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

14Ephraim et al. 1991; Bar-Tal et al. 1988; Lion et al. 1982; Baccini et al. 1982; Buerge-Weirich et al. 2002), sorbate/sorbent ratio (Gadde and Laitinen, 1974; Hayes and Leckie,1986; Weesner and Bleam, 1998; Webster et al. 1998), and competing <strong>metal</strong> ions(Benjamin and Leckie, 1981b; Voegelin et al. 2001; Christi and Kretzschmar, 1999;Christophi, 1998; Christophi and Axe, 2000; Gadde and Laitinen, 1974; Palmqvist, et al.1999; Swallow et al. 1980; Trivedi et al. 2001b; Vulava et al. 2000). Other factorsinvolve redox, temperature, supersaturated solutions of <strong>metal</strong> ions, and microbial effects.Generally, <strong>metal</strong> cation <strong>adsorption</strong> increases with increasing pH as the surface chargebecomes more negative. Ionic strength affects <strong>metal</strong> <strong>adsorption</strong> by influencing adsorbateactivity as well as the surface charge and double-layer capacitance and thickness.However, the effect of ionic strength is often relatively minor compared to pH.Complexing ligands may decrease or increase adsorbate uptake. Adsorption competitionoccurs when multiple species are present and compete for a limited number of surfacesites.Macroscopic studies include <strong>adsorption</strong> edges and isotherms. The formeraddresses pH and ionic strength dependence of <strong>adsorption</strong>, while the latter describes<strong>metal</strong> concentration dependence. In the following, both <strong>adsorption</strong> edges and isothermsas well as <strong>metal</strong> competition studies are introduced.2.2.1 Adsorption EdgeA plot of cation <strong>adsorption</strong> to iron oxides vs. pH is usually sigmoidal with percent <strong>metal</strong>sorbed increasing from 0 to 100% over a narrow pH range (Cornell and Schwertmann,1996; Hayes and Leckie, 1986). Sorbate/solid ratio influences the position of edge jump.Adding more solid causes the edge jump to shift to a lower pH; on the other hand,