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 Fe(3)O(4) nanoparticles (referred to as Fe(3)O(4)@ODPA) were prepared by condensation of hydroxyl groups between ODPA and Fe(3)O(4) 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 (θ(W)) of Fe(3)O(4)@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 (A(O)) (coating weight per gram Fe(3)O(4)) and specific surface area (S(O)) of Fe(3)O(4)@ODPA were 0.124–0.144 g·g(−1) and 78.65–91.01 m(2)·g(−1), 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 (C(S)), pH value of nanoemulsion, and NaCl or CaCl(2) electrolytes on the demulsification efficiency (R(O)) were investigated. The R(O) of Fe(3)O(4)@ODPA samples was found to be higher than that of bare Fe(3)O(4) samples (S0, ST, and SN) under all C(S) values. With the increase in C(S), the R(O) of Fe(3)O(4)@ODPA samples initially increased and then approached equilibrium value at Cs = 80.0 g·L(−1). A maximum R(O) of ~93% was achieved at C(S) = 100.0 g·L(−1) for the Fe(3)O(4)@ODPA sample S2. The pH and two electrolytes had a minor effect on R(O.) The Fe(3)O(4)@ODPA nanoparticles maintained high R(O) even after being reused for demulsification 11 times. This indicates that the hydrophobic Fe(3)O(4)@ODPA samples can be used as an effective magnetite demulsifer for oil-in-water nanoemulsions.