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

105theoretically sorbed and the experimental data. Modeling results for Pb/HFO CBCexperiment are shown as an example (Figure 7.9). The experimental data fall within twostandard deviations of the model suggesting that the diffusion model fits data reasonablywell. The best-fit D s was 2.5x10- 15 cm2 s-1 suggesting a slow and rate-limiting processfor Pb sorption to HFO. Results from many studies support that intraparticle diffusion isthe rate-limiting step in <strong>metal</strong> sorption to microporous oxides (Waychunas et al. 1993;Theis et al. 1994; Axe and Anderson, 1995, 1997; Papelis et al. 1995; Strawn et al. 1998;Scheinost et al. 2001; Trivedi and Axe, 2001a; Strathmann and Myneni, 2005; Fan et al.2005). The D s of this study (2.5x10-15 cm2 s -1) is consistent with the range obtained byFan et al. (2005) for Pb sorption in HAO, HMO, and HFO (10-16 to 10-14cm2 s-1..) Basedon this surface diffusivity and boundary condition, it will take more than 20 years for Pbsorption to approach equilibrium where the internal sites account for — 46% of the totaluptake.7.4 SummaryPb(II) <strong>adsorption</strong> mechanisms on freshly precipitated HFO as a function of time havebeen studied. XAS results reveal that the freshly precipitated HFO exhibits short-rangeorder possibly forming edge and face-sharing polymers. The sorbed Pb(II) ions do notsubstitute for Fe in the polymer structure. However, by binding to the edges of octahedrapolymers, Pb(II) ions inhibit crystallization of HFO, which is stable up to 21 months ofaging, thus maintaining its large capacity for Pb(II). EXAFS analysis of Pb(II)-sorbedamorphous HFO samples suggests that Pb(II) ions form mononuclear bidentate edgesharingsurface complexes on FeO6 octahedra. This mechanism appears to be invariant