Structural Features and Thermal Behavior of Ion-Exchanged Clinoptilolite from Beli Plast Deposit (Bulgaria)

The structural features and the thermal behavior of natural, Na-, Ca-, K-, Mg-, and Cd-exchanged clinoptilolite from the Beli Plast deposit (Bulgaria) were studied. Purified clinoptilolite sample was preliminary prepared and ion-exchanged at 100 degrees C for six days. DSC-TG analyses were performed for all studied forms. The effects in the DSC curves show differences with temperature due to release of weakly bound H2O molecules and strongly bound ones. The endotherm minima temperatures were between 78 and 115 degrees C decreasing in the sequence K- < Na- < Natural- < Ca- <= Mg- < Cd-clinoptilolite. The hydrate complexes around the exchanged cations also influenced the DSC curves. The cation-coordinating H2O molecules and the non-coordinating ones were determined by XRD structural refinement for all exchanged samples. The H2O molecules of the cation-hydrate complexes are released at higher temperatures than weakly bound ones and affected the DSC curves differently. The structural adjustments made by the Rietveld method, as well as the applied EDS analyzes for the chemical composition of the samples, allowed us to correlate these data to the thermal characteristics of the studied clinoptilolite samples.