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

129was highly correlated to site density. Based on residuals, all five sets provide equivalentfits to the titration data. Hayes et al. (1991) also found multiple solutions thatsuccessfully replicated acid-base behavior of a-FeOOH, a-Al2O3, and TiO2. Koopal et al.(1987) suggested that for crystalline oxides with few defects small ApKa values areneeded. Also molecular statics calculations have been performed to estimate stabilityconstants for proton binding to goethite (Rustad et al. 1996); 19 out of 26 intrinsic logKavalues were found to range between 7.7 and 9.4 where the pHpzc was calculated to be8.9. A ApKa value of one was chosen in this study. The model described theexperimental titration data between pH 4 and 10 very well (Figure 9.1). This set ofparameters was therefore used in modeling <strong>adsorption</strong>.9.2 Ni Adsorption on GoethiteIn this work, the objective was to model both <strong>adsorption</strong> edges and isotherms. Studies(Balistrieri and Murray, 1982; Rodda et al. 1993) have shown that parameters derivedfrom edge (pH dependence) modeling failed to account for the concentration dependenceof sorption; Palmqvist et al. (1997) and Dyer et al. (2003) also strongly emphasize theimportance of evaluating surface complexation models from data reflecting concentrationranges as wide as possible.Ni <strong>adsorption</strong> isotherms on goethite were conducted from pH 4 to 7 (Figure 9.2).Although <strong>adsorption</strong> increases with pH, isotherms between pH 4 and 5 are almostequivalent and within the experimental error. This result may be due to very low<strong>adsorption</strong> and thus the error is large. Overall, Ni <strong>adsorption</strong> increases significantlybetween pH 5.5 and 6.5. These isotherms also show that below 10 -6 M, the sorbed