Selective Capture of Nd(III) Using Novel <i>N</i> , <i>N</i> -Bis(phosphine hydroxymethyl)glycine-Activated MOF Composites: Kinetics, Mass Transfer Theory, and Mechanism Insight

Recovery of rare earths from secondary resources is of great strategic value and environmental significance. In this work, a phosphine-functionalized metal–organic framework (MOF) material (BPG@UiO-66-NH2) was prepared by modifying UiO-66-NH2 with the bis-phosphine-based ligand N,N-bis(phosphine hydroxymethyl)glycine (BPG) and applied to capture Nd(III) from wastewater. The results showed that BPG@UiO-66-NH2 could effectively capture Nd(III) at a pH range from 4 to 6, and the adsorption equilibrium was rapidly reached within 10 min with a maximum capture capacity of 190.51 mg/g. The kinetics and isotherms indicated that the adsorption of Nd(III) was more consistent with the Langmuir isotherm and pseudo-second-order model, and the adsorption belonged to monolayer adsorption on a homogeneous medium. The mass transfer results of the EDR-IDR and AOAS models illustrated that Nd(III) diffused rapidly into the adsorbent surface in solution and was dominated by the chemisorption reaction on the surface. Thermodynamic calculations indicated that the capture of Nd(III) was a spontaneous heat absorption process. Moreover, the KD value of BPG@UiO-66-NH2 for Nd(III) was up to 2278.53 mL/g, which was higher than those of the coexisting ions (Co(II), Zn(II), Ni(II), Mn(II)), demonstrating the superior adsorption selectivity for Nd(III). Meanwhile, BPG@UiO-66-NH2 exhibited excellent salt resistance and reusability with a high capture rate of 87.5%, even after five adsorption–desorption cycles. In addition, comprehensive characterization and density functional theory (DFT) calculations indicated that the adsorption mechanism was mainly attributed to the polydentate coordination binding of the phosphonic acid moiety in the BPG ligand to the Nd(III) “2:1” formula; electrostatic attraction and ion exchange also played an important role in the adsorption process. The superior performance of BPG@UiO-66-NH2 provides a practical solution for the efficient recovery of Nd(III).