ИСТИНА

In this study, the biochemical reduction of both nitrate and sulfate in U-containing aquifers of the Novosibirsk Plant of Chemical Concentrates (NPCC) has been investigated experimentally and thermodynamically. NPCC Sludge depository was built in 1960s and it is still in use. Uranium concentration in subsurface water in several zones reached 1-2 mg/l, the total nitrate concentration -up to 5000 mg/l, sulfates – 1200 mg/l. Uranium transport in subsurface depends on geochemical properties of the aquifer: sorption capacity of the rocks, TOC, Eh, oxidizers concentration (O2, NO3, Fe3+ etc.) and biogeochemical processes. We have studied chemical and microbiological characteristics of water samples in polluted and none polluted areas and got main data for biogeochemical modelling of Uranium migration capacity. We have found active microbial community with domination of aerobic organotrophic, denitrifying, sulphatereducing and ironreducing bacteria. In laboratory experiments we have studied the possibility and some technical aspects of aquifers in situ bioremediation process by organic compounds injection. It was observed that Eh decrease up to -397 mV (14 days after the start of the experiment) has a distinct effect on the denitrification and uranium precipitation as UO2(s). Nitrate was reduced to N2 with a temporary accumulation of the intermediate nitrite on the fourth day. According to the X-ray fluorescence analysis and thermodynamic calculations, more than half of the uranium is deposited in the first stage as UO2+x oxides, whereas the rest do so together with the sulfides in the reducing environment. After the laboratory stimulation test we found a formation of the significant amount of iron sulfides sediment (black colour) in all samples. After biogeochemical modelling in PHREEQC software we found that the most important microbiological process for Uranium immobilization is the oxidisers consumption in bacterial respiration processes. After that redox-dependent uranium reduction can be expected. In the presence of sulfate ions, further anaerobic processes of microbial sulfate reduction and iron reduction lead to formation of iron sulphide, which plays a significant role as an antioxidant buffer in case of dissolved oxygen migration. Thus, biotic sulfatereduction leads to formation of iron sulphidic phases, forming the sulphidic reductive barrier that has buffer properties to oxidation. This barrier can be considered as an antimigration barrier for metals and radionuclides.