Sorption of crude oil from a non-aqueous phase onto silica: the influence of aqueous pH and wetting sequence

Sorption of crude oil by mineral surfaces may affect its flow and recovery in porous media. Here, crude oil sorption from a toluene–heptane phase onto quartz and silica gel has been investigated both in the presence and absence of an aqueous phase, as a function of time, oil-to-sorbent ratio, pH, and wetting sequence. Steady-state is attained in roughly 24 h, with sorption following Freundlich isotherms. Maximum sorption capacity of initially dry quartz is roughly 2 mg oil per g; that of initially dry silica gel is greater than 8 mg per g. Sorption capacities of pre-wetted quartz and silica gel are approximately 0.5 and 0 mg oil per g, respectively. Aqueous pH only affects sorption by pre-wetted quartz, where sorption is greatest at pH 4, roughly 25% less at pH 7, and roughly 50% less at pH 2. The effect of pH and wetting sequence on oil sorption can be qualitatively described by calculating the electrostatic interaction energy between the mineral–water and oil–water interfaces.