Electric discharge by sulphide shuttling bacteria

Background: In biodesulphurisation processes, sulphide oxidising bacteria (SOB) convert toxic sulphide to sulphur. Haloalkalophilic SOB are known to anaerobically remove sulphide from solution and release electrons when subsequently exposed to an electrode. This makes SOB able to spatially decouple sulphide removal and current production, thereby acting as sulphide shuttles. Little is known about the kinetics of electron release by sulphide shuttling SOB. To gain more insight into the sulphide shuttling mechanism and electron release, current production of abiotic sulphide and sulphide shuttling SOB were compared.
Results: SOB communities dominated by Thioalkalivibrio sulfidophilus were incubated with sulphide in batch experiments. After sulphide was removed, SOB were discharged in an electrochemical cell. Anode potential, sulphide load and biomass concentration were varied. Current was produced at potentials of -0.2 V vs Ag/AgCl and higher, and at a potential of 0.1 V a coulombic efficiency of 70% was reached in 30 minutes. The maximum charge recovered from biologically stored sulphide was 4900 ±810 mC/mgN. Discharge kinetics were not affected by biomass concentration or degree of sulphide removal and kinetics of biotic and abiotic current production were similar.
Conclusion: Current production of sulphide shuttling SOB and abiotic sulphide were highly similar. This implies that sulphide shuttling is based on sorption to the biomass, and that shuttled sulphide converts to sulphur directly at the electrode surface. Therefore, the sulphide shuttling strategy is not sufficient to prevent electrode passivation, and different strategies need to be applied in the design of a sulphur-producing bioelectrochemical desulphurisation process.