| Abstract
| - Although the potential for KMnO4 to destroy chlorinatedethenes in situ was first recognized more than a decade ago,the geochemical processes that accompany the oxidationhave not previously been examined. In this study, aqueousKMnO4 solutions (10−30 g/L) were injected into an unconfinedsand aquifer contaminated by the dense non-aqueous-phase liquid (DNAPL) tetrachloroethylene (PCE). The effectsof the injections were monitored using depth-specific, multi-level groundwater samplers, and continuous cores.Two distinct geochemical zones evolved within severaldays after injection. In one zone where DNAPL is present,reactions between KMnO4 and dissolved PCE resulted inthe release of abundant chloride and hydrogen ions to thewater. Calcite and dolomite dissolved, buffering the pHin the range of 5.8−6.5, releasing Ca, Mg, and CO2 to thepore water. In this zone, the aqueous Ca/Cl concentrationratio is close to 5:12, consistent with the following reactionfor the oxidation of PCE in a carbonate-rich aquifer:3C2Cl4 + 5CaCO3(s) + 4KMnO4 + 2H+ → 11CO2 + 4MnO2(s)+ H2O + 12Cl- + 5Ca2+ + 4K+. In addition to Mg fromdolomite dissolution, increases in the concentration of Mgas well as Na may result from exchange with K at cation-exchange sites. In the second zone, where lesser amountsof PCE were present, KMnO4 persisted in the aquifer formore than 14 months, and the porewater pH increasedgradually to between 9 and 10 as a result of reaction betweenKMnO4 and H2O. A small increase in SO4 concentrationsin the zones invaded by KMnO4 suggests that KMnO4 injectionscaused oxidation of sulfide minerals. There are importantbenefits of carbonate mineral buffering during DNAPLremediation by in situ oxidation. In a carbonate-bufferedsystem, Mn(VII) is reduced to Mn(IV) and is immobilized inthe groundwater by precipitating as insoluble manganeseoxide. Energy-dispersive X-ray spectroscopy analysesof the manganese oxide coatings on aquifer mineral grainshave detected the impurities Al, Ca, Cl, Cu, Pb, P, K, Si,S, Ti, U, and Zn indicating that, similar to natural systems,precipitation of manganese oxide is accompanied bycoprecipitation of other elements. In addition, the consumptionof excess KMnO4 by reaction with reduced mineralssuch as magnetite will be minimized because the rates ofthese reactions increase with decreasing pH. Aquifercores collected after the KMnO4 injections exhibit darkbrown to black bands of manganese oxide reaction productsin sand layers where DNAPL was originally present.Mineralogical investigations indicate that the manganeseoxide coatings are uniformly distributed over the mineralgrains. Observations of the coatings using transmissionelectron microscopy indicate that they are on the order of1 μm thick, and consequently, the decrease in porositythrough the formation of the coatings is negligible.
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