Deep eutectic solvents assisted synthesis of MgAl layered double hydroxide with enhanced adsorption toward anionic dyes

Abstract

The chloride (Cl−)-intercalated MgAl layered double hydroxide (LDH) with a three-dimensional (3D) hierarchical structure was synthesized via a one-step solvothermal method based on deep eutectic solvents (DESs) including MgCl2·6H2O/urea (molar ratio of 1:2) and AlCl3·6H2O/PEG 200 (molar ratio of 1:3). The as-prepared Mg2Al-Cl LDH acquired high uptake capacities (1051.87, 889.76 and 512.55 mg g-1) for the anionic dyes of methyl orange (MO), congo red (CR) and indigo carmine (IC) at 25 °C, which are much higher than those of most reported LDHs in the literature. These high uptake capacities are not only associated with electrostatic attraction and hydrogen bonding, but also related to ion exchange between Cl− and dye molecules. The adsorption kinetics for all studied dyes were well-fitted to the pseudo-second order kinetic model well (R2 = 0.999), and the adsorption isotherms all conformed well to the Langmuir isotherm model (R2 > 0.97). Thermodynamic studies indicated that the adsorption process was endothermic in nature. In a word, the LDH was prepared with DESs using a directive and effective strategy to achieve the desirable Cl− intercalation and reduction of the CO32− contamination, which makes the adsorption rapid and highly effective for anionic dyes. Particularly, its micrometre-scale structure facilitates its separation from water, giving it extensive application prospect in wastewater treatment.