The influence of precipitate formation on the chemical oxidation of TCE DNAPL with potassium permanganate

A three-dimensional two-phase flow model is coupled to a non-linear reactive transport model to study the efficacy of potassium permanganate treatment on dense, non-aqueous phase liquid (DNAPL) source removal in porous media. A linear relationship between the soil permeability ( k ) and concentration of manganese dioxide precipitate ([MnO 2(s) ]), k = k o + S rind [MnO 2(s) ], is utilized to simulate nodal permeability reductions due to precipitate formation. Using published experimental column studies, an S rind = −5.5 × 10 −16 m 2 L/mg was determined for trichloroethylene (TCE) DNAPL. This S rind was then applied to treatment simulations on three-dimensional TCE DNAPL source zones comprising either DNAPL at residual saturations, or DNAPL at pooled saturations. DNAPL dissolution without oxidation treatment, simulated using equilibrium and the Nambi and Powers [Nambi I, Powers S. Mass transfer correlations for non-aqueous phase liquid dissolution from regions with high initial saturations. Water Resour Res 2003;39(2):1–11, SBH 4] mass transfer expression, required 31 and 36 years, respectively, to eliminate the residual source zone. For equilibrium dissolution with continuous treatment and no precipitate influence ( S rind = 0 m 2 L/mg), the residual source zone was removed after 11 years. However, when considering the precipitate influence (i.e., S rind = −5.5 × 10 −16 m 2 L/mg), 21 years of treatment were necessary to remove the DNAPL. When considering pulse treatments of 1 and 2 years duration followed by only dissolution, approximately 36 and 38 years, respectively, were required before the source zone was depleted, suggesting that there is no benefit to pulse treatment. Similar trends were observed when allowing 10 years of dissolution prior to treatment initiation. The treatment behaviour of the pooled TCE source, while slightly more efficient than the residual saturation source, was similar. Based on simulation findings, the precipitate (rind) formation significantly influences DNAPL treatment with permanganate; the most significant reductions in efficacy were observed for single pulse treatments (of 1 and 2 years), which exhibited times to source depletion similar to the case of dissolution without treatment.