Insight in ethanethiol degradation kinetics at biocathodes

New technologies to remove organosulfur compounds from industrial sources should focus on the recovery of sulfur rather than incineration and sorption processes. The removal of organosulfur compounds using bio electrochemical systems might form a sustainable alternative. The aim of this study was to analyse ethanethiol degradation at biocathodes under anaerobic conditions. This was done by operating two cells at different loading rates for >360 days. We observed a stable removal efficiency of >70% with a maximum elimination capacity of 2.25 mM/d. Initially, ethanethiol was present in the effluent. However, over time, diethyl disulfide became the dominant organosulfur compound. Sulfate and thiosulfate were formed in small quantities in the biocathode. SEM imaging demonstrated the presence of crystalline structures with the typical bipyramid shape of elemental sulfur. The images also demonstrated the presence of a microbial community in a scattered biofilm on the electrode surface. The biocathodes from the continuous experiments were used in the batch experiments to gain more insight in the degradation kinetics. No electron donor (other than ethanethiol) was added to these serum flasks. The experiments confirmed that the oxidation of ethanethiol into diethyl disulfide was mostly biocatalytic as ethanethiol oxidation rates were much lower in the controls. Dynamic modeling indicated that ethanethiol nor diethyl disulfide were further degraded under these conditions. We hypothesize that the oxidation of ethanethiol forms the first important step in the degradation of this compound and that diethyl disulfide can be further degraded under electrochemically controlled conditions.