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

7et al. 1996; Green-Pedersen et al. 1997; Uygur and Rimmer, 2000;). In addition, coatedminerals have been studied in recent years because of their potential application aseffective sorbents (Coston et al. 1995; Zachara et al. 1995; Lo and Chen, 1997; Kuan etal. 1998; Khaodhiar et al. 2000; Meng and Letterman, 1993a).Adsorption studies on both discrete iron oxide and iron oxide coatings areimportant for understanding and modeling heavy <strong>metal</strong> distribution and transport in theenvironment. While systematic studies are warranted in characterizing iron oxide-coatedsilica, an important representative surface in soil, methods and results of related studiesare summarized in the following sections.2.1.1 Oxide Coating MethodsAlthough iron oxide-coated media have been used to simulate natural systems or producenovel sorbents (Edwards and Benjamin, 1989; Zachara et al. 1995; Kuan et al. 1998;Khaodhiar et al. 2000; Meng and Letterman, 1993a; Lion et al. 1982; Dong et al. 2000;Lo et al. 1997; Szecsody et al. 1994; Joshi and Chaudhuri, 1996; Lai et al. 2000), littlehas been conducted in systematically evaluating factors that influence the degree ofcoating. Lo et al. (1997) found that higher coating temperatures enhanced the stability ofthe oxide coating. Furthermore, iron oxide coatings were amorphous at lowertemperatures (60 °C), while goethite and hematite formed at 150 °C and hematite formedexclusively at temperatures greater than 300 °C. Lo and Chen (1997) found nosignificant difference between the amount of iron oxide coated at 105 and 200 °C, whileScheidegger et al. (1993) observed increases in goethite coating with increasingtemperature (from 110 °C to 120 °C). In addition, coatings produced at pH 12 resulted ingreater <strong>metal</strong> <strong>adsorption</strong> efficiency than those produced at circum neutral; however, the