Making waves: Biological elemental sulfur disproportionation makes elemental sulfur-based biotechnologies more...

Elemental sulfur (S⁰)-based biotechnologies have emerged as sustainable solutions for nitrogen and/or metal removal from water/wastewater (e.g., industrial wastewater, contaminated groundwater, etc.), but their practical feasibility is controversial due to the unsatisfied effectiveness. The nature of this phenomenon is attributed to the low S⁰ bioaccessibility caused by its low water solubility. Bioaccessible polysulfide generated from the chemical reaction between S⁰ and sulfide has recently been introduced into biosystems, which was found to break the low efficiency bottleneck of S⁰-based processes and substantially improve system performance. However, addition of exogenous sulfide chemicals results in non-negligible issues related to high operational cost and environmental risks. A new approach to biologically generate sulfide and polysulfide by autotrophic S⁰-disproportionating bacteria (S⁰DB) has been recently employed during environmental engineering systems. With S⁰DB, biological elemental sulfur disproportionation (BESD) process was developed and proved to be a promising way to help S⁰-based biotechnologies address the critical challenges stated above. However, BESD is an emerging biotechnology and critical aspects influencing its widely practical feasibility needs a comprehensive evaluation. For instance, can SDB be easily accessible and cultivable? Can high-rate BESD be achieved? Can BESD be integrated into the existing S⁰-based process and cost-effectively enhance S⁰-based system effectiveness? Therefore, in this perspective, the worldwide distribution, cultivation, and regulation of S⁰DB were firstly analyzed, in which several S⁰DB have so far been determined to substantially enrich in artificial bioreactors accompanying with high-rate sulfide production, which is comparable to that produced by traditional sulfate reduction. Secondly, analysis of the response of S⁰DB to nitrate suggests, although nitrate exposure could shift the metabolic function of S⁰DB from BESD to dissimilatory nitrate reduction to ammonium, system configuration and operational parameters can be optimized to allow the harmonious co-existence of S⁰DB and denitrifiers in a single reactor. The denitrification rates can be substantially improved, reaching >1.8 times greater than that of conventional S⁰-driven autotrophic denitrification system. In addition, S⁰DB can tolerate the toxicities of common metals (e.g., Cu, Zn, Pd, Hg, etc.), indicating its feasibility for metal-contaminated water/wastewater treatment. Without organic consumption by S⁰DB, BESD could provide an organic-free solution for S⁰-based bioprocess to effectively treat organic-deficit metal-laden water/wastewaters. Finally, we highlight future research directions to strengthen BESD versatility and scale up BESD for low-carbon and cost-effective water/wastewater treatment.