In situ photoelectrochemical activation of sulfite by MoS2 photoanode for enhanced removal of ammonium nitrogen from wastewater

The advanced oxidation processes (AOPs) based on oxysulfur radicals (SO3 center dot-, SO4 center dot- and SO5 center dot-) has been receiving growing attention in wastewater treatment. In this study, we report the in situ photoelectrochemical activation of sulfite to produce oxysulfur radicals with MoS2 nanosheets as a wide spectrum absorptive photoanode. At alkaline condition, the selective and efficient conversion of ammonia to dinitrogen was exclusively achieved in the presence of sulfite electrolyte under visible light irradiation. The sulfite plays multiple roles such as working as hole scavenger for improving stability of MoS2 electrode by inhibiting photo-corrosion and serving as precursor of oxysulfur radicals in the meantime. The influences of radical scavenger, dissolved oxygen and electrolyte on the photoelectrochemical, electrochemical and photochemical conversion of ammonia verified that oxysulfur radicals are more powerful than hydroxyl radicals in terms of ammonia conversion. The proposed system appears to be applicable to in situ treatment of wastewater containing of ammonia and sulfite pollutants, such as wastewater from ammonia-absorption-desulfurization of combustion smoke. This work also provides a new protocol in the design of new AOPs, where oxysulfur radicals can work together with hydroxyl radicals for simultaneous pollutants degradation and detoxification.