Regulating the in situ generation of active chlorine species in a photoelectrochemical system for the efficient simultaneous removal of ammonia nitrogen and refractory pharmaceuticals

Abstract

The removal of chemical oxygen demand (COD) and ammonia nitrogen (NH4+-N) is always a critical problem in combined polluted sewage treatment. To simultaneously treat NH4+-N and refractory organics, a WO3/BiVO4-cobalt borate (WO3/BiVO4-CoBi) photoanode was applied in a photoelectrochemical (PEC) system. It used Cl- in highly-saline wastewater to generate strongly-oxidizing active chlorine species in situ, which were responsible for the system’s NH4+-N directional oxidation and simultaneous degradation ability. Thus, the PEC system with Cl- (PEC-Cl) simultaneously removed 99% of carbamazepine (CBZ) within 40 min and 75.4% of NH4+-N within 120 min. It also selectively converted NH4+-N into N2 (10% NO3–-N production rate). The {010} facet-oriented array of decahedral BiVO4 and WO3/BiVO4 heterostructure contributed to the effective separation of photo-generated electron-hole pairs. Coupled with the reduced charge-transfer resistance of the photoanode/solution interface and the greater conversion of photo-generated holes into redox equivalents by the CoBi co-catalyst, the system demonstrated efficient carrier transfer. Active chlorine species (Cl2, HClO, Cl•, and •ClO) were produced with the participation of photo-generated holes and •OH and could be regulated by controlling the system potential, Cl- concentration, and solution pH. This study provides a feasible method for the simultaneous removal of NH4+-N and refractory organics with a high efficiency and stability.