Dewatering poly(acrylic acid-co-acrylamide) hydrogels by ammonium bicarbonate for desalination

Charged hydrogels can swell in salty water, while a portion of salt ions are repelled by the fixed charged groups in the hydrogels. This characteristic of charged hydrogels grants them with excellent potential for desalination. Attributed to their high osmotic pressure, hydrogels can absorb large amounts of water. Conversely, when a swollen hydrogel is exposed to an environment with higher osmotic pressure, it releases absorbed water, yielding comparatively dilute product water. In this study, ammonium bicarbonate (NH4HCO3), which can be regenerated and recycled by waste heat, was introduced to provide high osmotic pressure to dewater swollen poly (acrylic acid-co-acrylamide) (PAAM) hydrogels. A water recovery of nearly 48 % and a salt rejection of 58 % were obtained for 0.85 g/L NaCl solutions. Even in real well water (~1.3 g/L NaCl), the hydrogel-NH4HCO3 system recovered 43 % of water and rejected 56 % of salt. Moreover, this method proved to be chemically gentle with regards to the hydrogel composition and stability; observations showed consistent water recovery and salt rejection values across >20 cycles. The mechanisms behind the dewatering and desalination process were elucidated through the calculation of osmotic pressure and Donnan ion distribution, offering valuable insights for the future utilization of hydrogels in desalination applications.