Adsorption of Charged Ionic Compounds

Cation exchange and selectivity are processes involving the cationic concentration in solntion, the cation dimensions, and the confignration of the exchange sites. For cationic molecules, the retention properties follow the relation 

When cations of differing charge are involved, the exchange equation (Eq. 2.44) is modified. Gapon (1933) proposed that in the case of an exchange between mono-and bivalent cations, the expression should be 

The selectivity of the exchange between two cations is specific to saline snbsnr-face systems, described by the sodium adsorption ratio (SAR). Based on Na content in soil solutions, the U.S. Salinity Laboratory (U.S. Salinity Laboratory 1954) derived the equation 

Metal cation adsorption processes include exchange, Coulombic, and site-specific adsorption. Heavy metal cations exhibit exchange reactions with negatively charged surfaces of clay minerals. Cationic adsorption is affected by the pH and in an acid environment (pH 5.5), and some heavy metals do not compete with Ca (a ubiquitous constituent in the subsurface) for mineral adsorption sites. At a higher pH, heavy metal adsorption increases abruptly and becomes irreversible. 

Negative adsorption occurs when a charged solid surface faces an ion in an aqueous suspension and the ion is repelled from the surface by Coulomb forces. The Coulomb repulsion produces a region in the aqueous solution that is depleted of the anion and an equivalent region far from the surface that is relatively enriched. Sposito (1984) characterized this macroscopic phenomenon through the definition of the relative surface excess of an anion in a suspension, by 

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Chapter 2 mentioned that the adsorption of charged ionic compounds on the solid phase is a result of a combination of chemical binding forces and electric fields at the interface. Here, we extend the discussion on this topic, focusing mainly on aspects relevant to behavior of ionic contaminants in the subsurface environment. 

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