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

28Surface complexation models have successfully described acid-base properties ofoxides (Hayes et al. 1991; Sahai and Sverjensky, 1997) and <strong>metal</strong> binding with variousmineral surfaces (Robertson et al. 1997, 1998; Katz and Hayes, 1995 a, b). For example,Dzombak and Morel (1990) used a two-site DLM and Hiemstra and van Riemsdijk(1996, 1999) used a charge distribution multi-site complexation (CD-MUSIC) model todescribe <strong>metal</strong> sorption to HFO and goethite successfully. Hayes et al. (1991) usedCCM, DLM, and TLM to fit goethite titration data and found that many combinations ofparameters can successfully replicate acid-base behavior. A unique set of parameters isstill difficult to achieve. Although a set of averaged parameters were used to predict<strong>adsorption</strong> behavior of complex assemblages of different bacterial types (Yee and Fein,2001, 2003), few studies have addressed both isotherm and edge data over a large rangeof conditions with the same model (e.g., Müller and Sigg, 1992; Waite et al. 1994;Heidmann et al. 2005). Furthermore, most models have been derived from <strong>adsorption</strong>edges (Dyer et al. 2003). The applicability of these models may be questionable in theevent there is an effect of adsorbate concentration with more than one type of site (Roddaet al. 1993; Dyer et al. 2003). Even with both edge and isotherm data, a single set of SCMparameters could not predict Pb(II) sorption onto ferrihydrite over a wide range ofconditions, the equilibrium "constant" was adjusted with pH to fit the isotherm data(Dyer et al. 2003). Robertson and Leckie (1998) also could not apply a unique model forCu <strong>adsorption</strong> isotherms with goethite, covering three pH values (4-6) and two ionicstrengths (10 -2-10 1 ).To summarize, surface complexation models can be used to describe <strong>adsorption</strong>data in a thermodynamically consistent way using physically realistic surface species.