The removal of nitrate from aqueous solutions by chitosan hydrogel beads

Abstract

A physico-chemical investigation of the adsorption of nitrate by chitosan hydrobeads was conducted. The adsorption of nitrate by chitosan hydrobeads was increased with a decrease in the pH of the solution. The adsorption process was found to be temperature dependant with an optimum activity at 30 °C. Adsorption capacity was found to decrease with increases in temperature after 30 °C, indicating the exothermic nature of this process. Theoretical correlation of the experimental equilibrium adsorption data for the nitrate–chitosan hydrobeads system was properly explained by the Langmuir isotherm model. This was supported by the fact that homogeneity index was close to unity (0.98–1.08) from Langmuir–Freundlich isotherm model. The maximum adsorption capacity was 92.1 mg/g at 30 °C. The kinetic results corresponded well with the pseudo-second-order rate equation. Intra-particle diffusion also played a significant role at the initial stage of the adsorption process. Thermodynamic parameters such as the Gibbs free energy (Δ G 0), enthalpy (Δ H 0), and entropy (Δ S 0) for the nitrate adsorption were estimated. Results suggest that the adsorption process is a spontaneous, exothermic process that has positive entropy. Desorption of nitrate from the loaded beads was accomplished by increasing the pH of the solution to the alkaline range, and a desorption ratio of 87% was achieved around pH 12.0.