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

151sites in the pH 6 and 7 isotherms. For these two conditions, the isotherm appearsapproach saturation (approximately 5x10 -7 mole Ni g -1 silica) for Ni concentrationsgreater than 10-6 M and less than 10 -4 M. Interestingly, using a non-electrostatic model,Delolme et al. (2004) obtained an optimized SiOH site concentration of 8x10 -7 mole g-1for a natural sand with a diameter of 0.3 to 2 mm, similar to that used in this study. Thesilica isotherms overlap at Ni concentrations greater than 10 -4 M; this may suggestformation of Ni polymers instead of a lower affinity site. The Ni/silica <strong>adsorption</strong>isotherm seems to reveal more information than the <strong>adsorption</strong> edges about surface sitessuggesting more than one mechanism. Coupled with electrophoretic information, Kuanet al. (2004) modeled Al(III) <strong>adsorption</strong> to an amorphous silica reasonably well with asurface complexation/precipitation model over Al(III) concentrations ranging from 10 4 ' 5to 10-2 M. The authors proposed Al(H2O)6 3+ complex at low pH (