Cation exchange hysteresis

Verburg K., Baveye Ph. (1995) Effect of Cation Exchange Hysteresis on a Mixing Procedure Used in the Study of Clay Suspensions. Clays and Clay Minerals. 

Barrer and Hinds [43] recorded the first observation of hysteresis created by cation exchange in a zeolite. The sodium form of analcime has one water molecule per cation in its framework, whereas the potassium form is anhydrous in its homoionic form. This K form is another phase, lcucite, and the hysteresis is a consequence of the co-existence of the two phases within the solid. Lcucite is now classified as a zeolite. 

Extensive intercalation of polar molecules takes place in this substance in an irreversible manner, and marked hysteresis results (Fig. 4.28). The driving force is thought to be the interaction between the polar molecules and the exchange cations present in the montmorillonitic sheets, since non-polar molecules give rise to a simple Type B hysteresis loop with no low-pressure hysteresis. 

In a similar study (Comans et al., 1990), the reversibility of Cs+ sorption on illite was studied by examining the hysteresis between adsorption and desorption isotherms and the isotopic exchangeability of sorbed Cs+. Apparent reversibility was found to be influenced by slow sorption kinetics and by the nature of the competing cation. Cs+ migrates slowly to energetically favorable interlayer sites from which it is not easily released. 

Examples of K+ and NH4 adsorption-desorption plots for the two-cation system (K-Ca NH4-Ca) and the three-cation system (K-NH4-Ca) are shown in Figure 4.42. These data show that there is a significant hysteresis or relaxation effect in the 24-hr desorption process. This hysteresis-relaxation effect in the case of K-Ca exchange is more pronounced without added NH4 than with added NH4 in the desorbing solution. 

A first stage of a systematic study of the natme of the low-pressure hysteresis loop observed in the nitrogen adsorption isotherms of some MFI zeolites is presented. It was shown that the pressure at which occurrence of this hysteresis loop takes place was linked to the presence of defect sites. The role of the exchangeable cation was also investigated, and it was suggested that in addition to the numbers of defect sites present, the strength of adsorbate-adsorbent interaction, and probably pore shape was also involved. 

Verburg K., and P. Baveye. 1994. Hysteresis in the binary exchange of cations on 2 1 clay minerals A critical review. Clays Clay Miner. 42 207-220. 

A fuller consideration of the normalisation, and its theoretical justification, can be found in Barrer et al. [10]. As emphasised earlier, to apply thermodynamic analyses to isotherm data it is imperative that the process be demonstrably reversible. In some zeolites the path of the reverse isotherm deviates markedly from that of the forward exchange (Fig.3). This is described as hysteresis and can arise from small errors in analysis, but often it is a consequence of changes in occupancy of heteroenergetic cation sites created by drying the zeolite samples prior to their use in generating the reverse curve. Other causes are the... 

Hysteresis is observed for a wide range of cations, from K (Figure 2.22) at ion exchange to heavy metals Cu, Cd (Figure 2.23), As, Sr, Cs, etc. Obviously, the property of hysteresis exists for linear and nonlinear kinetic reactions. 

In this presentation we discuss the effect of the presence of small amounts of template and of the nature of the exchangeable cation on the shape of the nitrogen adsorption isoOiecm of ZSM-S zeolite samples. It was found that for some exchangeable cations a low-pressure hysteresis loop is shown for Si A1 ratio of 120 but not 80, An explanation of the conditions under which the loop appears is given. 

Exchangeable Cation Heat Treatment ImY Pressure Hysteresis... 

It is evident that fire Si Al ratio is one ctor affecting the occurrence of the low-pressure hysteresis loop. It is fiuther evident that the rrature of the exchangeable cation also affects the shape, including the occurrence of a hyster is loop, of the isotherm at low partial pressure. It is now further shown tirat the presence of small amounts of impurities, such as remnants of... 

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