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

CHAPTER 2OXIDES AND THEIR EFFECT ON SORPTIONSorption of trace elements onto reactive mineral surfaces is an important processregulating contaminant mobility and bioavailability in natural systems (Forbes et al.1976; Balistrieri and Murray, 1982; Anderson and Christensen, 1988; Jackson and Inch,1989; Coston et al. 1995). This review focuses on the <strong>adsorption</strong> of solute(s) at theaqueous-mineral interfaces. Specifically in this chapter the following topics arediscussed: (1) iron oxides and iron oxide coatings in environment, (2) macroscopic<strong>adsorption</strong> studies, (3) XAS studies, and (4) surface complexation modeling.2.1 Iron Oxides and Iron Oxide Coatings in EnvironmentIron oxides are prevalent in natural environments as particles or surface coatings,exhibiting a high affinity and great capacity for <strong>metal</strong> ions (Schwertmann and Cornell,1991), and therefore impacting <strong>metal</strong> mobility significantly (e.g., Anderson andChristensen, 1988; Jackson and Inch, 1989; Johnson, 1990; Coston et al. 1995; Fuller etal. 1996; Green-Pedersen et al. 1997; Dong et al. 2002). These oxides in the environmentare present as crystalline and amorphous forms including goethite, hematite, andferrihydrite (also is referred to as hydrous ferric oxide, HFO). Goethite is the mostcommon and thermodynamically stable iron oxide under most soil conditions(Schwertmann and Taylor, 1989; Ainsworth et al. 1989). Although metastable,ferrihydrite is also an important sorbent because of its abundance in the subsurface.Ferrihydrite conversion to goethite and hematite is slow at temperatures below 20 °C, and5