New insight into increased toxicity during ozonation of chlorophenol: The significant contribution of oxidizing...

Biological safety evaluation and toxic by-products identification are critical issues in the partial oxidation process. Previous studies have shown that the whole toxicity increased in the effluent of an ozonation process for chlorophenols removal. Here, this study systematically investigated the changes of acute toxicity during the ozonation of 3-chlorophenol under four key operational conditions, including initial 3-chlorophenol concentration (20-60 mg/L), ozone concentration (14-42 mg/L), reaction pH (3-10) and ozonation time (0-50 min). The results found that the ozonation process induced a significant increase in the acute toxicity, followed by its gradual decrease. The observation of higher acute toxicity increase generally happened at higher initial 3-chlorophenol concentration, lower ozone concentration and lower reaction pH. At the toxicity peaks, the oxidizing intermediates posed acute toxicity equal to 65.8%-96.3% of the whole toxicity. Among them, free active chlorine (FAC) contributed 21.4%-51.6%, and its concentrations significantly correlated to the acute toxicity change. Therefore, two possible FAC generation pathways initiated by ozone molecule were proposed: (i) bond breaking of the oxychloride complex formed by the combination of chloride ion and zwitterion; or (ii) hydrolysis of ozonides formed by the electrophilic reaction of ozone molecule. Together, these results firstly revealed the significant toxicity contribution of oxidizing intermediates during the ozonation of chlorophenols, supporting further development of safe and effective ozone-based water treatment schemes.