An improved protocol for electrodialytic desalination yielding mineral-balanced potable water

Brackish water desalination through distillation produces essentially distilled water devoid of all minerals. In reverse osmosis (RO) all dissolved minerals get depleted but depletion of useful minerals (Mg²⁺, Ca²⁺, SO₄^2- and CO₃^2-/HCO₃^-) is even greater than of Na+ and Cl-. Conventional electrodialysis (ED_conv) too fails to meet the desired objective, all constituents being depleted in similar proportions with respect to feed. Consequently, re-mineralisation is necessary but in many cases not implemented. We report selective electrodialysis (ED_sel), for production of desalinated water containing relatively higher proportions of desirable minerals. Commercial cation- and anion exchange membranes (CEM_CNS and AEM_CNS) were coated with polyaniline (PANI), and the resultant membranes (PANI-CEM_CNS, PANI-AEM_CNS) were characterised by physico-chemical and electrochemical techniques. Due to sieving and hydrophobic effects, the PANI coating was demonstrated to improve the retention of Mg²⁺, Ca²⁺ and SO₄^2- during desalination. Retention of mineral constituents was further enhanced with PANI modified styrene-co-divinylbenzene-polyethylene-based interpolymer cation- and anion-exchange membranes (CEM_IP and AEM_IP). The total alkalinity of the treated stream increased during ED_sel, presumably due to concentration polarisation accompanied by preferential transport of H⁺ over OH^-. The process efficiency was only marginally lower (5%) for ED_sel, suggesting that this approach to desalination may be of practical importance.