Sorption and Binary Exchange of Nitrate, Sulfate, and Uranium on an Anion-Exchange Resin

Abstract

Competitive ion-exchange reactions were studied on a strong-base anion-exchange resin to remove NO₃^- and uranium from a contaminated groundwater containing high levels of NO₃^- (∼140 mM), SO₄^2- (∼10 mM), and U(VI) (∼0.2 mM). Results indicate that although SO₄^2- carries divalent negative charges, it showed the least selectivity for sorption by the Purolite A-520E resin, which is functionalized with triethylamine exchange sites. Nitrate was the most strongly sorbed. Sorption selectivity followed the order of NO₃^- > Cl^- > SO₄^2- under the experimental conditions. Nitrate competitively sorbed and displaced previously sorbed SO₄^2- in a column flow-through experiment and resulted in a high elution front of SO₄^2- in the effluent. Although the concentration of uranium in groundwater is orders of magnitude lower than that of NO₃^- or SO₄^2-, it was found to be strongly sorbed by the anion-exchange resin. Because the most stable uranium species in oxic and suboxic environments is the UO₂²⁺ cation, its strong sorption by anion-exchange resins is hypothesized to be the result of the co-ion effect of NO₃^- by forming anionic UO₂(NO₃)₃^- complexes in the resin matrix. These observations point out a potential alternative remediation strategy that uses strong-base anion-exchange resins to remove uranium from this site-specific groundwater, which has a low pH and a relatively high NO₃^- concentration.