Abstract

The mobility of anionic and cationic surfactants in unsaturated soil is an important consideration with respect to their potential use in removing or immobilizing contaminants located above the water table. Likewise, for situations in which surfactants are unintentionally released into the soil medium, mobility characteristics also need to be addressed to gauge possible adverse environmental impacts. For these reasons, transient unsaturated horizontal leaching tests were conducted to compare anionic and cationic surfactant transport characteristics in a loamy soil. These experiments utilized a computer-controlled syringe pump to inject 0.025 mol/kg surfactant solutions into the inlet of a dry soil column. Five anionic and five cationic surfactants were tested. Relative to the conservative behavior of soil moisture movement, adsorption mechanisms reduced surfactant transport in all cases. The mobilities of the anionic surfactants varied substantially and on average were greater than that of the cationic surfactants by a factor of five, based on the distance from the column inlet to the centroid of the concentration profile. Anionic surfactants also exhibited a far more pronounced impact on moisture distribution within the soil column in comparison to cationic surfactants. The results of this study serve as a meaningful guide in assessing the behavior of anionic and cationic surfactants in the unsaturated soil environment.

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