Preparation of Customizable Ligand-Functionalized Chitin Biopolymer Adsorbents Utilizing the Azide-Alkyne "Click" Reaction for the Adsorption of Aqueous Cu(II).

Abstract

Chitin is an abundant biopolymer, with 1-3 million tons potentially available annually from shellfish-processing waste, but remains underutilized because its insolubility in conventional solvents makes it difficult to process and functionalize. Herein, a three-step route to chitin adsorbents with interchangeable ligands is reported. Highly acetylated chitin was tosylated and converted to the C-6 azide, and amine- and hydroxyl-terminated ligands were attached to this common intermediate by the copper-catalyzed azide-alkyne cycloaddition (CuAAC) "click" reaction, yielding water-insoluble derivatives. Cu(II) adsorption to the adsorbents was evaluated using inductively coupled plasma optical emission spectrometry (ICP-OES analysis). Under standard conditions (pH 6.0, 4 h contact), amine- and alcohol-chitin adsorbed 30.1 ± 1.3 and 22.3 ± 1.9 mg Cu g^-1, respectively, approximately 4.9- and 3.6-fold more than unmodified chitin. Isotherm data were well described by the Langmuir model, with apparent capacities of 37.4 and 25.7 mg g^-1 at 4 h contact. The adsorbents could be regenerated with 7% (v/v) HCl, retaining 46% and 42% of their initial uptake after four cycles. The differences in derivatives' uptakes are attributable to the identity of the grafted ligand, illustrating how the click route allows chitin adsorbents to be tailored through the choice of alkyne.