Comparative assessment of a foam-based oxidative treatment of hydrocarbon-contaminated unsaturated and anisotropic soils.

In situ delivery of liquid reagents in vadose zone is limited by soil anisotropy and gravity. The enhanced delivery of persulfate (PS) as oxidant, using a new foam-based method (F-PS) was compared at bench-scale to traditional water-based (W-PS) and surfactant solution-based (S-PS) deliveries. The goal was to distribute PS uniformly in coal tar-contaminated unsaturated and anisotropic soils, both in terms of permeability and contamination. Water was the less efficiently delivered fluid because of the hydrophobicity of the contaminated soils. Surfactant enhanced PS-distribution into contaminated zones by reducing interfacial tension and inverting soil wettability. Regardless of coal tar contamination contrasts (0 vs. 5 and 1 vs. 10 g kg soil-1) or strong permeability contrasts, PS-solution injection after foam injection led to the most uniform reagents delivery. While PS-concentration varied more than 5-times between zones using W-PS and S-PS methods, it varied less than 1.6-times when the F-PS one was used. Finally, despite unfavorable conditions, the foam-based method did not show any detrimental effect regarding the oxidation of hydrocarbons compared to the W-PS and S-PS methods carried out in ideal conditions. Moreover, hydrocarbon degradation rates were slightly higher when using F-PS than S-PS due to a lower surfactant content in the targeted zone.

Enhanced remedial reagents delivery in unsaturated anisotropic soils using surfactant foam.

Homogeneous delivery of solution of oxidant in unsaturated soils is limited by soil anisotropy and gravity. An innovative injection strategy using foam was developed to improve in situ delivery. Primary foam injection before oxidant solution enhanced both the lateral and uniform delivery of reactant in isotropic and anisotropic (permeability, contamination) soils. The oxidant spread isotropically through the foam water network. This sequential injection heavily improved the delivery radius of influence (ROI), while limiting contact between surfactant and solution of oxidant in order to preserve the selective oxidation of petroleum hydrocarbons contaminant (TPH). Prior foam injection allowed uniform delivery of the solution of oxidant across the region occupied by the foam, regardless of the soil permeability contrast (1:18), whereas poor ROI were observed for the direct injection of oxidant. Experiments in contamination contrasted soils showed that foam was able to propagate in highly TPH contaminated soils (max 60% velocity reduction for 22 g.kgdry soil-1). As for permeability contrast, foam is expected to enhance reagents delivery in such contexts. This novel strategy was proven to be efficient, even for complex anisotropic conditions, and should allow to cut field costs and uncertainties associated to poor reagents delivery.