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

Abstract

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 (Ln³⁺ = 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 Ln³⁺ adsorption is relatively slow; it requires over 3 weeks to reach equilibrium. [M(dehp)₃] has different Ln³⁺ 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 Ln³⁺ adsorption progresses first by the central M³⁺/Ln³⁺ 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.