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Copper Inhibition of Triplet-Induced Reactions Involving Natural Organic Matter

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journal contribution
posted on 09.02.2018, 00:00 by Yanheng Pan, Shikha Garg, T. David Waite, Xin Yang
The triplet excited state of natural organic matter (3NOM*) is an important reactive intermediate in sensitizing transformation of a wide range of environmentally relevant organic compounds, but the impact of trace metals on the fate and reactivity of 3NOM* is poorly understood. In this study, we investigate the effect of low concentrations of copper on 3NOM*-mediated oxidation (electron transfer) and energy transfer reactions. The oxidative efficiency of 3NOM* from Suwannee River NOM (SRNOM) and the widely used model triplet sensitizer 4-carboxybenzophenone were determined by measuring the photooxidation of 2,4,6-trimethylphenol (TMP). The pseudo-first-order photooxidation rate constants of TMP decreased markedly in the presence of trace amounts of Cu­(II) (25–500 nM) with the decrease associated with the continuous reduction of the oxidation intermediates of TMP (i.e., TMP(−H)) by the photochemically produced Cu­(I). A kinetic model is developed that adequately describes the Cu inhibition effect in TMP photooxidation in irradiated SRNOM solutions. The 3NOM* energy transfer ability was assessed by measuring the isomerization of sorbic acid with the rate of this process markedly retarded in the presence of significantly higher (micromolar) concentrations of Cu­(II) than previously used. This result is attributed to (i) decreased formation of high energy 3NOM* due to formation of Cu–NOM complexes and (ii) increased loss of 3NOM* as a result of quenching by Cu. Since 3NOM* is the precursor to singlet oxygen (1O2) formation, the steady-state concentrations of 1O2 also decreased in the presence of micromolar concentrations of Cu­(II) with the quenching rate constant of 3NOM* by Cu calculated to be 1.08 × 1010 M–1 s–1.