Cadmium adsorption on modified chitosan‐coated bentonite: batch experimental studies

BACKGROUND: Several researchers have investigated the use of chitosan as an adsorbent for removal of heavy metals from aqueous streams. Chitosan flake or powder swells and crumbles making it unsuitable for use in an adsorption column. Chitosan also has a tendency to agglomerate or form a gel in aqueous media. The adsorption capacity can be enhanced by spreading chitosan on physical supports that can increase the accessibility of the metal binding sites. Although several attempts have been made to enhance the adsorption capacity of chitosan, using various chemicals, the sorption capacity for metal ions decreased after cross‐linking of chitosan. RESULTS: Bentonite was coated with chitosan (Chi) and its derivative, 3,4‐dimethoxy‐benzaldehyde (Chi/DMB). The product was then used as adsorbent for the removal of Cd²⁺ from aqueous solutions. The presence of imine groups resulting from chemical modification was confirmed using IR, DRS and SEM. The adsorption followed the Langmuir isotherm and could be described by pseudo‐second order kinetics. CONCLUSION: Chi/DMB coated on bentonite increased the accessibility of metal binding sites. The Chi/DMB/bentonite showed no significant pH dependence in the pH range 2–9, but bentonite coated with chitosan revealed very intensive pH dependence, which had a considerable effect on cadmium removal. As expected adsorption of Cd²⁺ by Chi/bentonite and Chi/DMB/bentonite is dependent on contact time and adsorbent dose. In addition, an EDTA solution is suitable for desorption of cadmium ions, and the reusability of Chi/DMB/bentonite is quite good. © 2012 Society of Chemical Industry