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.