Experimental determination of the hydrothermal solubility of ReS(2 )and the Re–ReO(2 )buffer assemblage and transport of rhenium under supercritical conditions

To understand the aqueous species important for transport of rhenium under supercritical conditions, we conducted a series of solubility experiments on the Re–ReO(2 )buffer assemblage and ReS(2). In these experiments, pH was buffered by the K–feldspar–muscovite–quartz assemblage; [Image: see text] in sulfur-free systems was buffered by the Re–ReO(2 )assemblage; and [Image: see text] and [Image: see text] in sulfur-containing systems were buffered by the magnetite–pyrite–pyrrhotite assemblage. Our experimental studies indicate that the species ReCl(4)(0 )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 ReCl(3)(+ )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 ReS(2 )is about two orders of magnitude less than that of ReO(2). This finding not only suggests that ReS(2 )(or a ReS(2 )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, TcS(2 )may be the stable Tc-bearing phase in deep geological repositories of radioactive wastes.