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DOI: 10.19047/0136-1694-2019-100-117-132 Cite this article as:
Sanzharova N.I., Ratnikov A.N., Fesenko S.V., Sviridenko D.G., Review on uranium in soil: levels, migration, Dokuchaev Soil Bulletin, 2019, V. 100, pp. 117-132, DOI: 10.19047/0136-1694-2019-100-117-132
Review on uranium in soil: levels, migration
© 2019 г. N. I. Sanzharova*, A. N. Ratnikov**, S. V. Fesenko , D. G. Sviridenko
Russian Institute of Radiology and Agroecology, 109 km Kievskoye shosse, 249032 Obninsk, Kaluga region, Russian Federation, e-mail: natsan2004@mail. ru, e-mail: ratnikov-51@mail. ru, e-mail: corwin [email protected], e-mail: sedelnikov 167@gmail. com.
Received 05.09.2019, Revised 05.09.2019, Accepted 18.12.2019
Abstract: The problems of radioactive contamination of biosphere with the 238U in recent years have attracted a large number of experts in various fields of knowledge. Natural radionuclides are an integral part of the biosphere. They are concentrated everywhere: in rocks, in soil, water, air and food. Various types of phosphorus fertilizers and other natural ameliorants used in agriculture are also an additional source of heavy natural radionuclides. The review describes the behaviour and migration
of 238U
in biosphere, which
depend on various factors acting simultaneously. It is noted that the behaviour of 238U in the soil depends on the forms of its presence in it and the physical and chemical properties of the soil. The processes of sorption and desorption of 238U by different soil types are significant indicators.
Keywords: U; radioactive contamination; soils, behaviour of U, uranium forms.
INTRODUCTION
Intensification of energy development on nuclear and fossil organic fuels, extraction of rare and non-ferrous metals, production of phosphorus fertilisers with a high content of heavy natural radionu-clides (HNR) can lead to further dispersion of uranium in the bio-
sphere. A sharp increase in technogenic flows of HNR in the biosphere, including in the field of agricultural production, necessitates the study of patterns of inclusion and migration of HNR in biological chains. To reliably assess the sanitary and hygienic danger of HNR, it is necessary to study the processes of interaction of these radionuclides with the soil. Migration of HNR in the terrestrial biosphere is a serious theoretical and practical problem, largely unsolved by modern science.
ROCKS AND SOILS
Isotopes of uranium are widely distributed in the Earth's crust. Uranium mass concentrations in parent rocks vary from 0.1 to 1,250 ppm with a typical range of 2.0-6.1 ppm or 25-75 Bq kg-1 238U (Table 1) (Rubtsov, Pravdina, 1971). The estimated average 238U concentration in the continental crust is 32.9 Bq kg-1.
Table 1. Typical U concentrations in various rocks (adapted from Gascoyne, 1992)
Rock types U (mg kg-1) 23SU (Bq kg-1)
Silicic rocks (granites-dacites) 2.2-6.1 27-75
Basalts 0.1-1 1-12
Gneiss 2 25
Schist 2-5 25-62
Limestone 2 25
Black shale 3-1,250 37-15,000
Phosphates 50-300 620-3,700
Relatively high uranium concentrations are observed in black shale and phosphate rocks. The high uranium concentrations in shale are likely due to associations of clay-rich material of organic origin, whereas phosphate rocks of sedimentary origin are composed of minerals rich in uranium. Silicic rocks, granites and basalts tend to have similar uranium concentrations.
Concentrations of uranium in the soil depend, to a large extent, on its concentration in the parent rock. Parent rocks are affected by many environmental factors, resulting in soil formation. In particular, because of uranium weathering from the parent rocks, uranium can be
transferred and deposited as loess, silt placers and Tertiary soils (Table 2).
Table 2. Uranium concentrations in soils (Alexakhin et al., 1990; Baranov, Morozova, 1971; Saric et al., 1995)
Landscape Type of soil Texture 238U mg kg1
Tundra Soddy-Podzolic* Loam 2.53 ± 0.6
Taiga Soddy-Podzolic* Loam 20.1 ± 2.5
Taiga Soddy-Podzolic Loam 4.7 ± 2.9
Mixed forests Typical chernozem Clay 2.7 ± 0.2
Deciduous forest Grey forest Loam 1.2 ± 0.1
Meadows, steppe Chernozem, Brown earthb Clay, clay-loam 3.03 ± 0.7
Chernozem Clay 2.6 ± 0.2
Light brown Clay; clay-loam 2.9 ± 0.7
Dry Subtropics Red earth Clay-loam 5.1 ± 0.3
Desert Light brown desert soil 5.33 ± 0.5
Area with elevated natural background, the Komi Republic; bSamples taken from the arable soil.
The data in table 2 show that 238U concentrations in soils in areas with normal natural radiation background vary from 0.4 to 20 mg kg"1. The highest mean values of 20.1 + 2.5 mg kg"1 were found in Soddy-Podzolic Loam soils of the taiga.
Available data on uranium measurements in soils of different countries of the world are also reported in an UNSCEAR report (Saric et al., 1995), table 3 shows that 238U concentrations in soil vary over a wider range than that reported in table 2, most likely because of the wider range of soil types investigated, inhomogeneity of the soils studied, the wider range of uranium concentrations in the parent rocks and other properties (in some cases localised) favouring uranium accumulation in the topsoil. Even within some countries, the ranges of reported
23 S ••• 23 8
U concentrations in soils covered a large fraction of U concentrations range presented in table 3. For example, this is the case for China (0.2-55 mg kg-1) and Thailand (0.2-29.7 mg kg-1).
Table 3. 238U-concentrations in soil on a country-by-country basis (adapted from Saric et al., 1995)
Region Country Bq kg-1 mg kg-1
Mean Range Mean Range
Africa Algeria 30 2-110 2.4 0.2-8.8
Egypt 37 6-120 3.0 0.5-9.6
North America Costa Rica 46 11-130 3.7 0.9-10.5
United States 35 4-140 2.8 0.3-11.3
East Asia China 33 2-690 2.7 0.2-55.5
India 29 7-81 2.3 0.6-6.5
Japan 29 2-59 2.3 0.2-4.7
Kazakhstan 37 12-120 3.0 1.0-9.6
Malaysia 66 49-86 5.3 3.9-6.9
Thailand 114 3-370 9.2 0.2-29.7
West Asia Armenia 46 20-78 3.7 1.6-6.3
Syrian Arab Republic 23 10-64 1.8 0.8-5.1
North Europe Lithuania 16 3-30 1.3 0.2-2.4
Norway 50 4.0
West Europe Ireland 37 8-120 3.0 0.6-9.6
Switzerland 40 10-150 3.2 0.8-12.1
East Europe Bulgaria 40 8-190 3.2 0.6-15.3
Hungary 29 12-66 2.3 1.0-5.3
Poland 26 5-120 2.1 0.4-9.6
Romania 32 8-60 2.6 0.6-4.8
Russian Federation 19 1-67 1.5 0.1-5.4
Slovakia 32 15-130 2.6 1.2-10.5
South Europe Albania 23 1.8
Croatia 110 21-77 8.8 1.7-6.2
Greece 25 1-240 2.0 0.1-19.3
Portugal 49 26-82 3.9 2.1-6.6
Median 35 17-60 2.8 1.4-4.8
The median value for the 23SU concentration in soil calculated based on these data is 35 Bq kg-1 or 2.8 mg kg-1. However, there are some areas in the world where the normal range of variation of naturally occurring radionuclides in soils or waters is greatly exceeded. These high radiation background regions are known as "Enhanced Natural Radiation Areas" or "ENRA"
Maximum values were found to be for some Azerbaijan sites, Ramsar, Iran and the Komi Republic of the Russian Federation, where 238U activity concentrations were two to three orders of magnitude higher than the average worldwide value of 32 Bq kg-1 reported in UNSCEAR (2008).
238U AND AGRICULTURAL ECOSYSTEMS
Heavy natural radionuclide uranium participates in the formation of the Earth crust. Atomic mass of the natural uranium is 238. It was discovered by the German chemist M.H. Klaproth in 1789. Uranium is widely spread in nature (Tananaev, 2011). It was discovered in considerable concentrations in earth material, oceans, the Earth mantle and meteorites. The half-life of 238U is 4.49 x 109 years (4.49 billion years) 238U is the a-emitter with maximum energy of 4.7 MeV.
In areas where rocks containing 238U and mine dumps come to the surface, there is an increased level of contamination of the biosphere with uranium. In the industrial production of phosphate fertilizers, there is also additional contamination of soils and vegetation with uranium (Ishchenko, 1988)
The content of mobile forms of 238U in ammophos and phos-phogypsum comes up to 5.95 and 23% from gross content in water-soluble form.
Content of uranium in earth crust is Average content of 238U in all earth soils is 2.4 x 10-4% (Alexakhin et al., 1992). The 238U content in grey forest soils is equal to 3.4 x 10-4%, in chernozems - 3.0 x 10-4%, in the upper layers of sierozemic soils of semideserts it comes up to 2.2 x 10-4%. The average content of 238U in soils all over the world is 24.4 Bq kg-1. Global technogenic flows of 238U in biosphere were approximately 1.5 x 1014 Bq at the operation of fuel cycle enterprises working on nuclear and fossil fuels and 1 x 1014 Bq at the application of phosphoric fertilizers. Volume of 238U in
ploughed horizon of agricultural areas is 1.4 * 1017 Bq (Alexakhin et al., 1992).
Uranium belongs to the class of water migrants, and in natural media, it exists in (IV) and (VI) valency forms. In continental surface waters, the distribution of 238U follows the natural and climatic zone sequence, where the concentration of this radionuclide varies from n * 10-8to n * 10-5 L g-1. In the waters of large rivers flowing through different climatic zones, the concentration of 238U for the Northern Hemisphere increases from north to south (Alexakhin et al., 1990). In continental surface waters, the distribution of 238U follows the natural and climatic zone sequence, where the concentration of this radionu-clide varies from n*10-8to n*10-5 lg-1. In the waters of large rivers flowing through different climatic zones, the concentration of 238U for the Northern Hemisphere increases from north to south (Alexakhin et al., 1990).
238U in natural waters exists as a uranyl ion UO2+, which forms complex compounds with inorganic and organic acids. The ability of the uranyl ion to form complexes with ions such as SO42-, Cl-, F-, NO3-, CO3-, CH3COO- etc., to become a part of the negatively charged colloidal sols of the hydroxide and to form complexes of uranium-organic compounds plays a major role in the migration of 238U to natural waters (Drichko, 1983). The forms of 238U in surface waters depend on pH and the content of CO2 and Corg. In neutral and slightly acidic waters of the cold and temperate zones of the humid zone, the dissolved 238U migrates in the form of fulvates, hydroxycomplexes or pseudocolloids. For weakly alkaline waters of the arid zone containing carbonate-ion, the main forms of finding 238U are stable carbonate complexes [UO2(CO3)2(H2O2)2]2- and [UO2(CO3)3]4-. The researchers also assume the presence of bicarbonate complex compounds 238U (Alexakhin et al., 1990).
Anthropogenic sources of environmental pollution by 238U, including agricultural ecosystems, are: extraction of uranium in mines and a formed uranium trace, the atomic and nuclear power industry, agriculture (mineral fertilizers, primarily phosphoric, may contain 238U in high concentrations), uranium content in irrigation water (Alexakhin et al., 1992; Drichko, 1983), the use of radioactive projectiles in military conflicts.
With the systematic use of waste (phosphogypsum) and complex phosphoric fertilizers, accumulation of gross and mobile forms of heavy natural radionuclides can occur in soils. It can reach the levels exceeding background concentrations by 1.5-2.0 times (Drichko, 1983).
The nature and direction of the soil formation processes determine the distribution of uranium in the soils of certain landscape zone. These factors result in the great variety of uranium content in soils. Uranium is accumulated mainly in the sod and the upper arable soil horizon.
238U IN SOILS OF AGRICULTURAL LANDS
The major factor determining the behaviour of the 238U, which enters the land cover, is its high mass concentration in the soils, whereby the chemical properties of the element itself play a more significant role in the behaviour of the radionuclide than the stable analogues (Alexakhin et al., 1990). At the same time, for 238U present in soils in ultramicroconcentrations, in some cases, the carrier may not be 238U,
but some chemical elements being typomorphic for individual land* • * * 234 238
scapes, which is confirmed by the deviation of the ratio 234U/238U from 1 (Titaeva, 1992).
Sorption of 238U in soils depends on many factors (Alexakhin et al., 1990). The degree of oxidation of 238U in soils is +4 and +6. In a highly reducing medium, 238U is predominantly in 4-valence, and in oxidative - in 6-valence forms.
Sorption of 238U in soils depends on the chemical properties, the physical and chemical state and concentration of elements, the granu-lometric and mineralogical composition of soils, the content of organic matter, the presence of certain ions in the solution, migratory-capable colloids, complexing agents, etc. (Alexakhin et al., 1990; Alexakhin et al., 1992; Drichko, 1983).
When pH varies from 5 to 6 238U appears in soils mainly in the form of well-sorbed hydrolysed forms of uranyl, and for this reason, the transition of the radionuclide into water extract from these soils is negligible. At a small (about 1M x 10-3) content of carbonates, hydrolysis forms of uranyl prevail in the pH range of 2-12 (Iskra, Bakhurov, 1981). Therefore, even at a high pH of the solutions, the recovery of
238U from the soils does not increase. Thus, at pH 11, the fraction of radionuclide that has passed from leached chernozem to water extract is only 1.5%. In neutral and alkaline media, carbonate and phosphate compounds predominate (Sultanbaev, Grigoryev, 1979), and this can reduce and weaken the absorption of 238U in the soil (Iskra, Bakhurov, 1981). At pH above 10, the equilibrium shifts again towards the formation of hydrolysis forms, the recovery of 238U decreases. In media, rich in CaCO3 and MgCO3, the absorption strength can increase due to the precipitation of Ca2(UO2(CO3)3 and Ca,Mg(UO2(CO3)3). The minimum absorption of 238U by sod horizons of sod-podzolic and sod-meadow soils was found at pH -3, maximum (97-98%) at pH 5-8.
A significant concentration of humus, as a rule, increases the sorption of radionuclides in the soil (Rusanova, 1971; Sultanbaev, Grigoryev, 1979). Organic substances of the soil solution bind 238U to complex compounds, differing in their solubility and stability (Evseeva et al., 1974). For example, the strength of the element complexes with humic acids is 1.5-2 times higher on average than with fulvic acids (Kichigin, Nosova, 2004). Under reducing conditions at pH 5, humic acids and, at pH 6.0-6.6, fulvic acids precipitate 238U, in oxidative conditions, soluble compounds are formed. The carbonates concentration of 1M x 10-3 almost eliminates the formation of soluble hydrolysis products and prevents the humate binding at the content of the humic acids lower than 0.1M x 10-6. At their concentration of 50M x 10-9 and low (0.5M x 10-3) carbonate content, the formation of uranium humates dominates (Rachkova et al., 2010). The effect of organic substances on the processes of absorption in the soil is due to the fact that they are themselves collectors of radioactive elements, or modify the absorption properties of other sorbents, and also form mobile compounds with radionuclides. Mechanisms of sorption and fixation of 238U by soils have been poorly studied. Absorption process includes adsorption, ion exchange, precipitation, particularly due to oxidation-reduction reactions. Some authors (Evseeva et al., 1974) believe that the last of the above mechanisms is the most important in the process of sorption of uranium in the soil. At the same time, it was experimentally established that hexavalent uranium retains its valence in complexes with natural humic acid (Teterin et al., 2001). The mechanism of sorption consists of the exchange of hydrogen cations and uranyl UO2+, that is why the
capacity and rate of absorption depend on the acidity of the medium. Fe3+ ions inhibit the formation of uranyl humates due to sorption on the surface of particles of colloidal humic acid (Teterin et al., 2001) that prevents the migration of 238U in the form of absorbed or molecular solutions of uranyl humates. At the same time, the addition of humic acid to the hematite suspension reduces the absorption of the radionu-clide, the degree of reduction depends on the pH of the medium and the amount of humic acid. In addition, the dominant factor is the fraction of 238U bound to humates. In the presence of fulvates, the sorption of the radionuclide by the clayey soil components is completely inhibited (Kornilovich et al., 2001).
The sorption of 238U in soil is related to the size and specific surface area of soil particles (Arkhipov et al., 1986; Ishchenko, 1988; Ishchenko, Butnik, 1990; Rubtsov, Pravdina, 1971). For soil with a high content of physical clay and silt, the high values of the distribution coefficients were set (Drichko, 1983; Rubtsov, Pravdina, 1971; Rud-nick, Gao, 2003; Saric et al., 1995; Titaeva, 1992). The content of 238U in the solid phase of clay soils is 200-7.9 * 105 times higher than in the equilibrium solution. For desert-sandy, meadow, meadow-swamp soils, light and typical sierozems, direct dependence of the absorption and mobility of 238U on the content of humus, potassium, calcium and physical clay has been established (Ishchenko, 1988; Ishchenko, Butnik, 1990). According to some estimates, the content
of 238U
in the clay
fraction is up to 3 times higher than its concentration in unfractionated soil. At the same time, little information indicates a lack of a close relationship between the sorption of radionuclides and the granulometric composition of soils. For example, for soils in the taiga zone, up to 50% of radionuclides of the uranium series are sorbed on the surface of particles with a size of 1-100 ^m, and consolidation of aggregates in this range is accompanied by a decrease in the content of radioelements.
Fixation of 238U in soils occurs due to its sorption by organic substances and clay minerals (illite, montmorillonite and kaolinite) (Alexakhin et al., 1990; Ratnikov et al., 2007; Rusanova, 1998; Taskaev, 1979). Limestone weakly absorbs 238U, and crystallised earth silicon does not absorb the uranium at all (Alexakhin et al., 1990; Sav-enko, 2001). The sorption centres of two types are significant for sorp-
tion of radionuclides by clay minerals. These centres are located on the basal surfaces of mineral particles, where ion exchange takes place, and on the lateral faces with the formation of strong sorption complexes. According to Kornilovich et al. (2001), uranium was primarily absorbed on the lateral faces of clay minerals The X-ray diffraction analysis (Iskra, Bakhurov, 1981) showed that binding takes place according to the ion-exchange mechanism. 238U competes with cations Ca2+, Ba2+, Mg2+ for sorption centres, but not with Na+ and K+. According to the structural data, uranyl forms dissolved and absorbed by clay minerals are identical. Under sub-acidic conditions, they correspond to the hex-ahydrated ion (Kornilovich et al., 2001). In the pH range 6-12, the sorbed forms of uranyl are represented by UO2OH+ and (UO2)2(OH). Absorption of UO2OH+ predominates at pH 6, with a decrease in acidity, the ratio of sorption of (UO2)2(OH) increases. The isotherms of 238U (VI) sorption by clay minerals have a maximum at pH 3-6 with a sharp decrease in the absorption parameters to the sides of this interval (Kornilovich et al., 2001).
Influence of the capacity of cation exchange on the sorption of 238U by soils is practically absent. For example, the removal of organic matter and free ferrous oxides from the clayey fraction of silty loam reduces its cation-exchange capacity, without changing the absorption rate of 238U. Along with this, there is an opinion (Kornilovich et al., 2001) that in surface media the Fe content controls the pH-dependent adsorption of 238U. Oxide forms of metals (goethite, hematite, boehm-ite, etc.), on the one hand, themselves serve as sorbents of a radionu-clide, and on the other - when they are adsorbed, these oxides form films on the surface of clay minerals changing their ion-exchange properties. The distribution coefficient of the radionuclide between solutions of its salts and Fe minerals reaches 2 * 106 ml g-1, which is 100 times greater than for secondary minerals.
Behaviour of 238U and its forms in soils are determined both by the state of the radionuclide itself and by the physical and chemical properties of the soils. The original form of the radionuclide fallout plays an essential role in the processes of sorption and desorption of
238 * 238 * * *
238U. For example, the sorption of 238U by 5 types of soil (17 soil differences) with contrasting physical and chemical properties and indicators of texture when introducing a radionuclide in the form of a solu-
tion of uranyl nitrate is 82.2-99.2%, desorption by water is 1.1-7.5%, by 1N ammonium acetate solution - 2.4-40.8%, and by 2N HNO3 solution - 51.3-79.2% (Alexakhin et al., 1985) (Table 4).
Table 4. Rate and strength of sorption of 238U in different types of soils (in relation to the original quantity), %
Soil type Sorption Desorption
H2O 1N CH3COONH4 2N HNO3
Sod-podzolic 97.0 1.7 2.4 54.6
Dark-grey forest 98.4 1.2 11.7 59.5
Grey forest 96.9 1.4 3.2 51.3
Brown forest 88.4 1.2 36.2 79.2
Chernozem
leached 98.5 1.2 13.4 68.4
podzolic 82.2 5.9 24.1 51.3
usual 85.9 7.5 9.6 57.0
southern 90.9 3.6 25.6 68.4
firm 98.1 1.1 22.2 76.4
Meadow chernozem 89.1 5.6 22.3 69.5
Sod 82.2 5.9 24.1 51.3
Meadow 99.2 1.5 23.6 60.8
Humic-gleyed 98.1 1.1 14.8 72.8
Meadow-swampy 87.5 3.6 23.0 69.2
Dark kastanozem 93.6 3.8 32.8 77.2
Kastanozem 87.0 4.8 40.8 61.2
Light kastanozem 87.4 4.4 26.4 72.0
In works of Arkhipov et al. (1986), there were established the limits of the absolute and relative content of gross, acid-soluble and exchangeable forms of natural radionuclides (238U, 232Th, 226Ra, 210Po and 210Pb) in the soils of the agricultural territory of the country. The gross content of 238U is 0.2-0.5 pCi g-1 in sod-podzolic soils, 0.6-1.0 pCi g-1 in chernozems, 0.7 pCi g-1in kastanozems, 0.8 pCi g-1in brown semi-desert soils, 0.7 pCi g-1in sierozems. The content of the acid-soluble forms of 238U is 6-26% of the gross content, depending on the agrochemical properties of the soil. The amount of exchangeable and
water-soluble forms of 238U in sod-podzolic soils varies between 1840%, in chernozem - 9-12% of gross soil content (Arkhipov et al., 1986).
By this means, a generalisation of the accumulated data indicates that intensive and strong absorption of 238U in soils occurs mainly due to organic matter and clay minerals. The processes of ion exchange, complexation, hydrolysis and oxidation-reduction designate the fixation of 238U in soils.
To date, there is a lack of the investigation of the temporal aspects of 238U absorption processes in various soils. The existing research methodologies do not allow evaluating fully the transformation of forms and mechanisms of radionuclide binding in soils. One of the ways to overcome these difficulties can be carrying out complex 238U full-scale and model studies.
Study of Ishchenko and Butnik (1990) conducted on different soil types in Central Asia gives a comprehensive assessment of the effect of 238U on the bioavailability, depending on the time of interaction of the radionuclide with the soil.
Transformation of the 238U forms in time was studied on a typical sierozem. Soil samples were taken from the plots, where cotton was grown in the first and second years, in the third year - wheat. The concentration of 238U was 53 mg kg-1 of soil.
238U from soil samples was successively extracted with distilled water, acetate-ammonium buffer solution (pH 4.8), solution of citric acid (0.033M), mixture of citric (0.033M) and oxalic (0.5M) acids, hydrochloric acid. The content of the 238U forms was calculated in %, assuming the initial value to be 100%.
It was found that uranium, introduced into the soil as a solution of 238U(NO3)2, comprises mainly exchangeable forms with soil components in desert-sandy soil, light and typical sierozems, and in meadow soils, the uranium interaction with soil components results in acid-soluble form. From such soils as desert-sandy, light and typical siero-zems, most of the 238U passed into aqueous and acetate-ammonium extracts (from 48.4 to 51.5%), into which readily soluble, exchangeable compounds and organomineral complexes are displaced. There is an inverse relationship between the physical and chemical soil properties and the amount of water-soluble and exchangeable forms of 238U, and
with the number of fixed 238U compounds, the relationship is direct. Correlation coefficients between the amount of radionuclide in water-soluble and exchangeable forms and all indicated soil parameters turned out to be rather high (R = 0.73-0.88).
The ratio of water-soluble, exchangeable, acid-soluble and fixed forms had been determined for three years in order to study the influence of the time factor on the transformation of the occurrence of 238U in a typical sierozem, the ratio of the forms of radionuclide was determined two weeks after its introduction into the soil. A year later, the number of exchangeable compounds of 238U increased, and the number of acid-soluble compounds decreased. After two years of radi-onuclide stay in the soil, no further changes in the content of the vari-
23 8 • • . я •
ous forms of 238U were detected. With a 3-year incubation of U in the soil, the number of fixed compounds increased significantly in comparison with their content in the first year, while the fraction of the exchangeable forms of the radionuclide remained unchanged (Ratnikov et al., 2019; Kornilovich et al., 2001) (Table 5).
Table 5. The content of various forms of 238U, depending on the time of their stay in the soil, % of application
Timing of determination Water soluble Exchangeable Acid-soluble Fixed
In 2 weeks 7.5 41.5 27.6 23.8
In 1 year 5.9 47.0 21.6 25.5
In 2 years 4.1 49.1 20.8 25.6
In 3 years 2.7 47.5 10.7 39.2
The amount of water-soluble and essentially acid-soluble forms has decreased. This occurs because acid-soluble compounds of 238U have been transformed into fixed ones with time. As for the water-soluble compounds of the radionuclide, it is possible that some of them migrated down the soil profile, and a part entered the plants, so their content in the plough horizon of the soil decreased.
Describing, in general, the behaviour and migration of 238U in biosphere, it should be noted that all this occurs under the influence of various factors acting simultaneously. For the chemical element as a whole, it will depend mainly on three main parameters: 1) the chemical
properties of the element; 2) the physical and chemical conditions of the environment; 3) the forms of finding the element in the initial rocks. The second factor is a function of climatic and partly geological conditions; the third one is completely determined by geological conditions.
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