Equilibrium and kinetics studies on the adsorption of substituted phenols by a Cu-Al layered double hydroxide intercalated with 1-naphthol-3,8-disulfonate

Cu-Al layered double hydroxides (Cu-Al LDHs) intercalated with 1-naphthol-3,8-disulfonate (1-N-3,8-DS^2-) were confirmed to easily take up substituted phenols with electron-poor benzene rings from aqueous solution. The uptake of the substituted phenols by the 1-N-3,8-DS•Cu-Al LDH was better expressed by the Langmuir-type than the Dubinin-Radushkevich (DR) adsorption model. The negative values of ΔG for all substituted phenols indicate that the adsorption process is spontaneous regardless of the temperature. The |ΔH| values for all substituted phenols are less than 20 kJ mol^-1, indicating that the phenol uptake by this LDH can be considered a physical adsorption process caused by π-π stacking interactions. Although the uptake of the substituted phenols by the 1-N-3,8-DS•Cu-Al LDH can be considered a physical adsorption process caused by π-π stacking interactions, it is closely related chemically to Langmuir-type adsorption. The uptake of various substituted phenols by 1-N-3,8-DS•Cu-Al LDH followed the pseudo-second-order kinetic model. By fitting the results of phenol uptake by 1-N-3,8-DS•Cu-Al LDH to the Eyring equation, it was found that positive values of ΔH^‡ and ΔG^‡ indicated the presence of an energy barrier in the adsorption process. Furthermore, the positive value of ΔH^‡ confirmed that the process was endothermic.