Oxidative degradation and kinetics of chlorinated ethylenes by potassium permanganate

摘要

The oxidative treatment of chlorinated ethylenes in ground water using permanganate was investigated in a series of batch kinetic tests. Five chlorinated ethylenes including tetrachloroethy- lene (PCE), trichloroethylene (TCE), and three isomers of dichloroethylenes (DCEs) were examined. The degradation process was rapid with pseudo-first-order rate constants ranging from 4.5 × 10~(-5) to 0.03 s~(-1) at MnO_4~- = 1 mM. The rate increased with a decreasing number of chlorine substituents on the ethylene. The higher reactivity of trans-DCE (k_obs = 30 × 10~(-3) s~(-1) at MnO_4~- = 1 mM) as compared to cis--DCE (k_obs = 0.9 × 10~(-3) s~(-1) at MnO_4~- = 1 mM) is thought to be caused by a significant steric effect due to the formation of a large cyclic activated complex. TCE oxidation as a second--order reaction was confirmed and the rate constant, k = 0.67 ± 0.03 M~(-1) s~(-1), is independent of pH over the range of 4-8. The activity of both Clˉ and hydrogen ions was monitored over time and suggests essentially complete dechlorination, making the degradation products less harmful than the parent compounds. Competition for MnO_4~- from other organic compounds in ground water or highly contaminated ground water was also evaluated in experiments. A simple and quick approach was demonstrated to estimate permanganate consump- tion by other organic compounds for field applications and to predict the TCE degradation rate in a system involving multiple contaminants. The modeling results suggest that the effect of autocatalysis by MnO_2 on TCE degradation is significant when the system contains high concentration levels of MnO_4~- and TOC.