Role of diffusion in chemical oxidation of PCE in a dual permeability system

The effect of in situ chemical oxidation (ISCO) with permanganate (MnO4-) on tetra chloroethene (PCE) in a dual permeability system consisting of low permeability clay with high permeability sand lenses was investigated by two-dimensional laboratory experiments. The experiments imitate a field remediation at a former dry cleaning facility in Denmark. Results from laboratory experiments and field observations both show that after an application of MnO4- in the sand layer, the diffusion rate into the matrix is decreased due to reaction with PCE and the natural oxidant demand (NOD) related to the clayey till. A narrow but very efficient reaction zone is created in the clayey till. Initially the zone developed rapidly followed by a slower expansion with time. PCE will counter diffuse into the reaction zone, where it will be degraded as long as MnO4- is present. A mass balance for the laboratory experiment revealed that the reaction between MnO4- and the clayey till was responsible for up to 90% of the total MnO4- consumption. Based on laboratory experiments, the high MnO4- consumption from reaction with clayey till appears to have been the limiting factor for the oxidation of PCE at the field site and is thus impairing the efficiency of ISCO as a remedy.