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

23concentration in their system was about one order of magnitude lower than in otherstudies where LDH precipitates were observed (Scheidegger et al. 1996, 1998; Scheinostet al. 1999; Elzinga and Sparks, 1999; Roberts et al. 1999; Nachtegaal and Sparks, 2003).Previous work with Ni and iron oxides has been limited to coprecipitation with goethiteand hematite; Ni 2+ was found to substitute for Fe 3+ at 3% and 6% (molar ratio) loadings,respectively (Manceau et al. 2000; Singh et al. 2000). Little has been reported on Ni<strong>adsorption</strong> to iron oxide surfaces. Although a number of researchers (Dzombak andMorel, 1990; Coughlin and Stone, 1995; Buerge-Weirich et al. 2002) have modeled Ni<strong>adsorption</strong> on goethite using SOM+ surface species, molecular-scale analyses are neededto corroborate this mechanism.Overall, the mobility, bioavailability, and toxicity of heavy <strong>metal</strong>s are greatlycontrolled by <strong>adsorption</strong> reactions taking place at the oxide/water interfaces. XASanalysis is an effective tool in deciphering mechanisms at the surface. Theseinvestigations demonstrate how the structure of sorbent, through the nature of the reactivesurface sites, may control the mobility and speciation of <strong>metal</strong>s in the environment(Manceau et al. 1992). Macroscopic observations can be more confidently explained,and as is shown by Trivedi (2001) and Dyer et al. (2003), XAS derived structuralinformation may be combined with macroscopic studies to predict heavy <strong>metal</strong> mobilityin aquatic/soil environments.2.4 Adsorption Modeling — Surface Complexation ModelsMany researchers used traditional Langmuir (e.g., Pierce and Moore 1980;Padmanabham, 1983; Jackson and Inch, 1989; Miller et al. 1989; Kooner, 1993;