Dichloroacetylene Formation from Azoles in the UV/Chlorine Process

The combined application of UV254 irradiation and chlorination (UV/chlorine) is considered to be a promising method for the disinfection and removal of organic pollutants. However, it is not clear how this process affects the formation of disinfection by-products (DBPs). In this study, the formation of DBPs from 11 of the most commonly used azoles in the UV/chlorine process has been investigated. The study demonstrated that dichloroacetylene, a previously unreported DBP, was identified as a coproduct of azoles. In addition to dichloroacetylene, another toxic DBP, cyanogen chloride (CNCl), was also detected in the process. However, neither dichloroacetylene nor CNCl was detected in the presence of azoles during chlorination alone. The results indicated that •OH and •Cl radicals resulting from UV/chlorine were the key factors in the production of dichloroacetylene and CNCl. Further studies have shown that dichloroacetylene was relatively stable in water with a hydrolysis half-life of 77 h and can be degraded in the presence of UV irradiation or high doses of chlorine. Additionally, bromochloroacetylene was observed in the UV/chlorine system in the presence of bromide. Furthermore, the presence of dichloroacetylene was detected in real UV/chlorine-treated water samples at concentrations ranging from 0.06 to 0.18 μM. This was found to be significantly increased by the addition of pyrazole, as was the case with CNCl. Dichloroacetylene and CNCl were detected in wastewater treatment plants at concentrations ranging from 0.15 to 0.31 μM. This finding suggested that the active species of the UV/chlorine process may alter the DBP formation pathway and lead to the emergence of novel toxic DBPs that have not yet been identified.