Biogeochemistry of uranium and thorium

Thorium and uranium reoffered to the group of actinides has been known along with protactinium as early as at the beginning of the past century (Arkhangelskiy, Rikhvanov, 2001). The concept about chemistry, occurrence and behavior of these natural nuclides was developed mostly in terms of advancement of nuclear engineering and achievements in chemistry. At present we have some information on their chemical properties (Spitsyn, Martynenko, 1991; Perelman, 1979; Titayeva, 2000; Rikhvanov, 2002), geochemical behavior in different types of soil, rocks and indicators (Yevseeva, Perelman, 1962 at al.) as well as on their influence on living organisms (“Harmful”, 1990; “Radioactivity…”, 1971; Balabukha, Fradkin, 1958; Moiseev, 1984; et al.).

Nevertheless, the problem of quantitative content of these elements in the segments of living environment as well as biogeochemical aspects of their behavior have not been studied yet up to now. Presumably, this fact is explained to a considerable extent by the analytical difficulties of their determination in living substance (LS). Contemporary kinds of analysis are favorable to accumulation of information on content of these elements in different natural objects.

The urgency of such research is conditioned by the fact that chemical toxicology has not been virtually investigated, radiation toxicology of those elements being well studied, which is a large gap in ecogeochemistry (Ivanov, 1997).

Thorium and uranium are referred to ultramicroelements the content of which in living organisms, as a rule, does not exceed 10^-5 %. Uranium and thorium are elements occurring in the environment in the amount being of practical interest in contrast to other actinides. In LS their content is incomparably less. The first quantitative characteristics of U and Th in living organisms are given in research of А.P. Vinogradov (1932), Е. Burkser (1931), J. Hoffman (1942, 1943). Thus, Burkser et al (1931) presents thorium content in ash of cancer forming 0,5 × 10^-3 %, but converting into LS 4,2 × 10^-5 %. In the same source there are data on fish (goby, Cottus gobio), in the ash of which the content of Th amounts 0,8 × 10^-3 % and 4 × 10^-5 % respectively. А.P. Vinogradov (1932) gives the results of analytical data on the content of this element in craw fish (Astacus fluviatilis) at the rate of 4 × 10^-6 % in LS. The author notices that uranium was found in the egg, but the order of its concentration was not defined. The earliest research giving the information on uranium concentration in LS is the work of German scientists of the 40’s of the past century. Thus, Hoffman (1942, 1943) determines uranium concentration in many organs and tissues of plants, animals, and humans. For example, in mushroom ash (Aspirigillus niger) uranium concentration is stated to be 6,37 × 10^-6 %, but in epiphyte causing grass disease Ustillago carbo – 6,08 × 10^-3 %, that, perhaps, indicates the process of uranium concentration in the given case. For eggs the author states its content in different parts: egg shell contains 4,03 × 10^-6 % of uranium, but its inner content (white and yolk) – 8,84 × 10^-8 %. Fluctuations in uranium concentrations make up orders in animal’s organisms depending on organ or tissue and change in the range from 3,17 × 10^-8 % in brains up to 1 × 10^-1 % in hypophysis (ash). The latest research has revealed the fact that ground plants are characterized by more stable values of uranium contect: 5 × 10^-5 % (D.P. Maluga), n × 10^-5 % (А.I. Perelman), 6 × 10^-5 % (P.L. Kanon) (ash, by (Beus, 1976), 2 – 30 mg/kg (dry matter, V.V. Yeramakov). V.V. Yermakov (2008) presents the data on fluctuation of uranium content in living organisms from 1 - 10 mg/kg of dry matter (dm) in animals to more than 40 000 mg/kg in bacteria (dm) according to literature review. A wide range of U content in plants is given by J. Bowen [6] – from 5 to 69 mkg/kg (dm). G.N. Sayenko (1992) points out that the scatter in actinide content in marine organisms makes up 1× 10^-6 - 4 × 10^-5 % (dm). A number of authors reveals the concentration of uranium in LS in points of natural abnormalities and sites of uranium industrial enterprise operation (Kabata-Pendias А., Pendias, 1989; Beus, 1976; Rikhvanov, 1997; Baranovskaya, Rikhvanov, 2002 et al.). In particular, uranium concentration in cade amounts up to 1100 mln^{-1 in ash in vicinity of uranium concentration plant. It is stated (Schaklett et al., 1989), that the maximum uranium content in trees growing on the soil with uranium mineralization amounts 2,2 mg/kg of ash, but in wormwood growing in vicinity of the plant producing phosphorous fertilizer uranium is accumulated up to 8 mg/kg of ash.}

At present, the information on thorium content in the environmental objects is less available than that for uranium. Thus, in the works of А. Kabat – Pendias and G. Pendias (1989), as well as in reference of V.V. Ivanov (1997) researches of different authors are pointed out where thorium concentration in ground plants are presented in the range from < 8 – to < 1300 mkg/kg (dm), as well as for vegetables from < 5 to 20 mkg/kg (dm). S.М. Tkalich (1963) points out that the average thorium content in ash of land plants amounts n × 10^-5 %, the highest content is – 0,1 %, with accumulation contrast in different species up to 2 000 times. Reference materials of C. Reimann, P. de Caritat (1998) present the results on some differences in thorium content in moss of Norway (0,08 mg/kg) and Germany (0,094 mg/kg) and lichen – 1,8 mg/kg (Germany). In the work of А.А. Kist (1987) the research results of N. J. M. Bowen, G.V. Iengar et al. (1978) and some other authors are summarized (Bowen, 1966; 1968; 1974) as to the content of that and other elements in organisms of plants, animals, and man. In fact it is stated that according to the standards Th concentration makes up: cabbage – 0,092 mg/kg (dm), spinach – 0,12 mg/kg (dm). In Siberian region numerous investigations on accumulation of some elements, including thorium, in plants are carried out by the group of researchers (“Geochemistry…”, 2002) on the territory of the Large Vasyugan marsh (Table 2).

Table 2.

Thorium content in the plants of the Large Vasyugan marsh,
mg/kg (dm) (“Geochemistry…”, 2002)

Plants	Eutrophic sites	Mesotrophic sites	Oligotrophic sites
Trees	0,07	0,09	0,16
Bushes	0,22	0,14	0,14
Grass	0,14	0,16	0,15
Moss	0,61	0,18	0,33

The analysis of the values as well as those presented in the latest reviews (Yermakov, 2008), has shown that concentration of radionuclides changes under the influence of numerous factors that affect the accumulation of other elements. In particular, it depends on species features, type of tissue or organ, age-related changes, properties of elements and a number of other factors mentioned earlier (Rikhvanov et al., 2006; Baranovskaya, 2009). Thus, one of rather significant causes for accumulation of those elements in living organisms is their high concentration in the environment. High levels of uranium and thorium accumulation in living substance are, as a rule, connected with the sites of their concentration in natural objects and reveal the presence of natural and man-made geochemical abnormalities. For example, uranium accumulation is known to achieve 110 mg/kg in ash of plants in the sites of deposits of this element (Kabata-Pendias, Pendias, 1989).

For such sites the indicating values of thorium and uranium ratio in soil, dust aerosols are stated (Shatilov, 2001; Arkhangelskiy, 2001; Yazikov, 2009). We have revealed the indicating values of element concentration and changes in their relationship in such living systems as human hair, blood, amphibious tissue, egg shell etc. (Baranovskaja, Rikhvanov, 2002; 2004; Kuranova et al., 2005) for the sites with the anthropogenic sources producing them. The study of environment and human biosubstrates in the territory of Tomsk region, Tomsk oblast with complex biogeochemical conditions of anthropogenic-natural origin proves the conclusion on specific accumulation of these elements as well (Rikhvanov et al., 2006).

Thus, the analysis of literature data as well as our own research has shown that the element concentration in living substance and their ratio values could be applied in ecological-geochemical zoning of the territory to define the presence of natural-anthropogenic abnormalities of natural radionuclides in the environment.

Analysis of spatial distribution of chemical elements in various media in the territory of Tomsk Oblast (Fig.1, 2) has shown that zones of their elevated concentrations in water, scale, and soil, as a rule, coincide with those stated by the results of accumulation in living substance – hair, blood, and bioptic material of pathologically changed thyroid glands. But in this case there are some peculiarities. Thus, elevated uranium concentration in blood, thyroid glands, water and scale in some regions of the oblast has a reversed character in comparison with their accumulation in soil and hair.

The maximum high concentrations of this element in pathologically changed thyroid glands are of linear zone character with East-West direction through the territory of Tomsk region characterized by the complex anthropogenic load.

High thorium concentrations are also connected with the territory of Tomsk region and typical for soil, blood, hair, and thyroid gland, but are not the case for drinking water scale.

Such features in element distribution result in the conclusion on migration of investigated elements in different forms along the «environment – living organism» chain. In fact, the intensity of uranium and thorium intake into living systems depends on the kind of chemical compounds (Th⁺⁴, U⁺⁴, U⁺⁶), medium conditions (Еh, рН), ways of their intake (breathing or along the food chain), as well as properties of living system itself and some other factors. Analysis of Th/U ration distribution in children’s hair composition of Tomsk oblast has shown that in contrast to other media its values higher than 0,7 are typical for narrow zone in the territory of Tomsk oblast connected with the zone of Siberian Chemical Combine impact – a nuclear fuel cycle plant (Fig.3). Taking into consideration the prevailing direction of wind transfer one could suggest that such a pattern of distribution is most likely explained by the effect of air constituent reflected in hair composition. Besides, this value is high not due to thorium constituent, against the background of both high uranium and thorium concentrations with some predominance of the latter in separate sites.

To solve the problem of potential mechanism of element entry to living system we calculated the water migration coefficient by А.I. Perelman (1962) for uranium. This index allowed for the statement of the fact that its maximum intensity is typical for Zyraynsk region of Tomsk oblast (Fig.4). One could assume that on the territory there is a favorable geochemical environment for more active uranium migration (elevated concentrations connected with lignite in the zone of aeration, hydrocarbon water composition is likely to result in fluid uranyl-carbonate complexes etc.).

А. Drinking water scale B. Soil

C. Human blood D. Children’s hair

E. Thyroid gland F. Drinking water

Fig. 1. Zoning of Tomsk oblast territory in terms of uranium content in the natural medium and human tissues

А. Scale B. Soil

C. Human blood D. Human hair

E. Thyroid gland

Fig. 2. Zoning of Tomsk oblast territory in terms of content in the natural medium and human tissues

Fig. 3. Zoning of Tomsk oblast territory in terms of uranium-thorium ratio in children’s hair composition

Fig.4. Coefficient of uranium water migration in the regions of Tomsk oblast.

Thorium does not show such migration capacity in this region. It is inert, and in its distribution map it is characterized by low accumulation level nearly in all media. Similar conditions are likely to be typical for Bakhchar, Asino, and Teguldet regions. Tomsk region is characterized by thorium accumulation in different media against the background of uranium water migration close to one.

Presumably, increase in fluid uranium forms is reflected in its significant accumulation in blood of the population living in these territories, that defines the region differentiation according to radioelement relationship in «water - blood» system and «hair - blood», «blood – thyroid gland» biosystems (Fig.5).

Fig. 5. Ranking regions of Tomsk oblast in terms of uranium relationship in different media: 1 – 16 – regions: 1 – Tomsk, 2 – Asino, 3 – Chainsk, 4 – Krivosheino, 5 – Shegarsk, 6 – Teguldet, 7 – Zyryansk, 8 – Parabel, 9 – Bakhchar, 10 – Kolpashevo, 11 – Verkhneketsk, 12 – Alexandrovo, 13 – Kargasok, 14 –Pervomayskiy, 15 – Kozhevnikovo, 16 – Molchanovo.

Special location of Zyryansk region (№ 7 in all graphs) with high concentration of uranium in such media as water and blood proves the data on significant intensity of the element migration and its possible intake with drinking water. For several residential areas of this region the presence of high uranium content in water has been known before, which probably comes into it from brown coals enriched with this component (Rikhvanov, 1997). Lower concentrations in hair and thyroid glands indicate that it is most likely to be a form of element in which it is excreted from organism and not accumulated in depository media.

Of particular interest is the fact that the residential areas are divided into three main groups of linear character in terms of features of uranium accumulation in water and human hair depending on its concentrations in these media.

It should be noted that for Tomsk region significant element concentrations are observed in thyroid tissue composition. That may be connected with selective uranium accumulation (Denisova et al., 2009) or peculiarities of its intake in this region, as the accumulation analysis of residual areas shows that the highest content is determined in composition of thyroid glands of residents from samus settlement as well as Seversk town located in the SCC impact zone.

Influence of Alexandrovo and Kargasok region location of Tomsk oblast in the analysis of «blood - hair» system permits the suggestion of prevailing other uranium intake mechanisms in living systems in comparison with those in Zyryansk region. These regions are described as oil-and-gas bearing ones that is likely to be reflected in features of uranium accumulation in living system composition in these areas.

Therefore, the analysis of data on spatial distribution of radioactive elements over the territory of Tomsk oblast as well as indicators of region ranking in terms of system interaction allows for suggestion of different intake, distribution, and accumulation mechanisms for uranium and thorium in living systems. Their accumulation in depository systems – hair, pathological forms of thyroid glands points to the fact that the elements entering with water, air, from soil along the food chains, are not excreted from organism, but stay in it. This fact requires for special analysis of medical health indicators in these regions, since uranium is known as a renal poison, but thorium has capacity of respiratory depression, hence, one could assume a higher specific disease rate for population. Therefore, it is necessary to arrange the prophylaxis of population disease in such areas.

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