Dynamics of radiostrontium leaching from radioactively contaminated floodplain soils of the Yenisei River
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ISSN 1066-3622, Radiochemistry, 2008, Vol. 50, No. 1, pp. 103108. Pleiades Publishing, Inc., 2008.Original Russian Text E.K. Legin, Yu.I. Trifonov, M.L. Khokhlov, D.N. Suglobov, E.E. Legina, V.K. Legin, 2008, published in Radiokhimiya, 2008,Vol. 50, No. 1, pp. 9196.
Dynamics of Radiostrontium Leaching from RadioactivelyContaminated Floodplain Soils of the Yenisei River
E. K. Legin, Yu. I. Trifonov, M. L. Khokhlov, D. N. Suglobov,E. E. Legina, and V. K. Legin
Khlopin Radium Institute, Research and Production Association, Federal State Unitary Enterprise,St. Petersburg, Russia
Received December 27, 2006
AbstractGleyzation-mediated leaching of radiostrontium from floodplain soils of the Krasnoyarsk Miningand Chemical Combine (MCC) activity zone [Atamanovskii Island (front part), Oseredysh Island (front part),and Berezovyi Island (rear part)] is studied with model systems. Leaching of radiostrontium from waterloggedsoils is analyzed in terms of the model of anaerobic biosolubilization of gel films. The leaching of radiostron-tium is found to correlate with that of iron, confirming the cosolubilization model. Addition of glucose (0.5%)as a stimulant for growth of iron-reducing microorganisms increases the dynamic coefficient of radiostrontiumleaching, particularly in soils with lower organic matter content. The model experiments showed that theradiostrontium leaching rate from floodplain soil is higher by 23 orders of magnitude than that of radio-cesium, suggesting the possibility of escape of radiostrontium from the floodplain of the Yenisei River withthe intrasoil runoff. This conclusion is supported by the experimental data on the 90Sr/137Cs ratio in the flood-plain of the Yenisei River downstream of MCC (0.010.1).PACS numbers: 89.60.-kDOI: 10.1134/S1066362208010177
Leaching of radionuclides from waterlogged soilsvia a long-term low-rate intrasoil runoff (infiltration ofthe soil solution) is an insufficiently explored problemof environmental radiochemistry. Such a water regimeis typical of radioactively contaminated floodplains ofrivers, e.g., the Techa and Yenisei Rivers contami-nated through activities of the Mayak Plant and MCC(Krasnoyarsk Mining and Chemical Combine), respec-tively. Floodplain soils are formed in the course ofperiodical floods accompanied by deposition of freshalluvium transferred from the catchment area and bot-tom sediment . As a result, floodplains play therole of a geochemical barrier to the migration ofchemical elements, among them radionuclides .Since the radioactively contaminated floodplain soilswere formed via deposition of radioactively contami-nated suspended matter, radionuclides in them aredistributed throughout the profile , and thosefrom each horizon could enter the intrasoil runoff.
Field study of the leaching of radionuclides fromwaterlogged soils is a very difficult task [6, 7]. There-fore, it is advisable to perform laboratory leachingexperiments with model systems. Previously we re-ported on leaching of 238U, 239, 240Pu, Co, and otherradionuclides from floodplain soils collected from the
MCCs activity zone downstream [8, 9]. Under gley-zation conditions (stagnant-drainage water regime),the leaching of MCC-derived radionuclides was dem-onstrated to correlate linearly with that of iron. Weconcluded that hydrolyzable radionuclides are pre-dominantly fixed on the soil gel films, and radionu-clides and iron are leached via cosolubilization underthe action of anaerobic microorganisms. Also wereported on biosolubilization of synthetic Pu-contain-ing Ca,Fe-fulvate gel films in aqueous soil extracts. Later  we have demonstrated that leaching ofradiostrontium from soils under the gleyzation condi-tions also correlates with that of iron. In terms of ourapproach this means that, like strongly hydrolyzableradionuclides, radiostrontium is fixed on the gel films,and, in the stagnant-drainage water regime, it mayenter the intrasoil runoff through solubilizationmediated by anaerobic microorganisms.
This model is consistent with the experimental dataon the speciation of radiostrontium in soils ,suggesting that 90Sr is preferentially associated withironhumus and iron hydroxide components of theclayhumus complex.
Keeping in mind that the intrasoil runoff takes agreat part in the formation of secondary radioactive
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Fig. 1. Correlation between the Fe loss and radiostrontiumleaching with simulated water of the Yenisei River (1) with-out and (2) with 0.5% glucose additive. Total volume ofthe liquid phase passed, ml: (1) 2050 and (2) 2140.
contamination of surface and ground waters, in thisstudy we examined the dynamics of radiostrontiumleaching from various soils of the floodplain of theYenisei River under the gleyzation conditions.
In carrying out experiments and interpreting data,we used experimental and conceptual approachesdescribed in [8, 11]. The experimental setup wasdesigned so as to monitor the leaching of radionu-clides from waterlogged soils in continuous many-month slow drainage of water through a soil sample. Colloidal and truly soluble species from theleachate move upward along a paper filter arranged atthe outlet, being accumulated at the top of the filter.After changing the filter, the leaching products areanalyzed.
The primary impact of waterlogging is the fillingof soil pores with water, resulting in poor oxygensupply (anaerobiosis). Under such conditions, thedominating microorganisms in the soil are anaerobesgrowing on the surface of the gel films [15, 16]. Asa result, the condition of the gel films formed on thesurface of clay minerals through coordination copoly-merization of Fe and Ca fulvates and humates is directly associated with vital functions of anaerobes[20, 21]. Thanks to the occurrence of a great numberof active sites, the gel films are the primary sorption-active component of the clayhumus complex.
In the gel films, Fe(III) is the most efficient ac-ceptor for the electrons liberated in anaerobic respira-
Table 1. Physicochemical characteristics of the soilsamples studied
Characteristic Sample Sample Sample no. 1 no. 2 no. 3
Calcination loss, % 3.4 3.4 12.6pH of aqueous suspension 7.5 7.4 8.0Cation-exchange capacity, 4.7 0.75 24.4mg-equiv/100 g Total Fe, wt % 4.2 4.5 2.1
tion of microorganisms . In the anaerobic respira-tion cycle, Fe(III) is reduced to Fe(II), which can formsoluble complexes with fatty acids (fermentation prod-ucts), fulvic acids, and carbonic acid.
Reduction of iron is the most significant chemicalprocess occurring in anaerobic soils . Iron(III) losspromotes gleyzation, a soil formation process typicalof hydromorphic soils . Gleyzation involves de-gradation of the gel films, followed by their solubili-zation with formation of truly soluble and colloidspecies. The solubilization involves both the macro-and microcomponents, including radionuclides as-sociated with organomineral complexes. The gleyza-tion dynamics may be monitored by the iron loss .Gleyzation can be promoted by adding a nutrient sub-strate capable of fermentation . Glucose, as a fer-mentation product of polysaccharides from plantresidues, is a particularly efficient additive promotinggleyzation. In the current study, glucose was used assuch an additive.
Figure 1 shows the interrelation between the leach-ing of radiostrontium and the iron loss from flood-plain-meadow peat soil under the gleyzation condi-tions. As a liquid phase, we used simulated water ofthe Yenisei River with glucose additive (0.5%) orwithout it.
Despite much more clearly pronounced iron loss inthe presence of glucose, in both systems (with andwithout additive), the radiostrontium leaching cor-relates linearly with the iron loss, confirming thecosolubilization mechanism .
As the gleyzation is being developed, Fe(II) isaccumulated in the system, inhibiting the electrontransfer to Fe(III) in anaerobic respiration. Further-more, our experiments showed that Fe(II) inhibitsfermentation of glucose, thus reducing the amount ofthe nutrition substrate for iron-reducing anaerobes.
In the experiments we used representative samplesof floodplain soil (horizon 1020 cm), collected fromthe front part of the Atamanovskii Island (sampleno. 1, 5 km downstream of MCC), front part of theOseredysh Island (sample no. 2, 44 km downstream ofMCC), and rear part of the Berezovyi Island (sampleno. 3, 21 km downstream of MCC). Sample nos. 1and 2: floodplain-meadow saturated layered soil; sam-ple no. 3: floodplain-meadow peat soil. Characteris-tics of the soil samples are given in Table 1.
The study performed at the Khlopin Radium Insti-tute revealed that the 90Sr activity in the floodplain
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DYNAMICS OF RADIOSTRONTIUM LEACHING 105
Table 2. Parameters of 85Sr leaching from floodplain soils of the Yenisei River*
Liquid phase t, days V, ml ASr, % A0,i103, Bq Vi, ml Kl,i 105, ml1 Kd,i, ml g1
Atamanovskii Island (front part)SW 12 136 8.5 43.6 136 62.4 12.2SW 21 260 14.2 39.9 124 50.5 15.5SW 49 583 23.4 37.4 323 33.1 22.5SW 95 953 30.0 33.4 370 23.5 32.4SW-G 117 1126 40.8 30.5 173 89.1 7.9SW-G 147 1400 52.8 25.8 214 94.2 7.1SW-G 178 1598 64.4 20.6 198 125.0 5.0
Oseredysh Island (front part)SW 12 240 3.3 43.6 240 13.8 58.6SW 21 376 6.0 42.2 136 20.7 39.1SW 49 713 12.4 41.0 337 20.1 38.6SW 95 1076 18.5 38.2 363 19.4 39.9SW-G 117 1274 30.1 35.5 198 71.6 10.0SW-G 147 1568 47.8 30.5 294 86.2 7.2SW-G 178 1715 57.6 22.8 147 127.1 5.3
Atamanovskii Island (rear part)SW 12 124 9.8 43.6 124 78.5 9.6SW 21 249 13.8 39.6 124 35.9 22.2SW 49 534 20.0 37.6 285 25.3 30.5SW 95 933 25.2 34.8 398 16.1 48.5SW-G 117 1100 30.0 32.6 166 39.0 19.9SW-G 147 1371 36.2 30.5 271 32.5 23.4SW-G 178 1611 39.9 27.8 240 24.1 32.6
* (SW) Simulated water of the Yenisei River, (SW-G) simulated water + 0.5% glucose, (t) leaching time, (V) volume of the liquidphase passed through the soil sample in time t, (ASr) fraction of radiostrontium leached from the sample in time t, (A0,i) 85Sr activ-ity in the sample at the origin of the ith section of the leaching curve, (Vi) volume of the liquid phase passed through the samplein the ith section of the leaching curve, (Kl,i) dynamic coefficient of radiostrontium leaching with the volume Vi, and(Kd,i) dynamic distribution coefficient of radiostrontium between the soil and the volume of the liquid phase Vi passed throughthe sample.
soils of the Yenisei River ranges from 1 to 20 Bq kg1. At such a low specific activity of radiostrontium,it is a difficult task to examine its leaching dynamics.Therefore, carrier-free 85Sr (4.36 104 Bq) was intro-duced into the slurry prepared by adding water to120 g of the sample. The resulting radioactively con-taminated soil sample was allowed to stand for twoweeks with intermittent stirring. Then the sample wasair-dried and placed in the corresponding compart-ment of the experimental setup.
Simulated calcium hydrocarbonate water, reproduc-ing the mineral composition of water of the YeniseiRiver , was prepared as follows. To 1 l of distilledwater, CaO (27 mg), MgO (6.63 mg), 0.1 M NaOH(0.65 ml), 0.1 M HCl (0.73 ml), and 0.05 M H2SO4(0.52 ml) were added, and the resulting suspensionwas saturated with CO2 until complete dissolution of
the salts. Excess CO2 was removed either by purgingthe solution with nitrogen gas or by vigorously stir-ring it until constant pH (8.28.5) was established.
In the leaching experiments, the collecting filterswere renewed at fixed intervals. The spent filters weretreated with 3 M HNO3, and the resulting solutionswere analyzed for 85Sr (-spectrometrically) and iron(colorimetrically in the form of the o-phenanthrolinecomplex).
To determine the 85Sr loss for sorption on the setupcomponents, after termination of the experiment, thesetup was disassembled, and radiostrontium sorbed onits parts was stripped with 3 M HNO3 and determined-spectrometrically. The 85Sr loss for sorption wasfound to be about 5% in both reference system andsystems with glucose additive.
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Fig. 2. Flow rate of the liquid phase through the soilsample as a function of time. Figures at the curves corre-spond to the sample numbering; the same for Fig. 4.
Fig. 3. 85Sr leaching curve. Soil: (AE) Atamanovskii Is-land (front part), (AF) Oseredysh Island (front part), and(AG) Berezovyi Island (rear part). Liquid phase: (AB, AC,AD) simulated water of the Yenisei River and (BE, CG,DF) simulated water + 0.5% glucose. (V) Volume of theliquid phase passed through the soil.
Fig. 4. Evolution of pH in the course of the leachingexperiments.
RESULTS AND DISCUSSION
In our experiments, the soil samples were continu-ously eluted for 6 months in the slow infiltrationmode (stagnant-drainage water regime). In the firstthree months, simulated water of the Yenisei Riverwas used as the liquid phase, and then it was changedfor simulated water containing 0.5% glucose to stimu-late gleyzation (this additive modeled the import oforganic nutrition substrate to the floodplain in thehigh-water period). The steady-state flow rate of theliquid phase through the soil sample insignificantlyfluctuated at about 10 ml day1 (0.14 ml h1) through-out the experiment (Fig. 2).
After 56 days, reducing conditions were es-tablished in the waterlogged soil. The redox potentialdecreased from +(450500) to (1020) mV in theabsence of glucose, and to (100120) mV after addi-tion of glucose (0.5%).
Parameters of radiostrontium leaching from flood-
plain soils of the Yenisei River are presented inTable 2 and Fig. 3.
Evolution in pH of the soil (aqueous suspension) inthe six-month experiment is shown in Fig. 4.
The decrease in pH, observed after introduction ofglucose into the liquid phase, was temporary, and sub-sequently pH returned to nearly the initial neutralvalue. The ability to maintain pH in the neutral orweakly basic range is typical of waterlogged soils.
Figure 3 shows that, in all the systems studied,both without and with glucose, we observed regularleaching of radiostrontium, suggesting continuoustransfer of its mobile forms from the soil to the liquidphase flow.
In three months of leaching with the simulatedwater containing no glucose (Fig. 3, sections AB, AC,and AD), the 85Sr loss from sample nos. 1, 2, and 3was 30, 18.5, and 25%, respectively. In the next threemonths of leaching with the simulated water contain-ing 5% glucose (Fig. 3, sections BE, DF, and CG),the corresponding values were 34.4, 39.1, and 14.7%.
As can be seen, the 85Sr leaching curve withthe simulated water from sample no. 3 (CEC24.4 mg-equiv/100 g) is arranged between the leach-ing curves from sample nos. 1 and 2 (CEC 4.7 and0.75 mg-equiv/100 g, respectively), suggesting nosimple interrelation between CEC and the le...