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

118be fit using either Fe or Ni atoms. Distinguishing between Fe and Ni atoms solely basedon EXAFS fitting is difficult, because of the difference in atomic number, because theyhave similar ionic radii (0.65 vs. 0.69 A, Richens, 1997), and because they form similarlocal structures — octahedral (Combes et al. 1989; Pandya et al. 1990; Scheidegger et al.1996). The Ni/HFO coprecipitation sample provides a good comparison. Thedifferences in x(k)•k 3 spectra between the sorption and coprecipitation samples (Figure8.4) suggest that the second shell in the former may be composed of Fe atoms. Also, asthe Ni loading increases, the (potential) number of Ni atoms does not (Table 8.3); theseresults are not self consistent for polymers or precipitates where the Ni coordinationnumber would increase as the structure grows. Therefore, the second shell is most likelyan Fe shell and the formation of a Ni polymer or Ni(OH)2 is ruled out.Using Fe in the second shell resulted in 3-7 O atoms at 2.03-2.06 A and 1-3 Featoms at 2.92-3.05 A. These results are consistent with two-shell fitting of acoprecipitated Ni/goethite sample (Manceau et al. 2000), where 5.3 O atoms at 2.07 Aand 2.1 Fe atoms at 3.00 A were obtained. However, two outer shells were also observedby Manceau et al. (2000) of 1.4 Fe atoms at 3.18 A and 2.5 Fe atoms at 3.62 A,suggesting substitution of Ni 2+ for Fe3+ in the goethite matrix. The absence of multipleFe shells in our study indicates Ni(II) ions form surface complexes with HFO. Althoughthe coordination number of the 8-month sample (3.36) may suggest tetrahedral structurefor Ni, the Ni-O distance of 2.03 A is consistent with octahedral structure. Additionally,the Ni-Fe distance of 2.92-3.05 A suggests formation of bidentate edge-sharingcomplexes. A more recent study (Strathmann and Myneni, 2005) reported similar surfacespecies (mononuclear bidentate edge-sharing surface complexes) for Ni 2+ sorption on