Enrichment of rare earth metal ions by the highly selective adsorption of phytate intercalated layered double hydroxide

Abstract

Phytate intercalated MgAl layered double hydroxide (MgAl-LDH) was prepared by an anion exchange method with the precursor NO₃⁻ containing MgAl-LDH. The final as-synthesized product [Mg_0.69Al_0.31(OH)₂] (phytateNa₆)_0.05 (NO₃)_0.01·mH₂O (phytate-LDH) has highly selective adsorption ability for some metal ions and can be used to enrich rare earth metal ions in mixed solution, such as Pr³⁺ and Ce³⁺ from a mixed solution of them with Pb²⁺ and Co²⁺. At first, phytate-LDH has good adsorption performance for these ions in single metal ion solutions. At low concentration (below 10 mg L⁻¹), all the capture rates of the four metal ions were more than 97%, for highly toxic Pb²⁺ it was even up to nearly 100%, and a high capture rate (99.87%) was maintained for Pb²⁺ at a high concentration (100 mg L⁻¹). When all the four metal ions are co-existing in aqueous solution, the selectivity order is Pb²⁺ ≫ Pr³⁺ ≈ Ce³⁺ > Co²⁺. In a solution containing mixtures of the three metal ions of Pr³⁺, Ce³⁺, and Co²⁺, the selectivity order is Pr³⁺ ≈ Ce³⁺ ≫ Co²⁺, and in a solution containing mixtures of Pr³⁺ with Co²⁺ and Ce³⁺ with Co²⁺, the selectivity orders are Pr³⁺ ≫ Co²⁺ and Ce³⁺ ≫ Co²⁺, respectively. The high selectivity and adsorption capacities for Pb²⁺, Co²⁺, Pr³⁺, and Ce³⁺ result in the efficient removal of Pb²⁺ and enrichment of the rare earth metal ions Pr³⁺ and Ce³⁺ by phytate-LDH. Based on the elemental analysis, it is found that the difference of the adsorption capacities is mainly due to the different coordination number of them with phytate-LDH. With molecular simulation, we believe that the adsorption selectivity is due to the difference of the binding energy between the metal ion and phytate-LDH. Therefore, the phytate-LDH is promising for the enrichment and/or purification of the rare earth metal ions and removal of toxic metal ions from waste water.