Efficient trimethoprim removal via cooperation of radical and non-radical pathways in UV/peroxymonosulfate: Kinetics, mechanisms and disinfection by-products-associated risks

Abstract

The efficient removal of trimethoprim (TMP) via cooperation of radical and non-radical pathways in UV/peroxymonosulfate (PMS) involving kinetics, mechanisms and disinfection by-products (DBPs)-associated risks were investigated. While UV or PMS alone showed negligible degradation, UV/PMS significantly improved the TMP elimination efficiency. The roles of involved radicals (SO4•− and HO•) and non-radical (singlet oxygen, 1O2) were distinguished. The contribution order was HO• (56.16%) > SO4•− (27.32%) > 1O2 (16.01%). The reaction rates of TMP with SO4•−, HO• and 1O2 were determined to be 2.05×109 M−1·s−1, 8.20×109 M−1·s−1 and 1.61×108 M−1·s−1. The effects of PMS dosages, pH, UV intensity, ions and nature organic matter (NOM) were explored comprehensively. Cl− had negligible influence, but HCO3−, SO42− and NH4+ all promoted the degradation rate, while NO3− displayed mild inhibition, which suggested that the non-radical 1O2 in UV/PMS can effectively resist the ions interference. The removal rate was certainly reduced by NOM and it correlated negatively with common organic indices. The energy demand of system was evaluated using electric energy parameters per order. The destruction pathways of TMP including hydroxylation, carbonylation, demethylation, hydrogen extraction and ipso substitution were proposed. Compared with UV, UV/Cl2 and PMS/Cl2 (for 1O2 scenario), UV/PMS displayed significant advantages for DBPs regulation in disinfection, while 1O2 induced more trichloromethane. It was specially highlighted the synergistic action of radical and non-radical pathways in UV/PMS. The data supported the advantages of UV/PMS for TMP-like pollutants in terms of removal efficacy, operational cost and DBPs-related toxicity control in water treatment.