Hydrophobic and positively charged magnetic nanoparticles for enhanced oil recovery from concentrated emulsion wastewaters

With the development of petroleum industry, a large amount of oil-in-water (O/W) emulsion wastewaters were produced, resulting in serious environmental pollution and resource waste. Up till now, magnetic nanoparticles (MNPs) were frequently reported in emulsion wastewater treatment and mainly focused on the water purification, but little effort was devoted to the facile recovery of oil resource. In the present work, a class of hydrophobic and positively charged MNPs, namely dimethyloctadecyl [3-(trimethoxysilyl) propyl] ammonium chloride (DOTAC)-coated MNPs (M-DOTAC), was carefully synthesized by regulating DOTAC anchoring density on Fe3O4@SiO2, and then employed to treat the concentrated emulsion wastewaters stabilized by anionic sodium dodecyl benzene sulfonate (SDBS) or nonionic Tween-80. M-DOTAC with relatively high DOTAC density (M-DOTAC 1.8) exhibited higher positive charge density and hydrophobicity, and hence could significantly promote the oil droplet coalescence and mergence via the electrostatic attraction and hydrophobic adsorption bridging as well as the hydrophilic-lipophilic transition of interfacial adsorption layer. Accordingly, with addition of moderate dosage of M-DOTAC 1.8, the emulsion was broken and divided into three layers: (1) a continuous oil layer was formed in the upper layer which could be directly recycled; (2) the middle layer was composed of MNPs-tagged oil droplets/flocs; (3) the bottom layer was the clearer water phase. The optimal recovery rate of oil resource reached similar to 60 % in SDBS stabilized system, and further increased to similar to 98 % in Tween-80 stabilized system. However, the oil recovery rate was declined at an excessive dosage of M-DOTAC 1.8 due the formation of double emulsion. Besides, M-DOTAC 1.8 exhibited good reusability. These results indicated the fabricated M-DOTAC had great application prospect in treating emulsion wastewaters containing high oil content.