Phosphate uptake studies of cross-linked chitosan bead materials

A systematic experimental study is reported that provides a molecular based understanding of cross-linked chitosan beads and their adsorption properties in aqueous solution containing phosphate dianion (HPO₄^2-) species. Synthetically modified chitosan using epichlorohydrin and glutaraldehyde cross-linkers result in surface modified beads with variable hydrophile-lipophile character and tunable HPO₄^2- uptake properties. The kinetic and thermodynamic adsorption properties of cross-linked chitosan beads with HPO₄^2- species were studied in aqueous solution. Complementary structure and physicochemical characterization of chitosan beads via potentiometry, Raman spectroscopy, DSC, and dye adsorption measurements was carried out to establish structure-property relationships. The maximum uptake (Q_m) of bead systems with HPO₄^2- at equilibrium was 52.1mgg^-1; whereas, kinetic uptake results for chitosan bead/phosphate systems are relatively rapid (0.111-0.113min^-1) with an intraparticle diffusion rate-limiting step. The adsorption process follows a multi-step pathway involving inner- and outer-sphere complexes with significant changes in hydration. Phosphate uptake strongly depends on the composition and type of cross-linker used for preparation of chitosan beads. The adsorption isotherms and structural characterization of bead systems illustrate the role of surface charge, hydrophile-lipophile balance, adsorption site accessibility, and hydration properties of the chitosan bead surface.