Effect of chlorine content of chlorinated phenols on their oxidation kinetics by Fenton's reagent

The effect of chlorine content of chlorinated phenols on their oxidation kinetics by Fenton's reagent was evaluated through different kinetic studies. First, elementary oxidation rate constants between hydroxyl radicals and 2,4-DCP,2,4,6-TCP as determined in this study are 7.2*10(9) M(-1)s(-1) and 6.3*10(9) M(-1)s(-1), respectively. The relative ratios between the rate constants of 2-CP and 2,4,6-TCP referred to the rate constant of 2,4-DCP indicates that the reactivity of chorophenols decreases with increasing substitution of chlorine on the aromatic ring. Second, the pseudo-first order oxidation kinetic constants at overdosed H2O2 and Fe2+ were proportional to the number of unsubstituted positions, while no correlation between dechlorination constants and the number of unoccupied sites on the aromatic ring was found. Third, dechlorination kinetic model as a function of H2O2, Fe2+, and chlorophenol concentration was developed. At constant H2O2 concentration of 5*10(-3) M, the observed dechlorination rate constants for 2-MCP, 2,4-DCP, and 2,4,6-TCP are 1.16*10(7) M(-1)s(-1), 2.28*10(7) M(-1)s(-1), and 4.36*10(8) M(-1)s(-1), respectively. All of these results demonstrated that the oxidation rate constants decrease with increasing content of chlorine. In addition, the initial dechlorination rates do not change significantly for all the chlorinated phenols studied at constant H2O2 and Fe2+ concentration, which suggests that the limiting step in Fenton chemistry is the generation of hydroxyl radicals through H2O2 and Fe2+. Copyright (C) 1996 Elsevier Science Ltd.