Adsorption selectivity of heavy metals by Na-clinoptilolite in aqueous solutions

Preferential adsorption is important to classification removal of heavy metals in wastewaters. In the present study, the adsorption forces of heavy metal cations M²⁺ (Pb²⁺, Zn²⁺, Cd²⁺ and Cu²⁺) onto the Na⁺-clinoptilolite were analyzed. The selectivity sequence of heavy metal cations was given as Pb²⁺ > Zn²⁺ > Cd²⁺ > Cu²⁺. The selectivity coefficient $\left(K_{M/\text{Na}}=\frac{a_{\text{Na}}{N}_M}{a_M{N}_{\text{Na}}}\right)$ between M²⁺ and Na⁺ on the natural zeolite decreased with increasing ion strength, which depends on the effects of hydration volume (steric) and electron configuration of cations (coordination and polarization). The contributions of main factors resulting in the Hofmeister series on porous zeolite were: the ionic steric effects > coordination > non-classical polarization. The interactions between Na⁺ and zeolite is relatively weak, the electric field near zeolite surface is strong in Na⁺ system, and the electron configurations of heavy metals are significantly enhanced by the strong electric field using Na-zeolite. Generally, the zeolite can be pretreated by ions with weak polarization, which is helpful to increase the contributions of coordination and polarization because of the strong electric field at the zeolite/water interface in these ionic solutions, and then improve the removal efficiency of heavy metal cations in wastewaters or soils using zeolite.