Kinetic modelling of the adsorption of nitrates by ion exchange resin

The capacity of ion exchange resins, Amberlite IRA 400, for removal of nitrates from aqueous solution was investigated under different initial concentrations. The suitability of the Freundlich, Langmuir and Dubinin-Radushkevich adsorption models to the equilibrium data was investigated. The equilibrium data obtained in this study were found to follow Freundlich adsorption isotherm. The maximum sorption capacity was 769.2 mg/g at 25 °C. Reversible-first-order, intraparticle diffusion, film diffusion and Bangham models were used to fit the experimental data. The adsorption of nitrates on Amberlite IRA 400 resin follows reversible-first-order kinetics. The overall rate constants were estimated for different initial concentrations. Results of the intra-particle diffusion and the film diffusion models show that the film diffusion was the main rate-limiting step. The low correlation of data to the Bangham's equation also confirms that diffusion of nitrates into pores of the resin was not the only rate-controlling step. The thermodynamic constants of adsorption phenomena, ΔH° and ΔS° were found to be −26.122 kJ/mol and −68.76 J/mol in the range 298–318 K and +19.205 kJ/mol and +68.76 J/mol in the range 318–343 K, respectively. The negative values of the Gibbs free energy (ΔG) demonstrate the spontaneous nature of adsorption of nitrates onto Amberlite IRA 400.