PNNL

Abstract

In-situ precipitation of autunite minerals has been proposed as a method for remediating groundwaters, deep within the subsurface, faced with uranium contamination. Recently, it has been to utilizes long-chain sodium polyphosphate compounds as a “time-released” source of phosphate for precipitation of uranium-phosphate minerals. Elevated sodium concentrations presented by this technique enhance formation of sodium autunite relative to the more common calcium autunite mineral phase. The goal of forming autunite minerals in-situ to remediate uranium contaminated groundwater requires a thorough understanding of the properties of sodium autunite minerals to evaluate the longevity and efficacy of a uranium-phosphate barrier. Research presented here is part of a larger effort to quantify the solubility and dissolution properties uranium-phosphate minerals, that may form due to remediation efforts, under environmentally relevant conditions. This paper focuses on the development of a direct synthesis route for precipitating sodium autunite, and a comparative analysis of the structural properties and differences presented in the direct versus previous indirect methods of precipitation using extended X-ray absorption fine structure (EXAFS) spectroscopy, chemical digestion followed by inductively-coupled plasma-optical emission spectroscopy and inductively-coupled plasma-mass spectroscopy for elemental analyses, X-ray diffraction (XRD), scanning electron microscopy (SEM), and multi point Brunauer-Emmett-Teller (BET) analyses.