Selectively recovering rare earth elements with carboxyl immobilized metal-organic framework from ammonium-rich wastewater

The material with high adsorption capacity and selectivity is essential for recovering rare earth elements (REE_S) from ammonium (NH₄⁺-N)-rich wastewater. Although the emerging metal-organic framework (MOF) has gained intensive attention in REE_S recovery, there are scientific difficulties unsolved regarding restricted adsorption capacity and selectivity, hindering its extensive engineering applications. In this work, a diethylenetriamine pentaacetic (DTPA)-modified MOF material (MIL-101(Cr)-NH-DTPA) was prepared through an amidation reaction. The MIL-101(Cr)-NH-DTPA showed enhanced adsorption capacity for La(III) (69.78 mg g^-1), Eu(III) (103.01 mg g^-1) and Er(III) (83.41 mg g^-1). The adsorption isotherm and physical chemistry of materials indicated that the adsorption of REEs with MIL-101(Cr)-NH-DTPA was achieved via complexation instead of electrostatic adsorption. Such complexation reaction was principally governed by -COOH instead of -NH₂ or -NO₂. Meanwhile, the resulting material remained in its superior activity even after five cycles. Such a constructed adsorbent also exhibited excellent selective adsorption activity for La(III), Eu(III), and Er(III), with removal efficiency reaching 70% in NH₄⁺-N concentrations ranging from 100 to 1500 mg L^-1. This work offers underlying guidelines for exploitation an adsorbent for REEs recovery from wastewater.