Generation of hydroxyl radical during chlorination of hydroxyphenols and natural organic matter extracts

The generation of hydroxyl radicals ((OH)-O-center dot) during the chlorination of air saturated solutions of different hydroxyphenols (hydroquinone, resorcinol, catechol, gallic and tannic acids) at pH 7 has been determined by the formation of phenol (in presence of benzene in excess) or 2-hydroxyterephthalic acid (in presence of terephthalic acid). Formation of (OH)-O-center dot was only detected during the chlorination of o- or p-hydroxyphenols, compounds that react with chlorine by electron transfer forming the corresponding semiquinones/quinones. In aerated solutions, oxygen is reduced by the semiquinone to the superoxide radical, O-2(center dot-), which reacts with HOCl to (OH)-O-center dot. Compared to the studied o-hydroxyphenols, the lower reactivity of hydroquinone towards chlorine favours the reaction between chlorine and O-2(center dot-), and its (OH)-O-center dot formation potential is -similar to 50 times higher. The extent of (OH)-O-center dot generated increased with the concentration of the hydroxyphenol and chlorine, but the (OH)-O-center dot yield (moles formed per mole of hydroxyphenol eliminated), decreased due to the formation of the quinone, that acts as O-2(center dot-) scavenger. The yield was almost not affected by the pH (6