Degradation of organic filter 2-Phenylbenzidazole-5-Sulfonic acid by light-driven free chlorine process: Reactive species and mechanisms

Abstract This study investigated the degradation of organic filter 2-phenylbenzidazole-5-sulfonic acid (PBSA) under ultraviolet and simulated sunlight activated free chlorine (UV/FC and SS/FC) in different water matrices. A mechanistic study revealed that ozone, HO•, Cl• and ClO• were responsible for PBSA degradation, and reactive chlorine species (RCS) played a major role. The developed mathematical model was utilized to estimate the rate constants of PBSA with representative radical species and predict the kinetics of PBSA attenuation well. Observed PBSA loss rate constants (kobs) decrease with pH during UV/FC due to the high quantum yield and weak radical scavenging ability of hypochlorite (pKa = 7.5), while there is little difference under SS/FC. Furthermore, formation of reactive oxidants (HO•, Cl•, ClO• and ozone) during FC photolysis as a function of pH (5-10) were quantified with probe compounds. In seawater matrix, kobs for PBSA degradation under UV/FC (0.0109 min-1) was two times lower than that under SS/FC (0.0244 min-1) because the highly abundant HCO3-, Br- and Cl- exhibited a stronger inhibition (76%) for the former. A screening test showed that NaBr and NaBr+NaHCO3 during SS/FC displayed the reinforcement effects, primarily due to the interaction between reactive bromine species and the imidazole moiety in PBSA. Based on these identified products, transformation pathways of PBSA included hydroxylation, ring cleavage and desulfonate. This work provided mechanistic insight into PBSA degradation by reactive species formed through FC photoactivation, and suggest a promise of SS/FC for efficient abatement of organic filters-caused seawater pollution for realistic application.