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

106of pH, ionic strength, Pb loading, and reaction time. Pb <strong>adsorption</strong> on HFO is a two-stepprocess — a fast initial uptake followed by a slow surface diffusion step. The <strong>adsorption</strong>mechanism does not appear to change with reaction time (as long as 21 months)suggesting that internal sites on micropore walls are no different than the external ones.Intraparticle diffusion modeling based on this observation described the Pb/HFO longtermsorption data reasonably well. The surface diffusivity obtained from the modelingverifies a slow and rate-limiting diffusion process, which may occur for decadesaccounting for approximately 46% of total Pb uptake.XAS results suggest Pb(II) forms similar surface complexes on a series of ironoxides exhibiting comparable surface structures and functional groups — amorphous HFO(this study and Manceau et al. 1992 for difference contact time), 2-line ferrihydrite(Scheinost et al. 2001; Trivedi et al. 2003 [for pH > 4.5]), goethite, and hematite (Bargaret al. 1997b). It is possible that sorption mechanisms of other heavy <strong>metal</strong>s also arecontrolled by the sorbent surface structures and similar surface complexes will form onHFO and goethite. Therefore, in the next chapter, XAS analysis on Ni/HFO sorptionsamples is presented and is followed by Ni/goethite modeling.