Mechanisms of sulfur selection and sulfur secretion in a biological sulfide removal (BISURE) system

Biological desulfurization technology is a sustainable process for the sulfide removal from biogas, which has multiple advantages. In this study, a biological sulfide removal (BISURE) system was established to investigate the working performances and process mechanisms. The results showed that the sulfide removal rate was 2.30 kg-S/(m(3) d), the sulfide removal efficiency was higher than 98%, the sulfur production rate was 1.76 kg-S/(m(3) d), the sulfur selectivity was 75.02 +/- 3.63% and the main form of products (sulfur compounds) was Rosickyite-S and S-8. The performance of BISURE system was supported by the dominant genus (abundance more than 60%) of sulfur-oxidizing bacteria (SOB) which shifted to Thiovirga at the high SLR. The sqr and dsrA genes could serve as the indicators for the pathway of two-step sulfide oxidation, i.e. "partial sulfide oxidation (PSO, sulfide -> sulfur)" and "complete sulfide oxidation (CSO, sulfur -> sulfate)". The sulfur selectivity was improved by enhancing PSO and inhibiting CSO with the indication of two genes. The cellular sulfur secretion was revealed, and the "outer-membrane vesicles (OMVs)-dependent" sulfur-secreting hypothesis was proposed to explain the transportation of elemental sulfur from inside to outside of SOB cells. The findings of this work provide a new perspective to understand the sulfur selection of sulfide bio-oxidation and the sulfur secretion of SOB cells so as to promote the development of biological desulfurization technology.