Heavy metal adsorption on iron oxide and iron oxide-coated silica ...

2Anderson and Christensen, 1988; Cowan et al. 1991; Ryan and Gschwend, 1994; Costonet al. 1995; Jenne, 1998; Barnett et al. 2002). As a multicomponent-multiphase system,the natural aquatic environments can only be described properly when competitivereactions/processes are considered (Nilsson et al. 1996).Adsorption of <strong>metal</strong> ions in aqueous solutions to mineral surfaces can bedescribed in terms of surface complexation models (Hsi and Langmuir, 1985; Hayes andLeckie, 1986; Hiemstra et al. 1989; Dzombak and Morel, 1990; Katz and Hayes, 1995 a,b; Hiemstra and van Riemsdijk, 1996; Sahai and Sverjensky, 1997; Robertson andLeckie, 1997, 1998). For the description of <strong>metal</strong> binding on surfaces, a series ofreaction equations need to be formulated (van Riemsdijk and Hiemstra, 1998). Sposito(1989) has summarized the principal mechanisms of <strong>adsorption</strong> as (1) inner-spherecomplexation, (2) outer-sphere complexation, and (3) association with a diffuse layer; therole of these three mechanisms in diverse reactions as well as their contribution to surfacecharge development have also been reviewed (Sposito, 1989). The more accurate themodel the more likely the prediction of <strong>metal</strong>/mineral interaction will be successful.However, describing ion binding on mineral surfaces in a way that is mechanisticallyreasonable is always a great challenge.One common practice is to assume that the surface is chemically homogeneousand to invoke a two step protonation reaction as the basis for the charging of <strong>metal</strong>(hydr)oxides (van Riemsdijk and Hiemstra, 1998; Sahai and Sverjensky, 1997; Palmqvistet al. 1997; Davis et al. 1998); ion binding reactions are generally proposed based oninformation from <strong>adsorption</strong> edges, isotherms, and/or number of H ± released (e.g., Forbeset al. 1976; Benjamin and Leckie, 1981a; Hayes et al. 1988; Lövgren et al. 1990; Müner