Effects of Electronegativity and Hydration Energy on the Selective Adsorption of Heavy Metal Ions by Synthetic NaX Zeolite

The adsorption capacity of synthetic NaX zeolite for Pb(2+), Cd(2+), Cu(2+) and Zn(2+) in single and multi-component systems were investigated. The effects of electronegativity and hydration energy on the selective adsorption, as well as potential selective adsorption mechanism of the NaX zeolite for Pb(2+), Cd(2+), Cu(2+) and Zn(2+) were also discussed. The maximum adsorption capacity order of the heavy metals in the single system was Pb(2+) > Cd(2+) > Cu(2+) > Zn(2+), and this could be related to their hydration energy and electronegativity. The values of the separation factors (α) and affinity constant (K(EL)) in different binary systems indicated that Pb(2+) was preferentially adsorbed, and Zn(2+) presented the lowest affinity for NaX zeolite. The selective adsorption capacities of the metals were in the order, Pb(2+) > Cd(2+) ≈ Cu(2+) > Zn(2+). The trend for the selective adsorption of NaX zeolite in ternary and quaternary systems was consistent with that in the binary systems. Pb(2+) and Cu(2+) reduced the stability of the Si-O-Al bonds and the double six-membered rings in the NaX framework, due to the high electronegativity of Pb(2+) and Cu(2+) than that of Al(3+). The selective adsorption mechanism of NaX zeolite for the high electronegative metal ions could mainly result from the negatively charged O in the Si-O-Al structure of the NaX zeolite, hence heavy metal ions with high electronegativity display a strong affinity for the electron cloud of the oxygen atoms in the Si-O-Al. This study could evaluate the application and efficiency of zeolite in separating and recovering certain metal ions from industrial wastewater.