Pressure Makes Diamonds: Enhanced Permeation Resistance to Petroleum Hydrocarbons of a Novel In-Situ Mixed Wall Material

Abstract

Petroleum hydrocarbons pose severe environmental hazards, forming source zones in soil and groundwater with varying petroleum hydrocarbons-to-soil ratios (1.0-8.1%), necessitating innovative containment. To mitigate the flow of petroleum hydrocarbons-contaminated groundwater, an in-situ mixed amended sand-bentonite (SB) wall material, composed of contaminated sand with different diesel-to-soil ratios and styrene ethylene butylene styrene (SEBS)-amended bentonite, was proposed. Free swell test results revealed that the free swell index change (FSIC) of amended bentonite in diesel gradually increased until it stabilized after 14 days, with an accelerated increase on the 4th day. Hydraulic conductivity test findings indicated that the hydraulic conductivity of amended wall specimens permeated with diesel (k_diesel) exhibited a three-stage variation trend: gradual increase, gradual decrease, and stabilization. The peak k_diesel at the transition decreased with increasing diesel-to-sand ratio. The final k_diesel remained as low as 2.5×10^-11-5.4×10^-11 m/s. Microscopic analyses demonstrated that the pores of amended specimens after permeating with diesel were effectively plugged by SEBS gel. Mercury intrusion porosimetry analyses indicated reduced macropore volumetric percentages (41-53% versus 63-73% in unamended specimens). The SEBS’s kinetic swelling behavior is essential to the k_diesel of amended specimens, and this unique swelling could mask the diesel-induced compression on the diffuse double layer of bentonite particles.