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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–1and for KUAs is −3000 ± 28 kJ mol–1derived 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.

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.

Surface water and sediments from the Jackpile mine, St. Anthony mine, Rio Paguate, Rio Moquino, and Mesita Dam areas near Pueblo of Laguna, New Mexico, were analyzed for 226Ra and U using gamma (γ) spectroscopy and inductively coupled plasma mass spectroscopy, respectively. Activity ratios for 226Ra/238U for solid samples range from 0.34 ± 0.13 to 16 ± 2.9, which reflect uranium transport and accumulation (<1), relatively pristine material in secular equilibrium (1), and removal of uranium by weathering (>1). Concentrations ranging from 80 to 225 μg L–1U were detected in unfiltered water samples near the Jackpile mine. Water samples upstream and downstream from the mine contained concentrations ranging from 12 to 15 μg L–1U. Water samples collected from the North Pit standing pond in the Jackpile mine contained as much as 1560 pCi L–1of 226Ra, and passing the water through a 0.2 μM filter did not substantially reduce the activity of 226Ra in the water. 234Th and 226Ra are in secular equilibrium in this water, while radon gas was lost from the water. The results of the current study provide insight into the distribution of U-series radionuclides in the Pueblo of Laguna area, including detection of high levels of radioactivity in water at some locations within the Jackpile mine.

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–1limestone. 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.