Copper Inhibition of Triplet-Induced Reactions Involving Natural Organic Matter

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 p...