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 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.