A combined ion exchange-nanofiltration process for water desalination: I. sulphate-chloride ion-exchange in saline solutions

Removal of chloride ions from saline water with seven different ion-exchange (IX) resins in sulphate form has been studied both on laboratory and pilot scales. It was found that sulphate-chloride exchange is very fast in aqueous solutions and that the feed salt concentration and the nature of the functional group of the resin play an important role in this process. It was shown that the chloride/sulphate separation factor depends on salt content in feed water and the higher the substitution of hydrogen atoms in amine functional group of anion exchange resin the higher chloride over sulphate selectivity. Exhausted IX resins were successfully regenerated using 0.2M Na₂SO₄ solution and multiple regeneration/saturation cycles proved that this did not affect the resin's performance on chloride ion removal. It was shown that the osmotic pressure of sea water was significantly reduced after SO₄/Cl ion exchange. Due to the drop of osmotic pressure lower energy consuming nanofiltration membranes compared with reverse osmosis membranes might be used for salty water desalination after IX treatment.