Self-enhanced ozonation of benzoic acid at acidic pHs

• Efficient ozonation of benzoic acid at pH = 2.3 was observed.
• BA degradation and ozone decomposition was enhanced by increasing BA concentration.
• Intermediates formed during BA ozonation served as an initiator to produce OH.
• Possible mechanism of ozone activation by BA was proposed.

Ozonation of recalcitrant contaminants under acidic conditions is inefficient due to the lack of initiator (e.g., OH−) for ozone to produce hydroxyl radicals (HO). In this study, we reported that benzoic acid (BA), which is inert to ozone attack, underwent efficient degradation by ozone at acidic pH (2.3). The kinetics of BA degradation and ozone decomposition were both enhanced by increasing BA concentrations. Essentially, it is a HO-mediated reaction. Based on the exclusion of possible contributions of H2O2 and phenol-like intermediates for HO production, the reaction mechanism involved the formation of ozone ion (O3·−O3·−), which is an effective precursor of HO, was thus proposed. The hydroxycyclohexadienyl-type radicals generated during the attack of BA by HO may lead to the formation of O3·−O3·−. Meanwhile, O3·−O3·− could also be possibly formed from the reaction between ozone and organic (e.g., ROO∙) or inorganic peroxyl radicals (e.g., HO2). In addition, the hydroxylated products like phenol-like intermediates also played a positive role in HO production. Consequently, HO was produced efficiently under acidic conditions, resulting in rapid degradation of BA. This study provides a new approach for ozone activation even at acidic pHs, and broadens the knowledge of ozonation in removal of micropollutants from water.

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