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Investigation of cadmium removal mechanisms by clinoptilolite

İpçi, İrem
Clinoptilolite is a natural zeolite which can be used favorably in heavy metal removal. The main mechanisms for metal removal via clinoptilolite are adsorption and ion exchange. Several sources propose to keep the normality constant to obtain equilibrium isotherms for ion exchange systems, while many studies use constant sorbent mass with varying normalities of the sorbate. The objective of this study is to investigate the prevailing mechanisms of clinoptilolite during cadmium removal and the impact of the methodology for obtaining equilibrium isotherms. Batch Cd2+ removal experiments were conducted by using the two different methodologies (i.e. keeping the sorbent mass constant vs. keeping the normality constant) with clinoptilolite in as-received (AsC) and conditioned form (CnC), an ion exchange resin and granulated activated carbon. Exchangeable and framework cations, conductivity and pH were monitored during experiments. The equilibrium results were then fitted to isotherm models. The prevailing mechanisms for Cd2+ removal with clinoptilolite are discussed considering all monitored parameters and isotherm model fits. Use of the methodology was seen to have an effect on the overall Cd2+-clinoptilolite interaction. For example, differences between methodologies are observed regarding maximum sorbed Cd2+ and the distribution of exchangeable cations. Conductivity profiles provided a good indication of presence of ion exchange and demonstrated that it is more dominating for CnC than for AsC. The Cd2+ removal capacities observed in this study (0.65 meq/g for AsC and 1.46 meq/g for CnC) are the highest recorded for a clinoptilolite sample, as can be found in the literature.