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To explore a simple and energy-efficient approach to regenerate draw agent is a key challenge for forward osmosis (FO) desalination. Electro-responsive hydrogels have attracted great attention due to their reversible swelling-shrinking behaviors under an electric stimulus. However, the developed electro-sensitive hydrogels based on 2-acrylamido-2-methyl-l-propanesulfonic acid (AMPS) so far exhibit low reusability due to their unstable electro-sensitive properties and poor degradability with potential environmental impact. This study explored the use of an electro-responsive superabsorbent hydrogel as a draw agent by grafting acrylic acid (AA) and acrylamide (AM) onto sodium carboxymethyl cellulose (CMC). The grafted hydrogel's morphology and structure were investigated by FTIR and SEM. The swelling behaviors and deswelling properties in an electrode contact system were examined. The swelling kinetics of the hydrogel could be well described by the Schott's pseudo-second-order model with a theoretical swelling ratio of 452.5 g/g. The grafted hydrogel exhibited stable electro-sensitive characteristics with a decrease of 7.3 % in the equilibrium swelling ratio after undergoing three cycles of deswelling and swelling. During the FO test, the dry hydrogel particles produced a water flux of 1.55 L·m−2·h−1 within the first hour, surpassing the performance of other carboxylic acid-based hydrogels used as draw agents. Significantly, it exhibited encouraging water recovery capabilities in the five recycling FO processes, retaining around 84 % of its initial water recovery ratio after 40 min of exposure to a voltage of 15 V. This indicated that the grafted hydrogel could be considered as an effective and sustainable draw agent for FO desalination. [Display omitted] [ABSTRACT FROM AUTHOR]

We explore the feasibility of utilizing electro-responsive hydrogels as novel forward osmosis (FO) draw agents for desalination. The chemically cross-linked hydrogels were synthesized via free radical copolymerization of common acrylamide (AM) with strong anionic comonomer 2-acrylamido-2-methyl- 1 -propanesulfonic acid (AMPS). The morphology, chemical structure, water-adsorbing capacity, and water state of the hydrogels were characterized by scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), a swelling test, and differential scanning calorimetry (DSC). The water-swollen hydrogels exhibited electroresponsiveness as shrinking and expanding reversibly with the on-off switching of the electric field in an electrode contact system. The magnitude of hydrogels' deswelling increased with the increase in the degree of swelling and applied voltages. This dehydration phenomenon was induced by the decrease in hydration power of microcounter ions due to the interaction with the electrodes by water electrolysis. The hydrogels were proved to be capable of generating a reasonable water flux of 2.76 L m−2 h−1 (LMH) from brackish water (2000 ppm NaCl solution) due to their high swelling pressure. In addition, the water flux was affected by the amount of hydrogel particles on the membrane surface. Importantly, the prepared hydrogels could effectively release around 71% of the adsorbed water at an applied voltage of 15 V for 40 min, and were able to maintain their water flux and water recovery performances up to three times regeneration. Results indicate that the electric field is an attractive alternative stimulus for extracting desirable water from the hydrogel draw agent in the FO desalination process. Image 1 • Electro-responsive hydrogels as draw agents for forward osmosis desalination. • Hydrogels exhibited electroresponsiveness as shrinking and expanding reversibly. • High water flux was achieved due to incorporation of strong anionic groups. • The hydrogel dewatering mechanism was associated with water electrolysis. [ABSTRACT FROM AUTHOR]

While forward osmosis (FO) has great potential to be the next generation water treatment technology, it is constrained by the development of ideal draw agents and membranes. Herein, a series of copolymerized hydrogels based on strong ionic monomer sodium 2-acrylamido-2-methylpropane sulfonate (AMPS-Na) and thermosensitive monomer -isopropylacrylamide (NIPAM) have been synthesized and used for the first time as draw agents in the FO process. The effects of the composition of poly(NIPAM- co-AMPS-Na) [P(NIPAM- co-AMPS)] hydrogels and operational conditions including draw agent concentrations, temperature, membrane orientation, salinity of feed solution, velocity of feed solution, and running times on the FO performance have been evaluated systematically. The results demonstrate that the water flux increases with increases of the content of AMPS-Na incorporated into the hydrogel and hydrogel concentration in the draw solution, e.g., from 0.40 LMH rising up to 2.85 LMH. Besides, the water flux increases by 18% as the temperature increases from 5 to 27°C. In addition, these hydrogels remained active even after five times regeneration and reuse with a total water flux decrease of less than 5%. Furthermore, these hydrogels have negligible reverse diffusion through the FO membrane as compared with the corresponding monomer mixture and NaCl, which may avoid possible contamination on the feed solution and lower the renewable cost. All these results indicate that these synthesized P(NIPAM- co-AMPS) hydrogels are promising draw agents for the FO process. [ABSTRACT FROM AUTHOR]