Carboxylic acid-assisted hydration for the preparation of mesoporous magnesium hydroxide/magnesium oxide with ultra-high adsorption performance and selective adsorption: Experiments and DFT investigations

Herein, mesoporous Mg(OH)₂ is prepared by one-step hydration method using microcrystalline magnesite as raw material and acetic acid as hydration agent, and the mesoporous MgO is obtained after thermal conversion. The adsorption experiments on three single dye systems (Congo red (CR), methyl orange (MO) and methylene blue (MB)) and three binary mixed dye systems (CR/MO, CR/MB and MO/MB) show that mesoporous MgO possesses superior adsorption selectivity towards anionic dyes. The mesoporous Mg(OH)₂ with cardroom-like structure, obvious pore channel and abundant edge and concern shows good adsorption capacity for CR (6591.96 mg/g) and MO (3087.04 mg/g). The mesoporous MgO obtained by calcination has high specific surface area (66 m²/g) and partially inherits the cardroom-like structure of Mg(OH)₂, exhibiting remarkable adsorption performance (CR: 10521.89 mg/g; MO: 6694.65 mg/g). CR and MO adsorption on the Mg(OH)₂/MgO accords with the Langmuir model and pseudo-second-order kinetic model. The density functional theory (DFT) is employed to evaluate the reactivity of CR/MO, such as molecular frontier orbital energies and energy gap, demonstrating that CR exhibits superior reactivity and adsorptive capacity in comparison to MO. The MgO sample is regenerated by calcination after CR/MO adsorption and the CR removal efficiency of MgO reaches above 85 % after 10 cycles.