Sulfur chemistry in a borosilicate melt Part 1. Redox equilibria and solubility

Abstract

The solubility and redox state of sulfur have been determined in an alkali borosilicate melt representative of those glass compositions under consideration for nuclear waste immobilization. The solubility of sulfur has been ascertained as a function of the melt temperature, imposed oxygen fugacity, and imposed sulfur fugacity. Sulfur dissolves predominantly as the sulfate (SO4 2-) ion under oxidizing conditions, and as the sulfide (S2-) ion under reducing conditions. There is only a very narrow range of oxygen fugacities at which both sulfate and sulfide ions coexist in the melt. The presence of sulfide ions in the melt becomes significant at an oxygen fugacity of about 10^-9 bar at 1150 °C, thus establishing a lower limit in terms of redox state for melter operation in nuclear waste immobilization.