Uranium reduction by Shewanella oneidensis MR-1 as a function of NaHCO3 concentration: surface complexation control of reduction kinetics

A review of remediation technologies for uranium-contaminated water

Uranium is a naturally existing radioactive element present in the Earth's crust. It exhibits lithophilic characteristics, indicating its tendency to be located near the surface of the Earth and tightly bound to oxygen. It is ecotoxic, hence the need for its removal from the aqueous environment. This paper focuses on the variety of water treatment processes for the removal of uranium from water and this includes physical (membrane separation, adsorption and electrocoagulation), chemical (ion exchange, photocatalysis and persulfate reduction), and biological (bio-reduction and biosorption) approaches. It was observed that membrane filtration and ion exchange are the most popular and promising processes for this application. Membrane processes have high throughput but with the challenge of high power requirements and fouling. Besides high pH sensitivity, ion exchange does not have any major challenges related to its application. Several other unique observations were derived from this review. Chitosan/Chlorella pyrenoidosa composite adsorbent bearing phosphate ligand, hydroxyapatite aerogel and MXene/graphene oxide composite has shown super-adsorbent performance (>1000 mg/g uptake capacity) for uranium. Ultrafiltration (UF) membranes, reverse osmosis (RO) membranes and electrocoagulation have been observed not to go below 97% uranium removal/conversion efficiency for most cases reported in the literature. Heat persulfate reduction has been explored quite recently and shown to achieve as high as 86% uranium reduction efficiency. We anticipate that future studies would explore hybrid processes (which are any combinations of multiple conventional techniques) to solve various aspects of the process design and performance challenges.

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.

The bioreduction of U(VI) and Pu(IV): Experimental and thermodynamic studies

The experimental and thermodynamic bioreduction of U(VI)aq and Pu(IV)am was studied in order to more accurately predict their transport velocities in groundwater and assess the contamination risks to the associated environments. The results obtained in this study emphasize the impact of carbonate-calcium and humic acids at 7.1 and anoxic solutions on the rate and extent of U(VI)aq and Pu(IV)am bioreduction by Shewanella putrefaciens. We found that the bioreduction rate of U(VI)aq became slow in the presence of NaHCO3/CaCl2. The more negative standard redox potentials of the ternary complexes of U(VI)-Ca2+-CO32- accounted for the decreased rate of bioreduction, e.g., [Formula: see text]  = -0.6797 V ≪ [Formula: see text]  = 0.3862 V. The bioreduction of Pu(IV)am seemed feasible, while humic acids accepted the adequate extracellular electrons secreted by S. putrefaciens, and the redox potential of Eh(HAox/HAred) was lower than Eh(PuO2(am)/Pu3+), e.g., Eh(HAox/HAred) ≦ Eh(PuO2(am)/Pu3+) if humic acids accepted ≧ 7.952 × 10-7 mol of electrons. The standard redox potentials, Eho(PuO2(am)/Pu3+) = 0.9295 V ≫ [Formula: see text]  = -0.6797 V, cannot explain the reduction extent of Pu(IV)am (8.9%), which is notably smaller than that of U(VI)aq (74.9%). In fact, the redox potential of Pu(IV)am was distinctly negative under the experimental conditions of trace-level Pu(IV)am (∼2.8 × 10-9 mol/L Pu(IV) if Pu(IV)am was completely dissolved), e.g., Eh(PuO2(am)/Pu3+) = -0.1590 V (α(Pu3+) = 10-10 mol/L, pH = 7.1). Therefore, the chemical factor of Pu3+ activity, leading to a rapid drop in Eh(PuO2(am)/Pu3+) at trace-level Pu(IV)am, was responsible for the relatively small reduction extent of Pu(IV)am.

The biochemical behavior and mechanism of uranium(Ⅵ) bioreduction induced by natural Bacillus thuringiensis

For a broader understanding of uranium migration affected by microorganisms in natural anaerobic environment, the bioreduction of uranium(Ⅵ) (U(Ⅵ)) was revealed in Bacillus thuringiensis, a dominant bacterium strain with potential of uranium-tolerant isolated from uranium contaminated soil. The reduction behavior was systematically investigated by the quantitative analysis of U(Ⅳ) in bacteria, and mechanism was inferred from the pathway of electron transmission. Under anaerobic conditions, appropriate biomass and sodium lactate as electron donor, reduction behavior of U(Ⅵ) induced by B. thuringiensis was restricted by the activity of lactate dehydrogenase, which was directly affected by the initial pH, temperature and initial U(Ⅵ) concentration of bioreduction system. Bioreduction of U(Ⅵ) was driven by the generation of nicotinamide adenine dinucleotide (NADH) from enzymatic reaction of sodium lactate with various dehydrogenase. The transmission of the electrons from bacteria to U(Ⅵ) was mainly supported by the intracellular NADH dehydrogenase-ubiquinone system, this process could maintain the biological activity of cells.

Amide Linkages in Pyrene-Based Covalent Organic Frameworks toward Efficient Photocatalytic Reduction of Uranyl

The bond linkages in covalent organic frameworks (COFs) partly determine its physical and chemical properties, thus affecting the photoreactive activity by influencing the generation of photoelectrons and the separation of excitons. Herein, pyrene-based amide COF 4,4',4″,4‴-(pyrene-1,3,6,8-tetrayl)tetrabenzaldehyde-3,8-diamino-6-phenylphenanthridine (TFPPy-DP) was synthesized by postsynthetic modification of imine COFs. Due to the introduction of oxygen atoms into the framework and the change in polarity, an increased number of photogenerated electrons and a wide band gap for amide COFs were found, hydrophilicity and dispersibility were prompted as well. Both imine and amide COF TFPPy-DP were applied in the photocatalytic reduction and removal of toxic U(VI) under visible light, the catalytic reduction equilibrium (91% removal percentage of 238 ppm U at pH 3) was achieved by imine COFs with 10 h of irradiation, while amide COFs only took 2 h of irradiation (82% removal percentage). The much faster photocatalytic reduction rate of U(VI) can be attributed to the fact that amide COF TFPPy-DP retained crystallinity and permanent porosity and exhibited lower electrochemical impedance and enhanced charge separation and accumulation. Further electronic excitation analysis based on time-dependent density functional theory calculations revealed that the intramolecular charge-transfer effect in amide TFPPy-DP enhanced its photocatalytic rate.

Accurate determination of tetravalent uranium reduced by microorganisms via a potentiometric titration procedure

Although bioreduction induced by microorganisms has been considered to play an important role in the chemical and migration behaviors of uranium in nature, the accurate determination of tetravalent uranium reduced by microorganisms is still difficult to achieve. In this work, potentiometric titration via K2Cr2O7 was used to quantitatively determine the microorganism reduced tetravalent uranium (U(IV)) for the first time. By evaluating the influence of microorganism substance content on the titration of U(IV), the appropriate determination range of U(IV) and biomass was confirmed, and U(IV) induced by bioreduction in three microorganisms was determined. With this method, U(IV) of more than 0.12 mg in microorganisms can be quantitatively measured with an accuracy of 2.2% and a precision of 1.3%, which has been established with the premise that the pretreatment biomass and quantity of U(IV) are in an appropriate range. Compared with the estimated values via the changes in hexavalent uranium (U(VI)) concentration in the bioreduction system, the results obtained by this method can more accurately reflect the quantity of U(IV) in microorganisms. This work can help us to better understand the bioreduction behavior of uranium in the environment.

Stable immobilization of uranium in iron containing environments with microbial consortia enriched via two steps accumulation method

The stable stabilization of uranium (U) in iron (Fe) containing environments is restricted by the reoxidation of UO₂. In the current study, based on air reoxidation tests, we propose a novel two steps accumulation method to enrich microbial consortia from paddy soil. The constructed microbial consortia, denoted as the Fe-U bacteria, can co-precipitate U and Fe to form stable Fe-U solids. Column experiments running for 4 months demonstrated the production of U(IV)-O-Fe(II) precipitates containing maximum of 39.51% uranium in the presence of Fe-U bacteria. The reoxidation experiments revealed the U(IV)-O-Fe(II) precipitates were more stable than UO₂. 16S rDNA high throughput sequencing analysis demonstrated that Acinetobacter and Stenotrophomonas were responsible for Fe and U precipitation, while, Caulobacteraceae and Aminobacter were crucial for the formation of U(VI)-PO₄ chemicals. The proposed two steps accumulation method has an extraordinary application potential in stable immobilization of uranium in iron containing environments.

Uranium bioremediation with U(VI)-reducing bacteria

Uranium (U) pollution is an environmental hazard caused by the development of the nuclear industry. Microbial reduction of hexavalent uranium (U(VI)) to tetravalent uranium (U(IV)) reduces U solubility and mobility and has been proposed as an effective method to remediate uranium contamination. In this review, U(VI) remediation with respect to U(VI)-reducing bacteria, mechanisms, influencing factors, products, and reoxidation are systematically summarized. Reportedly, some metal- and sulfate-reducing bacteria possess excellent U(VI) reduction capability through mechanisms involving c-type cytochromes, extracellular pili, electron shuttle, or thioredoxin reduction. In situ remediation has been demonstrated as an ideal strategy for large-scale degradation of uranium contaminants than ex situ. However, U(VI) reduction efficiency can be affected by various factors, including pH, temperature, bicarbonate, electron donors, and coexisting metal ions. Furthermore, it is noteworthy that the reduction products could be reoxidized when exposed to oxygen and nitrate, inevitably compromising the remediation effects, especially for non-crystalline U(IV) with weak stability.

Biogenic Sulfidation of U(VI) and Ferrihydrite Mediated by Sulfate-Reducing Bacteria at Elevated pH

Globally, the need for radioactive waste disposal and contaminated land management is clear. Here, gaining an improved understanding of how biogeochemical processes, such as Fe(III) and sulfate reduction, may control the environmental mobility of radionuclides is important. Uranium (U), typically the most abundant radionuclide by mass in radioactive wastes and contaminated land scenarios, may have its environmental mobility impacted by biogeochemical processes within the subsurface. This study investigated the fate of U(VI) in an alkaline (pH ∼9.6) sulfate-reducing enrichment culture obtained from a high-pH environment. To explore the mobility of U(VI) under alkaline conditions where iron minerals are ubiquitous, a range of conditions were tested, including high (30 mM) and low (1 mM) carbonate concentrations and the presence and absence of Fe(III). At high carbonate concentrations, the pH was buffered to approximately pH 9.6, which delayed the onset of sulfate reduction and meant that the reduction of U(VI)_(aq) to poorly soluble U(IV)_(s) was slowed. Low carbonate conditions allowed microbial sulfate reduction to proceed and caused the pH to fall to ∼7.5. This drop in pH was likely due to the presence of volatile fatty acids from the microbial respiration of gluconate. Here, aqueous sulfide accumulated and U was removed from solution as a mixture of U(IV) and U(VI) phosphate species. In addition, sulfate-reducing bacteria, such as Desulfosporosinus species, were enriched during development of sulfate-reducing conditions. Results highlight the impact of carbonate concentrations on U speciation and solubility in alkaline conditions, informing intermediate-level radioactive waste disposal and radioactively contaminated land management.

Conductive property of secondary minerals triggered Cr(VI) bioreduction by dissimilatory iron reducing bacteria

Although secondary minerals have great potential for heavy metal removal, their impact on chromium biogeochemistry in subsurface environments associated with dissimilatory iron reducing bacteria (DIRB) remains poorly characterized. Here, we have investigated the mechanisms of biogenic secondary minerals on the rate of Cr(VI) bioreduction with shewanella oneidensis MR-1. Batch results showed that the biogenic secondary minerals, schwertmannite and jarosite, appreciably increased the Cr(VI) bioreduction rate. UV-vis diffuse reflection spectra showed that schwertmannite and jarosite are semiconductive minerals, which can be activated by MR-1, followed by transferred conduction electrons toward Cr(VI). Cyclic voltammetry and Tafel analysis suggested that the resistance of secondary minerals is a dominant factor controlling Cr(VI) bioreduction. In addition, Cr(VI) adsorption on secondary minerals through ligand exchange promoted Cr(VI) bioreduction by decreasing the electron transfer distance between MR-1 and chromate. Fe(III)/Fe(II) cycling in schwertmannite and jarosite also contributed to Cr(VI) bioreduction as reflected by X-ray photoelectron spectroscopy and Fourier transform infrared spectrometer. Complementary characterizations further verified the contributions of Fe(III)/Fe(II) cycling, Cr(VI) adsorption, and conduction band electron transfer to enhanced Cr(VI) bioreduction. This study provides new insights on the understanding of Cr(VI) bioreduction by semiconductor minerals containing sulfate in subsurface environments.

The effect of plastic additives on Shewanella oneidensis growth and function

Plastic waste has the potential for significant consequences on various ecosystems; yet, there are gaps in our understanding of the interaction of bacteria with polymer additives. We studied the impact of representative additive molecules to the viability and cell function of Shewanella oneidensis MR-1. Specifically, we explored the toxicity of three bisphenols (bisphenol A (BPA), bisphenol S (BPS), and tetrabromo bisphenol A (TBBPA)) and two diesters (dibutyl sebacate (DBS) and diisobutyl phthalate (DIBP)) in order to evaluate the generalizability of toxicity based on similar molecular structures. TBBPA caused significant, dose-dependent decreases in viability for acute (4 h) exposures in aerobic and anaerobic conditions. While the other 4 additives showed no significant toxicity upon 4 h exposures, chronic (2 day) anaerobic exposures revealed a significant impact to growth. BPA and BPS cause a significant decrease in growth rates for all exposure doses (8-131 μM) while DBS and DIBP had decreases in growth for the lowest exposure concentrations, though recovered to growth rates similar to the control at the highest concentrations. This highlights that S. oneidensis may have the ability to use the diesters as a carbon source if present in high enough concentrations. Riboflavin secretion was monitored as a marker of cellular health. Most additives stimulated riboflavin secretion as a survival response. Yet, there was no generalizable trend observed for these molecules, indicating the importance of considering the nuances of molecular structure to toxicity responses and the need for further work to understand the consequences of plastic waste in our environment.

New insights into the role of calcium in the bioreduction of uranium(VI) under varying pH conditions

The effect of calcium in the uranium-contaminated groundwater on U(VI)_aq bioreduction remains uncertain. Some studies indicated that the presence of calcium may inhibit the bioreduction. However, our calculations show the negative standard molar Gibbs free energy of reduction. The bioreduction of the ternary uranyl-carbonate-calcium complexes seems thermodynamically favorable at specific pH. Sorption and reduction experiments were conducted to gain new insights of calcium into the bioreduction. The results show that the complexes were greatly reduced by Shewanella putrefaciens in the slightly acidic pH ~6.0 and alkaline pH ~7.9 solutions with the relatively high CaCl₂ (1.0-6.0 mmol/L) although the reduction was difficult at the nearly neutral pH ~6.9. At pH ~6.9, the removal percentage of U(VI)_aq decreased from 97.0% to 24.4% with increasing CaCl₂ from 0 to 6.0 mmol/L, in contrast to the increasing percentage from 50.9% to 89.7% at pH ~7.9. The obvious removal of U(VI)_aq was ascribed to the bioreduction instead of the biosorption, as evidenced by XPS, HRTEM and UV-vis spectra. The calculations such as [Formula: see text] and [Formula: see text] partially accounted for the reduction mechanisms. Accordingly, the U(VI)_aq bioreduction is a promising method to remediate the groundwater even rich in calcium and carbonate.

Biotic dissolution of autunite under anaerobic conditions: effect of bicarbonates and Shewanella oneidensis MR1 microbial activity

Uranium is a contaminant of major concern across the US Department of Energy complex that served a leading role in nuclear weapon fabrication for half a century. In an effort to decrease the concentration of soluble uranium, tripolyphosphate injections were identified as a feasible remediation strategy for sequestering uranium in situ in contaminated groundwater at the Hanford Site. The introduction of sodium tripolyphosphate into uranium-bearing porous media results in the formation of uranyl phosphate minerals (autunite) of general formula {X_1-2[(UO₂)(PO₄)]_2-1·nH₂O}, where X is a monovalent or divalent cation. The stability of the uranyl phosphate minerals is a critical factor that determines the long-term effectiveness of this remediation strategy that can be affected by biogeochemical factors such as the presence of bicarbonates and bacterial activity. The objective of this research was to investigate the effect of bicarbonate ions present in the aqueous phase on Ca-autunite dissolution under anaerobic conditions, as well as the role of metal-reducing facultative bacterium Shewanella oneidensis MR1. The concentration of total uranium determined in the aqueous phase was in direct correlation to the concentration of bicarbonate present in the solution, and the release of Ca, U and P into the aqueous phase was non-stoichiometric. Experiments revealed the absence of an extensive biofilm on autunite surface, while thermodynamic modeling predicted the presence of secondary minerals, which were identified through microscopy. In conclusion, the dissolution of autunite under the conditions studied is susceptible to bicarbonate concentration, as well as microbial presence.

Role of bacterial cell surface sulfhydryl sites in cadmium detoxification by Pseudomonas putida

Understanding bacterial metal detoxification systems is crucial for determining the environmental impacts of metal pollution and for developing advanced bioremediation and water disinfection strategies. Here, we explore the role of cell surface sulfhydryl sites in bacterial detoxification of Cd, using Pseudomonas putida with surface sulfhydryl sites mostly on its EPS molecules as a model organism. Our results show that 5 and 20 ppm Cd in LB growth medium affects the lag phase of P. putida, but not the overall extent of cell growth at stationary phase, indicating that P. putida can detoxify Cd at these concentrations. EXAFS analysis of Cd bound to biomass from the different growth stages indicates that Cd binds to both sulfhydryl and non-sulfhydryl sites, but that the importance of Cd-sulfhydryl binding increases from early exponential to stationary phase. Cell growth is positively correlated to the measured sulfhydryl concentration on different biomass samples, but is independent of the measured non-sulfhydryl binding site concentration on the cell surfaces. Taken together, our results demonstrate that the sulfhydryl binding sites on EPS molecules can play an important role in binding and detoxifying toxic metals, significantly decreasing the bioavailability of the metal by sequestering it away from the bacterial cells.

Uranium Isotope Fractionation (238U/235U) during U(VI) Uptake by Freshwater Plankton

Uranium contamination in the environment is a serious public health concern. Biotic U(VI) reduction and nonreductive U(VI) uptake by microorganisms (e.g., U(VI) biosorption by cyanobacteria) are effective U remediation techniques. Variations of ²³⁸U/²³⁵U have been extensively explored to track biotic U(VI) reduction in laboratory experiments and field applications. However, U isotope fractionation during nonreductive U(VI) uptake by microorganisms is poorly constrained. To investigate U isotope fractionation in this process, we cultured freshwater plankton in the presence of U(VI) and measured ²³⁸U/²³⁵U in the culture media and biomass. We found that nonreductive U(VI) uptake by freshwater plankton fractionated U isotopes in the opposite direction compared to biotic U(VI) reduction. δ²³⁸U values in freshwater plankton were consistently ∼0.23 ± 0.06‰ lighter than those in dissolved U in the culture medium at various fractions of U removal (12-30%), consistent with equilibrium isotope fractionation in a closed system. The equilibrium isotope fractionation observed in our experiments possibly results from changes in coordination geometry between dissolved U(VI) in the culture media and adsorbed U(VI) on cell surfaces. Our experimental results highlight the need to consider U isotope fractionation during nonredox U(VI) uptake by microorganisms and organic matter when applying variations of ²³⁸U/²³⁵U to track biogeochemical processes and evaluate U remediation.

Engineering and kinetic aspects of bacterial uranium reduction for the remediation of uranium contaminated environments

Biological reduction of soluble uranium from U(VI) to insoluble U(IV) coupled to the oxidation of an electron donor (hydrogen or organic compounds) is a potentially cost-efficient way to reduce the U concentrations in contaminated waters to below regulatory limits. A variety of microorganisms originating from both U contaminated and non-contaminated environments have demonstrated U(VI) reduction capacity under anaerobic conditions. Bioreduction of U(VI) is considered especially promising for in situ remediation, where the activity of indigenous microorganisms is stimulated by supplying a suitable electron donor to the subsurface to contain U contamination to a specific location in a sparingly soluble form. Less studied microbial biofilm-based bioreactors and bioelectrochemical systems have also shown potential for efficient U(VI) reduction to remove U from contaminated water streams. This review compares the advantages and challenges of U(VI)-reducing in situ remediation processes, bioreactors and bioelectrochemical systems. In addition, the current knowledge of U(VI) bioreduction mechanisms and factors affecting U(VI) reduction kinetics (e.g. pH, temperature, and the chemical composition of the contaminated water) are discussed, as both of these aspects are important in designing efficient remediation processes.

Mechanism study of photo-induced gold nanoparticles formation by Shewanella oneidensis MR-1

Shewanella oneidensis MR-1, a bioelectricity generating bacterium, is broadly used in bioremediation, microbial fuel cell and dissimilatory reduction and recovery of precious metals. Herein, we report for the first time that photo induction as a trigger to stimulate gold nanoparticles (Au@NPs) formation by MR-1, with wavelength and light intensity as two key variables. Results indicated that sigmoidal model is the best fit for Au@NPs formation at various wavelengths (with R² > 0.97). Light intensity in terms of photosynthetic photon flux density (PPFD) critically influences the rate constant in the low-light intensity region (PPFD < 20), while wavelength controls the maximum rate constant in the high-light region (PPFD > 20). By deletion of Mtr pathway genes in MR-1, we proposed the mechanism for light induced Au@NP formation is the excitation effect of light on certain active groups and extracellular polymeric substances (EPS) on the cell surface. Also, the release of electrons from proteins and co-enzyme complexes enhance electron generation. To the best of our knowledge, this is the first-attempt to explore the effect of photo-induction on Au@NPs production by MR-1, which provides an alternative cost-effective and eco-friendly process in green chemical industry.

Mechanism of Goethite Precipitation on Magnetite and Maghemite Nanoparticles Studied by Surface Complexation/Precipitation Modeling

Precipitation of goethite on magnetic nanoparticles (MNPs) has been proposed as an effective means to separate goethite from calcium sulfate in the iron removal process of zinc hydrometallurgy, which allows reuse of the hazardous residues. This study focuses on investigating the underlying mechanisms of goethite precipitation on magnetite and maghemite MNPs, providing insights on Fe(III)_aq adsorption and nucleation of goethite on MNPs. A predictive surface complexation/precipitation model of the system was developed based on the results from two different types of experiments: the potentiometric titration of MNPs to calculate proton binding constants ( K_a) of discrete MNP surface functional groups and the corresponding site concentrations; and adsorption of Fe(III)_aq onto MNP surfaces to determine metal binding constants ( K_f). The composition of the surface complexes on MNPs was determined by time-of-flight secondary ion mass spectrometry. The results indicated the formation of polynuclear surface complexes. The content of polynuclear surface complexes was found to be significantly higher on maghemite MNPs than on magnetite MNPs. This trend is consistent with our experimental results of a greater goethite precipitation on maghemite than on magnetite. Overall, the formation of Fe(III) polynuclear surface complexes correlates directly to the nucleation and precipitation of goethite on the surfaces of both types of MNPs.

The dynamic role of pH in microbial reduction of uranium(VI) in the presence of bicarbonate

The negative effect of carbonate on the rate and extent of bioreduction of aqueous U(VI) has been commonly reported. The solution pH is a key chemical factor controlling U(VI)_aq species and the Gibbs free energy of reaction. Therefore, it is interesting to study whether the negative effect can be diminished under specific pH conditions. Experiments were conducted using Shewanella putrefaciens under anaerobic conditions with varying pH values (4-9) and bicarbonate concentrations ( [Formula: see text] , 0-50 mmol/L). The results showed a clear correlation between the pH-bioreduction edges of U(VI)_aq and the [Formula: see text] . The specific pH at which the maximum bioreduction occurred (pH_mbr) shifted from slightly basic to acidic pH (∼7.5-∼6.0) as the [Formula: see text] increased (2-50 mmol/L). At [Formula: see text]  = 0, however, no pH_mbr was observed in terms of increasing bioreduction with pH (∼100%, pH > 7). In the presence of [Formula: see text] , significant bioreduction was observed at pH_mbr (∼100% at 2-30 mmol/L [Formula: see text] , 93.7% at 50 mmol/L [Formula: see text] ), which is in contrast to the previously reported infeasibility of bioreduction at high [Formula: see text] . The pH-bioreduction edges were almost comparable to the pH-biosorption edges of U(VI)_aq on heat-killed cells, revealing that biosorption is favorable for bioreduction. The end product of U(VI)_aq bioreduction was characterized as insoluble nanobiogenic uraninite by HRTEM. The redox potentials of the master complex species of U(VI)_aq, such as [Formula: see text] , [Formula: see text] , and [Formula: see text] , were calculated to obtain insights into the thermodynamic reduction mechanism. The observed dynamic role of pH in bioreduction suggests the potential for bioremediation of uranium-contaminated groundwater containing high carbonate concentrations.

pH-dependent microbial reduction of uranium(VI) in carbonate-free solutions: UV-vis, XPS, TEM, and thermodynamic studies

U(VI)_aq bioreduction has an important effect on the fate and transport of uranium isotopes in groundwater at nuclear test sites. In this study, we focus on the pH-dependent bioreduction of U(VI)_aq in carbonate-free solutions and give mechanistic insight into the removal kinetics of U(VI)_aq. An enhancement in the removal of U(VI)_aq with increasing pH was observed within 5 h, e.g., from 19.4% at pH 4.52 to 99.7% at pH 8.30. The removal of U(VI)_aq at pH 4.52 was due to the biosorption of U(VI)_aq onto the living cells of Shewanella putrefaciens, as evidenced by the almost constant UV-vis absorption intensity of U(VI)_aq immediately after contact with S. putrefaciens. Instead, the removal observed at pH 5.97 to 8.30 resulted from the bioreduction of U(VI)_aq. The end product of U(VI)_aq bioreduction was analyzed using XPS and HRTEM and identified as nanosized UO₂. An increasing trend in the biosorption of U(VI)_aq onto heat-killed cells was also observed, e.g., ~ 80% at pH 8.38. Evidently, the U(VI)_aq that sorbed onto the living cells at pH > 4.52 was further reduced to UO₂, although biosorption made a large contribution to the initial removal of U(VI)_aq. These results may reveal the removal mechanism, in which the U(VI)_aq that was sorbed onto cells rather than the U(VI)_aq complexed in solution was reduced. The decreases in the redox potentials of the main complex species of U(VI)_aq (e.g., [Formula: see text] and [Formula: see text]) with increasing pH support the proposed removal mechanism.

pH-dependent speciation and hydrogen (H2 ) control U(VI) respiration by Desulfovibrio vulgaris

In situ bioreduction of soluble hexavalent uranium U(VI) to insoluble U(IV) (as UO₂ ) has been proposed as a means of preventing U migration in the groundwater. This work focuses on the bioreduction of U(VI) and precipitation of U(IV). It uses anaerobic batch reactors with Desulfovibrio vulgaris, a well-known sulfate, iron, and U(VI) reducer, growing on lactate as the electron donor, in the absence of sulfate, and with a 30-mM bicarbonate buffering. In the absence of sulfate, D. vulgaris reduced  >90% of the total soluble U(VI) (1 mM) to form U(IV) solids that were characterized by X-ray diffraction and confirmed to be nano-crystalline uraninite with crystallite size 2.8 ± 0.2 nm. pH values between 6 and 10 had minimal impact on bacterial growth and end-product distribution, supporting that the mono-nuclear, and poly-nuclear forms of U(VI) were equally bioavailable as electron acceptors. Electron balances support that H₂ transiently accumulated, but was ultimately oxidized via U(VI) respiration. Thus, D. vulgaris utilized H₂ as the electron carrier to drive respiration of U(VI). Rapid lactate utilization and biomass growth occurred only when U(VI) respiration began to draw down the sink of H₂ and relieve thermodynamic inhibition of fermentation.

Effects of humic acid concentration on the microbially-mediated reductive solubilization of Pu(IV) polymers

The role of humic acid concentration in the microbially-mediated reductive solubilization of Pu(IV) polymers remains unclear until now. The effects of humic concentration (0-150.5mg/L) on the rate and extent of reduction of polymeric Pu(IV) were studied under anaerobic and pH 7.2 conditions. The results show that Shewanella putrefaciens, secreting flavins as endogenous electron shuttles, cannot notably stimulate the reduction of polymeric Pu(IV). In the presence of humic acids, the reduction rate of polymeric Pu(IV) increased with increasing humic concentrations (0-15.0mg/L): e.g., a 102-fold increase from 4.1×10^-15 (HA=0) to 4.2×10^-13mol Pu(III)_aq/h (HA=15.0mg/L). The bioreduced humic acids by S. putrefaciens facilitated the extracellular electron transfer to Pu(IV) polymers and thus the reduction of polymeric Pu(IV) to Pu(III)_aq became thermodynamically favorable. However, the reduction rate did not increase but decrease with increasing humic concentrations from 15.0 to 150.5mg/L. Humic coatings formed on the polymer surfaces at relatively high humic concentrations limited the electron transfer to the polymers and thus decreased the reduction rate. The finding of the dynamic role of humic acids in the bioreductive solubilization may be helpful in evaluating Pu mobility in the geosphere.

Enzymatic reduction of U60 nanoclusters by Shewanella oneidensis MR-1

In this study, a series of reduction experiments were conducted using a representative uranyl peroxide nanocluster, U60 (K16Li44[UO2(O2)OH]60) and a bacterial species, Shewanella oneidensis MR-1, that is capable of enzymatic U(VI) reduction. U60 was reduced by S. oneidensis in the absence of O2, but the reduction kinetics for U60 were significantly slower than was observed in this study for aqueous uranyl acetate, and were faster than was reported in previous studies for solid phase U(VI). Our results indicate that U60 aggregates bigger than 0.2 μm formed immediately upon mixing with the bacterial growth medium, and that these aggregates were gradually broken down during the process of reduction. Neither reduction nor dissolution of U60 was observed during 72 h of control experiments open to the atmosphere, indicating that the breakdown and dissolution of U60 aggregates is caused by the reduction of U60, and that S. oneidensis is capable of direct reduction of the U(VI) within the U60 nanoclusters, likely due to the adsorption of U60 aggregates onto bacterial cells. This study is first to show the reduction capacity of bacteria for uranyl peroxide nanoclusters, and the results yield a better understanding of the long term fate of uranium in environmental systems in which uranyl peroxide nanoclusters are present.

Microbial reduction of uranium (VI) by Bacillus sp. dwc-2: A macroscopic and spectroscopic study

The microbial reduction of U(VI) by Bacillus sp. dwc-2, isolated from soil in Southwest China, was explored using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge spectroscopy (XANES). Our studies indicated that approximately 16.0% of U(VI) at an initial concentration of 100mg/L uranium nitrate could be reduced by Bacillus sp. dwc-2 at pH8.2 under anaerobic conditions at room temperature. Additionally, natural organic matter (NOM) played an important role in enhancing the bioreduction of U(VI) by Bacillus sp. dwc-2. XPS results demonstrated that the uranium presented mixed valence states (U(VI) and U(IV)) after bioreduction, which was subsequently confirmed by XANES. Furthermore, the TEM and high resolution transmission electron microscopy (HRTEM) analysis suggested that the reduced uranium was bioaccumulated mainly within the cell and as a crystalline structure on the cell wall. These observations implied that the reduction of uranium may have a significant effect on its fate in the soil environment in which these bacterial strains occur.

Influence of bicarbonate on the abundance of microbial communities capable of reducing U(vi) in groundwater

7 experiments amended with 0, 5, 10, 15, 20, 25 and 30 mM initial concentrations of bicarbonate were conducted to investigate the influence of different concentrations of bicarbonate on the abundance of microbial communities capable of reducing U(vi) in groundwater.

The application of iron-based technologies in uranium remediation: A review

Remediating uranium contamination is of worldwide interest because of the increasing release of uranium from mining and processing, nuclear power leaks, depleted uranium components in weapons production and disposal, and phosphate fertilizer in agriculture activities. Iron-based technologies are attractive because they are highly efficient, inexpensive, and readily available. This paper provides an overview of the current literature that addresses the application of iron-based technologies in the remediation of sites with elevated uranium levels. The application of iron-based materials, the current remediation technologies and mechanisms, and the effectiveness and environmental safety considerations of these approaches were discussed. Because uranium can be reduced and reoxidized in the environment, the review also proposes strategies for long-term in situ remediation of uranium. Unfortunately, iron-based materials (nanoscale zerovalent iron and iron oxides) can be toxic to microorganisms. As such, further studies exploring the links among the fates, ecological impacts, and other environmentally relevant factors are needed to better understand the constraints on using iron-based technologies for remediation.

Interaction of Uranium with Bacterial Cell Surfaces: Inferences from Phosphatase-Mediated Uranium Precipitation

Deinococcus radiodurans and Escherichia coli expressing either PhoN, a periplasmic acid phosphatase, or PhoK, an extracellular alkaline phosphatase, were evaluated for uranium (U) bioprecipitation under two specific geochemical conditions (GCs): (i) a carbonate-deficient condition at near-neutral pH (GC1), and (ii) a carbonate-abundant condition at alkaline pH (GC2). Transmission electron microscopy revealed that recombinant cells expressing PhoN/PhoK formed cell-associated uranyl phosphate precipitate under GC1, whereas the same cells displayed extracellular precipitation under GC2. These results implied that the cell-bound or extracellular location of the precipitate was governed by the uranyl species prevalent at that particular GC, rather than the location of phosphatase. MINTEQ modeling predicted the formation of predominantly positively charged uranium hydroxide ions under GC1 and negatively charged uranyl carbonate-hydroxide complexes under GC2. Both microbes adsorbed 6- to 10-fold more U under GC1 than under GC2, suggesting that higher biosorption of U to the bacterial cell surface under GC1 may lead to cell-associated U precipitation. In contrast, at alkaline pH and in the presence of excess carbonate under GC2, poor biosorption of negatively charged uranyl carbonate complexes on the cell surface might have resulted in extracellular precipitation. The toxicity of U observed under GC1 being higher than that under GC2 could also be attributed to the preferential adsorption of U on cell surfaces under GC1. This work provides a vivid description of the interaction of U complexes with bacterial cells. The findings have implications for the toxicity of various U species and for developing biological aqueous effluent waste treatment strategies.

IMPORTANCE

The present study provides illustrative insights into the interaction of uranium (U) complexes with recombinant bacterial cells overexpressing phosphatases. This work demonstrates the effects of aqueous speciation of U on the biosorption of U and the localization pattern of uranyl phosphate precipitated as a result of phosphatase action. Transmission electron microscopy revealed that location of uranyl phosphate (cell associated or extracellular) was primarily influenced by aqueous uranyl species present under the given geochemical conditions. The data would be useful for understanding the toxicity of U under different geochemical conditions. Since cell-associated precipitation of metal facilitates easy downstream processing by simple gravity-based settling down of metal-loaded cells, compared to cumbersome separation techniques, the results from this study are of considerable relevance to effluent treatment using such cells.

Characterization of antibiotic resistant and enzyme producing bacterial strains isolated from the Arabian Sea

Marine bacteria are known to produce many bioactive molecules and extracellular enzymes of commercial importance. We have investigated the bacterial diversity of the coastal area of Karwar, Karnataka State, India. Among these bacterial isolates, five bacterial strains were selected and identified by their morphological, biochemical characteristics and phylogenetic analysis based on 16S rRNA gene sequences. The identified bacterial isolates, Bacillus toyonensis PNTB1, Lysinibacillus sphaericus PTB, Vibrio vulnificus PMD, Shewanella MPTDBS, and Pseudomonas chlororaphis PNTB were characterized for their tolerance to salt and antibiotics. Vibrio vulnificus PMD showed maximum tolerance at higher concentration of salt than other bacteria. These bacterial strains were screened for the production of extracellular enzymes such as lipase, cellulase, pectinase, tannase, chitinase, and l-glutaminase. Vibrio vulnificus showed maximum production of l-glutaminase enzyme. Bacillus toyonensis PNTB1 shows lipase, CM-cellulase and chitinase activities. These isolated bacterial cultures were also utilized most of the aromatic compounds at 7 mM. These findings indicate the organisms present in this zone may have more potential applications in bioremediation, agricultural, industrial, and therapeutics.

Bioreduction and reoxidation of uranium enhanced by thiol functional groups in natural organic matter

Although natural organic matter (NOM) is known to affect biological reduction of U(VI) and subsequent reoxidation of U(IV), the underlying mechanisms remain unclear. This study investigated the redox reactions of sulfide with NOM to form thiol functional groups, which can greatly enhance U(VI) bioreduction and U(IV) reoxidation. Results showed that humic acid (HA) was found to be more effective than fulvic acid (FA) in producing thiol groups, both U(VI) bioreduction and U(IV) reoxidation rates increased with the increase of thiols content in HA and FA. These findings suggested that among other redox sites, thiol groups in NOM may play an important role in the electron transport between uranium and microbial cells, and are of great environmental implications because they provided direct proof that thiol groups are responsible for bioremediation and immobilization of uranium when it enters into the natural environments such as soil and groundwater.

Binding of Shewanella FadR to the fabA fatty acid biosynthetic gene: implications for contraction of the fad regulon

The Escherichia coli fadR protein product, a paradigm/prototypical FadR regulator, positively regulates fabA and fabB, the two critical genes for unsaturated fatty acid (UFA) biosynthesis. However the scenario in the other Ɣ-proteobacteria, such as Shewanella with the marine origin, is unusual in that Rodionov and coworkers predicted that only fabA (not fabB) has a binding site for FadR protein. It raised the possibility of fad regulon contraction. Here we report that this is the case. Sequence alignment of the FadR homologs revealed that the N-terminal DNA-binding domain exhibited remarkable similarity, whereas the ligand-accepting motif at C-terminus is relatively-less conserved. The FadR homologue of S. oneidensis (referred to FadR_she) was over-expressed and purified to homogeneity. Integrative evidence obtained by FPLC (fast protein liquid chromatography) and chemical cross-linking analyses elucidated that FadR_she protein can dimerize in solution, whose identity was determined by MALDI-TOF-MS. In vitro data from electrophoretic mobility shift assays suggested that FadR_she is almost functionally-exchangeable/equivalent to E. coli FadR (FadR_ec) in the ability of binding the E. coli fabA (and fabB) promoters. In an agreement with that of E. coli fabA, S. oneidensis fabA promoter bound both FadR_she and FadR_ec, and was disassociated specifically with the FadR regulatory protein upon the addition of long-chain acyl-CoA thioesters. To monitor in vivo effect exerted by FadR on Shewanella fabA expression, the native promoter of S. oneidensis fabA was fused to a LacZ reporter gene to engineer a chromosome fabA-lacZ transcriptional fusion in E. coli. As anticipated, the removal of fadR gene gave about 2-fold decrement of Shewanella fabA expression by β-gal activity, which is almost identical to the inhibitory level by the addition of oleate. Therefore, we concluded that fabA is contracted to be the only one member of fad regulon in the context of fatty acid synthesis in the marine bacteria Shewanella genus.

Recovery of critical metals using biometallurgy

The increased development of green low-carbon energy technologies that require platinum group metals (PGMs) and rare earth elements (REEs), together with the geopolitical challenges to sourcing these metals, has spawned major governmental and industrial efforts to rectify current supply insecurities. As a result of the increasing critical importance of PGMs and REEs, environmentally sustainable approaches to recover these metals from primary ores and secondary streams are needed. In this review, we define the sources and waste streams from which PGMs and REEs can potentially be sustainably recovered using microorganisms, and discuss the metal-microbe interactions most likely to form the basis of different environmentally friendly recovery processes. Finally, we highlight the research needed to address challenges to applying the necessary microbiology for metal recovery given the physical and chemical complexities of specific streams.