Geochemical reactions resulting from in situ oxidation of PCE-DNAPL by KMnO4 in a sandy aquifer

Although the potential for KMnO4 to destroy chlorinated ethenes in situ was first recognized more than a decade ago, the geochemical processes that accompany the oxidation have not previously been examined. In this study, aqueous KMnO4 solutions (10-30 g/L) were injected into an unconfined sand aquifer contaminated by the dense non-aqueous-phase liquid (DNAPL) tetrachloroethylene (PCE). The effects of the injections were monitored using depth-specific, multilevel groundwater samplers, and continuous cores. Two distinct geochemical zones evolved within several days after injection. In one zone where DNAPL is present, reactions between KMnO4 and dissolved PCE resulted in the release of abundant chloride and hydrogen ions to the water. Calcite and dolomite dissolved, buffering the pH in the range of 5.8-6.5, releasing Ca, Mg, and CO2 to the pore water. In this zone, the aqueous Ca/Cl concentration ratio is close to 5:12, consistent with the following reaction for 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 from dolomite dissolution, increases in the concentration of Mg as well as Na may result from exchange with K at cation-exchange sites. In the second zone, where lesser amounts of PCE were present, KMnO4 persisted in the aquifer for more than 14 months, and the porewater pH increased graduallyto between 9 and 10 as a resultof reaction between KMnO4 and H2O. A small increase in SO4 concentrations in the zones invaded by KMnO4 suggests that KMnO4 injections caused oxidation of sulfide minerals. There are important benefits of carbonate mineral buffering during DNAPL remediation by in situ oxidation. In a carbonate-buffered system, Mn(VII) is reduced to Mn(IV) and is immobilized in the groundwater by precipitating as insoluble manganese oxide. Energy-dispersive X-ray spectroscopy analyses of the manganese oxide coatings on aquifer mineral grains have 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 by coprecipitation of other elements. In addition, the consumption of excess KMnO4 by reaction with reduced minerals such as magnetite will be minimized because the rates of these reactions increase with decreasing pH. Aquifer cores collected after the KMnO4 injections exhibit dark brown to black bands of manganese oxide reaction products in sand layers where DNAPL was originally present. Mineralogical investigations indicate that the manganese oxide coatings are uniformly distributed over the mineral grains. Observations of the coatings using transmission electron microscopy indicate that they are on the order of 1 microm thick, and consequently, the decrease in porosity through the formation of the coatings is negligible.