High positively charged FeO nanocomposites for efficient and recyclable demulsification of hexadecane-water micro-emulsion.

Oily wastewater not only causes major environmental issues, but also threatens human health. Magnetic nanoparticles (MNPs) are an attractively alternative commercial demulsifiers for their recyclability and high surface area. The wettability and surface charge of magnetic materials are significant factors in oily wastewater treatment. However, the specific influence of surface charge on the demulsification performance has not been rigorously investigated. Herein, a series of MNPs coated by dimethyl-diallyl-ammonium chloride (PDDA) and fulvic acid (FA) (FeO/FA/PDDA) with different surface positive charges were synthesized by adjusting the PDDA concentrations and applied in demulsification of hexadecane-water micro-emulsion. The oil-water separation efficiency (Es) was enhanced gradually with increasing the surface positive charge of demulsifiers. Derjaguin-Landau-Verwey-Overbeek (DLVO) theory confirmed that with increasing surface positive potential, the electrostatic attraction between demulsifiers and oil droplets increased, and thus, Es increased. In addition, the superior Es of FeO/FA MNPs for hexadecyl trimethyl ammonium bromide (CTAB)-stabilized micro-emulsions and FeO/FA/PDDA MNPs for sodium dodecyl sulfate (SDS)-stabilized micro-emulsions further confirmed that electrostatic force was critical in demulsification. The high positively charged FeO/FA/PDDA MNPs can be used as an efficient and recyclable demulsifier for hexadecane-water micro-emulsion. This study provides a theoretical basis for designing demulsifiers.