Preparation of Carboxyl-Functionalized Core–Shell Magnetic Nanoparticles and Their Forward Osmosis Desalination Performance

Forward osmosis (FO) technology has garnered considerable attention in seawater desalination owing to its low energy consumption and pressure-free operation. However, the lack of efficient and recoverable draw solutions has limited its practical application. In this study, Fe₃O₄ magnetic nanoparticles prepared via a hydrothermal method were used as cores, coated with a SiO₂ shell through the Stöber method, and then surface-functionalized with D-xylose and sodium citrate to yield two magnetic nanoparticle draw solutions of Fe₃O₄@SiO₂/D-Xylose and Fe₃O₄@SiO₂-COOH, respectively. The structure and magnetic properties of the materials were characterized by TEM, XRD, FT-IR, and VSM, and their desalination performance was evaluated using a self-built FO apparatus. The results demonstrated that the SiO₂ shell (thickness ~10 nm) effectively protected the Fe₃O₄ core. In a pH 4.5 acidic solution, the iron ion leakage was merely 0.07 mg/L, far below that of the uncoated sample (0.60 mg/L). After carboxyl functionalization, Fe₃O₄@SiO₂-COOH exhibited a saturation magnetization of 37 emu/g, enabling magnetic separation. When the draw solution concentration was 5 g/L and ultrapure water served as the feed solution, the water flux of Fe₃O₄@SiO₂-COOH reached 4.58 LMH, which was higher than those of Fe₃O₄@SiO₂ (2.98 LMH) and Fe₃O₄@SiO₂/D-Xylose (3.02 LMH). As the concentration increased to 50 g/L, the water flux rose to 20.32 LMH. Using 50 g/L Fe₃O₄@SiO₂-COOH as the draw solution and 35 g/L NaCl simulated seawater as the feed solution, an average water flux of 0.75 LMH was obtained in FO mode, and the diluted draw solution could be rapidly recovered by a permanent magnet. This study offers new insights into the application of magnetic nanomaterials in forward osmosis seawater desalination.