Formation of iodinated trihalomethanes during breakpoint chlorination of iodide-containing water

Abstract

This study investigated the formation of toxic iodinated trihalomethanes (I-THMs) during breakpoint chlorination of iodide-containing water. Impact factors including I− concentration, natural organic matter (NOM) concentration and type, pH as well as Br−/I− molar ratio were systematically investigated. Moreover, the incorporation of I− into I-THM formation was also calculated. The results showed that I-THM formation varied in different zones of the breakpoint curves. I-THMs increased with increasing chlorine dosage to breakpoint value and then dropped significantly beyond it. Iodoform (CHI3) and chlorodiiodomethane (CHClI2) were the major I-THMs in the pre-breakpoint zone, while dichloroiodomethane (CHCl2I) was the dominant one in the post-breakpoint zone. The formation of I-THMs increased remarkably with I− and dissolved organic carbon (DOC) concentrations. More bromine-containing species were formed as Br−/I− molar ratio increased from 0.5 to 5. In addition, the major I-THM compound shifted from CHCl2I to the more toxic CHClBrI. As pH increased from 6.0 to 8.0, I-THM formation kept increasing in the pre-breakpoint zone and the speciation of I-THMs changed alongside the breakpoint curves. The incorporation of I− during breakpoint chlorination was highly dependent on chlorine, I−, and NOM concentrations, NOM type, solution pH and Br−/I− molar ratio.