Kinetics and pathways of the degradation of PPCPs by carbonate radicals in advanced oxidation processes

The carbonate radical (CO₃^•-) is a typical secondary radical observed in engineering and natural aquatic systems. This study investigated the degradation kinetics of 20 pharmaceuticals and personal care products (PPCPs) by CO₃^•- and the transformation pathways of a typical PPCP (naproxen) that is susceptible to CO₃^•-. CO₃^•- is highly selective for compounds containing aniline and phenolic hydroxyl groups as well as naphthalene rings, such as sulfamethoxazole, sulfamethazine, salbutamol, propranolol, naproxen, and macrolide antibiotics such as azithromycin, for which the second-order rate constants range from 5.6 × 10⁷ M^-1s^-1 to 2.96 × 10⁸ M^-1s^-1. A good linear relationship is observed between the natural logarithms of k_CO3^•- and the negative values of the Hammett Σσ_p⁺ constant for aromatic PPCPs, indicating that electron-donating groups promote the attack of benzene derivatives by CO₃^•-. The contribution of CO₃^•- to naproxen degradation is significant in different processes such as UV/H₂O₂, UV/persulfate, UV/chlorine, and UV/monochloramine, in the presence of HCO₃^-, which compensates for the decreased contributions of primary radicals. In particular, the formation of CO₃^•- increases the first-order rate constant of naproxen by 127% in UV/monochloramine in the presence of 50 mM HCO₃^- compared to that without HCO₃^-. Natural organic matter (NOM) exerts a slight scavenging effect on CO₃^•-, decreasing the inhibition effect of NOM on the degradation of naproxen by UV/H₂O₂ in the presence of HCO₃^-. The pathways involved in the transformation of naproxen by CO₃^•- include decarboxylation, hydroxylation, ketonization, demethylation and aldolization. In addition, the alteration of the genotoxicity during naproxen degradation by CO₃^•- was negligible.