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

Abstract

Electrochem. oxidn. represents a versatile technique in treating wastewater onsite and has been proven effective in removing trace org. compds. including many pharmaceuticals and personal care products. Ni-Sb-SnO₂ based electrodes (NAT/AT) featuring co-prodn. of ozone and hydroxyl radicals have demonstrated high reactivity for treating toilet wastewater while achieving simultaneous disinfection and pharmaceutical removal, and certain arom. compds. showed accelerated degrdn. 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. Degrdn. 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 (k_(CBZ) = 3.0 x 10⁵ M⁻¹s⁻¹ vs. k_(FCZ) = 2.0 M⁻¹s⁻¹). Types and distributions of TPs were studied at NAT/AT and boron-doped diamond (BDD) to det. the influence of O₃ on degrdn. pathways. In addn., TP formation and distribution patterns were investigated when chloride and sulfate are present in the soln. The presence of chloride is an essential factor in detg. both degrdn. kinetics and TP distribution. For NAT/AT, very fast degrdn. of CBZ was obsd. 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 concn. For BDD, similar removal kinetics and TP distribution were obsd. in NaCl, while less TPs and slower removal rates were recorded in NaClO₄ due to the absence of O₃ prodn. The presence of sulfate, on the other hand, has less influence on removal kinetics but affects reaction pathways and thus distribution of certain TPs.

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