PNNL

Abstract

Previous in situ examination of U(VI) spiked concrete indicated that uranyl-oxyhydroxide phases that were initially formed, later led to the formation of mixed uranyl-oxyhydroxide/silicates, which subsequently transformed into uranyl-silicates, and finally altered into mixed uranyl-silicate/phosphate and uranyl-phosphate phases. We conducted solubility studies of the identified final uranyl-phosphate phase (calcium meta-autunite) in phosphate solutions ranging in concentration from 0.001 – 0.1M as a function of pH. These studies indicated a secondary phosphate phase that formed during the solubility of meta-autunite regulated the uranium concentrations at relatively low levels under high pH conditions (>12) typically encountered in cement pore waters. The importance of uranyl-phosphate minerals in concrete waste forms has, to date, been neglected because of the minimal amount of phosphorus present in most concrete compositions. However, because concrete is a continuously reacting solid, the thermodynamic stability of uranyl minerals that form at the later stages of reaction may have a substantial impact on the long-term fate of uranium in the waste forms. This study suggests that any future investigations should consider the potential benefit of including phosphorus in concrete waste forms.