An integrated, single-reactor for double-duty chemoautotrophic sulfide bio-oxidation and sulfate bio-reduction: Key parameters for unmatched efficiency in elemental sulfur production

The present study proposes an integrated system for the simultaneous removal of sulfide and sulfate, and determined the key parameters for high-level production of elemental sulfur (S⁰) as the desired end-product . Various modes of operation, including batch, semi-continuous, and continuous, were also examined. The packed bed bioreactor (PBBR) was operated under oxygen-limited conditions with different hydraulic retention times (HRT) near-neutral pH. At an HRT of 3 h, removal efficiencies greater than 99 % and 98 % were achieved for S^2- bio-oxidation and SO₄^2- bio-reduction, respectively. During the semi-continuous mode of PBBR operation, thiosulfate (S₂O₃^2-), a metabolite of sulfur oxidation, was detected in the effluent; however, when the bioreactor was operated in continuous mode at an HRT of 3 h, the concentration of thiosulfate was below the detection limit. The highest S⁰ selectivity (45 %) was achieved at an HRT of 1 h. Next-generation sequencing (NGS) analysis was used to identify the diversity of bacterial species in the biofilm; Thiobacillus sajanensis, Thiobacillus thioparus, and Rhizobium arsenicireducens were found to be the dominant species responsible for the simultaneous biological conversion of sulfide and sulfate. The mechanism of simultaneous sulfide and sulfate bioconversion was investigated and proposed. In summary, the developed PBBR was an efficient method for the concurrent bioconversion of sulfide and sulfate, with considerable selectivity towards elemental sulfur.