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

155M. Polymerization may occur at concentrations greater than 5x10 -6 M. Similar to Ni, Zn<strong>adsorption</strong> mechanisms need to be investigated further with techniques includingspectroscopic ones.10.3 SummaryOverall, GACS has a higher affinity for Ni and Zn than silica due to the presence ofgoethite. However, the electrostatic effect is controlled by silica. Both surfaces areneeded for successfully describing <strong>adsorption</strong> on this heterogeneous sorbent. Althoughedge studies potentially suggest outer-sphere <strong>adsorption</strong> for silica and GACS, Ni and Zn<strong>adsorption</strong> isotherms reveal more than one sorption mechanism. Silica exhibits high- andlow-affinity sites for Ni at low pH (4 and 5), but one type of site for Ni at high pH (6 and7) and for Zn at all pH values (4-7). GACS is composed of two types of surfaces,however, only one type of <strong>adsorption</strong> site was reflected in Ni and Zn isotherms up to 10 -6M. Although Ni and Zn <strong>adsorption</strong> on goethite were successfully modeled withoutinclusion of polymer formation, it may be needed for modeling silica and GACS system.Depicting accurate <strong>adsorption</strong> mechanisms under various conditions need to be addressedusing techniques including EXAFS. This work is challenging because <strong>metal</strong> loadings arelow (Figure 10.3 and Figure 10.5). Amorphous silica is a high surface area analogue forquartz (Manceau et al. 1999; Elzinga and Sparks, 2002), which may be a good substratefor studying <strong>metal</strong> <strong>adsorption</strong>. Overall, sorption on iron oxide-coated silica has not yetcome to the same level as discrete iron oxide, and therefore more work is needed in thisarea.