Copolymers-functionalized metal-organic framework composite for efficient adsorption of rare-earth elements

Based on the Hard-Soft-Acid-Base theory, we developed and synthesized a unique crystalline adsorbent called MIL-101(Cr)-SMA-ED-PMG (SMA: styrene maleic anhydride; ED: ethylenediamine; PMG: N,N-di(phosphonomethyl)glycine). This adsorbent consists of polymer chains that are abundant in carboxyl and phosphate Lewis hard base groups. This was performed to enhance the efficient recuperation of electrophilic Lewis hard acid rare earth elements. The Neodymium(III) (Nd3+) and Europium(III) (Eu3+) ions reached adsorption equilibrium in the MIL-101(Cr)-SMA-ED-PMG material after 6 h at pH 5. The adsorption of Nd3+ and Eu3+ follows the pseudo-second-order kinetic and Langmuir models. The maximum adsorption capabilities were 102.7 mg/g for Nd3+ and 110.4 mg/g for Eu3+. In addition, when applying a concentration of 0.3 mol/L of HNO3 or HCl, the adsorbent was nearly completely desorbed, and it remained stable for five cycles. The order of adsorption for the different REE ions was as follows: Eu3+ > Nd3+ > Ce3+ > La3+. The MIL-101(Cr)-SMA-ED-PMG compound demonstrated a remarkable 84 % selectivity in adsorbing Eu3+ compared to the other metal ions in the mixture (Na+, Mg2+, Co2+, Ni2+, and Zn2+). Furthermore, the adsorption of Eu3+ was not significantly influenced by changes in the aqueous conditions. The experiment conducted on a zinc ore simulated leachate exhibited the material remarkable capacity for Eu3+ separation and enrichment. MIL-101(Cr)-SMA-ED-PMG exhibits strong adsorption properties, exceptional selectivity, effective desorption, and significant potential for the recovery of rare earth elements (REEs).