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P O S I V A O Y
Olk i luo to
F I -27160 EURAJOKI , F INLAND
Te l +358-2-8372 31
Fax +358-2-8372 3709
Emmi My l l yky lä
Ka i j a O l l i l a
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
Recommended