Sulfide oxidizing bacteria (SOB) are widely applied in industry to convert toxic H2S into elemental sulfur. Haloalkaliphilic planktonic SOB can remove sulfide from solution under anaerobic conditions (SOB are ‘charged’), and release electrons at an electrode (discharge of SOB). The effect of this electron shuttling on product formation and biomass growth is not known. Here, we study and demonstrate a continuous process in which SOB remove sulfide from solution in an anaerobic ‘uptake chamber’, and shuttle these electrons to the
anode of an electrochemical cell, in the absence of dissolved sulfide. Two experiments over 31 and 41 days were performed. At a sulfide loading rate of 1.1 mmolS/day, electricity was produced continuously (3 A/m2) without
dissolved sulfide in the anolyte. The main end product was sulfate (56% in experiment 1% and 78% in experiment 2), and 87% and 77% of the electrons in sulfide were recovered as electricity. It was found that the current density was dependent on the sulfide loading rate and not on the anode potential. Biological growth occurred, mainly at the anode as biofilm, in which the delta-proteobacterial genus Desulfurivibrio was dominating. Our results demonstrate a novel strategy to produce electricity from sulfide in an electrochemical system.

In the biotechnological desulfurization process under haloalkaline conditions, dihydrogen sulfide (H ₂ S) is removed from sour gas and oxidized to elemental sulfur (S ₈ ) by sulfide-oxidizing bacteria. Besides S ₈ , the byproducts sulfate (SO ₄ ^2- ) and thiosulfate (S ₂ O ₃ ^2- ) are formed, which consume caustic and form a waste stream. The aim of this study was to increase selectivity toward S ₈ by a new process line-up for biological gas desulfurization, applying two bioreactors with different substrate conditions (i.e., sulfidic and microaerophilic), instead of one (i.e., microaerophilic). A 111-day continuous test, mimicking full scale operation, demonstrated that S ₈ formation was 96.6% on a molar H ₂ S supply basis; selectivity for SO ₄ ^2- and S ₂ O ₃ ^2- were 1.4 and 2.0% respectively. The selectivity for S ₈ formation in a control experiment with the conventional 1-bioreactor line-up was 75.6 mol %. At start-up, the new process line-up immediately achieved lower SO ₄ ^2- and S ₂ O ₃ ^2- formations compared to the 1-bioreactor line-up. When the microbial community adapted over time, it was observed that SO ₄ ^2- formation further decreased. In addition, chemical formation of S ₂ O ₃ ^2- was reduced due to biologically mediated removal of sulfide from the process solution in the anaerobic bioreactor. The increased selectivity for S ₈ formation will result in 90% reduction in caustic consumption and waste stream formation compared to the 1-bioreactor line-up.