Biosorption of azo dyes from aqueous solution by glutaraldehyde-crosslinked chitosans.

The crosslinked chitosan microparticles prepared through homogeneous coupling reaction and microparticle formation using a sodium hydroxide solution showed the largest adsorbed amounts toward the RB5 and 3R dyes than those from the three other methods through heterogeneous coupling reaction and microparticle formation using sodium hydroxide or sodium tripolyphosphate solutions. The dynamical experimental study showed that the dye adsorption accurately followed the second-order adsorption process. The experimental isotherm data were analyzed using three isotherm models, namely, Langmuir, Freundlich, and Dubinin-Radushkevich. The results revealed that the adsorption behavior of the RB5 and 3R dyes on the microparticles fitted well with the Langmuir model. In addition, the mean adsorption energy (E) from the Dubinin-Radushkevich isotherm and the activation energy (E(a)) from Arrhenius equation indicated that the adsorption process might be the dual nature of the process, physisorption and chemisorption, and was predominant on the chemisorption process The competitive adsorption showed that the adsorption of the 3R dye on the microparticles in the mixture solution was much more affected by the existence of the RB5 dye than the other way around. Furthermore, it was also found that the crosslinked chitosan microparticles can be regenerated and reused for dye adsorption.