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

147<strong>adsorption</strong> affinity of the coated media is strongly influenced by goethite coatings.Similar results were observed in Zn <strong>adsorption</strong> edges (Figure 10.2). Percent Znsorbed increases after pH 5 for silica and the edge jump was over a broad pH range from4 to 8 for GACS. The goethite coatings present a higher affinity for Zn than silica andthe pH values at which 50% <strong>adsorption</strong> occurs lowered by 1.7 pH units at IS 10 -3 and 1.4pH units at IS 10 -2. Both systems show ionic strength effects on Zn sorption; therefore,silica again appears to dominate electrostatic interactions of the coated media.Overall, similar to proton <strong>adsorption</strong>, both components of the coated silica affectNi and Zn <strong>adsorption</strong>. By distinguishing inner- and outer-sphere <strong>adsorption</strong> with XAS,the triple layer model is expected to be more advantageous in modeling than other modelssuch as CCM, DLM, and Stern model. This modeling work along with XAS isrecommended in future research.10.2 Ni and Zn Adsorption IsothermsNi <strong>adsorption</strong> isotherms with silica, GACS, and goethite were conducted from pH 4 to 7under ionic strength 10-3. Results (Figure 10.3) indicate that Ni <strong>adsorption</strong> to thesesorbents increases in the order of silica < GACS < goethite. For silica (Figure 10.4a), pH4 and 5 isotherms have slopes approximately equal to one below a Ni concentration of10-7 M and less than one between 10-7-10-6 M. Saturation of the higher affinity sites(approximately 5 x10 -9 mole Ni g-1 silica) may have occurred. The shape of the pH 4 and5 isotherms at Ni concentrations greater than 10 -6 M is consistent with presence of alower affinity site and/or formation of Ni polymer (Dzombak and Morel, 1990; Katz andHayes, 1995b; Hayes and Katz, 1996). However, there was no indication of low affinity