Forward osmosis with electro-responsive P(AMPS-co-AM) hydrogels as draw agents for desalination

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.