Effect of Leifsonia sp. on retardation of uranium in natural soil and its potential mechanisms.

Uranium mining and milling activities for many years resulted in release of uranium into the adjoining soil in varying degrees. Bioremediation approaches (i.e., immobilization via the action of bacteria) resulting in uranium bearing solid is supposed as an economic and clean in-situ approach for the treatment of uranium contaminated sites. This study purposes to determine the immobilization efficiency of uranium in soil by Leifsonia sp. The results demonstrated that cells have a good proliferation ability under the stress of uranium and play a role in retaining uranium in soil. Residual uranium in active Leifsonia-medium group (66%) was higher than that in the controls, which was 31% in the deionised water control, 46% in the Leifsonia group, and 47% in the medium group, respectively. This indicated that Leifsonia sp. facilitates the immobilization efficiency of uranium in soil by converting part of the reducible and oxidizable fraction of uranium into the residual fraction. X-ray photoelectron fitting results showed that tetravalent states uranium existed in the soil samples, which indicated that the hexavalent uranium was converted into tetravalent by cells. This is the first report of effect of Leifsonia sp. on uranium immobilization in soil. The findings implied that Leifsonia sp. could, to some extent, prevent the migration and diffusion of uranium in soil by changing the chemical states into less toxicity and less risky forms.

Influence on Uranium(VI) migration in soil by iron and manganese salts of humic acid: Mechanism and behavior.

Soil contains large amounts of humic acid (HA), iron ions and manganese ions, all of which affect U(VI) migration in the soil. HA interacts with iron and manganese ions to form HA salts (called HA-Fe and HA-Mn in this paper); however, the effects of HA-Fe and HA-Mn on the migration of U(VI) is not fully understood. In this study, HA-Fe and HA-Mn were compounded by HA interactions with ferric chloride hexahydrate and manganese chloride tetrahydrate, respectively. The influence of HA, HA-Fe and HA-Mn on U(VI) immobilization and migration was investigated by bath adsorption experiments and adsorption-desorption experiments using soil columns. The results showed that the presence of HA, HA-Fe and HA-Mn retarded the migration of U(VI) in soil. Supported by X-ray photoelectron spectroscopy (XPS) and BCR sequential extraction analyses, a plausible explanation for the retardation was that HA-Fe and HA-Mn could reduce hexavalent uranium to stable tetravalent uranium and increase the specific gravity of Fe/Mn oxide-bound uranium and organic/sulfide-bound uranium, which made it difficult for them to longitudinally migrate in soil. Scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), and surface area and pore size analyses indicated that the complex formed between the hydroxyl, amino and carboxyl groups of HA-Fe and U(VI) increased the crystallinity of HA-Fe. The reaction between U(VI) and the hydroxyl, amino, aldehyde, keto and chlorine-containing groups of HA-Mn had no effect on the crystallinity of HA-Mn. Notably, the column desorption experiment found that the U(VI) immobilized in the soil remigrated under the effect of rain leaching, and acid rain promoted uranium remigration better than neutral rain. The findings provide some guidance for the decommissioning disposal of uranium contaminated site and it's risk assessments.

Uranium adsorption and subsequent re-oxidation under aerobic conditions by Leifsonia sp. - Coated biochar as green trapping agent.

It has generally been assumed that the immobilization of U(VI) via polyphosphate accumulating microorganisms may present a sink for uranium, but the potential mechanisms of the process and the stability of precipitated uranium under aerobic conditions remain elusive. This study seeks to explore the mechanism, capacity, and stability of uranium precipitation under aerobic conditions by a purified indigenous bacteria isolated from acidic tailings (pH 6.5) in China. The results show that over the treatment ranges investigated, maximum removal of U(VI) from aqueous solution was 99.82% when the initial concentration of U(VI) was 42 µM, pH was 3.5, and the temperature was with 30 °C much higher than that of other reported microorganisms. The adsorption mechanism was elucidated via the use of SEM-EDS, XPS and FTIR. SEM-EDS showed two peaks of uranium on the surface. A plausible explanation for this, supported by FTIR, is that uranium precipitated on the biosorbent surfaces. XPS measurements indicated that the uranium product is most likely a mixture of 13% U(VI) and 87% U(IV). Notably, the reoxidation experiment found that the uranium precipitates were stable in the presence of Ca2+ and Mg2+, however, U(IV) is oxidized to U(VI) in the presence of NO3- and Na+ ions, resulting in rapid dissolution. It implies that the synthesized Leifsonia sp. coated biochar could be utilized as a green and effective biosorbent. However, it may not a good choice for in-situ remediation due to the subsequent re-oxidation under aerobic conditions. These observations can be of some guiding significance to the application of the bioremediation technology in surface environments.