# Anaerobic sulfide removal involves an intricate interplay between biomass, biosulfur, and solutes

## TL;DR

This study reveals that sulfide removal in biodesulfurisation involves not only bacteria but also biosulfur and solutes acting as electron shuttles.

## Contribution

The paper identifies biosulfur and solutes as novel participants in sulfide shuttling, beyond the previously known role of biomass.

## Key findings

- Sulfide removal occurs in two steps, with the second step lowering oxygen reduction potential (ORP).
- Biosulfur and solutes contribute to electron shuttling, increasing ORP even after sulfide is fully removed.
- The rate of ORP increase correlates with the sulfide removal capacity of the solution.

## Abstract

In the biodesulfurisation process harmful sulfide is converted to sulfur by sulfide oxidising bacteria (SOB), using oxygen as terminal electron acceptor. Surprisingly, in this process sulfide is already removed before oxygen is consumed. Therefore, sulfide and/or charge is being shuttled between sulfide removal and terminal electron transfer. Previously, it was thought that the bacteria themselves were the exclusive “electron shuttlers”. Patterns in sulfide concentration and oxygen reduction potential (ORP) during anaerobic sulfide removal tests in batch confirmed that SOB remove sulfide in two steps, of which the second lowered the ORP. However, we found that aside from biomass also biosulfur and certain solutes are involved in electron shuttling. Gradual removal of sulfide by interactions between sulfide, solutes, and biosulfur caused an increase in ORP, even after all sulfide was removed. The amplitude and rate of ORP increase rose with increasing sulfide removal capacity of the process solution. We hypothesise that organic thiol/disulfide redox couples are involved in electron shuttling.

In the biodesulfurisation process sulfide removal starts in the anaerobic section where biomass, organic solutes, and biosulfur together bind and transport sulfide equivalents to be oxidized in the aerobic reactor, thereby acting as sulfide shuttles.