The chemical stability of biogenic UO 2 , a nanoparticulate product of environmental bioremediation, may be impacted by the particles' surface free energy, structural defects, and compositional variability in analogy to abiotic UO 2+x (0 e x e 0.25). This study quantifies and compares intrinsic solubility and dissolution rate constants of biogenic nano-UO 2 and synthetic bulk UO 2.00 , taking molecular-scale structure into account. Rates were determined under anoxic conditions as a function of pH and dissolved inorganic carbon in continuous-flow experiments. The dissolution rates of biogenic and synthetic UO 2 solids were lowest at near neutral pH and increased with decreasing pH. Similar surface area-normalized rates of biogenic and synthetic UO 2 suggest comparable reactive surface site densities. This finding is consistent with the identified structural homology of biogenic UO 2 and stoichiometric UO 2.00 . Compared to carbonate-free anoxic conditions, dissolved inorganic carbon accelerated the dissolution rate of biogenic UO 2 by 3 orders of magnitude. This phenomenon suggests continuous surface oxidation of U(IV) to U(VI), with detachment of U(VI) as the rate-determining step in dissolution. Although reducing conditions were maintained throughout the experiments, the UO 2 surface can be oxidized by water and radiogenic oxidants. Even in anoxic aquifers, UO 2 dissolution may be controlled by surface U(VI) rather than U(IV) phases.

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U(VI) adsorption on aerosol-synthesized hematite particles ranging in size from 12 to 125 nm was studied to explore nanoscale size effects on uranium adsorption. Adsorption on 70 nm aqueous-synthesized particles was also investigated to examine the effect of the synthesis method on reactivity. Equilibrium adsorption was measured over pH 3-11 at two U(VI) loadings. Surface complexation modeling, combined with adjustment of adsorption equilibrium constants to be independent of site density and surface area, provided a quantitative reaction-based framework for evaluating adsorption affinity and capacity. Among the aerosol-synthesized particles, the adsorption affinity decreased as the particle size increased from 12 to 125 nm with similar intermediate affinities for 30 and 50 nm particles. X-ray absorption fine structure spectroscopy measurements suggest that the differences in adsorption affinity and capacity are not the result of substantially different coordination environments of adsorbed U(VI).

The molecular-scale immobilization mechanisms of uranium uptake in the presence of phosphate and goethite were examined by extended X-ray absorption fine structure (EXAFS) spectroscopy. Wet chemistry data from U(VI)-equilibrated goethite suspensions at pH 4-7 in the presence of ~100 μM total phosphate indicated changes in U(VI) uptake mechanisms from adsorption to precipitation with increasing total uranium concentrations and with increasing pH. EXAFS analysis revealed that the precipitated U(VI) had a structure consistent with the meta-autunite group of solids. The adsorbed U(VI), in the absence of phosphate at pH 4-7, formed bidentate edge-sharing, ≡ Fe(OH)(2)UO(2), and bidentate corner-sharing, (≡ FeOH)(2)UO(2), surface complexes with respective U-Fe coordination distances of ~3.45 and ~4.3 Å. In the presence of phosphate and goethite, the relative amounts of precipitated and adsorbed U(VI) were quantified using linear combinations of the EXAFS spectra of precipitated U(VI) and phosphate-free adsorbed U(VI). A U(VI)-phosphate-Fe(III) oxide ternary surface complex is suggested as the dominant species at pH 4 and total U(VI) of 10 μM or less on the basis of the linear combination fitting, a P shell indicated by EXAFS, and the simultaneous enhancement of U(VI) and phosphate uptake on goethite. A structural model for the ternary surface complex was proposed that included a single phosphate shell at ~3.6 Å (U-P) and a single iron shell at ~4.3 Å (U-Fe). While the data can be explained by a U-bridging ternary surface complex, (≡ FeO)(2)UO(2)PO(4), it is not possible to statistically distinguish this scenario from one with P-bridging complexes also present.

Chromite ore processing residue (COPR) is generated by the roasting of chromite ores for the extraction of chromium. Leaching of carcinogenic hexavalent chromium (Cr(VI)) from COPR dumpsites and contamination of groundwater is a key environmental risk. The objective of the study was to evaluate Cr(VI) contamination in groundwater in the vicinity of three COPR disposal sites in Uttar Pradesh, India, in the pre-monsoon and monsoon seasons. Groundwater samples (n = 57 pre-monsoon, n = 70 monsoon) were taken in 2014 and analyzed for Cr(VI) and relevant hydrochemical parameters. The site-specific ranges of Cr(VI) concentrations in groundwater were <0.005 to 34.8 mg L (Rania), <0.005 to 115 mg L (Chhiwali), and <0.005 to 2.0 mg L (Godhrauli). Maximum levels of Cr(VI) were found close to the COPR dumpsites and significantly exceeded safe drinking water limits (0.05 mg L). No significant dependence of Cr(VI) concentration on monsoons was observed.

Chromium pollution of soil and water is a serious environmental concern due to potential carcinogenicity of hexavalent chromium [Cr(VI)] when ingested. Eucalyptus bark biochar (EBB), a carbonaceous black porous material obtained by pyrolysis of biomass at 500 °C under oxygen-free atmosphere, was used to investigate the removal of aqueous Cr(VI) upon interaction with the EBB, the dominant Cr(VI) removal mechanism(s), and the applicability to treat Cr(VI)-contaminated wastewater. Batch experiments showed complete removal of aqueous Cr(VI) at pH 1-2; sorption was negligible at pH 1, but ~55% of total Cr was sorbed onto the EBB surface at pH 2. Detailed investigations on unreacted and reacted EBB through Fourier transform infrared spectroscopy and X-ray photoelectron spectrometry (XPS) indicate that the carboxylic groups in biochar played a dominant role in Cr(VI) sorption, whereas the phenolic groups were responsible for Cr(VI) reduction. The predominance of sorption-reduction mechanism was confirmed by XPS studies that indicated ~82% as Cr(III) and ~18% as Cr(VI) sorbed on the EBB surface. Significantly, Cr(VI) reduction was also facilitated by dissolved organic matter (DOM) extracted from biochar. This reduction was enhanced by the presence of biochar. Overall, the removal of Cr(VI) in the presence of biochar was affected by sorption due to electrostatic attraction, sorption-reduction mediated by surface organic complexes, and aqueous reduction by DOM. Relative dominance of the aqueous reduction mechanism depended on a critical biochar dosage for a given electrolyte pH and initial Cr(VI) concentration. The low-cost EBB developed here successfully removed all Cr(VI) in chrome tanning acidic wastewater and Cr(VI)-contaminated groundwater after pH adjustment, highlighting its potential applicability in effective Cr(VI) remediation.

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Groundwater contamination with fluoride (F) and uranium (U) has been reported in many parts of India. However, the sources and mobilization mechanisms of these contaminants remain poorly understood. The present study aimed to identify the processes governing the coexistence of elevated F and U in groundwater at a typical site in India’s middle Gangetic plain. Sustained groundwater sampling at 21 locations over two years indicated persistence of high F and U in a shallow aquifer (12 m depth), but absence of these pollutants in a deeper aquifer (30 m depth). For both the aquifers, Mn exhibited strong inverse correlations with F (−0.587; p < 0.01) and U (−0.581; p < 0.01). X-ray diffraction analysis of representative sediment cores indicated few differences in the mineralogy of the two aquifers, which consisted of fluorite (CaF2(s)) and calcite (CaCO3(s)), among others. Analysis of groundwater speciation and saturation state and sequential extraction on aquifer sediments suggested that elevated F in shallow groundwater occurred due to calcite precipitation-induced fluorite dissolution. The conditions in both the aquifers were oxidizing with respect to U, but reducing with respect to Mn. Elevated U was attributed to carbonate-promoted mobilization from iron–manganese (Mn) and residual sediment fractions. In the deeper groundwater, elevated Mn and lower pH levels persisted with conditions at saturation with respect to rhodochrosite (MnCO3(s)). Furthermore, medium (30 d) to long-term (300 d) batch experiments were performed to systematically evaluate the role of variable Mn on calcite precipitation under approximate in situ conditions. Precipitation of rhodochrosite outcompeted calcite precipitation and resulted in lower pH compared to pH of Mn-free systems, which (a) inhibited calcite precipitation and associated fluorite dissolution and (b) constrained pH and alkalinity in the deeper groundwater. These findings have implications for understanding F and U mobilization in comparable Mn-deficient sites and development of appropriate Mn-based amendments for in situ remediation.