Interaction between Darcy flow and electroosmosis for surfactant transport and hydrocarbon removal in heterogeneous...

Hydrocarbon contaminants in soil exhibit long-term persistence in low-permeability zones, where their low solubility and limited mobility make remediation challenging. Electrokinetic techniques provide a promising solution by enhancing surfactant delivery in low-permeability media through electroosmosis and electromigration. However, the role of heterogeneity remains poorly understood, arising not only from permeability differences that govern Darcy flow but also from variations in zeta potential that control electroosmotic flow. This study investigates how two-dimensional heterogeneity influences water flow, surfactant transport, and hydrocarbon removal under electrokinetic conditions. Laboratory experiments were carried out using heterogeneous soil-sand systems, with nonionic surfactants applied to enhance hydrocarbon solubility and mobility. Numerical models were developed to simulate water flow, surfactant transport, and hydrocarbon removal. In two-dimensional heterogeneous systems, distinct transport and removal behaviors were observed between electrokinetic and hydraulic gradient methods. Localized circulation flow around the soil block was observed and can be generated either by an externally imposed hydraulic gradient between the anode and cathode or by the intrinsic heterogeneity of the system, particularly the differences in hydraulic conductivity and zeta potential between soil and sand. This circulation, produced by the competition between electroosmotic pumping in high-zeta potential soils and Darcy flow in sand near the sand-soil interface, reduces surfactant delivery and limits hydrocarbon removal. Finally, a combined electrokinetic-hydraulic gradient method is proposed and demonstrated to enhance removal efficiency, offering a promising approach for field applications in heterogeneous media.