Metal Selectivity by Clay Minerals Using HSAB Principle.

Metal-clay features are the foundation of all interactions ranging from organo-metal-clay assemblages to larger soil structural properties to engineering applications. Commonly used bulk analysis methods such as CEC and isotherm studies provided useful information but are limited in their predictive abilities. Recent molecular level studies have offered valuable information about the atomic environments of metal adsorption.

Many investigators using XRD and XRA have found metal ions to coordinate with clays and water molecules in an octahedral and tetrahedral fashion. The type of interaction is sensitive to metal ion, clay mineral, exchanged cation and solution environment. Imaging techniques such as SEM and TEM have provided that metal adsorption onto clays occurs by homogenous adsorption with areas of localized concentration. The above instrumentation provides insight into metal-clay interaction but is limited due to their inability to measure valance properties. Additional information can be provided by PEEM on electronic valance states, which is of particular interest to the proposed research in its ability to measure the ionization energy (IE) and electron affinity (EA) of substances.

Previous studies have shown many inconsistencies surrounding metal adsorption by clay minerals, particularly in selectivity sequences. Theoretical models provide insight into molecular level interactions, but often require parameters that are not measurable. We propose to use the HSAB principle for prediction of clay-metal interactions that rely on measurable properties of valance states. Using PEEM to measure the IE and AE of clays the chemical hardness of clays can be calculated. Input of the chemical hardness into a modified Gibbs free energy equation will allow evaluation of the enthalpic parameter in metal-clay interactions, which can be use to predict metal selectivity by clays and increase our knowledge of clay mineral atomic scale properties that is central to understanding metal-clay mineral behavior.