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

100and Venturelli, 1999). Overall the fitting results are consistent with XRD data (Table7.4), except the distance of the second Fe subshell for hematite is about 0.18 A shorterthan the theoretical value (3.36 A). There are multiple Fe-Fe distances for hematite andgoethite (Table 4); the fitting results represent an averaged local structure. The beatfeatures in the x(k)•k 3 data at about 5-5.6, 6.2, and 8.2 A 1 in the hematite spectrum and3.8-4.4, 8.3-8.7, and 10.4-11.2 A -1 in the goethite spectrum represent contributions fromFe atoms beyond the second shell (Fe shell located at 2.93-3.29 A). For hematite andgoethite, the edge-sharing of FeO6 octahedra produces Fe-Fe distances of 2.97-3.28 Aand that for double corner-sharing is 3.37-3.70 A (Charlet and Manceau, 1992).Therefore, the beat features in the spectra suggest the presence of double corner-sharingbetween FeO6 octahedra, which is necessary for crystal growth.For HFO, many studies have reported an octahedral coordination environment(Manceau and Combes, 1988; Combes et al. 1989, 1990; Manceau et al. 1992; Charletand Manceau, 1992; Waychunas et al. 1993). In this study, the spectra (Figure 7.7) ofHFO show beats at 4.5, 5.2, 7.5, and 9.2-9.9 A -1 indicative of Fe atoms around the centralone. The RSF (Figure 7.8) reveals two well-defined peaks that can be fitted with O andFe atoms, respectively. Considering error, the fitted Fe-O distance is similar to those ofhematite and goethite (Table 7.4). The Fe-Fe distance is in between those of hematiteand goethite indicating existence of face-sharing octahedra, which produces the shortestFe-Fe distance of 2.90 A in hematite while there is no such sharing in goethite (Charletand Manceau, 1992). Manceau and Combes (1988) also observed the presence of facesharingFe(O,OH) 6 octahedra in HFO which necessitates the dissolution-reprecipitationmechanism in the transformation from HFO to goethite. In crystalline iron oxides