Kinetics of sulfur and pyrite oxidation by Thiobacillus thiooxidans. Competitive inhibition by increasing concentrations of cells

The oxidation of elemental sulfur by two strains of Thiobacillus thiooxidans was studied by measuring the rate of O2 consumption at various concentrations of substrate and cells. In both the laboratory strain ATCC 8085 and the mine isolate SM-6, sulfur oxidation was competitively inhibited by T. thiooxidans cells; the Ki values were 0.65 and 0.05 mg wet cells∙mL−1, respectively. The rate constants were 500 and 143 μM O2∙min−1∙mg wet cells−1∙mL−1 and the Km values for sulfur concentration were 7.5 and 0.32% pulp density, respectively. Mine isolate SM-6 was used also to study pyrite (FeS2) oxidation by measuring the rate of O2 consumption. Oxidation of both washed and unwashed pyrite samples was competitively inhibited by increasing concentrations of cells; with each sample the Ki values was 0.05 mg wet cells∙mL−1. The rate constants for each sample were also the same (100 μM O2∙min−1∙mg wet cells−1∙mL−1), but the Km values were different (1.11% pulp density for washed pyrite and 2.81% pulp density for unwashed pyrite). Based on the rate of Fe solubilization from the washed pyrite sample, T. thiooxidans cells oxidized the sulfide released from pyrite dissolution beyond the oxidation state of elemental sulfur. Key words: Thiobacillus thiooxidans, sulfur, pyrite, oxidation, kinetics.