5z-170 7040 1.63 1.44 3040 4.51 15.94 0.77 1.365z-l72 7.34 1.61 1.47 13.06 7.54 12044 1.26 1. 935'1:- J73 7.77 1.63 0.59 10.21 3.60 8044 1.31 1.895z-174 7044 1.61 0.57 13.00 22.53 15.26 1.54 2.305z-175 6044 1.36 0.54 8.11 5.57 11040 0.90 1045

~ 5z-176 7.38 1.64 1.24 9.85 4040 11.65 0.87 1.45VI 5z-179 7.63 1.65 1.03 14.70 22.22 12040 1.56 2.21cr-

5z-180 5.87 1.55 4.54 1.24 120.5 45.72 -0.20 1.200,-181 6.80 1045 0.69 6.66 5.21 11.13 0.88 1.440,-182 6.74 1.39 0.31 6049 2.95 5.92 0.96 1.315z-183 7.10 1041 0.38 6.50 3.93 4.12 1.11 IA95z-184 7.43 1.59 0.59 10.59 3.96 5.05 1.12 1.595z-186 6.86 1.65 2.13 1.26 94.24 24.91 -0.01 1.275z-187 6.36 1045 1.60 4.90 46.23 23.18 0.69 1.715z-188 6.64 1.38 0045 6.61 1.16 4.01 1.03 1.325z-190 6.67 1.46 0.70 4.11 5.67 16.27 0.68 1.405z-191 7.38 1.47 0.39 8.65 35.63 5.84 1.13 1.66

•Appendix III. Contillued.

•

Exp # Mg DM, DN• Ds• LIREEI",·. LIREEI...., Log(R.,,) Log(R)mol% ,100 '100 ,100 (nglg) (mglg)

sz-l92 6.93 1.39 0.43 8.67 36.59 5.57 1.20 1.650,-195 6.97 1.44 0.40 7.84 3.26 6.87 0.95 1.39sz-l96 7.06 1.40 0.32 8.02 3.74 3.43 1.15 1.60sz-l97 6.57 1.41 0.65 4.95 25.13 18.07 0.76 1.54sz-198 7.04 1.42 0.22 10.03 0.69 2.36 1.00 1.16sz-20 1 6.70 1.67 3.57 1.43 93.20 45.26 -0.12 0.91

...... I1s-203 7.03 1.43 0.31 6.00 3.52 5.95 0.94 1.21v, I1s-204 6.94 1.40 0.52 8.19 2.22 4.73 1.01 1.07-J

I1s-207 7.05 1.43 0.63 9.38 2.49 4.46 1.00 1.06I1s-209 7.20 1.48 0.64 7.14 2.15 5.38 0.99 1.210,-210 6.79 1.38 0.63 11.64 1.00 nIa 1.00 1.23sz-212 7.53 1.53 0.36 7.36 1.00 nIa 1.18 1.25

•Appcndix IV. Raw clata on calcite precipitation for the "O.6-g·' type experiments.

'1

•

Exp # (Cal At plI [CO,I n \Vurd Timc 1 Peo, [l\Igl..". \v.Wl Ratcmmollkg mt'q!kg mmol/kg g hr glmln l.og(alm) "glg mg 'lmol/(m l hr)

sz-113 10.87 5.165 7.803 0.230 5.69 0.6000 23.58 1.274 -2.59 89.7 4.38 5.95sz-114 10.96 5.326 7.797 0.234 5.85 0.6010 23.45 1.261 -2.57 78.0 3.98 5.44sz-115 10.94 5.305 7.780 0.225 5.61 0.6016 22.33 1.249 ·2.55 52.3 2.47 3.54sz-116 10.97 4.067 7.606 0.119 2.98 0.6007 91.28 0.346 -2.48 150.9 6.59 2.31sz·117 10.86 7.438 7.926 0.427 10.57 0.6013 5.48 5.458 ·2.56 54.9 2.87 16.74sz-118 10.78 7.390 7.919 0.419 10.28 0.6000 5.31 5.398 ·2.56 79.2 3.78 22.81s7.-119 10.80 3.943 8.034 0.278 6.85 0.6026 90.12 0.331 -2.97 209.0 9.14 3.24sz-120 10.77 7.308 8.171 0.675 16.57 0.6000 5.45 5.250 ·2.85 92.1 4.65 27.35

- s7.-122 10.75 6.497 7.825 0.3U3 7.43 0.6016 Il.73 2.655 ·2.51 95.6 4.56 12,42V1 sz-123 10.82 6.604 7.889 0.352 8.67 0.6024 Il.52 2.674 ·2.57 112.9 4.99 13.8400

s7.-124 1lJ.76 6,459 7.870 0.331 8.11 0.6009 12.05 2.672 -2.56 116,4 5.26 13.'JXsz·125 1lJ.73 7.523 7.771 0.314 7.68 0.6057 5.55 5.314 -2.39 82.8 4.59 26.26sz-126 10.82 4.052 7.532 0.101 2,49 0.6030 90.12 0.330 -2.40 66.9 3.U5 1.08sz·127 10.84 4.036 7.625 '. !J.123 3.04 0.6017 83.47 0.331 -2.50 88.9 4.16 1.59sz·139 10,44 13,44 7.392 0.248 5.89 0.6018 25.38 Ll58 -1.73 36.5 2.14 2.69sz-140 10.53 13.33 7.374 0.236 5.65 0.6028 26.00 LlI7 ·1.72 85.7 4.36 5.35sz-141 10.51 13.17 7.372 0.232 5.56 0.6015 24.72 1.095 -1.72 94.4 3.76 4.86

s7.·142 10.53 2.276 8.209 0.218 5.24 0.5999 27.32 1.078 ·3.42 51.2 2.fJ4 2.40

0,·143 10.83 5.845 7.827 0.274 6.75 0.6006 92.00 0.331 ·2.56 137.3 5.98 2.US

0,-144 10.83 5.720 7.883 0.301 7,41 0.6040 47.58 0.666 -2.63 92,4 4M, 2.98

0,-145 IU.78 5.878 7.845 0.286 7.02 0.6016 45.10 0.674 ·2.58 95.3 4.21 2.98

0,·146 10.75 5.901 7.849 0.289 7.08 0.6002 45.53 0.678 -2.58 107.5 4.53 3.19sz·147 11.23 4.566 7.748 0.181 4.63 0.6020 100.00 0.297 ·2.58 70.8 3.02 096

•AJlJlcndix IV. Conlil1ucd.

•

EXil # (Ca) Al pli (CO,) n \VUt4 Time [ Peo, IMgl"", \"eurl Rotemmo,n.g meq/kg mmol/kg g hr glmin Lo~(atm) "Wg m~ '111101f(m'hr)

--,•..l,'

570-148 10.90 8.453 8.032 0.600 14.90 0.5999 12.05 2.521 -2.63 188.2 7.95 21.15570-149 9.94 7.516 7.950 0.453 10.27 0.60[5 23.63 1.274 -2.59 174.6 6.69 9.05570-150 10.59 7.688 7.978 0.490 11.83 0.6029 23.57 1.271 -2.61 158.6 6.30 8.53570-151 10.63 7.320 7.954 0.445 10.77 0.6038 20.23 1.546 -2.60 145.6 6.93 10.91570-152 10.56 10.02 7.486 0.227 5.45 0.6003 23.88 1.236 -1.96 164.9 6.32 8.48570-159 10.37 4.333 7.798 0.191 4.50 0.6036 92.75 0.323 -2.66 79.1 3.52 1.21570-160 10.30 14.06 7.477 0.313 7.34 0.6002 44.17 0.630 -1.80 135.5 5.60 4.0657.-161 10.34 13.79 7.457 0.294 6.92 0.6052 46.08 0.629 -1.79 150.6 6.20 4.28

...... 570-162 10.29 4.122 7.723 0.155 3.63 0.6059 93.00 0.314 -2.60 109.6 4.51 1.54v, 0,-163 10.30 6.025 7.854 0.298 7.00 0.6050 45.00 0.620 -2.58 97.7 4.17 2.94'0

0,-164 10.32 6.015 7.845 0.292 6.88 0.6010 47.43 0.621 -2.57 84.6 3.79 2.550,-165 10.30 6.027 7.865 0.305 7.[6 0.6045 44.93 0.618 -2.59 105.4 4.39 3.1151.-166 10.29 13.80 7.476 0.306 7.17 0.6032 46.55 0.623 -1.81 90.2 3.49 2.39

57.-167 10.32 4.333 7.735 0.167 3.92 0.6027 93.67 0.314 -2.59 90.1 3.70 1.26

570-168 10.30 2.752 8.321 0.323 7.58 0.6027 45.20 0.617 -3.48 153.2 6.15 4.34

57.-169 10.31 2.732 8.351 0.337 7.90 0.6020 44.58 0.6[5 -3.52 124.9 4.80 3.44

57.-171 10.31 2.722 8.383 0.353 8.29 0.6014 45.47 0.609 -3.56 169.2 6.56 4.61

57.- 177 10.60 7.338 7.950 0.443 10.68 0.6040 24.07 1.206 -2.60 143.8 6.11 8.08

57.-178 10.62 7.422 7.961 0.458 11.07 0.6044 23.75 1.213 -2.60 147.8 6.13 8.21

57.-185 10.80 8.460 7.977 0.539 13.26 0.6037 12.05 2.404 -2.56 162.9 6.97 18.43

57.-189 10.67 6.094 7.873 0.314 7.63 0.6030 47.93 0.610 -2.59 193.2 7.66 5.10

57.-193 10.47 50.12 6.862 0.280 6.68 0.5993 47.45 0.610 -0.62 200.5 7.66 5.18

57.-194 10.22 50.72 6.863 0.284 6.62 0.6013 47.77 0.612 -0.61 112.5 4.20 2.81

57.-199 10.40 5.988 7.805 0.268 6.35 0.6033 48.58 0.609 -2.52 295.8 10.93 7.17

•Appcndix IV. Continued.

•

......0\o

EXil # (Cn) Al Il'' (CO,) n \V"'d Timc 1 Peo, [Mg]..". 'V'url Ralcmmnllkg mt'q/kg mmol!kg g hr glOlin Log(a.m) ,iglg mg 'UTIolI(m'hr)

s7.-20U 10.34 6.154 7.780 0.262 6.16 0.6052 48.43 0.606 -2.48 149.2 5.72 3.75sz-202 10.36 6.125 7.827 0.287 6.77 0.6070 47.88 0.605 -2.54 154.3 6.11 4.U4hs-205 10.40 6.266 7.827 0.294 6.96 0.6070 47.90 0.637 -2.53 167.8 6.33 4.19hs-2U6 10.31 6.157 7.791 0.268 6.29 0.6025 48.10 0.622 -2.50 166.4 6.20 4.11hs-208 10.32 6.170 7.817 0.283 6.66 0.6043 49.75 0.571 -2.52 135.7 5.U6 3.24

1.1

'/

• •Appcnllix V. Steady state REE concentrations (ng/g) in parent solutions for the "O.6-g" type cxpcrimcnts.

Exp # La Cc Pr Nd Sm Eu Gd Tb Dy 110 Er Yb rlnEEI

51.-113 2.21 2.30 2.56 1.53 3.29 3.89 5.14 7.84 8.26 20.2 25.0 28.3 110.55~1.-114 2.42 2.47 2.66 1.65 3.62 4.65 6.59 8.28 7.88 20.5 2').4 31.3 121.4951.-1 15 1.92 2.03 2.35 1.35 3.00 3.22 5.08 7.26 8.03 19.5 25.2 28.5 1117.31151.-116 1.83 1.81 2.02 1.21 2.66 2.66 5.15 7.27 7.97 20.2 26.6 30.8 1111.1251.-117 3.02 4.32 4.35 2.88 5.78 7.10 5.85 9.97 10.3 23.7 30.4 32.2 139.8551.-118 2.83 4.14 4.25 2.76 5.43 6.84 5.87 9.55 9.87 23.1 28.4 30.8 133.9851.-119 1.93 1.31 1.61 0.87 2.57 3.14 5.73 7.09 7.17 19.0 24.7 27.3 102.36- 51.-120 2.87 3.44 3.65 2.30 4.24 5.25 8.23 9.76 11.0 23.2 27.2 31.1 132.30

0\51.-122 2.65 2.84 3.08 1.89 4.28 4.72 5.52 12.4 14.1 30.9 32.7 35.2 150.23- 51.-123 2.81 3.02 3.14 VII 4.29 4.81 5.52 13.4 15.1 32.6 35.1 35.9 157.7351.-124 2.97 3.13 3.25 2.09 4.50 4.60 6.07 12.8 14.4 31.0 33.2 35.7 153.6451.-125 3.05 3.22 3.39 2.15 4.75 4.73 6.77 9.30 10.3 21.4 24.1 26.1 119.3651.-126 1.76 1.85 2.10 1.23 2.97 3.24 6.13 7.55 7.39 20.3 28.5 31.3 114.3751.-127 1.75 0.95 1.12 0.64 1.85 2.36 4.47 6.59 10.9 25.2 28.7 32.3 116.8951.-139 9.38 10.7 11.1 9.?~ 20.4 24.0 21.9 25.8 27.3 29.8 31.4 35.2 256.6751.-140 8.27 8.94 9.66 10.4 14.4 15.0 14.9 19.7 23.1 24.3 26.1 29.8 204.5051.-141 5.92 6.81 7.54 8.96 11.0 13.6 12.9 21.6 26.1 29.8 32.0 35.4 211.4451.-142 5.92 5.61 6.48 7.56 9.35 12.0 10.5 14.9 16.0 18.6 24.8 25.6 157.360,-143 4.88 4.59 4.89 6.61 8.90 10.9 9.88 28.0 40.5 53.5 64.4 79.8 316.770,-144 6.28 6.26 6.59 8.45 12.0 13.9 11.5 28.6 39.3 48.0 55.8 67.6 304.230,-lq5 5.36 5.12 5.43 7.12 10.1 12.7 10.8 31.6 45.0 56.8 68.0 83.6 341.730,-146 5.86 5.59 5.99 7.60 10.4 12.4 10.5 30.5 43.4 56.0 68.5 88.0 344.86

•Appendix V. Continued.

•

EXil # La Cc Pr Nd Sm Eu Gd Tb Dy 110 Er Yb ~IREEJ

57.-147 6.45 6.47 6.63 8.60 11.5 12.9 10.9 26.0 37.8 49.1 58.5 71.6 306.6757.-148 5.78 6.72 7.50 9.15 11.1 10.1 18.5 33.8 46.5 58.8 68.9 82.5 359.3357.-149 5.69 5.80 6.42 7.58 8.36 11.2 13.1 24.0 38.8 52.5 65.6 86.7 325.8257.-150 5.75 6.03 6.33 8.81 12.7 15.2 13.4 26.1 35.9 46.4 56.5 69.9 302.9757.-151 6.57 7.03 7.20 10.2 14.3 16.2 13.3 26.6 38.0 50.1 60.2 75.8 325.5057.-159 8.08 7.17 7.87 9.38 10.5 9.5 20.6 28.6 47.4 50.3 64.5 77.9 341.8157.-160 4.71 4.86 5.09 6.73 10.1 13.6 12.9 35.2 48.8 55.4 71.9 95.9 365.1757.-161 4.46 4.24 4.86 6.55 9.76 12.3 14.4 33.0 46.9 52.5 69.1 87.1 345.18

~ 57.-162 5.49 5.01 5.25 7.30 10.5 13.2 13.6 30.5 43.2 47.7 63.3 ï9.8 324.820\ 0,-163 4.12 4.01 4.25 5.54 8.20 10.1 12.0 34.2 47.9 54.3 70.5 93.2 348.431-..1

0,.164 4.04 3.88 4.16 5.02 6.98 8.29 10.6 33.6 47.2 53.8 71.1 96.2 344.800,.165 4.08 3.98 4.46 5.43 7.54 9.36 10.9 24.5 34.1 36.3 46.F 52.4 239.8457.-166 5.03 5.75 6.03 7.61 11.3 14.2 12.6 34.0 45.8 50.1 66.1 83.5 342.13Sl.·167 7.75 7.54 7.30 9.19 12.6 14.8 13.1 28.9 40.5 44.4 58.8 73.1 318.0857.-168 5.83 6.01 6.05 7.13 10.7 13.3 13.2 25.7 37.2 40.7 55.2 67.4 288.3457.-169 5.01 5.08 5.32 6.31 10.3 12.9 13.2 25.4 35.9 39.4 51.6 60.1 270.6257.-171 4.79 5.13 5.48 6.13 9.63 12.3 12.8 32.2 45.0 51.3 66.5 85.1 336.3457.-177 5.46 5.86 6.13 7.26 11.3 14.1 12.9 29.5 41.8 47.8 66.8 92.0 341.0257.-178 5.68 5.91 6.24 7.76 11.5 14.3 13.1 31.0 42.5 46.1 60.7 76.7 321.4557.-185 6.36 6.46 7.24 8.87 11.2 10.5 18.8 30.3 43.1 55.4 66.4 82.8 347.2857.-189 4.59 4.57 4.89 6.03 7.79 11.5 10.7 30.4 45.3 62.1 80.5 113 381.4757.-193 6.10 6.53 6.49 8.43 10.6 13.6 15.4 26.2 35.2 43.9 53.5 67.7 293.6257.-194 4.68 5.48 5.79 7.87 9.68 12.6 14.7 24.1 34.2 42.5 52.0 65.8 279.3457.-199 2.98 3.48 4.22 5.25 6.13 5.97 11.2 13.0 17.3 22.0 25.5 29.8 146.82

,'1

• •,'i Appcndix V. Continued.!

E"fI # Ln Cc Pr Nd Sm Eu Gd Tb D~' 110 Er Yb l:IHEEI

sz-200 3.90 4.52 4.91 5.68 7.65 9.56 12.5 20.9 31.3 41.0 51.7 67.4 260.9~

sz-202 4.82 4.71 5.04 5.n 7.17 8.99 12.0 26.4 36.1 46.1 57.0 73.8 287.99,bs-205 2.74 3.41 4.07 4.42 6.06 7.57 8.80 12.6 16.3 29.3 41.9 62.7 199.76"i)s-206 1.53 1.75 1.98 2.64 4.18 6.08 6.71 13.8 19.6 32.8 42.5 66.0 199.57

hs-208 3.04 3.62 4.72 5.38 5.06 7.13 9.49 17.0 25.5 38.5 48.6 78.9 24('.9.1

-a-v)

" ',r

• •Appcndix VI. REE concentrations (mg/g) in calcite overgrowths precipitated from the "O.6-g" type

experimcnts.

Ex)! # Ln Cc Pr Nd Sm Eu Gd Tb Dy 110 Er Yb

57.-113 1.93 1.92 1.83 1.74 1.59 1.37 1.33 2.25 1.95 1.62 1.37 1.0657.-114 2.21 2.15 2.07 1.99 1.82 1.59 1.53 2.62 2.23 1.82 1.52 1.1457.-115 2.41 2.28 2.13 2.03 1.85 1.65 1.75 3.10 2.79 2.40 2.17 1.8057.-116 0.55 0.51 0.50 0.50 0.48 0.41 0.52 0.87 0.82 0.69 0.64 0.5457.-117 3.23 3.12 2.95 2.67 2.40 2.42 2.01 3.60 3.19 2.61 2.31 1.8057.-118 2.27 2.14 1.93 1.85 1.69 1.52 1.45 2.54 2.16 1.87 1.59 1.25

~

57.-119 0.43 0.42 0.47 0.47 0.54 0.57 0.58 1.18 1.10 0.98 0.88 0.780\.j:>. 57.-120 2.25 2.14 2.04 1.95 1.73 1.82 1.84 3.32 3.33 2.77 2.49 2.10

57.-122 1.79 1.69 1.61 1.50 1.37 1.27 1.18 2.05 1.78 1.47 1.25 1.0157.-123 1.76 1.66 1.58 1.44 1.32 1.19 1.14 1.93 1.68 1.37 1.18 0.9157.-124 1.67 1.59 1.40 1.32 1.15 1.06 1.02 1.73 155 1.31 1.13 0.93

: :

57.-125 1.88 1.79 1.63 1.59 1.42 1.29 1.33 2.24 1.97 1.63 1.39 1.1057.-126 0.47 0.49 0.49 0.45 0.44 0.44 0.34 0.73 0.62 0.52 0.44 0.4657.-127 0.34 0.33 0.33 0.33 0.34 0.35 0.30 0.69 0.62 0.55 0.48 0.4757.-139 13.5 14.0 13.1 12.2 11.6 10.2 10.0 8.32 7.22 5.94 5.07 4.0657.-140 6.95 7.19 6.78 6.38 6.08 5.37 5.23 4.49 3.92 3.33 2.87 2.3557.-141 7.22 7.26 6.87 6.25 6.12 5.54 5.69 5.11 4.65 4.12 3.70 3.2057.-142 13.3 13.7 13.8 12.6 13.4 11.3 11.5 10.1 8.55 7.68 6.40 4.860,-143 13.3 13.7 13.9 13.7 14.5 12.1 13.6 12.1 10.3 9.05 7.08 4.780,-144 14.8 16.5 16.5 16.6 15.5 14.7 12.5 10.8 8.11 6.32 4.21 3.120,-145 18.7 19.0 18.9 18.4 17.9 15.7 16.7 14.8 13.0 11.2 8.87 5.920,-146 16.8 17.5 16.6 15.9 15.5 13.7 14.0 12.3 11.3 9.56 7.77 5.1J6

•Appendix VI. Continued.

•

Exp" La Cc Pr Nd Sm Eu Gd Tb D}' 110 Er Yh

51.-147 9.26 10.0 9.73 9.02 9.08 7.79 6.60 6.93 6.01 4.99 4.39 3.3751.-148 14.4 14.7 14.6 13.9 14.1 12.1 12.3 10.6 9.34 7.50 5.93 3.5351.-149 13.7 15.4 12.8 13.3 12.1 10.6 9.90 8.95 7.37 5.87 4.90 3.2351.-150 10.0 10.1 9.55 8.90 8.70 7.75 6.29 6.14 5.48 4.12 4.04 3.2451.-151 16.2 16.7 15.9 15.2 14.6 12.5 12.5 10.4 9.22 7.68 6.18 3.'1351.-152 7.92 7.83 7.54 7.15 7.06 6.27 6.14 5.67 5.28 4.78 4.21 3.5151.-159 21.6 21.2 21.1 21.3 21.2 18.9 19.0 16.8 14.6 10.9 9.58 6.7'1

...... 51.-160 12.9 13.1 12.4 12.4 12.1 Il.4 11.0 10.1 8.9, 6.71 6.15 4.330\

51.-161 12.8 13.2 12.8 12.9 12.9 Il.8 11.6 10.7 9.45 6.94 6.26 4.24v,51.-162 Il.9 Il.8 11.3 11.6 11.1 10.8 10.2 9.60 8.89 6.52 6.12 4.500,-163 16.6 17.4 17.2 17.2 17.2 16.5 15.4 14.3 13.0 9.76 8.91 6.110,-164 19.1 19.7 19.9 19.4 19.2 18.1 16.9 15.5 14.1 10.3 9.66 6.670,-165 15.7 16.0 J5.7 16.1 15.5 14.8 13.9 12.7 11.8 9.08 8.56 6.3151.-166 7.54 7.68 7.53 7.55 7.15 6.46 6.66 6.12 6.05 4.88 5.05 4.5651.-167 12.7 13.2 12.7 12.8 11.5 10.7 9.24 8.54 7.64 4.78 5.18 4.0851.-168 12.7 13.0 12.9 13.4 12.6 12.0 11.0 9.39 8.85 6.53 5.97 4.2851.-169 8.28 7.66 7.06 7.93 6.73 7.73 5.19 6.35 6.19 4.68 4.65 3.7051.-171 9.43 9.08 9.05 9.38 9.05 8.55 8.59 7.50 7.14 5.27 5.06 3.9351.-177 14.6 15.1 15.2 15.0 14.5 14.1 12.8 11.'1 10.4 7.53 6.58 4.4051.-178 11.2 12.3 12.1 Il.6 1J.4 11.3 9.59 9.42 8.88 6.63 6.39 5.0251.-185 14.6 14.8 14.7 14.9 13.7 13.0 12.4 11.0 9.47 7.71 6.20 3.9351.-189 7.98 7.67 7.56 7.42 7.01 6.66 6.25 6.04 5.61 4.81 4.10 2.86

• ','

Appcndix VI. Continued.

•

EXil # Ln Cc Pr Nd Sm Eu Gd Tb D)' 110 Er Yb

sz-I'93 6.26 6.82 6.41 6.35 5.94 5.60 5.00 4.89 4.64 4.09 3.75 3.11.)

sz-i94 5.64 6.05 5.60 5.53 5.36 4.88 4.98 5.05 4.97 4.56 4.35 4.00sz-199 2.57 2.56 2.58 2.58 2.49 2.32 2.45 2.48 2.45 2.38 2.25 2.01sl.-200 7.07 7.39 7.22 7.05 6.97 6.91 5.64 5.52 5.01 4.34 3.70 2.53sz-202 8.54 8.31 8.35 7.83 7.66 6.82 6.95 6.24 5.64 4.81 4.05 2.81hs-205 2.91 3.15 3.19 3.10 3.49 3.16 3.12 3.65 3.95 4.07 4.19 4.28hs-206 3.25 3.37 3.39 3.43 3.68 3.39 3.41 3.90 4.22 4.38 4.50 4.67

~ hs-208 3.67 3.05 3.05 2.73 2.96 2.91 2.83 3.03 3.13 3.04 3.01 2.900-.0\

•... Appendix VII. REE partition coefficients (i.e., Log(D)) for the "ü6-g" type expcrimcnts.

EXIl # La Cc Pr Nil Sm Eu Gd Tb Dl' 110 Er Yb

sz-113 3.13 3.11 3.05 3.25 2.88 2.74 2.60 2.65 2.56 2.10 1.93 1.76sz-114 3.17 3.15 3.10 3.29 2.91 2.74 2.58 2.71 2.66 2.16 1.92 1.77sz-115 3.28 3.23 3.14 3.36 2.97 2.89 2.71 2.81 2.72 2.27 2.11 1.98sz-116 2.62 2.59 2.54 2.76 2.40 2.33 2.15 2.22 2.15 1.67 1.52 1.39sz-II 7 3.25 3.08 3.05 3.19 2.84 2.75 2.76 2.78 2.71 2.26 2.10 1.97sz-118 3.09 2.90 2.84 3.01 2.67 2.53 2.58 2.61 2.52 2.09 1.93 1.79sz-119 2.48 2.65 2.60 2.88 2.47 2.39 2.14 2.36 2.32 1.85 1.69 1.59sz-120 3.10 3.00 2.95 3.14 2.82 2.75 2.56 2.74 2.69 2.28 2.17 2.04

..... sz-122 3.01 2.96 2.90 3.08 2.69 2.61 2.51 2.40 2.28 1.86 1.76 1.640\ sz-123 2.94 2.89 2.85 3.00 2.63 2.54 2.46 2.31 2.19 1.77 1.67 1.55-.l

sz-124 2.91 2.86 2.79 2.96 2.56 2.52 2.38 2.29 2.19 1.78 1.69 1.57sz-125 3.03 2.99 2.92 3. Il 2.72 2.68 2.54 2.62 2.52 2.13 2.00 1.87sz-126 2.59 2.58 2.53 2.72 2.33 2.30 1.91 2.15 2.09 1.57 1.35 1.33sz-127 2.46 2.71 2.65 2.89 2.44 2.34 2.01 2.19 1.93 1.52 1.40 1.33sz-139 3.43 3.39 3.35 3.37 3.03 2.90 2.93 2.78 2.69 2.57 2.48 2.33sz-140 3.13 3.11 3.05 3.00 2.83 2.76 2.75 2.57 2.44 2.34 2.25 2.10

sz-141 3.19 3.13 3.06 2.95 2.85 2.71 2.75 2.48 2.35 2.24 2.17 2.06sz-142 3.46 3.49 3.44 3.33 3.26 3.08 3.15 294 2.83 2.72 2.52 2.38

0,-143 3.58 3.61 3.60 3.46 3.35 3.19 3.28 2.78 2.55 2.37 2.18 1.92

0,-144 3.55 3.60 3.58 3.48 3.29 3.21 3.22 2.76 2.50 2.30 2.06 1.850,-145 3.69 3.71 3.69 3.56 3.40 3.24 3.34 2.82 2.61 2.44 2.26 2.00

0,-146 3.59 3.62 3.57 3.45 3.30 3.17 3.25 2.73 2.54 2.36 2.18 1.89

•

•Appcndix VII. Continued.

•

EXJI # La Cc Pr Nd Sm Eu Gd Tb Dy 110 Er Yb

51.-147 3.29 3.32 3.30 3.15 3.03 2.91 2.91 2.56 2.33 214 2.01 I.R 151.-148 3.52 3.47 3,42 3.31 3.23 3.21 2.95 2.62 2,43 2.23 2.06 1.7651.-149 3.46 3.51 3.38 3.33 3.24 3.06 2.96 2.66 2.36 2.13 1.96 1.6657.-150 3.34 3.32 3.28 3.10 2.93 2.81 2.77 2,47 2.28 2.05 1.95 1.7657.-151 3.57 3.55 3.52 3.35 3.19 3.06 3.15 2.77 2.56 2.36 2.19 I.R951.-152 3.32 3.23 3.19 3.01 2.84 2.71 2.73 2.80 2.60 2,40 2.30 1.9057.-159 3.58 3.62 3.58 3.50 3.45 3.45 3.11 2.92 2.64 2.49 2.32 2.09- 57.-160 3.56 3.55 3.51 3.38 3.20 3.04 3.05 2.58 2.38 2.20 2.05 1.77

C\00 57.-161 3.57 3.61 3.53 3,41 3.23 3.10 3.02 2.62 2,42 2.23 2.07 1.RO

51.-162 3.45 3.4R 3,44 3.31 3.14 3.03 2.99 2.61 2,43 2.25 2.10 I.R60,-163 3.73 3.77 3.74 3.62 J,45 3.34 3.24 2.75 2.56 2.38 2.23 1.950,-164 3.83 3.86 3.83 3.74 3.59 3.49 3.36 2.82 2.63 2,44 2.29 2.000,-165 3.70 3.72 3.67 3.59 3.43 3.32 3.22 2.R3 2.66 2.52 2.38 2.2057.-166 3.26 3.21 3.19 3.09 2.89 2.75 2.81 2.34 2.21 2.08 1.97 1.83

57.-167 3.33 3.36 3.36 3.26 3.08 2.97 2.96 2.59 2.39 2.15 2.06 1.86

51.-168 3,44 3.44 3,43 3.38 3.18 3.06 3.02 2.67 2.48 2.31 2.14 1.9157.-169 3.30 3.26 3.21 3.19 2.90 2.86 2.68 2,48 2.32 2.16 2.04 1.88

57.- i 71 3.38 3.34 3.31 3.27 3.06 2.93 2.92 2,46 2.29 2.10 1.97 1.75

57.-177 3.55 3.54 3.52 3.44 3.24 3.13 3.12 2.73 2.52 2.32 2.12 1.81

57.-178 3,41 3,44 3,41 3.29 3.11 3.01 2.98 2.60 2,44 2.27 2.14 1.93

57.-185 3,49 3,49 3,44 3.35 3.22 3.22 2.95 2.69 2.47 2.27 2.10 1.81

57.-189 3.34 3.12 3.29 3.19 3.05 2.86 2.86 2,40 2.19 1.99 1.80 1.50

•Appcndix VII. Continued.

•

I~

Ex)! # Ln Cc Pr Nd .' Sm Eu Gd Tb Dy 110 Er Yb

57.-193 3.10 3.10 3.08 2.96 2.83 2.70 2.60 2.36 2.20 2.05 1.93 1.7557.-194 3.16 3.12 3.06 2.92 2.82 2.66 2.61 2.40 2.24 2.11 2.00 1.8657.-199 3.00 2.93 2.85 2.76 2.67 2.65 2.41 2.34 2.21 2.10 2.01 1.8957.-200 3.34 3.30 3.25 3.18 3.04 2.94 2.74 2.50 2.29 2.11 1.94 1.6657.-202 3.34 3.34 3.31 3.22 3.12 2.97 2.86 2.47 2.29 2.11 1.95 1.68h5-205 3.10 3.04 2.97 2.92 2.83 2.69 2.62 2.53 2.46 2.22 2.07 1.91hs-206 3.40 3.36 3.30 3.19 3.02 2.82 2.78 2.52 2.40 2.20 2.10 1.92

~ h5-208 3.15 3.00 2.95 2.86 2.84 2.75 2.65 2.51 2.38 2.17 2.09 1.950\'0

i)

• •Appcndix VIII. REE adsorption by calcite: Variations of REE concentrations (ng/g) in calcite

equilibrated seawater solutions with reaction time (hf.).(Solid to solution ratio = 1 : 3000)

Timcthr) La Cc Pr Nd Sm Eu Gd Th Dy 110 Er Yh

0.00 87.6 91.9 89.2 91.7 91.1 92.8 89.3 96.4 97.3 98.4 96.5 95.90.13 84.6 89.5 89.7 92.8 90.4 96.6 87.7 97.1 92.4 93.5 91.7 95.80.33 84.6 86.7 87.2 88.7 88.8 90.0 85.9 96.7 95.3 96.4 945 94.2

/,

84.3 96.9 88.60.41 81.9 81.3 82.6 86.1 84.9 87.8 94.5 87.9 9l.5

...... 0.50 80.7 85.5 84.6 81.6 86.2 83.5 85.2 94.6 85.1 90.7 84.4 90.8-J 0.75 81.8 84.7 82.8 84.7 85.5 84.4 85.4 95.2 88.2 91.3 87.8 91.40

I.on 80.6 75.7 76.4 76.5 81.3 79.9 85.6 93.7 86.7 87.2 86.1 92.32.0n 80.6 73.9 72.0 72.5 73.2 72.6 84.4 89.3 83.1 89.7 85.4 88.92.50 75.9 69.4 73.4 72.3 75.7 73.5 80.7 85.1 87.8 85.1 89.5 89.23.00 63.3 67.6 67.1 71.6 70.5 68.0 73.7 80.5 85.7 86.1 87.4 89.75.00 67.4 66.3 64.7 69.9 71.1 68.7 70.3 83.9 83.6 83.9 84.2 88.39.00 61.6 63.5 63.5 66.8 68.7 66.8 68.7 82.4 82.8 87.4 88.0 87.1

11.00 62.1 63.3 63.6 69.2 70.1 69.1 69.7 85.0 83.5 87.1 84.3 88.623.00 60.6 61.6 61.4 64.8 66.6 65.3 66.2 82.7 84.3 80.2 87.1 86.725.00 60.4 63.1 62.6 67.0 69.4 69.5 66.4 75.8 82.6 82.6 85.0 84.842.00 59.2 60.3 59.0 61.7 64.7 67.5 65.2 78.6 83.8 84.7 87.9 86.077.00 55.9 57.2 54.8 58.6 60.7 65.1 60.7 75.4 80.2 80.7 84.5 84.994.00 55.6 56.0 58.0 57.2 6'2.7 66.9 61.0 78.3 83.0 83.5 85.2 84.2

•Appendix VIII. Continucd.

•

Timc(hr) La Cc Pr Nd Sm Eu Gd Tb D)' 110 Er Yh

0.00 87.7 88.5 87.3 85.6 84.2 87.3 88.3 88.4 93.6 98.3 98.6 98.50.25 84.0 83.2 82.5 82.0 81.2 78.9 84.5 88.2 92.8 92.5 94.2 95.40.52 83.0 78.9 78.8 77.8 77.7 77.8 80.3 85.9 89.7 88.3 91.0 91.30.83 80.3 75.4 74.5 75.5 76.5 74.5 81.0 84.9 89.4 91.0 93.6 94.61.92 78.2 72.3 71.1 74.2 73.7 73.6 80.0 85.3 89.5 90.7 93.9 93.22.92 74.1 68.1 66.6 70.5 70.9 69.0 77.5 82.4 87.3 90.4 94.9 94.04.00 73.2 65.1 64.0 68.1 68.0 68.3 74.8 81.4 86.2 89.8 93.3 93.2

~ 23.33 63.8 60.6 60.4 64.7 65.0 67.0 75.8 79.4 87.2 91.4 92.6 90.9-.l 24.17 64.7 62.2 60.4 66.7 64.6 64.6 75.2 72.3 87.0 91.3 92.5 87.4~

48.20 58.9 52.8 51.9 56.2 54.8 59.0 63.3 65.4 80.1 83.6 84.1 88.0

0.00 81.2 82.8 82.7 81.6 81.6 81.7 84.3 90.2 93.2 92.7 95.6 95.90.50 77.6 79.2 78.2 79.3 80.4 78.2 85.0 89.2 88.1 85.6 87.9 93.21.00 78.1 75.9 74.7 77.9 77.4 77.3 84.0 86.0 89.4 85.2 85.6 87.92.00 68.3 71.5 69.9 74.0 74.4 72.4 81.4 86.5 83.2 84.9 83.7 87.7

24.33 56.2 58.3 57.1 51.5 51.4 51.7 58.6 77.5 71.7 74.3 86.0 79.947.67 49.1 44.1 41.3 42.8 42.5 49.2 53.9 65.7 72.4 73.8 85.1 82.6