Solubility and Thermodynamic Investigation of Meta-Autunite Group Uranyl Arsenate Solids with Monovalent Cations Na and K.

We investigated the aqueous solubility and thermodynamic properties of two meta-autunite group uranyl arsenate solids (UAs). The measured solubility products (log Ksp) obtained in dissolution and precipitation experiments at equilibrium pH 2 and 3 for NaUAs and KUAs ranged from -23.50 to -22.96 and -23.87 to -23.38, respectively. The secondary phases (UO2)(H2AsO4)2(H2O)(s) and trögerite, (UO2)3(AsO4)2·12H2O(s), were identified by powder X-ray diffraction in the reacted solids of KUA precipitation experiments (pH 2) and NaUAs dissolution and precipitation experiments (pH 3), respectively. The identification of these secondary phases in reacted solids suggest that H3O+ co-occurring with Na or K in the interlayer region can influence the solubilities of uranyl arsenate solids. The standard-state enthalpy of formation from the elements (ΔHf-el) of NaUAs is -3025 ± 22 kJ mol-1 and for KUAs is -3000 ± 28 kJ mol-1 derived from measurements by drop solution calorimetry, consistent with values reported in other studies for uranyl phosphate solids. This work provides novel thermodynamic information for reactive transport models to interpret and predict the influence of uranyl arsenate solids on soluble concentrations of U and As in contaminated waters affected by mining legacy and other anthropogenic activities.

Removal of Aqueous Uranyl and Arsenate Mixtures after Reaction with Limestone, PO43-, and Ca2.

The co-occurrence of uranyl and arsenate in contaminated water caused by natural processes and mining is a concern for impacted communities, including in Native American lands in the U.S. Southwest. We investigated the simultaneous removal of aqueous uranyl and arsenate after the reaction with limestone and precipitated hydroxyapatite (HAp, Ca10(PO4)6(OH)2). In benchtop experiments with an initial pH of 3.0 and initial concentrations of 1 mM U and As, uranyl and arsenate coprecipitated in the presence of 1 g L-1 limestone. However, related experiments initiated under circumneutral pH conditions showed that uranyl and arsenate remained soluble. Upon addition of 1 mM PO43- and 3 mM Ca2+ in solution (initial concentration of 0.05 mM U and As) resulted in the rapid removal of over 97% of U via Ca-U-P precipitation. In experiments with 2 mM PO43- and 10 mM Ca2+ at pH rising from 7.0 to 11.0, aqueous concentrations of As decreased (between 30 and 98%) circa pH 9. HAp precipitation in solids was confirmed by powder X-ray diffraction and scanning electron microscopy/energy dispersive X-ray. Electron microprobe analysis indicated U was coprecipitated with Ca and P, while As was mainly immobilized through HAp adsorption. The results indicate that natural materials, such as HAp and limestone, can effectively remove uranyl and arsenate mixtures.

Kinetics of Na- and K- uranyl arsenate dissolution.

We integrated aqueous chemistry analyses with geochemical modeling to determine the kinetics of the dissolution of Na and K uranyl arsenate solids (UAs(s)) at acidic pH. Improving our understanding of how UAs(s) dissolve is essential to predict transport of U and As, such as in acid mine drainage. At pH 2, Na0.48H0.52(UO2)(AsO4)(H2O)2.5(s) (NaUAs(s)) and K0.9H0.1(UO2)(AsO4)(H2O)2.5(s) (KUAs(s)) both dissolve with a rate constant of 3.2 × 10-7 mol m-2 s-1, which is faster than analogous uranyl phosphate solids. At pH 3, NaUAs(s) (6.3 × 10-8 mol m-2 s-1) and KUAs(s) (2.0 × 10-8 mol m-2 s-1) have smaller rate constants. Steady-state aqueous concentrations of U and As are similarly reached within the first several hours of reaction progress. This study provides dissolution rate constants for UAs(s), which may be integrated into reactive transport models for risk assessment and remediation of U and As contaminated waters.

Interfacial Interactions of Uranium and Arsenic with Microplastics: From Field Detection to Controlled Laboratory Tests.

We studied the co-occurrence of microplastics (MPs) and metals in field sites and further investigated their interfacial interaction in controlled laboratory conditions. First, we detected MPs in freshwater co-occurring with metals in rural and urban areas in New Mexico. Automated particle counting and fluorescence microscopy indicated that particles in field samples ranged from 7 to 149 particles/L. The urban location contained the highest count of confirmed MPs, including polyester, cellophane, and rayon, as indicated by Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) spectroscopy analyses. Metal analyses using inductively coupled plasma (ICP) revealed that bodies of water in a rural site affected by mining legacy contained up to 332.8 µg/L of U, while all bodies of water contained As concentrations below 11.4 µg/L. These field findings motivated experiments in laboratory conditions, reacting MPs with 0.02-0.2 mM of As or U solutions at acidic and neutral pH with poly(methyl-methacrylate), polyethylene, and polystyrene MPs. In these experiments, As did not interact with any of the MPs tested at pH 3 and pH 7, nor U with any MPs at pH 3. Experiments supplied with U and MPs at pH 7 indicated that MPs served as substrate surface for the adsorption and nucleation of U precipitates. Chemical speciation modeling and microscopy analyses (i.e., Transmission Electron Microscopy [TEM]) suggest that U precipitates resemble sodium-compreignacite and schoepite. These findings have relevant implications to further understanding the occurrence and interfacial interaction of MPs and metals in freshwater.

From Adsorption to Precipitation of U(VI): What is the Role of pH and Natural Organic Matter?

We investigated interfacial reactions of U(VI) in the presence of Suwannee River natural organic matter (NOM) at acidic and neutral pH. Laboratory batch experiments show that the adsorption and precipitation of U(VI) in the presence of NOM occur at pH 2 and pH 4, while the aqueous complexation of U by dissolved organic matter is favored at pH 7, preventing its precipitation. Spectroscopic analyses indicate that U(VI) is mainly adsorbed to the particulate organic matter at pH 4. However, U(VI)-bearing ultrafine to nanocrystalline solids were identified at pH 4 by electron microscopy. This study shows the promotion of U(VI) precipitation by NOM at low pH which may be relevant to the formation of mineralized deposits, radioactive waste repositories, wetlands, and other U- and organic-rich environmental systems.

Uptake and Toxicity of Respirable Carbon-Rich Uranium-Bearing Particles: Insights into the Role of Particulates in Uranium Toxicity.

Particulate matter (PM) presents an environmental health risk for communities residing close to uranium (U) mine sites. However, the role of the particulate form of U on its cellular toxicity is still poorly understood. Here, we investigated the cellular uptake and toxicity of C-rich U-bearing particles as a model organic particulate containing uranyl citrate over a range of environmentally relevant concentrations of U (0-445 µM). The cytotoxicity of C-rich U-bearing particles in human epithelial cells (A549) was U-dose-dependent. No cytotoxic effects were detected with soluble U doses. Carbon-rich U-bearing particles with a wide size distribution (<10 µm) presented 2.7 times higher U uptake into cells than the particles with a narrow size distribution (<1 µm) at 100 µM U concentration. TEM-EDS analysis identified the intracellular translocation of clusters of C-rich U-bearing particles. The accumulation of C-rich U-bearing particles induced DNA damage and cytotoxicity as indicated by the increased phosphorylation of the histone H2AX and cell death, respectively. These findings reveal the toxicity of the particulate form of U under environmentally relevant heterogeneous size distributions. Our study opens new avenues for future investigations on the health impacts resulting from environmental exposures to the particulate form of U near mine sites.

Mine-site derived particulate matter exposure exacerbates neurological and pulmonary inflammatory outcomes in an autoimmune mouse model.

The Southwestern United States has a legacy of industrial mining due to the presence of rich mineral ore deposits. The relationship between environmental inhaled particulate matter (PM) exposures and neurological outcomes within an autoimmune context is understudied. The aim of this study was to compare two regionally-relevant dusts from high-priority abandoned mine-sites, Claim 28 PM, from Blue Gap Tachee, AZ and St. Anthony mine PM, from the Pueblo of Laguna, NM and to expose autoimmune-prone mice (NZBWF1/J). Mice were randomly assigned to one of three groups (n = 8/group): DM (dispersion media, control), Claim 28 PM, or St. Anthony PM, subjected to oropharyngeal aspiration of (100 µg/50 µl), once per week for a total of 4 consecutive doses. A battery of immunological and neurological endpoints was assessed at 24 weeks of age including: bronchoalveolar lavage cell counts, lung gene expression, brain immunohistochemistry, behavioral tasks and serum autoimmune biomarkers. Bronchoalveolar lavage results demonstrated a significant increase in number of polymorphonuclear neutrophils following Claim 28 and St. Anthony mine PM aspiration. Lung mRNA expression showed significant upregulation in CCL-2 and IL-1ß following St. Anthony mine PM aspiration. In addition, neuroinflammation was present in both Claim 28 and St. Anthony mine-site derived PM exposure groups. Behavioral tasks resulted in significant deficits as determined by Y-maze new arm frequency following Claim 28 aspiration. Neutrophil elastase was significantly upregulated in the St. Anthony mine exposure group. Interestingly, there were no significant changes in serum autoantigens suggesting systemic inflammatory effects may be mediated through other molecular mechanisms following low-dose PM exposures.

Effect of Bicarbonate, Calcium, and pH on the Reactivity of As(V) and U(VI) Mixtures.

Natural or anthropogenic processes can increase the concentration of uranium (U) and arsenic (As) above the maximum contaminant levels in water sources. Bicarbonate and calcium (Ca) can have major impacts on U speciation and can affect the reactivity between U and As. We therefore investigated the reactivity of aqueous U and As mixtures with bicarbonate and Ca for acidic and neutral pH conditions. In experiments performed with 1 mM U and As mixtures, 10 mM Ca, and without added bicarbonate (pCO2 = 3.5), aqueous U decreased to <0.25 mM at pH 3 and 7. Aqueous As decreased the most at pH 3 (∼0.125 mM). Experiments initiated with 0.005 mM As and U showed similar trends. X-ray spectroscopy (i.e., XAS and EDX) and diffraction indicated that U-As-Ca- and U-Ca-bearing solids resemble uranospinite [Ca(UO2)2(AsO4)2·10H2O] and becquerelite [Ca(UO2)6O4(OH)6·8(H2O)]. These findings suggest that U-As-Ca-bearing solids formed in mixed solutions are stable at pH 3. However, the dissolution of U-As-Ca and U-Ca-bearing solids at pH 7 was observed in reactors containing 10 mM bicarbonate and Ca, suggesting a kinetic reaction of aqueous uranyl-calcium-carbonate complexation. Our study provides new insights regarding U and As mobilization for risk assessment and remediation strategies.

Organic Functional Group Chemistry in Mineralized Deposits Containing U(IV) and U(VI) from the Jackpile Mine in New Mexico.

We investigated the functional group chemistry of natural organic matter (NOM) associated with both U(IV) and U(VI) in solids from mineralized deposits exposed to oxidizing conditions from the Jackpile Mine, Laguna Pueblo, NM. The uranium (U) content in unreacted samples was 0.44-2.6% by weight determined by X-ray fluorescence. In spite of prolonged exposure to ambient oxidizing conditions, ≈49% of U(IV) and ≈51% of U(VI) were identified on U LIII edge extended X-ray absorption fine structure spectra. Loss on ignition and thermogravimetric analyses identified from 13% to 44% of NOM in the samples. Carbonyl, phenolic, and carboxylic functional groups in the unreacted samples were identified by fitting of high-resolution X-ray photoelectron spectroscopy (XPS) C 1s and O 1s spectra. Peaks corresponding to phenolic and carbonyl functional groups had intensities higher than those corresponding to carboxylic groups in samples from the supernatant from batch extractions conducted at pH 13, 7, and 2. U(IV) and U(VI) species were detected in the supernatant after batch extractions conducted under oxidizing conditions by fitting of high-resolution XPS U 4f spectra. The outcomes from this study highlight the importance of the influence of pH on the organic functional group chemistry and U speciation in mineralized deposits.

Effect of Bicarbonate and Oxidizing Conditions on U(IV) and U(VI) Reactivity in Mineralized Deposits of New Mexico.

We investigated the effect of bicarbonate and oxidizing agents on uranium (U) reactivity and subsequent dissolution of U(IV) and U(VI) mineral phases in the mineralized deposits from Jackpile mine, Laguna Pueblo, New Mexico, by integrating laboratory experiments with spectroscopy, microscopy and diffraction techniques. Uranium concentration in solid samples from mineralized deposit obtained for this study exceeded 7000 mg kg-1, as determined by X-ray fluorescence (XRF). Results from X-ray photoelectron spectroscopy (XPS) suggest the coexistence of U(VI) and U(IV) at a ratio of 19:1 at the near surface region of unreacted solid samples. Analyses made using X-ray diffraction (XRD) and electron microprobe detected the presence of coffinite (USiO4) and uranium-phosphorous-potassium (U-P-K) mineral phases. Imaging, mapping and spectroscopy results from scanning transmission electron microscopy (STEM) indicate that the U-P-K phases were encapsulated by carbon. Despite exposing the solid samples to strong oxidizing conditions, the highest aqueous U concentrations were measured from samples reacted with 100% air saturated 10 mM NaHCO3 solution, at pH 7.5. Analyses using X-ray absorption spectroscopy (XAS) indicate that all the U(IV) in these solid samples were oxidized to U(VI) after reaction with dissolved oxygen and hypochlorite (OCl-) in the presence of bicarbonate (HCO3 -). The reaction between these organic rich deposits, and 100% air saturated bicarbonate solution (containing dissolved oxygen), can result in considerable mobilization of U in water, which has relevance to the U concentrations observed at the Rio Paguate across the Jackpile mine. Results from this investigation provide insights on the reactivity of carbon encapsulated U-phases under mild and strong oxidizing conditions that have important implication in U recovery, remediation and risk exposure assessment of sites.

Reactivity of As and U co-occurring in Mine Wastes in northeastern Arizona.

The reactivity of co-occurring arsenic (As) and uranium (U) in mine wastes was investigated using batch reactors, microscopy, spectroscopy, and aqueous chemistry. Analyses of field samples collected in proximity to mine wastes in northeastern Arizona confirm the presence of As and U in soils and surrounding waters, as reported in a previous study from our research group. In this study, we measured As (< 0.500 to 7.77 µg/L) and U (0.950 to 165 µg/L) in waters, as well as mine wastes (< 20.0 to 40.0 mg/kg As and < 60.0 to 110 mg/kg U) and background solids (< 20.0 mg/kg As and < 60.0 mg/kg U). Analysis with X-ray fluorescence (XRF) and electron microprobe show the co-occurrence of As and U with iron (Fe) and vanadium (V). These field conditions served as a foundation for additional laboratory experiments to assess the reactivity of metals in these mine wastes. Results from laboratory experiments indicate that labile and exchangeable As(V) was released to solution when solids were sequentially reacted with water and magnesium chloride (MgCl2), while limited U was released to solution with the same reactants. The predominance of As(V) in mine waste solids was confirmed by X-ray absorption near edge (XANES) analysis. Both As and U were released to solution after reaction of solids in batch experiments with HCO3 -. Both X-ray photoelectron spectroscopy (XPS) and XANES analysis determined the predominance of Fe(III) in the solids. Mössbauer spectroscopy detected the presence of nano-crystalline goethite, Fe(II) and Fe(III) in (phyllo)silicates, and an unidentified mineral with parameters consistent with arsenopyrite or jarosite in the mine waste solids. Our results suggest that As and U can be released under environmentally relevant conditions in mine waste, which is applicable to risk and exposure assessment.

Electrocatalytic Reduction of Nitrate Using Magnéli Phase TiO2 Reactive Electrochemical Membranes Doped with Pd-Based Catalysts.

This research focused on synthesis, characterization, and application of point-of-use catalytic reactive electrochemical membranes (REMs) for electrocatalytic NO3- reduction. Deposition of Pd-Cu and Pd-In catalysts to the REMs produced catalytic REMs (i.e., Pd-Cu/REM and Pd-In/REM) that were active for NO3- reduction. Optimal performance was achieved with a Pd-Cu/REM and upstream counter electrode, which reduced NO3- from 1.0 mM to below the EPAs regulatory MCL (700 µM) in a single pass through the REM (residence time ∼2 s), obtaining product selectivity of <2% toward NO2-/NH3. Nitrate reduction was not affected by dissolved oxygen and carbonate species and only slightly decreased in a surface water sample due to Ca2+ and Mg2+ scaling. Energy consumption to treat surface water was 1.1 to 1.3 kWh mol-1 for 1 mM NO3- concentrations, and decreased to 0.19 and 0.12 kWh mol-1 for 10 and 100 mM NaNO3 solutions, respectively. Electrocatalytic reduction kinetics were shown to be an order of magnitude higher than catalytic NO3- reduction kinetics. Conversion of up to 67% of NO3-, with low NO2- (0.7-11 µM) and NH3 formation (<10 µM), and low energy consumption obtained in this study suggest that Pd-Cu/REMs are a promising technology for distributed water treatment.

Effect of Calcium on the Bioavailability of Dissolved Uranium(VI) in Plant Roots under Circumneutral pH.

We integrated field measurements, hydroponic experiments, microscopy, and spectroscopy to investigate the effect of Ca(II) on dissolved U(VI) uptake by plants in 1 mM HCO3- solutions at circumneutral pH. The accumulation of U in plants (3.1-21.3 mg kg-1) from the stream bank of the Rio Paguate, Jackpile Mine, New Mexico served as a motivation for this study. Brassica juncea was the model plant used for the laboratory experiments conducted over a range of U (30-700 µg L-1) and Ca (0-240 mg L-1) concentrations. The initial U uptake followed pseudo-second-order kinetics. The initial U uptake rate ( V0) ranged from 4.4 to 62 µg g-1 h-1 in experiments with no added Ca and from 0.73 to 2.07 µg g-1 h-1 in experiments with 12 mg L-1 Ca. No measurable U uptake over time was detected for experiments with 240 mg L-1 Ca. Ternary Ca-U-CO3 complexes may affect the decrease in U bioavailability observed in this study. Elemental X-ray mapping using scanning transmission electron microscopy-energy-dispersive spectrometry detected U-P-bearing precipitates within root cell walls in water free of Ca. These results suggest that root interactions with Ca and carbonate in solution affect the bioavailability of U in plants. This study contributes relevant information to applications related to U transport and remediation of contaminated sites.

Metal Reactivity in Laboratory Burned Wood from a Watershed Affected by Wildfires.

We investigated interfacial processes affecting metal mobility by wood ash under laboratory-controlled conditions using aqueous chemistry, microscopy, and spectroscopy. The Valles Caldera National Preserve in New Mexico experiences catastrophic wildfires of devastating effects. Wood samples of Ponderosa Pine, Colorado Blue Spruce, and Quaking Aspen collected from this site were exposed to temperatures of 60, 350, and 550 °C. The 350 °C Pine ash had the highest content of Cu (4997 ± 262 mg kg-1), Cr (543 ± 124 mg kg-1), and labile dissolved organic carbon (DOC, 11.3 ± 0.28 mg L-1). Sorption experiments were conducted by reacting 350 °C Pine, Spruce, and Aspen ashes separately with 10 µM Cu(II) and Cr(VI) solutions. Up to a 94% decrease in Cu(II) concentration was observed in solution while Cr(VI) concentration showed a limited decrease (up to 13%) after 180 min of reaction. X-ray photoelectron spectroscopy (XPS) analyses detected increased association of Cu(II) on the near surface region of the reacted 350 °C Pine ash from the sorption experiments compared to the unreacted ash. The results suggest that dissolution and sorption processes should be considered to better understand the potential effects of metals transported by wood ash on water quality that have important implications for postfire recovery and response strategies.

Investigation of patterned and non-patterned poly(2,6-dimethyl 1,4-phenylene) oxide based anion exchange membranes for enhanced desalination and power generation in a microbial desalination cell.

Quaternary ammonium poly(2,6-dimethyl 1,4-phenylene oxide) (QAPPO) anion exchange membranes (AEMs) with topographically patterned surfaces were assessed in a microbial desalination cell (MDC) system. The MDC results with these QAPPO AEMs were benchmarked against a commercially available AEM. The MDC with the non-patterned QAPPO AEM (Q1) displayed the best desalination rate (a reduction of salinity by 53 ± 2.7%) and power generation (189 ± 5 mW m- 2) when compared against the commercially available AEM and the patterned AEMs. The enhanced performance with the Q1 AEM was attributed to its higher ionic conductivity and smaller thickness leading to a reduced area specific resistance. It is important to note that Real Pacific Ocean seawater and activated sludge were used into the desalination chamber and anode chamber respectively for the MDC - which mimicked realistic conditions. Although the non-patterned QAPPO AEM displayed better performance over the patterned QAPPO AEMs, it was observed that the anodic overpotential was smaller when the MDCs featured QAPPO AEMs with larger lateral feature sizes. The results from this study have important implications for the continuous improvements necessary for developing cheaper and better performing membranes in order to optimize the MDC.

Reactive Transport of U and V from Abandoned Uranium Mine Wastes.

The reactive transport of uranium (U) and vanadium(V) from abandoned mine wastes collected from the Blue Gap/Tachee Claim-28 mine site in Arizona was investigated by integrating flow-through column experiments with reactive transport modeling, and electron microscopy. The mine wastes were sequentially reacted in flow-through columns at pH 7.9 (10 mM HCO3-) and pH 3.4 (10 mM CH3COOH) to evaluate the effect of environmentally relevant conditions encountered at Blue Gap/Tachee on the release of U and V. The reaction rate constants (km) for the dissolution of uranyl-vanadate (U-V) minerals predominant at Blue Gap/Tachee were obtained from simulations with the reactive transport software, PFLOTRAN. The estimated reaction rate constants were within 1 order of magnitude for pH 7.9 (km = 4.8 × 10-13 mol cm-2 s-1) and pH 3.4 (km = 3.2 × 10-13 mol cm-2 s-1). However, the estimated equilibrium constants (Keq) for U-V bearing minerals were more than 6 orders of magnitude different for reaction at circumneutral pH (Keq = 10-38.65) compared to acidic pH (Keq = 10-44.81). These results coupled with electron microscopy data suggest that the release of U and V is affected by water pH and the crystalline structure of U-V bearing minerals. The findings from this investigation have important implications for risk exposure assessment, remediation, and resource recovery of U and V in locations where U-V-bearing minerals are abundant.

Spectroscopic Investigation of Interfacial Interaction of Manganese Oxide with Triclosan, Aniline, and Phenol.

We investigated the reaction of manganese oxide [MnOx(s)] with phenol, aniline, and triclosan in batch experiments using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and aqueous chemistry measurements. Analyses of XPS high-resolution spectra suggest that the Mn(III) content increased 8-10% and the content of Mn(II) increased 12-15% in the surface of reacted MnOx(s) compared to the control, indicating that the oxidation of organic compounds causes the reduction of MnOx(s). Fitting of C 1s XPS spectra suggests an increase in the number of aromatic and aliphatic bonds for MnOx(s) reacted with organic compounds. The presence of 2.7% Cl in the MnOx(s) surface after reaction with triclosan was detected by XPS survey scans, while no Cl was detected in MnOx-phenol, MnOx-aniline, and MnOx-control. Raman spectra confirm the increased intensity of carbon features in MnOx(s) samples that reacted with organic compounds compared to unreacted MnOx(s). These spectroscopy results indicate that phenol, aniline, triclosan, and related byproducts are associated with the surface of MnOx(s)-reacted samples. The results from this research contribute to a better understanding of interactions between MnOx(s) and organic compounds that are relevant to natural and engineered environments.

Post Gold King Mine Spill Investigation of Metal Stability in Water and Sediments of the Animas River Watershed.

We applied spectroscopy, microscopy, diffraction, and aqueous chemistry methods to investigate the persistence of metals in water and sediments from the Animas River 13 days after the Gold King Mine spill (August 5, 2015). The Upper Animas River watershed, located in San Juan Colorado, is heavily mineralized and impacted by acid mine drainage, with low pH water and elevated metal concentrations in sediments (108.4 ± 1.8 mg kg-1 Pb, 32.4 ± 0.5 mg kg-1 Cu, 729.6 ± 5.7 mg kg-1 Zn, and 51 314.6 ± 295.4 mg kg-1 Fe). Phosphate and nitrogen species were detected in water and sediment samples from Farmington, New Mexico, an intensive agricultural area downstream from the Animas River, while metal concentrations were low compared to those observed upstream. Solid-phase analyses of sediments suggest that Pb, Cu, and Zn are associated with metal-bearing jarosite and other minerals (e.g., clays, Fe-(oxy)hydroxides). The solubility of jarosite at near-neutral pH and biogeochemical processes occurring downstream could affect the stability of metal-bearing minerals in river sediments. This study contributes relevant information about the association of metal mixtures in a heavy mineralized semiarid region, providing a foundation to better understand long-term metal release in a public and agricultural water supply.

Long-term in situ oxidation of biogenic uraninite in an alluvial aquifer: impact of dissolved oxygen and calcium.

Oxidative dissolution controls uranium release to (sub)oxic pore waters from biogenic uraninite produced by natural or engineered processes, such as bioremediation. Laboratory studies show that uraninite dissolution is profoundly influenced by dissolved oxygen (DO), carbonate, and solutes such as Ca(2+). In complex and heterogeneous subsurface environments, the concentrations of these solutes vary in time and space. Knowledge of dissolution processes and kinetics occurring over the long-term under such conditions is needed to predict subsurface uranium behavior and optimize the selection and performance of uraninite-based remediation technologies over multiyear periods. We have assessed dissolution of biogenic uraninite deployed in wells at the Rifle, CO, DOE research site over a 22 month period. Uraninite loss rates were highly sensitive to DO, with near-complete loss at >0.6 mg/L over this period but no measurable loss at lower DO. We conclude that uraninite can be stable over decadal time scales in aquifers under low DO conditions. U(VI) solid products were absent over a wide range of DO values, suggesting that dissolution proceeded through complexation and removal of oxidized surface uranium atoms by carbonate. Moreover, under the groundwater conditions present, Ca(2+) binds strongly to uraninite surfaces at structural uranium sites, impacting uranium fate.

Elevated Concentrations of U and Co-occurring Metals in Abandoned Mine Wastes in a Northeastern Arizona Native American Community.

The chemical interactions of U and co-occurring metals in abandoned mine wastes in a Native American community in northeastern Arizona were investigated using spectroscopy, microscopy and aqueous chemistry. The concentrations of U (67-169 µg L(-1)) in spring water samples exceed the EPA maximum contaminant limit of 30 µg L(-1). Elevated U (6,614 mg kg(-1)), V (15,814 mg kg(-1)), and As (40 mg kg(-1)) concentrations were detected in mine waste solids. Spectroscopy (XPS and XANES) solid analyses identified U (VI), As (-I and III) and Fe (II, III). Linear correlations for the release of U vs V and As vs Fe were observed for batch experiments when reacting mine waste solids with 10 mM ascorbic acid (∼pH 3.8) after 264 h. The release of U, V, As, and Fe was at least 4-fold lower after reaction with 10 mM bicarbonate (∼pH 8.3). These results suggest that U-V mineral phases similar to carnotite [K2(UO2)2V2O8] and As-Fe-bearing phases control the availability of U and As in these abandoned mine wastes. Elevated concentrations of metals are of concern due to human exposure pathways and exposure of livestock currently ingesting water in the area. This study contributes to understanding the occurrence and mobility of metals in communities located close to abandoned mine waste sites.