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

25Surface complexation modeling (SCM) has been demonstrated to be one of themost promising methods in modeling sorption. It offers a distinct advantage byrepresenting surface chemical reactions with a set of quasi-thermodynamic constants,which are independent of changes in solution conditions (Hayes, 1987). A number ofstudies have shown that <strong>adsorption</strong> can be described in terms of SCMs accounting for theeffect of pH, ionic strength, and surface-coverage (Hsi and Langmuir, 1985; Hayes andLeckie, 1986; Hiemstra et al. 1989; Dzombak and Morel, 1990; Katz and Hayes, 1995 a,b; Hayes and Katz, 1996; Hiemstra and Riemsdijk, 1996; Sahai and Sverjensky, 1997;Robertson and Leckie, 1997, 1998). One requirement of using SCMs is that controllingreactions and surface species must be known or assumed. The more accurate the surfacecomplex, the more likely the model can predict surface interactions successfully.However, describing ion binding on mineral surfaces in a way that is mechanisticallyreasonable is always a great challenge and discussed below.2.4.1 Surface Reactions and SpeciesIn a review, Sposito (1989) summarized that regardless of the molecular structure,polymeric constituents of natural aqueous colloidal systems present reactive functionalgroups of two kinds: siloxane ditrigonal cavities and inorganic or organic hydroxylgroups. One common practice is to assume that the surface is chemically homogeneousand a two-step protonation reaction accounts for the charging of <strong>metal</strong> (hydr)oxides(Palmqvist et al. 1997; Sahai and Sverjensky, 1997; Davis et al. 1998; van Riemsdijk andHiemstra, 1998). Adsorbate reactions are generally proposed based on <strong>adsorption</strong> edges,isotherms, and/or the number of H+ released (e.g., Forbes et al. 1976; Benjamin andLeckie, 1981a; Hayes and Leckie, 1987; Hayes et al. 1988; Lövgren et al. 1990; Müner