Humic Acid with Vertical Adsorption Conformation Enhanced the Transport of Petroleum Hydrocarbon-Contaminated Colloids

Humic acid (HA) enhances colloidal transport in porous media, yet the mechanisms by which the HA adsorption conformation affects colloid transport remain unclear. This study investigated the influence of HA on the transport of petroleum-hydrocarbon-contaminated soil colloids (TPHs-SC) in saturated sand columns. The presence of TPHs on the colloidal surface occupied adsorption sites, hindering HA from forming a horizontal adsorption conformation, as observed on uncontaminated soil colloids (SC). Instead, a vertical adsorption conformation was formed, reducing the overall adsorption of HA. Vertically adsorbed HA increased the colloidal diffuse double-layer potential and extended the Derjaguin-Landau-Verwey-Overbeek energies between colloids and water-bearing media. This was evidenced by higher ζ potentials (-28.5 to -34.0 mV) and enhanced TPHs-SC transport compared to SC (ζ potentials ranging from -25.2 to -29.5 mV) in the presence of HA, particularly under alkaline conditions. Additionally, weak van der Waals and electrostatic interactions between TPHs near colloidal surfaces and free HA/TPHs formed a zonal distribution, facilitating the cotransport of colloids with TPHs. These findings underscore the significance of the HA adsorption conformation in TPHs-SC transport and provide insights into the critical mechanisms from an environmental structural chemistry perspective.