Adsorptive behaviors of phenol in water by MWCNT modified by ZnCl₂, KOH and HNO₃

The texture and surface chemistry of carbon nanotubes before and after chemical treatment using ZnCl₂, KOH and HNO₃ were determined by scanning electronic microscope, X-ray diffraction, N₂ adsorption, and Boehm titration; and the effect of adsorptive conditions (contact time, initial concentration and temperature) on phenol removal and the thermodynamic and kinetic behavior and adsorption mechanism were investigated by tests and data fitting with three kinetic models (pseudo-first order, pseudo-second order and the Elovich kinetic equations) as well as thermodynamic equation. The results show that the treatment by HNO₃, ZnCl₂ or KOH less changes the BET surface area of carbon nanotubes, but obviously changes the surface chemical property. Specifically, the treatment by HNO₃ obviously enhances surface acidic groups and slightly increases basic groups, whereas the treatment by ZnCl₂ or KOH greatly decreases surface carboxyl groups and lactonic groups but obviously increases surface basic groups, which affects the phenol removal by carbon nanotubes. It is found that the phenol removal by carbon nanotubes treated with ZnCl₂ or KOH increases due to a decrease in surface carboxyl groups of carbon nanotubes, but HNO₃ treatment slightly reduces the phenol removal possibly because the adsorption is influenced by both structure and surface chemical property. Moreover, the adsorption of phenol by carbon nanotubes is spontaneous, exothermic and physically controlled, and the adsorption process of phenol by carbon nanotubes complies with the pseudo-second order equation.