An Innovative in Situ Monitoring of Sulfate Reduction within a Wastewater Biofilm by H 2 S and SO 4 2− Microsensors

Microelectrodes can be used to obtain chemical profiles within biofilm microenvironments. For example, sulfate (SO 4 2− ) and hydrogen sulfide (H 2 S) microelectrodes can be used to study sulfate reduction activity in this context. However, there is no SO 4 2− microelectrode available for studying sulfate reduction in biofilms. In this study, SO 4 2− and H 2 S microelectrodes were fabricated and applied in the measurement of a wastewater membrane-aerated biofilm (MAB) to investigate the in situ sulfate reduction activity. Both the SO 4 2− and H 2 S microelectrodes with a tip diameter of around 20 micrometers were successfully developed and displayed satisfying selectivity to SO 4 2 − and H 2 S, respectively. The Nernstian slopes of calibration curves of the fabricated SO 4 2− electrodes were close to −28.1 mV/decade, and the R 2 values were greater than 98%. Within the selected concentration range from 10 − 5 M (0.96 mg/L) to 10 − 2 M (960 mg/L), the response of the SO 4 2− microelectrode was log-linearly related to its concentration. The successfully fabricated SO 4 2− microelectrode was combined with the existing H 2 S microelectrode and applied on an environmental wastewater biofilm sample to investigate the sulfate reduction activity within it. The H 2 S and SO 4 2− microelectrodes showed stable responses and good performance, and the decrease of SO 4 2− with an accompanying increased of H 2 S within the biofilm indicated the in situ sulfate reduction activity. The application of combined SO 4 2− and H 2 S microelectrodes in wastewater biofilms could amend the current understanding of sulfate reduction and sulfur oxidation within environmental biofilms based on only H 2 S microelectrodes.