Research On Hydrogel And Sulfated Cyclodextrin Sodium Salt As Draw Solutes In Forward Osmosis

In recent years, forward osmosis (FO) has been viewed as a sustainable and efficient non-traditional membrane technology for water treatment, leading to significant public concerns. However, the practical FO applications are highly dependent on the selection of an appropriate draw solute. Hydrogel, a smart macromolecular, has been reported to be capable of absorbing and releasing water reversibly in response to external stimuli, which may be a class of promising FO draw solutes after functionalized design. Besides, since P-cyclodextrin sodium salt (NaCDS), a sulfated cyclodextrin derivative, has rich hydrophilic sulfonic groups (-SO3Na) and a ring-like structure, it is expected to provide high osmotic pressure and allow minimal reverse solute leakage. These characteristics ensure the suitability of NaCDS used as draw solute. Therefore, the project proposes the use of highly water-absorbing and environment-sensitive hydrogels as well as a ring-like macromolecule NaCDS as draw solutes in FO, and investigates the inner mechanism between the physical and chemical properties of draw solutes and their water-absorbing behaviors in FO. This work will provide a direction for the design and selection of a draw solute in FO, and extend the development and application of intelligent macromolecular materials in the environmental field.Herein, a series of copolymerized hydrogels with different monomer ratios based on strong ionic monomer sodium2-acrylamido-2-methylpropane sulfonate (AMPS) and thermosensitive monomer N-isopropylacrylamide (NIPAM) were synthesized via the free-radical polymerization. The characterization results showed that the swelling ratios and rates of hydrogels increased with the increasing of the content of AMPS incorporated into the hydrogels, showing certain thermosensitivity. The effects of operational conditions including the composition and concentrations of draw solute, temperature, velocity of feed solution, membrane orientation mode, salinity of feed solution, and running times on the FO performance had been evaluated systematically. The results showed that the water flux increased with the increasing of the AMPS content incorporated into the hydrogel and draw solute concentration, e.g., from0.40LMH rising up to2.85LMH. The water flux increased by18%as the temperature increased from5to27℃. The hydrogels could produce a negligible difference of water flux between different membrane orientation modes, causing ICP minimization. In addition, these hydrogels remained active even after five times regeneration and reuse with a total water flux decrease of less than5%. Furthermore, the hydrogels just had negligible reverse solute diffusion through the FO membrane when compared with the corresponding monomer mixture and NaCl under the same conditions.To improve the dewatering efficiency of hydrogel draw solute, magnetic hydrogels poly(N-isopropylacrylamide-co-sodium2-acrylamido-2-methylpropane sulfonate)(P(NIPAM-co-AMPS)/MNPs) were prepared by introducing Fe3O4magnetic nanoparticles (MNPs) into hydrogels. The characterization results showed that these hydrogels exhibited superparamagnetic property, and the saturation magnetization values increased linearly with the increasing content of MNPs in the hydrogels. Results from FO experiments demonstrated that the incorporation of MNPs into the hydrogels had no clear influence on water flux with a range from2.22to2.52LMH. The change trend of water flux with time for magnetic hydrogels decreased sharply in the initial stage and subsequently smoothly, which might be ascribed to the water-absorbing mechanism of hydrogels. Under different membrane orientation modes, the hydrogels showed a negligible distinction of water flux, causing minimal ICP. In addition, these hydrogels had good reusability, and the total water flux decrease was less than5%even after three times reuse. Under the thermal stimuli (65℃), the inclusion of MNPs could improve the dewatering efficiency of magnetic hydrogels. The dewatering rate of the hydrogel containing9.0wt%MNPs was3.2times higher than that of the hydrogel without MNPs within the first20minutes.To further enhance the performance of hydrogel-driven FO processes, magnetic thermoresponsive poly(N-isopropylacrylamide-co-sodium2-acrylamido-2-methylpropane sulfonate)(Fe3O4@P(NIPAM-co-AMPS)) nanogels were prepared by copolymerization of NIPAM and AMPS by emulsion polymerization at the present of Fe3O4nanoparticles. The characterization results showed that these magnetic nanogels exhibited a core-shell structure, thermosensitivity and superparamagnetic property with the saturation magnetization value of25.296emu/g. The osmotic pressures of magnetic nanogels dispersions were measured by the freezing-point depression method, indicating that the osmotic pressure increased with an increasing concentration of the nanogels. Results from FO experiments showed that strong ionic Fe3O4@P(NIPAM-co-AMPS) nanogels could produce a higher water flux than weak ionic magnetic poly(N-isopropylacrylamide- co-acrylic acid)(Fe3O4@P(NIPAM-co-AA)) nanogels, and the water flux yielded by the former was2.4times higher than the later. Furthermore, the water flux increased with the increasing concentration of magnetic nanogels. Especially, due to the existence of thermosensitive poly(N-isopropylacrylamide)(PNIPAM), after FO the diluted draw solution of magnetic nanogels could be dewatered quickly under an external magnetic field combined with a thermal stimuli, improving the recovery efficiency of draw solute.Finally, considering the poor water flux produced by hydrogels due to the limited osmotic pressure, a draw solute, sulfated β-cyclodextrin sodium salt (NaCDS), was proposed for use in FO. The osmotic pressures of NaCDS with different concentrations were determined by the freezing-point depression method. The effects of different operating conditions on the FO performance were evaluated systematically. Results showed that the osmotic pressure of NaCDS solution increased with the increasing concentration. The NaCDS could yield a relatively high water flux and a minimal reverse solute leakage. When the active layer of FO membrane was faced against draw solution, the NaCDS could produce a higher water flux and the water flux decreased with the increase of feed salinity. The water flux increased by23%when the velocities increased from0.184to0.552m/s. In addition, the temperature difference by elevating only the temperature of draw solution while keeping the temperature of feed solution constant could also increase the water flux significantly. The water flux gain was25%when the temperature difference between draw solution and feed solution was increased by20℃.