Selective removal of sulfate ion from different drinking waters

Abstract

The sulfate anion is a stable, oxidized form of sulfur that is an essential nutrient for plants and animals. Due to its stability, sulfate is part ofa large number ofnatural minerals, the most common being gypsum (CaSO4·2H2O), anhydrite (CaSO4), baryte (BaSO4) and celestine (SrSO4). Sulfate is an important component of non-polluted waters and is part of the six major ions in surface waters (Na+, Ca2+, Mg2+, Cl−, HCO3 −, SO42−). Depending on its concentration, in association with calcium and magnesium, sulfate can give water a bitter taste and can provoke a laxative effect. Consequently, its concentration in drinking waters is regulated by the World Health Organization (WHO) in the Guidelines for Drinking-water Quality at a maximum value of 250 mg/L. Here, we report a new method applied for the selective removal of sulfate from various kinds of water. The method employed a new material obtained by loading an organic reagent (4,4’- (ethane-1,2- diyl)bis[1-(2-oxo-2-phenylethyl)pyridinium] dibromide) on DOWEX 50W cation exchange resins having different grades of reticulation (×2 and ×10) that were previously activated by addition of HCl or NaCl. We found that the resins with the lowest grade of reticulation could bind all of the added reagents. In their reagent-bound form, the resins are capable ofretaining the sulfate anion, with the greatest quantity retained by the reagent-bound form of the DOWEX 50WX2 resin that had previously been activated by HCl. The fixation of sulfate was selective in the presence of chloride and nitrate. Collectively, these results show that the method is applicable for the separation and selective concentration of the sulfate anion in water samples, by comparison to non-selective methods that use anion exchange resins.