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

967.2 Fe XAS of Iron Oxides and Pb/HFO SamplesThree iron oxides, freshly prepared HFO, goethite (a-FeOOH), and hematite (a-Fe2O3),and two Pb/HFO samples (reacted for 4 hours and 21 months respectively) were analyzedwith XAS to study the crystallization of HFO. In the XANES analysis (Figure 7.6), thespectra of HFO and goethite were adjusted to correct for energy calibration at the Fe K-edge. All samples (Figure 7.6) have a similar pre-edge feature suggesting existence ofdefective FeO6 octahedra (Combes et al. 1989). The more pronounced pre-edge structurefor HFO samples indicates a greater degree of defects. Tetrahedrally coordinated Featoms produce a well-resolved shoulder (at about three fifths of the edge height) beforemaximum edge peak (Combes et al. 1989). The absence of such a feature in the spectraof HFO and its similarity to that of goethite, which exhibits octahedral (FeO6) structure(Szytula et al. 1968), suggest HFO and Pb/HFO samples too have octahedral structure.This result is consistent with other studies (Manceau and Combes, 1988; Combes et al.1989, 1990; Manceau et al. 1992; Charlet and Manceau, 1992; Manceau and Drits, 1993;Waychunas et al. 1993). Compared to hematite, goethite and HFO appear to have lessfeatures in the XANES spectra. Although relatively low in intensity, the shape of theedge crest of HFO is closer to that of goethite suggesting similarities. Also, similar edgecrests were observed for HFO and Pb-sorbed HFO samples (aged for 4 hours and 21months) indicating a similar local structure even as a function of reaction time.The EXAFS spectra (Figure 7.7) of hematite and goethite reveal backscatteringfrom multiple Fe atoms at different distances. The first two RSF peaks of each spectrum(Figure 7.8) can be fitted with O and two subshells of Fe atoms, respectively, for whichthe coordination numbers were fixed based on XRD results (Blake et al. 1966; Gualtieri