Electrochemical oxidation represents a versatile technique in treating wastewater onsite and has been proven effective in removing trace organic compounds including many pharmaceuticals and personal care products. Ni-Sb-SnO2 based electrodes (NAT/AT) featuring co-production of ozone and hydroxyl radicals have demonstrated high reactivity for treating toilet wastewater while achieving simultaneous disinfection and pharmaceutical removal, and certain aromatic compounds showed accelerated degradation kinetics from anodic O3 activation even though themselves being inert to O3 attack. In this study, the mechanism and the applicability of NAT/AT in treating pharmaceuticals were further probed. Degradation kinetics and transformation products (TPs) were studied for carbamazepine (CBZ) and fluconazole (FCZ), two of the top 100 commonly used pharmaceuticals with significantly different reaction rates with O3 (kCBZ = 3.0 × 105 M-1s-1 vs. kFCZ = 2.0 M-1s-1). Types and distributions of TPs were studied at NAT/AT and boron-doped diamond (BDD) to determine the influence of O3 on degradation pathways. In addition, TP formation and distribution patterns were investigated when chloride and sulfate are present in the solution. The presence of chloride is an essential factor in determining both degradation kinetics and TP distribution. For NAT/AT, very fast degradation of CBZ was observed in both NaClO4 and NaCl electrolytes, though a lot more TPs were identified in NaCl than in NaClO4 (19 vs. 4). Fast removal of FCZ were also recorded in NaClO4, which, combined with kinetic modeling, indicated significant contribution from O3 activation. The rate was greatly retarded in NaCl (60-70% removal in 1 h vs. 100% removal in 5 min), the degree of which depends on chloride concentration. For BDD, similar removal kinetics and TP distribution were observed in NaCl, while less TPs and slower removal rates were recorded in NaClO4 due to the absence of O3 production. The presence of sulfate, on the other hand, has less influence on removal kinetics but affects reaction pathways and thus distribution of certain TPs.

Ozone and hydroxyl radical-mediated oxidation of pharmaceuticals using Ni-Sb-SnO2 electrodes: Reaction kinetics and transformation product identification

ACS Fall 2021

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