Photosensitizer Method to Determine Rate Constants for the Reaction of Carbonate Radical with Organic Compounds

Carbonate radical (CO3•-) is a powerful oxidant that is present in sunlit surface waters and in waters treated by advanced oxidation processes. The production of CO3•- in aqueous solution through oxidation of carbonate anion by excited triplet states of aromatic ketones was investigated in this study to provide new methods for the determination of rate constants and to explore a possible photoinduced pathway of CO3•- formation in the aquatic environment. Rate constants for triplet quenching by carbonate anion of up to 3.0 × 107 M-1 s-1 and CO3•- yields approaching unity, determined using laser flash photolysis, allowed us to conclude that such a formation mechanism might be significant in sulit natural waters. Kinetic methods based on either flash photolysis or steady-state irradiation and on the use of aromatic ketones as photosensitizers gave bimolecular rate constants in the range of 4 × 106 to 1 × 108 M-1 s-1 for the reaction of CO3•- with several s-triazine and phenylurea herbicides. For various anilines and phenoxide anions, rate constants determined by these methods agreed well with published values. Moreover, it could be shown for the first time by a direct method that dissolved natural organic matter (DOM) reduces the lifetime of CO3•- and a second-order rate constant of (280 ± 90) (mg of C/L)-1 s-1 was obtained for Suwannee River fulvic acid.