Isotope Effects in the Chemical and Bacterial Reduction of Sulphur Compounds.

Abstract

Equilibrium exchange constants were calculated for exchange of sulphite with other sulphur compounds. The equilibrium constant for sulphur isotope exchange between sulphate in solution and solid calcium sulphate was calculated and measured experimentally· In the chemical reduction of sulphate to sulphide reacted 2.56 faster than s34O4 , in agreement with the calculated kinetic isotope effect for the step sulphate to sulphite. The isotope effect in the reduction of sulphate by Desulphovibrio desulphuricans was found to vary from 0.0 to 2∙5%. The results were intcrpreted on tiιe basis of a mechanism involving two consecutive steps, pick-up of sulphate and reduction of sulphate to sulphite, competing for control of the rate. The isotope effect in bacterial reduction of sulphite was studied briefly.