Preparation of NH₂-MIL-125(Ti) Nanocrystals Regulated by Deep Eutectic Solvents and Its Adsorption Properties for Cationic Dyes

Compared with NH₂-MIL-125(Ti) microcrystals (m-NMT) prepared by conventional solvothermal method, this work uses a deep eutectic solvent composed of choline chloride and methanol as an additive to conventional solvothermal method for regulating the synthesis of NH₂-MIL-125(Ti) nanocrystals (n-NMT), which have tunable dimensions, abundant oxygen vacancies and ligand defects, as well as enhanced hydrophilicity. The materials were applied to adsorb cationic dyes Rhodamine B (RhB), malachite green (MG), mixed dyes composed of anionic and cationic dyes in aqueous solutions. Kinetic fitting of RhB and MG adsorption was performed, with further investigations on the effects of initial concentration and pH of RhB solution on the adsorption performance, and the recyclability of n-NMT-53 for RhB removal. Results demonstrate that the adsorption of cationic dyes RhB and MG by NH₂-MIL-125(Ti) follows the pseudo-second-order kinetic model, with n-NMT exhibiting superior adsorption capabilities over m-NMT. The pseudo-second-order rate constants (k₂) of n-NMT-53 for RhB and MG reached 18.14×10^-2 and 7.41×10^-3 g/(mg·min), respectively, representing 16.3 times and 4.2 times that of m-NMT (1.11×10^-2 and 1.76×10^-3 g/(mg·min)). At 298.15 K, the saturated adsorption capacity of RhB on n-NMT-53 (79.87 mg/g) was 2.3 times that of m-NMT (34.03 mg/g), and the efficient adsorption capacity of n-MTT-53 for RhB has a wide pH application range and good cycling stability. n-NMT-53 exhibited efficient selective adsorption for cationic dyes (RhB and MG) over anionic dye methyl orange in mixed solutions. The enhanced RhB adsorption mechanism primarily involves the synergistic effect of hydrogen bonding interactions, π-π interactions and Ti⁴⁺ centers. This work proposes a facile strategy for synthesizing NH₂-MIL-125(Ti) nanocrystalline adsorbents, demonstrating significant potential for efficient dye removal and wastewater treatment applications.