Adsorption toward Trivalent Rare Earth Element from Aqueous Solution by Zeolitic Imidazolate Frameworks

Zeolite imidazolate frameworks (ZIFs) have been applied to gas adsorption, but there remain no reports detailing the adsorption–desorption process for lanthanum metal ion (LMI). This work first testifies that ZIF-8 has a better adsorption property than ZIF-90 for remediation of LMI from water. Adsorption results demonstrate that ZIF-8 removes 100.0% of the metals ions, whose concentrations are below 20 mg L–1, and the saturated adsorption capacity is 385 mg g–1. Characterization by P-XRD, ICP, and FT-IR indicates that the main structure of ZIF-8 remained unchanged to a large extent during the adsorption process. Density functional theory (DFT) calculations illustrated that the adsorption process is spontaneous, and the adsorption site is in the center of two imidazole rings. La-ZIF-8 was subjected to two polar desorbents, acetonitrile and methyl alcohol, possessing a desorption ratio of 94.4% and 61.0%, respectively. Recycling the adsorption/desorption process using acetonitrile resulted in significant crystal structure collapse after three recycles reducing the adsorption capacity to 150 mg g–1. However, acetonitrile still maintains a desorption ratio of over 90.0%. Our work shows that ZIF-8 can be used as an efficient adsorbent in the removal of aquatic LMI, especially at trace concentration levels.