Sulfide oxidation at halo-alkaline conditions in a fed-batch bioreactor

A biotechnological process is described to remove hydrogen sulfide (H2S) from high-pressure natural gas and sour gases produced in the petrochemical industry. The process operates at halo-alkaline conditions and combines an aerobic sulfide-oxidizing reactor with an anaerobic sulfate (SO) and thiosulfate (S2O) reducing reactor. The feasibility of biological H2S oxidation at pH around 10 and total sodium concentration of 2 mol L-1 was studied in gas-lift bioreactors, using halo-alkaliphilic sulfur-oxidizing bacteria (HA-SOB). Reactor operation at different oxygen to sulfide (O2:H2S) supply ratios resulted in a stable low redox potential that was directly related with the polysulfide (S) and total sulfide concentration in the bioreactor. Selectivity for SO formation decreased with increasing S and total sulfide concentrations. At total sulfide concentrations above 0.25 mmol L-1, selectivity for SO formation approached zero and the end products of H2S oxidation were elemental sulfur (S0) and S2O. Maximum selectivity for S0 formation (83.3±0.7%) during stable reactor operation was obtained at a molar O2:H2S supply ratio of 0.65. Under these conditions, intermediary S plays a major role in the process. Instead of dissolved sulfide (HS-), S seemed to be the most important electron donor for HA-SOB under S0 producing conditions. In addition, abiotic oxidation of S was the main cause of undesirable formation of S2O. The observed biomass growth yield under SO producing conditions was 0.86 g N mol-1 H2S. When selectivity for SO formation was below 5%, almost no biomass growth was observe