Towards temperature driven forward osmosis desalination using Semi-IPN hydrogels as reversible draw agents

• Temperature cycle driven FO desalination using responsive semi-IPN hydrogels as draw agents.
• Desalination driven by a temperature modulation, e.g., between room temperature and 40 °C.
• FO desalination without the need of draw solute recovery.
• A conceptual design is advanced for a hydrogel incorporated into a quasi-continuous FO module.
• A parameter is defined as a metric to guide in the search for better and better hydrogel in future.

We report a study to explore new materials and a new concept for temperature driven quasi-continuous desalination using hydrogels as draw agents in forward osmosis (FO). This concept is enabled by the design and preparation of thermally responsive hydrogels having a semi-interpenetrating network (semi-IPN) structure. Thermally responsive semi-IPN hydrogels were synthesized by polymerization of N-isopropylacrylamide (NIPAm) in the presence of polysodium acrylate (PSA) or polyvinyl alcohol (PVA). Their functions as draw agents in FO were systematically studied and compared with hydrogels prepared from the PNIPAm homopolymer or the NIPAM-SA copolymer. While the semi-IPN hydrogels displayed the desirable balanced thermally responsive swelling and dewatering behavior, the NIPAm-SA copolymer hydrogels were found to have poor dewatering behavior, making them unsuitable for a continuous temperature driven desalination process. At 40 °C, the semi-IPN hydrogels rapidly release nearly 100% of the water absorbed during the FO drawing process carried out at room temperature. Results clearly indicate the potential of semi-IPN hydrogels as semi-solid draw agents in the FO process, in which quasi-continuous desalination could be achieved by cyclic heating and cooling within a moderate temperature change.