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

CHAPTER 6SYNTHESIS AND CHARACTERIZATION OF IRON OXIDE-COATED SILICAAND ITS EFFECT ON METAL ADSORPTIONTwo methods were used for producing goethite-coated silica, a modified <strong>adsorption</strong>method based on Szecsody et al. (1994) and a modified precipitation method based onMeng and Letterman (1993a). To evaluate factors that influence the degree of coating,experiments were designed using a three-level fractional factorial method (Anderson andMclean, 1974) to reduce the number of studies and isolate effects (Table 6.1). Factorsinvestigated include coating temperature (T), mean diameter of silica particle (D), Feconcentration initially applied (Feini), and aging time (t, for <strong>adsorption</strong> method only).6.1 Synthesis of Goethite-Coated SilicaThe Fe., before and after the abrasion study (Figure 6.1) demonstrated thatapproximately 0.5 to 11 mg Fe g-1 silica detached from the coating constituting as muchas 80% of the initial loading. Scheidegger et al. (1993) separated coated sand withstrongly bound goethite from weakly attached goethite aggregates by washing the systemin a 1 M NaNO3 solution. Stenkamp and Benjamin (1994) and Khaodhiar et al. (2000)washed the iron oxide-coated sand with DI water until the supernatant pH was about 7.Others (Edwards and Benjamin. 1989; Zachara et al. 1995; Kuan et al. 1998; Joshi andChaudhuri, 1996) have rinsed until the runoff was visually clear. Abrasion is necessarybefore quantifying the degree of coating, and coating characteristics discussed below arethose measured after the process.62