Effect of pH on the oxidation rate of organic compounds by FeII/H2O2.: Mechanisms and simulation

In this work, the kinetics of oxidation of Fe-II and of organic compounds (atrazine, 1,2,4-trichlorobenzene) in dilute aqueous solution by Fe-II/H2O2 have been investigated. Experiments were carried out at 25 degrees C, at initial pH between 1 and 8, and in the presence of an excess of Fe-II ([Fe-II](0) greater than or equal to 2 [H2O2](0)). Data obtained at pH less than or equal to 3 showed that the rates of oxidation of Fe-II and of organic compounds were predicted well by a kinetic model based on the Fe-II/H2O2 mechanism proposed by Barb et al.(1) (1951). The following values were found for the kinetic rate constants for the reaction of OH. with atrazine (molecular form: k = 3.0 x 10(9) L mol(-1) s(-1);protonated form: k = 1.2 x 10(9) L mol(-1) s(-1)) and 1,2,4-trichlorobenzene (k = 6.2 x 10(9) L mol(-1) s(-1)). At pH > 4 and in the absence of dissolved oxygen, the kinetic model was found to overestimate the rate of oxidation of both atrazine and 1,2,4-trichlorobenzene. These data suggest that the reaction of H2O2 with Fe-II probably goes through the formation of an intermediate. This intermediate leads to the formation of OH. radicals and Fe-III (major route under acidic pH) or leads to the formation of another intermediate which may react with Fe-II to produce Fe-III without formation of OH..