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 [1]. As a result, floodplains play therole of a geochemical barrier to the migration ofchemical elements, among them radionuclides [2].Since the radioactively contaminated floodplain soilswere formed via deposition of radioactively contami-nated suspended matter, radionuclides in them aredistributed throughout the profile [35], 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[10]. Later [11] 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 [1214],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

  • RADIOCHEMISTRY Vol. 50 No. 1 2008

    104 LEGIN et al.

    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[11]. 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 [1719]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 [20]. 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 [22]. Iron(III) losspromotes gleyzation, a soil formation process typicalof hydromorphic soils [23]. 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 [23].Gleyzation can be promoted by adding a nutrient sub-strate capable of fermentation [22]. 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 [11].

    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.

    EXPERIMENTAL

    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

  • RADIOCHEMISTRY Vol. 50 No. 1 2008

    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 2