Tuning the adsorption behaviour of β-structure chitosan by metal binding

Chitosan, which is commonly extracted from squid pens of the Loligo genus, has a beta-type structure. Chitosan has potential application to the adsorption of pollutants but has received little study. We investigate the adsorption ability of beta-structure chitosan as well as Fe-III and Al-III chitosan-metal complexes. Pristine beta-chitosan shows lower adsorption abilities for dye, Cr-VI and fluoride ions compared with those for alpha-chitosan, mainly owing to having fewer -NH3+ groups on its surface. However, the anionic pollutant adsorption efficiency of beta-chitosan is clearly enhanced when chelated with metal ions. A beta-structure chitosan-Fe-Al complex displayed adsorption capacities of 621.45 mg g-(1) and 144.53 mg g(-1) for Acid Red 73 dye and fluoride ions, respectively, according to the fitted Langmuir-Freundlich model; and of more than 173.03 mg g(-1) for Cr-VI, according to the Freundlich model. These values are much higher than those observed for alpha-structure chitosan-metal complexes. This enhancement effect on the sorptive behaviour of beta-chitosan can be attributed to its loose structure. The polymer chains of beta-chitosan are arranged in parallel with relatively weak intermolecular forces, which allows them to easily chelate metal ions. Anionic pollutants can then be efficiently adsorbed by the chelated metal ions in the chitosan-metal complex if the electrostatic attraction of the -NH3+ groups is weak. This investigation provides a better understanding of beta-chitosan-based adsorbents for application to anionic pollutant adsorption and removal.