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

12reflects kinetics or mineralogical form. A number of coating studies (Khaodhiar et al.2000; Edwards and Benjamin, 1989; Stahl and James, 1991a, b; Wang, 1995; Fan, 1996)have shown that the degree of coating increased with increasing temperature.Earlier studies (Table 2.2) have involved varying coating temperature, initialapplied <strong>metal</strong> concentration, and silica (with different particle size, surface area,roughness and other characteristics). Although a relationship between the amount ofcoating and temperature was observed in earlier work, no such result was found withrespect to particle size. Most likely, the degree of coating is a function of a number offactors. To systematically evaluate factors that influence the degree of coating,experiments will be designed using a three-level fractional factorial method (Andersonand Mclean, 1974) to reduce the number of studies and isolate effects. Factorsinvestigated will include coating temperature (T), mean diameter of silica particle (D), Feconcentration initially applied (Feini), and aging time (t, for <strong>adsorption</strong> method only).In summary, <strong>adsorption</strong> studies on discrete iron oxides and iron oxide-coatedsilica are important in investigating heavy <strong>metal</strong> distribution in environment. However,for iron oxide-coated silica, factors affecting coating processes have not been fullyevaluated.2.2 Macroscopic Adsorption StudiesMetal sorption to hydrous oxides is affected by the solution pH (Gadde and Laitinen,1974), ionic strength, (Swallow et al. 1980), inorganic and organic ligands present in thesystem (Forbes et al. 1974; Ali and Dzombak, 1996; Weng et al. 2002, Scheinost et al.2001; Sauvé et al. 2000; Brooks and Herman, 1998; Düker et al. 1995; Ledin et al. 1994;