Comparison of Microbial Sulfuric Acid Production in Sewage Sludge from Added Sulfur and Thiosulfate

Microbial leaching is one of the most attractive methods of removing toxic metals from sewage sludge. Sulfuric acid produced by indigenous microflora by the oxidation of elemental sulfur and sulfur compounds solubilizes toxic metals. The oxidation of sulfur compounds can be achieved by the direct oxidation to sulfates or by indirect oxidation, through the production and accumulation of soluble intermediate (S₂ O^2- ₃ , S₃ O^2- ₆ , S₄ O^2- ₆ ) compounds. The production of these intermediates may create a potential danger of acidification of the receiving waters or the agricultural soil where the leached sludge is ultimately destined, via slow oxidation of the intermediates with subsequent sulfuric acid production. The objective of this research was to investigate the formation of S₂ O^2- ₃ , S₃ O^2- ₆ , and S₄ 2^- ₆ during metal bioleaching using elemental sulfur and thiosulfate as energy substrates for growth of indigenous thiobacilli (sulfur-oxidizing microorganisms) in sludge. The results obtained showed that intermediates were not formed when elemental sulfur was used as a substrate, whereas trithionate and tetrathionate accumulated in the sludges when thiosulfate was used as substrate. Moreover, the metabolism of thiosulfate was much slower than that of elemental sulfur in sludge medium. Therefore, the utilization of thiosulfate for the growth of indigenous thiobacilli in sewage sludge is a less attractive alternative for the metal bioleaching.