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

76Furthermore, because discrete goethite particles are much smaller (0.2100 pm,Trivedi and Axe, 2001a) than silica (average 900 pm) and the coatings accumulate inrough concave areas, the coating does not appear to impact the particle size distribution(Figure 6.9). However, the surface area of the coated systems increased by one to twoorders of magnitude (Table 6.2) due to the attachment of fine iron oxide particles.Scheidegger et al. (1993) reported that the surface area of the goethite-coated sand isconsistent with the theoretical sum of the surface areas of the pure silica and thecorresponding weight fraction of goethite. The surface areas of goethite coatings wereback calculated and resulted in 40 m 2 g-1 for the adsorbed and 78 m 2 g-1 for theprecipitated forms. As observed in ESEM images the goethite coating is composed ofsub-micrometer crystals (Figure 6.7) and is not comparable with aqueous aggregates(Trivedi and Axe, 2001a). In this study, although porosity is equivalent for all systems(about 8 — 9%, Table 2), smaller pores were observed with the coated silica versus theuncoated system (Figure 6.9). Intraparticle diffusion is an important process in <strong>metal</strong>sorption to amorphous hydrous ferric oxide (Trivedi and Axe, 2001a), however, with 8%porosity, this phenomenon is not expected to be significant for the goethite-coated silica.Nevertheless, many studies have demonstrated the importance of Fe-oxidecoatings in sequestrating <strong>metal</strong>s (Uygur and Rimmer, 2000; Fuller et al. 1996; Lo andChen, 1997; Khaodhiar et al. 2000; Stenkamp and Benjamin, 1994; Lo et al. 1997).Surface charge characteristics impact <strong>adsorption</strong> capacities (Scheidegger et al. 1993).Therefore, potentiometric titrations were conducted to investigate the surface chargedistribution of uncoated and goethite-coated silica. The pHpzc for the silica shifts to ahigher pH with iron oxide coatings and is between that of pure silica and goethite