Magnetic demulsification of diluted crude oil-in-water nanoemulsions using oleic acid-coated magnetite nanoparticles

Most produced crude oil and oily wastewater from oilfields typically exist in the form of emulsions; this makes their demulsification a very challenging process. Chemical demulsifiers are commonly employed to enhance demulsification efficiency (ED). However, there is still an urgent need to develop more efficient techniques and demulsifiers to meet eco-friendly and economically competitive requirements. In this study, magnetic demulsification of single-layer oleic acid-coated magnetite (Fe3O4@OA) nanoparticles as a demulsifier for cyclohexane-diluted crude oil-in-water nanoemulsions (denoted as CO-NEs) was investigated under an external magnetic field. The effects of the dosage and wettability of the demulsifier, the pH of the CO-NEs, and the demulsification operation process on the ED were examined. The ED increased with increasing Fe3O4@OA dosage and a high ED of ∼97% was reached, which demonstrates the potential of Fe3O4@OA nanoparticles as a magnetic demulsifier for crude oil-containing emulsions from oilfields. The ED is related to the wettability (or water contact angle) of the magnetic nanoparticles, and a maximum ED was observed at a water contact angle of ∼90°. The ED was almost unchanged in the pH range of 4.0–7.5, while it gradually decreased as the pH rose from 8.0 to 11.0. Multistep demulsification exhibited a higher ED than the single-step operation when the same amounts of demulsifier were used. The Fe3O4@OA nanoparticles exhibited good recyclability; no significant change in the ED of the recycled Fe3O4@OA after demulsification was observed over five cycles. This work improves the understanding of demulsification behaviors of magnetic demulsifiers.