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

131concentration is linearly related to the bulk aqueous concentration suggesting one type ofsite. Palmqvist et al. (1997) did not observe a set of "high-affinity" surface sites in thesorption of Pb, Cd, Zn, and Cu onto goethite below a concentration of —10-6 M.However, the slope of the isotherms became less than one for Ni concentrations greaterthan 10-6 M, suggesting surface site heterogeneity (Robertson and Leckie, 1997). Theslight curvature seen in this same range of the isotherm may indicate an approach tosaturation of the stronger binding sites. These asymptotes may correspond to site densityvarying with pH. Trivedi et al. (2001b) also observed that site density is a function of pH.Surface heterogeneity has to be taken into account in modeling isotherms over theentire range of Ni concentrations tested. However, as discussed earlier, only one type ofsite was observed in XAS analysis. Because XAS data are averaged over the entiresurface, there may be a small fraction of other site types present, which are not detectedin the analysis. Benjamin and Leckie (1980, 1981b) proposed that the oxide surfaceexhibits a spectrum of binding energies; as the surface binds more ions, the higher energysites may become limiting and the overall binding energy of the surface decreases.Although different binding energies exist on the goethite surface, the reactionstoichiometry proposed may still reflect an equivalent surface complex. A two-sitemodel was used as employed by Dzombak and Morel (1990). XAS derived (SO)2Nicomplexes were applied for both high affinity (KH) and low affinity (KO sites.Surface complexation modeling resulted in seven best-fit sets of parameters(Table 9.2), which correspond to each constant-pH Ni <strong>adsorption</strong> isotherm. The sitedensity of high affinity sites (NSH) increased with pH to attain convergence; these wereconstrained from the isotherm studies. Overall, the TLM accurately reproduced the