Adsorption of Asphaltenes at Oil–Water and Oil–Clay Interfaces in the Presence of Humic Acids

Humic substances in the soil and underground water are important media for the environmental fate and transport of oil pollutants, but direct experimental evidence is lacking on the effects of humic acids on the interfacial activity and adsorption properties of oil asphaltenes in the soil. In this study, the oil–water interfacial tension (IFT) was measured by optical contact angle instruments, while the isothermal adsorption of asphaltenes on two montmorillonites and one kaolinite was fitted using four classical models. Results demonstrated that the oil–water IFT decreased by 37.5% when the asphaltenes (500 mg L−1) were present in the oil, which further decreased by 62.7% when the humic acids (25 mg L−1) were added in the water. The best-fitted form of isotherm equation (Langmuir model) and the adsorption capacity were not changed by coating humic acids on the clay surface prior to asphaltene adsorption, but the presence of humic acids on the clay surface doubled the adsorption rate. Results also revealed that the asphaltenes could coaggregate with the humic acids at the oil–water interface or in the bulk water, but they were unlikely to coaggregate with the humic acids binding on the clay surface.