Two-dimensional lithium-intercalated Ti3C2Tx MXene for highly selective neodymium (Ⅲ) adsorption

Abstract

Recovering rare earth elements (REEs) from low-concentration REE-bearing waste streams has been deemed as a sustainable approach to diversity REE supply and reduce environmental burden. However, highly selective separation of trivalent Fe3+ and Al3+ impurities during low-concentration REE processing still remains a critical challenge. In this study, two-dimensional multilayered MXene adsorbents TCFs were successfully prepared and utilized for Nd3+ adsorption via selective etching of Al element in Ti3AlC2. It was found that LiF/HCl-etched TCF-2 can simultaneously realize effective Al etching, interlayer spacing expansion and in-situ Li+ intercalation. The intercalated Li+ amount was up to 4.44 wt% (6.40 mmol/g). Compared with HF-etched TCF-1, TCF-2 demonstrated boosted adsorption kinetics (2 h), large adsorption capacity (517.79 mg/g), impressively high selectivity (Nd3+/Fe3+ SF = 221, Nd3+/Ca2+ SF = 858, Nd3+/Mg2+ SF = 3545), and broad working pH range (2–7). Various experimental characterizations reveal that selective Nd3+ recovery is owing to the ion-exchange and surface complexation induced by interlayer Ti-O/Ti-OH of TCF-2. Specifically, the extended X-ray sorption fine structure (EXAFS) results, in combination with density functional theory (DFT) calculations, further confirm the Nd3+ is selectively adsorbed by forming surfaced complexes structure in the interlayer. And this is also evidenced by the decrease of Nd-O distances and high coordination number. The present results illustrate that alkaline metal ion-intercalated MXene could serve as a promising candidate to efficiently remove trivalent Fe3+ impurities for REEs recovery.