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Generation of Hydroxyl Radical During Chlorination of Hydroxyphenols and Natural Organic Matter Extracts

Generation of Hydroxyl Radical During Chlorination of Hydroxyphenols and Natural Organic Matter Extracts

Eva M Rodríguez, Urs von Gunten

Water Res. 2020 Jun 15;177:115691.

PMID: 32304908

Abstract:

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.

Chemicals Related in the Paper:

Catalog Number	Product Name	Structure	CAS Number	Price
AP636942	2-Hydroxyterephthalic acid		636-94-2	Price

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