Ion adsorption on variable charge materials and thermodynamics of ion exchange

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Abstract
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.