Phosphonic Acid-Functionalized MIL-53(Al) As an Efficient Sorbent for Trace Rare Earth Elements Preconcentration, Storage and Their Determination by X-ray Fluorescence Spectrometry

In this work, a simple and novel method coupling solid phase extraction (SPE) with X-ray fluorescence spectrometry (XRF) is proposed for the simultaneous determination of 15 kinds of trace rare earth elements (REEs) in water samples. A phosphonic acid functionalized metal–organic framework named BPG-MIL-53(Al) was prepared via postsynthetic modification and served as an efficient adsorbent for these REEs. The prepared BPG-MIL-53(Al) could almost completely adsorb REEs in 5 min under neutral conditions. After filtration, REEs-adsorbed BPG-MIL-53(Al) was deposited on a filter membrane to form a thin film, which was directly analyzed by XRF. The XRF intensities of the REEs-retained MOF disc remained almost unchanged after six months. Taking advantage of this strategy, XRF was able to quantitate ng mL–1 levels of REEs in water samples, achieving impressive limits of detection in the range of 0.4–4.7 ng mL–1. The proposed method was applied to the on-site collection and analysis of REEs in real water samples with desirable accuracy and spike recoveries obtained.