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

The generation of hydroxyl radicals (^•OH) 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 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, O₂^•-, which reacts with HOCl to ^•OH. Compared to the studied o-hydroxyphenols, the lower reactivity of hydroquinone towards chlorine favours the reaction between chlorine and O₂^•-, and its ^•OH formation potential is ∼50 times higher. The extent of ^•OH generated increased with the concentration of the hydroxyphenol and chlorine, but the ^•OH yield (moles formed per mole of hydroxyphenol eliminated), decreased due to the formation of the quinone, that acts as O₂^•- scavenger. The yield was almost not affected by the pH (6 ≤ pH ≤ 7.5), whereas a strong impact of dissolved O₂ was observed. The ^•OH production was null in absence of O₂ and 2.5-3 times higher at oxygen saturated conditions compared to air-saturated. Contrary to chlorination, during bromination of hydroquinone ^•OH was not formed, which can be attributable to a much faster consumption of the oxidant, with no chance for O₂^•- to react with bromine. Formation of ^•OH during the chlorination of different NOM extracts (SRHA, SRFA, PLFA and Nordic Lake NOM) and water from Lake Greifensee (Switzerland) was also studied using terephthalic acid as ^•OH scavenger. For SRHA, SRFA and Nordic Lake NOM (all of allochthonous origin and presenting high electron-donating capacity, EDC), ^•OH yields expressed as moles formed per mole of DOC₀ (%), were between 1.1 and 2.0, similar to that of hydroquinone (∼1.5). For PLFA and Lake Greifensee water (autochthonous, lower EDC) much lower ^•OH yields were observed (0.1-0.3). Both chlorination rate and EDC, the later favouring the formation/stabilization of O₂^•-, seem to be key factors involved in ^•OH generation during the chlorination of NOM. A mechanism for these findings is proposed based on kinetic simulations of hydroquinone chlorination at pH 7.