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ISSN 1070-3632, Russian Journal of General Chemistry, 2011, Vol. 81, No. 9, pp. 2018–2028. © Pleiades Publishing, Ltd., 2011. Original Russian Text © A.P. Novikov, S.N. Kalmykov, T.A. Goryachenkova, B.F. Myasoedov, 2010, published in Rossiiskii Khimicheskii Zhurnal, 2010, Vol. 54, No. 3, pp. 111–119.

2018

Colloid Transport of Radionuclides in Soils A. P. Novikova, S. N. Kalmykovb, T. A. Goryachenkovaa, and B. F. Myasoedova

a Vernadskii Institute of Geochemistry and Analytical Chemistry (GEOKHI), Russian Academy of Sciences (RAS), ul. Kosygina 19, Moscow, 119991 Russia

e-mail: [email protected] b Lomonosov Moscow State University, Leninskie gory 1, Moscow, 119899 Russia

e-mail: [email protected]

Received April 20, 2010

Abstract—Radionuclides undergo redistribution and change existence forms (and, therefore, migration dynamics) not only immediately after they enter into the environment, but also during migration. The latter can be associated with changes in the delivery medium (for example, as strongly contaminated wastewaters is diluted by natural), decrease in the concentration of radionuclides (during their sorption and coprecipitation on soil or host rock microparticles), or change in carrier forms (dissolution of fuel matrices). In view of the multifactor nature and complexity of these processes, we set ourselves the task to summarize results obtained at the GEOKHI RAS on the forms of existence and migration dynamics of radionuclides in radioactively contaminated soils. As objects for study we used soils typical of the forest-steppe zone of the Eastern Ural Radioactive Trace (EURT) and taken at a distance of 2 through 8 km from the Trace axis and 2–4 km from the accident place, as well as samples of the high-water bed soils and sediments of the Enisey River, taken 60 km downstream from the Mining Chemical Combine (MCC).

Soils of the EURT Zone

The principal feature differentiating migration of radionuclides in EURT soils from migration of those from global fallout was that in the first case radionuclides were initially present as readily soluble nitrate-acetate compounds [1]. As a result, over 50 years in an undisturbed soil layer, radionuclides have penetrated to depths of up to 50 cm (radionuclides from global fallout generally penetrate to depths of no more than 10 cm). The penetration depth and power depend on the type of the biogenic landscape and the radionuclide (Fig. 1).

In black-earth and black-earth meadow soils, radionuclides are mostly retained in the upper layers. The most intense migration was observed in peat bog, strongly waterlogged soils. The deepest penetration in all soil types was characteristic of strontium radionuclides, and the least deep penetration was characteristic of plutonium. The density of plutonium contamination of EURT soils at a distance of 2–8 km of the Trace axis was estimated at 590–12300 Bq m–3.

Irrespective of the source of entry, specifically global fallout or accident (EURT soils), plutonium

migrates by two mechanisms: slow (prevailing) and fast [2, 3]. Therewith, if during the first years after the accident formed the EURT, the contribution of the second mechanism was appreciable (up to 20%), now it is no more than a few percent. This is explained by the fact that plutonium, being involved in biochemical processes, is incorporated into slow humate complexes (Fig. 2).

As seen from the presented data, in EURT soils radioactive cesium is primarily associated with mineral soil components, plutonium is associated with the organic mineral component, and plutonium is mainly present in mobile forms.

The commonly accepted [4] semiempirical mathematical models for assessment of the vertical migration ability of radionuclides in soils fairly adequately describe the time dependences of the vertical radionuclide profiles in soils. According to Pavlotskaya [5], the divergence of calculated from actual profiles, for example, for cesium-137, was no larger than 20% in more than half of cases, which is quite acceptable for predictions.

DOI: 10.1134/S1070363211090489

Fig. 1. Depth distribution of radiostrontium, radiocesium, and plutonium in soils of different genetic structures.

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As shown in [5], the vertical transport of radionuclides is a quasi-diffusion process: It is non-diffusion in nature but is fairly described by diffusion equations. Actually, the tranport coefficient is empirically established and reflects the rates of concurrent processes: directed transport on the

infiltration of atmospheric precipitates (convective transport), transport on migrating colloid particles, and diffusion mass transfer depthward from the surface due to the concentration gradient dq/dx. Therefore, it would be more correctly named the migration coefficient.

COLLOID TRANSPORT OF RADIONUCLIDES IN SOILS

RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 81 No. 9 2011

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The mean migration coefficient (М) of global plutonium in different natural climatic zones and landscape and geochemical conditions was estimated at (12±10)×10–8 cm2 s–1 (t0 was set at 1963). The migration coefficients for the global contamination of Trans-Urals forest-steppe soils were found to be as follows: М1 = (2.34±1.0)×10–8 cm2 s–1 and М2 = (3.4±1.4)×10–8 cm2 s–1 (at t0 1954 and 1963, respectively). The following migration coefficients were obtained for the EURT: М1 = 1.9×10–8 cm2 s–1 (90.9 %) and М2 = 10.7×10–8 cm2 s–1 (9.1%) for the black earths leached under the meadow vegetation; М1 = 1.1×10–8 cm2 s–1 (76.3%) and М2 = 10.3×10–8 cm2 s–1 (23.7 %) in the black earths leached in the birch forest; and М1 = 1.0×10–8 cm2 s–1 (96.3%) and М2 = 1.5×10–8 cm2 s–1 (3.7%) for a gray forest soil. Downwards the profile the migration coefficients tend to increase with simultaneously increasing fluctuation range. In the USA, 7 years after local contamination the М1 and М2 were found to be (8±1)×10–8 cm2 s–1 and (15±4)×10–8 cm2 s–1, respectively [6]. The prin-cipal migration mechanism of plutonium in the subsurface horizons of all our studied soils from different zones was slow transport: on the average, 87 % (65–100%); the contribution of fast transport was about 13%.

The observed differences in the migration coef-ficients of plutonium both in soils in general and in separate genetic horizons, as well as the slow/fast transport ratios depend in the type of soils and vegetation, weather conditions, forms of plutonium and mechanism of their interaction with soils, etc. The soil solution is one of the categories of soil waters including various types of moisture (film, adsorbed, pore, capillary, and gravitation). The soil solution in itself is arbitrarily considered to include pore and capillary waters [7]. Together with a really soluble fraction, the soil solution contains colloids and suspended soil particles.

Solutions containing particles less than 0.001 µm are arbitrarily classed with real solutions. Particles 0.45–0.001 µm in size are classed with colloid particles, and particles 0.45 µm in size are referred to as suspended matter [8]. It was found that in surface, ground, and soil waters most chemical elements are present as coarse suspensions and real solutions, and the colloid form is largely characteristic of readily hydrolyzed elements (Fe, Cu, Zn, Ni, Co, V, Cr, Mn) [7]. Exposure to hydromechanical effects results in washing-out intact fine earth fractions and colloid particles from the surface humic and eluvial horizons and their accumulation in the illuvial horizons of the soil profile (the lessivage process) [9]. Colloid transport accelerates with increasing soil humidity. The size of colloid particles much affects the rate of their tangential transport downward over the soil profile. However, this process is not described by the classical Stokes–Einstein equation (diffusion coef-ficient is inversely related to particle radius). In certain cases, fairly coarse colloids incapable of diffusion in a porous material migrate faster than smaller particles which are better sorbed on the surface of soil particles [10]. It is because of the presence of various-size plutonium particles in the soil solution that explains the observation of two mechanisms of plutonium transport in soils (fast and slow).

To estimate the distribution of transuranium elements between different soil fractions, we made use of ultrafiltration [11]. Separation of the organic matter from the black-earth and sod-podzolic soils taken in the EURT zone and its grouping and fractionation were performed following Orlov’s procedure [12].

For further research we took the fraction of humic acids, obtained by treatment of preliminarily decal-

Fig. 2. Radionuclide speciations in leached black earth.

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cified soil with 1 М NaOH. This fraction represents humic acids associated with mobile iron and aluminum hydroxides, as well as calcium. The resulting alkaline extract was acidified with dilute HCl to separate humic (HA) and fulvic acids (FA). The isolated HAs were washed with 6М HCl, the HA precipitate was centrifuged and transferred, using distilled water, into a dialysis cell. Dialysis was performed against distilled water, using a MEMBRA-CEL-MD-44 (14 kDa) membrane for 12 days, changing the water at intervals ranging from 2 h to several days.

To separate humic acids with different molecular weights, we used centrifugal ultrafiltration at 8000 rpm through MILLIPORE Amicon filters (100–3 kDa), taking a separate 0.5-ml sample for each size membrane.

It was found that the aqueous extracts from the soils studied had different рНs: 7.2, 5.1, 4.7, and 4.4 for black earth, sod-podzolic, and podzolic soils (0–3 and 16–21 cm), respectively.

Even though black earths have more organic matter, their extracts contain less organic matter than those from sod-podzolic soils. This is explained by the fact that sod-podzolic soil contain more low-molecular fulvic acids fairly readily extracted with water. The contents of chemical elements in aqueous soil extracts is different and increases in the series: Fe (0.03%) < K, Na (0.1%) < Mn (0.3%) < Ca, Mg (0.4–0.6%).

For extraction we used distilled water, natural river water, and ground water prepared under laboratory conditions according to the Standard PNCTN141298-013(1998). The effect of humic acid on the recovery of radionuclides from soil was studied using the artificial ground water spiked with an alkaline solution containing 0.5 mg of a purified humic acid (com-mercial sample from Aldrich, obtained from brown coals). The liquid/solid ratio in all cases was 1:5, phase contact time 1 day with intermittent stirring.

The river and artificial ground waters differ from each other in composition. The contents of anions, cations, and organic carbon (mg l–1) in the river and artificial ground water, respectively, are as follows: Na+ (20.8 and 97.4), K+ (3.3 and 2.3), Ca2+ (70.3 and 2.9), Mg2+ (13.3 and 0), Fe3+ (0.3 and 0), HCO– (204.3 and 180.0), SO2–(14.0 and 12.6), Cl– (51.1 and 17.8), NО3

– (4.5 and 0), рН (7.8 and 8.5), Сorg (9.6 and 0). The artificial ground water has a sodium bicarbonate composition and no iron and organic carbon, whereas the river water has a calcium bicarbonate composition.

To separate suspended particles in aqueous extracts, we used filters with pore sizes of 0.45 and 0.05 µm. The 0.45–0.05-µm particles can be related to suspensions or fine suspensions, as well as coarse and medium-size colloids, and the filtrates (<0.05 µm) contain fine colloids and water-soluble organic and inorganic compounds. It was found that the distribution of Pu, Am, and Сorg between particles of different size, present in aqueous extracts, is controlled by a number of factors: first, on the composition and nature of the extractant water; second, on the chemical properties of radionuclides; and third, on the type of soil.

The distributions of radionuclides and organic carbon over particles in aqueous extracts have both common features and differences.

Organic carbon in aqueous extracts. The transfer of organic carbon into the initial aqueous extracts depends on the type of soil. The Сorg content in the aqueous extracts from sod-podzolic soils is higher (2.2–2.5%) compared to black earths (1.0–1.7%), which is associated with higher contents in the former of readily water-soluble low-molecular organic compounds and mobile fulvic acids. The nature of water does not appreciably affect the extraction of organic substances from soil.

The Сorg contents in the aqueous extracts from black-earth and sod-podzolic soils are 12–30 and 5–16 %, respectively.

The most part of organic carbon in the colloid fraction of the soil extracts (64–100%, on the average, 83%), irrespective of the soil type, was found in fine colloid soil particles and soluble substances of dif-ferent nature, including soluble organic and inorganic compounds (fraction <0.05 µm).

Plutonium and americium in aqueous extracts. It was found that the recoveries of Pu and Am depend of the nature and compositions of water and soil. The recoveries of radionuclides increase in the series: distilled water ≅ river water < ground water < ground water + humic acid. In terms of plutonium contents in the aqueous extracts of soils, the following series can be formed: podzolic (16–21 cm, 2.4% Pu) > podzolic (0–3 cm, 0.8% Pu) > sod-podzolic (0.5% Pu) > black earth (0.05% Pu). The same tendency is observed with americium: podzolic (16–21 cm, 1,7% Аm) > podzolic (0–3 cm, 0.5% Аm) > sod-podzolic (0.14% Аm) > black earth (0.06% Аm) (Tables 3, 4). The reverse order is characteristic of Сorg (68.0, 34.3, 19,0, and 8.0 mg g–1, respectively).



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The presence of humic acid in the ground water increases 6–8 times the recoveries of plutonium and americium from sod-podzolic soils compared with distilled water and 4 times compared to black-earth soils, due to complex formation of radionuclides and HA.

In the suspension of the sod-podzolic soil and black earth ( >0.45 µm) we found 20–40% (on the average, 31%) of Pu and Am. In the colloid fraction of the aqueous extracts 13–40% (on the average, 17%) of radionuclides are associated with coarse and medium-size colloid particles (0.45–0.05 µm). For the most part of radionuclides (60–100%, on the average, 83%), irrespective of soil type, are associated particles and soluble compounds less than 0.05 µm in size.

Almost the same, irrespective of soil type, radio-nuclide distribution between particles of different sizes was observed in the aqueous soil extracts.

We also studied the distribution of Pu, Am, and Сorg between the groups and fractions of the organic matter in the soils studied. In the humic acids of sod-podzolic soils most Pu, Am, and Сorg were found in relatively mobile hydroxides R2O3·nН2О (up to 36%). In fulvic acids, too, most radionuclides are associated with the most mobile low-molecular fulvic acids. No associa-tion of radionuclides with Са2+ fulvates and humates was detected.

In black-earth soils, compared to sod-podzolic, a different Pu, Am, and Сorg distribution between the groups and fractions of humic acids was observed. It was found that 34.0% of plutonium, 78.8% of americium, and 62.2% of organic carbon in the black earth are associated with relatively mobile humic acids. Plutonium, compared to americium, in more associated with the insoluble organic mineral fraction of the black earth (66 and 21%, respectively). The most part of Pu, Am, and Сorg in humic acids, unlike what is characteristic of sod-podzolic soils, is associated with relatively mobile R2O3·nН2О and calcium (72–81% for humic acids and 45–61% for fulvic acids). This fraction was used for dialysis and study of radionuclide association with humic acids of different molecular weights (MW) by ultrafiltration.The dialytic study of the dynamics of membrane transport of radionuclides showed that most plutonium and americium pass through the membrane within the first two days after the initiation of dialysis; with time the rate of membrane transport sharply decreases. Compared to organic carbon (44.0–63.8%), only little Pu and Am (0.1–

0.2 and 3.0–4.3%, respectively) pass through the mem-brane from humic acid solutions. Americium faster diffuses through the membrane, than plutonium, which is associated with the higher content of Am in relatively mobile compounds.

In the dialysis of fulvic acids, 20.4–25.1% of plu-tonium and 66.3–68.3% of americium were found in the receiving solution, on account of the fact that these radionuclides are mostly present in low-molecular fractions of fulvic acids and also in nonspecific organic and inorganic compounds. The HA fraction of black-earth soils was purified by reprecipitation with HCl and used to study radionuclide association with different-size HAs and FAs.

By means of ultrafiltration we could assess the distribution of radionuclides between with different-MW groups of humic acids. Most transuranium elements were found in the groups of humic acids with MW > 100 kDa (67.5 and 74.6% for Pu and Am, respectively). Low-molecular compounds of humic acids with MW < 3 kDa associate no more than 4.4% of Pu and 1.7% of Am. Humic acids with MW 10– 50 kDa (the fractions with MW 50, 30, and 10 kDa) associate a total of 16.7 and 18.6% for plutonium and americium, respectively. In the HA fraction with MW 3-10 kDa we found 11.4 and 5.1% of plutonium and americium, respectively. In the fulvic acids, 44.1–48.8% of radionuclides are present in the group with MW > 100 kDa, and 42.4–51.0% in the group with MW < 3 kDa.

Membrane filters with known pore sizes make it possible to study the association of radionuclides with particles of various complex substances isolated from environmental matrices. Thus, after consecutive filtra-tion through a series of membrane filters and cellulose membrane of a sample of a solution of the humic acid extracted from the black earth we found that 66% of Pu is associated with particles larger than 0.2 µm, and 26% of Pu remains in the solution, associated with HA particles less than 0.001 µm in size. In Tamm’s extracts [24.8 (NH4)2C2O4 + 12.6 H2C2O4 g l–1] [13] from podzolic soils, most Pu (87–89%) was found in the filtrate, which points to the association of Pu with oxalate complexes of iron and aluminum compounds.

The resulting data allow us to suggest mechanisms of plutonium and americium transport to soils. Thus, the prevailing slow transport of actinides is in essence their migration with micro- and colloid particles during filtration of atmospheric precipitates into the soil


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depth. The fast transport of Pu and Am in soils is due to their migration, first, with fine suspensions and coarse colloids indisposed to sorption on soil microminerals and, second, as soluble complexes with low-molecular and fulvic acids.

Sediments and High-Water Bed Soils of the Enisey River Near the MCC

The migration behavior of americium and Pluto-nium in the high-water bed soils and sediments of the Enisey River is important to study to assess and predict the nuclear environmental consequences of operation of the MCC in this region.

At present monitoring results for transuranium elements (TUE) and other long-lived radionuclides in the region adjacent to the MCC and beyond it up to the Enisey Gulf and Kara Sea [14–17]. The surface and vertical distribution of TUEs was studied, and first assessments of their fixation in high-water bed soils and sediments [14, 18]. However, data on radionuclide contents allow no more than nuclear risk assessment but not enough for reliable analysis of the migration behavior of radionuclides in the high-water bed soils and sediments of the Enisey River. The information on the effects of natural organic substances on the mobility of americium and plutonium in stream ecosystems is quite scarce. The same relates to the effect of excess water logging of high-water bed soils during spring floods on the transfer of americium and plutonium into the soluble state and their wash-out from high-water bed soils and sediments into water.

To this end, in the present work we studied the distribution of TUEs between the organic and in-organic components, groups, and fraction of the organic matter of soil and sediments. The organic-mineral colloid particles can function as effective sorption centers for radionuclides, and, therewith, their mobility under the conditions of the powerful hydrodynamic regime of the river is much higher than in underground hydraulic systems.

Concentrations and Distribution of Radionuclides in Soils Around Nuclear Waste Repositories

The concentrations of radionuclides in the soil samples taken from every 5-cm layer up to depths of 15–30 cm nearby waste repositories are different and depend on the type of the repository. The highest soil concentrations of 239,240Pu were found south-eastward from the boundary of the liquid-waste repository (up to 20 Bq kg–1 on air-dry weight basis) and the lowest

concentrations (within the global background range), at the Severnyi landfill, where liquid nuclear wastes are injected into clay aquifers at depths of 130–220 and 400–500 m [17]. The highest soil concentrations of 137Cs, too, were found in the vicinity of the liquid-waste repository. The distributions of plutonium and americium radionuclides over soil depth differ from each other. Thus, increased 239,240Pu concentrations were sometimes found at depths of 5–10 cm. The 238Pu and 241Am isotopes are distributed more uniformly, but feature more intense downward migration, which implies increasing 238Pu/239,240Pu and 241Am/239,240Pu ratios (from 0.03 to 0.27 and from 0.03 to 0.43, respectively).

It should be noted that the observation of increased 238Pu/239,240Pu ratios in soils around the object 353 and Severnyi landfill [19] compared to the global back-ground due to the radioactive fallout after experimental nuclear explosions suggest that the MCC contributes into the radioactive pollution of the territory. Thus, the 238Pu/239,240Pu ratio in the global nuclear fallout is not higher than 0.02–0.05 [20].

The 239,240Pu concentration in most of the soils studied decreases with depth. For example, at depths of 5–10, 10–15, and 25–30 cm it is 1.1–2.8, 3.4, and 4.4–6.6 times lower than in the upper layer (0–5 cm). The 137Сs concentration, too, decreases with depth. The concentrations of plutonium and cesium radionuclides in soils and the 238Pu/239,240Pu ratios around almost all the repositories studied were above the global background. Thus, the background soil concentrations of 239,240Pu and 137Cs at 50°–60° N are estimated at 0.2–2.7 and 1.0–7.2 Bq kg–1, respectively [20]. This suggests that TUE and 137Сs “escape” from the boundaries of liquid-waste repositories and can migrate further.

Fig. 3. Plutonium and americium contents in Enisey River sediments against distance from the Krasnoyarsk MCC.

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Concentrations and Distribution of Radionuclides in High-Water Bed Soils

The radioactive pollution of the Enisey high-water beds, including islands, spreads over the coastal shore 5–50 m in width, confined with the highest spring flood level [17, 19]. We found that the TUE concentration in high-water bed soils decreases with distance from the MCC and varies from horizon to horizon [21].

Thus, the 238Pu, 239,249Pu, and 241Am contents in the 0–5 cm layer span the ranges 0.1–0.8, 1.2–8.1, and 0.4–2.1 Bq kg–1 on the air-dry weight basis (it was only one sample where the americium content was 7.9 Bq kg–1). The 238Pu/239,240Pu ratio in the 0–5 cm layer of the high-water bed soil was virtually the same, but it markedly increases at a depth of 10–15 cm (from 0.05–0.1 to 0.2–0.96). According to other authors, the 238Pu and 239,249Pu pollution densities of the high-water bed soils 5–10 km downstream from the discharge of radioactive waters was 0.1–0.8 and 0.4–1.7 kBq m–2, respectively, decreasing downstream and repeating the picture of the high-water bed pollution with 137Cs [21].

The distribution of all the TUEs over the depth of high-water bed soils is nonuniform: The concentrations both increase and decrease with depth.

Arbitrary assessment of the intensity of 239,249Pu migration in the high-water bed soils of the Enisey River revealed two types of transport: slow (prevailing) and fast with the migration coefficients (1.4–5.3)×10–8 and (24–132)×10–8 cm2 s–1. In our previously studied nonhydromorphic soil samples taken in the impact areas of the Beloyarsk and Leningrad nuclear power plants, in the EURT, too, two types of transport with migration coefficients close to those obtained for the high-water bed soils of the Enisey River [22]. The higher plutonium migration coefficients in the high-water bed soils can be explained by the hydromorphicity of these soils and increased contents in them of natural water-soluble complex-forming ligands.

Radionuclide Concentrations in the Sediments of the Enisey River

The bottom of the Enisey River is covered mostly by coarse sand, pebbles, gravel, and silty sands of different sizes [17]. We found that the concentration of plutonium in the sediments of the the Enisey River tends, like with high-water bed soils, to decrease with increasing distance from the pollution source (Fig. 3).

An exception are the sediments in the Balchugov-skaya and Khloptunoskaya Canals [21], where the

highest plutonium concentration were determined, on account, probably, with stagnant phenomena in these canals. There is some information that the hydro-graphic and hydrologic features of the Enisey River result in the accumulation of sediments in such places as canals, tailings, etc., where sediment damping occurs to the greatest extent. Increased concentrations of other radionuclides, too, are observed in silty sediments. It can be suggested that the high concentrations of Pluto-nium in the sediments at the right shore of the Atama-novskiygin Island is associated with the discharge of the water cooling the reactor core 50–100 m apart from the right shore 6 km upstream the Atamanovo Village.

The concentration of plutonium in sediments is higher than in upper layers of high-water bed soils. The concentrations of 239,240Pu in the sediments around the Atamanovskiy and Tary Islands were found to be much higher than in the high-water bed soils in the same sites.

The concentrations of 241Am are almost the same over the entire river part in focus. The 241Am/239,240Pu ratio increases with distance, implying a higher mobility of americium (Fig. 3).

Radionuclide Speciations in the High-Water Bed Soils and Sediments

of the Enisey River To characterize the geochemical mobility of

radionuclides in high-water bed and sediments, we determined their contents in the following fractions: water-soluble (distilled water), exchangeable and readily soluble (1 M CH3COONH4, pH 4.8), mobile (1 M HCl), acid-soluble (6 M HCl), and poorly soluble (8 M HNO3 + 0.2 M KBrO3).

The resulting data show that in the high-water bed soils of the Atamanovskiy Island (tailings) the distributions of Am and Pu have similar orders of distribution between the mobile forms: water-soluble < exchangeable and readily soluble < mobile < acid-soluble < poorly soluble, even though the contents of different forms are quite different.

Americium, compared to plutonium, shows a stronger tendency to prefer the first three mobile forms (35 and 21%, respectively). The 241Am/239,240Pu ratio for the potentially mobile forms at different depths varied in the range 1.5–3.9. At the same time, markedly higher plutonium contents were found in the acid-soluble and poorly soluble forms which are strongly bound with the solid phase (78% and 65%, respectively).

For the nonhydromorphic high-water bed soil at the upstream edge of the Atamanovskiy Island, the


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239,240Pu distribution order between the mobile forms is the same as above. However, this soil features in-creased contents of potentially mobile forms of plutonium at the depths 5–10, 20–25, and 35–40 cm. Moreover, unlike the hydromorphic soil, essential fluctuations in the contents of the same form in samples from different depths and in the relative contents of different forms are observed. The fractions of water-soluble, exchangeable, and readily soluble plutonium in the nonhydromorphic high-water bed soil are 1.1, 2.3, and 1.3 times higher than in the hydromorphic soil. In spite of the scarcity of data, we can note lack of considerable differences in the 238Pu and 239,240Pu speciations in the non-hydromorphic soil.

The fact that plutonium and americium are present in mobile forms suggests the possibility of transfer of these radionuclides from the solid phase into the soluble state and their involvement in migration processes solutes. With variations in the levels of soil waterlogging, chemical composition of the aqueous phase, and other natural conditions, the relative contents of different forms can vary either in favor of formation of mobile forms, which increases the migration ability of radionuclides, or in favor of solid-state forms due to formation of poorly soluble compounds and their incorporation into clay minerals.

In the river sediments, plutonium and americium are present in the same forms as in the soils of this ecosystem, but in different ratios. In the fractions related to the exchangeable and acid-soluble forms, the plutonium contents in sediments are respectively 2.4 and 1.7 times higher than in high-water bed soils.

The content of the water-soluble form of americium in soil, by analogy with what is observed with plutonium, is higher (37.5 times) than in sediments. At the same time, a feature which distinguishes americium from plutonium is a much higher (2.7 times) content of its mobile form in sediments compared to high-water bed soils. Notable is a markedly lower, compared to soils, content of the most mobile water-soluble forms of plutonium and americium in sediments, which can be explained by washing-out of soluble forms from the solid phase under conditions of a percolative water regime.

As seen from the reported data, americium is more prone than plutonium to be present in potentially mobile forms both in soils and in sediments. This finding suggests that colloid-associated americium is more probable to pass from solid formations to the

aqueous phase, and, therefore, it more actively migrates in the ecosystem. Probably, it is enhanced migration ability of americium that explains its almost equal concentrations (0.2 Bq kg–1) in the 0–5 cm layer of sediments in the zone ranging from 7 to 28 km from the MCC wastewater discharge site.

It was found that the prevailing part of macroelements in sediments are present in the poorly soluble form, i.e. as hydroxides, oxides, high-mole-cular organic-mineral substances; or are incorporated into the crystal structure of clay minerals. The dif-ferences in the distribution of macroelements between forms, on the one hand, and microquantities of Pluto-nium and americium, on the other, are largely as-sociated with the fact that the former are involved in the solid phase of soils and sediments and determine their chemical composition and structure, whereas radionuclides are involved in the forming (or already formed) soils and sediments in their intrinsic specific forms [23, 24].

Apart from the above-mentioned forms of TUEs, there are two more forms considered in geochemistry, pedology, and radiogeochemistry: amorphous and silicate. The first represents a group of potentially soluble organic and inorganic substances forming films on the solid phase and containing, along with the mineral fraction, poorly soluble hydroxides and high-molecular organic compounds (humines). The amor-phous form (without separation into constituents) is most commonly isolated by treatment of samples with Tamm’s solution [24.8(NH4)2C2O4 + 12.6H2C2O4, g l–1].

To separate the amorphous phase into constituents, it was consecutively treated with a mixture of alkali and sodium oxalate solutions and with Tamm’s reagent. The components extracted at the first stage include organic acids (low-molecular, amino, humic, etc.) and their compounds with iron, aluminum, alkaline-earth, and other elements. The stage stage involves extraction of iron and aluminum hydroxides. It was found that the contents of plutonium and americium in the organic components of high-water bed soils and sediments are on average 24–30 and 16–31%. The largest difference in the Am and Pu contents, both in high-water bed soils and in sediments, is observed in the hydroxide fractions and the residue. Therewith, in the low-mobility hydroxides of high-water bed soils and sediments we found, on the average, 4.0 and 2.6 times more plutonium, than americium. Noteworthy, nearly the same plutonium



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distribution between the organic and hydroxide fractions was found in the sediments of the coastal zones of the Irish Sea and Bombay Bay, polluted by nuclear power plant discharges: 32–37 and 20–30% in the Irish Sea and 33–51% and 32–55% in the Bombay Bay [25].

According to our data, organic carbon in comparable quantities is present in all horizons of the high-water bed soil and sediment profiles studied. Even though organic carbon is nonuniformly distri-buted in high-water bed soils and sediments, a positive correlation is observed, like with the high-water bed of the Techa River (Mayak Production Corporation), between the contents of 239,240Pu and organic carbon [26, 27]. This circumstance imparts urgency to research on the effect of organic matter on the migration behavior of radionuclides and, in the final analysis, high-water bed self-cleaning.

The organic matter of soil comprises strong complex-forming agents for TUEs (humic and fulvic acids and their organic-mineral derivatives). There-with, the resulting complexes, depending on conditions, can both suppress (mostly humic acids) and drive (fulvic acids) radionuclide migration. Model experiments showed that the mobility of Eu(Am) in the high-water bed soils of the Enisey River is largely controlled by the strength of the coordination poly-meric structure of metal fulvate gel phases [27–29], which, in its turn, depends on the soil concentrations of principal metals, first of all, iron and calcium. It is important to note calcium adversely affects the solubility of iron fulvate gel phases. This fact should be taken into account in considering the migration behavior of TUEs in the Enisey ecosystem whose aqueous component relates to the calcium hydro-carbonate type.

Fractionation results showed that americium and plutonium are present in all groups and fractions of the organic matter of the high-water bed soils and sediments of the Enisey River. However, the distributions of these radionuclides between groups and fractions much differ from each other and are almost depth-independent. For example, the contents of americium in the most mobile fractions are on average 5.4 and 7.1 times higher than those of plutonium for the high-water bed soils and sediments, respectively. This is probably explained by a weaker bond of americium with organic and inorganic com-ponents of the solid phase and its transition into the

soluble state by the ion-exchange mechanism. More plutonium than americium was found in all humic acid (FA+HA) samples; therewith, this effect is much better expressed in HAs compared to FAs. The fraction of plutonium in the nonhydrolyzed residue of high-water bed soils and sediments is respectively 5.5 and 4.7 times higher than the fraction of americium.

The distributions of plutonium and americium are differently distributed both between the fractions of the organic matter of soil, and between the fractions of humic acids. The plutonium concentrations in HA fractions of the high-water bed soil decrease in the order: calcium humates > free humic acids and their compounds with mobile iron and aluminum hyd-roxides > humates and low-mobility hydroxides. In humic acids of sediments plutonium is preferentially associated with less soluble compounds. Compared with the high-water bed soil, the plutonium con-centrations in mobile fractions are lower 1.6 and 1.2 times, and in low-mobility fractions, higher 1.9 times.

Americium, too, was detected in all HA fractions. In the high-water bed soil it is preferentially associated with more mobile compounds.

In the FA group of the high-water bed soil, most plutonium and americium were detected in potentially mobile fractions which also contained nonspecific organic components (low-molecular and amino acids, polysaccharides, etc.).

Comparison of Am and Pu distributions between HA fractions shows that americium is always, both in the high-water bed soil and in sediments, associated with more mobile compounds than plutonium.

The key factor responsible for the behavior of metals in waterlogged soils is the development of restorative processes due to decomposition of plant residues by heterotrophic anaerobic microorganisms [30, 31]. Under oxygen deficit conditions, the role of electron acceptors during fermentative transformation of plant residues in soil largely falls on active forms of Fe(III) (amorphous hydroxides and organic-mineral complexes). Their reduction gives rise to mobile complexes of Fe(II) with humic acids, as well as with various nonspecific organic acids formed by transformations of plant residues. This process leads to gradual dissolution (solubilization) of iron hydroxide organic-mineral films on the surface of clay soil minerals [32]. Along with organic acids, bicarbonate anions form readily soluble complexes with Fe(II).


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This is the major form in which iron enters the ground water. Uncompensated washout of iron from soil under a stagnant-percolative water regime causes soil loosen-ing (gleization) [31].

As known [33], functioning anaerobic micro-organisms in a waterlogged soil increases the fraction of exchangeable iron forms and partial conversion of crystalline iron into amorphous. Surface soil films most actively accumulate radionuclides [34]. There-fore, one can expect that their dissolution will be accompanied by joint washout of iron and plutonium. By now these processes have scarcely been explored.

CONCLUSIONS

The examples of the EURT soils and high-water bed soils of the Enisey River were used to show that Pu and Am are mostly associated with low-mobility Са, Fe, and Al humates and poorly soluble humines, and, therewith, Pu is associated with humates to a greater extent than Am. More Am than Pu was found to be associated with mobile fulvic acids, which points to a higher migration ability of Am in soils, compared to Pu.

Up to 70–75% of plutonium and americium were found in the group of high-molecular soil humic acids with MW > 100 kDa. In fulvic acids, 40–50% of Pu and Am are associated with the group of relatively mobile low-molecular fulvic acids with МW < 3 kDa. Both the mentioned groups of organic matter can be considered as potentially mobile. The first is such in highly waterlogged soils and when abundant river flow or rainfall run-off take place, whereas the second is the principal transport mechanism under conditions of a low water infiltration.

ACKNOWLEDGMENTS

The work was performed in the framework of the Scientific and Scientific-Pedagogical Personnel of the Innovative Russia in 2009–2013 Federal Targeted Program (State Contract nos. P2358, P2191, P2259, and P2220).

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