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

152one for GACS (2.77x10 -6 mole sites g -1 solid) based on the degree of coating and siteconcentrations of goethite (3.09x10 -4 mole g-1 goethite, Chapter 9) and silica (8x10 -7mole g -1 , Delolme et al. 2004). At Ni concentrations greater than 10-3 M, the increasedslope in the isotherms may be due to polymerization. Overall, GACS exhibits one typeof site over a large range of Ni concentrations (10 -10 to 10 -3 M). Polymers may form onGACS at Ni concentrations greater than 10 -3 M in contrast to that observed for silicawhere polymerization may occur at concentrations greater than 10 -4 M. Although a lowaffinity site was not reflected in the GACS isotherms, the surface is heterogeneous bynature. Metal sorption mechanisms therefore need to be investigated with othertechniques including XAS.Zn <strong>adsorption</strong> isotherms were performed on silica, GACS, and goethite at pH 5and 6 and ionic strength 10 -3 (Figure 10.5). Adsorption on coated silica is less than thaton goethite and slightly greater than silica due to the presence of goethite. The Znisotherms with silica and GACS are similar over the concentration of 10 -1° to 10 -5 M(Figure 10.5) with slopes approximately equal to one below 10-7 M and less than one atgreater concentrations up to —5x10 -6 M (Figure 10.6a and b). Site saturation may beapproached at approximately 10-7 mole Zn g -1 solid for both systems at pH 5 and 6. Theabrupt increase in <strong>adsorption</strong> near 10 -5 M may suggest precipitation of a new phase. Inan EXAFS analysis of Zn complexation on an amorphous silica, Roberts et al. (2003)observed inner-sphere <strong>adsorption</strong> complexes for pH 5.1 to 7.45, a loading 6.3x10 -6 to2.63x10 -4 mole Zn g-1 solid, and a reaction time of 24 hours to 18 months. They foundamorphous Zn(OH)2 precipitation occurring at the highest pH 7.51. Overall, isothermswith silica and GACS suggest one type of site over a Zn concentration of 10 -1° to 5x10-6