Oil sorption by exfoliated graphite from dilute oil–water emulsion for practical applications in produced water treatments

• Oil in emulsion can be reduced from 100 to 0.1 ppm by exfoliated graphite (EG).
• Concentration of oil in EG was 200 times larger than that of the initial emulsion.
• Magnetic EG (MEG) was prepared from EG-iron chloride intercalation compound.
• The MEG was successively collected by a magnet after oil sorption in emulsion.

We used engine oil–water emulsion as a model produced water to evaluate the oil sorption properties of exfoliated graphite (EG) and showed that 100 mg/L of oil in emulsion can be reduced to a concentration of a 0.1-few mg/L using as-received commercially available EG and surface modified EG subjected to an additional oxidation treatment. Oil at a concentration of less than 15 mg/L, the standard of typical water treatment processes. Although EG also sorbs water, it sorbs oil preferentially and the oil concentration of the sorbed phase in EG was estimated to be about 200 times that of the initial untreated emulsion. The sorption mechanism was discussed with the aid of the ab-initio relaxation calculation and classical molecular dynamics (MD) simulation for n-Hexane (C6H14) adsorption on graphene. When EG was incorporated with micron-size iron particles, the oil sorbed EG was found to be effectively recovered from emulsion by using magnetic field. The oil concentration in emulsion treated with EG was able to reduce down to the level comparable to that achieved by nanofiltration (NF) or reverse osmosis (RO) membrane treatment. We concluded that EG from natural graphite can effectively remove oil component during the treatment of emulsion and exhibits high potential for practical use.