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

Abstract

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+, SO42− and CO32−/HCO3−) is even greater than of Na+ and Cl–. Conventional electrodialysis (EDconv) 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 (EDsel), for production of desalinated water containing relatively higher proportions of desirable minerals. Commercial cation- and anion exchange membranes (CEMCNS and AEMCNS) were coated with polyaniline (PANI), and the resultant membranes (PANI-CEMCNS, PANI-AEMCNS) 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 SO42− during desalination. Retention of mineral constituents was further enhanced with PANI modified styrene-co-divinylbenzene-polyethylene-based interpolymer cation- and anion-exchange membranes (CEMIP and AEMIP). The total alkalinity of the treated stream increased during EDsel, presumably due to concentration polarisation accompanied by preferential transport of H+ over OH−. The process efficiency was only marginally lower (5%) for EDsel, suggesting that this approach to desalination may be of practical importance.