Dynamic interplay between uranyl phosphate precipitation, sorption, and phase evolution

Abstract

Natural examples demonstrate uranyl-phosphate minerals can maintain extremely low levels of aqueous uranium in groundwaters due to their low solubility. Therefore, greater understanding of the geochemical factors leading to uranyl phosphate precipitation may lead to successful application of phosphate-based remediation methods. However, the solubility of uranyl phosphate phases varies over >3 orders of magnitude, with the most soluble phases typically observed in lab experiments. To understand the role of common soil/sediment mineral surfaces in the nucleation and transformation of uranyl phosphate minerals under environmentally relevant conditions, batch experiments were carried out with goethite and mica at pH 6 in mixed electrolyte solutions ranging from 1–800μM U and 1–800μM P. All experiments ended with uranium concentrations below the USEPA MCL for U, but with 2–3 orders of magnitude difference in uranium concentrations. Despite the presence of many cations that are well known to incorporate into less soluble autunite-group minerals, chernikovite rapidly precipitated in all experiments containing U and P, except for solutions with 1μM U and 1μM P that were calculated to be undersaturated. Textures of uranyl phosphates observed by AFM and TEM indicate that nucleation was homogenous and independent of the initial mineral content. Comparison of time-course U and P concentrations from the experiments with thermodynamic modeling of solution equilibria demonstrated that aqueous uranium concentrations in the experimental systems evolved as increasingly sparingly soluble uranyl phosphate phases nucleated over time, with sorption accelerating the transition between phases by influencing solution chemistry. Aqueous uranium concentrations consistent with partially dehydrated (meta-) autunite were achieved only in experiments containing goethite and/or mica. These dynamic nucleation-growth-sorption-nucleation-growth-sorption cycles occur over the time scales of weeks, not hours or days at room temperature. Lab experiments and field-based investigations of uranium phosphate should consider these or longer time scales for the greatest long-term relevance.