Hydraulic Conductivities of Cationic Surfactant Modified Soil Mixtures

Surfactant-enhanced sorbent zones have potential utility for contaminant plume management in the subsurface. Changes in the hydraulic properties associated with the formation of such zones were studied by measuring hydraulic conductivities of sandyloam soil mixtures with different fine contents (6, 12, 18, and 24%) that were either untreated, or treated in batch systems with hexadecyltrimethylammonium (HDTMA). Experimental conditions included treatment to 0.1, 0.7, 1.0, and 2.3 times the cation exchange capacities (CEC), and effective stresses from 0 to 766 kPa. Achieved treatment levels and the kinetics of HDTMA adsorption were evaluated using 14Clabeled HDTMA. Soil suspension turbidity was used to infer the degree of clay dispersion or flocculation; clays were dispersed in 0, 0.1, and 2.3 CEC treated soils and flocculated in 0.7 and 1.0 CEC treated soils. Consistent with their flocculated clay structures, the 0.7 and 1.0 CEC treated soils showed equal or higher conductivities compared to 0, 0.1, and 2.3 CEC treated soils. The 0.7, 1.0, and 2.3 CEC treated soils also showed higher compressibility. HDTMA adsorption occurred rapidly at levels up to 1.0 CEC but was kinetically limited at the high treatment level (2.3 CEC). Creation of sorbent zones by ex situ batch treatment with cationic surfactants was deemed hydraulically feasible as evidenced by equal or higher conductivity of HDTMA-treated soils to 0.7 and 1.0 CEC treatment levels, as compared to untreated soil, under all loads.