Preparation and characterization of cross-linked chitosan beads for phosphate adsorption in aqueous solution
Chitosan beads were cross-linked with glutaraldehyde (GA) and epichlorohydrin (EP), respectively, at variable composition. The cross-linked chitosan beads were further modified and doped using calcium chloride. Several techniques were used to characterize the structural and chemical properties of chitosan bead materials. These materials possess unique structural and chemical properties such as hydrophile-lipophile characteristics with variable chemical functionalities that affect the adsorption properties. The general features of the adsorption and textural properties of the bead systems were studied using model dye compounds such as p-nitrophenolate (PNP) and phenolphthalein (PHP). The results from the uptake study of the model dyes was used to complement a systematic adsorption study of hydrogen phosphate ion (HPO42-) species and p-nitrophenolate phosphate (PNPP) at pH 8.5 and 295 K. Several model equations were used to analyze the adsorption data, as follows: Langmuir, Sips, and Dubinin–Radushkevich (D-R) isotherms. The Sips model provided adsorption parameters for the chitosan bead systems: (i) the monolayer adsorption capacity (Qm) for PNP ranged from 0.30 to 0.52 mmol g -1, and (ii) the Qm values for the bead systems with HPO42- ranged from 0.22–0.53 mmol g-1, (iii) the Qm values for the bead systems with PNPP ranged from 0.18–1.07 mmol g -1. The kinetic adsorption results for hydrogen phosphate ions (HPO42-) onto chitosan beads was analyzed at variable temperature to obtain the respective thermodynamic adsorption parameters using the pseudo-first order (PFO) and pseudo-second order (PSO) order kinetic models. The experimental rate constants for the bead systems with HPO42- ranged from 0.04–0.11 (min-1) for the PFO model and 0.0021–0.0045g.mg-1.min-1 for the PSO model. A molecular level understanding of the adsorption mechanism for the uptake of phosphate onto chitosan beads was proposed from these experimental results. Bead materials in this research have shown favorable adsorption–desorption properties and represent a promising tunable adsorbent system for the effective removal of phosphate anion species in aqueous solution.
Chitosan bead, Cross-linking, Phosphate, Adsorption
Master of Science (M.Sc.)