Geochemistry of uranium in organic sediments of small lakes in the Baikal region

Vosel Yu.S.

V.S. Sobolev Institute of Geology and Mineralogy sb ras, Novosibirsk, Russia

Vosel@yandex.Ru

Ways of migration and forms of accumulation of elements in the components of the environment - one of the major issues in modern geochemistry. A very good object of these studies is the sediments of small continental of lakes, as they are the unbroken record of events of the recent geological past of the earth.

The main goal of this work - comparison of the ways uranium accumulates and distribution of the forms of its existence in the sediments of two lakes with organogenic and mixed organogenic-clastic type of sediments and in soils of catchment areas. Baikal region.

To solve the problem two lake systems located in the Baikal region were analyzed. They differ from each other by various signs: landscape, climatic conditions, hydrological conditions, hydro-chemical parameters of water and amounts of organic matter in sediments.

The lake Skazka is located in taiga landscape of the southern tip of Lake Baikal in the north-western slope of the granite Hamar-Daban mountain range. This lake is fed solely by precipitation: rain, snow.

Lake Dukhovoye is located on the eastern shore of Lake Baikal in the northwest foot Golondinsky ridge, on the metamorphic complex folded by granitoids. This lake is flowing, and is fed also by groundwater and precipitation.

Analytical work was carried out in the IGM SB RAS, using appropriate methods of sample preparation and analysis. Determination of ions (Ca²⁺, Mg²⁺, Na⁺, K⁺) in water was measured by atomic absorption. In the analytic center of the same Institute made ​​X-ray diffraction analysis of mineral matter. The form of U in the sediments were determined by the method of sequential extraction developed by E. Klempt [3]. Next, the samples after sequential extraction were measured by alpha-spectrometry, the same method were using during the measurement of total concentration of uranium.

The water in both lakes is the first type of Alekin classification (HCO₃^2-> Ca²⁺, Mg²⁺), and has a low total mineralization (Lake Skazka - 14mg/l and the lake Dukhovoye - 90mg/l). In the lake Skazka the water has pH = 6.9, in the lake Dukhovoye it has pH = 9.6. In the bottom sediments of the lake Dukhovoye conditions is no hydrogen sulfide, and in the lake Skazka we are observers a slight presence of H₂S.

According to X-ray analysis the studied lake sediments can be attributed to terrigenous-organogenic and organogenic type. Lake Skazka - terrigenous-organogenic: in the sample is dominate quartz, plagioclase, mica, chlorite, but also quite a number of organic matter. Lake Dukhovoye - organogenic: the sediment is dominate organic matter, but there are traces of terrigenous material.

These two lakes are quite different from the total concentration (table). In the sediments of lake Skazka we have a reduction of uranium content as compared with soils of watershed (sediments - 0.76 ppm; soil - 1.46 ppm). In Lake Dukhovoye we observed a strong increase in sediments with respect to catchment soils (sediments - 10.1 ppm; soil - 1.48 ppm). Now consider the distribution of uranium in different fractions. In the case of Lake Skazka increase in individual fractions was observed only in the case of exchangeable ions fraction, of organic matter and amorphous silicates. In this case of ²³⁸U/²³⁴U activity ratio in the sediments does not increase much with respect to soils, indicating that the intense precipitation of uranium in sediments from the water does not occur [1, 2, 4]. The other fractions are observed to reduce the uranium content relative to the soil. Also, there are no increase in ²³⁸U/²³⁴U activity ratio, which indicates that the precipitation of uranium from water in to these fractions is not happening. In the case of insoluble residues, on the basis that there is a reduction of the total content of uranium, and that the activity ratio of uranium isotopes ²³⁸U/²³⁴U = 1, it is safe to say that in the insoluble residues enters the uranium, which is in part of the terrigenous sediment. In the case of Lake Dukhovoye observed increase in the total contents of uranium in all fractions of the sediment with respect to the soil. In addition, an increase in activity ratio of isotopes in sediments relative to soils. The greatest accumulation of uranium occurs in the organic matter, as a result of formation of organic complexes of the uranyl ion. This indicates precipitation of uranium to the sediments from the solution. Based on the fact that there is a strong increase of the uranium content in lake sediments with respect to soils it can be assumed that the most of uranium in lake water gets together with the groundwater. This is an interesting fact that the analysis showed an increase in uranium content in the sediments relative to soils in the insoluble residue. We can assume that in this case part of the uranium associated with organic matter fell into the insoluble residue. This issue requires further study.

From the above we can conclude the following: increasing the concentration of uranium in organic sediments relative to soils occurs mainly through the loss of uranium from the solution in the form of organic complexes of the uranyl ion.

	238U(Bk)	234U(Bk)	238U (%)	234U (%)	238U/234U
Lake Skazka (lake sediments)
Exchangeable ions	0.07	0.02	15	3	0.2
Carbonates	0.01	0.02	2	3	1.6
Oxides and hydrox. of Fe and Mn	0.02	0.03	5	5	1.1
Organic matter	0.15	0.21	32	40	1.4
Amorphous silicates	0.05	0.07	11	13	1.3
Residuals	0.16	0.18	34	35	1.1
Σ (Bk)	0.47	0.52	100	100	1.1
Total conc. (ppm)	0.76	1.46
Lake Skazka (soils of catchment areas)
Exchangeable ions	0.05	0.09	6	9	1.8
Carbonates	0.13	0.13	15	13	1.0
Oxides and hydrox. of Fe and Mn	0.13	0.16	15	16	1.2
Organic matter	0.09	0.12	10	12	1.3
Amorphous silicates	0.03	0.05	3	5	1.7
Residuals	0.45	0.45	51	45	1.0
Σ (Bk)	0.88	1	100	100	1.1
Total conc. (ppm)	1.46
Lake Dukhovoye (lake sediments)
Exchangeable ions	0.1	0.15	2	2	1.5
Carbonates	0.27	0.46	5	5	1.7
Oxides and hydrox. of Fe and Mn	0.96	1.53	16	16	1.6
Organic matter	1.8	2.87	30	30	1.6
Amorphous silicates	0.06	0.09	1	1	1.5
Residuals	2.86	4.52	42	47	1.6
Σ (Bk)	6.05	9.62	100	100	1.6
Total conc. (ppm)	10.1
Lake Dukhovoye (soils of catchment areas)
Exchangeable ions	0.02	0.02	2	2	1.0
Carbonates	0.1	0.1	11	10	1.0
Oxides and hydrox. of Fe and Mn	0.12	0.15	14	16	1.3
Organic matter	0.1	0.11	11	12	1.1
Amorphous silicates	0.09	0.13	10	14	1.4
Residuals	0.46	0.45	52	47	1.0
Σ (Bk)	0.89	0.96	100	100	1.1
Total conc. (ppm)	1.48

Table. The results of sequential extraction of U in the sediments of the studied lakes

This work was supported by Interdisciplinary Integration Project no. 125 from the Siberian Branch of the RAS.

References:

1. Chabaux, F., Riotte, J., Dequincey, O., (2003) U-Th-Ra fractionation during weathering and river transport. Rev. Mineral. Geochem. 52, 533-576.

2. Chalov P.I. (1991) Development of the ideas of V. I. Vernadsky in studies of the natural separation of the even isotopes of uranium. Nauka, Moscow, 192-206pp.

3. Klemt E. et al. // Proc. 5^th Int. Conf. “Environmtbtal Radioactivity in the Arctic and Antarctic”. St. Petersburg (Russia). – 2002. – p. 67-70.

4. Titaeva N. A. (2000) Nuclear geochemistry. Publ. Moscow State University, Moscow, 336 pp.