An initial study on soil wettability effects during entrapped LNAPL removal by surfactant flooding in coarse-grained sand media

Purpose: Non-ideal or fractional wettability conditions may exist at field sites. It was hypothesized that fractional wettability could cause larger entrapped LNAPL saturations and unexpected soil interactions during surfactant remediation. Soil wettability effects during entrapped LNAPL recovery by surfactant flooding in coarse-grained sand were investigated through a comparative study. The main objective was to identify the impacts of soil wettability on LNAPL removal via submicellar concentration surfactant flooding to remobilize entrapped LNAPL. Materials and methods: A baseline for comparison was established by testing an ideal water-wet soil (silica sand) along with a fractionally wet soil. Two LNAPLs were tested: LNAPL from a contaminated field site and heptane, which represented an ideal lab-grade fluid. Contact angle, interfacial tension, capillary pressure-saturation, and column tests were performed to characterize wettability and identify the effects of soil wettability during entrapped LNAPL recovery. Two anionic surfactants were used for the column experiments: sodium dodecyl-benzene-sulfonate and a field site anionic surfactant. To further investigate the effects of LNAPL contact time within the soil, columns tests were performed at two different LNAPL contact times. Results and discussion: Contact angle measurements and column tests conducted with the field LNAPL revealed its potential to establish non water-wet wettability conditions. Column test results indicated that fluid entrapment was independent of fluid type, and the wettability and contact time conditions studied. Entrapped LNAPL saturations after water flooding were approximately 19 %. Entrapped LNAPL removal by mobilization occurred at the predetermined submicellar surfactant concentration (0.5 g/L) with a maximum removal of 43 %. Entrapped LNAPL removal from fractionally wet columns was higher in comparison to water-wet columns and was found to increase with contact time. Conclusions: Entrapped LNAPL saturations after water imbibition were not impacted by the wettability conditions studied. The fractionally wet soil behaved differently during the surfactant-flood; higher LNAPL removal was achieved suggesting that non-ideal wettability had a positive impact. The presence of NAPL-wet mineral grains might have favored pore scale interactions causing NAPL redistribution and increasing NAPL-surfactant solution interfacial areas contributing to LNAPL removal. Pore scale studies and subsequent testing is recommended to further this study's findings