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

133isotherm data (Figure 9.2). The results suggest that although polynuclear surfacecomplexes and/or precipitation can account for isotherms with slopes not equal to one(Lützenkirchen and Behra, 1994; Katz and Hayes, 1996), multisite <strong>adsorption</strong> isapplicable when supported by spectroscopic data. More than one type of surface site hasbeen experimentally observed on ferric oxyhydroxide (Bottero et al. 1993) and supportedin crystallographic analysis (van Riemsdijk and Hiemstra, 1999). As Robertson andLeckie (1997) note, even when slopes equal one, this does not guarantee that only onetype of site is present; it is more likely that several site types exist.Even if site density is a function of pH, it appears that at least in this study, thehigh affinity site density falls within an order of magnitude of each other. Therefore, interms of applying the TLM, a single set of parameters describing both <strong>adsorption</strong> edgeand isotherm data would be more practical. Adsorption edges (Figure 9.3) are consistentwith isotherm data, the % Ni sorbed increased significantly from pH 5 to 6.5 at low Niconcentrations (Figure 9.3); however, at the highest [Ni]0 (Fig. 3a) the typical sigmoidcurve is not observed potentially due to site saturation. All seven sets of TLM parametersderived from isotherms were used to predict edge data (Figure 9.3). Overall, only the pH6.5 set of parameters was found to predict all edge data reasonably well (represented bythe solid line in each edge plot in Figure 9.3). These parameters were further tested onthe isotherms, and results (Figure 9.4) indicate that this one set of parameters successfullymodeled <strong>adsorption</strong>. Therefore, a single set of TLM parameters was found to describe Ni<strong>adsorption</strong> on goethite over a broad range of conditions, covering almost seven orders ofmagnitude in concentration (10 -10 to 10-3 M), ionic strength from 10 -3 to 10-2, and anenvironmentally relevant pH range from 5 to 6.5.