Removal of crude oil from kaolinite by water flushing at varying salinity and pH

The ability of crude oil to adhere to kaolinite in an aqueous environment by adsorbing or depositing its polar components (asphaltenes and resins), and the ability of subsequent water flushing to remove this bound oil, were compared for two oils and varying salinity and pH of the flushing solution. A scanning electron microscopy technique was used to image the locations of oil residues on and between kaolinite platelets, while fluorescence spectroscopy yielded the corresponding amount of asphaltenics. Consideration of the mechanisms of electrostatic interfacial attraction and surface precipitation giving rise to the adsorption/deposition of asphaltenics on kaolinite were used to interpret the extent of their removal by flushing. The oil exhibiting greater surface precipitation was fairly unresponsive to flushing, with displacement of the bulk oil leaving residues substantially lining the kaolinite platelets and filling the pores between them. The oil which adsorbed by electrostatic attraction was more amenable to removal of residues, to partially reinstate the water-wetness of the pristine kaolinite, by flushing with salt solutions which weakened this attraction. In particular, higher salinity of the flush minimized the attractions between oppositely charged sites on the heterogeneous interfaces, and pH shift to higher or lower levels rendered the interfacial charges more similar and mutually repulsive.