Adsorption behavior and mechanism of acid orange 7 and methylene blue on self-assembled three-dimensional MgAl layered double hydroxide: Experimental and DFT investigation

Abstract

The removal of toxic ionic dyes is an urgent issue in the water pollution control field. Herein, the self-assembled three-dimensional MgAl layered double hydroxide (3D-MA-LDH) have been synthesized via the soft-template synthesis employing sodium dodecyl sulfate (SDS) as soft template and can be successfully applied in the anionic azo dye Acid Orange 7 (AO7) and cationic heteroaromatic compound methylene blue (MB) removal. The maximum removal capacities were 485.6 and 58.3 mg·g−1 for AO7 and MB, respectively. The removal behavior for AO7 and MB fitted the Langmuir and Freundlich model, respectively, and revealed a diffusion process contained chemical adsorption. X-ray diffraction, Fourier transform spectroscopy, and X-ray photoelectron spectroscopy demonstrated that anion exchange dominated the AO7 removal, while MB was adsorbed via hydrophobic interaction between MB and DS. Density functional theory further confirmed the interaction, wherein the bonding distance was 1.622 Å between AO7 and 3D-MA-LDH’s layer. MB showed strong affinity toward DS with no dependence of bond connection to be stably anchored on the 3D-MA-LDH. In addition, the recyclability experiments exibited that 3D-MA-LDH could be stably reused for at least five times. This work provided a new approach for cleaning ionic dyes and broadened the application field of 3D- LDHs.