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

70substrate diameter of 0.2 mm, an initial Fe concentration of 3% (by weight as Fe), and for<strong>adsorption</strong> an aging time of 4 days (Figure 6.4).6.2 Characterization of Iron Oxide-Coated SilicaThe coated silica with the highest iron concentration (21.36 mg Fe g-1 solid, coated by<strong>adsorption</strong> method at pH 7.5 and 60 °C) was analyzed with FUR to investigateinteractions between the substrate and coating. Overall, the spectrum of goethite-coatedsilica is very similar to that of silica (Figure 6.5); therefore a difference spectrum wascalculated based on the approach described by Scheidegger et al. (1993). Despite theweak signal as a result of the low iron concentration, the difference spectrum showedcharacteristic goethite absorption bands at 892, 795, and 455 cm -1 , suggestingunambiguously that goethite was coated on the silica surface. The appearance of the1080 cm-1 peak in the difference spectrum may indicate enhanced asymmetric stretchingof SiO4. The Fe-0 stretching band was found to shift from 640 to 620 cm -1 and anabsorption band at 825 cm -1 appeared in the difference spectrum as well (Figure 6.5).Interestingly, absorption bands at similar positions, 580 and 825 cm -1 (olivine,Christensen et al. 2000), 612 and 848 cm -1 (olivine, Gadsden, 1975), and 566 and 828 cm -1 (fayalite, Gadsden, 1975; Fabian et al. 2001) have been observed for iron silicateminerals. Additionally, other researchers have reported that substitution of Fe 3+ for Si4+caused a substantial shift of the Si-O-Si stretching band (1080 cm -1) to lower frequencies,for example, a shift to 750 cm -1 for a lignitic coal ash slag (Nowok et al. 1993) and 940cm-1 for a ferrihydrite sample precipitated with 4.35% Si by weight (Schwertmann andThalmann, 1976). Comparing the coating spectrum to that of the physically mixed