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Znucalite is a zinc uranyl-carbonate mineral that was until recently only partially characterized with a formula originally given as Zn12Ca(UO2)(CO3)3(OH)22·4H2O, with an unknown crystal structure and ambiguous symmetry determinations. We have reinvestigated this mineral using three-dimensional electron diffraction (3D ED) and powder X-ray diffraction and revealed for the first time its structural details. Znucalite is unambiguously monoclinic, P21/m, with a = 10.722(2) Å, b = 6.259(1) Å, c = 25.355(1) Å, β = 101.13(1)°, and V = 1669.54(9) Å3. The structure refinement of the 3D ED data using the dynamical approach (Robs = 0.1594 for 3579 observed reflections and 244 parameters) provided the following structure model. Znucalite possesses a layered structure, with a [Zn10(OH)14(CO3)2] double sheet (with Zn2+ both in octahedra and tetrahedra), which is connected to a thick interlayer that hosts U6+, Ca2+, and H2O molecules. The linkage between structural units and the interlayer occurs via the vertices of ZnO4 tetrahedra protruding from the sheet. In the interlayer, differences in ordering between U and Ca take place and likely cause the difficulties encountered during the attempts to solve the structure. The refined structural formula of znucalite, Zn10Ca0.828[UO2]0.828[CO3]4(OH)15.312(H2O)5.484, corresponds well to the composition obtained from the electron-microprobe analyses, (Zn9.84Al0.16)Σ10.00 Ca0.83(UO2)0.80[(CO3)3.96(SO4)0.04]Σ4.00(OH)15.42(H2O)5.48. Raman spectroscopy evidenced the presence of several non-equivalent CO3 groups, as well as OH and H2O. The U-O bond lengths obtained from the stretching frequencies of U O 2 2 + vibrations are in line with the structural model. A discussion on the environmental importance of znucalite is appended, based on geochemical calculations with an estimate of the solubility product for this mineral. [ABSTRACT FROM AUTHOR]

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Uranium was immobilized with hydroxyapatite and the dissolution stability of the immobilization product was investigated. The immobilization efficiency and capacity of U were respectively almost 100% and 105 mg/g at the optimal conditions that were experimentally determined. Meta-autunite was found in the mineral phases of the immobilization product, and its dissolved concentration of U was below 0.1 mg/L pH of 5–7 and only 34 μg/L from the toxicity characteristic leaching procedure (TCLP) test. A thermodynamic model of dissolution was established to reveal the mechanism of high dissolution stability of the immobilization product. [ABSTRACT FROM AUTHOR]

The mineralization of uranium (U(VI)) has emerged as a highly effective strategy for mitigating its potential hazards to biological organisms. This study focused on synthesizing two hydroxyapatite nanoparticles labeled as nHAP 2h and nHAP 48h , varying in size and microstructure by adjusting the stirring time, to investigate their U(VI) mineralization behavior and mechanisms. Notably, the maximum U(VI) adsorption capacity of nHAP 2h reached 1643 mg/g at room temperature, surpassing that of nHAP 48h (1465 mg/g) and most other known calcium phosphate powder materials. Through X-ray diffraction analysis, the transformation of U(VI) ions into meta-autunite [Ca(UO 2) 2 (PO 4) 2 ∙2H 2 O] was confirmed, highlighting a chemisorption-dominant immobilization process, substantiated by scanning electronic microscopy. The phosphate release rate of nHAP 48h exhibited a substantial increase after 10 min, followed by a deceleration, whereas that of nHAP 2h demonstrated a continuous escalation. Corresponding to their phosphate release kinetics, both nHAPs exhibited distinct phases in their adsorption kinetics: a swift phosphate dissolution-precipitation regulated adsorption stage followed by a prolonged inner particle diffusion-driven adsorption stage. The phosphate release behavior emerged as the critical determinant for the U(VI) immobilization efficiency of the nHAPs, offering crucial insights into nano-mineral crystal growth and the development of highly efficient adsorbents. [Display omitted] • Two nHAPs with different sizes and microstructures were synthesized by varying the wet-precipitation stirring time. • The Q m of nHAP 2h reached 1643 mg/g at room temperature, surpassing that of the majority of reported calcium phosphate powder materials. • The chemisorption kinetics of U(IV) were influenced by the phosphate release characteristics of the nHAPs. [ABSTRACT FROM AUTHOR]

A novel resin (DBGR) with α-hydroxy ketone groups was easily synthesized via condensation of 2,5-dihydroxy-1,4-benzoquinone with glyoxal, which was characterized by XPS, FTIR, SEM, TGA, EDS and BET. Experiments show that DBGR has a good uranium adsorption capacity of 741 mg g−1 and a high selectivity of uranyl ion in the presence of competing metal ions such as Ag+, Ca2+, Co2+, Mg2+, Sr2+ and Cs+. Adsorption isothermal and kinetic studies indicate that the uranium adsorption process of DBGR fits well with Langmuir isotherm and pseudo-second-order models. Analysis shows DBGR binds the uranyl ion via C=O, O–C–O and C–OH functional groups. [ABSTRACT FROM AUTHOR]

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

Uranium containing radioactive wastewater is seriously hazardous to the natural environment if it is being discharged directly. Herein, nano-flake like Fe loaded sludge carbon (Fe-SC) is synthesized by carbothermal process from Fe-rich sludge waste and applied in the immobilization of uranium in aqueous. Batch isotherm and kinetic adsorption experiments are adopted to investigate the adsorption behavior of Fe-SC to uranium in aqueous. XPS analyses were conducted to evaluate the immobilized mechanism. It was found that the carbonized temperature played significant role in the characteristics and immobilization ability of the resulted Fe-SC. The Fe-SC-800 carbonized at 800 °C takes more advantageous ability in immobilization of uranium from aqueous than the commercial available AC and powder zero valent iron. The adsorption behavior could be fitted well with the Langmuir isotherm adsorption model and pseudo-second order model. The equilibrium adsorption amount and rate for Fe-SC-800 is high to 148.99 mg g −1 and 0.015 g mg −1 min −1 , respectively. Both reductive precipitation and physical adsorption are the main mechanisms of immobilization of uranium from aqueous by Fe-SC-800. [ABSTRACT FROM AUTHOR]

[Display omitted] • A synergistic heterostructure-defect strategy is proposed for U(VI) photoreduction. • The g-C 3 N 4 /CeO 2-x shows efficient U(VI) reduction under visible light. • The separation rate and lifetime of the photoinduced carrier were greatly improved. • The photoreaction mechanism of U(VI) by g-C 3 N 4 /CeO 2-x is proposed. Photocatalytic technology can reduce easily soluble U(VI) to insoluble U(IV) particles to realize the separation of uranium. However, the complex composition in seawater or the organic matter in uranium-containing wastewater severely restricts the separation of U(VI). Herein, a C 3 N 4 -CeO 2-x heterojunction with abundant oxygen vacancies was constructed for the photoreduction of U(VI) in organic radioactive wastewater under visible light. Kinetic characterization and density functional theory (DFT) imply that the photoelectrons were transferred from g-C 3 N 4 to CeO 2-x through the built-in electric field generated by the heterostructure and were trapped by shallow traps generated by surface vacancies to achieve spatial separation. The tremendously enhanced separation rates and lifetime (∼125 %) of photoinduced carriers grant the exceptionally improved photocatalytic activity for U(VI) reduction (up to a 39-fold increase over the bulk g-C 3 N 4). The as-prepared C 3 N 4 -CeO 2-x exhibits good resilience to a variety of competing ions and over a wide range of pH values, thus it maintains excellent performance in uranium-spiked seawater and the organics (RhB) contained water. X-ray absorption fine structure (XAFS) analysis reveals that the reduction and inner-sphere surface complexation contributed to uranium immobilization. This work provides a new strategy for the separation of uranium from organic radioactive wastewater. [ABSTRACT FROM AUTHOR]

Carbonate-bound uranium (U) is critical in controlling the migration of U in circumneutral to alkaline conditions. The potential release risk of carbonate-bound U should be concerned due to the contribution of mineral replacement. Herein, we explored the fate of U during the conversion process from microbial-induced calcite to hydroxylapatite (HAP) and investigated the phase and morphology evolution of minerals and the immobilization efficiency, distribution, and stability of U. The results showed that most calcite could convert to HAP during the conversion process. The aqueous residual U was below 1.0 mg/L after U-HAP formation, and the U removal efficiencies were enhanced by 20.0–74.4% compared to the calcite precipitation process. XRD and TEM results showed that the products were a mixture of HAP and uramphite. The elemental mapping results showed that most U concentrated on uramphite while a handful of U distributed homogeneously in calcite and HAP matrixes. The stability test verified that U-bearing HAP decreased the U solubility by 98–100% relative to calcite due to the uramphite formation and U incorporation into HAP. Our findings demonstrated that the combinations of microbial-induced calcite precipitation and calcite-HAP conversion could facilitate the U immobilization in treating radioactive wastewater and soil. [Display omitted] • Microbially precipitated U-bearing calcite was prepared and converted to hydroxylapatite. • The distribution of uranium was spatially homogeneous in U-bearing calcite but heterogeneous in the conversion products. • Residual uranium reacted with phosphate to form platy uramphite and enhanced uranium removal efficiencies by 20.0–74.4%. • U-bearing calcite and hydroxylapatite are more soluble than the U-free phases. [ABSTRACT FROM AUTHOR]

• The immobilization efficiency of uranium (0.52 mmol/L) by co-precipitation is up to 92.6%. • Uranium is stably immobilized into the structure of carbonated hydroxyapatite. • Carbonate inhibits the immobilization of uranium. Due to the randomly distributed nature of uranium contamination, remediation strategies often concentrate on in-situ immobilization. Limestone can limit the migration of uranium through its spontaneous replacement reaction with phosphate, and achieve uranium immobilization. In this study, the immobilization of uranium in the deposition process of hydroxyapatite was investigated. The effect of carbonate concentration on the reaction process was studied by adding Na 2 CO 3. The deposition process is accompanied by the immobilization of uranium by co-precipitation. The immobilization (92.6%) of uranium (0.52 mmol/L) is due to the formation of calcium uranyl phosphate, increasing carbonate concentration has a significant inhibitory effect on uranium immobilization, and the 50 Mm carbonate can reduce the uranium immobilization efficiency by nearly 20%. XRD, FTIR, Raman, XPS, EA and fluorescence spectroscopy proved that co-precipitation, complexation, and encapsulation were the main mechanisms of uranium immobilization. Through determining the ratios of type-A and type-B substitutions, it reveals the immobilization of uranium weakens due to the competition of carbonate and phosphate. Our study provides the deposition of uranium in the geosphere should be taken into account for remediation and radioactive pollution disposal. This diagram shows the reaction mechanism and does not represent the exact atomic positions. [Display omitted] [ABSTRACT FROM AUTHOR]

Betafite of composition [(Ca,U) 2 (Ti,Nb,Ta) 2 O 7 ] was prepared via a solid state synthesis route. The synthesis was shown to be sensitive to initial reactant ratios, the atmosphere used (oxidising, neutral, reducing) and time. The optimum conditions for the synthesis of betafite were found to be heating the reactants required at 1150 °C for 48 h under an inert gas atmosphere. XRD characterisation revealed that the synthesised betafite contained minor impurities. EPMA analysis of a sectioned surface showed very small regions of Ca-free betafite on grain boundaries as well as minor rutile impurities. Some heterogeneity between the Nb:Ta ratio was observed by quantitative EPMA but was generally within the nomenclature requirements stated for betafite. SEM analysis revealed the synthesised betafite was comprised mostly of hexaoctohedral crystals of ∼3 μm in diameter. XPS analysis of the sample showed that the uranium in the synthesised betafite was predominately present in the U 5+ oxidation state. A minor amount of U 6+ was also detected which was possibly due to surface oxidation. [ABSTRACT FROM AUTHOR]

Uranium is a heavy metal with both chemotoxicity and radiotoxicity. Due to the increasing consumption of uranium, the remediation of uranium contamination and recovery of uranium from non-conventional approach is highly needed. Microorganism exhibits high potential for immobilization of uranium. This study for the first time isolated a marine Pseudomonas stutzeri strain MRU-UE1 with high uranium immobilization capacity of 308.72 mg/g, which is attributed to the synergetic mechanisms of biosorption, biomineralization, and bioreduction. The uranium is found to be immobilized in forms of tetragonal chernikovite (H 2 (UO 2) 2 (PO 4) 2 ·8H 2 O) by biomineralization and CaU(PO 4) 2 by bioreduction under aerobic environment, which is rarely observed and would broaden the application of this strain in aerobic condition. The protein, phosphate group, and carboxyl group are found to be essential for the biosorption of uranium. In response to the stress of uranium, the strain produces inorganic phosphate group, which transformed soluble uranyl ion to insoluble uranium-containing precipitates, and poly-β-hydroxybutyrate (PHB), which is observed for the first time during the interaction between microorganism and uranium. In summary, P. stutzeri strain MRU-UE1 would be a promising alternative for environmental uranium contamination remediation and uranium extraction from seawater. [Display omitted] • Pseudomonas stutzeri strain is used for uranium accumulation for the first time. • P. stutzeri strain MRU-UE1 exhibits uranium immobilization capacity of 308.72 mg/g. • Synergy of biosorption, biomineralization, and bioreduction is observed. • Bioreduction of uranium by bacteria happens under aerobic environment. • Production of PHB is observed for the first time to resist the toxicity of uranium. [ABSTRACT FROM AUTHOR]

Abstract: Regions within the U.S. Department of Energy Hanford 300 Area (300 A) site experience periodic hydrologic influences from the nearby Columbia River as a result of changing river stage, which causes changes in groundwater elevation, flow direction and water chemistry. An important question is the extent to which the mixing of Columbia River water and groundwater impacts the speciation and mobility of uranium (U). In this study, we designed experiments to mimic interactions among U, oxic groundwater or Columbia River water, and 300 A sediments in the subsurface environment of Hanford 300 A. The goals were to investigate mechanisms of: 1) U immobilization in 300 A sediments under bulk oxic conditions and 2) U remobilization from U-immobilized 300 A sediments exposed to oxic Columbia River water. Initially, 300 A sediments in column reactors were fed with U(VI)-containing oxic 1) synthetic groundwater (SGW), 2) organic-amended SGW (OA-SGW), and 3) de-ionized (DI) water to investigate U immobilization processes. After that, the sediments were exposed to oxic Columbia River water for U remobilization studies. The results reveal that U was immobilized by 300 A sediments predominantly through reduction (80–85%) when the column reactor was fed with oxic OA-SGW. However, U was immobilized by 300 A sediments through adsorption (100%) when the column reactors were fed with oxic SGW or DI water. The reduced U in the 300 A sediments fed with OA-SGW was relatively resistant to remobilization by oxic Columbia River water. Oxic Columbia River water resulted in U remobilization (∼7%) through desorption, and most of the U that remained in the 300 A sediments fed with OA-SGW (∼93%) was in the form of uraninite nanoparticles. These results reveal that: 1) the reductive immobilization of U through OA-SGW stimulation of indigenous 300 A sediment microorganisms may be viable in the relatively oxic Hanford 300 A subsurface environments and 2) with the intrusion of Columbia River water, desorption may be the primary process resulting in U remobilization from OA-SGW-stimulated 300 A sediments at the subsurface of the Hanford 300 A site. [Copyright &y& Elsevier]

Abstract: The chemical and biological processes underlying in situ bioremediation of uranium-contaminated groundwater have been studied in the laboratory and in the field. This article focuses on the long-term stability of uraninite (UO2) in the underground. A large tailings pond, ‘Dänkritz 1’ in Germany, was selected for this investigation. A single-pass flow-through experiment was run in a 100-liter column: bioremediation for 1year followed by infiltration of tap water (2.5years) saturated with oxygen, sufficient to oxidize the precipitated uraninite in two months. Instead, only 1wt.% uraninite was released over 2.4years at concentrations typically less than 20μg/L. Uraninite was protected against oxidation by the mineral mackinawite (FeS0.9), a considerable amount of which had formed, together with uraninite. A confined field test was conducted adjacent to the tailings pond, which after bio-stimulation showed similarly encouraging results as in the laboratory. Taking Dänkritz 1 as an example we show that in situ bioremediation can be a viable option for long-term site remediation, if the process is designed based on sufficient laboratory and field data. The boundary conditions for the site in Germany are discussed. [Copyright &y& Elsevier]

Abstract: Microbial melanin production by autochthonous bacteria was explored in this study as a means to increase U immobilization in U contaminated soil. This article demonstrates the application of bacterial physiology and soil ecology for enhanced U immobilization in order to develop an in situ, U bio-immobilization technology. We have demonstrated microbial production of a metal chelating biopolymer, pyomelanin, in U contaminated soil from the Tims Branch area of the Department of Energy (DOE), Savannah River Site (SRS), South Carolina, as a result of tyrosine amendments. Bacterial densities of pyomelanin producers were >106 cells per g wet soil. Pyomelanin demonstrated U complexing and mineral binding capacities at pH 4 and 7. In laboratory studies, in the presence of goethite or illite, pyomelanin enhanced U sequestration by these minerals. Tyrosine amended soils in a field test demonstrated increased U sequestration capacity following pyomelanin production up to 13 months after tyrosine treatments. [Copyright &y& Elsevier]

Hexavalent uranium [U(VI)] was immobilized in biofilms composed of the sulfate reducing bacteria (SRB), Desulfovibrio desulfuricans G20. The biofilms were grown in two flat-plate, continuous-flow reactors using lactate as the electron donor and sulfate as the electron acceptor. The growth medium contained uranium U(VI) and the pH was maintained constant using bicarbonate buffer. The reactors were operated for 5 months, and during that time biofilm activity and uranium removal were evaluated. The efficiency of uranium removal strongly depended on the concentration of uranium in the influent, and was estimated to be 30.4% in the reactor supplied with 3 mg/L of U(VI) and 73.9% in the reactor supplied with 30 mg/L of U(VI). TEM and SAED analysis showed that uranium in both reactors accumulated mostly on microbial cell membranes and in the periplasmic space. The deposits had amorphous or poor nanocrystalline structures. [ABSTRACT FROM AUTHOR]

• Reduction of U(VI) to insoluble U(IV) is the major driving force for U immobilization by ZVI nanoparticles. • Carboxymethyl cellulose works better than starch to enhance the transport ability of ZVI nanoparticles. • No re-mobilization of treated U was detected when aged for 6 months under anoxic conditions. • CMC-ZVI nanoparticles treated U had almost no inhibition for the natural bacteria activity. Uranium is one of the most commonly detected radionuclides in the environment. Of the two most predominant oxidation states, U(VI) is much more soluble, mobile and toxic than U(IV). Consequently, converting U(VI) to U(IV) can facilitate the removal of U from water and reduce its mobility and biological exposure. In this work, stabilized zero-valent iron (ZVI) nanoparticles were prepared using starch or carboxymethyl cellulose (CMC) as stabilizers and then tested for reductive removal of U(VI) from simulated groundwater. Nearly 100% removal of U(VI) (initial U = 25 mg/L) was achieved using CMC-stabilized ZVI (Fe = 35 mg/L) at pH 6. In pH range of 6–9, the lower pH favored the reaction. CMC-ZVI nanoparticles presented better deliverability than starch-ZVI, while bare ZVI nanoparticles was almost trapped in the soil column. CMC-ZVI worked effectively in the presence of a model humic acid (up to 10 mg/L as TOC) and bicarbonate (1 mM), though higher dosages of the ligands inhibited U(VI) removal. After treatment, no re-mobilization of U was detected when aged for 6 months under anoxic conditions and the addition of strong ligands only remobilized U(VI). When exposed to oxic conditions, the immobilized U will be partially oxidized and remobilized due to the ingress of atmospheric O 2 and CO 2. In terms of toxicity reduction, the ZVI treated U had almost no inhibition for natural bacteria activity, while dissolved U(VI) showed significant inhibitive effects. The CMC-ZVI nanoparticles may serve as effective reactive materials to facilitate immobilization of U(VI) in groundwater, which in turn can greatly mitigate the human exposure and toxic effects of U on biota. [ABSTRACT FROM AUTHOR]