The carbonate radical (hco3·/co3–·) as a reactive intermediate in water chemistry: kinetics and modelling

Abstract

Results from kinetic laboratory studies of reactions of the carbonate radical anion (CO - 3 .) with aromatic compounds in aqueous solution at T = 298 K are presented. Data were obtained in using a laser photolysis laser long-path absorption (LP-LPLA) apparatus which was designed for direct time-resolved studies of radical reactions. For the reactions of CO - 3 . with hydroquinone dimethyl ether (2), methyl anisole (3), benzene (4), p-xylene (5), toluene (6), chlorobenzene (7), nitrobenzene (8), and benzonitrile (9), rate coefficients of k 2 = (3.0 ± 0.6). 10 7 M-' s -1 , k 3 = (9.7 ± 1.7). 10 5 M -1 s -1 , k 4 = (3.2 ± 0.7). 10 5 M -1 s -1 , k 5 = (3.8 ± 0.9). 10 4 M -1 s -1 , k 6 = (6.8 ± 2.3). 10 4 M -1 s -1 , k 7 = (2.7 ± 0.6). 10 5 M -1 s -1 , k 8 = (1.4 ± 0.5). 10 4 M -1 s -1 , and k 9 < 1.3. 10 2 M -1 s -1 were obtained. In further studies the effect of temperature on the reactions (2), (4), and (5) has been studied. The kinetic data obtained for the reaction of the carbonate radical anion with aromatic compounds were compared to the corresponding reactions of the hydroxyl radical. Finally, these kinetic data were used within a simple model system to investigate the implications of carbonate radical anion kinetics within water treatment processes. It is shown that the degradation of organic pollutants in. OH-radical based water treatment may proceed via the CO - 3 ./HCO 3 . radical under certain conditions.