Lanthanide ion exchange properties of a coordination polymer consisting of di (2-ethylhexyl) phosphoric acid and trivalent metal ions (Ce3+, Fe3+, or Al3+)

Three kinds of coordination polymers ([M(dehp)(3)], M = Ce, Fe, or Al) were prepared by mixing the sodium form (Na(dehp)) of di(2-ethylhexyl) phosphoric acid and MCl3 in an ethanol-water binary mixture. They have monoclinic crystalline structure with similar lattice parameters. The lanthanide ion (Ln(3+) = La3+, Sm3+, Dy3+, or Yb3+) exchange properties were studied in a 20 : 80 vol% ethanol-water binary mixture containing 2 mM Ln(NO3)(3) at room temperature. The rate of Ln(3+) adsorption is relatively slow; it requires over 3 weeks to reach equilibrium. [M(dehp)(3)] has different Ln(3+) affinities depending on the kind of central metal ions: the affinity order at 3 week adsorption is Yb3+ < La3+ < Dy3+ < Sm3+ for [Ce(dehp)(3)], La3+ < Sm3+ < Dy3+ < Yb3+ for [Fe(dehp)(3)], and La3+ < Sm3+, Dy3+, Yb3+ for [Al(dehp)(3)]. The difference in affinity order can be explained by two factors: the coordination preference and steric strain caused by the polymeric structure. The chemical and structural analyses suggested that the Ln(3+) adsorption progresses first by the central M3+/Ln(3+) exchange, followed by a morphological change to a rod-like or fibrous form by a solid phase reaction. In the case of [Fe(dehp)(3)], the eluted Fe3+ may be hydrolyzed and precipitated as amorphous iron hydroxide.