Sorption behavior of perrhenate ion on Reillex(TM)-HP anion exchange resin from nitric acid and sodium nitrate/hydroxide solutions

The distribution coefficients (K'(d)) are reported for perrhenate on the nitrate form of Reillex(TM)-HP, a weak base anion exchange resin, as a function of nitric acid concentration from 0.100 to 10.0 M. Perrhenate K'(d) values were determined in 1.00 M NaNO3 from pH 2 to 13. The K'(d) values were determined in solutions containing 1.35 M NaNO3 and variable NaOH, 0.155 to 4.66 M, and in solutions containing 0.655 M NaOH and variable NaNO3, 0.46 to 5.35 M. Maximum perrhenate loadings were <2.7% of theoretical capacity. The K'(d) is defined by the following equation: K'(d)=Re-total-[Re](sol)xV(sol)/mass of dry resin/[Re](sol)xV(sol)/V-sol mL/g The total mmol of rhenium initially added is Re-total and [Re](sol) is the analytical molarity of rhenium in V-sol mL of solution after a batch contact. At 1.0 M HNO3 the value of K'(d) is 73 mL/g. The behavior of the perrhenate system in acid can be described by assuming that perrhenic acid, HReO4, is formed with a derived acid dissociation constant of about 6.5 M. In 1.00 M NaNO3 from pH 2 to 13 the sorption behavior is initially what is expected for a weak base resin with an acid dissociation constant of 1.24 x 10(-5) M. Thus, the K'(d) value decreases with pH values above the pK(a) value. However, instead of the value of K'(d) decreasing to zero at higher pH values, it becomes constant at 15 mL/g. In a solution containing 1.00 M NaOH and 1.00 M NaNO3, the value of K'(d) is 25 mL/g. The unusual behavior of the Reillex(TM)-HP anion exchange resin suggests that the resin's unprotonated pyridine functionality is solvating a {Na+,ReO4-} ion pair. The interaction of the perrhenate ion with Reillex(TM)-HP is more involved than a simple ion exchange phenomenon and the presence of NaOH affects the resin-perrhenate interactions.