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

95previous studies. Bargar et al. (1997b) proposed that the expected Pb-Fe distances are2.91-3.49 A for bidentate <strong>adsorption</strong> to edges of FeO6 octahedra. Therefore, EXAFSanalysis of Pb/HFO samples suggests that upon <strong>adsorption</strong>, Pb ions lose their waters ofhydration and form mononuclear bidentate complexes at the edges of FeO6 octahedra.Pb/HFO sorption samples were prepared at pH 5 and 6, ionic strength from 10 -3 to10 -2, loading from 104 to 10-3 mole Pb g-1 HFO, and reaction time of 4 hours to 21months; the similarity in x(k)-k3 spectra and the fitting results of all these samplesindicate similar local structures. Therefore Pb <strong>adsorption</strong> to HFO may be described withone average type of mechanism under the experimental conditions tested. This result alsosuggests that Pb(II) forms similar surface complexes on the micropore walls as on theexternal particle surface as 23% of additional sites were occupied during intraparticlediffusion. In other words, internal and external <strong>adsorption</strong> sites are approximatelyequivalent. One assumption in earlier modeling efforts (Axe and Anderson, 1995, 1997;Trivedi and Axe, 2001a; Fan et al. 2005) was that the <strong>adsorption</strong> sites located along themicropore walls are no different from ones on the external surface; this assumption isverified in this current Pb/amorphous HFO EXAFS study.The conversion of HFO to goethite and hematite has been reported to be slow attemperatures below 20 °C, and is further retarded by adsorbed species (Schwertmann etal. 1985; Cornell, 1987; Cornell et al. 1987). The effect of Pb(II) on the transformationof HFO to goethite or hematite during the long-term CBC studies is addressed in thefollowing section.