Abstract

To understand the aqueous species important for transport of rhenium under supercritical conditions, we conducted a series of solubility experiments on the Re–ReO₂ buffer assemblage and ReS₂. In these experiments, pH was buffered by the K–feldspar–muscovite–quartz assemblage; f_O2 in sulfur-free systems was buffered by the Re–ReO₂ assemblage; and f_O2 and f_S2 in sulfur-containing systems were buffered by the magnetite–pyrite–pyrrhotite assemblage. Our experimental studies indicate that the species ReCl₄⁰ is dominant at 400 °C in slightly acidic to near-neutral, and chloride-rich (total chloride concentrations ranging from 0.5 to 1.0 M) environments, and ReCl₃⁺ may predominate at 500 °C in a solution with total chloride concentrations ranging from 0.5 to 1.5 M. The results also demonstrate that the solubility of ReS₂ is about two orders of magnitude less than that of ReO₂. This finding not only suggests that ReS₂ (or a ReS₂ component in molybdenite) is the solubility-controlling phase in sulfur-containing, reducing environments but also implies that a mixing process involving an oxidized, rhenium-containing solution and a solution with reduced sulfur is one of the most effective mechanisms for deposition of rhenium. In analogy with Re, TcS₂ may be the stable Tc-bearing phase in deep geological repositories of radioactive wastes.