Quaternary ammonium siloxane-decorated magnetic nanoparticles for emulsified oil-water separation

Abstract

Rapid demulsification of “oil-in-water” (O/W) and “water-in-oil” (W/O) emulsions is extremely important for oil utilization and environmental protection. Although some siloxane-modified magnetic nanoparticles (MNPs) are applicable to separate oil–water mixture, there is no report on the demulsification performance of quaternary ammonium siloxane-coated MNPs in O/W or W/O emulsion. Herein, a series of dimethyloctadecyl [3-(trimethoxysilyl) propyl] ammonium chloride (DOTAC)-modified magnetic nanoparticles (MNPs) has been successfully synthesized and results showed that the optimized MNPs exhibited higher demulsification efficiency than our previously reported siloxane-modified MNPs. The synthetic MNPs were characterized in detail and their wettability and surface charge were facilely regulated by just adjusting DOTAC anchoring density on Fe3O4@SiO2 surface. Demulsification performance in O/W emulsion was enhanced at first and then declined with increasing DOTAC anchoring density at various pH levels. When the MNPs were weakly positively charged, both electrostatic attraction and hydrophobic interactions could promote the nanoparticle adsorption on oil droplet surface, thereby accelerating subsequent magnetic flocculation and separation. Specifically, M3-DOTAC 0.06, M3-DOTAC 0.12 and M3-DOTAC 0.6 exhibited best demulsification performance at pH 4.0, 7.0 and 10.0, respectively. The oil removal efficiency exceeded 94 % when 200 mg/L of MNPs introduced. Moreover, its demulsification performance was degraded with increasing surfactant concentration, and declined at first and then enhanced with increasing salinity. In W/O emulsion, demulsification effect was gradually increased with increasing DOTAC anchoring density. For an extremely stable W/O emulsion, dehydration efficiency of the optimized M3-DOTAC 1.8 exceeded 93% at 20 g/L of MNPs. Furthermore, the synthetic MNPs with suitable DOTAC anchoring density exhibited good recyclability in either O/W or W/O emulsion. Considering its simple synthetic procedure and high demulsification efficiency, DOTAC-coated MNPs could potentially find practical applications in demulsifying O/W or W/O emulsion.