Inhibition of ferrous ion oxidation in Thiobacillus ferrooxidans in the presence of oxyanions of sulfur and phosphorus

Abstract

Ferrus iron oxidation by growing cultures of Thiobacillus ferrooxidans was sensitive to negative catalysis (inhibition) by selected oxyanios of sulfur. The relative order of negative catalytic activity for the sulfoxyanions was S_20_5^(2-) > S_20_3^(2-) > S_20_4^(2-) > S_40_6^(2-). Inhibition by the sulfoxyanions was greater at μm 1.5 than at pH 2.0. A mechanism involving the formation of radical intermediates, HO_2,-OH, HSO_3, HSO_5, H_2O_4, and HS_3O_3, induced by Fe(II) and Fe(III) complexes is proposed to account for the observed inhibitory effects. Orthophosphate, pyrophosphate, and tripolyphosphate compounds were also inhibitory to ferrous iron oxidation by T. ferrooxidans. Orthophosphates were increasingly at high concentrations (200-300 mM) whereas the apparent toxicity of pyrophosphate and tripolyphosphate was inversely related to concentration over the range of 50-300 mM with no apparent at pH dependency at pH ≤ 2. Equilibrium models are presented to explain the relative effect of the phosphates in terms of phosphate precipitates, which were actually observed , and soluble complexes, which have an effect on the thermodynamics of electron transfer.