Comparison of the Rate Constants of •OH, SO4•−, CO3•−, Cl2•−, Cl•, ClO• and H• Reactions with Organic Water Contaminants
László Wojnárovits, Erzsébet Takács
Abstract
The reactions of 7 radicals, which play a key role in the degradation of water contaminants in Advanced Oxidation Processes, were discussed and compared. For evaluation of the reactivities and comparisons, the chemical reactivity rate constants were used, i.e., the rate constant that would be measured if diffusion of the species was not rate-influencing (kchem). By selecting appropriate diffusion-controlled rate constants (kdiff) and using the measured rate constant (k) values, kchem was calculated by the Noyes equation: 1/k = 1/kchem + 1/kdiff. When k and kdiff were close to each other, kchem was much higher than k. log kchem values showed good correlation in the cases of the •OH-H•, SO4•−-•OH, and Cl2•−-CO3•− radical pairs, moderate correlation was found in the case of the CO3•−-ClO• pair. The correlations may reflect, at least partly, similar reaction mechanisms, and allow estimation of unknown rate constant values. No correlation was found for the •OH-Cl• pair; this indicates highly different reaction mechanisms. The reactivity of oxidizing radicals decreases with decreasing reduction potential in the order: Cl• > •OH > SO4•− > Cl2•− > CO3•− > ClO•. The reductive H• reactions with organic molecules are similar to those of •OH, although the H• rate constants are 0.5–1 order of magnitude smaller.