Preparation of Hydrophobic Octadecylphosphonic Acid-Coated Magnetite Nanoparticles for the Demulsification of n-Hexane-in-Water Nanoemulsions.

To design more environmentally friendly, economical, and efficient demulsifiers for oily wastewater treatment, hydrophobic octadecylphosphonic acid (ODPA)-modified FeO nanoparticles (referred to as FeO@ODPA) were prepared by condensation of hydroxyl groups between ODPA and FeO nanoparticles using the co-precipitation method. The prepared magnetite nanoparticles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscope (SEM), Fourier transform infrared (FTIR) spectroscopy, and thermogravimetric/differential thermogravimetric (TG/DTG) analysis. The water contact angles () of FeO@ODPA nanoparticles were more than 120°, indicating hydrophobic nature, and the diameter of the obtained spherical-shaped magnetite nanoparticles was 12-15 nm. The ODPA coating amount () (coating weight per gram FeO) and specific surface area () of FeO@ODPA were 0.124-0.144 g·g and 78.65-91.01 m·g, respectively. To evaluate the demulsification ability, stability, and reusability, the magnetite nanoparticles were used to demulsify an n-hexane-in-water nanoemulsion. The effects of the magnetite nanoparticle dosage (), pH value of nanoemulsion, and NaCl or CaCl electrolytes on the demulsification efficiency () were investigated. The of FeO@ODPA samples was found to be higher than that of bare FeO samples (S0, ST, and SN) under all values. With the increase in , the of FeO@ODPA samples initially increased and then approached equilibrium value at Cs = 80.0 g·L. A maximum of ~93% was achieved at = 100.0 g·L for the FeO@ODPA sample S2. The pH and two electrolytes had a minor effect on The FeO@ODPA nanoparticles maintained high even after being reused for demulsification 11 times. This indicates that the hydrophobic FeO@ODPA samples can be used as an effective magnetite demulsifer for oil-in-water nanoemulsions.