Impact of phosphate on U(VI) immobilization in the presence of goethite

Abstract

Past mining, processing, and waste disposal activities have left a legacy of uranium-contaminated soil and groundwater. Phosphate addition to subsurface environments can potentially immobilize U(VI) in-situ through interactions with uranium at mineral-water interfaces. Phosphate can induce the precipitation of low solubility U(VI)-phosphates, and it may enhance or inhibit U(VI) adsorption to iron(III) (oxy)hydroxide surfaces. Such surfaces may also facilitate the heterogeneous nucleation of U(VI)-phosphate precipitates. The interactions among phosphate, U(VI), and goethite (α-FeOOH) were investigated in a year-long series of experiments at pH 4. Reaction time, total U(VI), total phosphate, and the presence and absence of goethite were systematically varied to determine their effects on the extent of U(VI) uptake and the dominant uranium immobilization mechanism. Dissolved U(VI) and phosphate concentrations were interpreted within a reaction-based modeling framework that included dissolution-precipitation reactions and a surface complexation model to account for adsorption. The best available thermodynamic data and past surface complexation models were integrated to form an internally consistent framework. Additional evidence for the uptake mechanisms was obtained using scanning electron microscopy and X-ray diffraction. The formation and crystal growth of a U(VI)-phosphate phase, most likely chernikovite, UO 2HPO 4·4H 2O (s), occurred rapidly for initially supersaturated suspensions both with and without goethite. Nucleation appears to occur homogeneously for almost all conditions, even in the presence of goethite, but heterogeneous nucleation was likely at one condition. The U(VI)-phosphate solids exhibited metastability depending on the TOTU:TOTP ratio. At the highest phosphate concentration studied (130 μM), U(VI) uptake was enhanced due to the likely formation of a ternary surface complex for low (∼1 μM) to intermediate (∼10 μM) TOTU concentrations and to U(VI)-phosphate precipitation for high TOTU (∼100 μM) concentrations. For conditions favoring precipitation, the goethite surface acted as a sink for dissolved phosphate that resulted in higher dissolved U(VI) concentrations relative to goethite-free conditions. Based on the total uranium and available sorption sites, a critical phosphate concentration between 15 μM and 130 μM was required for preferential precipitation of uranium phosphate over U(VI) adsorption.