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) from ammonium (NH-N)-rich wastewater. Although the emerging metal-organic framework (MOF) has gained intensive attention in REE 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), Eu(III) (103.01 mg g) and Er(III) (83.41 mg g). 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 -NH or -NO. 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 NH-N concentrations ranging from 100 to 1500 mg L. This work offers underlying guidelines for exploitation an adsorbent for REEs recovery from wastewater.