Novel Two-Dimensional Magnetic Titanium Carbide for Methylene Blue Removal over a Wide pH Range: Insight into Removal Performance and Mechanism.

Two-dimensional (2D) layer-structured titanium carbide MXenes (e.g., 2D Ti3C2 MXene) have received tremendous attention owing to their excellent properties and unique 2D planar topology. Nevertheless, there are still several challenges to be addressed for well dispersibility and easy separation from a heterogeneous system, hindering the practical applications. Herein, 2D Ti3C2 MXene, as the most typical member of 2D MXenes, is functionalized with magnetic Fe3O4 nanoparticles via an in situ growth approach (designated as MXene@Fe3O4), which exhibits the intriguing phenomenon on methylene blue (MB) adsorption in the environmental remediation realm. The maximum adsorption capacity of the MXene@Fe3O4 composites for MB is calculated to be 11.68 mg·g-1 by a Langmuir isotherm model. A thermodynamic study of the adsorption demonstrates that the reaction process is exothermic and entropy-driven. Attractively, the removal process is a pH-independent process, and the optimal MB adsorption capacity is achieved at pH = 3 or 11, which is ascribed to electrostatic interactions and the hydrogen bond effect. X-ray diffraction, Fourier transform spectroscopy, X-ray photoelectron spectroscopy, and density functional theory calculation results reveal that the adsorption process is based on a combination of Ti-OH···N bonding, electrostatic attraction, and reductivity. Furthermore, multiple cycle runs demonstrate an excellent stability and reusability of MXene@Fe3O4 composites. This study provides a promising approach for the alternative removal of MB and broadens the potential application of 2D MXene for the treatment of practical acidic or alkaline wastewater.