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

72goethite and silica results in only the two distinct spectra (Figure 6.5). In attempt tounderstand the oxide-silica interfacial interactions, other samples coated by precipitation(pH 12) or with differing temperature (i.e., 35, 90, and 110 °C) were evaluated; becauseof the lower iron concentrations, however, only one goethite band was discernible in thedifference spectra. Interactions between silica and the oxide coatings potentially involveelectrostatic, van der Waals, and/or chemical bonds. Goethite was coated at pH 12 (as inprecipitation method), where both goethite and silica are negatively charged, suggestingelectrostatic repulsion. Furthermore, fluid shear from the abrasion study is approximatelyone to two orders of magnitude greater than van der Waals forces. Therefore, theseresults along with FTIR suggest the iron oxide-silica interaction may involve chemicalforces.Optically, red-brown goethite coatings were observed to be non-uniform andmostly located in concave rough areas (even before abrasion) (Figure 6.6), indicating thatrougher areas of silica may result in higher loadings of coating material. ESEM images(Figure 6.7) reveal needle-shaped goethite crystals located mostly in rough areas as well.The precipitation method produced smaller crystals of goethite (< 150 nm) than the<strong>adsorption</strong> method (100-1000 nm) (Figure 6.7), and is somewhat consistent with theextraction analysis in that ferrihydrite was present suggesting the presence of silica mayinhibit crystal growth. Elemental mapping from EDX analysis (Figure 6.8) shows Feconcentrating in rough areas, and the relative distribution of Fe and Si demonstrates anon-uniform coating on the silica surface, which is in agreement with others (Edwardsand Benjamin, 1989; Scheidegger et al. 1993).