speciation of trace elements in acidic pore waters

8/19/2019 Speciation of Trace Elements in Acidic Pore Waters

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Aquatic Geochemistry 6: 347–366, 2000.

© 2000 Kluwer Academic Publishers. Printed in the Netherlands. 347

Speciation of Trace Elements in Acidic Pore Watersfrom Waste Rock Dumps by Ultrafiltration and Ion

Exchange Combined with ICPMS and ICPOES

ALEXANDER PLEßOW and HARTMUT HEINRICHS∗

Geochemisches Institut Georg-August-Universität, Goldschmidtstraße 1, D-37077 Göttingen,

Deutschland (∗ Author for correspondence: E-mail: [email protected])

(Received: 2 July 1999; accepted: 22 February 2000)

Abstract. A speciation procedure developed on reactive acidic pore water samples from mining

areas is presented. Methods with low consumption of solution are required that allow rapid samplepreparation to avoid equilibrium changes as far as possible. The entire procedure includes only three

parallel separation steps. One aliquot is filtered through an 1 kd ultrafiltration membrane to separate

trace elements adsorbed or complexed by colloids. One cation and one anion exchange are performed

with two additional aliquots to determine simple hydrated ions and small inorganic complex ions.

Commonly used procedures of ion exchange seem to be problematic. This new technique is based

on a novel ion exchanger. Subsequently the three fractions obtained from the separation procedures

and the original pore water sample are analysed by ICPMS, ICPOES, ET-AAS, Flame-AAS, FES

and IC to determine the concentrations of the major ions and additionally up to 50 trace elements.

The influence of pH-values and several dissolved compounds is controlled in experiment series with

synthetically prepared solutions to reveal potential artifacts.

Key words: acidic mine drainage, adsorption, artifact, colloid, complex, element species, organic

ligand, separation

1. Introduction

Acidic mine drainage caused by the exposure and oxidation of sulfides, mainly

pyrite, in waste rock dumps is a serious problem in mining areas (e.g., Alpers and

Blowes, 1994; Jambor and Blowes, 1994; Murad et al., 1994; Wisotzky, 1994;

van Berk and Wisotzky, 1995; Bigham et al, 1996, Wisotzky, 1996). Particularly,

surface mining of brown coal produces large overburden dumps: In 1993 2.25 ×

109 t of sediments were moved and dumped due to brown coal mining in Ger-

many (Umweltbundesamt, 1997). Pyrite concentrations range from 0.37 wt.% FeS2

to 1.57 wt.% FeS2 in rubbish from Garzweiler (Rhineland, Germany) and from2.81 wt.% FeS2 to 7.86 wt.% FeS2 in rubbish from Zwenkau (Central Germany),

both locations representing open pit mines. During mining and deposition con-

tact of such sulfide containing sediments with oxygen from the air is inevitable.

Thereupon, high amounts of sulfuric acid and iron are released to seepage waters.

Apart from iron several other metals and trace elements are mobilised due to ac-


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348 ALEXANDER PLEßOW AND HARTMUT HEINRICHS

celerated weathering of acid sensitive sediments. Contamination of ground water

is an often observed consequence. Mean concentrations found in 7 seepage waters

from a waste rock dump near Leipzig, Germany are shown in Table I.

Enrichment factors (EF) of about 10–10,000 compared to not anthropogenically

affected ground waters are calculated for many elements, for example: EF = 1,000–

3,500 for V > Co, Cr, EF = 100–1,000 for As > Sr, Li, Ni > Pb > Zn and EF =

10–100 for Cd > Cu > Mn > Sb. The extent of trace element release and transport

depends on their geochemical behavior. Thus, knowledge about the speciation of

trace elements in pore solutions is required to enable forecasts concerning further

developments in ground water quality.

Calculations based on thermodynamic data (Garrels and Christ, 1965; Turner

et al., 1981; Brookins, 1988; Al et al., 1997) are often unsatisfactory for mainly

two reasons: (1) kinetic effects are usually not considered and (2) natural systems

are very complex (Florence and Batley, 1980; Florence et al., 1992). Thus, supple-

mentary to numerical modelling, analytical approaches are advisable for speciation

purposes. Application of several techniques has been reported (Florence and Bat-ley, 1980; Buffle, 1981; Morrison, 1987; Florence et al., 1992; Schwedt, 1997). For

a variety of compounds especially with low concentrations of individual species,

it is usually neither possible nor meaningful to characterise all of them exactly

and to quantify each species separately. Often all essential information can be de-

rived from an operational speciation approach instead: Groups of different species

are first separated by appropriate techniques as chromatography, filtration or ion

exchange. Afterwards the obtained fractions are analysed by sensitive analytical

methods as ICPMS, ICPOES or AAS. The analyses give information about the

distribution of the elements in the separated groups referring to the different kinds

of species.

Commonly applied and recommended procedures are often a comprehens-ive combination of numerous methods (Florence and Batley, 1980; Sauer, 1990;

Sauer and Lieser, 1994; Mach et al., 1996) with high sample consumptions. Pore

solutions can be obtained by squeezing sediment samples (Böttcher et al., 1997;

Heinrichs et al., 1996) only in small amounts and have to be handled swiftly

without any changes in pH-values. Therefore a simple speciation scheme is re-

quired. Furthermore, the applicability of the choosen methods to reactive acidic

pore water solutions with extremely high sulfate concentrations has to be veri-

fied. To separate complexed or colloidal species from hydrated trace elements, an

one-step ultrafiltration and ion exchanges are investigated.

2. Experimental2.1. ULTRAFILTRATION

Ultrafiltration is the most appropriate method to separate colloidal matter quantit-

atively from water samples (Buffle et al., 1978; Thurman et al., 1982; Staub et al.,

1984; Marley et al., 1992) and is therefore well established in speciation (Florence


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TRACE ELEMENT SPECIATION IN ACIDIC SOLUTIONS 349

Table I. Mean concentration (µg/l) of 7

seepage waters from a waste rock dump

near Leipzig, Germany.


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and Batley, 1980; Sauer, 1990; Buffle et al., 1993; Sauer and Lieser, 1994; Aster et

al., 1996; Schwedt, 1997). To avoid artifacts the concentrations of hydrated ions

and low-molecular, subcolloidal compounds should not be influenced by ultra-

filtration. Since adsorption especially of trace elements and retention of sulfate has

been reported (Florence and Batley, 1980; Buffle et al., 1978; Buffle et al., 1993;

Amicon 1993), experiments are performed with solutions of inorganic salts. The

concentrations are adjusted to values found in acidic pore water samples from a

mining area. Merely organic compounds and sulfate are at first omitted in the stock

solution for experimental purpose because of their complexing character. To ensure

almost quantitative separation of colloids the membranes cut-off should not exceed

1 kd according to about 1 nm pore diameter. The lowest available molecular weight

cut-off is at 0.5 kd.

Ultrafiltrations were performed with stirred ultrafiltration cells (Amicon Inc.

Beverly USA, model 8200TM) at 400 kPa pressure in argon atmosphere. A mag-

netic stirrer drove 100 rotations per minute. Two different membrane types were

used: YC05TM by Amicon Inc. Beverly USA (cut off 0.5 kd) and OmegaTMby Pall-Filtron USA (cut off 1 kd). The entire filtration procedure followed the

manufacturers instructions. In each experiment a volume of 50 ml was filtered.

2.2. ION EXCHANGE

Speciation procedures often include ion exchanges for the separation of dissolved

compounds based on their different charge densities (Florence and Batley, 1980;

Morrison, 1987; Sauer, 1990; Florence et al., 1992; Sauer and Lieser, 1994;

Schwedt, 1997). Actually speciation is often limited by changing chemical equilib-

ria due to ion exchange because processing is too slow (Florence and Batley, 1980).

Furthermore, standard methods seem to be problematic because distinct pH-valuesare often recommended, wet package of a column may influence the concentrations

in the sample solution or, if a batch exchange is performed, an additional filtration

step is required. To avoid all these disadvantages a new technique is developed.

Main characteristics are easy handling and extremely short contact phases for the

sample aliquots. Contrary to common methods, self-contained membrane modules

(SartobindTM by Sartorius AG, Göttingen, Germany) are used for ion exchanges.

The ion exchanging media are non-compressible, synthetic membranes with re-

generable active groups attached on the inner surface of the membrane pores. The

membranes are made of regenerated cellulose. Currently four types with different

functional groups are available. Due to high total ionic contents and low pH-values

expected in the sample solutions, strongly acidic S100

TM

with sulfonic acid asfunctional group for cation exchange and strongly basic Q100TM with quarternary

ammonium as functional group for anion exchange were chosen. Each has an ef-

fective adsorption area of 100 cm2. Pore diameters are 3 µm. Thus, no filtration

effect occurs with 450 nm prefiltered solutions. Solutions were passed through

the modules with a constant flow rate of 5 ml/min by a peristaltic pump (Desaga


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Multipurpose STA 131900); within 10 minutes an adequate volume for analyses

(50 ml) is obtained. The dead volume of the S100TM / Q100TM adsorber units

comes up to about 4 ml. S100TM were used in H+-form, Q100TM in OH−-form.

The latter causes an increase of pH-values due to anion exchanges. Therefore the

eluates were stabilised with 1 ml nitric acid (10 mol/l, Merck KGaA, Darmstadt,

Germany, purified by sub-boiling distillation) per 50 ml sample volume. Regener-

ation was performed with 1 mol/l hydrochloric acid, or 1 mol/l sodium hydroxide,

respectively, according to the manufacturers instructions.

2.3. MATERIALS

The stock solution for the ultrafiltration and ion exchange experiments was pre-

pared from ICP standard solutions manufactured by Bernd Kraft GmbH, Duisburg,

Germany, and Merck KGaA, Darmstadt, Germany and high purity grade deionized

water. Its composition was adapted to the concentrations that were found in water

samples from a mining area but in general the concentrations were chosen high

enough to enable analytical determinations with appropriate accuracy: 10 mg/l Na,

5 mg/l mg and Ca, 2 mg/l Al and Fe, 1 mg/l K and Zn, 500 µg/l Sr, 200 µg/l Mn

and Ni, 100 µg/l Li, Be, Sc, Ti, V, Cr, Cu, As, Rb, Mo, Cs, Ba, La, Ce, Nd and

Pb, 50 µg/l Hg and 20 µg/l B, Ga, Ge, Y, Zr, Nb, Ru, Rh, Pd, Ag, Cd, Sn, Sb,

Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, W, Os, Ir, Pt, Au, Tl, Bi, Th

and U. Most of the used standard solutions were stabilised with nitric acid, only

few elements were dissolved in hydrochloric acid. The stock solution had a pH-

value of 1.4 and an electric conductivity of 17.6 mS/cm. Some experiments were

performed with additional compounds. For that purpose 0.16 mol sulfate/l (24 g/l

Na2SO4, Merck KGaA, Darmstadt, Germany, pro analysi), 30 g/l meta-tartaric

acid (Merck KGaA, Darmstadt, Germany, for synthesis), 30 g/l tri-sodium citratedihydrate (Merck KGaA, Darmstadt, Germany, pro analysi), or 6.9 g/l fulvic acid

(prepared by Dr. Krüger, Leipzig; Krüger 1995) were dissolved in separate aliquots

of the stock solution. Changes in pH were balanced with sodium hydroxide (Merck

KGaA, Darmstadt, Germany, pro analysi), or nitric acid (Merck KGaA, Darmstadt,

Germany, suprapure), respectively, to values below 2. To study the influences of

pH-values, another aliquot of the stock solution was brought to pH = 5.8 with

sodium hydroxide (Merck KGaA, Darmstadt, Germany, pro analysi).

2.4. ANALYSES

All samples were analysed in dublicate by F-AAS/AES (Unicam PU 9200X with50 mm slit burner) for Na, K, Fe, Zn, As, in combination with STAT (slotted tube

atom trap) for Zn or hydride system (Unicam PU 9360X) for As, by ET-AAS

(Unicam 939 Solaar System with graphit furnace GF90 and autosampler FS90)

for Al, Cr, Co, Ni, Cu, Zn, Ag, Cd, by ICPOES (Perkin Elmer Optima 3300 DV

with autosampler AS91) for Li, Be, Na, mg, Al, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co,


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Ni, Cu, Zn, As, Sr, Sr, Y, Zr, Mo, Ba, La and by ICPMS (Fisons VG Plasma Quad

2+ STE, 10 µg/l In + Re as internal standards) for Li, Be, B, Sc, Ti, V, Cr, Co, Ni,

Cu, Zn, Ga, Ge, As, Rb, Sr, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Sn, Sb, Cs, Ba, La,

Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, W, Os, Ir, Pt, Au, Hg,

Tl, Pb, Bi, Th, U. The analytical methods are described in detail by Welz (1985),

Potts (1987), Guo et al. (1990), Skoog and Leary (1992), Boss and Fredeen (1997),

Rocholl et al. (1997) and Hinners et al. (1998).

Calibrations were performed with multi-element standard solutions (ICPMS

quality, Bernd Kraft GmbH, Duisburg, Germany). Several certified reference ma-

terials (GSJ-JA-2, GSJ-JB-3, NRC-SLRS-3, SABS-SARM-7, SABS-SARM-11,

SABS-SARM-12) and four in-house standards were analysed as quality controls.

The analytical errors generally were below 10% for most elements but rose up to

50% for low concentrations near the detection limits. The accuracy of F-AAS/AES

determination is judged better than ±5% for Na, K, and ±10% for Fe, Zn, and

better than 15% for As (hydride). For ET-AAS values of less than ±10% are found

for Co, Zn, Cd, less than ±15% for Cr, Ni, Cu, Ag, Pb, and less than 20% for Al.Based on analysis of a solution of GSJ-JA-2 with ICPOES an accuracy of ±5%

was calculated for Mg, Al, K, Ca, Sc, V, Mn, Fe, Co, Ni, Sr, Zr, Ba and La, of

±10% for Na, Ti, Cu, Zn and Y, of ±15% for Be, of ±20% for Li, Cr and Mo,

and of ±25% for As. For a solution of GSJ-JB-3 the accuracy of the analytical

determinations with ICPMS is judged better than ±5% for Sc, Rb, Sr, Ba, Sn, Sb,

La, Gd, Tb, Yb, Tl and Pb, better than ±10% for Li, Co, Ni, Zn, Ga, Zr, Mo, Cs,

Ce, Pr, Nd, Sm, Eu, Dy, Ho, Er, Hf and U, better than ±15% for Ti, Cu, As, Tm,

and Lu better than ±20% for Be, V, Y, Nb, Cd, and Bi and better than ±25% for Cr,

Ge and Th. Many problems that occur for example during analyses of the platinum

group elements (Ru, Rh, Pd, Os, Ir, Pt) in natural water samples could be avoided

here because relatively high concentrations were choosen. Viscosity changes dueto separation procedures possibly affect the results of continuous-flow analytical

methods. However, such deviations could be detected and corrected by means of

internal standards.

3. Results

3.1. ULTRAFILTRATION

Concentrations of major and trace elements were determined in the first 50 ml

aliquots of filtrates that were obtained from ultrafiltration experiments. Aliquots

of the unfiltered test solutions, which have been described above in detail, were

analysed parallel to enable precise comparisons. With only few exceptions of equalconcentration values, in most cases a deficit was found in the filtrate. The decreases

in element concentrations due to ultrafiltration, expressed as fractional part in % of

the initial values, are shown in Table II.

Even without additional admixtures, high amounts of many dissolved elements

were removed from the test solution during ultrafiltration through a 0.5 kd mem-


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Table II. Losses on ultrafiltration for different membrane types and solutions. Concentration

deficits in the first 50 ml aliquots of the filtrates compared to the dissolved concentrations

before ultrafiltration are given as ratios in % of the initial values (n a = not analysed). See text

for further explanations.


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brane. For example, deficits of 80–90% compared to the initial Al, Sc, Fe, Ga, Y,

La, the lanthanoides, Hf, Ta, Th concentrations and 50–80% compared to the initial

Be, Mg, Ca, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Sr, Zr, Ru, Cd, Sn, Sb, Ba, Ir, Au, U

concentrations occurred in the filtrates. Only Li, B, Rb, Ag, Cs, Hg and Bi passed

through this membrane with losses below 10%. Contrary, the 1 kd membrans did

not provoke significant decreases of 10% or more of the element concentrations

except for Nb, Pd, Ta, W, Os, Au, Hg and Tl. However, in presence of sulfate,

tartaric acid or fulvic acid, this membrane retained more than 10% of the dissolved

amounts of nearly all investigated elements. Generally, next to the stock solution

without additional admixtures the lowest deficits of about 20% for many elements

were observed with sulfate. With tartaric acid and fulvic acid losses ran up to 20–

50% and 70% or more, respectively, for most elements. An increase of the pH-value

from 1.4 to 5.8 resulted in significant losses as well: 10–60% of the 3d-transition

metals and 50–80% of the lanthanoides were removed from the less acidic solution.

3.2. ION EXCHANGE

The cation exchangers capacity was checked by analyses of the processed solution

in steps of 10 ml aliquots. The break through curves obtained by plotting the

elements concentrations versus the processed volume can be classified as shown

in Figure 1: three groups are formed. The first consists of elements as for example

the alkali metals, that cannot be removed quantitatively from such acidic solutions

even in the first 50 ml. Many transition metals and the alkaline-earth metals belong

to a second group that broke through after 50–250 ml of processed volume. A

remarkable exception are V and Cr, which remain in the first aliquots of the solution

in low amounts of less than 5% of the initial values. Finally, some other metals

especially La and the lanthanoides (REE) are the last to be found in the eluatefrom the cation exchange. Under the chosen conditions they appear after 300 ml

processed volume.

Cation exchange results for the first 50 ml of several test solutions passed

through the membrane adsorber units are listed in Table III. All data are given as

ratios in % of element concentrations in the eluates compared to the initial values.

Corresponding to the break through curves shown in Figure 1, Be, Mg, Al, Ca, Sc,

Ti, Mn, Fe, Co, Ni, Cu, Zn, Ga, Sr, Y, Zr, Nb, Cd, Sn, Ba, La, Ce, Pr, Nd, Sm,

Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, Pb, Bi, Th and U could be removed

quantitatively from 50 ml of the basic solution. Admixtures of sulfate, tartaric acid,

fulvic acid or sodium hydroxide (pH = 5.8) resulted in more or less sharp increases

of many elements concentrations in the eluate. Citric acid yielded extraordinaryhigh amounts of about 70–100% of the initial values for all elements, except Ta

and Hg (Table III).

Analog to the cation exchanges, anion exchange results for the first 50 ml are

given in Table IV, again as ratios in % of the elements concentrations in the eluates

compared to the initial values. Because in aqueous solutions most of the invest-


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Table III. Cation exchange with SartobindTM S100 for 50 ml of the test solutions. Elements

concentrations in the eluates expressed as ratios in % of the initial values (n a = not analysed).

See text for further explantions.


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Figure 1. Scheme of breakthrough curves for cation exchanges with SartobindTM S100 at pH

= 1.4. Concentrations in the eluate as ratios of initial values in the test solution (c/c 0 in %)

versus processed volume. See text for further explanations.

igated elements exist as cationic species, only few elements concentrations were

reduced by an anion exchange. Significant decreases were observed for Mo, Pd,

Ag, W, Au, Hg and Tl. Moreover, the concentrations of V, Cr, As, Ru, Ir and Pt were

slightly reduced. Admixtures of sulfate did not have strong influences and yieldedsimilar results, whereas at pH = 5.8 or with fulvic acid in the stock solution many

elements as for instance the transition metals or the lanthanoides were removed

completely by the anion exchange procedure. The two other tested compounds,

tartaric acid and citric acid, were less effective. Both admixtures resulted in signi-

ficantly reduced concentrations (


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Table IV. Anion exchange with SartobindTM Q100 for 50 ml of the test solutions. Elements

concentrations in the eluates expressed as ratios in % of the initial values (n a = not analysed).

See text for further explanations.


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4. Discussion

4.1. ULTRAFILTRATION

For speciation purposes the 0.5 kd ultrafiltration membranes are supposed to bemost appropriate. At this low cut off the entire colloidal fraction can be separated

almost quantitatively and even concentrations of subcolloidal compounds with

diameters below 500 pm are significantly reduced in the filtrates. Unfortunately,

these membranes retain unacceptable high amounts of dissolved trace elements

from the stock solution, that did not contain any colloids or complexing agents

apart from nitrate and chloride. For this reason, filtration with 0.5 kd ultrafilters

would inevitably lead to overestimations of the complexed fraction. Losses on

ultrafiltration are decisively lower for the 1 kd membranes: As long as there are

no additional compounds in the solution, concentration decreases of most elements

do not exceed 10% of the initial values (Table II). This is almost within the range

of analytical errors. Consequently, 1 kd membranes were used for the following

speciation experiments.The reasons for the distinct filtration properties of the two membranes used are

not clear. Due to different membrane materials the 0.5 kd ultrafilters possibly tend

much more to adsorb dissolved elements than the 1 kd membranes. In this case

concentration decreases during ultrafiltration should not appear any more after a

surface layer has formed. Actually, no significant difference was observed with

0.5 kd membranes that had been pretreated with 500 ml of the stock solution. Thus,

it seems more likely that the observed losses are a result of complex formation even

with ligands such as nitrate, chloride or simply water.

No significant losses occurred during 1 kd ultrafiltration of the stock solu-

tion whereas several admixtures yielded different results: Sulfate, tartaric acid

and especially fulvic acid appear to form sufficiently stable and ultrafilterablecomplexes because for many elements concentration decreases are found during

1 kd ultrafiltration in presence of these potential ligands (Table II). Unfortunately,

organic complexes cannot be differed from complexes with sulfate ligands by ul-

trafiltration. Furthermore, remarkable losses during ultrafiltration are observed at

pH-values of 5.8 which is relative high compared to pH = 1.4 in the stock solution

or pH ≤ 2.0, respectively, in the other test solutions. Most likely adsorption effects

become more important as proton concentrations decrease because hydrogen ions

compete with other cations for surface adsorption. Thus, ultrafiltration at 1 kd

seems to be appropriate for speciation purposes only in strong acidic solutions.

4.2. ION EXCHANGE

The capacities of the used Sartobind S100TM and Q100TM units are proved to be

sufficient for 50 ml sample volumes even under extreme experimental conditions.

As expected, most analysed elements are separated as cations (Table III). Only V,

Cr, As, Se, Mo, Ru, Rh, Pd, Ag, Te, W, Ir, Pt, Au, Hg and Tl are removed by anion


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Figure 2. Ion exchange balance with Sartobind S100TM and Sartobind Q100TM. Exchange

rates (c/c0 in %) compared to initial concentration values for 50 ml of the test solution.

exchange to different extents (Table IV). Anions formed by elements with positive

oxidation states are mainly hydrolysis products as H2VO4−, HCrO4

−, H2AsO4−

and HMoO4−, chloro-complexes as AgCl2

−, AuCl2−, AuCl4

2− and HgCl42−or

even complexes with nitrate (Turner et al., 1981; Brookins, 1988; Greenwood andEarnshaw, 1988; Al et al., 1997). Deficits between the initial concentrations and

the total amount of cationic plus anionic species indicate neutral compounds. As

shown by the ion exchange balance (Figure 2) most elements could be separated

quantitatively either by cation exchange or by anion exchange.

Besides the alkali metals there are only few exceptions: B, Ge, As, Ru, Rh,

Pd, Sb, Te, W, Os, Ir, Pt, Au. Most likely B, Ge, As, Sb and Te are at least partly

present as not dissociated oxygen acids as B(OH)3, H4GeO4, As(OH)3, Sb(OH)3,

H2TeO4. Dissociation is suppressed due to the low pH-value. Thus, these neutral

species pass the ion exchanging membrane. Furthermore, W, Au and the platinum

group metals form neutral compounds as H2OsO5 or highly coordinated complexes

with low charge densities, but there is little information available concerning theseelements species. However, concentrations in natural samples usually will be in the

range of detection limits and therefore too low for speciation studies.

Due to their low charge densities alkali metals are separated by ion exchanges

only in small amounts, particularly at high total ionic contents and low pH-values.

On the other hand, it is because of this specific feature that alkali metals are always


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dissolved as hydrated cations and hardly tend to adsorption, hydrolysis or complex

formation. Thus, further investigations seem to be unnecessary.

4.3. CATION EXCHANGE IN PRESENCE OF COMPLEXING AGENTS

Sulfate is the dominating anion in water samples from sulfide bearing waste rock

dumps. It is supposed to form complexes (Johannesson et al., 1996, Johannesson

and Lyons, 1995; Turner et al., 1981) and therefore has a significant influence

on the ion exchange results. Compared to experiments with the sulfate free stock

solution, cation exchanges with sulfate containing solutions were less effective and

higher amounts of many elements, especially of transition metals, remained in the

eluates (Table III). Even the alkali metals were found in slightly higher concen-

trations, most likely not as a result of complexation but as a consequence of the

increased total ionic content due to the sodium sulfate admixture. The same effect

may occur in presence of tri-sodium citrate. Further information is obtained from

the complementary anion exchange experiments.The three organic compounds used as complexing agents yielded different res-

ults (Table III). While citrate as a strong tridental ligand forms stable, non cationic

complexes and fulvic acid causes similar but less distinct effects, an admixture

of tartaric acid seems to be ineffectual. Because the results of the ultrafiltration

experiments indicated complex formations even with tartaric acid, these complexes

are supposed to be cationic and therefore retained by the cation exchanger or to be

decomposed during the ion exchange process.

4.4. ANION EXCHANGE IN PRESENCE OF COMPLEXING AGENTS

Admixtures of sulfate to the stock solution lead to concentration decreases in therange of 5% to 10% of the initial values after anion exchanges. This reveals the

formation of anionic complexes with sulfate, under certain circumstances even

with 2 or more sulfate ions per central cation, which is for instance reported for

uranium (Turner et al., 1981). The trivalent citrate anions form negatively charged

complexes to an extent of 20% to 80% for transition metals and


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responding ions as well contribute to the total ionic content of the solution and

compete with the other dissolved ions. This can partly be compensated by increas-

ing the exchange capacities, but ion exchange will be efficiently suppressed by

high concentrations of the ions that should be set free from the exchangers func-

tional groups. (2) At low pH-values the mentioned competition between dissolved

ions prevent adsorption losses of other cations than hydrogen ions. Consequently,

at increasing pH-values adsorption may become significant. (3) Many chemical

equilibria are directly influenced by pH-values. The formation of several species of

dissolved elements, especially hydrolysis but also complexation often depends on

the pH-value.

The ion exchange experiments with the stock solution proved that the first

problem is not relevant under these specific conditions. Unfortunately, this does

not apply to the second. Cation exchanges at pH = 5.8 yielded different results

compared to exchanges at pH = 1.4 only for alkali metals, but anion exchanges at

pH = 5.8 removed almost all elements quantitatively from the solution. Most likely

this is caused by precipitation of iron and coprecipitation of many other elements.Furthermore, in the near neutral pH-range apart from the functional groups the

basic frame material of the (an)ion exchanging membranes possibly behaves as an

cation exchanger. This effect is reported for cellulose and some other substances

(Meisch and Gauer, 1998; Höll, 1991; Wegscheider and Knapp, 1981; Helfferich et

al., 1977; Dorfner, 1964). However, the results clearly indicate, that severe artifacts

have to be considered for anion exchanges at pH-values around 6. Ions are usually

exchanged with hydrogen or hydroxide ions. That inescapably decreases pH-values

during cation exchanges and increases it during anion exchanges. To avoid this,

buffers can be used or ion exchanges can be performed with sodium and chlor-

ide ions, respectively. Both would keep pH constant but unfortunately influence

solution chemistry and equilibria. This leads to the third problem mentioned abovewhich is illustrated by ion exchanges of three pore waters with pH-values from 4.4

to 7.1 in the original samples. For all analysed elements rates of cation exchange

with Na+ are plotted versus rates of cation exchange with H+ in Figure 3a. The data

points scatter around the diagonal, indicating that in many cases the results from

both procedures are not consistent. Deviations at high exchange rates can partly be

explained by increasing analytical errors at low concentration ranges. Nevertheless,

exchange rates are predominantly higher, when ion exchangers are used in the

sodium form. Na+ seems to reduce the complexation of several metals even more

strongly than H+, because it matches better to complexing metal ions and competes

with them more effectively. Thus, cation exchanges with H+ are preferred to keep

influences on the solution chemistry as weak as possible. Contrary to the above

mentioned cations, the anions used for anion exchanges tend to react more or less asligands themselves. Hydroxide not only neutralises H+ but also forms many stable

compounds, of which some are soluble, as for instance Al(OH)4− or HCrO4

−,

while others, like iron hydroxides, are precipitated. Chloride generally affects only

few elements. Thus, distinctly higher amounts of most elements are removed from


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Figure 3. Comparison ion exchange results for cation exchanges (CE) with Na+ versus cation

exchanges with H+; (a) and anion exchanges (AE) with Cl− versus cation exchanges with

OH−, (b) in 3 pore water samples from a waste rock dump. Data are given in % exchange

compared to initial concentrations.

the solutions by anion exchanges with OH− than by anion exchanges with Cl−

(Figure 3b).

From this point of view anion exchanges with Cl− are advantageous, however

chloride will cause some analytical difficulties. First of all, many interferences

will occur in mass spectrometry (e.g. May and Wiedmeyer, 1998). Apart from As,

whose only existing isotope interferes with 40Ar35Cl, for example interferences

with lanthanoides are reported (Dulski, 1994). Therefore, additional preparation

steps or appropriate corrections are required and some elements like As have to be

determined by other methods when anion exchangers are used in the chloride form.

5. Conclusions

The results discussed above prove that ultrafiltration and ion exchange with mem-

brane adsorber units are suitable separation techniques for operationally defined

speciation approaches particularly concerning acidic mine drainage. Figure 4

shows the complete speciation procedure schematically for pore waters squeezed

from sediment samples. It consists of 3 parallel separation steps for each sample

followed by analyses of an original sample aliquot and the 3 fractions obtained

from ultrafiltration and ion exchanges. Additionally the solid residue may be

fractionated and determined.

Experiment series performed with synthetically prepared solutions nevertheless

demonstrated that separation techniques used for speciation have to be checkedvery carefully. Depending on the specific conditions in the investigated samples

several phenomena might cause severe artifacts and misinterpretations. Significant

adsorption losses during ultrafiltration occurred when 0.5 kd membranes were used

or proton concentrations exceeded a pH of 5, most likely causing additional con-

centration decreases due to precipitation of iron and coprecipitation of several trace


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Figure 4. Speciation scheme for pore water investigations.

elements. The main advantage in the use of membrane adsorber units for speciation

purposes is, that the time required for ion exchanges can be distinctly diminished.

This way the risk of further chemical reactions and shifting equilibria is reduced.

Furthermore contact of sensitive water samples with oxygen from the air can be

avoided easily. However, some difficulties in species separations by ion exchanges

remain. Ion exchange processes like ultrafiltration are closely connected to thesolution chemistry, especially the pH-value, and even under optimum conditions

unwelcome effects can just be minimised. Advantageous are cation exchanges with

H+ whereas Cl− seems preferable for anion exchanges although high amounts of

chloride unfortunately cause analytical problems. Apart from this, interpretations

of ion exchange results often require additional information on the sample. Consid-

ering all these limitations of separation techniques regarding speciation purposes,

speciation schemes based on operationally defined separations by no means can

be applied on any kind of sample as universal method. They rather have to be

adapted to the specific conditions. Many speciation procedures proposed in the

literature as generally suitable obviously have never been checked or applied to real

samples. For example, multi stage ultrafiltrations will hardly yield reliable resultswith regard to trace metal speciation because all unwelcome effects are multiplied

while the elements concentrations in each single fraction are inevitably reduced and

analytical errors increase. Another absolutely useless speciation method has been

recommended to separate organic compounds together with adsorbed or complexed

elements quantitatively from water samples by means of activated charcoal. Unfor-


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tunately, activated charcoal not only removes organic molecules but also adsorbs

high amounts of not complexed dissolved trace elements, as could be expected.

Therefore this approach of course had to be given up. However, since alternatives

are seldom available, some speciation methods as described above are useful as

long as artifacts and analytical errors are considered earnestly.

Acknowledgements

This paper represents publication no. 34 of the priority program 546 “Geo-

chemical processes with long-term effects in anthropogenically-affected seepage-

and ground water”. Financial support was provided by Deutsche Forschungsge-

meinschaft. The authors would like to thank further Dr. W. Demmer, Sartorius AG

Göttingen, for efficient cooperation and Dr. A. Krüger, Universität Leipzig, for

isolation of fulvic acids. We also appreciate the help of Dr. B. Bock to improve the

text.

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