Analysis of metronidazole removal and micro-toxicity in photolytic systems: Effects of persulfate dosage, anions and reactor operation-mode

The efficiency of persulfate (PS) alone, UV-C activated TiO2 (UV/TiO2), UV-C activated PS (UV/PS) and UV/PS with TiO2 (UV/PS/TiO2) in the oxidative removal of metronidazole (MNZ) was studied. In addition, applicability of UV/PS process in batch and continuous photo-reactors (BPR and CPR) was investigated. The performance of BPR and CPR was assessed in terms of energy consumed and electrical energy per order (E-EO), which indicated that both UV/PS and UV/PS/TiO2 systems were energy efficient. On the other hand, MNZ removal kinetics has revealed a higher MNZ removal rate of 0.232 min(-1) in UV/PS/TiO2 system and showed a pseudo-first-order kinetics (r(2) > 0.96). Furthermore, the effect of influent anion concentration (i.e. Cl-, SO42-, NO3- and their combinations) on UV/PS system was examined. The sequence of MNZ removal (from highest to lowest) in the presence of anions was (SO42- + Cl- + NO3-) > (SO42- + NO3-) > SO42- > (SO42- + Cl-) > Cl- > NO3-. An oxidative degradation pathway of MNZ in UV/TiO2 system was proposed. The toxicity of treated samples was evaluated using micro-toxicity assay against V. fischeri. The percentage inhibition of effluent samples to V. fischeri was within 15%, which denotes the suitability of CPR for real-time application.