High-capacity adsorptive removal and recovery of trivalent europium ions using zinc-based metal-organic frameworks: Adsorption and mechanism study

The presence of trivalent lanthanides and actinides in water is a global environmental concern that is receiving considerable attention worldwide. Herein, zinc-terephthalate (Zn-BDC) MOF was used to study its potential for the adsorption of Eu(III) ions from water. The effect of solution pH, the adsorbent dosage, initial concentrations, and contact time was studied on the removal efficiency of Zn-BDC towards Eu(III). The Zn-BDC removed Eu(III) efficiently in a pH range of 4-8 with a maximum adsorption capacity of 598 mg g-1. The kinetic and adsorption data were well fitted by the pseudo-second-order and intra-particle diffusion models, respectively, which suggested an involvement of chemisorption and diffusion mechanism in the adsorption process. The adsorption mechanism of Eu(III) ions onto the Zn-BDC adsorbent involved the binding of the ions with the -OH/-COOH groups and, at the same time, Eu(III) ions exchange with the Zn(II) ions. The adsorbent material showed an excellent recovery (92.9%) after four cycles, which made the MOF effective for Eu adsorption and recovery. Thus, this work evidences the preparation of a non-functionalized MOF adsorbent with a low-cost synthesis and recyclability as an effective approach to removing trivalent lanthanide ions from aqueous solutions.