Desalination of Seawater Using Cationic Poly(acrylamide) Hydrogels and Mechanical Forces for Separation

In this study, the ability of cationic poly(acrylamide-co-(3-acrylamidopropyl)trimethylammonium chloride) hydrogels to desalinate seawater is explored, where the salt separation is based on the partial rejection of mobile salt ions by the fixed charges along the polymer backbone. Water absorbency measurements reveal that artificial seawater-containing divalent ions (Mg$^{2+}$, Ca$^{2+}$, and SO$_{4}$$^{2-}$) drastically decrease the swelling capacity of previously employed anionic poly(acrylic acid-co-sodium acrylate) hydrogels, whereas no influence on the swelling behavior of the synthesized cationic hydrogels is found. The swelling behavior and mechanical properties are studied by varying the degree of crosslinking and degree of ionization systematically in the range of 1–5 and 25–75 mol%, respectively. Finally, artificial seawater (c$_{sea}$= 0.171 mol L$^{-1}$) is desalinated in a custom-built press setup with an estimated efficiency of E$_{m³}$= 17.6 kWh m$^{-3}$ by applying an external pressure on the swollen hydrogels.