Improving the treatment of aqueous and non-aqueous phase TCE in low permeable zones with permanganate

Treating aqueous and non-aqueous phase trichloroethene (TCE) in the low permeable zones (LPZs) of contaminated aquifers is a particularly challenging issue for injection-based remedial treatments. This is because injected oxidants typically stay in the more porous zones of an aquifer and bypass LPZs. Once the porous zones surrounding a LPZ are treated, contaminants diffuse out of the LPZ and contaminate flowing water, a process known as rebound. When permanganate (MnO4-) is used as the chemical oxidant, three hurdles to reducing rebound include: getting the MnO4 - to penetrate the LPZs, minimizing MnO2 rind formation, and overcoming the kinetic constraints of treating a DNAPL with an aqueous-phase oxidant. Our objective was to improve the sweeping efficiency of permanganate into LPZs to treat TCE. This was accomplished by conducting 2D transport experiments that quantified the ability of MnO4 - to penetrate and sweep through a LPZ that was spiked with either aqueous or non-aqueous phase 14C-TCE. The remedial treatments evaluated included pairing MnO4- with: (i) a shear-thinning polymer (xanthan); (ii) stabilization aids that minimized MnO2 formation, and (iii) a phase-transfer catalyst. In addition, we quantified the ability of these flooding solutions to improve TCE destruction under batch conditions by developing miniature LPZ cylinders that were spiked with 14C-labeled TCE. Transport experiments showed that MnO4- alone was inefficient in penetrating the LPZ and caused significant MnO2 rind formation when non-aqueous phase TCE was present. By including xanthan with MnO4-, the sweeping efficiency increased significantly and the total percentage of dissolved-phase TCE oxidized increased by 12% over MnO4- alone. Although xanthan improved dissolved-phase TCE oxidation, MnO2 rind formation was still evident when non-aqueous phase TCE was present. By including xanthan and the stabilization aid, sodium hexametaphosphate (SHMP), sweeping efficiency was 100% and MnO2 precipitation in the LPZ was minimal. By differentiating transport 14C-effluent into “TCE” versus “oxidized-TCE”, results showed the MnO4-+xanthan+SHMP treatment oxidized 8% more non-aqueous phase TCE than MnO4- alone. Batch experiments also confirmed that the stabilization aids tested improved TCE destruction by 11 to 21%. These results support the use of xanthan and stabilization aids in treating both aqueous and non-aqueous phase TCE in contaminated aquifers