pH influence on polysulfide, internal cell-bound sulfane, and implications for design of the biological desulfurization process at haloalkaline conditions

Biological desulfurization, utilizing sulfide-oxidizing bacteria (SOB) to convert hydrogen sulfide (H 2 S) in gas into elemental sulfur, has been implemented at a large scale since the 1990s. The recent addition of a sulfidic bioreactor between the absorber column and micro-oxic bioreactor in the process line-up has increased the selectivity for sulfur production. However, when applied for gas streams with a low carbon dioxide to H 2 S ratio (i.e., high pH conditions), the sulfur selectivity was found to decrease while the biological uptake of sulfide in the sulfidic bioreactor increased, hypothesized to be related to polysulfide (S x 2− ). This study investigated the effect of pH on the total S x 2− concentration, total uptake of (poly)sulfide, and internal cell-bound sulfane to elucidate the influence on sulfur selectivity in the process line-ups with and without a sulfidic bioreactor. Total S x 2− concentration increased from ∼1.5 mmol-S L −1 at pH 8.5 to ∼5.0 mmol-S L −1 at pH ∼9.3 in the sulfidic reactor. The maximum biological absolute (poly)sulfide uptake of ∼72% and the maximum internal cell-bound sulfane (0.05 ± 0.01 mg-S mg-N −1 ) were obtained in the line-up with the sulfidic bioreactor at pH ∼9.3. However, the dual-reactor process at pH ∼9.3 had the lowest sulfur selectivity (44.1% ± 28.7%) and highest thiosulfate selectivity (53.7% ± 28.4%), indicating increased chemical oxidation of S x 2− . This study provides insight into (poly)sulfide-SOB interactions, the influence these interactions have on sulfur selectivity, and the implications for the design of new processing schemes.