Phosphate Adsorption onto Chitosan-Based Hydrogel Microspheres

Phosphate discharges are closely linked to eutrophication, algal and cyanobacterial blooms in natural water bodies. Chitosan hydrogel microspheres (CHMs) were used as an adsorbent to remove phosphate (PO43–) from aqueous solution. The CHMs were prepared by a sequential methodology consisting of spraying and gelation processes. The physicochemical properties of the hydrogel microspheres were determined by size and zeta-potential analysis. Batch experiments, to investigate the kinetics of phosphate adsorption onto CHMs, show that equilibrum was reached within 30 minutes. The kinetic model was pseudo-second-order (R2 > 0.999). The effect of varying operating conditions (initial anions concentration, adsorbent loading, pH and temperature) on the experiments were also studied. According to the Boyd model, the rate of adsorption is controlled by the diffusion in the boundary layer. The Langmuir, Freundlich and Dubinin–Radushkhevic (D–R) adsorption models were applied to describe the equilibrium isotherms, and the data correlated better with the Freundlich model (R2 = 0.975). The mean free-energy values (obtained from the D–R model) indicate that the adsorption was physical in nature. The values of ΔG and ΔH demonstrated the spontaneous and exothermic nature of the reaction and the negative value of ΔS indicates a decrease in entropy.