Ion adsorption on variable charge materials and thermodynamics of ion exchange

Some of the problems associated with the application of the existing thermodynamics of ion exchange for the description of ion adsorption on variable charge soils are discussed. A major problem is the difficulty in saturating the surface functional groups of variable charge materials with ions other than H⁺ or OH⁻, i.e., the difficulty in obtaining complete exchange isotherms. It was shown that this difficulty could be overcome by choosing appropreate standard states of exchanger phase components. A set of equations that relate the exchanger phase activity coefficients and thermodynamic equilibrium constant to the selectivity coefficient were derived based on a nonconventional asymmetric reference system. The terms related to adsorbed water activity that appear in the derived equations were found to be non-significant for some variable charge materials.