Contamination of groundwater by uranium, nitrate, ammonium and sulfate near uranium sludge storages due to degradation of engineering safety barriers is an urgent problem during their long-term operation. The purification of such multicomponent contaminants is a complex task and one of the promising methods for it is in situ bioremediation using the metabolic potential of aborigenic microflora. The work focused on the geochemical, geological, and microbiological parameters of groundwater with multi-component contamination near uranium sludge storages of four chemical plants. In conditions of extreme nitrate contamination (up to 15 g/L), denitrifying bacteria were found to be the dominant group of microorganisms in all cases. In conditions of nitrate-ammonium contamination, bacteria responsible for the Anammox process were found. In laboratory, optimal conditions were selected to stimulate microflora to promote nitrate removal. To do this, sources of carbon and phosphorus (sucrose, acetate, whey) were added to the water samples in concentrations necessary for the complete nitrate removal by microbial denitrification. The experiment was carried out at a temperature of 10 C in hermetically sealed vials. Uranyl nitrate was added to the samples at a concentration of 5 mg/l for uranium. It has been found that nitrate removal contributes the cycle of anaerobic processes of authigenic sedimentation due to sulfate and iron reduction processes, which provide the formation of a mineral geochemical barrier for uranium immobilization. As a result of the experiment, after 3-6 months, depending on the concentration of nitrate in the groundwater sample, the uranium content in the liquid phase decreased by 92-98%. The most significant uranium removal effect was achieved by adding a sample of sodium dihydrogen phosphate (100 mg/l). Elemental analysis of the resulting amorphous biogenic phases showed the presence of sulfur, iron, phosphorus, and uranium in them. Modeling using HCh 4.4 (MSU) and PHREEQC 2.18 (USGS) software packages was carried out to determine the optimal conditions for the formation of a biogeochemical barrier for in situ uranium immobilization in the aquifers near uranium sludge storages of four different plants. Based on the modeling, zones optimal for in situ biogeochemical barrier and the optimal composition of solutions for microorganisms stimulating were selected. Conducting field trials at two enterprises showed the high efficiency of biogeochemical barriers for removing nitrates and immobilizing uranium (95-100%) in reduced, poorly soluble forms.
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In situ biogeochemical barriers for contaminated groundwater treatment near uranium sludge storages
Published: 16 March 2023 by MDPI in The 7th International Electronic Conference on Water Sciences session Water Quality and Advanced Water Treatment and Reuse
Keywords: groundwater, contamination, biogeochemical barriers, uranium