Hydrophilic Siloxane-capped Magnetic Nanoparticles as Separable and Reusable Draw Solute in Forward Osmosis Water Treatment

Abstract

Forward osmosis (FO) is an emerging technology for water treatment. Finding of feasible draw solute which is the origin of the driving force for FO is one of the most important challenges of the whole research of FO. High osmotic pressure generation and easy recycling are most required properties of draw solute. Hydrophilic magnetic nanoparticle (HMNP) has full of promise as feasible draw solute. However, gradual decline of osmotic performance after recycling process, caused by particle core aggregation, is one of the major problems of HMNP draw solute. In this work, we have synthesized HMNP capped with hydrophilic siloxane (HS-MNP) for draw solute. Be compared to the carboxylate ligands of conventional HMNP draw solutes, siloxane linkages formed by condensation of hydrophilic trimethoxysilane are considered as a good protector of particle core against aggregation on high strength magnetic fields. Ligands of MNP/oleic acid synthesized by thermal decomposition method were exchanged to hydrophilic siloxane. Two trimethoxysilanes, 2-[methoxy-(polyethyleneoxy)propyl]trimethoxysilane (Silane-PEG) and n-(trimethoxysilylpropyl) ethylenediamine triacetic acid (Silane-COOH), were used as hydrophilic ligands for HS-MNP draw solutes. Physical properties of HS-MNP draw solutes were thoroughly characterized and osmotic pressure generation of HS-MNPs were analyzed by freezing point depression osmometer. HS-MNPs were applied to a batch scale FO to determine the water flux generation. After that, variation of water flux generation and mean particle size of HS-MNPs were measured after 5-times-repeated magnetic recycle. Osmotic pressure and water flux generation of PEG-MNP were 7.6 atm and 2.13 LMH at 0.038 mol/L of ligand concentration. Similarly, osmotic pressure and water flux generation of COOH-MNP were 6.3 atm and 1.81 LMH at same concentration. These results confirm that synthesized HS-MNPs were feasible to apply to the FO water treatment of mild brackish water or wastewater. In addition, water flux generation and particle size PEG-MNP draw solutes were well-maintained by 5-tmies of magnetic recycle. Finally, 0.038 mol/L of PEG-MNP + DI water draw solution was applied to the FO process with a feed solution with 500 ppm of methylene blue. Water flux generation with the feed solution was up to 1.58 LMH. Results of this work shows that HS-MNPs are feasible draw solutes in FO water treatment. Especially, properties of PEG-MNP as draw solute were well-maintained on magnetic recycle, and acceptable to FO water treatment of mild brackish water.