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

CHAPTER 10NI AND ZN SORPTION TO IRON OXIDE-COATED SILICAIron oxide-coated silica represents an important surface in soils and sediments. Theeffect of iron oxide coatings on <strong>adsorption</strong> needs to be addressed for better understandingof heavy <strong>metal</strong> distribution in the environment. Macroscopic <strong>adsorption</strong> studies havebeen conducted on goethite-coated silica and compared with those on discrete silica andgoethite. Results are discussed below.10.1 Ni and Zn Adsorption EdgesNi <strong>adsorption</strong> edges on 5 g L -1 goethite-coated silica (GACS) and silica were performedover the pH range from 2 to 8 as a function of ionic strength (IS, 10 -3 to 10-2). Thesigmoid-shaped edge was not found for silica, possibly due to site saturation, and wasonly observed for GACS at IS 10 -3 (Figure 10.1). The edge jump begins at pH 6 for silicaand covers a range of 4-8 for GACS. Compared to silica, 50% <strong>adsorption</strong> for the GACSsystem occurred 1.4 pH units lower at IS 10 -3 and 0.6 pH units lower at IS 10 -2. TheGACS surface clearly shows a greater affinity for Ni. Ionic strength effects are observedfor the silica surface (Figure 10.1) potentially suggesting Ni sorbed as outer-spherecomplexes. Although inner-sphere Ni complexes were found on discrete HFO andgoethite, as presented in Chapter 7, the distinct edges at ionic strength 10 -3 and 10-2 forNi/GACS system suggest that the silica surface significantly impact the electrostaticcharacter of coated silica. This observation is consistent with the small amount ofgoethite coatings, which are non-uniformly distributed (Figure 6.6 and Figure 6.7).However, the significant increase in % Ni sorbed for GACS (Figure 10.1) suggests that145