Comparative assessment of a foam-based method for ISCO of coal tar contaminated unsaturated soils

In situ delivery of liquid reagents in vadose zone is limited by gravity and soil anisotropy. A new foam-based delivery method of persulfate (PS) solutions in unsaturated soils was previously shown to overcome these limitations. The goal of this paper is to demonstrate the efficiency of this method regarding contaminant removal. Hence, the comparative oxidation of 200 mg.kg(-1) coal tar artificially contaminated soils with thermally activated PS was carried out after PS-delivery using foam, surfactant solution and pure water. The foam-based method was compared in unfavorable conditions to the reference methods. Especially, in the latter, soil and oxidant solutions were thoroughly mixed to ensure good contact between pollutant and reagent. Degradation kinetics of PS, surfactant and semi-volatile hydrocarbons (HCs) were followed. PS-concentration of 9.1 g.kg(soil)(-1), based on the stochiometric molar ratio between oxidant and HCs, resulted in high HCs oxidation rates (> 90%) for the three delivery fluids and degradation rate constants (k(obs),(HC)) were calculated. The presence of 1%w surfactant led to the decrease of k(obs,HC) by 10 to 14-times compared to water. However, using the foam-based method, k(obs,HC) showed intermediate results, being 6 to 8-times higher than for the surfactant solution, because of 80% lower surfactant content in soils after PS-injection within foam. This was confirmed through the calculated selectivity of HCs oxidation, which was 38-times higher using foam than for the surfactant solution. Finally, considering the toxicity of leachates from treated soils, lower detrimental effects were observed for the foam-based PS-delivery in contrasts to the reference methods.