Sulfur chemistry in a borosilicate melt. Pt. 2

The diffusion of gaseous sulfur compounds under both oxidizing and reducing conditions has been determined in an alkali borosilicate melt representative of compositions under consideration for nuclear waste immobilization. The diffusion coefficients D of these sulfur gases as a function of melt temperature T (in K) can be expressed as: log D = -3.08-(3300/T) regardless of whether the sulfur is being incorporated as the sulfate ion in oxidized melts or as the sulfide ion in reduced melts. The rate-determining step for the diffusion is probably the diffusion of SO2 gas into the melt. Even though sulfate and sulfide ions are the two equilibrium redox states of sulfur in this melt, transient sulfur species of intermediate redox state can be produced in the melts by placing an initially oxidized melt in a reduced sulfur atmosphere. Metastable polysulfide ions of the general formula Sx2- and Sy- are formed which slowly decay to the more stable sulfide ions under these conditions. (orig.)