Ozone- and Hydroxyl Radical-Mediated Oxidation of Pharmaceutical Compounds Using Ni-Doped Sb–SnO2 Anodes: Degradation Kinetics and Transformation Products

Electrochemical oxidation provides a versatile technique for treating wastewater streams onsite. We previously reported that a two-layer heterojunction Ni-Sb-SnO2 anode (NAT/AT) can produce both ozone (O3) and hydroxyl radical (•OH). In this study, we explore further the applicability of NAT/AT anodes for oxidizing pharmaceutical compounds using carbamazepine (CBZ) and fluconazole (FCZ) as model probe compounds. Details of the oxidation reaction kinetics and subsequent reaction products are investigated in the absence and presence of chloride (Cl-) and sulfate (SO42-). In all cases, faster or comparable degradation kinetics of CBZ and FCZ are achieved using the double-layered NAT/AT anode coupled with a stainless steel (SS) cathode in direct comparison to an identical setup using a boron-doped diamond anode. Production of O3 on NAT/AT enhances the elimination of both parent compounds and their transformation products (TPs). Very fast CBZ degradation is observed during NAT/AT-SS electrolysis in both NaClO4 and NaCl electrolytes. However, more reaction products are identified in the presence of Cl- than ClO4- (23 TPs vs 6). Rapid removal of FCZ is observed in NaClO4, while the degradation rate is retarded in NaCl depending on the [Cl-]. In SO42--containing electrolytes, altered reaction pathways and transformation product distributions are observed due to sulfate radical generation. SO4·- oxidation produces fewer hydroxylated products and promotes the oxidation of aldehydes to carboxylic acids. Similar trend in treatment performance is observed in mixtures of CBZ and FCZ with other pharmaceutical compounds in latrine wastewater and secondary WWTP effluent.