Kinetics and Mechanisms for TCE Oxidation by Permanganate

Abstract

The oxidation of trichloroethylene (TCE) by permanganate proceeds in three sequential reaction steps. In the initial step, a cyclic hypomanganate ester is formed via an activated organometallic complex. The activation parameters for this step were determined to be Ea = 41.46 kJ/mol, ΔH⧧ = 39 kJ/mol, and ΔS⧧ = −14 J/mol. The initial reaction is a rate-limiting step (second-order rate constant k1p = 0.65−0.68 M-1 s-1 at 21 °C) and independent of pH. In the second step, the decomposition of the cyclic ester with complete chlorine liberation proceeds quickly via various reaction pathways to form four carboxylic acids. Approximately 77% of the TCE was transformed to formic acid at pH 4, while 95−97% of the TCE was transformed to oxalic and glyoxylic acids at pH values of 6−8. Kinetic data suggest that the decomposition rate of the cyclic ester is at least 100 times higher than its formation rate. In the final step, all carboxylic acids are oxidized by permanganate to the final product, CO2. Second-order rate constants of k3ap = 0.075−0.35 M-1 s-1, k3bp = 0.13−0.37 M-1 s-1, and k3cp = 0.073−0.11 M-1 s-1 over a pH range of 4−8 at 21 °C were estimated for oxidation of formic, glyoxylic, and oxalic acids, respectively. The oxidation rate of carboxylic acids and accumulation rate of CO2 increase with decreasing pH. The kinetic model that was developed, formulated, and solved analytically on the basis of the understanding of various processes is consistent with results obtained in the kinetic experiments.