The effect of pH on thiosulfate formation in a biotechnological process for the removal of hydrogen sulfide from gas streams

In a biotechnological process for hydrogen sulfide (H(2)S) removal from gas streams, operating at natronophilic conditions, formation of thiosulfate (S(2)O(3)(2-)) is unfavorable, as it leads to a reduced sulfur production. Thiosulfate formation was studied in gas-lift bioreactors, using natronophilic biomass at [Na(+)]+[K(+)] = 2 mol L(-1). The results show that at sulfur producing conditions, selectivity for S(2)O(3)(2-) formation mainly depends on the equilibrium between free sulfide (HS(-)) and polysulfide (S(X)(2-)), which can be controlled via the pH. At pH 8.6, 21% of the total dissolved sulfide is present as S(X)(2-) and selectivity for S(2)O(3)(2-) formation is 3.9-5.5%. At pH 10, 87% of the total dissolved sulfide is present as S(X)(2-) and 20-22% of the supplied H(2)S is converted to S(2)O(3)(2-), independent of the H(2)S loading rate. Based on results of bioreactor experiments and biomass activity tests, a mechanistic model is proposed to describe the relation between S(2)O(3)(2-) formation and pH.