Demulsification of surfactant-rich emulsion systems by using amphiphilic or positively charged magnetic nanoparticles

Emulsified oily wastewater comes from many fields, such as oil exploitation, processing, storage and transport, mechanical processing and food and catering industries, causing serious environmental pollution. Considerable part of emulsified oily wastewater was extremely stable and difficult to be treated due to the existence of high-concentration surfactants. Magnetic demulsification of oil-in-water (O/W) emulsion by using magnetic nanoparticles (MNPs) attracted more and more attentions because of its flexible and efficient process; however, the study regarding the treatment of emulsified oily wastewater containing high-concentration surfactant was still few. In this study, amphiphilic dimethyloctadecyl [3-(trimethoxysilyl) propyl] ammonium chloride (DOTAC)-coated MNPs (M-DOTAC) and hydrophilic but highly positively charged quaternized chitosan (QC)-coated MNPs (M-QC) were employed to treat the O/W emulsions containing various contents of anionic and nonionic surfactants. Influence of surfactant type and concentration on the demulsification performance of the two MNPs was investigated in detail. Results showed that M-DOTAC could more effectively demulsify the nonionic surfactant-stabilized emulsion mainly via hydrophobic interaction, while M-QC could more efficiently demulsify the anionic surfactant-stabilized emulsion mainly via electrostatic interaction. With increasing surfactant concentration, the MNPs were still able to completely demulsify the emulsion at a relatively higher dosage. Moreover, when the two classes of MNPs were used together, highly stable emulsion system with containing both anionic and nonionic surfactants could be completely demulsified. These results suggested the synthesized two MNPs could provide simple but powerful approach to treat different emulsified oily wastewaters with containing high-concentration surfactants.