Effects of Ozone on the Photochemical and Photophysical Properties of Dissolved Organic Matter

This study focused on the effects of ozonation on the photochemical and photophysical properties of dissolved organic matter (DOM). Upon ozonation, a decrease in DOM absorbance was observed in parallel with an increase in singlet oxygen (O-1(2)) and fluorescence quantum yields (Phi(1O2) and Phi(F)). The increase in Phi(1O2) was attributed to the formation of quinone-like moieties during ozonation of the phenolic moieties of DOM, while the increase in Phi(F) can be explained by a significant decrease in the internal conversion rate of the first excited singlet state of the DOM ((DOM)-D-1*). It is a consequence of an increase in the average energy of the first electronic transition (S-1 -> S-0) that was assessed using the wavelength of maximum fluorescence emission (lambda(F,max)). Furthermore, ozonation did not affect the ratio of the apparent steady-state concentrations of excited triplet DOM ((DOM)-D-3*) and O-1(2), indicating that ozonation does not affect the efficiency of O-1(2) production from (DOM)-D-3*. The consequences of these changes for the phototransformation rates of micropollutants in surface waters were examined using photochemical model calculations. The decrease in DOM absorbance caused by ozonation leads to an enhancement of direct photolysis rates due to the increased transparency of the water. Rates of indirect photooxidation induced by O-1(2) and (DOM)-D-3* slightly decrease after ozonation.