Magnetophoretically driven demulsification of oil-in-water emulsions

Employing magnetophoresis, we investigate the demulsification of oil-in-water emulsions using SDBS-functionalized magnetic nanoparticles (MNP@SDBS) prepared by two routes: coprecipitation from commercial iron salts (SMNPs) and an alternative sand-derived route using Fe-rich natural sand as the iron source (NMNPs). Demulsification performance was evaluated for pristine MNP@SDBS and for thermally recovered nanoparticles after one demulsification cycle (MNP@SDBS-R). Upon applying a magnetic-field gradient to the MNP@SDBS–oil-in-water mixture ( \(27^∘ \) API, 5% v/v), SMNP@SDBS and NMNP@SDBS achieve 98% hydrocarbon removal at 2500 ppm, both within 1 min. These results demonstrate that magnetic demulsifiers prepared from Fe-rich sands have separation efficiencies comparable to those synthesized from commercial iron salts, supporting a magnetophoretically assisted process involving droplet coalescence and nanoparticle precipitation. The recovered MNP@SDBS-R materials retained high demulsification efficiency with only modest performance losses. In contrast, control tests performed in the absence of both nanoparticles and an external magnetic field exhibit neither precipitation nor demulsification even after 10 days. Comprehensive structural, magnetic, microstructural, optical, and elemental characterizations were performed at all stages (as-prepared MNPs, functionalized MNP@SDBS, and recovered MNP@SDBS-R) to guide optimization of MNP use and recovery. In all cases, the analyses confirm inverse-spinel iron oxides. Sand-derived materials were predominantly magnetite ( \({Fe}₃{O}_4\) ) and salt-derived materials were predominantly maghemite ( \(γ -{Fe}_2{O}_3\) ). All samples exhibited room-temperature superparamagnetic-like behavior. These results support the above-proposed mechanism in which interfacial adsorption of MNP@SDBS and field-gradient-driven magnetophoretic collection promote droplet bridging and coalescence, while also enabling rapid magnetic recovery and reuse of the demulsifier.