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

6is further retarded by adsorbed species (Scheinost et al. 2001; Schwertmann et al. 1985;Cornell, 1987; Cornell et al. 1987, 1992). Given localized increases in ferricconcentrations, its precipitation is accompanied by a high partitioning of trace <strong>metal</strong>s insoils (Manceau et al. 2003). Therefore, goethite and HFO have been widely used asmodel sorbents in <strong>adsorption</strong> studies (Cornell and Schwertmann, 1996). Dzombak andMorel (1990) comprehensively compiled and modeled <strong>metal</strong> <strong>adsorption</strong> on HFO usingthe diffuse double layer model. However, since 1990, spectroscopic evidence has provento be critical in mechanistic model development.In natural aquatic environments, many reactions such as dissolution, <strong>adsorption</strong>,precipitation, and colloidal adhesion result in multi-component solids with bulk andsurface characteristics different from the original pure phases (Anderson and Benjamin,1990). Specifically, oxide mineral coatings are often observed in soils and sediments(Ryan and Gschwend, 1994; Coston et al. 1995; Wang et al. 1993). For example, Ryanand Gschwend (1994) found that in oxic sediments, quartz grains are coated by colloidalkaolinite and microcrystalline iron (III) oxide, primarily goethite (a-FeOOH) at 39 ± 2μmol Fe g-1 sediment (0.22% by weight). Coston et al. (1995) reported that in a shallow,unconfined aquifer of glacial out wash, sand and gravel were extensively coated with AlandFe-bearing minerals and that the iron concentrations ranged between 0.0736 and 4.96mg Fe g-1 sand. Iron oxide coatings were also found on ped surfaces and pores of paddyand non-paddy soils at concentrations of 10 to 44 mg Fe g -1 soil and were characterizedas ferrihydrite, lepidocrocite, and goethite (Wang et al. 1993). Many studies have shownthe importance of these surface coatings in controlling <strong>metal</strong> distribution in soils andsediments (Edwards and Benjamin, 1989; Coston et al. 1995; Zachara et al. 1995; Fuller