Pradeep Chheda

Kinetics and mechanism of oxidation of tetrachloroethylene with permanganate.

The kinetics, reaction pathways and product distribution of oxidation of tetrachloroethylene (PCE) by potassium permanganate (KMnO4) were studied in phosphate-buffered solutions under constant pH, isothermal, completely mixed and zero headspace conditions. Experimental results indicate that the reaction is first-order with respect to both PCE and KMnO4 and has an activation energy of 9.3+/-0.9 kcal/mol. The second-order rate constant at 20 degrees C is 0.035+/-0.004 M(-1) s(-1), and is independent of pH and ionic strength (I) over a range of pH 3-10 and I approximately 0-0.2 M, respectively. The PCE-KMnO4 reaction may proceed through further oxidation and/or hydrolysis reaction pathways, greatly influenced by the acidity of the solution, to yield CO2(g), oxalic acid, formic acid and glycolic acid. Under acidic conditions (e.g., pH 3), the further oxidation pathway will dominate and PCE tends to be directly mineralized into CO2 and chloride. Under neutral (e.g., pH 7) and alkaline conditions (e.g., pH 10), the hydroxylation pathway dominates the reaction and PCE is primarily transformed into oxalic acid prior to complete PCE mineralization. Moreover, all chlorine atoms in PCE are rapidly liberated during the reaction and the rate of chloride production is very close to the rate of PCE degradation.

Oxidation of chlorinated ethenes by potassium permanganate: a kinetics study

The kinetics of oxidation of perchloroethylene (PCE), trichloroethylene (TCE), three isomers of dichloroethylene (DCE) and vinyl chloride (VC) by potassium permanganate (KMnO(4)) were studied in phosphate-buffered solutions of pH 7 and ionic strength approximately 0.05 M and under isothermal, completely mixed and zero headspace conditions. Experimental results have shown that the reaction appears to be second order overall and first order individually with respect to both KMnO(4) and all chlorinated ethenes (CEs), except VC. The degradation of VC by KMnO(4) is a two-consecutive-step process. The second step, being the rate-limiting step, is of first order in VC and has an activation energy (E(a)) of 7.9+/-1 kcal mol(-1). The second order rate constants at 20 degrees C are 0.035+/-0.004 M(-1) s(-1) (PCE), 0.80+/-0.12 M(-1) s(-1) (TCE), 1.52+/-0.05 M(-1) s(-1) (cis-DCE), 2.1+/-0.2 M(-1) s(-1) (1,1-DCE) and 48.6+/-0.9 M(-1) s(-1) (trans-DCE). The E(a) and entropy (DeltaS(*)) of the reaction between KMnO(4) and CEs (except VC) are in the range of 5.8-9.3 kcal mol(-1) and -33 to -36 kcal mol(-1) K(-1), respectively. Moreover, KMnO(4) is able to completely dechlorinate CEs, and the increase in acidity of the solution due to CE oxidation by KMnO(4) is directly proportional to the number of chlorine atoms in CEs.

Chemical oxidation of trichloroethylene with potassium permanganate in a porous medium

Abstract The extent of oxidation of dissolved-phase and pure-phase trichloroethylene (TCE) by potassium permanganate (KMnO4) in a sandy aquifer matrix and the impact of reaction products (H+ and MnOx) on pH, metal ion leaching and permeability of the aquifer medium near TCE source zones during KMnO4 flushing were investigated using laboratory-scale column experiments. The results of three column experiments indicated that KMnO4 completely dechlorinated TCE, evidenced by ∼100% chloride recovery, when TCE was present in dissolved phase or pure phase in the aquifer matrix. Two other column experiments were conducted to investigate the impact of H+ and MnOx on the aquifer medium near TCE source zones. KMnO4 flushing of the aquifer medium with residual pure TCE showed significant decreases in the pH levels (e.g. from 6.7 to ∼2.0) of the column effluents, and large quantities of MnOx precipitates were retained in the columns. The decrease in the pH levels in the columns led to an increase in the iron content of the column effluents. Two bromide tracer tests indicated that MnOx precipitates reduced approximately 20% of the pore space in the columns. In addition, characterization of MnOx by X-ray diffraction (XRD) and infrared (IR) spectroscopy demonstrated that the TCE–KMnO4 reaction yielded birnessite-type manganese oxide.

Impact of ozone on stability of montmorillonite suspensions

Abstract A sodium montmorillonite (NaM) suspension (150 mg/liter) was treated with various ozone doses up to 97 μ M (0.65 μmol O 3 /mg NaM). Suspension stability increased with increasing levels of ozonation as evidenced by increases in critical coagulation concentration (CCC) values of Na + , Ca 2+ , and La 3+ . The increase in induced stability of NaM was found to be most pronounced with Na + as an indifferent electrolyte and least noticeable when La 3+ was used. The enhanced stability of NaM can in large measure be attributed to an increase in the surface charge as a result of ozone-induced transformations. The conductivity of the suspending medium (water) was found to increase with ozonation indicating a leaching of ions from the crystal structure. DLVO theory was utilized to interpret the stability behavior of NaM suspensions; however, it underestimated CCC values. This discrepancy was attributed to an additional force resulting from hydrogen-bonding interactions. These interactions were found to be repulsive (hydration pressure) in nature. Hydration pressure increased with ozonation while Liftshitz—van der Waal forces remained largely unaffected. Electrostatic forces were found to be the major component responsible for increased stability of NaM as a result of ozonation, supporting a crystal dissolution hypothesis.