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 ac company the oxidation have not previously been examined. In this study, aqu eous KMnO4 solutions (10-30 g/L)were injected into an unconfined sand aquif er contaminated by the dense non-aqueous-phase liquid (DNAPL) tetrachloroet hylene (PCE). The effects of the injections were monitored using depth-spec ific, multilevel groundwater samplers, and continuous cores. Two distinct g eochemical zones evolved within several days after injection. In one zone w here DNAPL is present, reactions between KMnO4 and dissolved PCE resulted i n 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 concent ration ratio is close to 5:12, consistent with the following reaction for t he oxidation of PCE in a carbonate-rich aquifer: 3C(2)Cl(4) + 5CaCO(3)(s) 4KMnO(4) + 2H(+) --> 11CO(2) + 4MnO(2)(s) + H2O + 12Cl(-) + 5Ca(2+) + 4K(). In addition to Mg from dolomite dissolution, increases in the concentrat ion 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 incr eased gradually to between 9 and 10 as a result of reaction between KMnO4 a nd H2O. A small increase in SO4 concentrations in the zones invaded by KMnO 4 suggests that KMnO4 injections caused oxidation of sulfide minerals. Ther e are important benefits of carbonate mineral buffering during DNAPL remedi ation by in situ oxidation. In a carbonate-buffered system, Mn(VII) is redu ced to Mn(IV) and is immobilized in the groundwater by precipitating as ins oluble manganese oxide. Energy-dispersive X-ray spectroscopy analyses of th e manganese oxide coatings on aquifer mineral grains have detected the impu rities Al, Ca, Cl, Cu, Pb, P, K, Si, S, Ti, U, and Zn indicating that, simi lar to natural systems, precipitation of manganese oxide is accompanied by coprecipitation of other elements. In addition,the consumption of excess KM nO4 by reaction with reduced minerals such as magnetite will be minimized b ecause the rates of these reactions increase with decreasing pH. Aquifer co res collected after the KMnO4 injections exhibit dark brown to black bands of manganese oxide reaction products in sand layers where DNAPL was origina lly present.