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

1208.3 EXAFS Analysis of FeThe x(k)•k3 spectra of discrete, sorbed, and co-precipitated samples (Figure 8.6) revealsimilar structures. Fitting resulted in (Table 8.4) 2-3 O atoms at 2.03-2.04 A and 3-4 Featoms at 2.95 to 2.98 A suggesting edge-sharing FeO6 polymers. Double corner- orsingle corner-sharing with Fe-Fe distances longer than 2.98 A are characteristic ofcrystalline iron oxides (Charlet and Manceau, 1992) but were not observed. Even at thehighest loading of 8.1x10 -3 mole Ni g-1 HFO, no evidence of crystallization was seenwith XRD after aging the sample for 1 year. Ni(II) ions bound to surfaces orcoprecipitated with HFO may have inhibited crystallization of the amorphous iron oxide.Similar results have been observed for the Pb/HFO system (Chapter 7).In studying crystal chemistry of Ni(II) in synthetic goethite (Manceau et al. 2000;Carvalho-E-Silva et al. 2003) and hematite (Singh et al. 2000), Ni(II) was coprecipitatedwith amorphous iron oxides and went through an aging process (3, 14, and 93 days)under elevated temperatures (70-90 °C) and pH (7.5 to —13). Subsequent EXAFSanalysis revealed Ni2+ substitution for Fe 3+. However, the transformation of HFO togoethite and hematite has been reported to be slow at temperatures below 20 °C, and isfurther retarded by adsorbed species (Schwertmann et al. 1976; Cornell et al. 1987;Cornell and Giovanoli, 1988; Paige et al. 1997; Martinez and McBride, 1998). In thisstudy, the HFO samples sorbed or coprecipitated with Ni(II) were maintained under roomtemperature at pH less than or equal to 7. Ni2+ was not observed to substitute for Fe3+possibly because HFO did not undergo long range ordering and therefore the Ni(II)complexes were stable up to 8 months. In other words, Ni 2+ substitution for Fe 3+ mayrequire crystal growth of iron oxides. In soils and sediments, trace <strong>metal</strong>s could be