A study of oxidation of chlorinated ethenes with permanganate in aqueous and porous media

Laboratory-scale batch and column experiments were conducted to study the kinetics and reaction pathways of oxidation of chlorinated ethenes (CE) with permanganate and to investigate the extent of reaction between CE and permanganate in soil environments and the impact of the reaction products [protons and manganese dioxide] on the soil system. ^ The kinetics study demonstrates that the reaction is first order in both CE and potassium permanganate (KMnO4). The second order rate constants for the reactions between KMnO4 and tetrachloroethylene (PCE), trichloroethylene (TCE), cis-1,2-dichloroethylene ( cis-DCE), 1,1-DCE, and trans-1,2-DCE (trans -DCE) at 20°C are 0.035 ± 0.004 M−1s −1 (PCE), 0.80 ± 0.12 M−1s −1 (TCE), 1.52 ± 0.05 M−1s −1 (cis-DCE), 2.1 ± 0.2 M−1 s−1 (1,1-DCE), and 48.6 ± 0.9 M −1s−1 (trans-DCE), respectively. Over the range of pH 3∼10 and ionic strength 0∼0.2 M, both pH and ionic strength have little effect on the reaction rate. The activation energy (Ea) of the reaction between KMnO4 and CE [except vinyl chloride (VC)] ranges from 5.8 ± 0.1 kcal/mol to 9.3 ± 0.9 kcal/mol. The degradation of VC by KMnO4 is a two-step process. The rate-limiting step is the second step involving the decomposition of an intermediate formed at the first step of the reaction. The second step is first order in VC and has an Ea of 7.9 ± 1 kcal/mol. ^ The reaction pathway study indicates that the oxidation of PCE with KMnO 4 may proceed through various pathways to yield CO2(g), oxalic acid, formic acid, and glycolic acid. In the solution of pH 3.1, PCE is quickly mineralized into CO2(g) and Cl−, while at pH 7 and pH 10.3, PCE is primarily transformed into oxalic acid first that is further oxidized by permanganate at a relatively slow rate. All chlorine atoms in PCE are rapidly liberated accompanied by protons during the reaction. ^ The study of TCE-KMnO4 reactions in a sandy soil indicates that KMnO4 can completely dechlorinate TCE when it is present in either dissolved phase or pure phase in the soil, as evidenced by ∼100% recovery of chlorine atoms in TCE. A significant decrease in pH [e.g., from 6.7 (initial pH) to ∼2.0 (the lowest pH during the run)] is observed in two columns with pure-phase TCE (7.6 g and 16.3 g) present. MnO2 is the predominant reduced manganese species of TCE-KMnO4 reactions. The MnO2 precipitate retained in the columns has caused a ∼20% reduction in mean retention time (e.g., from 123 min to 100 min) of the columns, implying a potential decrease in permeability of the soil due to MnO 2 deposition.