Adsorption sites and electron transfer characteristics of methyl orange on three-dimensional hierarchical flower-like nanostructures of Co-Al-layered double hydroxides: Experimental and DFT investigation

The dried layered double hydroxides (LDHs), have poor dispersibility, and are difficult to recycle in water. In this study, the cobalt-aluminum LDHs were prepared by hydrothermal reaction (BC@Co-Al-LDHs), and then further calcined to obtain hollow-structured BC@Co-Al-LDOs. The surface morphologies and physicochemical characteristics of these two materials were characterized. Simultaneously, batch tests were conducted to study their adsorption performances for methyl orange (MO). The maximum removal rate (90.24 %) and adsorption capacity (147.68 mg/g) of BC@Co-Al-LDOs for MO were 37.11 % and 60.72 mg/L higher than those of BC@Co-Al-LDHs, respectively. Based on the fitting results of the adsorption kinetic model, the type of MO adsorption by the two materials was chemisorption. Furthermore, density functional theory (DFT) was used to further study the adsorption mechanism of MO on the surface of these materials at the molecular level. The theoretical calculation results demonstrated that the [sbnd]SO₃⁻ in the MO was more prone to electrophilic reactions and accepted electrons from BC@Co-Al-LDHs, simultaneously, the Co adsorption site had the most accessible adsorption energy (−5.342 eV) contributing to the vertical adsorption. In addition, the partial density of states (PDOS) measurement revealed that MO interacted with the BC@Co-Al-LDHs essentially through s and p orbital hybridization.