Patent ID: 8840793

Claim:
A method for selective removal of sulfate anions from aqueous solutions bearing calcium cations, the method comprising: (a) contacting a wastewater stream with an anion exchange resin in a reactor, to form a resin loading solution, wherein: i) the wastewater has an initial dissolved sulfate concentration that is higher than a desired discharge sulfate concentration, wherein the initial dissolved sulfate concentration is equal to or lower than the saturated concentration of sulfate as gypsum in the wastewater; ii) the dissolved sulfate is a major anion requiring treatment in the wastewater comprising at least 50% of the total anions requiring treatment in the wastewater; iii) the pH of the wastewater is in the range of 6 to 12; and, (b) adding carbon dioxide to the wastewater stream upstream of the reactor or in the reactor so as to treat the resin loading solution with a carbon dioxide gas stream so that the pH of the resin loading solution is maintained below a contacting pH of 9.5 while sulfate anions are loaded onto the resin in exchange for hydroxyl anions, to produce a treated water stream and a loaded resin, wherein: (i) the treated water stream has a lower dissolved sulfate concentration than the wastewater stream and the sulfate removed is stoichiometrically replaced by bicarbonates or carbonates, said bicarbonates or carbonates generated from a reaction between carbon dioxide, hydroxyl anions, and water; (ii) an amount of dissolved calcium in the treated water stream is equal to the amount of dissolved calcium in the wastewater, less the amount that precipitates and is removed as calcium carbonate solids; and, (iii) amounts of dissolved magnesium and dissolved sodium in the treated water stream are substantially equal to respective amounts of dissolved magnesium and dissolved sodium in the wastewater; wherein the pH of the resin loading solution is maintained within the range of pH 6 to 9.5.