Preparation and Alkali Metal Ion Exchange Properties of Protonated Rb8Nb22O59 Compound

An ion-exchanger H8Nb22O59·8H2O was prepared by extracting Rb+ from Rb8Nb22O59, and its ion-sieve properties for alkali metal ions were investigated. The Rb+ can be effectively extracted using a concentrated HNO3 solution at room temperature, leading to a protonated compound with composition of Rb0.14H7.9Nb22O59·8.2H2O. The protonated (hydronium ion form) crystal has the lattice parameters (a = 7.5104(2) and c = 43.0780(8) Å of hexagonal symmetry, R3̄m), very close to the Rb+-form precursor. The exchange capacity increased in the order Cs+ ≪ Rb+ < K+, Na+ < Li+, which agrees with the decreasing order of ionic radius. The capacities for Li+, Na+, and K+ were close to the theoretical value of 2.55 mmol/g calculated from the composition of protonated compound. The distribution coefficients (K d) of alkali metal ions were determined in the condition of microamount of ion loading. The selectivity sequence was Cs+, Li+ ≪ Na+ < Rb+ ≪ K+ at pH around 3; the protonated samples showed markedly high selectivity for the adsorption of K+. The pH titration curves showed a mono-base acid character toward Na+, K+, and Rb+ ions. The analysis based on the ion-exchange model showed that it has a large amount of uniform acidic sites up to the neutralization degree of 0.5. The intrinsic acidities, pK0 values, were evaluated as 1.6 for Na+, K+, and Rb+ exchanges; this indicates that the ion-exchange sites have relatively strong acidity. The removal of Na+ and K+ impurities from Li+-concentrated solutions was studied, and the removal rates for Na+ and K+ were found to reach 98 and 99.9%, respectively, from a solution containing 410 mM LiCl, 20 mM LiOH, 4.3 mM NaCl, and 1.0 mM KCl.