Nitrates removal on PUROLITE A 520E resin: Kinetic and thermodynamic studies

Ion-exchange technology was studied to remove nitrates using PUROLITE A 520E. Predicting the rate at which adsorption takes place for a given system is probably the single most important factor for adsorber design, with adsorbate residence time and ultimately the reactor dimensions controlled by the system's kinetics. A fixed volume stirred tank reactor was used to study the kinetics of adsorption in a single-component system. Results of the intraparticle diffusion and the film diffusion models show that the film diffusion was the main rate-limiting step at high concentrations of nitrates. Process parameters including the rate of agitation, pH, and initial concentrations of nitrates were examined and the obtained data were modeled using three kinetic models including the pseudo-first-order equation, secondorder equation, and intraparticular diffusion model. The best fit of experimental adsorption data was obtained by means of the pseudo-second-order models. Equilibrium data were fitted to the Freundlich, Langmuir, and Dubinin-Radushkevich isotherm equations, and the equilibrium data were found to be well represented by the Langmuir isotherm equation. The thermodynamic constants of adsorption phenomena, ΔH⁰ and ΔS⁰ were found to be - 14.88 kJ/mol and 1.08 J/mol in the range 300-343 K respectively. The negative values of the Gibbs free energy ΔG⁰ demonstrate the spontaneous nature of adsorption of nitrates onto PUROLITE A 520E.