Reactive transport modeling of multicomponent cation exchange at the laboratory and field scale

As pointed out by Appelo (1996), multicomponent ion exchange models have not been widely used in geochemical or geochemical transport modeling of subsurface processes, despite their recognized importance in regulating soil and sediment pore water compositions. This is also somewhat surprising considering the demonstrated success of ion exchange models in describing sorption and subsurface transport (Ritchie, 1966; Pope et al., 1978; Valocchi et al., 1981a and 1981b; Griffioen, 1993; Appelo and Willemsen, 1987; Appelo, 1994; Lichtner et al., 2004; Steefel, 2004). The key strength of the ion exchange models over “simpler” formulations like the linear distribution coefficient (Kd), the Langmuir isotherm, or even the Freundlich isotherm, is that it captures competitive adsorption behavior through the use of a mass action expression. The total exchange capacity, which can not be exceeded in classical ion exchange, is incorporated as well. Since the ion exchange process involves electrically balanced exchange, complexities associated with the development of surface charge such as appear in the case of electrostatic surface complexation models are avoided. In some cases, it has been necessary to introduce multi-site models to account for the strong dependence of exchange on cation ion concentration (Brouwer et al., 1983; Comans et al., 1991; Poinssot et al., 1999; Zachara et al., 2002; Steefel et al., 2003). Other difficulties have arisen in heterovalent systems where the non-ideality of exchange has required the use of activity coefficients for exchangers (Sposito, 1981; Appelo, 1996; Liu et al., 2004).