Electro-washing of pipelines spills: On-site strategies for different soil matrices

Crude oil remains a dominant global energy source but spills from pipelines, and reservoirs pose significant environmental and health risks. Remediating petroleum hydrocarbon (PHC)-contaminated soils is critical yet challenging, especially in fine-grained matrices where conventional methods underperform. In this context, electro-washing (EW), either in-situ or ex-situ with surfactant enhancement, offers a tunable, energy-efficient solution that is adaptable to varying soil textures and voltage gradients. This study demonstrates a low-voltage EW methodology for PHC-polluted soils containing 0-5 % bentonite clay, utilizing a zwitterionic surfactant. Laboratory-scale EW cells treated 1 kg soil samples under 1-3 V/cm for four days. Results show that increasing both the clay fraction and voltage gradient significantly improved PHC removal, achieving up to 88 % reduction in soils with 5 % clay at 3 V/cm. Electroosmotic flow transported pore fluid toward the cathode, while negatively charged surfactant micelles migrated toward the anode via electrophoresis, facilitating the delivery of heavier oil fractions. Additionally, in-situ electro-demulsification enabled oil recovery and clean water separation. A polynomial empirical model correlating clay content and voltage to removal efficiency was developed, yielding an excellent fit (R² ≈ 0.93, p < 0.01) and indicating that both factors significantly influence PHC reduction. This work highlights a novel low-energy electrokinetic-surfactant system capable of overcoming the limitations of conventional washing, especially in clayey soils. The method demonstrates near-regulatory cleanup within days, with the added benefit of resource recovery. A preliminary scale-up analysis confirms the feasibility of EW as a sustainable and field-adaptable remediation strategy.