The impact of sulfate ions on the ion exchange of rare earth elements

Abstract

The influence of sulfate versus chloride media on the adsorption of rare earth elements (REE) by strong acid and iminodiacetic acid resins was investigated. For the strong acid resin, Lewatit MonoPlus SP 112, the adsorption constants recorded for La, Sm and Er from MgSO4 solution were much lower compared to the values recorded from MgCl2 solution. The adsorption isotherms constructed for Sm from 0.5 M MgSO4 and 0.5 M MgCl2 solution also showed a substantial decrease in the equilibrium resin capacity that could be attained from sulfate solution. In addition, it was determined that the extraction strength of Sm from 0.5 M MgSO4 solution was unaffected by a change in solution pH from 1 to 3, despite the significant change in the aqueous speciation of SO42− and Sm3+ over this range. For the iminodiacetic acid resin, Purolite S930Plus, a similarly large decrease in the La adsorption constants was recorded from MgSO4 solution compared to MgCl2 solution which resulted in a large shift in the extraction curve to higher pH (pH50 = 1.9 in 1 M MgCl2 solution and 2.8 in 1 M MgSO4 solution). For both resins the decreased adsorption strength of the REE in sulfate solution also resulted in a decreased selectivity for the REE over key impurity ions. For instance the selectivity of the strong acid resin for Sm over Al was more than halved in MgSO4 versus MgCl2 solution, whereas the selectivity of the iminodiacetic acid resin for La over Mg decreased by an order of magnitude in MgSO4 versus MgCl2 solution (pH 4).