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

102(hematite, goethite, akaganeite, and lepidocrocite), edge-sharing and corner-sharingoctahedra produce an Fe-Fe distance ranging 2.97-3.28 A and 3.37-3.87 A, respectively(Charlet and Manceau, 1992). The 2.95 A Fe-Fe distance for HFO sample (Table 7.4)suggests the presence of face and edge-sharing polymer in HFO without corner-sharing.The x(k)•k3 spectra (Figure 7.7), RSF (Figure 7.8), and the fitting results (Table7.4) of 4-hour and 21-month Pb/HFO samples are similar to those of HFO. Fits were notobtained with Pb in the second shell, therefore Pb(II) ions do not appear to substitute intoFe positions in freshly prepared HFO even with Pb/Fe molar ratio of 0.37 (correspondingto 3.41 x 10 -3 mole Pb g-1 HFO) or aged for 21 months. The similarity in EXAFS resultsof 4-hour and 21-month Pb/HFO with pure HFO suggests that HFO does not transforminto a more crystalline form in the presence of Pb(II) during aging up to 21 months. Theadsorbed Pb(II) ions binding to the edges of HFO polymers may block the edge andcorner-sharing between HFO polymers thus inhibiting crystal growth. This observationis important for situations where <strong>adsorption</strong> occurs not long after ferric oxideprecipitation (possibly in wastewater treatment), the large binding capacity of theamorphous oxide is maintained resulting in more effective removal of Pb (II).7.3 Intraparticle Surface Diffusion ModelingResults for Pb/HFO <strong>adsorption</strong> at pH 5 (Figure 7.9) show that approximately 72% sorbedwithin 30 min (Figure 7.9 insert). This fraction represents that sorbed to the externalsurface. The remaining Pb was sorbed through a much slower process. As discussedearlier, this second step was observed by maintaining a constant boundary condition withthe Pb(II) in the bulk aqueous phase and therefore on the external surface. This slow