Heavy Metals Uptake by Sardinian Natural Zeolites: Experiment and Modeling

Sardinian natural clinoptilolites are examined to evaluate their performance for heavy metals, i.e., lead, cadmium, copper, and zinc, removal. The natural material is either used as received or once converted into the sodium homoionic form. Equilibrium data for heavy metals in aqueous solutions and the natural material are obtained. The corresponding behavior is quantitatively correlated using classical adsorption isotherms as well as the ion-exchange equilibria model developed by Melis et al. (Ind. Eng. Chem. Res. 1995, 34, 3916), whose parameters are estimated by fitting the equilibrium data. To evaluate removal performance under dynamic conditions, breakthrough experiments are also conducted. The latter ones are compared with different ion-exchange rate laws (namely, equilibrium model, linear driving force (LDF), and film model rate approximations, along with Nernst-Planck model) with the aim of pointing out the importance of the hindrance effect of intra- and interphase mass transport when using the natural materials examined in this work. On the basis of the results reported and discussed in the present paper, the natural zeolite named ROM1 may be selected, from the equilibrium point of view as well as in terms of bed volumes at breakthrough point, for application in a potentially competitive low-cost process of lead removal from aqueous solutions.