Heavy metal removal by clinoptilolite. An equilibrium study in multi-component systems

Ternary and quaternary ion-exchange equilibria have been studied between heavy metal solution (Pb(2+), Cd(2+), Cu(2+)) and Na-form of clinoptilolite. The value of the ion-exchange equilibrium constant was estimated using the Langmuir, Competitive Langmuir, and thermodynamic sorption models. For each isotherm, calculations were done taking into account the concentration of ions in both phases. Additionally, for the thermodynamic isotherm, two other cases were considered: activity of ions in the liquid phase and concentration in the solid phase; activity of ions in both phases. The activity coefficients of ions in the liquid phase were determined using Pitzer's model; activity coefficients in the solid phase were estimated by Wilson's model. It was found that the exchange capacity for a given M(2+) is not constant and differs in one- or multi-component systems. The results show that the equilibrium model based on the law of mass action, which considers nonideal behavior of both phases, allows one to achieve the best approach to the real multi-component equilibrium data in all studied systems.