Uranyl Phosphate Minerals Research Articles

Synergistic effects of hydrogen peroxide and phosphate on uranium(VI) immobilization: implications for the remediation of groundwater at decommissioned in situ leaching uranium mine.

The processes of acid in situ leaching (ISL) uranium (U) mines cause the pollution of groundwater. Phosphate (PO43-) has the potential to immobilize U in groundwater through forming highly insoluble phosphate minerals, but the performance is highly restricted by low pH and high sulfate concentration. In this study, hydrogen peroxide (H2O2) and PO43- were synergistically used for immobilizing U based on the specific properties of groundwater from a decommissioned acid ISL U mine. The removal mechanisms of U and the stability of U on the formed minerals were elucidated by employing X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and kinetic experiments. Our results indicated that the removal of U by simultaneously adding H2O2 and PO43- was significantly higher than the removal of U by individually adding H2O2 or PO43-. The removal of U increased with increasing PO43- concentration from 20 to 200 mg L-1 while decreased with increasing H2O2 concentration from 0.003 to 0.3%. Specifically, the removal efficiency of U from groundwater reached 98% after the application of 0.003% H2O2 and 200 mg L-1 PO43-. Amorphous iron phosphate that preferentially formed at low H2O2 and high PO43- concentrations played a dominant role in U removal, while the formations of schwertmannite and crystalline iron phosphates may be also contributed to the removal of U. This was significantly different from the immobilization mechanism of U through the formation of uranyl phosphate minerals after adding phosphate. The kinetic experimental results suggested that the immobilized U had a good stability. Our research may provide a promising method for in situ remediating U-contaminated groundwater at the decommissioned acid ISL U mines.

Structure, Stability, and Acidity of the Uranyl Arsenate Dimer in Aqueous Solution.

The migration of uranium (U) in the surficial environment has received considerable attention. Due to their high natural abundance and low solubility, autunite-group minerals play a key role in controlling the mobility of U. However, the formation mechanism for these minerals has yet to be understood. In this work, we took the uranyl arsenate dimer ([UO2(HAsO4)(H2AsO4)(H2O)]22-) as a model molecule and carried out a series of first-principles molecular dynamics (FPMD) simulations to explore the early stage of the formation of trögerite (UO2HAsO4·4H2O), a representative autunite-group mineral. By using the potential-of-mean-force (PMF) method and vertical energy gap method, the dissociation free energies and the acidity constants (pKa's) of the dimer were calculated. Our results show that the U in the dimer holds a 4-coordinate structure, which is consistent with the coordination environment observed in trögerite mineralogy, in contrast to the 5-coordinate U in the monomer. Furthermore, the dimerization is thermodynamically favorable in solution. The FPMD results also suggest that tetramerization and even polyreactions would occur at pH > 2, as observed experimentally. Additionally, it is found that trögerite and the dimer have very similar local structural parameters. These findings imply that the dimer could serve as an important link between the U-As complexes in solution and the autunite-type sheet of trögerite. Given the nearly identical physicochemical properties of arsenate and phosphate, our findings suggest that uranyl phosphate minerals with the autunite-type sheet may form in a similar manner. This study therefore fills a critical gap in atomic-scale knowledge of the formation of autunite-group minerals and provides a theoretical basis for regulating uranium mobilization in P/As-bearing tailing water.

Exploring the Role of Organic Functional Groups in the Ionothermal Synthesis of Uranyl Phosphate Materials

AbstractFour new hybrid organic‐inorganic uranyl phosphate compounds were structurally characterized after single crystals were grown via ionothermal synthesis using the ionic liquids 1‐ethyl‐3‐methylimidazolium dimethyl phosphate and 1‐ethyl‐3‐methylimidazolium dibutyl phosphate. Three of the new crystal structures presented here incorporate dimethyl or monomethyl phosphate ligands into the structural unit to form one chain‐based compound and two sheet‐based compounds. One of the structures utilizes dibutyl phosphate to form a previously reported uranyl compound, a structural polymorph that crystallized in space groupP21/cin contrast to the previously reportedP‐1 structure. The structural and topological relationships are compared to those of uranyl phosphate minerals as well as previously reported organophosphate compounds. The roles of the organics on the phosphate anions in impacting uranyl coordination and stabilizing the crystal structures are discussed.

Mechanisms and kinetics of citrate-promoted dissolution of a uranyl phosphate mineral

The formation of uranyl phosphate precipitates is a remediation strategy to limit the mobility of uranium in contaminated soils. However, exposure to organic ligands, like the plant exudate citrate, can remobilize the uranium. The purpose of this study is to provide a more thorough comprehension of citrate-promoted dissolution of a uranyl phosphate mineral, chernikovite [(H3O)(UO2)(PO4)·3H2O], by determining the extent of uranium release from chernikovite at a wide range of citrate concentrations. We have quantified the kinetics of dissolution and proposed potential mechanisms of chernikovite dissolution to gain a better understanding of the fate of uranyl phosphate precipitates in the environment. Batch dissolution and continuously stirred tank reactor (CSTR) experiments indicate that increasing citrate concentrations from 0.1 mM to 50 mM increases the concentration of dissolved uranium. However, at citrate concentrations of 10 mM and greater, the effectiveness of the ligand to enhance the dissolution decreases. Density functional theory modeling and Raman spectroscopy indicate a strong interaction between citrate and the uranyl ions at the surface of the uranyl phosphate. The interaction between citrate and uranyl suggests the formation of an alteration layer of uranyl-citrate surface complexes, which could impede uranium release at citrate concentrations of 10 mM or higher. Together, these results indicate that the citrate-promoted dissolution of chernikovite is a fast reaction that is hindered by a combination of surface saturation and a secondary-phase precipitation reaction at higher concentrations of citrate and reaction times greater than or equal to 72 min based on the hydraulic residence time of CSTR experiments. These findings improve our understanding of the stability of uranyl phosphate for use in sequestration of uranium groundwater plumes at contaminated sites.

Traces of hydrocarbon-bearing fluid and microbial activities and their implications for uranium mineralization in southern Ordos basin, China

The sandstone-type uranium mineralization in the Shuanglong area of the Ordos basin, has been studied from the point view of organic geochemistry and mineralogy. Acidolysis hydrocarbon compositions within sandstones, and occurrences of uranium minerals were analyzed by using optical microscope, gas chromatograph, scanning electron microscopy coupled with energy dispersive spectroscopy, and electron probe. The results show the methane is dominant among acidolysis hydrocarbon components, and the natural gas is oil-type gas. According to the escape time of oil -gas, the main mineralization period, and spatial relations of oil–gas and ore bodies, it is inferred that the hydrocarbon-bearing fluid is mainly from the lacustrine source rocks of the Triassic Yanchang Formation (T3y), and the uranium-bearing stratum might undergo hydrocarbon reduction. Uranium minerals occur in forms of microspheres outlining at the cell lumens of organic matter, and uranyl phosphate minerals are also observed not alongside Fe-oxyhydroxides, indicating that bio-mineralization exits. The fact that carbonaceous debris (i.e., CD) partly underwent biodegradation is evidenced by the front-peak type n-alkanes compounds with the presence of unknown complex mixtures. Besides, numbers of framboidal pyrites with the diameter of 1 µm occur in CD, implying that microbial activities exist in uranium-bearing stratum. The maximum paleogeothermal temperature of the Jurassic Zhiluo Formation (J2z) shows that the geological environment is favorable for the activity of microorganisms. Under the combined actions of deep hydrocarbon-bearing fluid, microbial activities, CD, and pyrites, aqueous uranium is precipitated and enriched in forms of adsorption and reduction, which is helpful for further understanding the mechanism of sandstone-type uranium mineralization, thus providing certain guide for uranium exploration.

Dissolved Carbonate and pH Control the Dissolution of Uranyl Phosphate Minerals in Flow-Through Porous Media.

Uranyl phosphate minerals represent an important secondary source of uranium release at contaminated sites. In flow-through column experiments with background porewater (BPW) of typical freshwater aquifer composition (pH 7.0, ∼0.2 mM total carbonate (TC)), dissolution of K-ankoleite (KUO2PO4·3H2O), Na-autunite (NaUO2PO4·3H2O), and Ca-autunite (Ca(UO2)2(PO4)2·6H2O) was controlled by mineral solubility at steady-state U release. Effluent concentrations indicated exchange with BPW cations, and postreaction characterization showed alteration of the initial mineral composition, changes in structure (decreased crystallinity, increased disorder, and distortion of U-P mineral sheets) and possible neoformation of phases of similar structure. Increasing the BPW pH and TC to 8.1-8.2 and 2.2-3.7 mM, respectively, resulted in mineral undersaturation and produced ca. 2 orders-of-magnitude higher U and P release without reaching steady state. Minerals incorporated less BPW cations into their structures compared to low carbonate BPW experiments but showed structural disorder and distortion. Faster dissolution rates were attributed to the formation of binary and ternary uranyl carbonate complexes that accelerate the rate-determining step of uranyl detachment from the uranyl-phosphate layered structure. Calculated dissolution rates (log Rs between -8.95 and -10.32 mol m-2 s-1), accounting for reaction and transport in porous media, were similar to dissolution rates of other classes of uranyl minerals. In undersaturated solutions, dissolution rates for uranyl phosphate, oxyhydroxide, and silicate minerals can be predicted within 1-2 orders-of-magnitude from pH ∼5-10 on the basis of pH/carbonate concentration.

Krystalová struktura phurcalitu, Ca2[(UO2)3O2(PO4)2]·7H2O, z Jáchymova

A rare supergene uranyl phosphate mineral, phurcalite, was found on a few specimens originating from the dump material of the Eduard shaft, the Jáchymov ore district, Czech Republic. Phurcalite forms yellow to yellowish-orange perfect prismatic crystals, reaching up to 3 - 4 mm in cavities of vuggy quartz-dominated gangue. Phurcalite was found in the association with walpurgite, uranophane-α, and members of the metatorbernite-metazeunerite series. According to single-crystal X-ray data phurcalite is orthorhombic, space group Pbca, with a 17.3785(8), b 15.9864(6), c 13.5477(6) Å, and V 3763.8(3) Å3. Its crystal structure has been refined to R = 3.56 % for 3488 unique observed reflections [Iobs>3σ(I)] collected on a Rigaku SuperNova X-ray diffractometer with an Atlas S2 CCD detector and focused MoKα radiation. The results of the structure refinement are in line with the recently published structure refinement of phurcalite from Shinkolobwe (Africa). Nevertheless, in phurcalite from Jáchymov, the substitution of As for P takes place at greater extent. The structural formula obtained for the crystal from Jáchymov is Ca2[(UO2)3O2(PO4)1.753(AsO4)0.247]·7H2O, Z = 8, Dcalc. = 4.409 g/cm3.

Autunite-Group Minerals and Their Paragenesis from the Sheared Granite of Gabal El Sela, South Eastern Desert, Egypt

G. El Sela is located in the Southern Eastern Desert of Egypt cropping as two parts, occupied by monzogranites that were categorized as biotite granite, muscovite granite and two mica granites. The northern part is more significant according its high concentrations of uranium that controlled by complicated structure regime; faulting, infrastructures and shearing are the most common structural criteria of this part. The Egyptian Nuclear Materials Authority (NMA) mined this part to produce the uranium ore. The previous mineralogical studies indicated that this granite was dominated by primary uranium minerals (pitchblende and coffinite) and secondary minerals belong to the autunite group (autunite, metautunite, phurcalite) in addition to uranophane. In the present work, petrographic and mineralogical studies are applied for the granites using the polarized and stereo microscopes and followed by electron microscope and XRD. The result of the microscopic examinations revealed the tectonic regime controlling the radioactivity and recognized the sodic autunite (meta-natroautunite) beside the pre-mentioned autunite group minerals completing the paragenetic sequrnce of these minerals. In this study, it is concluded that the sheared biotite granite is monzogranite originated during the episode of the continental plate collision (syncollision). The study finished to presence of two main types of the alteration corresponding to the two high levels of radioactivity (moderate and anomalous). The first is the thermal alteration (saussiritization, sericitization, kaolinization, silicification and hematization) and the second is the chemical transformation (oxidation, dehydration, ion substitutions and confusion) responsible for formation of the secondary uranium minerals. The temperature needed for the thermal alteration is sourced by the hydrothermal solutions, while the temperature needed for the uranium minerals transformation may be generated during the episode of the continental plate collision (syncollision). Paragenesis of these minerals indicates that they represent a series of uranyl phosphate minerals (autunite group) with paragenetic sequence starting by autunite (calcic uranyl phosphate) and ends by meta-natroautunite (sodic uranyl phosphate). An advanced process of dehydration accompanies the process of mineral transformation from autunite to meta-natroautunite leading to formation of the anhydrous uranyl mineral (phurcalite) formed by oxidation and dehydration of autunite. Meta-autunite is recorded as a transitional mineral composed of sodic-calcic uranyl phosphate. Uranophane is created by thermal confusion of autunite with the silica.

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 {X1-2[(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.

Phosphorus Stress-Induced Changes in Plant Root Exudation Could Potentially Facilitate Uranium Mobilization from Stable Mineral Forms.

Apparent deficiency of soil mineral nutrients often triggers specific physio-morphological changes in plants, and some of these changes could also inadvertently increase the ability of plants to mobilize radionuclides from stable mineral forms. This work, through a series of sand-culture, hydroponics, and batch-equilibration experiments, investigated the differential ability of root exudates of Andropogon virginicus grown under conditions with variable phosphorus (P) availability (KH2PO4, FePO4, Ca3(PO4)2, and no P) to solubilize uranium (U) from the uranyl phosphate mineral Chernikovite. The mineral form of P, and hence the bioavailability of P, affected the overall composition of the root exudates. The lower bioavailable forms of P (FePO4 and Ca3(PO4)2), but not the complete absence of P, resulted in a higher abundance of root metabolites with chelating capacity at 72 hrs after treatment application. In treatments with lower P-bioavailability, the physiological amino acid concentration inside of the roots increased, whereas the concentration of organic acids in the roots decreased due to the active exudation. In batch dissolution experiments, the organic acids, but not amino acids, increase the dissolution U from Chernikovite. The root exudate matrix of plants exposed to low available forms of P induced a >60% increase in U dissolution from Chernikovite due to 5-16 times greater abundance of organic acids in these treatments. However, this was ca. 70% of the theoretical dissolution achievable by this exudate matrix. These results highlight the potential of using active management of soil P as an effective tool to alter the plant-mediated mobilization of U in contaminated soil.

Thermodynamic properties of phosphate members of the meta-autunite group: A high-temperature calorimetric study

Samples of synthetic analogs of uranyl phosphate minerals have been prepared at room temperature by slow mixing of reactants by a diffusion method. Reaction products were analyzed using powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), inductively coupled plasma optical emission spectrophotometry (ICP-OES), and inductively coupled plasma mass spectrometry (ICP-MS). Calorimetric measurements have been performed in a Calvet-type twin calorimeter using sodium molybdate (3Na2O-4MoO3) solvent at 976K as a flux. The enthalpy of formation from the binary oxides, ΔHf-ox, at 298K was calculated for each compound from the respective drop solution enthalpies, ΔHds. Calculated standard enthalpies of formation from the elements, ΔH0f, at 298K are −3425±9kJ/mol for meta-ankoleite (KUP), −6233±17kJ/mol for meta-autunite (CaUP), −6921±13kJ/mol for meta-torbernite (CuUP), −7254±17kJ/mol for meta-saléeite (MgUP), −3264±12kJ/mol for Rb-meta-autunite (RbUP), −3580±7kJ/mol for meta-natro-autunite (NaUP), −3692±11kJ/mol for Li-meta-autunite (LiUP), −6402±5kJ/mol for meta-uranocircite (BaUP), −3277±6kJ/mol for Cs-meta-autunite (CsUP), and −7109±19kJ/mol for Co-meta-autunite (CoUP). The results exhibit trends of the thermodynamic stability of these compounds. The normalized charge deficiency per anion (NCDA) approach relates the thermodynamic stability of these compounds to their crystal structures. The thermodynamic stability of uranyl phosphate minerals is important for understanding their formation in Nature, as well as their fate in a geological repository for nuclear waste, and their existence in the subsurface of anthropogenically contaminated environments.

Groundwater uranium stabilization by a metastable hydroxyapatite

In-situ remediation of groundwater uranium (U) contamination via the precipitation of uranyl phosphate (U-P) minerals is a promising, passive remedial approach for aquifers impacted by mobile hexavalent U (U(VI)). We demonstrate the efficacy of U stabilization in a contaminated aquifer using a metastable form of hydroxyapatite (mHAP) derived from fish bone. This material was reacted with depleted uranium (DU) contaminated groundwater both under ambient flow in-situ, and under pumped flow ex-situ. The U immobilized under both ambient and accelerated flow conditions was strongly bound in solid phases, with greater than 99% U removal from groundwater. Stable U uptake in excess of 50 g U/kg solid was achieved due to the precipitation of the crystalline U-P mineral chernikovite. Prior field trials yielded U immobilization by sorption alone [Fuller et al., 2003, Env. Sci. Technol. 37, 4642.], likely due to higher pH and alkalinity groundwater conditions, which increase the solubility of U-P phases via aqueous complexation. Our study is the first to demonstrate the feasibility of U immobilization by U-P precipitation from natural groundwater. These findings suggest that in groundwaters contaminated by U(VI) with circumneutral pH and low carbonate alkalinity, fish bone-derived hydroxyapatite is an effective material for in-situ U remediation that can be readily implemented, requiring no redox manipulation.

Uranyl phosphates and associated minerals in the Koprubasi (Manisa) uranium deposit, Turkey

The Koprubasi uranium deposit is the largest known uranium deposits in Turkey. Uranium mineralization is hosted in Neogene fluvial sediments that consist of predominantly sandstone and conglomerate interlayered with siltstone, claystone and mudstone. Sediments were primarily derived from metamorphic rocks adjacent to the basins of deposition, with some volcanic contribution in the form of tuffs. The sandstones and conglomerates are the most widespread sediments within the study area and host the majority of the U ore.X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX) analysis were used to identify the ore and associated minerals. The uranium ore minerals of the host sedimentary rocks are torbernite, meta-torbernite and meta-autunite. These coexist with jarosite, various clays (chlorite/kaolinite, illite, and smectite), and Fe- and Mn-(hydr) oxides with minor titanium oxide. Additionally, quartz, feldspars and a minor amount of mica (muscovite and biotite) are the main primary minerals in the sedimentary rocks.The uranyl phosphates coat the surfaces of pebbles and some mineral grains, fill cracks, and are disseminated within the pores of the host sedimentary rocks. Uranyl phosphates are commonly associated with Fe-(hydr) oxides and clays, although some were observed in millimeter- or centimeter wide gaps within Koprubasi sediments. Oxides of manganese are also locally abundant and associated with uranyl phosphates.The uranyl phosphates in the sediments of the Koprubasi area most likely formed when oxidizing groundwater moved through the host rock. The common association of uranyl phosphates with Fe-(hydr) oxides and clays suggests that the precipitation of uranyl phosphate minerals is mainly due to adsorption of the uranyl and phosphate to the surface of Fe-(hydr) oxides and clays.

Uranium Biominerals Precipitated by an Environmental Isolate of Serratia under Anaerobic Conditions.

Stimulating the microbially-mediated precipitation of uranium biominerals may be used to treat groundwater contamination at nuclear sites. The majority of studies to date have focussed on the reductive precipitation of uranium as U(IV) by U(VI)- and Fe(III)-reducing bacteria such as Geobacter and Shewanella species, although other mechanisms of uranium removal from solution can occur, including the precipitation of uranyl phosphates via bacterial phosphatase activity. Here we present the results of uranium biomineralisation experiments using an isolate of Serratia obtained from a sediment sample representative of the Sellafield nuclear site, UK. When supplied with glycerol phosphate, this Serratia strain was able to precipitate 1 mM of soluble U(VI) as uranyl phosphate minerals from the autunite group, under anaerobic and fermentative conditions. Under phosphate-limited anaerobic conditions and with glycerol as the electron donor, non-growing Serratia cells could precipitate 0.5 mM of uranium supplied as soluble U(VI), via reduction to nano-crystalline U(IV) uraninite. Some evidence for the reduction of solid phase uranyl(VI) phosphate was also observed. This study highlights the potential for Serratia and related species to play a role in the bioremediation of uranium contamination, via a range of different metabolic pathways, dependent on culturing or in situ conditions.

Dynamic interplay between uranyl phosphate precipitation, sorption, and phase evolution

Natural examples demonstrate uranyl-phosphate minerals can maintain extremely low levels of aqueous uranium in groundwaters due to their low solubility. Therefore, greater understanding of the geochemical factors leading to uranyl phosphate precipitation may lead to successful application of phosphate-based remediation methods. However, the solubility of uranyl phosphate phases varies over >3 orders of magnitude, with the most soluble phases typically observed in lab experiments. To understand the role of common soil/sediment mineral surfaces in the nucleation and transformation of uranyl phosphate minerals under environmentally relevant conditions, batch experiments were carried out with goethite and mica at pH 6 in mixed electrolyte solutions ranging from 1–800μM U and 1–800μM P. All experiments ended with uranium concentrations below the USEPA MCL for U, but with 2–3 orders of magnitude difference in uranium concentrations. Despite the presence of many cations that are well known to incorporate into less soluble autunite-group minerals, chernikovite rapidly precipitated in all experiments containing U and P, except for solutions with 1μM U and 1μM P that were calculated to be undersaturated. Textures of uranyl phosphates observed by AFM and TEM indicate that nucleation was homogenous and independent of the initial mineral content. Comparison of time-course U and P concentrations from the experiments with thermodynamic modeling of solution equilibria demonstrated that aqueous uranium concentrations in the experimental systems evolved as increasingly sparingly soluble uranyl phosphate phases nucleated over time, with sorption accelerating the transition between phases by influencing solution chemistry. Aqueous uranium concentrations consistent with partially dehydrated (meta-) autunite were achieved only in experiments containing goethite and/or mica. These dynamic nucleation-growth-sorption-nucleation-growth-sorption cycles occur over the time scales of weeks, not hours or days at room temperature. Lab experiments and field-based investigations of uranium phosphate should consider these or longer time scales for the greatest long-term relevance.

Spectroscopic markers for uranium(vi) phosphates: a vibronic study

Uranyl phosphate minerals are studied by optical and vibrational spectroscopy which provide robust markers to discriminate among environmentally hazardous uranium phases.

A spectroscopic study on U(VI) biomineralization in cultivated Pseudomonas fluorescens biofilms isolated from granitic aquifers.

The interaction between the Pseudomonas fluorescens biofilm and U(VI) were studied using extended X-ray absorption fine structure spectroscopy (EXAFS), and time-resolved laser fluorescence spectroscopy (TRLFS). In EXAFS studies, the formation of a stable uranyl phosphate mineral, similar to autunite (Ca[UO2]2[PO4]2•2-6H2O) or meta-autunite (Ca[UO2]2[PO4]2•10-12H2O) was observed. This is the first time such a biomineralization process has been observed in P. fluorescens. Biomineralization occurs due to phosphate release from the cellular polyphosphate, likely as a cell's response to the added uranium. It differs significantly from the biosorption process occurring in the planktonic cells of the same strain. TRLFS studies of the uranium-contaminated nutrient medium identified aqueous Ca2UO2(CO3)3 and UO2(CO3)3 (4-) species, which in contrast to the biomineralization in the P. fluorescens biofilm, may contribute to the transport and migration of U(VI). The obtained results reveal that biofilms of P. fluorescens may play an important role in predicting the transport behavior of uranium in the environment. They will also contribute to the improvement of remediation methods in uranium-contaminated sites.

Decrease of U(VI) immobilization capability of the facultative anaerobic strain Paenibacillus sp. JG-TB8 under anoxic conditions due to strongly reduced phosphatase activity.

Interactions of a facultative anaerobic bacterial isolate named Paenibacillus sp. JG-TB8 with U(VI) were studied under oxic and anoxic conditions in order to assess the influence of the oxygen-dependent cell metabolism on microbial uranium mobilization and immobilization. We demonstrated that aerobically and anaerobically grown cells of Paenibacillus sp. JG-TB8 accumulate uranium from aqueous solutions under acidic conditions (pH 2 to 6), under oxic and anoxic conditions. A combination of spectroscopic and microscopic methods revealed that the speciation of U(VI) associated with the cells of the strain depend on the pH as well as on the aeration conditions. At pH 2 and pH 3, uranium was exclusively bound by organic phosphate groups provided by cellular components, independently on the aeration conditions. At higher pH values, a part (pH 4.5) or the total amount (pH 6) of the dissolved uranium was precipitated under oxic conditions in a meta-autunite-like uranyl phosphate mineral phase without supplying an additional organic phosphate substrate. In contrast to that, under anoxic conditions no mineral formation was observed at pH 4.5 and pH 6, which was clearly assigned to decreased orthophosphate release by the cells. This in turn was caused by a suppression of the indigenous phosphatase activity of the strain. The results demonstrate that changes in the metabolism of facultative anaerobic microorganisms caused by the presence or absence of oxygen can decisively influence U(VI) biomineralization.

Uranium(VI) bioprecipitation mediated by a phosphate solubilizing Acinetobacter sp. YU-SS-SB-29 isolated from a high natural background radiation site

Bioprecipitation of uranium (U) into uranyl phosphate (U-P) mediated by soluble ortho-phosphate is an attractive proposition for U bioremediation. As an alternative to the microbial phosphatase, we have investigated the dissolution of phosphate by the organic acids produced by bacteria to aid in U precipitation. The bacterium Acinetobacter sp. YU-SS-SB-29, isolated from monazite sand of natural background radiation site solubilized 952.0 ± 46.7 mg L−1 phosphate from tri-calcium phosphate (TCP) in the Pikovskaya's medium and showed tolerance to 120 ppm U(VI). U(VI) bioprecipitation was investigated by adding different concentrations of U(VI) to a cell-free culture supernatant containing ortho-phosphate released from TCP by the bacterium. A yellow precipitate was immediately formed following which there was a reduction in U(VI) concentration. A strong positive correlation (R2 = 0.98) was observed between % decrease in phosphate and U(VI) concentration (up to 750 ppm U) added. FTIR and EDX spectra of the yellow precipitate demonstrated the involvement of phosphate groups in U(VI) binding. Furthermore, the XRD pattern of the precipitate agrees well with that of chernikovite, a uranyl phosphate mineral. The results from this study demonstrate the potential of the U tolerant, phosphate solubilizing bacterium Acinetobacter sp. YU-SS-SB-29 for non-reductive in situ bioprecipitation of uranium.

Biosorption and Biomineralization of U(VI) by the Marine Bacterium Idiomarina loihiensis MAH1: Effect of Background Electrolyte and pH

The main goal of this study is to compare the effects of pH, uranium concentration, and background electrolyte (seawater and NaClO4 solution) on the speciation of uranium(VI) associated with the marine bacterium Idiomarina loihiensis MAH1. This was done at the molecular level using a multidisciplinary approach combining X-ray Absorption Spectroscopy (XAS), Time-Resolved Laser-Induced Fluorescence Spectroscopy (TRLFS), and High Resolution Transmission Electron Microscopy (HRTEM). We showed that the U(VI)/bacterium interaction mechanism is highly dependent upon pH but also the nature of the used background electrolyte played a role. At neutral conditions and a U concentration ranging from 5·10−4 to 10−5 M (environmentally relevant concentrations), XAS analysis revealed that uranyl phosphate mineral phases, structurally resembling meta-autunite [Ca(UO2)2(PO4)2 2–6H2O] are precipitated at the cell surfaces of the strain MAH1. The formation of this mineral phase is independent of the background solution but U(VI) luminescence lifetime analyses demonstrated that the U(VI) speciation in seawater samples is more intricate, i.e., different complexes were formed under natural conditions. At acidic conditions, pH 2, 3 and 4.3 ([U] = 5·10−4 M, background electrolyte = 0.1 M NaClO4), the removal of U from solution was due to biosorption to Extracellular Polysaccharides (EPS) and cell wall components as evident from TEM analysis. The L III-edge XAS and TRLFS studies showed that the biosorption process observed is dependent of pH. The bacterial cell forms a complex with U through organic phosphate groups at pH 2 and via phosphate and carboxyl groups at pH 3 and 4.3, respectively. The differences in the complexes formed between uranium and bacteria on seawater compared to NaClO4 solution demonstrates that the actinide/microbe interactions are influenced by the three studied factors, i.e., the pH, the uranium concentration and the chemical composition of the solution.