Multifunctional hyperbranched polyglycerol-grafted silica-encapsulated superparamagnetic iron oxide nanoparticles as novel and reusable draw agents in forward osmosis process

Forward osmosis (FO) has gained attention recently due to its low cost and energy consumption while it happens naturally. However, finding a proper draw agent for this process is a challenging task. Magnetic nanoparticles, especially with modified surfaces, have been reported as a promising draw agent, which can be easily recovered by using a magnetic field. Here, an attempt was made to study the effect of different superparamagnetic iron oxide nanoparticles (SPIONs) surface engineering on the osmotic pressure, hydrophilicity, degree of agglomeration of nanoparticles and also water flux. In this study, the osmotic potential of naked Fe3O4, silica-coated superparamagnetic iron oxide nanoparticles (SPION@SiO2), hyperbranched polyglycerol/carboxylate-functionalized SPION (SPIOWN@SiO2@HPG and SPION@SiO2@HPG-CO2H) were evaluated for FO process. The effect of ionized SPIONs on the water flux has been studied for the first time, too. The SPIONs were characterized by scanning electron microscopy, transmission electron microscopy, X-ray powder diffraction, Fourier transform infrared, vibrating sample magnetometer and thermogravimetric analysis. The average of water flux in a long-term performance of FO for the introduced draw agents increased in the order of SPION < SPION@SiO2 < SPION@SiO2@HPG < SPION@SiO2@HPG-CO2H due to higher hydrophilicity and lesser agglomerations and precipitation. It was surprisingly observed that water flux of ionic magnetic draw solution behaves differently comparing with the non-ionic ones. It was revealed that ionic SPIONs at high concentration led to the formation of non-ideal polyelectrolyte solution that produces a high osmotic pressure. However, by the permeation of water, a transition from non-ideal to ideal solution at some point was observed during FO process.