Optimization of Environmental Conditions for Microbial Stabilization of Uranium Tailings, and the Microbial Community Response

Potential risks of in-situ microbial remediation of uranium-contaminated groundwater: Uranium release and remigration

After in-situ microbial remediation of uranium-contaminated groundwater, the environmental problems caused by the remigration of uranium immobilized in the aquifer due to microbial decay require attention. In this study, uranium-containing Leifsonia sp. spoilage was produced by natural decay of uranium-adsorbed Leifsonia sp.. Batch experiments were used to investigate the influence on uranium release from the Leifsonia sp. spoilage under the conditions of different pH, action time, and concentrations of the metal ions K+, Ca2+, Na+, Mg2+, and Zn2+. The remigration of immobilized uranium was simulated by the Leifsonia sp. spoilage sand column experiment. The release rate of uranium initially decreased with increasing pH, increased with increasing contact time, and then remained unchanged with increasing time; the release rate of uranium peaked at 4.98 % at pH 3 and 120 h. Compared to the absence of metal ions, the release rate of uranium increased by >10 % under the action of metal ions, in which Ca2+had the greatest effect, up to 18.4 %. Furthermore, U(IV) in the spoilage was oxidized to U(VI), resulting in uranium release, and uranium release was related to hydroxyl, carboxyl, amino, and amide groups in the spoilage. The release kinetics of uranium were consistent with those of the Elovich and double constant models, indicating that the release of uranium was a multifactorial integrated chemical desorption process. In addition, the remigrated uranium in groundwater had two components: some of the uranium was released from the spoilage and migrated independently as uranyl ions, and some was present in the spoilage and migrated with the spoilage, the amount of the former being much greater than that of the latter. This study provides a theoretical basis for the rational use of microbial in-situ remediation of uranium-contaminated groundwater by in-situ leaching of uranium.

Biomineralization of uranium by Desulfovibrio desulfuricans A3-21ZLL under various hydrochemical conditions

Uranium pollution in groundwater environment has become an important issue of global concern. In this study, a strain of Desulfovibrio desulfuricans was isolated from the tailings of acid heap leaching, and was shown to be able to remove uranium from water via biosorption, bio-reduction, passive biomineralization under uranium stress, and active metabolically dependent bioaccumulation. This research explored the effects of nutrients, pH, initial uranium and sulfate concentration on the functional groups, uranium valence, and crystal size and morphology of uranium immobilization products. Results showed that tetravalent and hexavalent phosphorus-containing uranium minerals was both formed. In sulfate-containing water where Desulfovibrio desulfuricans A3-21ZLL can grow, the sequestration of uranium by bio-reduction was significantly enhanced compared to that with no sulfate loading or no growth. Ungrown Desulfovibrio desulfuricans A3-21ZLL or dead ones released inorganic phosphate group in response to the stress of uranium, which associated with soluble uranyl ion to form insoluble uranium-containing precipitates. This study revealed the influence of hydrochemical conditions on the mineralogy characteristics and spatial distribution of microbial uranium immobilization products. This study is conducive to the long-term and stable bioremediation of groundwater in decommissioned uranium mining area.

A bidirectional kinetic reaction model to predict uranium distribution in permeable reactive bio-barrier with high-sulfate environment

The use of permeable reactive bio-barriers (Bio-PRBs) is a developing method for remediation of uranium groundwater pollution. However, some remediation effects are difficult to estimate when because of the subsurface environment. Advanced knowledge of uranium migration and reactions in Bio-PRBs is crucial for successful practical application. In this study, a bidirectional kinetic reaction model was developed for predicting uranium reduction in a Bio-PRB system. The research demonstrates that the model is able to predict the uranium migration and rapidly evaluate the Bio-PRBs performance. The results show that contact period and microbial growth are the key factors that affect the remediation performance of Bio-PRBs. Microbial growth could lead to a decrease in hydraulic conductivity (K). The hydraulic conductivity loss in the free microorganisms (FM) group was 0.8-2.3 m/d, which was significantly smaller than the immobilized microorganism (IM) group (0.8-5.2 m/d). Compared to the IM group, the simulation results reveal that longer contact reaction period improves the remediation performance of SO42- and uranium by 32.6% and 21.7%, respectively. The bidirectional reaction between microorganisms and pollutants leads to a decrease in the remediation efficiency. In addition, the model can be used to design standard Bio-PRBs for real field of uranium contanminated groundwater. To meet the remediation goal of groundwater, the width of IM group needs to be increased to 250 cm while 500 cm for FM group. Therefore, IM-PRBs save costs significantly. The model has successfully optimized Bio-PRBs and predicted uranium contaminant-plume evolution and microbial growth inhibition in different Bio-PRBs.

Community ecological study on the reduction of soil antimony bioavailability by SRB-based remediation technologies

Sulfate-reducing bacteria (SRB) were effective in stabilizing Sb. However, the influence of electron donors and acceptors during SRB remediation, as well as the ecological principles involved, remained unclear. In this study, Desulfovibrio desulfuricans ATCC 7757 was utilized to stabilize soil Sb within microcosm. Humic acid (HA) or sodium sulfate (Na2SO4) were employed to enhance SRB capacity. The SRB+HA treatment exhibited the highest Sb stabilization rate, achieving 58.40%. Bacterial community analysis revealed that SRB altered soil bacterial diversity, community composition, and assembly processes, with homogeneous selection as the predominant assembly processes. When HA and Na2SO4 significantly modified the stimulated microbial community succession trajectories, shaped the taxonomic composition and interactions of the bacterial community, they showed converse effect in shaping bacterial community which were both helpful for promoting dissimilatory sulfate reduction. Na2SO4 facilitated SRB-mediated anaerobic reduction and promoted interactions between SRB and bacteria involved in nitrogen and sulfur cycling. The HA stimulated electron generation and storage, and enhanced the interactions between SRB and bacteria possessing heavy metal tolerance or carbohydrate degradation capabilities.