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

27Overall, surface complexation modeling is heavily dependent on the accuratechemical and physical representation of surface complexes and the oxide surface (Hayesand Leckie, 1987; Cowan et al. 1991). Spectroscopic studies can provide qualitative andquantitative information on <strong>adsorption</strong> modeling, connecting molecular details tomacroscopic measurements and significantly improving the ability to model thesolid/liquid interface (e.g., Katz and Hayes, 1995 a, b; Hayes and Katz, 1996; Dyer et al.2003, 2004).2.4.2 Surface Complexation Models and Their ApplicationsA variety of SCMs have been used for predicting sorption of <strong>metal</strong> ions and other specieson minerals (Westall and Hohl, 1980; Hayes and Katz, 1996; Lützenkirchen, 2002).Although these models may be expressed with similar mass law and material balanceequations, they involve different descriptions of the electrical double layer (Westall andHohl, 1980). The non-electrostatic model (NEM) neglects the electrostatic effects; thediffuse layer model (DLM) uses the Gouy-Chapman equation for the relationshipbetween surface charge and potential in the interfacial region; the constant capacitancemodel (CCM) has one sorption plane with an inner-layer capacitance, C 1 ; and the triplelayer model (TLM) incorporates the full Gouy-Chapman-Stern-Grahame equation for theinterfacial region with two planes for surface complexation, an inner- and outer-layercapacitance, C1 and C2 (Westall and Hohl, 1980; Hayes and Katz, 1996). The DLM andCCM are often limited to lower and higher (>0.01 M and constant) ionic strengthconditions, respectively. The TLM can be applied to a wide range of ionic strengths withan upper limit of applicability being that for the Davies Equation (Davies, 1962), which isemployed to account for activity corrections (Hayes et al. 1991).