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November 2008

Work ing Repor t 2008 -74

Interaction Experiments Between U(VI)and Fe(II) in Aqueous SolutionUnder Anaerobic Conditions

November 2008

Working Reports contain information on work in progress

or pending completion.

The conclusions and viewpoints presented in the report

are those of author(s) and do not necessarily

coincide with those of Posiva.

Emmi My l lyky lä

Ka i j a O l l i l a

V T T

Work ing Report 2008 -74

Interaction Experiments Between U(VI)and Fe(II) in Aqueous SolutionUnder Anaerobic Conditions

Interaction Experiments Between U(VI) and Fe(II) in Aqueous Solution Under Anaerobic Conditions

ABSTRACT

This report describes interaction tests between uranium (VI) hydroxyl- or carbonate

complexes and iron(II) in aqueous phase. The tests were conducted under N2

atmosphere of the glove box. The aim was to investigate the effect of aqueous Fe(II) on

these complexes and to observe possible sings of U(VI) reduction by Fe(II).

The 0.01 M NaCl and 0.002 M NaHCO3 solutions were spiked with a U(VI) solution.

Fe(II) was added to the solutions. The initial uranium concentration in reaction vessels

was varied (8.4·10-8

- 4.2·10-7

M). In all tests, the initial concentration of Fe(II) was

2 · 10-6

M. The reaction times varied from 19 to 72 days. The reaction vessels were

maintained closed and the concentration of U in solution was measured with periodical

samplings after the addition of Fe(II). The concentrations of Fe(II) and total iron were

also determined. During the tests, the pH values were measured periodically for all test

solutions. In one test, the redox potential (Eh) of the solution was followed with

continuous measurements with Au and Pt electrodes, which were incorporated into the

test vessel. The oxidation states of uranium were preliminarily analysed for the solution

samples two tests. The method included the separation of the U(IV) and U(VI) with

anion exchange chromatography.

In all test solutions, the concentrations of uranium decreased during the experiments.

The decrease can be a consequence of a redox reaction between U(VI) and Fe(II). The

consumption of Fe(II) seemed to be inversely proportional to the initial concentration of

U in solution. It is also possible that Fe(II) was partly oxidized by the trace oxygen of

the N2 atmosphere during tests and formed Fe(III) precipitates, which may sorb U(VI)

species. In a separate stability test with Fe(II) in 0.002 M NaHCO3 solution, some

oxidation (30 %) was observed. Any visible precipitates could not be observed.

However, the filtrations with different pore sizes revealed a heterogeneous nature of test

solutions. Additional tests with more detailed solution analyses, including potential

precipitates, are needed to clarify mechanisms.

Keywords: uranium, reduction, iron, Fe2+

(aq), anoxic conditions, colloids, spent fuel

U(VI):n ja Fe(II) vuorovaikutuskokeet vesiliuoksessa hapettomissa olosuhteissa

TIIVISTELMÄ

Tässä raportissa kuvataan liuosfaasissa tehtyjä U(VI):n ja Fe(II):n välisiä vuoro-

vaikutuskokeita. Kokeet tehtiin hapettomissa olosuhteissa olosuhdekaapin typpi-

atmosfäärissä. Tarkoituksena oli tutkia liuoksessa olevan Fe(II):n vaikutusta uraanin

hapetustilaan, ja tehdä mahdollisia havaintoja U(VI):n pelkistymisestä.

U(VI) lisättiin ensin liuokseen. Koeliuokset olivat 0.01 M NaCl ja 0.002 M NaHCO3

Näissä liuoksissa U(VI) esiintyy hydroksyyli- ja karbonaattikomplekseina. Tasapaino-

tuksen jälkeen lisättiin rauta(II). Uraanin alkupitoisuudet vaihtelivat välillä 8.4 · 10-8

-

4.2 · 10-7

M. Fe(II):n alkupitoisuus oli kaikissa kokeissa sama (2 · 10-6

M). Reaktioaika

vaihteli 19 päivästä 72 päivään. Koeliuosten pH, sekä uraani- ja rautapitoisuudet

mitattiin sopivin väliajoin Fe(II)-lisäyksen jälkeen. Rauta-analyysit sisälsivät kokonais-

raudan ja Fe(II):n pitoisuusmittaukset. Yhdessä kokeessa pH- ja redox-potentiaali-

mittaukset suoritettiin jatkuvana mittauksena koeastiaan kiinteästi liitetyillä elektro-

deilla. Redox- mittaus tapahtui Au- ja Pt-elektrodeilla. Uraanin hapetustilat analysoitiin

alustavasti yhdestä koeliuoksesta. Käytetyssä menetelmässä U(VI) ja U(IV) erotetaan

toisistaan anioninvaihto kromatografian avulla.

Kokeiden aikana uraanikonsentraatiot laskivat kaikissa koeliuoksissa. Lasku voi olla

seuraus redox-reaktiosta U(VI):n ja Fe(II):n välillä. Fe(II)-konsentraatio laski saman-

aikaisesti. Fe(II):n kulutus näytti olevan käänteisesti verrannollinen uraanin alku-

konsentraatioon liuoksessa. On myös mahdollista, että kokeiden aikana tapahtuu

Fe(II):n hapettumista kaapin N2-atmosfäärin sisältämän jäännöshapen vaikutuksesta.

Hapettumisen seurauksena voi muodostua niukkaliukoisia Fe(III)-saostumia, joilla on

taipumus sorboida U(VI)-spesieksiä. Erillisessä Fe(II):n stabiilisuus testissä 0.002 M

NaHCO3-liuoksessa Fe(II)-pitoisuus laski 30 %. Tämä viittaa Fe(II):n hapettumiseen,

koska kokonaisraudan pitoisuudessa ei havaittu muutosta. Saostumia ei voitu havaita.

Varsinaisissa vuorovaikutuskokeissa suodatukset eri huokoskoon suodattimilla paljasti-

vat koeliuosten heterogeenisen luonteen. Reaktiomekanismien ymmärtämiseksi tarvi-

taan lisäkokeita kontrolloiduissa olosuhteissa, joissa pyritään eliminoimaan jäännös-

hapen mahdollisia vaikutuksia, sekä tehdään tarkempia liuoksen ja mahdollisesti

saostuvien kiinteiden faasien analyyseja (esim. U yhdisteet).

Avainsanat: uraani, pelkistyminen, rauta, Fe2+

(aq), anaerobiset olosuhteet, kolloidit,

käytetty polttoaine

1

TABLE OF CONTENTS

ABSTRACT TIIVISTELMÄ

1 INTRODUCTION ................................................................................................. 3

2 EXPERIMENTAL METHODS .............................................................................. 4

2.1 Preparation of solutions .............................................................................. 5

2.2 Experimental procedure .............................................................................. 6

2.3 Eh and pH measurements ........................................................................... 7

2.4 Elemental analyses..................................................................................... 8

2.5 Uranium oxidation state analyses ............................................................. 10

3 RESULTS AND DISCUSSION ........................................................................... 12

3.1 The evolution of Eh and pH values ............................................................ 12

3.2 Influence of Fe(II) addition on the concentration of uranium in solution ..... 15

3.3 The effects of ultrafiltration ........................................................................ 24

3.4 Uranium oxidation states .......................................................................... 25

3.5 Stability of Fe(II) in test conditions ............................................................ 26

4 SUMMARY AND PRELIMINARY CONCLUSIONS ............................................ 28

5 ACKNOWLEDGEMENTS .................................................................................. 30

REFERENCES ........................................................................................................... 31

APPENDICES............................................................................................................. 33

2

LIST OF SYMBOLS USED

HDPE High density polyethylene

ICP-MS Inductively Coupled Plasma – Mass Spectrometry

MWCO Molecular weight cut-off

s.p. Supra pure

p.a. Pro analysis

PE Polyethylene

ppm parts per million

3

1 INTRODUCTION

Finland plans to dispose the spent nuclear fuel in deep geological repositories sited in

crystalline rock. In the repository, the fuel bundles will be surrounded by an engineered

barrier system consisting of the fuel matrix itself, the iron-copper canister and the

deposition hole filled with a bentonite buffer. The granitic bedrock is also part of the

barrier system. (Pastina and Hellä, 2006)

The spent fuel itself consists mainly of uranium dioxide, UO2 (c) (~96 %). Uranium

dioxide represents the reduced form (of uranium), in which uranium has the oxidation

state of U(IV). Hence it is sparingly soluble. UO2 remains stable in the reducing

groundwater conditions, which should prevail in the repository in the long term.

However, in the case of water intrusion in a potentially defective canister, locally

oxidising conditions might be induced by the radiolysis of water on the surface of the

fuel (Shoesmith, 2000). Alpha radiolysis is the most important after a few hundred

years, producing equal amounts of oxidizing and reducing products, e.g. hydrogen

peroxide and hydrogen, respectively. H2O2 is expected to be more reactive than

hydrogen. Under oxidizing conditions, the dissolution of uranium dioxide to more

soluble U(VI) species could occur. If carbonate is available, the U(VI) forms very

soluble U(VI) carbonate complexes (e.g. UO2(CO3)22-

, UO2(CO3)34-

, (UO2)2CO3(OH)3-)

(Langmuir, 1997). These complexes may become stable already in mildly reducing

conditions. In the absence of carbonate, U(VI) forms hydroxyl complexes (e.g UO2OH+,

UO2(OH)2, UO2(OH)3-, (UO2)3(OH)5

+, (UO2)2(OH)2

2+) above the pH 5. The dissolution

of uranium dioxide might lead to the mobilization of other actinides and soluble

radionuclides from spent fuel matrix. If the U(VI) is reduced to U(IV), it might also

coprecipitate other actinides and radionuclides, thus preventing their release.

The iron insert of the canister contains of 10-18 tons (Raiko, 2005) of metallic iron. In

the case of water intrusion to the canister, the anaerobic corrosion of iron produces

Fe(II) ions and hydrogen gas to the surrounding solution. Both Fe(II) and hydrogen

could function as an reductant for uranium inside the canister.

In the experimental studies (Butorin et al. 2003, Ollila et al. 2003, Farrell et al. 1999,

Fiedor et al. 1998), the concentration of uranium in solution has been observed to

decrease strongly in the presence of metallic iron under anoxic conditions. The reduced

uranium precipitates were observed to form on the iron surface. However, some

experiments (Butorin et al. 2003) indicated that the reduction of U(VI) species might

take place also in solution. This suggests, that aqueous Fe(II) or hydrogen could

function as a reductant for U(VI) in the solution phase. The purpose of the interaction

tests of this work was to study the potential effects of Fe(II).

The interaction tests between aqueous Fe(II) and U(VI) were performed in 0.01 M NaCl

and 0.002 M NaHCO3 solutions in the glove box under N2 atmosphere. The test

solutions were selected to investigate the effects of Fe(II) on the behaviour of uranyl

hydroxyl and uranyl carbonate complexes in solutions, respectively. One aim of these

preliminary experiments was to test experimental and analytical techniques in the glove

box for the future experiments.

4

2 EXPERIMENTAL METHODS

The interactions between U(VI) and aqueous Fe(II) were studied in 0.002 M NaHCO3

and 0.01 M NaCl solutions under N2 atmosphere in the glove box. First, three

preliminary tests in 0.002 M NaHCO3 solutions were carried out, see Table 1. These

included a test in duplicate with aqueous Fe(II) and a reference test with metallic iron

(UFE1-UFE3), see Table 1.

This was followed by experiments in 0.002 M NaHCO3 or 0.01 M NaCl solutions, in

which the initial concentration of uranium was varied (UFE4-UFE10). One experiment

was performed in each test solutions without uranium in order to study the stability of

the Fe(II) state in solution in the experimental conditions (UFE 9 and 10).

Finally, two additional tests were performed in 0.002 M NaHCO3. The solution volume

was increased to 250 ml. The first one was conducted with continuous pH and redox

(Eh) measurements (UFE 11). This reaction vessel was equipped with incorporated

electrodes and the vessel was not opened during the test. Samples for U and Fe analyses

were taken only at the end of the test. The parallel test (UFE 12) was performed with

frequent sampling for U and Fe analyses. The pH was measured with external electrode

from the separate samples of the test solution. The purpose was to get information about

the effect of the trace oxygen of the N2 atmosphere on the redox conditions during the

opening of the vessel for samplings by comparing the results of these two tests.

All the experiments were performed in high-density polyethylene vessels under

anaerobic conditions (N2) in the glove box, see Figure 1.

The test solutions used in the experiments are described in more detail in Table 1.

Table 1. The compositions of the test solutions.

Test [U(VI)] initial [Fe(II)] initial Solution Solution

volume

Duration

(mol/l) (mol/l) (ml) (d)

UFE 1 4.2·10-7

2·10-6

0.002 M NaHCO3 200 72 UFE 2 4.2·10

-7 2·10

-6 0.002 M NaHCO3 200 72

UFE 3 4.2·10-7

Fe coupon 0.002 M NaHCO3 200 72

UFE 4 4.2·10-7

2·10-6

0.002 M NaHCO3 100 59 UFE 5 2.1·10

-7 2·10

-6 0.002 M NaHCO3 100 59

UFE 6 8.4·10-8

2·10-6

0.002 M NaHCO3 100 59

UFE 7 2.1·10-7

2·10-6

0.01 M NaCl 100 59

UFE 8 8.4·10-8

2·10-6

0.01 M NaCl 100 59

UFE 9 0 2·10-6

0.002 M NaHCO3 100 59

UFE 10

101010

0 2·10-6

0.01 M NaCl 100 59

UFE 11 8.4·10-8

2·10-6

0.002 M NaHCO3 250 19 UFE 12 8.4·10

-8 2·10

-6 0.002 M NaHCO3 250 19

5

Figure 1. Reaction vessels in the glove box. The detailed test conditions are given in

table 1.

2.1 Preparation of solutions

The Fe(II) stock solution was prepared in the glove box by dissolving FeCl2 · 4H2O

(Merck, pro analysis quality) in deaerated deionized water. The Fe(II) chloride had been

stored in the glove box in order to prevent the oxidation of the material. Fresh Fe(II)

stock solution was used at the start of the three separate phases of the experiments.

In UFE 3 test vessel, two Fe strips (0.9 g), 1.5 x 3 cm, were used as a source of metallic

iron. The strips originated from the larger Fe foil with purity of 99.5 % (Goodfellow

Cambridge Limited, 100 mm x 100 mm x 0.125 mm).

The U(VI) stock solution was a standard uranium solution in 2 % HNO3 (Accu TraceTM

Reference Standard, 1002 mg U/l). The stock solution (1002 mg U/l) was diluted with

deaerated 1 M HNO3 in the glove box. An aliquot of the dilution was added to the test

solutions.

The NaHCO3 solutions were prepared by dissolving NaHCO3 (Merck, p.a. quality) salt

in deaerated deionized water in the glove box. The NaHCO3 solutions were stored in

tightly closed polyethylene containers in the glove box.

The NaCl solutions were prepared from deaerated stock solution (1 M) by dilution with

deaerated deionized water in the glove box. The stock solution was prepared by

dissolving NaCl (s.p. quality) in deionized water under aerobic conditions.

6

2.2 Experimental procedure

The tests were performed in a glove box as follows.

UFE 1, UFE 2

The 200 ml of 0.002 M NaHCO3 solution was added to a polyethylene vessel (250 ml).

An aliquot of uranyl nitrate solution (200 µl) was added to the test vessel. 0.1 M NaOH

was added to neutralize the acid from uranyl nitrate solution. The solution was allowed

to equilibrate for 24 hours. Next an aliquot of the Fe(II) chloride solution was added to

the solution. After stirring, the test vessel was closed tightly. The vessel remained

closed in the glove box. The evolution of the U, Fe(II) and total Fe concentrations were

followed by taking samples periodically for analyses. The pH was also measured. The

duration of the tests was 72 days.

UFE 3

Two Fe strips (1.5 x 3 cm) were added to the solution instead of aqueous Fe(II).

Otherwise, the test was performed similarly with the previous tests.

UFE 4-10

The 100 ml of 0.002 M NaHCO3 or 0.01 M NaCl solution was added to a polyethylene

vessel (125 ml). An aliquot of uranyl nitrate solution was added to the test vessel. 0.1 M

NaOH was added to neutralize the acid from uranyl nitrate solution. After overnight

equilibration, the ferrous iron was added as an aliquot of Fe(II) chloride solution. The

solutions were maintained in closed vessels in the glove box. The duration of the tests

was 59 days. The solutions in the test vessels were sampled periodically as in the

previous experiments.

UFE 11

The 250 ml of 0.002 M NaHCO3 solution was added to a polyethylene vessel (250 ml).

At next day, an aliquot of uranyl solution was added to the test vessel. 0.1 M NaOH was

added to neutralize the acid from uranyl nitrate solution. The solution was allowed to

equilibrate for 3 days, after which an aliquot of the Fe(II) solution was added. After this,

the reaction vessel was maintained closed in the glove box. The samplings were made at

the end of the test (18 days).

UFE 12

The test was performed in the same way with UFE 11, except the solution samples were

taken periodically.

The sampling procedures, as well as pH and redox measurements are described in detail

in Chapter 2.3.

7

2.3 Eh and pH measurements

The pH of the test solutions was measured with an Orion ROSS pH electrode (UFE 1-

10, UFE 12). The samples, which were used for pH measurements, were returned to the

reaction vessels in the UFE 4-10 tests and rejected in the other tests.

In the UFE 11 test, the pH and Eh were measured continuously throughout the test. The

reaction vessel was equipped with integrated electrodes for the measurements. Solid

IrOx wire worked as pH electrode. The Eh was measured with two electrodes. One was

prepared from Pt wire and the other from Au wire. The preparation of these electrodes is

described more detailed in Muurinen and Carlsson (2007). A commercial Ag/AgCl

electrode (LF-27, Innovative Instruments, Inc) was used as reference electrode for all

measuring electrodes. The electrodes were placed in four holes in the lid of the vessel

and attached with epoxy glue. The vessel with the electrodes was delivered to the glove

box.

Before the measurements, the electrodes were calibrated. The potential of the reference

electrode (LF-27) was checked with an additional commercial reference electrode

(Ag/AgCl, Orion 900100) in 0.1 M NaCl. The pH electrode (IrOx) was calibrated

measuring the potential between it and the reference electrode in four different buffer

solutions (pH 4, 7, 10 and 12). The calibration was performed before the start and after

the termination of the test.

The time table for the measurements is given in Table 2.

Table 2. The time table for pH ja Eh measurements in UFE experiments.

Test pH Eh

(days) (days)

UFE 1 1, 23, 67 -

UFE 2 1, 23, 67 -

UFE 3 1 -

UFE 4 1, 16, 53 -

UFE 5 1, 16, 53 -

UFE 6 1, 16, 53 -

UFE 7 1, 16, 53 -

UFE 8 1, 16, 53 -

UFE 9 1, 16, 53 -

UFE 10 1, 16, 53 -

UFE 11 (-3,-2,-1)*, 1,2,3,4,5, 8,9,10,11,12,15,16,17,18,19**

(-3,-2,-1)*, 1,2,3,4,5, 8, 9,10,11,12,15,16,17,18,19**

UFE 12 (-3)*, 1, 19 -

* Measured before the addition of Fe(II)

** Continuous measurement, readings daily

8

2.4 Elemental analyses

Uranium

The total concentrations of U in test solutions were analysed with ICP-MS (VG Plasma

Quad 2+). The uranium concentration was measured in unfiltered (1 ml), microfiltered

(2 ml) and ultrafiltered (800 μl) samples. The samples were acidified before

measurements (1 M HNO3). Microfiltration was performed with the help of a syringe

and a Millex-GV filter (0.22 m pore size, 13 mm). Ultrafiltered samples were taken

to evaluate a possible colloid formation in the test solutions. Ultrafiltration was carried

out by centrifuging the samples in Whatman’s Vectraspin Micro 2 ml centrifuge tubes

(8000-9000 g, 15-40 min). These were equipped with cellulose triacetate filter

membrane with 12 K molecular weight cut off (MWCO). All the filtrations were

performed under nitrogen atmosphere in the glove box.

The time table for uranium samplings is given in Table 3.

Table 3. Uranium samplings in UFE experiments.

Test Unfiltered Microfiltered Ultrafiltered Oxidation state

vessel (days) (days) (days) (days)

UFE 1 1,2,3,7,9,11,23,71 8,10,22,70 - -

UFE 2 1,2,3,7,9,11,23,71 8,10,22,70 - -

UFE 3 1,2,3,7,9 8,10 - -

UFE 4 1,15,34,43,56 1,15,34,43,56 55 -

UFE 5 1,15,34,43,56 1,15,34,43,56 55 -

UFE 6 1,15,34,43,56 1,15,34,43,56 55 -

UFE 7 1,15,34,43,56 1,15,34,43,56 55 -

UFE 8 1,15,34,43,56 1,15,34,43,56 55 -

UFE 9 1,15,34,43,56 1,15,34,43,56 - -

UFE 10 1,15,34,43,56 1,15,34,43,56 - -

UFE 11 18 18 19 -

UFE 12 1,2,3,4,5,8,10,15,18 4,5,8,10,15,18 19 12,16

9

Iron

The analyses of total iron and ferrous iron contents in solution were made by using a

spectrophotometric method illustrated by Dimmock et al. (1979) and developed further

by Ruotsalainen et al. (1994). The basic idea of the method is that the ferrous iron in

solution is allowed to react with ferrozine (3-(2-pyridyl-5,6,bis(4-phenylsulfonic acid)-

1,2,4 triazene, disodium salt). The concentration of developed iron(II)- ferrozine

complexes is measured with a spectrophotometer.

The total iron content was analysed by first reducing all iron in solution to Fe(II), which

was followed by the reaction of Fe(II) with ferrozine. Thioglycolic acid was used as

reducing agent. The content of Fe(III) can be calculated by substracting the Fe(II)

content from the total Fe content. The criterion of detection for iron with this method is

9·10-9

to 3·10-8

mol/l depending on the spectrophotometer used for the measurements

(Dimmock et al. 1979).

The combined ferrozine reagent was prepared by dissolving 0.2 g of ferrozine in a few

millilitres of ultra pure water in a volumetric flask (100 ml). Then 25 ml of acetic acid

and 6 of ml ammonia were added with stirring. The flask was filled with ultra pure

water, after it had cooled down to room temperature (~20ºC). The pH of the solution

was checked and adjusted to 4 with few drops of ammonia.

The samples (5 ml) for Fe(II) and Fetot (5 ml) analyses were taken from test solutions in

the glove box. The ferrozine reagent buffer (400 l) was added to the Fe(II) samples in

the glove box. Thioglycolic acid (50 μl) was added to the Fetot samples also in the glove

box, before they were exported.

For calibration curve, seven solutions with different iron contents (8 – 400 l) were

prepared by dilution from a stock solution of iron in ultrapure water. The Fe stock

solution was prepared by dissolving FeCl3· 6 H2O in ultrapure water. The iron in

solutions was reduced to Fe(II) with thioglycolic acid at 90 C (30 min). After cooling,

the ferrozine reagent buffer was added and pH adjusted to 4.1. The absorbancies of

solutions were measured with a VIS-spectrophotometer at the wavelength of 562 nm.

The samples from test solutions were taken in the following way:

UFE 1-3

The samples for Fe analyses were taken 7, 22 and 72 days after the start of the

experiments in order to evaluate the development of Fe(II) and Fetot concentrations.

Two samples (3 ml) were pipeted from each test solution to polypropylene tubes in the

glove box. The sample tubes had been maintained under nitrogen atmosphere at least a

few days before use to deoxidize the surface of the tubes. The deaerated ferrozine

reagent buffer (240 μl) was added to the Fe(II) samples and respectively the deaerated

thioglycolic acid (30 μl) was added to the Fetot samples. Next, the sample tubes were

taken out from the glove box.

The pH of the Fe(II) samples was adjusted to 4.1. The absorbancies were measured.

The Fetot samples were kept in the water bath at 90 C (30 min). After the cooling of the

samples, the ferrozine reagent buffer was added and the pH value was adjusted to 4.1

with ammonia. The absorbancies were measured.

10

The blank samples for spectrophotometric determinations were prepared from 0.002 M

NaHCO3 solution in the similar way with the Fetot samples.

UFE 4-10

The samples for Fe analyses were taken 1, 15, 31 and 59 days after the start of the

experiments. The samples were treated otherwise in the similar way with the UFE 1-3

samples, but the sample volume was increased to 5 ml. Thus the added amounts of

thioglycolic acid and ferrozine reagent buffer were 50 μl and 400 μl, respectively.

The blank samples for spectrophotometric determinations were prepared from 0.002 M

NaHCO3 or 0.01 M NaCl solution in the similar way with the Fetot samples

UFE 11-12

The samples for Fe analyses from the UFE 12 test solution were taken 4, 8, 15 and 18

days after the start of the experiments. The UFE 11 test solution was sampled only near

the end of the experiment (18 d). The sample volume was 5 ml. The samples were

treated in the similar way with the UFE 4-10 samples.

The blank samples for spectrophotometric determinations were prepared from 0.002 M

NaHCO3 solution in the similar way with the Fetot samples.

2.5 Uranium oxidation state analyses

The oxidation state of U was preliminarily analysed for the samples, which were taken

from the UFE 12 test solution at the experimental days of 12 and 16. Duplicate samples

were taken at both times. The method included the separation of the tetravalent and

hexavalent states by anion-exchange chromatography in HCl medium, followed by the

analyses of the uranium contents of each of the fractions by ICP-MS (Hussonnois et al.

1989, Ollila 1996). The separation of U(IV) and U(VI) is based on the fixation of U(VI)

chloride complexes on the anionic resin in strongly acidic HCl medium.

The separation of U(IV) and U(VI) was performed under N2 atmosphere in the glove

box. All the acid solutions were of suprapure quality and they were dearated with N2

prior to the use. The separation included the following steps:

1. The sample (0.5 ml) was taken from the test solution.

2. Concentrated HCl was added to the sample. The resulting sample solution should

have 4.5 M Cl-. The acid solution breaks up colloids and/or unknown hydrolyzed and

complexed species present in the sample and forms U chloride complexes. The strongly

acidic solution was allowed to react for a couple of hours.

3. Anionic resin (Dowex 1 x 8, 200-400 mesh Cl) was added to an empty column

(2 ml, Eichrom). The resin was washed with 4.5 M HCl (4 ml) and 0.1 M HCl (8 ml) to

remove trace uranium from the resin. Finally, the resin was treated with 4.5 M HCl

(4 ml).

4. The sample solution was pipetted into the column reservoir and allowed to flow

through the column (0.2 ml/min) to test tubes. At this stage, the U(VI) chloride

complexes fix on the resin and the U(IV) flows through the column.

11

5. 4 ml of 4.5 M HCl was pipetted into the column and allowed to flow through in

order to flush the rest of the U(IV) to test tubes.

6. 0.7 ml of 0.1 M HCl was pipetted into the column and allowed to flow through to

rinse 4.5 M HCl out of the column.

7. 6 ml of 0.1 M HCl was pipetted into the column and allowed to flow through to

test tubes. At this stage, the U(VI) is eluted from the resin.

Next, the test tubes with the U(VI) in 0.1 M HCl solution from the step 7, and the test

tubes with the U(IV) in 4.5 M HCl solution from the steps 4 and 5 were brought out of

the glove box.

Concentrated nitric acid (s.p.) was added to the U(VI) test tubes (1 M HNO3). The

samples were stored in the refrigerator until they were analysed in U with ICP-MS.

The 4.5 M HCl solution samples with the U(IV) were too acidic for ICP-MS analyses. It

was necessary to separate the U from the solution before the measurements. This was

performed by oxidizing the U(IV) in the samples to U(VI), which was followed by the

fixation of the U(VI) on the anionic resin and the elution with 0.1 M HCl as follows:

1. The 4.5 M HCl solution samples were maintained in air over a night in order to

oxidize the U(IV) to U(VI).

2. The 4.5 M HCl samples were combined in a flask. An aliquot of 233

U solution was

added to the sample solution for yield determination (1.6 ng 233

U). The solution was

allowed to equilibrate for a couple of hours.

3. A column with anionic resin was prepared and treated with 4.5 and 0.1 M HCl in

the same way with the step 3 above.

4. The sample solution was pipetted into the column reservoir and allowed to flow

through the column to fix the U(VI) on the resin.

5. 0.6 ml of 0.1 M HCl was pipetted into the column and allowed to flow through to

rinse 4.5 M HCl out of the column.

6. 8 ml of 0.1 M HCl was pipetted into the column and allowed to flow through to

test tubes to elute the U(VI) from the resin.

Conc. nitric acid (s.p.) was added to the U(VI) test tubes (1 M HNO3). The samples

were stored in the refrigerator until they were analysed in U with ICP-MS.

The U(IV) and U(VI) amounts in the original sample were calculated on the basis of the

results of the ICP-MS analyses.

12

3 RESULTS AND DISCUSSION

3.1 The evolution of Eh and pH values

Preliminary tests in NaHCO3 solution (UFE1-UFE3)

In these tests, U(VI) was added to 0.002 M NaHCO3 solution (Table 1). Under these

conditions, the predominant aqueous species for U(VI) are carbonate complexes, e.g.

UO2(CO3)34-

and UO2(CO3)22-

(Bruno et al. 1997, Ollila & Ahonen 1998). The

measured pH in uranyl carbonate solutions was rather stable, see Table 4. The pH

increased slightly after the addition of Fe(II) to solution. In the reference test with Fe

strips (UFE 3), the pH was measured only in the beginning of the test. The Eh was not

measured in these tests.

Table 4. Measured pH values for NaHCO3 solutions before and after the addition of

Fe(II) to the solution.

day UFE1 UFE2 UFE3

after the equilibration of the uranyl with

carbonate solution (24 hours) 8.8 8.9 8.6 after the Fe(II) addition to the uranyl

carbonate solution (30 hours) 8.9 9.0 -

23 days 9.3 9.3 -

67 days 9.2 9.2 -

Tests in NaHCO3 and NaCl solutions with varying initial U concentration (UFE 4-10)

In these tests, a varying concentration of U(VI) was added to 0.002 M NaHCO3 or

0.01 M NaCl solutions. In the latter solution, U(VI) forms uranyl hydroxyl complexes

in the absence of carbonate (Langmuir 1997, Ollila & Ahonen 1998). As in the previous

preliminary tests, there was only a slight increase in pH in NaHCO3 solutions after the

addition of Fe(II) (Figure 2). In NaCl solutions, the change in pH was very small.

The pH of the 0.01 M NaCl solution without uranium differed from those with U

(Figure 2, B). The pH of this solution was approximately 7.5. The pH values of the

solutions with U varied between 8.8 and 9.6 and tended to increase during the tests.

13

A

Time / d0 10 20 30 40 50 60

pH

7.0

7.5

8.0

8.5

9.0

9.5

10.0

4.2x10-7

mol/l U

2.1x10-7

mol/l U

8.4x10-8

mol/l U

no U

B

Time / d

0 10 20 30 40 50 60

7.0

7.5

8.0

8.5

9.0

9.5

10.0

2.1x10-7

mol/l U

8.4x10-8

mol/l U

no U

Figure 2. Measured pH values after the addition of Fe(II) to 0.002 M NaHCO3

solutions (A) and to 0.01 M NaCl solutions (B). The initial uranium concentrations are

given in the legends.

Additional tests in NaHCO3 solutions (UFE 11 and 12)

The measured pH values for the uranyl carbonate solutions before and after the addition

of Fe(II) are presented in Figure 3. There is a very slow increasing trend in pH. The pH

values measured with the Ir electrode in the UFE 11 test were based on the calibrations

before and after the test, Ir (1) and Ir (2), respectively. The results were almost equal

(see Figure 3). The parallel measurements with the Ross electrode in the UFE 11 test

gave the pH values, which was 0.3 units higher. One possible reason for the higher

value with Ross could be the escape of CO2 from the solution to the atmosphere of the

glove box during the measurement in the open test vessel. Depending on the used

electrode the final pH varied from 8.8 to 9.1.

The pH in the parallel test (UFE 12) was measured three times during the reaction

period, see Figure 3. Samples were taken from the test solution for the measurements

with the Ross electrode. The final pH value was in good agreement with the one

measured with the same electrode for the parallel test solution (UFE 11).This suggests,

that the opening of the test vessel for U and Fe samplings in the UFE 12 test did not

have effect on pH. The final pHs were also at the same level with the measured values

for the test solution in the previous UFE 6 experiment, which was performed under

similar conditions.

14

Time (d)

0 5 10 15 20

pH

7.5

8.0

8.5

9.0

9.5

Addition of U(VI)

Addition of Fe(II)

UFE 11 Ir (1)

UFE 11 Ir (2)

UFE 11 (Ross)

UFE 12 (Ross)

Figure  3. Measured  pH  values  for  NaHCO3  solutions  in  UFE 11  and  UFE 12experiments. The plots Ir(1) and Ir(2) represent the measured pH with the incorporatedIr electrode according to the electrode calibrations before (1) and after (2) the test. ThepH values measured with the Ross pH electrode are presented as comparison.

The results of the continuous Eh measurements for the UFE 11 test solution before andafter  the  addition  of  Fe(II)  are  shown  in  Figure  4.  After  the  addition  of  the  Fe(II)aliquot,  the  Eh decreased  rapidly  from  the  initial  value  of  100­200 mV  to  below­300 mV, and was stable for a few days. Afterwards, it was increasing slowly. The testvessel was kept closed throughout the experiment.

There was a difference of 100 mV  between the  initial Eh values  measured with  the Ptand Au electrodes. The Eh values were almost similar during the period of 10 days afterthe  addition  of  Fe(II),  but  they  differed  in  the  later  stages  of  the  experiment.  Thepotential given by Au electrode was higher. It seemed like that the Au electrode wouldhave given a slightly faster response to the changes in the solution.

15

Time (d)

0 5 10 15 20

Eh

(mV

)

­400

­300

­200

­100

0

100

200Addtion of U

Addtion of Fe

Au electrode

Pt electrode

Figure  4. Measured  Eh  values  for  the  test  solution  with  the  incorporated  Au  and  Ptelectrodes in the UFE 11 test vessel.

3.2  Influence of Fe(II) addition on the concentration of uranium insolution

Preliminary tests in NaHCO3 solution (UFE 1­3)

Figure 5 illustrates the observed changes in the uranium and iron concentrations in theuranyl carbonate solutions after the addition of Fe(II) to solution. The U was analysedfor  unfiltered  and  selected  microfiltered  samples  (see  Tables  A1­1  and  A1­2  inAppendix 1). There was a rapid initial drop in the concentration of U during the first 10days after the addition of the Fe(II) aliquot to the solution. Simultaneously with the U,the concentrations of both Fe(II) and total Fe in solution decreased. In the presence of astrip  of  metallic  iron,  the  U  decreased  more  rapidly.  It  reached  the  value  below  thedetection  limit  of  ICP­MS  (8.4 · 10­11  mol/l)  in  11 ­ 23  days  (Figure  6).  A  blackprecipitate was observed to form in the solution.The  initial decrease  in  the  U  concentration  suggests  the  reduction  of  U(VI)  by  Fe(II)which is followed by the precipitation of U(IV) due to its low solubility. If the reductionoccurs,  the  oxidation  of  Fe(II)  to  Fe(III)  also  takes  place.  The  developed  Fe(III)probably precipitates due to its low solubility under these conditions. This could explainthe decrease in both total Fe and Fe(II) concentration.

16

UFE 2

Time (d)

0 20 40 60 80

U (

mo

l/l)

0.0

1.0e­7

2.0e­7

3.0e­7

4.0e­7

5.0e­7

Fe

 (m

ol/

l)

0.0

5.0e­7

1.0e­6

1.5e­6

2.0e­6

Uunfiltered

Ufiltered

Fe2+totFe

Time (d)

0 20 40 60 80

U (

mo

l/l)

0.0

1.0e­7

2.0e­7

3.0e­7

4.0e­7

5.0e­7

Fe

 (m

ol/

l)

0.0

5.0e­7

1.0e­6

1.5e­6

2.0e­6UFE 1

Uunfiltered

Ufiltered

totFeFe2+

Figure 5. Development of uranium and iron concentrations in 0.002 M NaHCO3 afterthe addition of Fe(II) to solution. The results are for parallel tests UFE 1 and UFE 2.

17

UFE 3

Time (d)

0 20 40 60 80

U (

mo

l/l)

10-11

10-10

10-9

10-8

10-7

10-6

U (unfiltered)

U (microfiltered)

Detection limit for U

Figure 6. Development of uranium concentration in 0.002 M NaHCO3 after the

addition of metallic Fe to solution (UFE 3).

Another reason for the decrease in the U concentration could be the sorption onto iron

precipitates and/or the coprecipitation of U(VI) with these precipitates. It is possible that

small amounts of Fe(II) may become oxidized due to the trace oxygen of the nitrogen

atmosphere of the glove box, and thus cause the formation of iron (III) precipitates. The

oxygen content in the atmosphere of the glove box normally stays below 1 ppm. The

stability of Fe(II) in 0.002 M NaHCO3 and 0.01 M NaCl solutions in the glove box was

tested with the help of parallel tests without U (UFE 9 and 10), see paragraph 3.5

(p. 31). A decrease of 30 % in the concentration of Fe(II) was observed in the carbonate

solution during the test period of 60 days. The total concentration of Fe in the same

solution seemed to remain stable.

After the rapid initial decrease, the U concentration in solution seemed first to stabilize

but increased slightly between the last two sampling points, see Figure 5. The

concentrations of Fe(II) and total Fe decreased slowly until the end of the experimental

time. The trends for U and Fe concentrations were similar in the unfiltered and

microfiltered samples.

Tests in NaHCO3 and NaCl solutions with varying initial U concentration (UFE 4-10)

The uranium concentrations in 0.002 M NaHCO3 and 0.01 M NaCl solutions after the

addition of Fe(II) to solutions are given in Table A2-1 in Appendix 2. The measured

contents are for unfiltered and microfiltered samples. As expected on the basis of the

preliminary tests, there was a clear difference between unfiltered and microfiltered

samples showing the solutions to be inhomogeneous. This suggests the formation of

precipitates in the solutions. Precipitates were not visible to the eye. The difference

between unfiltered and filtered samples was greater in NaCl solutions. Figure 7 gives

the measured contents in different samples for the test with an initial U content of

8.4 · 10-8

M in 0.002 M NaHCO3 solution.

18

UFE 6

Time/d

0 20 40 60

U (

mo

l/l)

0

2e­8

4e­8

6e­8

8e­8

1e­7

Fe

 (mo

l/l)

0.0

5.0e­7

1.0e­6

1.5e­6

2.0e­6

2.5e­6

U(unfiltered)

U(filtered)

Fe(II)

Fe(tot)

Figure 7. Development of the concentrations of uranium, Fe(II) and total Fe in 0.002 MNaHCO3 solution after the addition of Fe(II) to solution (UFE 6).

The measured concentrations of uranium in microfiltered samples in 0.002 M NaHCO3

and  0.01 M  NaCl  solutions  are  given  in  Figures  8  and  9,  respectively.  Theconcentrations  probably  represent  the  U  in  soluble  or  colloidal  form.  The  Uconcentration  in solution decreased  in all  tests after  the addition of Fe(II). The 34 daysamples gave somewhat higher results. This may be due to the use of a filter membranewith  a  larger  pore  size  (0.45 µm),  which  was  used  instead  of  the  normal  filter  in  thissampling. A filter with a pore size of 0.22 µm was used in all other samplings.

The decrease of  the U concentration  in  solution  was greater  in 0.01 M NaCl  solution,especially  in  the  test  with  the  initial  U  content  of  2.1 · 10­7 M  (Figure  9).  The  Udecreased four orders of magnitude. In this test, the U concentrations in the samples for1 and 15 days were below the detection limit of the ICP­MS. The first points show theamounts of U additions at the start of the tests. In the test with the initial U content of8.4 · 10­8   M,  the  U  concentration  in  the  1 day  sample  was  two  orders  of  magnitudelower  than  the  added  amount  at  the  start.  The  results  suggest  faster  reactions  in  theabsence of carbonate.

19

0.002 M NaHCO3

Time (d)

0 10 20 30 40 50 60

U (

mo

l/l)

10­9

10­8

10­7

10­6

Initial U: 4.2x10­7

 M

Initial U: 2.1x10­7 M

Initial U: 8.4x10­8

M

Figure 8. Development of uranium concentration in 0.002 M NaHCO3 after the additionof Fe(II) to solution. The results are for microfiltered samples from tests UFE 4­6.

0.01 M NaCl

Time  (d)

0 10 20 30 40 50 60

U (

mo

l/l)

10­11

10­10

10­9

10­8

10­7

10­6

Initial U: 2.1x10­7

 M

Initial U: 8.4x10­8

 M

c(U) < 8.4x10­11

 M

Detection limit for U

?

Figure 9. Development of uranium concentration in 0.01 M NaCl after the addition ofFe(II) to solution. The results are for microfiltered samples from tests UFE 7­8.

20

Figure 10 gives the analysed Fe(II) concentrations during the test period, respectively.

The detailed data are given in Table A2-2, Appendix 2. The initial concentration of

U(VI) seemed to affect the consumption of Fe(II) in solution. The decrease in the Fe(II)

concentration was inversely proportional to the initial concentration of U(VI) both in

0.002 M NaHCO3 and in 0.01 M NaCl solutions. The higher was the initial

concentration of U(VI), the lower was the final concentration of Fe(II). The last

sampling point in the tests in 0.002 M NaHCO3 made an exception. The final Fe(II)

concentrations in the tests with the initial U contents of 2.1 · 10-7

and 8.4 · 10-8

M were

equal. The results suggest some U(VI) reduction to occur during the tests. Figure 11

presents the formation of Fe(III) in the tests. The concentration of the Fe(III) was

calculated subtracting the Fe(II) content from the total Fe content. The reason for the

negative values at the start of the tests is, that some Fe(II) analyses gave higher results

than the corresponding total Fe analyses (see App. 2). There was some decrease in the

total Fe concentrations during the test periods (Figure 12). The decrease seemed to be

smaller than in the preliminary tests (Figure 5).

0.0E+00

5.0E-07

1.0E-06

1.5E-06

2.0E-06

2.5E-06

3.0E-06

0 10 20 30 40 50 60 70

Time (d)

Fe(I

I) / (

mo

l/l)

4.2E-7 M U (UFE4)

2.1E-7 M U (UFE5)

8.4E-8 M U (UFE6)

2.1E-7 M U (UFE7)

8.4E-8 M U (UFE8)

Figure 10. The Fe(II) concentrations as a function of time in the tests in 0.002 M

NaHCO3 (UFE4-7) and 0.01 M NaCl solutions (UFE 7-8) with different initial U

concentrations.

21

­5.0E­07

0.0E+00

5.0E­07

1.0E­06

1.5E­06

2.0E­06

0 10 20 30 40 50 60 70

Time (d)

Fe(I

II) 

/ (m

ol/l)

4.2E­7 M U (UFE4)

2.1E­7 M U (UFE5)

8.4E­8 M U (UFE6)

2.1E­7 M U (UFE7)

8.4E­8 M U (UFE8)

Figure  11. The  Fe(III)  concentrations  as  a  function  of  time  in  the  tests  in  0.002  MNaHCO3  (UFE4­7) and  0.01  M  NaCl  solutions  (UFE 7­8)  with  different    initial  Uconcentrations.

0.0E+00

5.0E­07

1.0E­06

1.5E­06

2.0E­06

2.5E­06

3.0E­06

0 10 20 30 40 50 60 70

Time (d)

Fe(t

ot)

 / (

mo

l/l)

4.2E­7 M U (UFE4)

2.1E­7 M U (UFE5)

8.4E­8 M U (UFE6)

2.1E­7 M U (UFE7)

8.4E­8 M U (UFE8)

Figure 12. The  total  Fe  concentrations as a  function  of  time  in  the  tests  in  0.002  MNaHCO3  (UFE4­7) and  0.01  M  NaCl  solutions  (UFE 7­8)  with  different  initial  Uconcentrations.

22

Additional tests in NaHCO3 solutions (UFE 11 and 12)

Finally,  two  parallel  interaction  tests  between  U(VI)  and  Fe(II)  were  performed  in0.002 M NaHCO3 solution (Table 1, p. 7). The duration of the tests was relatively short(19  days).  The  first  one  (UFE 11)  was  conducted  in  a  closed  reaction  vessel  withcontinuous pH and redox measurements (p. 10). The vessel was not opened during thetest period. Samples for U and Fe analyses were taken only at the end of the test. In theparallel test (UFE 12), samples were taken periodically for analyses. The purpose was tosee if the frequent exposure of the test solution to the nitrogen atmosphere of the glovebox  by  opening  the  experimental  vessel  has  effect on  the  results.  The  N2 atmospherecontains trace amounts of oxygen (< 1 ppm). No effect was observed on pH results.The measured U concentrations after the addition of Fe(II) to solution in both tests aregiven in Figure 13. Both microfiltered and ultrafiltered samples were taken from the testsolutions.  In  the  UFE 12  test  with  frequent  sampling,  the  U  concentrations  inmicrofiltered  samples  were  clearly  lower  than  in  unfiltered  samples  after  5 days’reaction period,  suggesting  the  formation of precipitates  in  the  test  solution. The U  inmicrofiltered samples was also decreasing more  rapidly. This  is  in agreement with theresults  of  the  UFE 4­6  tests  in  0.002 M  NaHCO3.  The  U concentration  in  theultrafiltered  sample  at  the  end  of  the  test  was  lower  than  in  the  microfiltered  sample(Figure 13). This refers to the presence of particles with different size, e.g. colloids  inthe solution.

Time (d)

0 2 4 6 8 10 12 14 16 18 20

U (

mo

l/l)

10­10

10­9

10­8

10­7

UFE12 (unfiltered)

UFE12 (microfiltered)

UFE12 (ultrafiltered)

UFE11 (unfiltered)

UFE11 (microfiltered)

UFE11 (ultrafiltered)

Figure  13. The  evolution  of  uranium  concentrations  in  0.002 M  NaHCO3  after  theaddition  of  Fe(II)  to  solution.  The  initial  Fe(II)  and  U  concentrations  were  1.8 · 10­6

and  8.4 · 10­8  M,  respectively.  UFE 11:  Samplings  at  the  end  of  the  test.  UFE 12:Samplings periodically.

23

The  final  U  concentrations  measured  for  the  samples  at  the  end  of  the  UFE 11  testdiffered from those of the previous test (UFE 12). The U in the unfiltered sample was atthe same level, while the U concentrations in the filtered samples were higher. Filteringhad only a small effect on the result, in contrast to the U results of UFE 12. The UFE 11test  vessel  was  kept  closed  throughout  the  experiment.  This  test  vessel  also  had  theintegrated  electrodes  immersed  in  the  test  solution  for  continuous  pH  and  Ehmeasurements.  The  reason  for  the  difference  between  the  tests  is  not  known.  On  theother hand, the duration of these tests was short (< 20 days). Tests with longer durationare needed in order to be able to interpret the results.

The results of the analyses for Fe(II) and total Fe contents are given in Figure 14. Theconcentration  of  the  total  Fe  was  almost  constant  during  the  test  period  in  both  tests.The  concentration  of  Fe(II)  decreased  very  slowly  during  the  first  15  days  and  moreclearly  afterwards.  The  difference  between  the  Fe(II)  concentrations  in  test  vesselsUFE 11  and 12  was  quite  great  in  the  end  of  the  experiment.  The  reason  for  thedifference  is unknown. Tests with  longer duration should  be performed  in order  to beable to interpret the results more properly.

Time (d)

2 4 6 8 10 12 14 16 18 20

Fe

 (m

ol/l)

10­8

10­7

10­6

10­5

UFE12 Fe(total)

UFE12 Fe(II)

UFE11 Fe(total)

UFE11 Fe(II)

Figure  14. The  evolution  of  Fe(II)  and  total  Fe  concentrations  in  the  UFE 11  andUFE 12 tests. Test conditions: see above (Figure 13).

24

3.3 The effects of ultrafiltration

Selected samples from the test solutions were ultrafiltrated and the filtrates were

analysed for uranium. The concentrations of uranium in these filtrates were compared

with the concentrations of uranium in microfiltrated samples to see if some uranium

containing colloids had been formed in the test solutions, see Table 5. The filtrations

were performed under anaerobic conditions in the glove box.

The results of the uranium analyses showed a great difference in the amounts of

uranium for some samples, which were filtered with different pore size. The U

concentrations were 1-2 orders of magnitude higher in the microfiltrated samples, which

had been taken from the UFE 4-6 tests (see Figure 15)

After the ultrafiltration procedure, a lustrous colourless gel-like material was observed

on the filter surface. The amount of this gel seemed to be greater on the filters, which

were used for the samples from the test solutions UFE 5, UFE 6 and UFE 8.

The results gained for the test solutions UFE 1 and UFE 2 differed from the other

results. Any significant difference was not observed in uranium concentrations between

the ultrafiltered and microfiltered samples. The reaction time in these tests was longer

(Table 5).

Table 5. Uranium concentrations in ultra- and microfiltrated samples from 0.002 M

NaHCO3 (UFE 1-6) and 0.01 M NaCl (UFE 7-8) test solutions.

Test Time

(days) Ultrafiltered (12 K MWCO)

U(mol/l)

Time

(days) Microfiltered (0.22 m)

(U/mol)

UFE1 70 2.22·10-7

71 2.22·10-7

UFE2 70 1.62·10-7

71 1.78·10-7

UFE4 55 2.46·10-9

56 3.82·10-8

UFE5 55 4.84·10-10

56 6.72·10-8

UFE6 55 3.40·10-10

56 4.92·10-9

UFE7 55 8.97·10-10

56 8.40·10-11

UFE8 55 1.05·10-9

56 2.60·10-9

25

UFE4 UFE5 UFE6 UFE8

U (

mo

l/l)

10-10

10-9

10-8

10-7

Ultrafiltrated

Microfiltrated

Figure 15. Comparison of the uranium concentrations in ultra- and microfiltrated

samples. The samples were taken from 0.002 M NaHCO3 (UFE 4-6) and 0.01 M NaCl

(UFE 8) test solutions 55 days after the start of the tests.

3.4 Uranium oxidation states

The oxidation state of uranium was preliminarily analysed for samples, which were

taken from the test solution of the UFE 12 test. The aqueous phase was 0.002 M

NaHCO3. The method was based on the separation of the U(IV) and U(VI) states by

anion-exchange chromatography (see p.14-15). The duplicate samples were taken from

the UFE 12 test solution 12 and 16 days after the start of the experiment. The sample

volume was 0.5 ml. The uranium contents of eluted fractions were analysed by ICP-MS.

The concentration of uranium in the U(IV) fractions was under the detection limit of

ICP-MS (0.02 μg/l). The yield in the separations of U(IV) from 4.5 M HCl solution,

which were performed after the U(IV)/U(VI) separations, was around 50 % (for the

method, see p. 14-15). U-233 was used as a tracer. On the other hand, U(IV) has a high

tendency to precipitate due to its low solubility. The measured U(VI) contents were at

the same level with the measured U contents in microfiltered samples. This suggests

that all the uranium in the solution had the oxidation state of U(VI).

Table 6. Measured U in unfiltrated and microfiltrated solution samples, and in the

separated U(IV) and U(VI) fractions.

Time U unfiltered

(mol/l)

U microfiltered

(mol/l)

U(IV) (mol/l)

U(VI) (mol/l)

12 days 3.7·10-8

6.8·10-9

< 8.4·10-11

3.3·10-9

“ “ “ < 8.4·10-11

4.8·10-9

16 days 1.1·10-8

2.1·10-9

< 8.4·10-11

2.5·10-9

“ “ “ < 8.4·10-11

4.5·10-9

26

3.5  Stability of Fe(II) in test conditions

The stability of the Fe(II) oxidation state  in test solutions was studied  in parallel withthe  interaction  tests.  100  ml  of  0.002  M  NaHCO3  or  0.01 M  NaCl  containing  Fe(II)were  added  to  polyethylene  vessels  (125  ml).  The  initial  concentration  of  Fe(II)  was2.3 · 10­6 M. The vessels were closed and  left  in  the glove box. The samples  for Fe(II)and total Fe analyses were taken at certain intervals, see Figure 16.Some  decrease  was  observed  in  the  concentration  of  Fe(II)  in  0.002  M  NaHCO3solution.  The  concentration  decreased  30 %  within  the  test  period  (60  days).  Thissuggests  the oxidation to Fe(III) to occur. The concentration of  total  iron stayed at theinitial  level.  In  the  0.01  M  NaCl  solution,  the  concentrations  of  Fe(II)  and  total  Feseemed to remain stable. The observed changes in the concentrations can be consideredas variation of the analysis results.The results suggest that the ferrous state of iron is more stable in 0.01 M NaCl solutionthan in 0.002 M NaHCO3 solution under anaerobic conditions used in our experiments.The  ferrous  iron  or  its  complexes  seemed  to  oxidize  partly  in  0.002 M  NaHCO3solution.  Precipitates  did  not  seem  to  form  because  the  concentration  of  total  ironremained at the same level during the test period.

 UFE 9 (NaHCO3) and UFE 10 (NaCl)

Time/d

0 10 20 30 40 50 60 70

Fe

 (m

ol/l)

0.0

5.0e­7

1.0e­6

1.5e­6

2.0e­6

2.5e­6

Fe(tot) in NaHCO3

Fe(II) in NaHCO3

Fe(tot) in NaCl

Fe(II) in NaCl

Figure 16. The stability of Fe(II)  in 0.002 M NaHCO3 and  in 0.01 M NaCl solutionsunder test conditions.

27

Generally, Fe(II) forms weak complexes and ion pairs and thus it occurs as free ion in

most natural waters. However, Fe(II) ions might form carbonate and bicarbonate

complexes in carbonate solution. In chloride solutions, the formation of chloride

complexes can not be totally excluded. (Vuorinen et al. 1998, Langmuir 1997)

The calculations were performed to provide a better evaluation of the prevailing form of

ferrous iron in the test solutions. The speciation of Fe(II) in the test solutions under the

experimental conditions (Figure 16) were modelled with a geochemical solution-

mineral equilibria code EQ3. The results are given in Table 7. The results indicate that

Fe(II) forms carbonate and bicarbonate complexes in 0.002 M NaHCO3 solution, but it

partially stays in the form of free Fe(II) ions in the solution. In the 0.01 M NaCl

solution, Fe(II) is mainly present as uncomplexed ion.

Table 7. The speciation of Fe(II) (2.4·10-6

M) in test solutions under the N2 atmosphere.

0.002 M NaHCO3 pH = 9.0, Eh = -300 mV

0.01M NaCl pH = 7.5, Eh = -300 mV

Main species Percentage (%) Main species Percentage (%)

FeCO3(aq) 63.82 Fe2+

98.83

Fe2+

17.41 FeOH+ 0.722

FeHCO3+ 14 FeCl

+ 0.451

FeOH+ 4.71

Fe(OH)2(aq) 0.0355

28

4 SUMMARY AND PRELIMINARY CONCLUSIONS

The objective of these interaction experiments between Fe(II) and U(VI) was to see, if

these preliminary tests showed any signs of the reduction of U(VI) by aqueous Fe(II).

U(VI) was added to deaerated 0.01 M NaCl and 0.002 M NaHCO3 solutions to compare

the behaviour of uranyl hydroxyl and uranyl carbonate complexes, respectively. The

tests were conducted under N2 atmosphere in the glove box. Another aim was to study

the analytical and experimental methods inside the glove box, e.g if the redox

conditions are sufficiently low in oxygen to maintain iron at the Fe(II) state.

In the tests, Fe(II) ions were added to the solution, which contained uranyl ions either as

carbonate or hydroxyl complexes. After the addition of Fe(II), the concentration of

uranium was measured as a function of time. The aim was to investigate the possible

changes in the solubility and speciation of uranium after the addition of Fe(II). The

development of Fe(II) and total iron concentrations were also examined. If uranium(VI)

is reduced to U(IV), the concentration of Fe(II) should also decrease due to the

oxidation reaction to Fe(III).

After the preliminary tests, the initial concentration of U(VI) was varied. The initial

concentration of Fe(II) was kept constant. In the additional test in 0.002 M NaHCO3, a

continuous Eh and pH measuring with integrated electrodes in a closed reaction vessel

was conducted to see if any changes caused by the reduction reaction can be seen in

these conditions. A parallel experiment with frequent sampling was performed. The

other target was to find out if the traces of oxygen potentially introduced during the

sampling affected the experimental conditions.

Minor changes were observed in measured pH values during the reaction period in all

tests. The measured pH in both aqueous phases with uranium and iron was around 9.

The pH remained almost stable showing slight increase after the addition of Fe(II) tot

the solution. The results of the continuous Eh measurements in a closed reaction vessel

with Pt and Au electrodes were in agreement. First, the Eh decreased below -300 mV

after addition of Fe(II) to the solution. During the reaction period with uranium (19 d),

there was a slow increasing trend in the Eh until the end of the experimental time.

In all tests the concentrations of U and Fe(II) in the aqueous phase decreased after the

addition of Fe(II) to solution. This can be a consequence of a redox reaction between

uranium and iron leading to the precipitation of the sparingly soluble U(IV). The

decrease in the concentration of Fe(II) in solution seemed to be inversely proportional to

the initial uranium concentration. The results suggested faster reactions stronger

reactions in the absence of carbonate in the NaCl solution.

It is possible that the Fe(II) was partly oxidized by trace oxygen in the N2 atmosphere of

the glove box. and formed Fe(III) precipitates, which tend to sorb U(VI) species. The

stability tests of Fe(II) showed that the trace oxygen may have an influence. Some

decrease in the Fe(II) content of the solution was observed to occur in the test in

0.002 M NaHCO3 solution. The concentration of Fe(II) decreased by 30 % within the

period of two months, probably due to the oxidation of Fe(II) to Fe(III). In a parallel

tests in 0.01 M solution, Fe(II) seemed to remain stable.

The samplings for U without filtration and with micro- or ultrafiltration revealed the

heterogeneous nature of the test solutions, although any visible precipitates could not be

29

seen. The results of ultrafiltration showed the presence of colloids in solutions. The

formation of colloids could be a preliminary phase before precipitation.

30

5 ACKNOWLEDGEMENTS

These experiments were done at VTT (Technical Research Center of Finland). We

thank Piia Juhola (Posiva Oy), Margit Snellman (Saanio & Riekkola), and Virginia

Oversby (VMO Konsult) for their cooperation during this process. We thank also Arto

Muurinen (VTT) for help and advices with the experimental set-up in the continuous pH

and Eh measurements and Maija Lipponen (VTT), Jaana Rantanen (VTT) and Riitta

Zilliacus (VTT) for the ICP-MS analyses of uranium.

Posiva Oy and VTT are gratefully acknowledged for their financial support.

31

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33

APPENDICES

APPENDIX 1

Table A1-1. Uranium concentrations in unfiltered and filtered UFE 1-3 samples.

Unfiltered samples (mol/l) Filtered samples (mol/l)

Time(d) UFE1 UFE2 UFE3 UFE1 UFE2 UFE3

1 4.3·10-7

4.4·10-7

4.2·10-7

2 4.·10-7

4.5·10-7

3.8·10-7

3 4.7·10-7

3.8·10-7

3.3·10-7

7 3.2·10-7

1.6·10-7

1.4·10-8

9 2.8·10-7

1.6·10-7

4.8·10-9

1.3·10-7

3.2·10-7

4.6·10-9

11 2.4·10-7

1.4·10-7

< 8.4·10-11

1.2·10-7

8.4·10-8

3.3·10-10

23 1.9·10-7

1.7·10-7

” 1.3·10-7

9.2·10-8

< 8.4·10-11

71 2.5·10-7

2.0·10-7

” 2.2·10-7

1.8·10-7

”

Table A1-2. Iron concentrations in UFE 1-3 samples.

Fe(total) (mol/l) Fe(II) (mol/l)

Time(d) UFE1 UFE2 UFE3 UFE1 UFE2 UFE3

7 1.6·10-6

5.6·10-7

7.6·10-5

7.7·10-7

3.8·10-7

6.7·10-5

22 5.4·10-7

5.2·10-7

2.7·10-7

3.4·10-7

72 9.0·10-8

2.0·10-7

9.0·10-8

9.0·10-8

34

APPENDIX 2

The 34 day samples gave somewhat higher results. This may be due to the use of a filter

membrane with a larger pore size (0.45 m) instead of the normal filter in this sampling.

A filter with a pore size of 0.22 µm was used in all other samplings.

35

APPENDIX 3

UFE 2

Time/d

0 20 40 60 80

U (

mo

l/l)

0.0

1.0e­7

2.0e­7

3.0e­7

4.0e­7

5.0e­7

Fe (

mo

l/l)

0.0

5.0e­7

1.0e­6

1.5e­6

2.0e­6

Uunfiltered

U filtered

Fe2+totFe

Time/d

0 20 40 60 80

U (

mo

l/l)

0.0

1.0e­7

2.0e­7

3.0e­7

4.0e­7

5.0e­7

Fe (

mo

l/l)

0.0

5.0e­7

1.0e­6

1.5e­6

2.0e­6UFE 1

Uunfiltered

U filtered

totFeFe2+

UFE 3

Time / d

0 20 40 60 80

U / (

mo

l/l)

10­11

10­10

10­9

10­8

10­7

10­6

U (unfiltered)

U (microfiltered)

Detection limit for U