Effect of sulfate, carbonate, and phosphate on the uranium(VI) sorption behavior onto bentonite

Summary

Batch experiments were conducted to study the uranium U(VI) sorption onto bentonite as a function of pH (3 to 8), and initial U(VI) concentrations (5 × 10⁻⁶ and 5 × 10⁻⁵ M) in the presence and absence of sulfate, carbonate, and phosphate. Uranium sorption onto bentonite depended on the initial U(VI) concentration with a stronger sorption at lower concentrations and was high over a wide range of pH in the absence of complexing ligands. In the presence of 0.005 M sulfate, U(VI) sorption was reduced at low pH values due to the competition between SO₄²⁻ and the uranyl ion for sorption sites on the bentonite surface, or the formation of uranyl-sulfate complexes. In the presence of 0.003 M carbonate, U(VI) sorption decreased sharply at a pH above 7, because of the formation of negatively charged uranyl-carbonate complexes, which are weakly adsorbed onto the bentonite. Uranium sorption onto bentonite was greatly enhanced in the presence of 0.003 M phosphate. Kinetic batch experiments carried out for 5 × 10⁻⁵ M U(VI) at pH values of 3, 5, and 8 revealed that the sorption rate was generally rapid over the first 10 min of the experiments, then slowed down appreciably after 1 to 24 h. Sulfate had little effect on the kinetics of U(VI) sorption; both in the absence and presence of sulfate, sorption equilibrium was attained after 4 h. In the presence of carbonate, attainment of sorption equilibrium required more time than in its absence. The presence of 0.003 M phosphate reduced the time required to reach sorption equilibrium across a wide range of pH compared to phosphate-free systems.