The triplet states of dissolved organic matter (3DOM*) have been well known to oxidize various organic contaminants, but evidence of their reducing properties are largely scarce. In this work, chlorine dioxide (ClO2) as a single-electron oxidant was used as a probe to evaluate the reduction property of 3DOM*. The reduction of ClO2 to chlorite was observed in the solutions of model photosensitizers (i.e., 4-carboxybenzophenone, benzophenone, acetophenone, 3-methoxyacetophenone, naphthalene, and xanthone) during UV irradiation with the presence of ClO2, though they are resistant to ClO2 oxidation in the dark. The reducing property of the triplet states of photosensitizers was verified and their second-order reaction rate constants with ClO2 were determined to be in the range of 1.45(± 0.03)× 109 - 2.18(± 0.06) × 109 M-1 s-1 at pH 7.0. The quenching tests excluded the role of other reactive species (e.g., HO•, O(3P), Cl•, ClO• and HOCl/OCl-, O2•- and eaq-) in ClO2 reduction to chlorite when using model photosensitizers and DOM isolates. Chlorite formation was 48.1-90.4% and 4812.8-7721.8% higher during UV irradiation with the presence of ClO2 and DOM than those without UV irradiation or without DOM present, respectively. The enhancement was attributed to the enhanced electron donating capacity (chlorite precursors) of DOM upon UV irradiation and also to 3DOM* acting as an electron donor reducing ClO2 to chlorite. This study highlighted the important role of 3DOM* as a reductant.
Keywords: Chlorine dioxide; Chlorite; Dissolved organic matter (DOM); Triplet state; UV irradiation; Water treatment.
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