Acid-salt treated CoAl layered double hydroxide nanosheets with enhanced adsorption capacity of methyl orange dye

In this study, the ultra-thin CoAl layered double hydroxide (LDH) nanosheets with Cl^- intercalation were prepared by a hydrothermal method following acid-salt treatment. The morphology, structure and composition information of CoAl LDHs were identified by a variety of characterization methods including SEM, XRD, XPS, FT-IR, and BET. The effects of initial adsorbent concentration, pH, competitive anions, and temperature were also measured. The fitted adsorption curves are matched well with Langmuir isotherm model and pseudo-second-order kinetic model, suggesting that the removal of methyl orange (MO) is governed by monolayer heterogeneous adsorption and chemical adsorption, respectively. The measurements of adsorption capacity at various temperatures revealed that the adsorption process is spontaneous. Co-existing anions have different negative effects on the adsorption capacity. The Co₄Al₁-Cl LDH adsorbent shows the maximum experimental and theoretical adsorption capacity of 801.08 and 827.50 mg g^-1 (pH = 7 and T = 293 K), respectively. An adsorption mechanism called "ion exchange and expansion-extrusion" is proposed to explain the change of interlayer spacing. These results indicate that a novel avenue for designing LDH-type adsorbent has been proposed with excellent adsorption capacity for anionic dyes like MO.