The effect of bacteria on uranium sequestration stability by different forms of phosphorus.

Immobilisation of uranium (U (VI)) by direct precipitation of uranyl phosphate (U-P) exhibits a great potential application in the remediation of U (VI)-contaminated environments. However, phosphorus, vital element of bacteria's decomposition, absorption and transformationmay affect the stability of U (VI) with ageing time. The main purpose of this work is to study the effect of bacteria on uranium sequestration mechanism and stability by different forms of phosphorus in a water sedimentary system. The results showed that phosphate effectively enhanced the removal of U (VI), with 99.84%. X-Ray Diffraction (XRD), Scanning Electron Microscopy and Energy Dispersive Spectrometer (SEM-EDS), and X-ray Photoelectron Spectroscopy (XPS) analyses imply that U (VI) and U (IV) co-exist on the surface of the samples. Combined with BCR results, it demonstrated that bacteria and phosphorus have a synergistic effect on the removal of U (VI), realising the immobilisation of U (VI) from a transferable phase to a stable phase. However, from a long-term perspective, the redissolution and release of uranium immobilisation of U (VI) by pure bacteria with ageing time are worthy of attention, especially in uranium mining environments rich in sensitive substances. This observation implies that the stability of the uranium may be impacted by the prevailing environmental conditions. The novel findings could provide theoretical evidence for U (VI) bio-immobilisation in U (VI)-contaminated environments.

Organophosphorus mineralizing-Streptomyces species underpins uranate immobilization and phosphorus availability in uranium tailings.

Phosphate-solubilizing bacteria (PSB) are important but often overlooked regulators of uranium (U) cycling in soil. However, the impact of PSB on uranate fixation coupled with the decomposition of recalcitrant phosphorus (P) in mining land remains poorly understood. Here, we combined gene amplicon sequencing, metagenome and metatranscriptome sequencing analysis and strain isolation to explore the effects of PSB on the stabilization of uranate and P availability in U mining areas. We found that the content of available phosphorus (AP), carbonate-U and Fe-Mn-U oxides in tailings was significantly (P < 0.05) higher than their adjacent soils. Also, organic phosphate mineralizing (PhoD) bacteria (e.g., Streptomyces) and inorganic phosphate solubilizing (gcd) bacteria (e.g., Rhodococcus) were enriched in tailings and soils, but only organic phosphate mineralizing-bacteria substantially contributed to the AP. Notably, most genes involved in organophosphorus mineralization and uranate resistance were widely present in tailings rather than soil. Comparative genomics analyses supported that organophosphorus mineralizing-Streptomyces species could increase soil AP content and immobilize U(VI) through organophosphorus mineralization (e.g., PhoD, ugpBAEC) and U resistance related genes (e.g., petA). We further demonstrated that the isolated Streptomyces sp. PSBY1 could enhance the U(VI) immobilization mediated by the NADH-dependent ubiquinol-cytochrome c reductase (petA) through decomposing organophosphorous compounds. This study advances our understanding of the roles of PSB in regulating the fixation of uranate and P availability in U tailings.

Study on uranium leaching from uranium purification residue with ammonium hydrogen fluoride.

Residues generated from the uranium purification process, characterized by a high uranium content, pose a significant challenge for recovery through leaching and present a considerable environmental threat. After using XRD and SEM-mapping characterization analysis combined with the BCR continuous graded extraction test to analyze the content of different states of uranium, it was found that the main reason why the uranium in the residue was difficult to leach because it was encapsulated by SiO2 crystals. Using NH4HF2 as a leaching agent, a leaching study of uranium in the residue was carried out, and the results showed that the H+ and F- produced by NH4HF2could react with SiO2, destroying the crystal lattice of SiO2 and causing the encapsulated uranium to come into contact with the leaching agent, facilitating the leaching of uranium in the residue. The optimum conditions for uranium leaching were 10% mass fraction of NH4HF2, a liquid-solid ratio of 30:1, a reaction temperature of 30 °C and a reaction time of 120 min, and the leaching efficiency of uranium from the residue was as high as 98.95%. The leaching kinetics of uranium by NH4HF2 were consistent with the mixed controlled model in the shrinking core models, indicating that the surface chemical reaction and mass diffusion dominated both uranium leaching processes. This may provide a viable method for resource recovery and the treatment of uranium purification residues.

Utilization of nickel-graphite electrode as an electron donor for high-efficient microbial removal of solved U(VI) mediated by Leifsonia sp.

Enzymatically catalyzed reduction of metals by bacteria has potential application value to uranium-mine wastewater. However, its practical implementation has long been restricted by its intrinsic drawbacks such as low efficiency and long treatment-time. This study aims to explore the effect of electrodes on U (VI) removal efficiency by a purified indigenous bacteria isolated from a uranium mining waste pile in China. The effects of current intensity, pH, initial U (Ⅵ) concentration, initial dosage of bacteria and contact time on U (Ⅵ) removal efficiency were investigated via static experiments. The results show that U(VI) removal rate was stabilized above 90% and the contact time sharply shortened within 1 h when utilized nickel-graphite electrode as an electron donor. Over the treatment ranges investigated maximum removal of U (Ⅵ) was 96.04% when the direct current was 10 mA, pH was 5, initial U (Ⅵ) concentration was 10 mg/L, and dosage of Leifsonia sp. was 0.25 g/L. In addition, it is demonstrated that U (VI) adsorption by Leifsonia sp. is mainly chemisorption and/or reduction as the quasi-secondary kinetics is more suitable for fitting the process. FTIR results indicated that amino, amide, aldehyde and phosphate -containing groups played a role in the immobilization of U (VI) more or less. SEM and EDS measurements revealed that U appeared to be more obviously aggregated on the surface of cells. A plausible explanation for this, supported by XPS, is that U (VI) was partially reduced to U (IV) by direct current then precipitated on the cells surface. These observations reveal that Nickel-graphite electrode exhibited good electro-chemical properties and synergistic capacity with Leifsonia sp. which potentially provides a new avenue for uranium enhanced removal/immobilization by indigenous bacteria.

Microbial removal of uranyl from aqueous solution by Leifsonia sp. in the presence of different forms of iron oxides.

Immobilization of uranyl by indigenous microorganisms has been proposed as an economic and clean in-situ approach for removal of uranium, but the potential mechanisms of the process and the stability of precipitated uranium in the presence of widespread Fe(III) (hydr)oxides remain elusive. The potential of iron to serve as a reductant and/or an oxidant of uranium indicates that bioemediation strategies which mainly rely on the reduction of highly soluble U(VI) to poorly soluble U(IV) minerals to retard uranium transport in groundwater may be enhanced or hindered under different environmental conditions. This study purposes to determine the effect of ubiquitous Fe(III) (hydr)oxides (two-line ferrihydrite, hematite and goethite) on the removal of U(VI) by Leifsonia sp. isolated from an acidic tailings pond in China. The removal mechanism was elucidated via SEM-EDS, XPS and Mössbauer. The results show that the removal of U(VI) was retarded by Fe(III) (hydr)oxides when the initial concentration of U(VI) was 10 mg/L, pH was 6, temperature was 25 °C. Particularly, the retardatory effect of hematite on U(VI) removal was blindingly obvious. Also, it is worth noting that the U(VI) in the precipitate slow-released in the Fe(III) (hydrodr) oxide treatment groups, accompanied by an increase in Fe(II) concentration. SEM-EDS results demonstrated that the ferrihydrite converted to goethite may be the reason for U(VI) release in the process of 15 days culture. Mössbauer spectra fitting results further imply that the metastable iron oxides were transformed into stable Fe3O4 state. XPS measurements results showed that uranium product is most likely a mixture of Iron-U(IV) and Iron-U(VI), which indicated that the hexavalent uranium was converted into tetravalent uranium. These observations imply that the stability of the uranium in groundwater may be impacted on the prevailing environmental conditions, especially the solid-phase Fe(III) (hydr)oxide in groundwater or sediment.

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.

Low concentration of Fe(II) to enhance the precipitation of U(VI) under neutral oxygen-rich conditions.

Immobilization of U(VI) by naturally ubiquitous ferrous ions (Fe(II)) has been considered as an efficient and ecofriendly method to retard the migration of aqueous U(VI) at many nuclear sites and surface environments. In this study, we conducted Fe-U coprecipitation experiments to investigate the mechanism and stability of uranium (U) precipitation induced by a small quantity of Fe(II) under oxygen-rich conditions. The experimental results suggest that the sedimentation rates of U(VI) by Fe(II) under neutral oxygen-rich conditions are more than 96%, which are about 36% higher than those without Fe(II) and 16% higher than those under oxygen-free conditions. The Fe-U coprecipitates were observed to remain stable under slightly acidic to neutral and oxygen-rich conditions. Fe(II) primarily settles down as low-crystalline iron oxide hydroxide. U(VI) mainly precipitates as three forms: 16-20% of U forms uranyl hydroxide and metaschoepite, which is absorbed on the surface of the solids; 52-56% of U is absorbed as discrete uranyl phases at the internal pores of iron oxide hydroxide; and 27-29% of U is probably incorporated into the FeO(OH) structure as U(V) and U(VI). The U(V) generated via one-electron reduction is somewhat resistant to the oxidation of O2 and the acid dissolution. In addition, nearly 70% of U and only about 15% of Fe could be extracted in 24 h by a hydrochloric acid solution with the H+ concentration ([H+]) of 0.01 M, revealing that U(VI) immobilization by low concentration of Fe(II) combined with O2 has potential applications in the separation and recycling of aqueous uranium.

Correction to: Influence of Leifsonia sp. on U(VI) removal efficiency and the Fe-U precipitates by zero-valent iron.

The original publication of this paper contains a mistake.

Influence of Leifsonia sp. on U(VI) removal efficiency and the Fe-U precipitates by zero-valent iron.

Zero-valent iron (ZVI) has been widely applied to the remediation of uranium (U)-contaminated water. Notably, indigenous bacteria may possess potential positive or unfavorable influence on the mechanism and stability of Fe-U precipitates. However, the focus of the researches in this field has mainly been on physical and/or chemical aspects. In this study, batch experiments were conducted to explore the effects of an indigenous bacterium (Leifsonia sp.) on Fe-U precipitates and the corresponding removal efficiency by ZVI under different environmental factors. The results showed that the removal rate and capacity of U(VI) was significantly inhibited and decreased by ZVI when the pH increased to near-neutral level (pH = 6~8). However, in the ZVI + Leifsonia sp. coexistence system, the U(VI) removal efficiency were maintained at high levels (over 90%) within the experimental scope (pH = 3~8). This revealed that Leifsonia sp. had a synergistic effect on U(VI) remove by ZVI. According to scanning electron microscope and energy dispersive X-ray detector (SEM-EDX) analysis, dense scaly uranium-phosphate precipitation was observed on ZVI + Leifsonia sp. surface. The X-photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) analysis indicated that Leifsonia sp. facilitated the generation of U(VI)-phosphates precipitates. The X-ray diffraction (XRD) analyses further revealed that new substances, such as (Fe(II)Fe(III)2(PO4)2(OH)2), Fe(II)(UO2)2(PO4)2·8H2O, Fe(II)Fe(III)5(PO4)4(OH)2·4H2O, etc., were produced in the coexisting system of ZVI and Leifsonia sp. This study provides new insights on the feasibility and validity of site application of ZVI to U(VI)-contaminated subsurface water in situ. Graphical abstract.

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.

Understanding the solid phase chemical fractionation of uranium in soil profile near a hydrometallurgical factory.

Uranium (U) contamination of soil has become a major concern with respect to its toxicity, accumulation in the food chain, and persistence in the environment. Anthropogenic activities like mining and processing of U ores has become pressing issues throughout the world. The aim of the work is to understand the chemical fractionation of U in polluted soil and the mechanism involved. U-free soils samples of eluvial (E), illuvial (B), and parent-material (C) horizons from a hydrometallurgical factory area were used. The experimental results showed that the U adsorption capacity decreased with depth, and its mobility in the upper soil is better than the lower. It was closely related to distribution coefficient (Kd), pH, organic-matter (OM), and carbonate content of soil horizons. The chemical fractionation of U was studied using the BCR sequential extraction scheme for soils after saturated adsorption. It was noted that the U reducible and oxidizable fraction in the E and B horizons can vertically transfer to the C horizon and occurs a significant rearrangement of U in different horizons. BET, SEM, XRD, and FT-IR analyses showed that different U distribution and migration in soil profile is mainly affected by specific surface area, soil particle size, mineral composition, and active groups. The XPS data further indicated that U (VI) is gradually converted to U (IV) with decreased depth and fixed in deeper soil becoming insoluble and immobile. It is the first step to investigate potential migration and plan U mining and milling area long-term management.

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.