Lifetimes of Triplet Dissolved Natural Organic Matter (DOM) and the Effect of NaBH_4 Reduction on Singlet Oxygen Quantum Yields:Implications for DOM Photophysics

摘要

The natural lifetimes of triplet dissolved organic matter (~3DOM~*) were determined by an O_2 saturation kinetics study of singlet oxygen quantum yields (Φ_(102)) in buffered D_2O. At least two distinct ~3DOM~* pools are present, and the observed lifetime range (～20 to 80 μs) leads to a dependence of Φ_(102) on O_2 concentrations between 29 and 290 μM. Thus, steady-state O_2 concentrations will depend on [O_2] in natural waters. The lifetimes are essentially identical for DOM samples of different origins and do not vary with excitation wavelength. However, Φ_(102) varies greatly between samples and decreases with excitation wavelength. These data strongly suggest that ~3DOM~* quantum yields decrease with excitation wavelength, which gives rise to the Φ_(102) variation. Borohydride reduction of several samples in both D_2O and H_2O lowers the absorbance and ~1O_2 production rates, but it does not alter Φ_(102). This is consistent with a model in which ~O_2 sensitizing chromophores are borohydride reducible groups in DOM, such as aromatic ketones. Interpreted in the framework of a charge transfer (CT) model for DOM optical properties, the collective data suggest a model in which electron acceptor moieties are Important ~O_2 setuitizers and where CT interactions of these moieties disrupt their ability to produce ~1O_2.