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

Abstract

Environmental contextChitosan is an abundant natural component of marine life with potential applications as an adsorbant material for pollutants. We investigate the binding behaviour of chitosan, and show that the β-type structure readily chelates metal ions leading to enhanced adsorption of anionic pollutants in the chitosan-metal complex. The results are highly relevant to the removal of anionic organic pollutants from water. AbstractChitosan, which is commonly extracted from squid pens of the Loligo genus, has a β-type structure. Chitosan has potential application to the adsorption of pollutants but has received little study. We investigate the adsorption ability of β-structure chitosan as well as FeIII and AlIII chitosan-metal complexes. Pristine β-chitosan shows lower adsorption abilities for dye, CrVI and fluoride ions compared with those for α-chitosan, mainly owing to having fewer –NH3+ groups on its surface. However, the anionic pollutant adsorption efficiency of β-chitosan is clearly enhanced when chelated with metal ions. A β-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 CrVI, according to the Freundlich model. These values are much higher than those observed for α-structure chitosan-metal complexes. This enhancement effect on the sorptive behaviour of β-chitosan can be attributed to its loose structure. The polymer chains of β-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 β-chitosan-based adsorbents for application to anionic pollutant adsorption and removal.