Kinetics, thermodynamics, and structural investigations on the removal of Pb²⁺, Cd²⁺, and Zn²⁺ from multicomponent solutions onto natural and Fe(III)-modified zeolites

The adsorption of Pb²⁺, Cd²⁺, and Zn²⁺ from equimolar multicomponent solutions onto natural and Fe(III)-modified zeolites was studied by batch experiments in order to determine the influence of the competitive effect on the efficiency ions removal at different initial concentrations and different temperatures. Kinetic and equilibrium studies showed that affinity of both zeolites was the highest for Pb²⁺ and much lower for Zn²⁺ and Cd²⁺. The competitive effect was negligible at lower concentrations, while Zn²⁺ and especially Cd²⁺ ions adsorption was suppressed as the concentrations were increased. Affinity of natural zeolite toward the ions was lower than that of the modified zeolite, as was shown for adsorption from single-component solutions. External film diffusion was the rate-controlling step at lower concentrations and for ions of higher affinity for the adsorbent, while intraparticle diffusion was dominant at higher concentrations and for ions of lower affinity for the adsorbent. The increasing spontaneity of the adsorption process with increasing temperature and the positive values of enthalpy of adsorption indicated the endothermic nature of the adsorption process. Structural investigations of the adsorbents loaded with Pb²⁺, Cd²⁺, and Zn²⁺ ions by X-ray photoelectron spectroscopy (XPS) confirmed the higher content of the ions in the surface and subsurface regions of the Fe(III)-modified than of the natural zeolite. XPS depth profiles of the loaded Fe(III)-modified zeolite showed that surface mechanisms of adsorption were dominant in the case of lead and cadmium, while zinc loading on the zeolite sample was governed mainly by an ion-exchange process.