5 Adsorption Models Incorporated into Chemical Equilibrium Models

Adsorption is the net accumulation of a chemical species at the interface between a solid phase and an aqueous solution phase, leading to a loss from the solution phase (Sposito, 1989). Adsorption reactions can be described by various models. Empirical models provide descriptions of adsorption data without theoretical basis. Examples are the distribution coefficient, the Freundlich adsorption isotherm, and the Langmuir adsorption isotherm. Chemical models provide a molecular description of adsorption using an equilibrium approach. Surface complexation models are chemical models designed to calculate values of thermodynamic properties mathematically. Examples are the constant capacitance model (Stumm et al., 1980) the diffuse layer model, and the triple layer model (Davis et al., 1978). MICROQL (Westall, 1979) and FITEQL (Westall, 1982) are computer programs that have been used to optimize parameters for the surface complexation models. Equilibrium speciation models that contain the surface complexation models are MINTEQ (Felmy et al., 1984; Allison et al., 1990), SOILCHEM (Sposito & Coves, 1988) and HYDRAQL (Papelis et al., 1988). Transport models containing surface complexation models include TRANQL (Cederberg et al., 1985), HYDROGEOCHEM (Yeh & Tripathi, 1990), and the model of Jennings et al. (1982). The purpose of this article is to review the common adsorption models and their incorporation into chemical equilibrium models. Model characteristics such as surface species, chemical reactions, equilibrium constant expressions, and surface activity coefficients will be described. Adjustable parameters and numerical methods for obtaining their values will be discussed. Limitations in the application of models to soil systems will be indicated.