ClO2 pre-oxidation changes the yields and formation pathways of chloroform and chloral hydrate from phenolic precursors during chlorination

Abstract

Phenolic moieties in natural organic matter (NOM) are important precursors of disinfection by-products (DBPs). In this study, the formation of chloral hydrate from chlorination of seventeen phenolic compounds, including mono-, di- and tri-hydroxybenzenes, were evaluated and the role of chlorine dioxide (ClO2) pre-oxidation on its formation pathways was explored. Chloroform, was also evaluated for comparison. Chlorination of resorcinol exhibited the highest chloral hydrate yield (2.83 ± 0.13%) followed by chlorination of 2,4,6-trichlorophenol (0.61 ± 0.03%). The median of chloral hydrate yields from the tested phenolic compounds was 0.22%. ClO2 pre-oxidation reduced the yields of chloroform from phenol derivatives by 37–97%, except 4-methoxyphenol, catechol and 2,3-dihydroxyphenol. On the contrary, ClO2 pre-oxidation of di- and tri-hydroxybenzenes tended to increase chloral hydrate yields in post-chlorination. Mixed results (both increases and decreases) were observed in chloral hydrate formation from chlorination of mono-hydroxybenzenes after ClO2 pre-oxidation. The changes of their formation were dependent on ClO2 pre-oxidation time and dosages. Identification of transformation products suggested that phenolic compounds were mainly converted to unsaturated carbonyl structures by ClO2. Chlorine substituted benzoquinones and cyclopent-4-ene-1,3-diones were important transformation products after a series of ring open, decarboxylation, hydrolysis and chlorine substitution reactions. The changes in the formation yields of chloral hydrate and chloroform were governed by the difference in initial phenolic precursors and the transformation products after ClO2 pre-oxidation. ClO2 pre-oxidation in water treatment can effectively reduce chloroform formation but may have a risk of increasing chloral hydrate formation.