Thermal study of APTES-functionalized magnetite nanoparticles with citric acid and polyacrylic acid for advanced forward osmosis systems

Iron-based magnetic nanoparticles (MNPs) have attracted considerable attention as promising draw solutes in forward osmosis processes due to various advantages. In the present study, magnetite (Fe3O4) nanoparticles (MNPs) with diameters in the range of 12 +/- 2 nm were synthesized by applying the coprecipitation method and stabilized using (3-aminopropyl)triethoxysilane (APTES). The amino groups of APTES were further functionalized with various amounts of citric acid (CA) and poly(acrylic acid) (PAA) to achieve higher osmotic pressure. The as-prepared and functionalized nanoparticles were characterized using different methods: Fourier-transform infrared spectroscopy (FTIR) confirmed the binding of APTES and CA/PAA on the surface of the MNPs, while the size of the particles was determined by transmission electron microscopy (TEM) and X-ray powder diffraction. The isoelectric point, zeta potential, and the particle size in suspension were determined by dynamic light scattering (DLS)/zeta potential measuring system. The amount of free -COOH groups was determined by conductometric titrations. The amount of organic matter (APTES, CA, and PAA) bound on the surface was determined by thermogravimetric analysis (TG), giving results between 7.6% for samples stabilized with APTES only and 17.5% for samples functionalized with APTES + PAA. TG provides a simple and efficient method for determining the amount of organic compounds bound to the surface of MNPs. The osmotic pressure measurements of double-layer coated MNPs provided encouraging results of up to 28 bar, enabling using the samples as draw solutions in an experimental forward osmosis system.