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

910.08 A. This octahedral structure represents that of fully hydrated Pb(II), as greater than99% of the total Pb exists as Pb 2+ in this aqueous sample. This result is consistent withother studies (Bargar et al. 1997a; Elzinga and Sparks, 2002).All spectra of Pb/HFO sorption samples are different from those of PbO, PbCO3,and aqueous Pb(NO3)2 (Figure 7.1 and Figure 7.2), suggesting unique structures fromthese standards. Overall, the spectra are dominated by the backscattering from a firstshell of O atoms and beat features at 4.5 — 5.0 A 1 and 6.5 — 8.5 A-1 indicative of a heavierbackscatter from second shell contributions. The x(k)•k 3 spectra were Fouriertransformed over the range 2.8 — 9.5 A -1 and fitted over the range 0.45 — 3.58 A with aFe-substituted PbO model generated theoretically using FEFF7 (Zabinsky et al. 1995).The RSFs show two peaks at —1.9 A and —2.9 A (Figure 7.4 and Figure 7.5), and fitting(Table 7.3) reveals 1.27 — 2.92 O atoms at 2.25 — 2.30 A for the first shell. The secondshell could only be fit with Fe atoms and resulted in 0.92 — 2.05 Fe atoms at 3.29 — 3.34A. The experimental amplitude and phase were reproduced by the fitting (Figure 7.4 andFigure 7.5). However, the larger Debye-Waller factor for the second shell (Table 7.3)suggests greater variation in the Pb-Fe distances than those for Pb-O.The absence of Pb backscatters around the central atom suggests that the adsorbedPb(II) ions on HFO are isolated mononuclear complexes. The absence of O as a secondshell and the tighter Pb-O distances as compared to the hydrated form observed indicatethat the waters of hydration were lost upon <strong>adsorption</strong> to HFO. Other EXAFSinvestigations (Manceau et al. 1992; Scheinost et al. 2001; Trivedi et al. 2003) reportedthat the adsorbed Pb(II) on ferrihydrite results in two O atoms at 2.3 A and less than oneto two Fe atoms at around 3.3 A. The fitting results of this study are similar with those