Electrochemical oxidation of pharmaceuticals in synthetic fresh human urine: Using selective radical quenchers to reveal the dominant degradation pathways and the scavenging effects of individual urine constituents

Electrochemical oxidation of fresh human urine is a promising method to prevent pharmaceuticals from being discharged into the environment. Here, we evaluate the importance of electro-generated oxidants and direct anodic oxidation for degradation of four pharmaceutical (cyclophosphamide (CP), carbamazepine (CBZ), sulfamethoxazole (SMX) and ibuprofen (IBP)) accounting for the scavenging effect of urine constituents using boron-doped diamond (BDD) and IrO2 electrodes. Allyl alcohol and tert-butanol were used as selective quenchers for adsorbed and dissolved radicals, respectively. In electrolyte containing only chloride and pharmaceuticals, we found that CBZ and SMX are primarily oxidized by electro-generated Cl2 in the fluid boundary layer , and CP and IBP are primarily oxidized by physisorbed •OH or chemisorbed chlorine (IrO3-Cl). Regarding the effects of other fresh urine constituents, urea, creatinine, and uric acid quench the dissolved reactive chlorine species (Cl•/Cl2•‒, HOCl, Cl2, etc.). However, SO42‒ shows no effect on pharmaceutical degradation while H2PO4‒ and citrate ions quench IrO3-Cl resulting in a mixed kinetic and mass-transfer limiting oxidation of pharmaceuticals on IrO2. Citrate ions only quench the dissolved oxidants (surface adsorbed radicals are the dominant oxidants) leading to the pharmaceutical degradation limited by the mass transfer of pharmaceutical to BDD surface. This work provides an understanding of the significance of various pathways for pharmaceutical degradation, scavenging effect of urine constituents, and strategies for rapid pharmaceutical degradation in human urine.