Magnetic thermoresponsive ionic nanogels as novel draw agents in forward osmosis

Magnetic poly(N-isopropylacrylamide-co-sodium 2-acrylamido-2-methylpropane sulfonate) (denoted as Fe3O4@P(NIPAM-co-AMPS)) nanogels were prepared based on strong ionic monomer AMPS and thermosensitive monomer NIPAM via precipitation polymerization in the presence of Fe3O4 nanoparticles and investigated as draw solutes in forward osmosis (FO). The magnetic nanogels were characterized by Fourier-transform infrared spectroscopy, transmission electron microscopy, dynamic light scattering, X-ray diffraction, and vibrating sample magnetometry, respectively, indicating that they exhibited a core-shell structure, thermosensitivity and superparamagnetic properties. These properties would provide benefits for recovering these nanogels after FO. The water flux yielded by Fe3O4@P(NIPAM-co-AMPS) nanogels in FO was investigated compared with magnetic weak ionic nanogels based on poly(N-isopropylacrylamide-co-acrylic acid) under the same operating conditions. The results show that the water flux yielded by Fe3O4@P(NIPAM-co-AMPS) nanogels is 2.4 times higher than the later. Furthermore, the water flux increases with the increase of nanogel concentration in the draw solution. Especially, due to the existence of thermosensitive poly(N-isopropylacrylamide) segments in Fe3O4@P(NIPAM-co-AMPS) nanogels, these nanogels can be recovered from the diluted draw solution quickly under an external magnetic field combined with a thermal stimulus, resulting in an improvement of the recovery efficiency.