Removal of Zn2+, Pb2+, Cd2+, and Cu2+ from aqueous solution by synthetic clinoptilolite

Removal of heavy metal ions from wastewater by natural clinoptilolite has been well investigated. However, the removal efficiency of heavy metals with natural clinoptilolite is not consistent due to its uncertain quality. Furthermore, the impurities in natural clinoptilolite significantly affect the investigation on the sorption behavior and mechanism of clinoptilolite. In this study, we investigated the adsorption (ion-exchange) and regeneration behavior of modified synthetic clinoptilolite including the influence of pH, solid/liquid ratio, and ion exchange temperature on the adsorption process, adsorption kinetics and isotherms, and competitive adsorption behavior of Zn2+, Pb2+, Cd2+, and Cu2+. The maximum adsorption capacity of NaCl-modified synthetic clinoptilolite for Zn2+, Pb2+, Cd2+, and Cu2+ is as high as 31.47, 181.8, 44.64, and 33.76 mg/g, respectively, which is much higher than the reported values in the studies on modified and unmodified natural clinoptilolite. The present study shows that the amount of adsorption of NaCl-modified synthetic clinoptilolite for Zn2+, Pb2+, Cd2+, and Cu2+ is less affected by the operation temperature. Kinetics study shows that the pseudo-second-order kinetic equation has a better description for the adsorption behavior of NaCl-modified synthetic clinoptilolite. Isotherms study suggests that the adsorption process of NaCl-modified synthetic clinoptilolite follows the Langmuir model. Competitive adsorption investigation suggests that some cation sites in the open-framework of clinoptilolite can only be exchanged by selective alien cations and this phenomenon reveals the origin of the selectivity of zeolites in ion-exchange.