Rationally Engineered Phosphonate-Functionalized UiO-66 Nanotrap Materials for Highly Selective Recovery of Rare-Earth Elements

To ensure supply chain security, highly selective recovery of rare-earth elements (REEs) from industrial solid wastes is essential. In this study, UiO-66-NH2 was functionalized with N-(phosphonomethyl) iminodiacetic acid (PMIDA) and iminodi(methylenephosphonic acid) (IDMP) to prepare phosphonate nanotraps UiO-66-PMIDA and UiO-66-BTATPA (benzene-1,3,5-triamido-tetraphosphonic acid adduct, formed by the amidation reaction between trimesoyl chloride (TMC) and IDMP). Both exhibited Langmuir-type adsorption and pseudo-second-order kinetics for Nd3+, Dy3+, and Er3+, with adsorption capacities in the order UiO-66-NH2 < UiO-66-PMIDA < UiO-66-BTATPA. Under multicomponent competition (Cd2+, Ni2+, Zn2+, Ca2+, and Mg2+), the Kd values of REEs were over 104 times higher than those of interfering ions, and BTATPA performed better. Characterization and density functional theory (DFT) analysis showed that higher phosphonate density enhanced surface electronegativity, enabling selective binding of REE3+ via multidentate coordination. After 5 adsorption–desorption cycles, the adsorbents retained ∼80% of their initial capacity, providing an experimentally and computationally supported design strategy for large-scale REE recovery from real waste leachates.