Various types of ion-exchange models have been developed to simulate cation adsorption in clay minerals. However, many complex assumptions and parameters are involved in most models. In this study, a strict ion-exchange model was developed to predict alkali cation (i.e. Cs+, K+, Na+, Li+) adsorption in Na-montmorillonite. Cation adsorption envelopes and isotherms were conducted. Then strict ion-exchange model was built, which involves three basic assumptions: (1) all reactions are ion-exchange reactions; (2) all reactions follow mass balance, and (3) charge balance. An IExfit software (http://www.alfisol.com) was specifically developed for the strict ion-exchange model. The strict ion-exchange model simulated the cation adsorption on Na-montmorillonite (NaMt) very well. Two types of cation adsorption sites (site S1 and S2) were predicted from the model. The cation adsorption equilibrium constant K at site S1 was much higher than at S2 for all the cations. The cation adsorption equilibrium constant K increased in the sequence of Li+ &lt; Na+ &lt; K+ &lt; Cs+ at both sites. The Na+ adsorption equilibrium constant K increased in the presence of other co-adsorbing alkali cations at both sites, and the enhancement effect followed the sequence Li+ &lt; K+ &lt; Cs+. This increase in Na+ adsorption strength can be explained by the nano-pore inner-sphere enhancement effect1, 2. X-ray diffraction analysis of the clay minerals showed that NaMt with Cs+ adsorbed had the smallest interlayer d001 spacing compared to other cations, which was consistent with the strong adsorption strength of Cs+. The interlayer d001 spacing suggested that all the cations were partially dehydrated in the interlayer and were probably adsorbed via an inner-sphere mechanism. This study showed that the strict ion-exchange model provides a feasible and efficient tool to simulate alkali cation adsorption on montmorillonite. Moreover, Na+ is a significant competitor in ion-exchange reactions in montmorillonite instead of an inert background electrode. The adsorption strength of Na+ is enhanced by the presence of other alkali cations as is qualified by the increase in its equilibrium constant K values.

Strict ion-exchange modeling for cation selective adsorption on Na-montmorillonite

256th National Meeting (2018)

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