Effect of uranium (VI) on two sulphate-reducing bacteria cultures from a uranium mine site

Effects and driving mechanisms of bioremediation on groundwater after the neutral in situ leaching of uranium

The remediation of groundwater subject to in situ leaching (ISL) for uranium mining has raised extensive concerns in uranium mill and milling. This study conducted bioremediation through biostimulation and bioaugmentation to the groundwater in an area in northern China that was contaminated due to uranium mining using the CO2 + O2 neutral ISL (NISL) technology. It identified the dominant controlling factors and mechanisms driving bioremediation. Findings indicate that microorganisms can reduce the uranium concentration in groundwater subject to NISL uranium mining to its normal level. After 120 days of bioaugmentation, the uranium concentration in the contaminated groundwater fell to 0.36 mg/L, achieving a remediation efficiency of 91.26 %. Compared with biostimulation, bioaugmentation shortened the remediation timeframe by 30 to 60 days while maintaining roughly the same remediation efficiency. For groundwater remediation using indigenous microbial inoculants, initial uranium concentration and low temperatures (below 15 °C) emerge as the dominant factors influencing the bioremediation performance and duration. In settings with high carbonate concentrations, bioremediation involved the coupling of multiple processes including bioreduction, biotransformation, biomineralization, and biosorption, with bioreduction assuming a predominant role. Post-bioremediation, the relative abundances of reducing microbes Desulfosporosinus and Sulfurospirillum in groundwater increased significantly by 10.56 % and 6.91 %, respectively, offering a sustainable, stable biological foundation for further bioremediation of groundwater.

Recovery of gold(0) nanoparticles from aqueous solutions using effluents from a bioremediation process

Biological processes for the recovery of gold from low-concentration media derived from leaching of secondary sources are of great importance due to the scarcity of the primary resources and the economic and environmental advantages of these methods.

Metallomics of two microorganisms relevant to heavy metal bioremediation reveal fundamental differences in metal assimilation and utilization

Although as many as half of all proteins are thought to require a metal cofactor, the metalloproteomes of microorganisms remain relatively unexplored. Microorganisms from different environments are likely to vary greatly in the metals that they assimilate, not just among the metals with well-characterized roles but also those lacking any known function. Herein we investigated the metal utilization of two microorganisms that were isolated from very similar environments and are of interest because of potential roles in the immobilization of heavy metals, such as uranium and chromium. The metals assimilated and their concentrations in the cytoplasm of Desulfovibrio vulgaris strain Hildenborough (DvH) and Enterobacter cloacae strain Hanford (EcH) varied dramatically, with a larger number of metals present in Enterobacter. For example, a total of 9 and 19 metals were assimilated into their cytoplasmic fractions, respectively, and DvH did not assimilate significant amounts of zinc or copper whereas EcH assimilated both. However, bioinformatic analysis of their genome sequences revealed a comparable number of predicted metalloproteins, 813 in DvH and 953 in EcH. These allowed some rationalization of the types of metal assimilated in some cases (Fe, Cu, Mo, W, V) but not in others (Zn, Nd, Ce, Pr, Dy, Hf and Th). It was also shown that U binds an unknown soluble protein in EcH but this incorporation was the result of extracellular U binding to cytoplasmic components after cell lysis.

Uranium removal and microbial community in a H2-based membrane biofilm reactor

We evaluated a hydrogen-based membrane biofilm reactor (MBfR) for its capacity to reduce and remove hexavalent uranium [U(VI)] from water. After a startup period that allowed slow-growing U(VI) reducers to form biofilms, the MBfR successfully achieved and maintained 94-95% U(VI) removal over 8 months when the U surface loading was 6-11 e(-) mEq/m(2)-day. The MBfR biofilm was capable of self-recovery after a disturbance due to oxygen exposure. Nanocrystalline UO2 aggregates and amorphous U precipitates were associated with vegetative cells and apparently mature spores that accumulated in the biofilm matrix. Despite inoculation with a concentrated suspension of Desulfovibrio vulgaris, this bacterium was not present in the U(VI)-reducing biofilm. Instead, the most abundant group in the biofilm community contained U(VI) reducers in the Rhodocyclaceae family when U(VI) was the only electron acceptor. When sulfate was present, the community dramatically shifted to the Clostridiaceae family, which included spores that were potentially involved in U(VI) reduction.

Phosphate promotes uranium (VI) adsorption in Staphylococcus aureus LZ-01

Staphylococcus aureus LZ-01 was isolated from the Yellow River upstream from Lanzhou which can resist and reduce chromium (VI) to chromium (III). In this study, strain LZ-01's uranium (VI) resistance and adsorption abilities were investigated. Our results showed that it can resist 2 mmol l(-1) U(VI) and adsorb 96% of 2 mmol l(-1) U(VI) after 6 h incubation. Transmission electron microscopy (TEM) images showed that precipitates were formed on the surface of the cells. Energy dispersive X-ray spectroscopy (EDX) analysis indicated that the precipitates contained uranium and phosphorus. The U(VI) adsorption rate of strain LZ-01 was promoted by 20 mmol l(-1) phosphate. It adsorbed 45% of 2·5 mmol l(-1) U(VI) in 30 min compared to 36% without phosphate (P < 0·05). Strain LZ-01 can resist heavy metals and survive in nuclear waste-contaminated environments. Strain LZ-01 might be a potential candidate for nuclear waste remediation with phosphate added.

SIGNIFICANCE AND IMPACT OF THE STUDY

Staphylococcus aureus LZ-01 can resist 2 mmol l(-1) U(VI). It could adsorb more than 90% of the 2 mmol l(-1) U(VI) in 6 h. Uranium is precipitated with phosphorus on the surface of the cells. Phosphate promotes uranium adsorption in strain LZ-01, and its U(VI) adsorption capacity is related to its cell availability. These results indicate that the strain LZ-01 might be a potential candidate for remediation of nuclear waste when phosphate is added.

Green synthesis of covellite nanocrystals using biologically generated sulfide: potential for bioremediation systems

This work describes the synthesis of CuS powders in high yield and via an environmentally friendly and straightforward process, under ambient conditions (temperature and pressure), by adding to aqueous copper (II) a nutrient solution containing biologically generated sulfide from sulfate-reducing bacteria (SRB). The powders obtained were composed of CuS (covellite) nanoparticles (NPs) exhibiting a spheroid morphology (<5 nm). The relevance of this method to obtain CuS supported solid substrates has been demonstrated by performing the synthesis in the presence of TiO2 and SiO2 submicron particles. We further extended the work carried out, which substantiates the potential of using biogenic sulfide for the production of covellite nanocrystals and composites, using the effluent of a bioremediation column. Hence, such process results in the synthesis of added value products obtained from metal rich effluents, such as metallurgical and industrial ones, or Acid Mine Drainage (AMD), when associated with bioremediation processes.

Aerobic uranium immobilization by Rhodanobacter A2-61 through formation of intracellular uranium-phosphate complexes

Severe environmental problems arise from old uranium mines, which continue to discharge uranium (U) via acid mine drainage water, resulting in soil, subsoil and groundwater contamination. Bioremediation of U contaminated environments has been attempted, but most of the conceptual models propose U removal by cell suspensions of anaerobic bacteria. In this study, strain Rhodanobacter A2-61, isolated from Urgeiriça Mine, Portugal, was shown to resist up to 2 mM of U(vi). The conditions used (low nutrient content and pH 5) potentiated the interaction of the toxic uranyl ion with the tested strain. The strain was able to remove approximately 120 μM of U(vi) when grown aerobically in the presence of 500 μM U. Under these conditions, this strain was also able to lower the phosphate concentration in the medium and increased its capacity to take up inorganic phosphate, accumulating up to 0.52 μmol phosphate per optical density unit of the medium at 600 nm, after 24 hours, corresponding approximately to the late log phase of the bacterial culture. Microscopically dense intracellular structures with nanometer size were visible. The extent of U inside the cells was quantified by LS counting. EDS analysis of heated cells showed the presence of complexes composed of phosphate and uranium, suggesting the simultaneous precipitation of U and phosphate within the cells. XRD analysis of the cells containing the U-phosphate complexes suggested the presence of a meta-autunite-like mineral structure. SEM identified, in pyrolyzed cells, crystalline nanoparticles with shape in the tetragonal system characteristic of the meta-autunite-like mineral structures. U removal has been reported previously but mainly by cell suspensions and through release of phosphate. The innovative Rhodanobacter A2-61 can actively grow aerobically, in the presence of U, and can efficiently remove U(vi) from the environment, accumulating it in a structural form consistent with that of the mineral meta-autunite inside the cell, corresponding to effective metal immobilization. This work supports previous findings that U bioremediation could be achieved via the biomineralization of U(vi) in phosphate minerals.

Biologically-induced precipitation of sphalerite-wurtzite nanoparticles by sulfate-reducing bacteria: implications for acid mine drainage treatment

Several experiments were conducted to evaluate zinc-tolerance of sulfate-reducing bacteria (SRB) obtained from three environmental samples, two inocula from sulfide-mining districts and another inoculum from a wastewater treatment plant. The populations of SRB resisted zinc concentrations of 260 mg/L for 42 days in a sulfate-rich medium. During the experiments, sulfate was reduced to sulfide and concentrations in solution decreased. Zinc concentrations also decreased from 260 mg/L to values below detection limit. Both decreases were consistent with the precipitation of newly-formed sphalerite and wurtzite, two polymorphs of ZnS, forming <2.5-μm-diameter spherical aggregates identified by microscopy and synchrotron-μ-XRD. Sulfate and zinc are present in high concentrations in acid mine drainage (AMD) even after passive treatments based on limestone dissolution. The implementation of a SRB-based zinc removal step in these systems could completely reduce the mobility of all metals, which would improve the quality of stream sediments, water and soils in AMD-affected landscapes.

Bromate removal by anaerobic bacterial community: mechanism and phylogenetic characterization

A highly bromate resistant bacterial community and with ability for bromate removal was obtained from a sulphate-reducing bacteria enrichment consortium. This community was able to remove 96% of bromate and 99% of sulphate from an aqueous solution containing 40 μM bromate and 10 mM sulphate. Moreover, 93% of bromate was removed in the absence of sulphate. Under this condition bromate was reduced stoichiometrically to bromide. However, in the presence of sulphate only 88% of bromate was reduced to bromide. Although, bromate removal was not affected by the absence of sulphate, this anion promoted a modification on the structure of the bacterial community. Phylogenetic analysis of 16S rRNA gene showed that the community grown in the presence of bromate and sulphate was mainly composed by bacteria closely to Clostridium and Citrobacter genera, while the community grown in the absence of sulphate was predominantly composed by Clostridium genus. It is the first time that Clostridium and Citrobacter genera are reported as having bromate removal ability. Furthermore, bromate removal by the consortium predominantly composed by Clostridium and Citrobacter genera occurred by enzymatic reduction and by extracellular metabolic products, while the enzymatic process was the only mechanism involved in bromate removal by the consortium mainly composed by Clostridium genus.

Influence of uranium on bacterial communities: a comparison of natural uranium-rich soils with controls

This study investigated the influence of uranium on the indigenous bacterial community structure in natural soils with high uranium content. Radioactive soil samples exhibiting 0.26% - 25.5% U in mass were analyzed and compared with nearby control soils containing trace uranium. EXAFS and XRD analyses of soils revealed the presence of U(VI) and uranium-phosphate mineral phases, identified as sabugalite and meta-autunite. A comparative analysis of bacterial community fingerprints using denaturing gradient gel electrophoresis (DGGE) revealed the presence of a complex population in both control and uranium-rich samples. However, bacterial communities inhabiting uraniferous soils exhibited specific fingerprints that were remarkably stable over time, in contrast to populations from nearby control samples. Representatives of Acidobacteria, Proteobacteria, and seven others phyla were detected in DGGE bands specific to uraniferous samples. In particular, sequences related to iron-reducing bacteria such as Geobacter and Geothrix were identified concomitantly with iron-oxidizing species such as Gallionella and Sideroxydans. All together, our results demonstrate that uranium exerts a permanent high pressure on soil bacterial communities and suggest the existence of a uranium redox cycle mediated by bacteria in the soil.

A bacterial consortium isolated from an Icelandic fumarole displays exceptionally high levels of sulfate reduction and metals resistance

The soils of three fumaroles and one mining site, all with high metal content, were surveyed for the presence of metal-resistant sulfate-reducing bacteria and their potential application in the bioremediation of acid mine drainages. By means of selective soil enrichments a bacterial consortium was isolated from an Icelandic fumarole that displayed very high sulfate reduction in the presence of a mixture of 0.75 g/L of Fe, 0.20 g/L of Zn and 0.080 g/L of Cu. Under these conditions the bacterial consortium reduced 91% of the added 3.9 g/L of sulfate after 28 days, precipitating 100% of the Fe, 96% of the Zn and 97% of the Cu during the same time. Both total bacterial numbers and numbers of culturable sulfate-reducing bacteria remained unchanged when grown in media containing metals, suggesting low or absent inhibitory effects of the metals on the bacterial consortium. PCR-DGGE profiles of the sulfate reducing bacterial communities obtained from the Icelandic fumarole sample showed that bacterial diversity decreased significantly after metal addition: from the original 12 ribotypes only two were detected in the metal-tolerant culture. Phylogenetic analysis of 16S ribosomal RNA gene sequences revealed that these two ribotypes were affiliated with the genera Clostridium and Desulfovibrio, with C. subterminale, C. pascui, C. mesophilum and C. peptidovorans and D. desulfuricans identified as their closest relatives.