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desorption of radioactive contaminants by sediment from the Kara Sea
the !Science of the ntd Izlwblment An-JmurE-*bwtbeE9-~9smuiNmp.la* ELSEVIER
The Scienceof the Total Environment
202 (1997)
5-24
Sorption/desorption of radioactive contaminants by sediment from the Kara Sea Mark Fuhrmanna,* , Huan Zhou”, James Neiheiselb, Martin A.A. Schoonenc, Robert Dyer b ‘Environmental and Waste Technology Center, Building 830, Brookhaven National Laboratory, Upton, New York, NY11973-5000, USA bUS Environmental Protection Agency, Washington D.C., USA ‘DepartMent of Earth and Space Sciences, State University of New York at Stony Brook Stony Brook, New York, USA
Abstract
To understandthe long term impact of the disposalof radioactive wasteon the Kara Sea, partition coefficients ( Kd) for severalimportant radionuclides,the mineralogyof the sediment,and the relationshipof Kd to liquid-to-solid ratio were quantified. Sedimentwasobtained from four locationsin the Kara Seaarea. Slow sorption kinetics were observedfor 85Sr,232U,lzsI and “Tc, whilst sorptionwasrapid (lessthan 50 h to steady-state)for 137Cs, **‘Pb, and %lAm. Partition coefficients(K,) were determinedusingbatch type experimentsand sorption isothermswhich were Partition coefficientsfor 137Cs were approx. 350ml/g for sedimentfrom developedfor *‘Sr, WTc, lzI, U and 137Cs. the Trough and 180 ml/g for StepovogoFjord. This difference may be causedby the lower fraction of expandable clay in sedimentfrom the fjord. Uptake of *?Sr,“Tc, lzI and U were all similarfor both locations,with K, values averaging4, 3, 17 and 60 ml/g, respectively. The K, fdr 137Cs varied non-linearly from 40 to 3800 ml/g as the liquid-to-solid ratio varied from 3.4 to 6500,but only when the sorption capacitywas high comparedto the massof 13’Csin the closedsystemof the experiment. Under identical conditions,sedimentwith lower K, valuesshowedno effect. Oxidation of sedimenteffectively desorbed“Tc from the solidphase,whilst it causedincreaseduptake of 85Sr and U. In sequentialrinseswith fresh seawater,desorptionwas limited to 60% of 137Csand “Sr, and 35% of uranium. 0 1997Elsevier ScienceB.V. Keywords:
Kara Sea;Sediment;Radionuclides;Sorption; Desorption; Partition coefficient (K,)
1. Introduction Large quantities of radioactive materials have been disposedof either directly into the Kara Sea
*Corresponding author.Tel.: + 1 5163442224; fax: + 1516 3444486.
or in terrestrial areas that are drained by rivers, such as the Ob and Yenesei, that flow to the Kara Sea. The material has been introduced into the sea in many forms, including solutions that were pumped into the sea and as solids and liquids in dumped containers. A large fraction of the inventory is associatedwith 16 reactor pressure vessels, six of which contain nuclear fuel, which will re-
004%9697/97/$17.00 0 1997ElsevierScienceB.V. All rightsreserved. PZZ SOO48-9697(97)00101-O
M. Fuhrmann et al. / The Science of the Total Environment 202 (1997) 5-24
6
lease contaminants into solution over long timeperiods. The inventory of radionuclides from these fuel loads (as well as the activation products contained in the vessels), in 1993, has been estimated as 0.2-0.9 PBq of actinides, 18.2-20.0 PBq of fission products, and 4.6 PBq of activation products (Mount et al., 1993). The fission products present in greatest quantities are 137Cs and 9oSr, with approx. 4.4 PBq each. Two fission products, 99Tc and 1291, are present in small quantities and have relatively little impact on human health when present in low concentrations. However, their long half-lives will make them the dominant fission products after the others have decayed away. The purpose of this study is to quantify the sorption and desorption of radionuclides on sediment from the Kara Sea, to determine the longterm fate of these contaminants in the arctic. The parameters of interest are: the partition coefficients (K,) for several important radionuclides for sediment taken from the Kara Sea; the mineralogy of the sediment; and the relationship of K, to liquid-to-solid ratio. Values of K, were determined for the dominant radionuclides present in the waste, with the longest half-lives, and/or of articular environmental concern. These are 90Sr, f& 1291 137~~ natU and 241Am. The gannna3 emitters “Sr and “‘1 were substituted for 90Sr and 1291(both beta-emitters), respectively. Partition coefficients, describing the distribution of contaminants between water and sediment, indicate their relative mobility and are key parameters used to model the transport and distribution of radionuclides in environmental systems containing fluids and particulates. The critical parameter, K,, used to quantify this process is defined as the concentration of the species of interest on the solid divided by its concentration in the liquid, at steady-state. Operationally it is: K = (1 -Fgv d
F,M
for experiments in which the concentration on the solid is determined by difference from a reference solution. FL is the fractional difference in concentration in the liquid compared to the starting concentration prior to sorption, v is the
volume of liquid in the experiment and M is the mass of solid in the experiment. The partition coefficient is affected by factors such as grain size of the solid, its mineralogy and organic matter content, solution chemistry, and speciation of the contaminant in solution. Because of these influences, to determine Kd values appropriate to the site under study, it is necessary to conduct experiments that come as close as reasonably possible to site-specific chemical conditions and materials. Partitioning can be a non-linear function of the concentration of the contaminant in solution. Typically this becomes apparent at higher masses of sorbed contaminant; whilst at lower fractional uptake the relationship between the concentration of the contaminant in solution and its concentration on the solid is assumed to be linear. Isotherms are used to determine if the K, varies with solution concentration, allowing an evaluation of the relationship between sorption at differing concentrations of contaminant. An isotherm can be generated from experimental data by plotting contaminant concentrations on the solid on the Y-axis and its concentration in solution on the X-axis. For simple linear systems the slope is Kd. If the relationship is linear at the relatively high tracer concentrations used in laboratory experiments, then it is appropriate to use that K, for the lower concentrations typically found in the environment. 2. Materials Sediment from four locations in the Kara Sea was sampled by the Joint Russian-Norwegian Expeditions of 1992 and 1993. Sampling stations in the Kara Sea area were: Station 2 (73” OO’N, 58” OO’E in 287 ml, Station 3 (74” 30’N, 62” OO’E at a depth of 334 ml, Station 8 (72” 40’N, 58” 10’E at a depth of 360 m) and Station 6 in the Stepovogo Fjord (72” 33.04’N, 55” 22.15’E at a depth of 45 ml. The locations are shown on Fig. 1. Sediment from all stations consisted of green/black mud. Samples from Stations 2, 3, and 6 were subcores of box cores, while sediment from Station 8 was from a surface grab. The subcores were frozen intact whilst sediment from
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M. Fuhrmann et al. / The Science of the Total Environment 202 (1997) 5-24
Station 8 was freeze-dried. They were kept frozen until they were sectioned in an argon atmosphere to preserve their anoxic condition, which is necessary to maintain the presence of authigenic minerals containing reduced metals (such as iron sulphides) that may be important controls on the sorption/precipitation of some elements. The seawater used for experiments was ob-
-
__---,
---
---
50 ,,
56 \!
i---e-I 1 \ I , I , I , I , I I / I \ I
tained from the Atlantic Ocean, offshore of Long Island, New York. It was filtered before use and stored in a refrigerator. Its salinity was 32.8%0. Tracers were prepared from commercially obtained radionuclide solutions. Serial dilutions were initially done with distilled/deionized water. The pH was adjusted to close to neutral and the final dilution made with seawater. This tracer was
62 \\
i
66 (\
76
\ 1
-72
---
_-
56 Fig. 1. The Kara Sea near Novaya Zemlya, showing the location from which the sediment sample was taken.
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then added to the seawater used for the experiments, resulting in only a slight reduction of salinity in the experiments. All experiments were conducted with single tracers. The 137Cs, 210Pb, 125I and 99Tc tracers were carrier free, whilst the s5Sr and %iArn tracers had specific activities of 457 MBq/mg of strontium and 102 MBq/mg of americium, respectively. Uranium experiments were conducted with either 232U tracer or natural uranium diluted from a standard obtained from the National Institute of Standards and Technology. A dilution of the acidic standard was prepared in seawater and the pH adjusted with NaOH, causing a precipitate to appear. After standing overnight the precipitate redissolved. This solution was brought to 100 ml and filtered through a O.l-pm syringe filter. The solution was analyzed by Inductively Coupled Plasma Emission Spectroscopy and had a uranium concentration of 82.6 mg/l. Subsequent analysis of this tracer showed that it was a stable solution. 3. Methods 3.1. Sorption aperiments The general method used in our sorption experiments is given here. Volumes and masses of sediment are given in the sections describing specific experiments. Sorption experiments were conducted under argon to preserve redox conditions and any sulphide minerals. For each experiment, wet sediment was weighed into a polyethylene centrifuge tube, in a glove box. Separate subsamples were also taken for water content determinations. The contact solution was filtered seawater to which tracer was added. The experiments were sampled by shaking the specimen and removing approx. 5 ml of slurry with a plastic syringe. The slurry was then filtered with a 0.45pm syringe filter and the effluent was weighed into counting vials. Analysis was performed by liquid scintillation counting for %Tc and by gamma spectroscopy with an intrinsic germanium gamma detector for the other radionuclides. Count times were adjusted to provide all samples with similar counting errors of approx. 7%. Concentration on the solid was determined by
202 (1997) 5-24
difference from a reference, consisting of seawater and tracer, that was made and handled in a manner identical to the samples (including fIltration), but contained no sediment. These values were then used to determine the concentration in the beginning solution from which the quantity of radionuclide sorbed was calculated by subtracting the concentration in the sample liquid from that of the reference. In this way, any sorption on containers and filters was automatically corrected. Count rates of the samples were corrected relative to the reference count rate to allow for dilution caused by the porewater in the wet sediment. All results were normalized to the mass of the counting sample as well as the dry weight of sediment and the total liquid in each experiment. 3.2. Sorption kinetics Uptake of tracers on the sediment was determined from single batch experiments which were sampled periodically. For each experiment, approx. 16 g of wet sediment were contacted with 40.0 g of seawater and between 1 and 3 ml of tracer. Samples were taken periodically by shaking the specimen and removing approx. 2.5 ml of slurry with a plastic syringe and filtering the slurry. Samplings started at 0.04 days and, depending on the results, in some cases extended to 48 days. Uptake was determined as a fraction of concentrations present in the reference solutions. 3.3. Sorption kinetics of sterilized sediment The focus of this work is on inorganic processes. Nevertheless, active metabolic uptake by microbiota may have significant influence on the sorption of iodine. To determine the extent of biological control of iodine sorption in natural materials, sorption kinetics of iodine was examined for two samples of sediment, comparing sorption rates for samples that had been sterilized by gamma-irradiation to those that were not irradiated. Sterilizing the sediment eliminates metabolic uptake, allowing other sorption processes to be observed. In each experiment, 2 g of wet sediment, from Station 6, were weighed into plastic centrifuge
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tubes. One of each pair of samples was irradiated with @‘Co three times, several days apart, with each irradiation giving an exposure of approx. lo6 R. Each sediment sample (irradiated or unirradiated) and 40 g of sterilized (by boiling) seawater containing radioactive tracer were mixed together and the bottles were placed on a shaker table. Periodically, approximately 2.5 g of slurry were removed with plastic syringes and filtered. Analysis was conducted as described above. 3.4. Sorption experiments for K,
In this study, K, values were determined for 85~~ 99Tc 1251 > 232U (or natural uranium) and ‘2b1Am. Measurements for 210Pb were not made since the kinetics experiment showed complete removal of lead. Each test was conducted as a single experiment consisting of sediment/ seawater/tracer in a plastic bottle. Tests run at single tracer concentrations (generally in replicate) are referred to as ‘batch’ tests. Tests with varying concentrations of tracer (typically at least five different concentrations) are referred to as isotherms experiments. In either case, approx. 1.0 g of wet sediment was weighed into pre-weighed plastic bottles. For batch tests, 25 g of seawater were added and 0.5 ml of radioactive tracer solution. For isotherms, between 10 and 11.8 g of seawater were then weighed in; the quantity of seawater varying depending on how much tracer was added, the sum of the two being approx. 12.0 g. The kinetics data obtained earlier were used to determine the minimum time required for the batch tests. Determination of the partition coefficient is influenced by uncertainties in the analysis. Ideally, the activity on the solid should be similar to that in the liquid. In most cases, in this set of experiments, activities in both phases were acceptable. However, in the case of 241Am, the K, was sufficiently high that little %lArn remains in solution. Due to the way in which K, is calculated, when the count rate in the liquid is low compared to the rate on the solid, small changes of activity in the liquid can result in large changes in K,. Consequently, some of the results reported for 241Am are minimum values of K,, because the 137~~
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activities of 241Am in the liquid phase are below the minimum activity that was detectable. 3.5. Desorption
experiments
Sediment which previously had been used for uptake experiments (for uranium, 137Cs, “Sr and WTc> under reducing conditions was exposed to oxidizing conditions to determine effects of oxidation on sorption/desorption. Four millilitres of seawater/sediment suspension were taken from earlier sorption experiments (Station 2) that had been stored under Ar for 5.5 months. One ml of 30% hydrogen peroxide was added to each sample and allowed to react for 3 days. They were then opened to the air and allowed to slowly evaporate back to their original weight (before addition of H,O,). Each experiment was sampled after the peroxide treatment and the count rates of the tracers in the liquid phase were compared to previous samplings taken during anoxic conditions. In another set of experiments, previously contaminated sediment was exposed to clean seawater. At intervals of 0.2, 1, 3, 12, and 28 days, the slurry was centrifuged and decanted. New seawater was then added. The decanted water was filtered and analyzed for desorbed tracer. This is analogous to contaminated sediment being exposed to clean seawater (seawater containing no contaminant) by resuspension. 3.6. Liquid-to-solid
ratio experiments
Experiments were conducted to determine if a large difference in liquid-to-solid ratio can significantly alter the uptake of a set of contaminants. These experiments were batch sorption tests, similar to those described above with six and eight sorption tests conducted for 137Cs and 99Tc respectively. The ratio of liquid to solid was varied from 4 to 6400 for the 99Tc experiment and from 3.4 to 6500 for 137Cs. The concentration of the tracer was initially the same in each experiment. The experiment bottles were kept under Ar, on a shaker table before sampling. The experiments for 137Cs and 99Tc ran for 17 days and 54 days, respectively.
IO
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3.7. Sediment parameters
Mineralogical analysis was done by optical petrographic methods and by X-ray diffraction. Grain size was determined by wet sieving through a 63-pm screen and then by pipette analysis (Folk, 2974). Moisture content was determined by difference in weights after drying at 50°C under vacuum for 5 days. Surface-area analysis was done by the BET method (Brunauer et al., 1939) using nitrogen on air-dried samples. Sediment from Station 6 was not analyzed for surface area because of its elevated radionuclide concentrations. 4. Results 4.1. Mineralogy and other sediment parameters
Mineralogy, grain size and other sediment parameters are shown in Table 1. Total organic carbon (TOC) at Station 2 is 93 mg/g in the O-6 cm fraction and 80 mg/g in the 26-28 cm fraction. This value is high, even for a coastal sediment. In a recent worldwide assessment of the relationship of total organic carbon to surface area in sediments, only sediment from the Peru slope had higher values (Meyer, 1994). Mineralogy is dominated by quartz and feldspar in the sand and coarse silt fractions and by illite/mica, chlorite and kaolinite in the fine silt, The clay fraction is comprised primarily of mixed layer
202 (1997) 5-24
smectite, illite, chlorite and kaolinite. Mineralogy is similar for all stations, with the exception of Station 6 which has only traces of smectite. Black nodules and black coatings on sand-sized particles occur in approx. half of the sand fraction of Station 6. These are probably the result of manganese forming small nodules and coatings. 4.2. Sorption kinetics
In the first set of experiments we determined the times required for the different radionuclides to reach steady-state concentrations in the water-sediment system. Results of the kinetics experiments are given in Table 2. Sorption kinetics for 137Cs, s5Sr, uranium, and 99T~, on sediment from Station 2, are shown in Fig. 2. Most of the lead was removed rapidly; 98.6% was taken up in 1 h, and it was below detection limits (0.1 counts per min, cpm, or approx. 100 rig/l) in the contact solution in approx. 50 h. Similarly, sorption of 24*Am was rapid, with steady-state being attained within 1 h. Uranium had much slower kinetics, requiring approx. 50 h for 50% removal from solution and steady-state was achieved after approx. 350 h, probably as a result of a redox reaction converting the uranium tricarbonate, which is the predominant species in seawater, to a reduced form with low solubility (Barnes and Cochran, 1993). Sediment from Station 6 sorbed iodine slowly, reaching steady-state after approx.
Table 1 Sediment parameters, Kara Sea
---- .-..--. Parameter
Station 2
Station 3
Station 6
Station 8
Location Depth (m) Sand % Silt % Clay % Organics Surface Area m’/g Quartz % Chlorite % Mite/mica % Kaolinite % Feldspar % Smectite %
73”OO’N 58”OO’E 287 15.5 60.9 23.6 TOC = 93 mg/g 42 30 13 21 10 11 9
74”30’N 62”OO’E 334 2.5 44.2 53.8 LOI = 14.9% 27.7 2.5 14 26 8
72”33.04’N W22.15’E 45 3.5 67.0 29.5 Lo1 = 7.1%
72”40N 58”lO’E 360 7.4 60.5 32.1 LO1 = 10.9%
23
32 18 30 5 10 Trace
31 10 20 13 20
11
M. Fuhnnann et al. / The Science of the Total Envirvnment 202 (1997) 5-24 Table 2 Sorption rates and K,, (ml/g) values for Kara Sea sediment determined by batch tests (Bl and by isotherms (Iso) Element Caesium Strontium Technetium Iodine Uranium Americium
Sorption kinetic2 50 150 200 600b > 350
Station 2
Station 3
Station 6
360 Is0 3 Is0 41x1 21 Iso 56 B 23 B -
330 Is0 3 Is0 1 Is0 18 Is0
180 Iso 5 Iso 6 Iso 9 Is0
110 Is0 130000
31 Iso >280000
Station 8 1260 B 22 B 1B 19 B 73 B -
IAEA, coastal 3000 1000 100 20 mm 2000000
aHours to steady-state on sediment from Station 2. bSediment from Station 6.
600 h. However, iodine was not retained on the sediment from Station 2, after what appeared to be a brief period of sorption and then desorption. This lack of uptake may be the result of high
* Cs-137
150
concentrations of iodine found in reducing sediments (Price and Calvert, 1973) causing a flux of iodine out of the sediment. Technetium exhibited slow sorption kinetics that became steady-state
f Sr-85 * U-232 * Tc-99
200
250
Time (hours) Fig. 2. Sorption kinetics for caesium, strontium, technetium, and uranium are shown for sediment from Station 2. All data have been normalized to reference tracer solutions.
M. Fuhrmann et al. /The Science qf the Total Environment 202 (1997) 5-24
I?
after 170 h on sediment from Station 2. Caesium concentrations in the contact solution became steady-state by 50 h, whilst strontium became steady-state after approximately 150 h. 4.3. Sorption kinetics of sterilized sediment
Results of the irradiation experiment to examine the impact of metabolic processes on iodine uptake are shown in Fig. 3. The sediment that was irradiated (and presumably sterilized) showed no sorption. The scatter in the data is the result of a 10% experimental error but the regression line shows that 100% of the iodine remained in solution. Sediment that was not irradiated sorbed iodine over a period of 20-35 days, with uptake following the square root of time. Uptake stopped by day 38. 4.4. Partition coeficients
Table 2 gives results of the batch tests (labeled
..5
B) and the isotherms (labeled Iso) for each of the four stations. The observed values are compared to those recommended by the IAEA for coastal sediment (International Atomic Energy Agency). The K, values determined for the Kara Sea sediment are significantly lower than the IAEA values in all cases, with the exception of iodine. 4.5. Sorption results
Isotherms for uptake of 137Cson sediment from several stations in the Kara Sea are shown in Fig. 4. Uptake of 137Cs is proportional to its concentration in solution; consequently the isotherms are linear with a positive slope. The slope, therefore, is the K,. Stations 2 and 3, located in the Trough, have similar K, values, approx. 350 ml/g, as determined by the slope of the isotherm. Station 6, in Stepovogo Fjord, had a K, that was half of that at Stations 2 and 3, 180 ml/g. The value for Station 8 was 1260 ml/g, which is significantly higher than the other Trough stations and possi-
60--
E
0
1
2
3
4
5
6
7
Square Root of Time (Days) Fig. 3. Sorption kinetics of “‘1 on irradiated and unirradiated (natural) sediment from Station 6.
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bly a result of freeze-drying and oxidation of reduced iron, generating iron oxyhydroxide that has a relatively large capacity to sorb these radionuclides (Cerling and Turner, 1982). To compare results from the triplicate set of batch tests these points have been plotted on the isotherm. Noting that the liquid-to-sediment (solid) ratio is somewhat different (5O:l vs. 25:l) the values are comparable (effects of liquid-to-sediment ratio are discussed below). Strontium isotherms for Stations 2, 3 and 6, shown in Fig. 5, are all similarin slope and are linear. Values for K, from these experiments average 4 ml/g. Sediment from Station 8 had a higher value, averaging 22 ml/g, again probably an artifact of freeze-drying. These results are low compared to the IAEA recommended value of 1000 ml/g (International Atomic Energy Agency). Isotherms for 125I are shown in Fig. 6 for Stations 6 and 2 along with batch results (replicates at one concentration) for Stations 2, 3 and
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8. Although they are both linear, the two isotherms have different slopes. For Station 2, the K, is approx. 20 ml/g, whilst for Station 6 the Kd is approx. 10. Batch results for Station 2 average 56 ml/g and for Station 8 the average is 20 ml/g. Sorption isotherms for 99Tc are illustrated in Fig. 7 for Stations 2,3, and 6, along with duplicate batch data for sediment from Station 8. The ZQ are low, with values averaging 4 ml/g. The IAEA recommended coastal value is 100 ml/g. Uptake of uranium is illustrated in Fig. 8 for Stations 3 and 6. The statistics for the linear regressions of the isotherms are poor but the results indicate relatively low Kd values, averaging for all samples (including batch tests) approx. 55 ml/g. 4.6. Desorption
A critically important aspect for this study is the level of oxygenation of the water in the Kara
t
A Station 2 x Station 2 batch Station 6 Slope = 181
+ Station 3
h
v Station 6 + Station 8
5 la7Cs Activity
10
15
in Liquid (cpm/mL)
Fig. 4. Isotherms are shown for 13’Cs on sediment from Stations 2, 3, and 6. Also shown are the points for the triplicate K, experiments for Station 2 and duplicate points for Station 8.
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Sea. The oxygen content of the water column is determined by the balance of the rate of oxygen advection and oxygen consumption. The advection of oxygen is complicated because of the variable ice coverage and the complex currents. Intuitively one may expect that the rate of gas transfer through ice is virtually nil, but in fact there is gas transfer through ice. Most of the transfer occurs, however, along brine channels in the sea ice (Anderson and Dryssen, 1989). A second requirement for oxygenation of the water column is vertical mixing, which occurs preferentially during the winter months when ice forms and brines sink to the deeper parts. The consumption of oxygen is limited due to the cold temperature and therefore, overall the waters are well oxygenated (Jones et al., 1990). Even the bottom waters have high 0, concentrations. The rate of new organic carbon production has
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been estimated at 45 gC/m2/year. As a result, the sediment, as it rests on the ocean bottom, is reducing. If the sediment has sorbed radionuelides and it is then resuspended, it will be exposed to an oxidizing environment. What is the fate of the previously sorbed contaminant if the particle stays in the water column sufficiently long that it oxidizes? Results of the peroxide oxidation experiment are shown in Table 3. With the exception of 99Tc there was no remobilization of radionuclides after oxidation had taken place. Moreover, *‘Sr and u2U were sorbed to a greater extent after addition of oxygen than by the reduced sediment. While the count rates of 137Cs and 12’1 in solution were unchanged, the count rate of 99Tc increased by 67%. Results of desorption experiments using sequential decanting and addition of fresh seawater, are shown in Fig. 9. Approximately 60% of previ-
800 m Station 2
/-
+ Station 2 batch
/
/
s z8 600
+ Station
6
A Station 3 + Station
/ /
8
Station 6 ,/’ Slope = 4.7 ,-‘,
20
40
60
85Sr Activity
80
jkation _. slope
100
.,
I
, , .* /
3
_ = 4.p
120
140
in Liquid (cpm/mL)
Fig. 5. Isotherms and batch experiment results for %r, showing that sediment horn all stations (except Station 8) had similar values.
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ously sorbed 85Sr was desorbed rapidly, with the remaining fraction staying fixed on the sediment. Desorption of “‘1 was less rapid but approx. 90% was released over 12 days. Similarly 137Cs was also desorbed with 46% released after 12 days and 50% after 27 days. The release rate for 137Cs slowed significantly after day 5, implying that only approx. 60% would be released. Approximately 35% of uranium was released, with desorption stopping after 2 days. 4.7. Sorption as a jimction of liquid-to-solid ratio
To effectively model the near-field impacts of radioactive waste disposal it is necessary to determine if the effects of a varying ratio of liquidto-solid can significantly influence sorption of the contaminants. The K, for 137Cs for sediment from Station 2 (Trough) varied non-linearly from
*Station
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40 to 3800 ml/g as the liquid-to-solid ratio varied from 3.4 to 6500. However, for sediment from Station 6 (Stepovogo Fjord), the K, was unchanged while the liquid-to-solid ratio ranged from 4 to 2250. This is shown in Fig. 10 which also provides a level of confidence that the K, is quite reproducible in laboratory experiments. Fig. 11 shows how the masses of ‘s7Cs and ““Tc that are sorbed per gram of sediment varied as a function of the liquid-to-solid ratio. For both ‘j7Cs and 99Tc this is a linear relationship. The slope for 99Tc’from Station 2 is lower than that for 137Cs from the same station, indicating that changing the liquid-to-solid ratio has less of an effect on 99Tc than on 137Cs. The mass of ‘j’Cs sorbed on sediment from Station 6 did not change as the liquid-to-solid ratio increased. For both 137Cs and 99Tc on sediment from Station 2, the masses sorbed at the highest ratio of liquid to
6
X Station 3 + Station 2 v Station 2 Batch *Station
8
Station 6 Slope = 9.4
0
50
100
150 lz51 Activity
200
250
300
350
400
on Liquid (cpm/mL)
Fig. 6. Isotherms for lz51 on sediment from Stations 2 and 6 are shown along with data from the batch experiments.
16
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+ Station
2
_?.Station.
3.
a- Station 5 E e ; .-
800
3
400
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6
/ / /
- 4 Stat.ion. 6.
/ ,
/ /
, ’ ’ Station -# 6 Slope = 5.z 4 ’
600
::-i;i;;i$/ $&L&y--
s iz g
et al. / The Science of the Total Environment
200
20
Station # 3 Slope = 1.2
m
/ /-
0
;
40
60
80
100
ggTc in Liquid
120
140
160
180
200
(cpm/mL)
Fig. 7. Isotherms for “Tc are shown along with results of batch tests.
solid are significantly below the trend lines, indicating that no more of either tracer can be sorbed. 5. Discussion 5.1. Solption /desolption of caesium
Partition coefficients for 13’Cs average approx. 350 ml/g for sediment from the Trough and are less, approx. 180 ml/g, for sediment from Stepovogo Fjord. The difference in Kd is probably caused by the difference in mineralogy between the two sites, with more smectite present in sediment from the Trough (see Table 1). To put these K, values into context, the IAEA generic K, for caesium on coastal sediment is much higher, 3000 ml/g. A summary of reported Kds for caesium, from a broad range of sediment types is given by Fuhrmann et al. (1992) in which Kds, range from
10 to 9000 ml/g; showing the need for site specific values. Isotherms for 13’Cs are linear over the range tested. The activity of the starting concentrations of tracer used in this series of experiments ranged from approx. 18 to 100 Bq/ml; concentrations that are in a range that could be expected in leachate from radioactive waste dumped into the ocean. At these concentrations the mass of caesium in the tracer is extremely low (the specific activity of carrier-free 137Cs is calculated to be 2.3 X lo-i5 mol/Bq; we have added to the samples between 5 x lo-l3 and 3 x lo-” mol). This concentration is trivial compared to the concentration of caesium in seawater (1.5 X lo-* mol/kg). The implication of this difference in concentration and the observation of rapid uptake, is that 137Cs is taken up through isotope exchange with stable caesium. Desorption of 13’Cs by seawater appears to
M. Fuhmuznn et al. / The Science ofthe Total Environment 202 (1997) 5-24
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500 * Station 3 * Station 6 . Station 6
400
~.
s 9
Station # 3
g300 5: c ; 200 3 E 2 = 100
o-
L
0
0.5
1
1.5
Uranium
2 in Liquid
2.5
3
3.5
4
(ug/mL)
Fig. 8. Isotherms are shown for uranium at Stations 3 and 6 and batch experiment results for Stations 8.
become asymptotically low at approx. 60% release. A significant proportion of 137Cs can be eluted on exposure to clean water. This is consistent with observations that while sequential extractions failed to remove most 137Cs from contaminated sediment taken from Abrosimov Fjord (which had been contaminated for many years), newly sorbed 134Cs was mobile and only became Table 3 Activity in seawater in contact with sediment (Station 2) showing effects of oxidation on sorption/desorption Tracer
ssSr wTc 232U 125 I 137cs
Activity before oxidation
Activity after oxidation
(cpm/ml)
@pm/ml)
19.3 f 1.8 135 3.3 *0.7 8.9 k 0.8 0.46 f 0.12
14.7 f 1.3 214 -0.1 zko.1 8.8 f 1.4 0.58 f 0.14
Change
Sorption Desorption Sorption No change No change
fixed over time (Borretzen et al., 1995). Oxygenation of sediment (which contained 137Cs at steady-state with seawater) by H,O, added to the seawater caused no additional uptake of 137Cs from the solution. 5.2. Sorption / desorption of strontium Whilst the K, for “Sr was only 4 ml/g, desorption of ‘?3r with fresh seawater, took place only during the ‘first interval (0.2 days) and subsequent exposure to seawater released no more 85Sr. Of the previously sorbed 85Sr, 60% was desorbed. To ascertain that our source-term calculations were correct, we analyzed the solid phase, and confirmed that 85Sr was still present. These results are similar to those of Barretzen et al. (1995) who found that 50-70% of the 90Sr present in surflcial sediment from Stepovogo Fjord was associated with a mobile exchangeable fraction. In our experiment in which H,O, in seawa-
18
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Time (days) Fig. 9. Desorption of radionuclides from previously contaminated sediment by exposure of the sediment to seawater over a series of intervals.
ter was added to a steady-state sediment/seawater system containing 85Sr, additional 85Sr was sorbed from the water. Formation of small quantities of iron oxyhydroxides apparently caused uptake of more strontium. Preferential uptake of ?Sr has been observed for newly formed coating of iron and manganese oxides in freshwater systems (Cerling and Turner, 1982). Formation of iron oxides during freeze drying is also a likely explanation for the high K, values for sediment from Station 8. 5.3. Sorption /desorption of iodine
Results of our iodine sorption experiments are not defhritive. We observed slow uptake in one experiment in which approx. 75% of the iodine was sorbed on sediment from Station 6. However, in another experiment with sediment from Station 2, no uptake was observed. The large differ-
ence in Kds between Stations 3 and 6 could be the result of different amounts of metabolic uptake. The irradiation experiment demonstrated that metabolic activity is important for iodine sorption in this sediment. Another possibility is that the difference in mineralogy (the lack of smectite at Station 6) between the two sites has resulted in different masses of iodine sorbed. Since we did not test sorption of iodine on irradiated samples from Station 2, it is not possible to distinguish between the two possibilities. In recent work, Fuhrmann and Bajt (1994) have shown, using X-ray absorption near edge structure CXANES) of iodine, iodide, and iodate, that uptake of iodine species on mineral separates can be strong and (depending on the mineral) can be associated with a heterogenous redox reaction. This was particularly the case for iodate in the presence of pyrite. Considering the strongly reducing conditions of the sediment and its color,
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we can assume that iron sulfide minerals are present. There may be a relationship between the presence of these minerals and sorption of iodine. In addition, as mentioned earlier, reduced sediment is typically enriched in iodine and as a result, competition between the radionuclide and the natural iodine may also have an effect on uptake. In our experiment with sequential desorption using fresh seawater, most iodine was readily removed from the sediment. Oxygenation of the sediment by H,O, caused no change in the partitioning of iodine between the liquid and solid phases, indicating that reduction of iodate and subsequent sorption, is probably not taking place. This result favors interpretation of uptake by metabolic processes.
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5.4. Sorption / desorption of technetium
Uptake kinetics for 99Tc are slow, requiring approx. 200 h to come to steady-state. Results of the kinetics experiments were anomalous, with greater uptake of technetium occurring in this experiment than in any other. The isotherms consist of a set of uncoupled data points that show trends. Therefore, the isotherm data are more reliable than the kinetics data for assessing K, values. Nevertheless, the conclusion that technetium sorbs slowly is still valid. Values of K, ranged from 1 to 6 ml/g. Isotherms appear to be linear, although most have relatively poor correlation coefficients. Technetium exists in valence states from - 1 to + 7 (with 0, + 7 and +4 being
5
* Station 2 r- f
Station 6
6
Liquid
to Solid Ratio (Thousands)
Fig. 10. The relationship between the K, for 137Csand the liquid-to-solid ratio (L/S) for sediment from Station 2 is different than that for Station 6. Station 2 shows increasing K, as L/S increases, while sediment from Station 6 shows no change in K, as a function of L/S.
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2
A Cs-137 Station 2 v- Cs-137 Station 6
2
Liquid
3
4
5
6
7
to Solid Ratio (Thousands)
Fig. 11. The relationships between the masses of 13’Cs and 99Tc sorbed by the solid and the liquid-to-solid high ratios are attained:
the most stable). Under most natural conditions it is generally found as TcO; which forms highly soluble compounds. The removal of technetium from solution in our experiments was effective, which is surprising since the pertechnetate anion is generally considered to be mobile. As with uranium, this is probably a redox process with technetium being converted from TcO; to either a reduced oxide or a sulphide (Lee and Bondietti, 1983). The experiments were conducted with the sediment under reducing conditions which may have a sufficiently low Eh value to immobilize technetium. Alternatively, since the ionic radius of T&V) is similar to that of Fe(III), it may substitute for Fe in some minerals. Processes involving soil microbiota and organic matter appear to influence the behavior of technetium (Wildung et al., 1979). There is evidence from short-term experiments (24 h) that indicate reactions of technetium with organic matter: (1) soils with low carbon content tend to have low sorption
ratio are linear, unti1
capacities for technetium; (2) sorption is reduced following digestion of soil in peroxide; and (3) some technetium can be recovered from sorption on soils by extraction with NaOH. Longer duration experiments also implicate microbiological processes in the uptake of technetium (Landa et al., 1977). Oxidation of sediment very effectively desorbs 99Tc from the solid phase. The 9gTc content of water in contact with sediment that had previously sorbed “Tc increased by a factor of 1.6 on exposure to H,O, in seawater. The implication is that resuspension of sediment contaminated with !@Tc into well-oxyg enated seawater (as in the Arctic) will cause rapid desorption of this radionuclide. 5.5. Sotption / desopion Uptake
of uranium
of uranium
was slow, requiring
more
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than 350 h to come to steady-state. The average K, value was 55 ml/g, but isotherms were rather scattered, with poor statistics. Desorption with seawater released approx. 35% of previously sorbed uranium over 3 days. Desorption became asymptotic quickly, with little uranium being released in the last interval. As with strontium, in the H,O, experiment, additional uranium was taken up after the addition of the peroxide/ seawater solution such that no uranium was left in solution. 5.6. Effect of liquid-to-solid
ratio
A set of experiments examined how sorption of 13’Cs and 99Tc is related to the ratio of liquid to solid. This is an important parameter when various scenarios are developed for the way that contaminants are introduced and transported in the marine environment. For example, contaminants have been introduced to the Kara Sea under a variety of conditions which result in large differences in the amount of sediment that the contaminant is exposed to (and potentially sorbed on) in the near-field of the disposal site. Simply considering one type of disposal; that of a drum of waste onto the seabed, there are two regimes of liquid-to-solid ratio (i.e. ratio of seawater to sediment in a volume of material where the release takes place). The contaminant may be released as a solution directly to the seawater above the bottom, in which case the liquid-to-solid ratio is large (on the order of mg/kg or 1061. In the other extreme, the container may leak from a corroded area beneath the sediment. In this case the ratio of liquid to solid is quite small (perhaps 0.3-0.6). This effect has been observed in the Stepovogo Fjord, where radionuclide and metal concentrations at depth in the sediment next to waste disposal containers are similar to, or higher than at the surface of the sediment (Van Weers, 1994). The relationship of K, of 13’Cs to the liquidto-solid ratio is non-linear for material from Station 2 while it is much lower and linear for sediment from Station 6 (see Fig. 9). The difference in behaviour may be the result of different total capacities of the two sediment samples to sorb 13’Cs. This is illustrated in Fig. 11 showing
202 (1997) 5-24
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the relationship between liquid-to-solid ratio and the total activity of 137Cs and 99Tc sorbed per gram of sediment from the two stations. The mass of 13’Cs on each gram of sediment increases linearly with increasing liquid-to-solid ratio. At the highest liquid-to-solid ratio, the capacity of the sediment from Station 2 to sorb either tracer was apparently reached. However the capacity of sediment from Station 6 was attained at much lower liquid-to-solid ratios. This means that even a very dilute slurry, with respect to solid content, can sorb significant quantities of i3’Cs, without reducing the proportionality between the mass of 13’Cs on the sediment and the mass of 13’Cs available (since the amount of caesium that can be sorbed is directly related to amount of liquid in the experiment). Given that seawater contains much more caesium than is present in carrier-free radioactive tracers, we can assume that the capacity of sediment to sorb caesium is saturated by stable caesium. If isotope exchange is the uptake process involved, then uptake of 13’Cs tracers will be proportional to the mass of stable caesium present on the sediment (determined by its sorption capacity). Linear behaviour is expected if the quantity of 13’Cs, that is available in solution, is high compared to the sorption capacity of the sediment. The non-linear curve for Station 2 is reasonable if the sediment has a great capacity to sorb caesium (and therefore has a relatively high concentration of stable caesium already sorbed), and the mass of 13’Cs tracer available is limited by a closed experimental system so that the sediments supply of stable caesium was much greater than the supply of 13’Cs that could be exchanged for it. In the literature, a number of examples have been identified in which sorption of contaminants has shown an inverse proportionality between the K, and the mass of solid in the experimental system. Most of this work focuses on hydrophobic organic liquids but some of it examined metal sorption (Di Toro et al., 1986; Honeyman and Santschi, 1988; McKinley and Jenne, 1991). This has been called the ‘particle concentration’ effect. Benoit et al. (1994) observed the particle concentration effect for metals in samples from six estuaries in Texas, and concluded that the presence of colloidal material in the filtered fraction was
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the cause. Voice et al. (1983) suggested that this effect for hydrophobic organic liquids is caused by the presence of microparticles and associated organic carbon remaining in suspension after the separation process. Gschwend and Wu (1985) demonstrated that the quantity of non-settling particles (NSP) or macroparticles (as measured by weight of dissolved solids) was proportional to the solid-to-liquid ratio; concluding that any observed change in K, as a function of liquid-to-solid ratio is an experimental artifact caused by incomplete phase separation. For hydrophobic organic liquids they developed a three ‘phase’ model that includes the dissolved phase, the material sorbed on settling particles and on non-settling particles. Although the literature focuses on organic liquids, some of the issues, such as incomplete phase separation, are equally relevant to metals. McKinley and Jenne (1991) reviewed the literature and performed a set of experiments examining the sorption of cadmium on iron oxyhydroxide. They grouped the causes of the ‘particle concentration’ effect as: 1. sorption by non-settling (or unfilterable) colloids that remain in suspension after separation processes; 2. competition for the species being sorbed by complexing agents (e.g. organic carbon) being released from the solid phase; 3. competition by other sorbing species; 4. increased aggregation of solids resulting in reduced numbers of sorption sites; 5. chemical reactions such as dissolution of sulphide minerals or precipitation of metal oxyhydroxides, as well as reactions caused by bacterial activity resulting in changes in pH or Eh. In our study the ‘particle concentration’ effect has significant control over the K, for 137Cs at Station 2, while it does not effect K, from Station 6. The effect also influenced the Kd for 99Tc. Depending on the cause of this effect for the Kara Sea sediment, there may be significant impact on the transport of radionuclides in the marine environment. Several of the causes that are described above can be eliminated from con-
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sideration because the experiments were conducted in seawater. Since seawater is a relatively high ionic strength solution, it readily flocculates particles. This should significantly reduce the concentration of colloidal particles that cannot be filtered. Since all of our filtrations were done the same way, and the effect only influenced data from one station, we believe that poor filtration is not a cause of the effect observed at Station 2. Seawater buffers the pH of the system, so there was little if any change in pH. Our experiments used single isotope tracers and therefore there were no other sorbing species to compete with the tracer other than those that may have been in contact with the sediment under natural conditions, specifically, stable caesium in the seawater. For 99Tc the issue is less clear, depending on its speciation. While there is no direct competition from natural technetium there may be competition from other oxyanions, since technetium is probably present in solution as TcO;. The contrast in behaviour of ‘37Cs K, with respect to liquid-to-solid ratio between Stations 2 and 6, may illustrate another way that K, is influenced by local conditions. The difference in behaviour is probably caused by the relative capacity of the two sediments to sorb ‘37Cs, coupled to the closed system of the laboratory experiments. In both cases, the 137Csactivity in solution at steady-state should have been adequate to support more sorption (several hundred cpm/ml). The sediment from Station 6, having a relatively low K,, showed no change of K, (as well as no change in the mass of ‘37Cs sorbed per gram of sample) with increasing ratio of liquid to solid (L/S). However, with a greater K, and total sorption capacity, material from Station 2 was able to sorb more activity from solution. In the high L/S samples, this was supplied by the greater mass of 137Cs present in the greater volume of water. The increasing uptake of 137Cs and 99Tc per gram of sediment stopped only in the samples with the highest (65001 liquid-to-solid ratios. 6. Conclusions
125
Uptake kinetics were determined for ‘$r, 99Tc, I 7 137Cs, 232U, and %‘Am. Slow kinetics were
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observed for uranium, iodine and technetium implying that the rate-limiting process was probably not adsorption but a reaction prior to uptake. Partition coefficients were determined for these radionuclides (lead was excluded since its removal from the aqueous phase was complete) using batch type experiments. In addition the Kd values for 137Cs9 ‘$r, U, 1251,and 99Tc were also determined using isotherms. These isotherms were linear, indicating that it is appropriate to use the K, determined with higher activity tracers to those activities encountered in the environment. Mineralogy of the sediment influenced K, for caesium and iodine. The presence of more smectite in the Trough stations appears to result in higher K, values for these two radionuclides, but not for strontium, technetium or uranium. Oxidation of sediment that was at steady-state with seawater containing tracers, effectively desorbed 99Tc from the solid phase, whilst it caused increased uptake of 85Sr and uranium. There were no changes in the concentrations of 137Cs and 1251.In sequential rinses with fresh seawater, desorption was limited to 60% of 137Cs and 85Sr, and 35% of uranium, whilst 90% of the 125I was desorbed. We observed that the K, for 137Csvaried nonlinearly from 40 to 3800 ml/g as the liquid-to-solid ratio varied from 3.4 to 6500. The K, values of ‘37Cs are influenced by the liquid to solid ratio, but apparently only when the sorption capacity is high compared to the mass of 13’Cs in the closed system of the experiment. Under identical conditions, sediment with lower K, values showed no particle concentration effect. Acknowledgements
This work was funded by the United States Office of Naval Research, through the Arctic Nuclear Waste Assessment Program, with Robert Edson as the Program Manager. We thank Per Strand of the Norwegian Radiation Protection Authority for his help to obtain samples. We also thank the anonymous reviewers of this paper for their helpful comments.
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References Anderson L, Dryssen D. Chemical oceanography of the Arctic Ocean. In: Herman Y, editor. The Arctic Seas. New York: Van Nordstrand Reinhold, 1989:93-114. Barnes CE, Cochran JK. Uranium geochemistry in estuarine sediments: controls on removal and release processes. Geochim Cosmochim Acta 1993;57:555-563. Benoit G, Oktay-Marshall SD, Cantu A et al. Partitioning of Cu, Pb, Ag, Zn, Fe, Al, and Mn between filter-retained particles, colloids and solution in six Texas estuaries. Mar Chem 1994;45:307-336. Berretzen P, Fjelldal H, Lien H, Oughton DH, Salbu B. Mobility of radionuclides in sediment from Abrosimov and Stepovogo Fjords. In: Strand P, Cooke A, editors. Environmental radioactivity in the Arctic, Proceedings of the Second International Conference on Environmental Radioactivity in the Arctic. Osteras, Norway: Norwegian Radiation Protection Board, 1995:168-172. Brunauer S, Emmett PH, Teller E. Adsorption of gases in multi-molecular layers. J Am Chem Sot 1939;60:309. Cerling TE, Turner RT. Formation of freshwater Fe-Mn coatings on gravel and the behavior of @)Co,‘%r and 13’Cs in a small watershed. Geochim Cosmochim Acta 1982;42:1333-1343. Di Toro D, Mahoney J, Kirchgraber P, O’Byrne A, Pasqale L, Piccirilli D. Effects of nonreversibility, particle concentration, and ionic strength on heavy metal sorption. Environ Sci Technol 1986;20:55-61. Folk RL. Petrography of sedimentary rocks. Hemphill Publishing, 1974:182. Fuhrmann M, Pietrzak R, Neiheisel J, Dyer R. Partitioning of Cs-137 between sediment and water from the Black Sea. Chem Ecol 1992;7:3-17. Fuhrmann M, Bajt S. Iodine oxidation states in contact with minerals. In: Rothman E, editor. National synchrotron light source activity report. Upton, New York: Brookhaven National Laboratory, 1994:B-182, abstract only. Gschwend P, Wu S. On the constancy of sediment-water partition coefficients of hydrophobic organic pollutants. Environ Sci Technol 1985;19:90-96. Honeyman B, Santschi P. Metals in aquatic systems. Environ Sci Technol 1988;22:862-871. International Atomic Energy Agency (IAEA). Sediment K,s and concentration factors for radionuclides in the marine environment, Technical Report Series No. 247. Vienna: IAEA. Jones EP, Nelson DM, Tregur P. Chemical oceanography. In: Smith WO, editor. Polar oceanography, Part B. Academic Press, 1990:407-476. Landa ER, Tholvig IJ, Gast RG. Effect of selective dissolution, electrolytes, aeration and sterilization on technetium99 on soils. J Environ Qua1 1977;6:181-187. Lee SY, Bondietti EA. Technetium behavior in sulfide and ferrous iron solutions. Material Research Social Sympe sium Proceedings. Vol 15. 1983:315-321.
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McKinley JP, Jenne E. Experimental investigation and review of the ‘solids concentration’ effect in adsorption studies. Environ Sci Technol 1991;25:2082-2087. Meyer LM. Relationship between mineral surfaces and organic carbon concentrations in soils and sediments. Chem Geol 1994;114:347-363. Mount M, Shaeffer MK, Abbot DT. Estimated inventory of radionuclides in former Soviet Union naval reactors dumped in the Kara Sea. In: Strand P, Holm E, editor. Environmental radioactivity in the Arctic and Antarctic, Proceedings of the International Conference on Environmental Radioactivity in the Arctic and Antarctic. Osteras, Norway: Norwegian Radiation Protection Board, 1993:81-87. Price NB, Calvert SE. The geochemistry of iodine in oxidized
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and reduced recent marine sediments. Geochim Cosmochim Acta 1973;37:2149-2158. Van Weers AW. Results of gammaspectrometric analysis of sediment samples from the 1994 Norwegian-Russian expedition to the Kara Sea. In: Strand P, Cooke A, editors. Environmental radioactivity in the Arctic, Proceedings of the Second International Conference on Environmental Radioactivity in the Arctic. Osteras, Norway: Nonvegian Radiation Protection Board, 1995:163-167. Voice T, Rice CP, Weber Jr W. Effects of solid concentration on the sorptive partitioning of hydrophobic pollutants in aquatic systems.Environ Sci Technol 1983;17:513-518. Wildung RE, McFadden KM, Garland TR. Technetium sources and behavior in the environment. J Environ Qua1 1979;8:156-161.
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Sorption/desorption of radioactive contaminants by sediment from the Kara Sea Mark Fuhrmanna,* , Huan Zhou”, James Neiheiselb, Martin A.A. Schoonenc, Robert Dyer b ‘Environmental and Waste Technology Center, Building 830, Brookhaven National Laboratory, Upton, New York, NY11973-5000, USA bUS Environmental Protection Agency, Washington D.C., USA ‘DepartMent of Earth and Space Sciences, State University of New York at Stony Brook Stony Brook, New York, USA
Abstract
To understandthe long term impact of the disposalof radioactive wasteon the Kara Sea, partition coefficients ( Kd) for severalimportant radionuclides,the mineralogyof the sediment,and the relationshipof Kd to liquid-to-solid ratio were quantified. Sedimentwasobtained from four locationsin the Kara Seaarea. Slow sorption kinetics were observedfor 85Sr,232U,lzsI and “Tc, whilst sorptionwasrapid (lessthan 50 h to steady-state)for 137Cs, **‘Pb, and %lAm. Partition coefficients(K,) were determinedusingbatch type experimentsand sorption isothermswhich were Partition coefficientsfor 137Cs were approx. 350ml/g for sedimentfrom developedfor *‘Sr, WTc, lzI, U and 137Cs. the Trough and 180 ml/g for StepovogoFjord. This difference may be causedby the lower fraction of expandable clay in sedimentfrom the fjord. Uptake of *?Sr,“Tc, lzI and U were all similarfor both locations,with K, values averaging4, 3, 17 and 60 ml/g, respectively. The K, fdr 137Cs varied non-linearly from 40 to 3800 ml/g as the liquid-to-solid ratio varied from 3.4 to 6500,but only when the sorption capacitywas high comparedto the massof 13’Csin the closedsystemof the experiment. Under identical conditions,sedimentwith lower K, valuesshowedno effect. Oxidation of sedimenteffectively desorbed“Tc from the solidphase,whilst it causedincreaseduptake of 85Sr and U. In sequentialrinseswith fresh seawater,desorptionwas limited to 60% of 137Csand “Sr, and 35% of uranium. 0 1997Elsevier ScienceB.V. Keywords:
Kara Sea;Sediment;Radionuclides;Sorption; Desorption; Partition coefficient (K,)
1. Introduction Large quantities of radioactive materials have been disposedof either directly into the Kara Sea
*Corresponding author.Tel.: + 1 5163442224; fax: + 1516 3444486.
or in terrestrial areas that are drained by rivers, such as the Ob and Yenesei, that flow to the Kara Sea. The material has been introduced into the sea in many forms, including solutions that were pumped into the sea and as solids and liquids in dumped containers. A large fraction of the inventory is associatedwith 16 reactor pressure vessels, six of which contain nuclear fuel, which will re-
004%9697/97/$17.00 0 1997ElsevierScienceB.V. All rightsreserved. PZZ SOO48-9697(97)00101-O
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6
lease contaminants into solution over long timeperiods. The inventory of radionuclides from these fuel loads (as well as the activation products contained in the vessels), in 1993, has been estimated as 0.2-0.9 PBq of actinides, 18.2-20.0 PBq of fission products, and 4.6 PBq of activation products (Mount et al., 1993). The fission products present in greatest quantities are 137Cs and 9oSr, with approx. 4.4 PBq each. Two fission products, 99Tc and 1291, are present in small quantities and have relatively little impact on human health when present in low concentrations. However, their long half-lives will make them the dominant fission products after the others have decayed away. The purpose of this study is to quantify the sorption and desorption of radionuclides on sediment from the Kara Sea, to determine the longterm fate of these contaminants in the arctic. The parameters of interest are: the partition coefficients (K,) for several important radionuclides for sediment taken from the Kara Sea; the mineralogy of the sediment; and the relationship of K, to liquid-to-solid ratio. Values of K, were determined for the dominant radionuclides present in the waste, with the longest half-lives, and/or of articular environmental concern. These are 90Sr, f& 1291 137~~ natU and 241Am. The gannna3 emitters “Sr and “‘1 were substituted for 90Sr and 1291(both beta-emitters), respectively. Partition coefficients, describing the distribution of contaminants between water and sediment, indicate their relative mobility and are key parameters used to model the transport and distribution of radionuclides in environmental systems containing fluids and particulates. The critical parameter, K,, used to quantify this process is defined as the concentration of the species of interest on the solid divided by its concentration in the liquid, at steady-state. Operationally it is: K = (1 -Fgv d
F,M
for experiments in which the concentration on the solid is determined by difference from a reference solution. FL is the fractional difference in concentration in the liquid compared to the starting concentration prior to sorption, v is the
volume of liquid in the experiment and M is the mass of solid in the experiment. The partition coefficient is affected by factors such as grain size of the solid, its mineralogy and organic matter content, solution chemistry, and speciation of the contaminant in solution. Because of these influences, to determine Kd values appropriate to the site under study, it is necessary to conduct experiments that come as close as reasonably possible to site-specific chemical conditions and materials. Partitioning can be a non-linear function of the concentration of the contaminant in solution. Typically this becomes apparent at higher masses of sorbed contaminant; whilst at lower fractional uptake the relationship between the concentration of the contaminant in solution and its concentration on the solid is assumed to be linear. Isotherms are used to determine if the K, varies with solution concentration, allowing an evaluation of the relationship between sorption at differing concentrations of contaminant. An isotherm can be generated from experimental data by plotting contaminant concentrations on the solid on the Y-axis and its concentration in solution on the X-axis. For simple linear systems the slope is Kd. If the relationship is linear at the relatively high tracer concentrations used in laboratory experiments, then it is appropriate to use that K, for the lower concentrations typically found in the environment. 2. Materials Sediment from four locations in the Kara Sea was sampled by the Joint Russian-Norwegian Expeditions of 1992 and 1993. Sampling stations in the Kara Sea area were: Station 2 (73” OO’N, 58” OO’E in 287 ml, Station 3 (74” 30’N, 62” OO’E at a depth of 334 ml, Station 8 (72” 40’N, 58” 10’E at a depth of 360 m) and Station 6 in the Stepovogo Fjord (72” 33.04’N, 55” 22.15’E at a depth of 45 ml. The locations are shown on Fig. 1. Sediment from all stations consisted of green/black mud. Samples from Stations 2, 3, and 6 were subcores of box cores, while sediment from Station 8 was from a surface grab. The subcores were frozen intact whilst sediment from
7
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Station 8 was freeze-dried. They were kept frozen until they were sectioned in an argon atmosphere to preserve their anoxic condition, which is necessary to maintain the presence of authigenic minerals containing reduced metals (such as iron sulphides) that may be important controls on the sorption/precipitation of some elements. The seawater used for experiments was ob-
-
__---,
---
---
50 ,,
56 \!
i---e-I 1 \ I , I , I , I , I I / I \ I
tained from the Atlantic Ocean, offshore of Long Island, New York. It was filtered before use and stored in a refrigerator. Its salinity was 32.8%0. Tracers were prepared from commercially obtained radionuclide solutions. Serial dilutions were initially done with distilled/deionized water. The pH was adjusted to close to neutral and the final dilution made with seawater. This tracer was
62 \\
i
66 (\
76
\ 1
-72
---
_-
56 Fig. 1. The Kara Sea near Novaya Zemlya, showing the location from which the sediment sample was taken.
8
M Fuhrmann
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then added to the seawater used for the experiments, resulting in only a slight reduction of salinity in the experiments. All experiments were conducted with single tracers. The 137Cs, 210Pb, 125I and 99Tc tracers were carrier free, whilst the s5Sr and %iArn tracers had specific activities of 457 MBq/mg of strontium and 102 MBq/mg of americium, respectively. Uranium experiments were conducted with either 232U tracer or natural uranium diluted from a standard obtained from the National Institute of Standards and Technology. A dilution of the acidic standard was prepared in seawater and the pH adjusted with NaOH, causing a precipitate to appear. After standing overnight the precipitate redissolved. This solution was brought to 100 ml and filtered through a O.l-pm syringe filter. The solution was analyzed by Inductively Coupled Plasma Emission Spectroscopy and had a uranium concentration of 82.6 mg/l. Subsequent analysis of this tracer showed that it was a stable solution. 3. Methods 3.1. Sorption aperiments The general method used in our sorption experiments is given here. Volumes and masses of sediment are given in the sections describing specific experiments. Sorption experiments were conducted under argon to preserve redox conditions and any sulphide minerals. For each experiment, wet sediment was weighed into a polyethylene centrifuge tube, in a glove box. Separate subsamples were also taken for water content determinations. The contact solution was filtered seawater to which tracer was added. The experiments were sampled by shaking the specimen and removing approx. 5 ml of slurry with a plastic syringe. The slurry was then filtered with a 0.45pm syringe filter and the effluent was weighed into counting vials. Analysis was performed by liquid scintillation counting for %Tc and by gamma spectroscopy with an intrinsic germanium gamma detector for the other radionuclides. Count times were adjusted to provide all samples with similar counting errors of approx. 7%. Concentration on the solid was determined by
202 (1997) 5-24
difference from a reference, consisting of seawater and tracer, that was made and handled in a manner identical to the samples (including fIltration), but contained no sediment. These values were then used to determine the concentration in the beginning solution from which the quantity of radionuclide sorbed was calculated by subtracting the concentration in the sample liquid from that of the reference. In this way, any sorption on containers and filters was automatically corrected. Count rates of the samples were corrected relative to the reference count rate to allow for dilution caused by the porewater in the wet sediment. All results were normalized to the mass of the counting sample as well as the dry weight of sediment and the total liquid in each experiment. 3.2. Sorption kinetics Uptake of tracers on the sediment was determined from single batch experiments which were sampled periodically. For each experiment, approx. 16 g of wet sediment were contacted with 40.0 g of seawater and between 1 and 3 ml of tracer. Samples were taken periodically by shaking the specimen and removing approx. 2.5 ml of slurry with a plastic syringe and filtering the slurry. Samplings started at 0.04 days and, depending on the results, in some cases extended to 48 days. Uptake was determined as a fraction of concentrations present in the reference solutions. 3.3. Sorption kinetics of sterilized sediment The focus of this work is on inorganic processes. Nevertheless, active metabolic uptake by microbiota may have significant influence on the sorption of iodine. To determine the extent of biological control of iodine sorption in natural materials, sorption kinetics of iodine was examined for two samples of sediment, comparing sorption rates for samples that had been sterilized by gamma-irradiation to those that were not irradiated. Sterilizing the sediment eliminates metabolic uptake, allowing other sorption processes to be observed. In each experiment, 2 g of wet sediment, from Station 6, were weighed into plastic centrifuge
M. Fuhrmann
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tubes. One of each pair of samples was irradiated with @‘Co three times, several days apart, with each irradiation giving an exposure of approx. lo6 R. Each sediment sample (irradiated or unirradiated) and 40 g of sterilized (by boiling) seawater containing radioactive tracer were mixed together and the bottles were placed on a shaker table. Periodically, approximately 2.5 g of slurry were removed with plastic syringes and filtered. Analysis was conducted as described above. 3.4. Sorption experiments for K,
In this study, K, values were determined for 85~~ 99Tc 1251 > 232U (or natural uranium) and ‘2b1Am. Measurements for 210Pb were not made since the kinetics experiment showed complete removal of lead. Each test was conducted as a single experiment consisting of sediment/ seawater/tracer in a plastic bottle. Tests run at single tracer concentrations (generally in replicate) are referred to as ‘batch’ tests. Tests with varying concentrations of tracer (typically at least five different concentrations) are referred to as isotherms experiments. In either case, approx. 1.0 g of wet sediment was weighed into pre-weighed plastic bottles. For batch tests, 25 g of seawater were added and 0.5 ml of radioactive tracer solution. For isotherms, between 10 and 11.8 g of seawater were then weighed in; the quantity of seawater varying depending on how much tracer was added, the sum of the two being approx. 12.0 g. The kinetics data obtained earlier were used to determine the minimum time required for the batch tests. Determination of the partition coefficient is influenced by uncertainties in the analysis. Ideally, the activity on the solid should be similar to that in the liquid. In most cases, in this set of experiments, activities in both phases were acceptable. However, in the case of 241Am, the K, was sufficiently high that little %lArn remains in solution. Due to the way in which K, is calculated, when the count rate in the liquid is low compared to the rate on the solid, small changes of activity in the liquid can result in large changes in K,. Consequently, some of the results reported for 241Am are minimum values of K,, because the 137~~
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9
activities of 241Am in the liquid phase are below the minimum activity that was detectable. 3.5. Desorption
experiments
Sediment which previously had been used for uptake experiments (for uranium, 137Cs, “Sr and WTc> under reducing conditions was exposed to oxidizing conditions to determine effects of oxidation on sorption/desorption. Four millilitres of seawater/sediment suspension were taken from earlier sorption experiments (Station 2) that had been stored under Ar for 5.5 months. One ml of 30% hydrogen peroxide was added to each sample and allowed to react for 3 days. They were then opened to the air and allowed to slowly evaporate back to their original weight (before addition of H,O,). Each experiment was sampled after the peroxide treatment and the count rates of the tracers in the liquid phase were compared to previous samplings taken during anoxic conditions. In another set of experiments, previously contaminated sediment was exposed to clean seawater. At intervals of 0.2, 1, 3, 12, and 28 days, the slurry was centrifuged and decanted. New seawater was then added. The decanted water was filtered and analyzed for desorbed tracer. This is analogous to contaminated sediment being exposed to clean seawater (seawater containing no contaminant) by resuspension. 3.6. Liquid-to-solid
ratio experiments
Experiments were conducted to determine if a large difference in liquid-to-solid ratio can significantly alter the uptake of a set of contaminants. These experiments were batch sorption tests, similar to those described above with six and eight sorption tests conducted for 137Cs and 99Tc respectively. The ratio of liquid to solid was varied from 4 to 6400 for the 99Tc experiment and from 3.4 to 6500 for 137Cs. The concentration of the tracer was initially the same in each experiment. The experiment bottles were kept under Ar, on a shaker table before sampling. The experiments for 137Cs and 99Tc ran for 17 days and 54 days, respectively.
IO
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3.7. Sediment parameters
Mineralogical analysis was done by optical petrographic methods and by X-ray diffraction. Grain size was determined by wet sieving through a 63-pm screen and then by pipette analysis (Folk, 2974). Moisture content was determined by difference in weights after drying at 50°C under vacuum for 5 days. Surface-area analysis was done by the BET method (Brunauer et al., 1939) using nitrogen on air-dried samples. Sediment from Station 6 was not analyzed for surface area because of its elevated radionuclide concentrations. 4. Results 4.1. Mineralogy and other sediment parameters
Mineralogy, grain size and other sediment parameters are shown in Table 1. Total organic carbon (TOC) at Station 2 is 93 mg/g in the O-6 cm fraction and 80 mg/g in the 26-28 cm fraction. This value is high, even for a coastal sediment. In a recent worldwide assessment of the relationship of total organic carbon to surface area in sediments, only sediment from the Peru slope had higher values (Meyer, 1994). Mineralogy is dominated by quartz and feldspar in the sand and coarse silt fractions and by illite/mica, chlorite and kaolinite in the fine silt, The clay fraction is comprised primarily of mixed layer
202 (1997) 5-24
smectite, illite, chlorite and kaolinite. Mineralogy is similar for all stations, with the exception of Station 6 which has only traces of smectite. Black nodules and black coatings on sand-sized particles occur in approx. half of the sand fraction of Station 6. These are probably the result of manganese forming small nodules and coatings. 4.2. Sorption kinetics
In the first set of experiments we determined the times required for the different radionuclides to reach steady-state concentrations in the water-sediment system. Results of the kinetics experiments are given in Table 2. Sorption kinetics for 137Cs, s5Sr, uranium, and 99T~, on sediment from Station 2, are shown in Fig. 2. Most of the lead was removed rapidly; 98.6% was taken up in 1 h, and it was below detection limits (0.1 counts per min, cpm, or approx. 100 rig/l) in the contact solution in approx. 50 h. Similarly, sorption of 24*Am was rapid, with steady-state being attained within 1 h. Uranium had much slower kinetics, requiring approx. 50 h for 50% removal from solution and steady-state was achieved after approx. 350 h, probably as a result of a redox reaction converting the uranium tricarbonate, which is the predominant species in seawater, to a reduced form with low solubility (Barnes and Cochran, 1993). Sediment from Station 6 sorbed iodine slowly, reaching steady-state after approx.
Table 1 Sediment parameters, Kara Sea
---- .-..--. Parameter
Station 2
Station 3
Station 6
Station 8
Location Depth (m) Sand % Silt % Clay % Organics Surface Area m’/g Quartz % Chlorite % Mite/mica % Kaolinite % Feldspar % Smectite %
73”OO’N 58”OO’E 287 15.5 60.9 23.6 TOC = 93 mg/g 42 30 13 21 10 11 9
74”30’N 62”OO’E 334 2.5 44.2 53.8 LOI = 14.9% 27.7 2.5 14 26 8
72”33.04’N W22.15’E 45 3.5 67.0 29.5 Lo1 = 7.1%
72”40N 58”lO’E 360 7.4 60.5 32.1 LO1 = 10.9%
23
32 18 30 5 10 Trace
31 10 20 13 20
11
M. Fuhnnann et al. / The Science of the Total Envirvnment 202 (1997) 5-24 Table 2 Sorption rates and K,, (ml/g) values for Kara Sea sediment determined by batch tests (Bl and by isotherms (Iso) Element Caesium Strontium Technetium Iodine Uranium Americium
Sorption kinetic2 50 150 200 600b > 350
Station 2
Station 3
Station 6
360 Is0 3 Is0 41x1 21 Iso 56 B 23 B -
330 Is0 3 Is0 1 Is0 18 Is0
180 Iso 5 Iso 6 Iso 9 Is0
110 Is0 130000
31 Iso >280000
Station 8 1260 B 22 B 1B 19 B 73 B -
IAEA, coastal 3000 1000 100 20 mm 2000000
aHours to steady-state on sediment from Station 2. bSediment from Station 6.
600 h. However, iodine was not retained on the sediment from Station 2, after what appeared to be a brief period of sorption and then desorption. This lack of uptake may be the result of high
* Cs-137
150
concentrations of iodine found in reducing sediments (Price and Calvert, 1973) causing a flux of iodine out of the sediment. Technetium exhibited slow sorption kinetics that became steady-state
f Sr-85 * U-232 * Tc-99
200
250
Time (hours) Fig. 2. Sorption kinetics for caesium, strontium, technetium, and uranium are shown for sediment from Station 2. All data have been normalized to reference tracer solutions.
M. Fuhrmann et al. /The Science qf the Total Environment 202 (1997) 5-24
I?
after 170 h on sediment from Station 2. Caesium concentrations in the contact solution became steady-state by 50 h, whilst strontium became steady-state after approximately 150 h. 4.3. Sorption kinetics of sterilized sediment
Results of the irradiation experiment to examine the impact of metabolic processes on iodine uptake are shown in Fig. 3. The sediment that was irradiated (and presumably sterilized) showed no sorption. The scatter in the data is the result of a 10% experimental error but the regression line shows that 100% of the iodine remained in solution. Sediment that was not irradiated sorbed iodine over a period of 20-35 days, with uptake following the square root of time. Uptake stopped by day 38. 4.4. Partition coeficients
Table 2 gives results of the batch tests (labeled
..5
B) and the isotherms (labeled Iso) for each of the four stations. The observed values are compared to those recommended by the IAEA for coastal sediment (International Atomic Energy Agency). The K, values determined for the Kara Sea sediment are significantly lower than the IAEA values in all cases, with the exception of iodine. 4.5. Sorption results
Isotherms for uptake of 137Cson sediment from several stations in the Kara Sea are shown in Fig. 4. Uptake of 137Cs is proportional to its concentration in solution; consequently the isotherms are linear with a positive slope. The slope, therefore, is the K,. Stations 2 and 3, located in the Trough, have similar K, values, approx. 350 ml/g, as determined by the slope of the isotherm. Station 6, in Stepovogo Fjord, had a K, that was half of that at Stations 2 and 3, 180 ml/g. The value for Station 8 was 1260 ml/g, which is significantly higher than the other Trough stations and possi-
60--
E
0
1
2
3
4
5
6
7
Square Root of Time (Days) Fig. 3. Sorption kinetics of “‘1 on irradiated and unirradiated (natural) sediment from Station 6.
6
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bly a result of freeze-drying and oxidation of reduced iron, generating iron oxyhydroxide that has a relatively large capacity to sorb these radionuclides (Cerling and Turner, 1982). To compare results from the triplicate set of batch tests these points have been plotted on the isotherm. Noting that the liquid-to-sediment (solid) ratio is somewhat different (5O:l vs. 25:l) the values are comparable (effects of liquid-to-sediment ratio are discussed below). Strontium isotherms for Stations 2, 3 and 6, shown in Fig. 5, are all similarin slope and are linear. Values for K, from these experiments average 4 ml/g. Sediment from Station 8 had a higher value, averaging 22 ml/g, again probably an artifact of freeze-drying. These results are low compared to the IAEA recommended value of 1000 ml/g (International Atomic Energy Agency). Isotherms for 125I are shown in Fig. 6 for Stations 6 and 2 along with batch results (replicates at one concentration) for Stations 2, 3 and
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8. Although they are both linear, the two isotherms have different slopes. For Station 2, the K, is approx. 20 ml/g, whilst for Station 6 the Kd is approx. 10. Batch results for Station 2 average 56 ml/g and for Station 8 the average is 20 ml/g. Sorption isotherms for 99Tc are illustrated in Fig. 7 for Stations 2,3, and 6, along with duplicate batch data for sediment from Station 8. The ZQ are low, with values averaging 4 ml/g. The IAEA recommended coastal value is 100 ml/g. Uptake of uranium is illustrated in Fig. 8 for Stations 3 and 6. The statistics for the linear regressions of the isotherms are poor but the results indicate relatively low Kd values, averaging for all samples (including batch tests) approx. 55 ml/g. 4.6. Desorption
A critically important aspect for this study is the level of oxygenation of the water in the Kara
t
A Station 2 x Station 2 batch Station 6 Slope = 181
+ Station 3
h
v Station 6 + Station 8
5 la7Cs Activity
10
15
in Liquid (cpm/mL)
Fig. 4. Isotherms are shown for 13’Cs on sediment from Stations 2, 3, and 6. Also shown are the points for the triplicate K, experiments for Station 2 and duplicate points for Station 8.
14
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M. Fuhrmann
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Sea. The oxygen content of the water column is determined by the balance of the rate of oxygen advection and oxygen consumption. The advection of oxygen is complicated because of the variable ice coverage and the complex currents. Intuitively one may expect that the rate of gas transfer through ice is virtually nil, but in fact there is gas transfer through ice. Most of the transfer occurs, however, along brine channels in the sea ice (Anderson and Dryssen, 1989). A second requirement for oxygenation of the water column is vertical mixing, which occurs preferentially during the winter months when ice forms and brines sink to the deeper parts. The consumption of oxygen is limited due to the cold temperature and therefore, overall the waters are well oxygenated (Jones et al., 1990). Even the bottom waters have high 0, concentrations. The rate of new organic carbon production has
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been estimated at 45 gC/m2/year. As a result, the sediment, as it rests on the ocean bottom, is reducing. If the sediment has sorbed radionuelides and it is then resuspended, it will be exposed to an oxidizing environment. What is the fate of the previously sorbed contaminant if the particle stays in the water column sufficiently long that it oxidizes? Results of the peroxide oxidation experiment are shown in Table 3. With the exception of 99Tc there was no remobilization of radionuclides after oxidation had taken place. Moreover, *‘Sr and u2U were sorbed to a greater extent after addition of oxygen than by the reduced sediment. While the count rates of 137Cs and 12’1 in solution were unchanged, the count rate of 99Tc increased by 67%. Results of desorption experiments using sequential decanting and addition of fresh seawater, are shown in Fig. 9. Approximately 60% of previ-
800 m Station 2
/-
+ Station 2 batch
/
/
s z8 600
+ Station
6
A Station 3 + Station
/ /
8
Station 6 ,/’ Slope = 4.7 ,-‘,
20
40
60
85Sr Activity
80
jkation _. slope
100
.,
I
, , .* /
3
_ = 4.p
120
140
in Liquid (cpm/mL)
Fig. 5. Isotherms and batch experiment results for %r, showing that sediment horn all stations (except Station 8) had similar values.
M. Fuhrmann
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ously sorbed 85Sr was desorbed rapidly, with the remaining fraction staying fixed on the sediment. Desorption of “‘1 was less rapid but approx. 90% was released over 12 days. Similarly 137Cs was also desorbed with 46% released after 12 days and 50% after 27 days. The release rate for 137Cs slowed significantly after day 5, implying that only approx. 60% would be released. Approximately 35% of uranium was released, with desorption stopping after 2 days. 4.7. Sorption as a jimction of liquid-to-solid ratio
To effectively model the near-field impacts of radioactive waste disposal it is necessary to determine if the effects of a varying ratio of liquidto-solid can significantly influence sorption of the contaminants. The K, for 137Cs for sediment from Station 2 (Trough) varied non-linearly from
*Station
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40 to 3800 ml/g as the liquid-to-solid ratio varied from 3.4 to 6500. However, for sediment from Station 6 (Stepovogo Fjord), the K, was unchanged while the liquid-to-solid ratio ranged from 4 to 2250. This is shown in Fig. 10 which also provides a level of confidence that the K, is quite reproducible in laboratory experiments. Fig. 11 shows how the masses of ‘s7Cs and ““Tc that are sorbed per gram of sediment varied as a function of the liquid-to-solid ratio. For both ‘j7Cs and 99Tc this is a linear relationship. The slope for 99Tc’from Station 2 is lower than that for 137Cs from the same station, indicating that changing the liquid-to-solid ratio has less of an effect on 99Tc than on 137Cs. The mass of ‘j’Cs sorbed on sediment from Station 6 did not change as the liquid-to-solid ratio increased. For both 137Cs and 99Tc on sediment from Station 2, the masses sorbed at the highest ratio of liquid to
6
X Station 3 + Station 2 v Station 2 Batch *Station
8
Station 6 Slope = 9.4
0
50
100
150 lz51 Activity
200
250
300
350
400
on Liquid (cpm/mL)
Fig. 6. Isotherms for lz51 on sediment from Stations 2 and 6 are shown along with data from the batch experiments.
16
M. Fuhrmann
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2
_?.Station.
3.
a- Station 5 E e ; .-
800
3
400
202 (1997) 5-24
6
/ / /
- 4 Stat.ion. 6.
/ ,
/ /
, ’ ’ Station -# 6 Slope = 5.z 4 ’
600
::-i;i;;i$/ $&L&y--
s iz g
et al. / The Science of the Total Environment
200
20
Station # 3 Slope = 1.2
m
/ /-
0
;
40
60
80
100
ggTc in Liquid
120
140
160
180
200
(cpm/mL)
Fig. 7. Isotherms for “Tc are shown along with results of batch tests.
solid are significantly below the trend lines, indicating that no more of either tracer can be sorbed. 5. Discussion 5.1. Solption /desolption of caesium
Partition coefficients for 13’Cs average approx. 350 ml/g for sediment from the Trough and are less, approx. 180 ml/g, for sediment from Stepovogo Fjord. The difference in Kd is probably caused by the difference in mineralogy between the two sites, with more smectite present in sediment from the Trough (see Table 1). To put these K, values into context, the IAEA generic K, for caesium on coastal sediment is much higher, 3000 ml/g. A summary of reported Kds for caesium, from a broad range of sediment types is given by Fuhrmann et al. (1992) in which Kds, range from
10 to 9000 ml/g; showing the need for site specific values. Isotherms for 13’Cs are linear over the range tested. The activity of the starting concentrations of tracer used in this series of experiments ranged from approx. 18 to 100 Bq/ml; concentrations that are in a range that could be expected in leachate from radioactive waste dumped into the ocean. At these concentrations the mass of caesium in the tracer is extremely low (the specific activity of carrier-free 137Cs is calculated to be 2.3 X lo-i5 mol/Bq; we have added to the samples between 5 x lo-l3 and 3 x lo-” mol). This concentration is trivial compared to the concentration of caesium in seawater (1.5 X lo-* mol/kg). The implication of this difference in concentration and the observation of rapid uptake, is that 137Cs is taken up through isotope exchange with stable caesium. Desorption of 13’Cs by seawater appears to
M. Fuhmuznn et al. / The Science ofthe Total Environment 202 (1997) 5-24
17
500 * Station 3 * Station 6 . Station 6
400
~.
s 9
Station # 3
g300 5: c ; 200 3 E 2 = 100
o-
L
0
0.5
1
1.5
Uranium
2 in Liquid
2.5
3
3.5
4
(ug/mL)
Fig. 8. Isotherms are shown for uranium at Stations 3 and 6 and batch experiment results for Stations 8.
become asymptotically low at approx. 60% release. A significant proportion of 137Cs can be eluted on exposure to clean water. This is consistent with observations that while sequential extractions failed to remove most 137Cs from contaminated sediment taken from Abrosimov Fjord (which had been contaminated for many years), newly sorbed 134Cs was mobile and only became Table 3 Activity in seawater in contact with sediment (Station 2) showing effects of oxidation on sorption/desorption Tracer
ssSr wTc 232U 125 I 137cs
Activity before oxidation
Activity after oxidation
(cpm/ml)
@pm/ml)
19.3 f 1.8 135 3.3 *0.7 8.9 k 0.8 0.46 f 0.12
14.7 f 1.3 214 -0.1 zko.1 8.8 f 1.4 0.58 f 0.14
Change
Sorption Desorption Sorption No change No change
fixed over time (Borretzen et al., 1995). Oxygenation of sediment (which contained 137Cs at steady-state with seawater) by H,O, added to the seawater caused no additional uptake of 137Cs from the solution. 5.2. Sorption / desorption of strontium Whilst the K, for “Sr was only 4 ml/g, desorption of ‘?3r with fresh seawater, took place only during the ‘first interval (0.2 days) and subsequent exposure to seawater released no more 85Sr. Of the previously sorbed 85Sr, 60% was desorbed. To ascertain that our source-term calculations were correct, we analyzed the solid phase, and confirmed that 85Sr was still present. These results are similar to those of Barretzen et al. (1995) who found that 50-70% of the 90Sr present in surflcial sediment from Stepovogo Fjord was associated with a mobile exchangeable fraction. In our experiment in which H,O, in seawa-
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Time (days) Fig. 9. Desorption of radionuclides from previously contaminated sediment by exposure of the sediment to seawater over a series of intervals.
ter was added to a steady-state sediment/seawater system containing 85Sr, additional 85Sr was sorbed from the water. Formation of small quantities of iron oxyhydroxides apparently caused uptake of more strontium. Preferential uptake of ?Sr has been observed for newly formed coating of iron and manganese oxides in freshwater systems (Cerling and Turner, 1982). Formation of iron oxides during freeze drying is also a likely explanation for the high K, values for sediment from Station 8. 5.3. Sorption /desorption of iodine
Results of our iodine sorption experiments are not defhritive. We observed slow uptake in one experiment in which approx. 75% of the iodine was sorbed on sediment from Station 6. However, in another experiment with sediment from Station 2, no uptake was observed. The large differ-
ence in Kds between Stations 3 and 6 could be the result of different amounts of metabolic uptake. The irradiation experiment demonstrated that metabolic activity is important for iodine sorption in this sediment. Another possibility is that the difference in mineralogy (the lack of smectite at Station 6) between the two sites has resulted in different masses of iodine sorbed. Since we did not test sorption of iodine on irradiated samples from Station 2, it is not possible to distinguish between the two possibilities. In recent work, Fuhrmann and Bajt (1994) have shown, using X-ray absorption near edge structure CXANES) of iodine, iodide, and iodate, that uptake of iodine species on mineral separates can be strong and (depending on the mineral) can be associated with a heterogenous redox reaction. This was particularly the case for iodate in the presence of pyrite. Considering the strongly reducing conditions of the sediment and its color,
M. Fuhrmann
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we can assume that iron sulfide minerals are present. There may be a relationship between the presence of these minerals and sorption of iodine. In addition, as mentioned earlier, reduced sediment is typically enriched in iodine and as a result, competition between the radionuclide and the natural iodine may also have an effect on uptake. In our experiment with sequential desorption using fresh seawater, most iodine was readily removed from the sediment. Oxygenation of the sediment by H,O, caused no change in the partitioning of iodine between the liquid and solid phases, indicating that reduction of iodate and subsequent sorption, is probably not taking place. This result favors interpretation of uptake by metabolic processes.
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5.4. Sorption / desorption of technetium
Uptake kinetics for 99Tc are slow, requiring approx. 200 h to come to steady-state. Results of the kinetics experiments were anomalous, with greater uptake of technetium occurring in this experiment than in any other. The isotherms consist of a set of uncoupled data points that show trends. Therefore, the isotherm data are more reliable than the kinetics data for assessing K, values. Nevertheless, the conclusion that technetium sorbs slowly is still valid. Values of K, ranged from 1 to 6 ml/g. Isotherms appear to be linear, although most have relatively poor correlation coefficients. Technetium exists in valence states from - 1 to + 7 (with 0, + 7 and +4 being
5
* Station 2 r- f
Station 6
6
Liquid
to Solid Ratio (Thousands)
Fig. 10. The relationship between the K, for 137Csand the liquid-to-solid ratio (L/S) for sediment from Station 2 is different than that for Station 6. Station 2 shows increasing K, as L/S increases, while sediment from Station 6 shows no change in K, as a function of L/S.
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of the
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2
A Cs-137 Station 2 v- Cs-137 Station 6
2
Liquid
3
4
5
6
7
to Solid Ratio (Thousands)
Fig. 11. The relationships between the masses of 13’Cs and 99Tc sorbed by the solid and the liquid-to-solid high ratios are attained:
the most stable). Under most natural conditions it is generally found as TcO; which forms highly soluble compounds. The removal of technetium from solution in our experiments was effective, which is surprising since the pertechnetate anion is generally considered to be mobile. As with uranium, this is probably a redox process with technetium being converted from TcO; to either a reduced oxide or a sulphide (Lee and Bondietti, 1983). The experiments were conducted with the sediment under reducing conditions which may have a sufficiently low Eh value to immobilize technetium. Alternatively, since the ionic radius of T&V) is similar to that of Fe(III), it may substitute for Fe in some minerals. Processes involving soil microbiota and organic matter appear to influence the behavior of technetium (Wildung et al., 1979). There is evidence from short-term experiments (24 h) that indicate reactions of technetium with organic matter: (1) soils with low carbon content tend to have low sorption
ratio are linear, unti1
capacities for technetium; (2) sorption is reduced following digestion of soil in peroxide; and (3) some technetium can be recovered from sorption on soils by extraction with NaOH. Longer duration experiments also implicate microbiological processes in the uptake of technetium (Landa et al., 1977). Oxidation of sediment very effectively desorbs 99Tc from the solid phase. The 9gTc content of water in contact with sediment that had previously sorbed “Tc increased by a factor of 1.6 on exposure to H,O, in seawater. The implication is that resuspension of sediment contaminated with !@Tc into well-oxyg enated seawater (as in the Arctic) will cause rapid desorption of this radionuclide. 5.5. Sotption / desopion Uptake
of uranium
of uranium
was slow, requiring
more
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than 350 h to come to steady-state. The average K, value was 55 ml/g, but isotherms were rather scattered, with poor statistics. Desorption with seawater released approx. 35% of previously sorbed uranium over 3 days. Desorption became asymptotic quickly, with little uranium being released in the last interval. As with strontium, in the H,O, experiment, additional uranium was taken up after the addition of the peroxide/ seawater solution such that no uranium was left in solution. 5.6. Effect of liquid-to-solid
ratio
A set of experiments examined how sorption of 13’Cs and 99Tc is related to the ratio of liquid to solid. This is an important parameter when various scenarios are developed for the way that contaminants are introduced and transported in the marine environment. For example, contaminants have been introduced to the Kara Sea under a variety of conditions which result in large differences in the amount of sediment that the contaminant is exposed to (and potentially sorbed on) in the near-field of the disposal site. Simply considering one type of disposal; that of a drum of waste onto the seabed, there are two regimes of liquid-to-solid ratio (i.e. ratio of seawater to sediment in a volume of material where the release takes place). The contaminant may be released as a solution directly to the seawater above the bottom, in which case the liquid-to-solid ratio is large (on the order of mg/kg or 1061. In the other extreme, the container may leak from a corroded area beneath the sediment. In this case the ratio of liquid to solid is quite small (perhaps 0.3-0.6). This effect has been observed in the Stepovogo Fjord, where radionuclide and metal concentrations at depth in the sediment next to waste disposal containers are similar to, or higher than at the surface of the sediment (Van Weers, 1994). The relationship of K, of 13’Cs to the liquidto-solid ratio is non-linear for material from Station 2 while it is much lower and linear for sediment from Station 6 (see Fig. 9). The difference in behaviour may be the result of different total capacities of the two sediment samples to sorb 13’Cs. This is illustrated in Fig. 11 showing
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the relationship between liquid-to-solid ratio and the total activity of 137Cs and 99Tc sorbed per gram of sediment from the two stations. The mass of 13’Cs on each gram of sediment increases linearly with increasing liquid-to-solid ratio. At the highest liquid-to-solid ratio, the capacity of the sediment from Station 2 to sorb either tracer was apparently reached. However the capacity of sediment from Station 6 was attained at much lower liquid-to-solid ratios. This means that even a very dilute slurry, with respect to solid content, can sorb significant quantities of i3’Cs, without reducing the proportionality between the mass of 13’Cs on the sediment and the mass of 13’Cs available (since the amount of caesium that can be sorbed is directly related to amount of liquid in the experiment). Given that seawater contains much more caesium than is present in carrier-free radioactive tracers, we can assume that the capacity of sediment to sorb caesium is saturated by stable caesium. If isotope exchange is the uptake process involved, then uptake of 13’Cs tracers will be proportional to the mass of stable caesium present on the sediment (determined by its sorption capacity). Linear behaviour is expected if the quantity of 13’Cs, that is available in solution, is high compared to the sorption capacity of the sediment. The non-linear curve for Station 2 is reasonable if the sediment has a great capacity to sorb caesium (and therefore has a relatively high concentration of stable caesium already sorbed), and the mass of 13’Cs tracer available is limited by a closed experimental system so that the sediments supply of stable caesium was much greater than the supply of 13’Cs that could be exchanged for it. In the literature, a number of examples have been identified in which sorption of contaminants has shown an inverse proportionality between the K, and the mass of solid in the experimental system. Most of this work focuses on hydrophobic organic liquids but some of it examined metal sorption (Di Toro et al., 1986; Honeyman and Santschi, 1988; McKinley and Jenne, 1991). This has been called the ‘particle concentration’ effect. Benoit et al. (1994) observed the particle concentration effect for metals in samples from six estuaries in Texas, and concluded that the presence of colloidal material in the filtered fraction was
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the cause. Voice et al. (1983) suggested that this effect for hydrophobic organic liquids is caused by the presence of microparticles and associated organic carbon remaining in suspension after the separation process. Gschwend and Wu (1985) demonstrated that the quantity of non-settling particles (NSP) or macroparticles (as measured by weight of dissolved solids) was proportional to the solid-to-liquid ratio; concluding that any observed change in K, as a function of liquid-to-solid ratio is an experimental artifact caused by incomplete phase separation. For hydrophobic organic liquids they developed a three ‘phase’ model that includes the dissolved phase, the material sorbed on settling particles and on non-settling particles. Although the literature focuses on organic liquids, some of the issues, such as incomplete phase separation, are equally relevant to metals. McKinley and Jenne (1991) reviewed the literature and performed a set of experiments examining the sorption of cadmium on iron oxyhydroxide. They grouped the causes of the ‘particle concentration’ effect as: 1. sorption by non-settling (or unfilterable) colloids that remain in suspension after separation processes; 2. competition for the species being sorbed by complexing agents (e.g. organic carbon) being released from the solid phase; 3. competition by other sorbing species; 4. increased aggregation of solids resulting in reduced numbers of sorption sites; 5. chemical reactions such as dissolution of sulphide minerals or precipitation of metal oxyhydroxides, as well as reactions caused by bacterial activity resulting in changes in pH or Eh. In our study the ‘particle concentration’ effect has significant control over the K, for 137Cs at Station 2, while it does not effect K, from Station 6. The effect also influenced the Kd for 99Tc. Depending on the cause of this effect for the Kara Sea sediment, there may be significant impact on the transport of radionuclides in the marine environment. Several of the causes that are described above can be eliminated from con-
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sideration because the experiments were conducted in seawater. Since seawater is a relatively high ionic strength solution, it readily flocculates particles. This should significantly reduce the concentration of colloidal particles that cannot be filtered. Since all of our filtrations were done the same way, and the effect only influenced data from one station, we believe that poor filtration is not a cause of the effect observed at Station 2. Seawater buffers the pH of the system, so there was little if any change in pH. Our experiments used single isotope tracers and therefore there were no other sorbing species to compete with the tracer other than those that may have been in contact with the sediment under natural conditions, specifically, stable caesium in the seawater. For 99Tc the issue is less clear, depending on its speciation. While there is no direct competition from natural technetium there may be competition from other oxyanions, since technetium is probably present in solution as TcO;. The contrast in behaviour of ‘37Cs K, with respect to liquid-to-solid ratio between Stations 2 and 6, may illustrate another way that K, is influenced by local conditions. The difference in behaviour is probably caused by the relative capacity of the two sediments to sorb ‘37Cs, coupled to the closed system of the laboratory experiments. In both cases, the 137Csactivity in solution at steady-state should have been adequate to support more sorption (several hundred cpm/ml). The sediment from Station 6, having a relatively low K,, showed no change of K, (as well as no change in the mass of ‘37Cs sorbed per gram of sample) with increasing ratio of liquid to solid (L/S). However, with a greater K, and total sorption capacity, material from Station 2 was able to sorb more activity from solution. In the high L/S samples, this was supplied by the greater mass of 137Cs present in the greater volume of water. The increasing uptake of 137Cs and 99Tc per gram of sediment stopped only in the samples with the highest (65001 liquid-to-solid ratios. 6. Conclusions
125
Uptake kinetics were determined for ‘$r, 99Tc, I 7 137Cs, 232U, and %‘Am. Slow kinetics were
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observed for uranium, iodine and technetium implying that the rate-limiting process was probably not adsorption but a reaction prior to uptake. Partition coefficients were determined for these radionuclides (lead was excluded since its removal from the aqueous phase was complete) using batch type experiments. In addition the Kd values for 137Cs9 ‘$r, U, 1251,and 99Tc were also determined using isotherms. These isotherms were linear, indicating that it is appropriate to use the K, determined with higher activity tracers to those activities encountered in the environment. Mineralogy of the sediment influenced K, for caesium and iodine. The presence of more smectite in the Trough stations appears to result in higher K, values for these two radionuclides, but not for strontium, technetium or uranium. Oxidation of sediment that was at steady-state with seawater containing tracers, effectively desorbed 99Tc from the solid phase, whilst it caused increased uptake of 85Sr and uranium. There were no changes in the concentrations of 137Cs and 1251.In sequential rinses with fresh seawater, desorption was limited to 60% of 137Cs and 85Sr, and 35% of uranium, whilst 90% of the 125I was desorbed. We observed that the K, for 137Csvaried nonlinearly from 40 to 3800 ml/g as the liquid-to-solid ratio varied from 3.4 to 6500. The K, values of ‘37Cs are influenced by the liquid to solid ratio, but apparently only when the sorption capacity is high compared to the mass of 13’Cs in the closed system of the experiment. Under identical conditions, sediment with lower K, values showed no particle concentration effect. Acknowledgements
This work was funded by the United States Office of Naval Research, through the Arctic Nuclear Waste Assessment Program, with Robert Edson as the Program Manager. We thank Per Strand of the Norwegian Radiation Protection Authority for his help to obtain samples. We also thank the anonymous reviewers of this paper for their helpful comments.
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