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

158surfaces are needed for successfully describing <strong>adsorption</strong> on this heterogeneous sorbent.Ni and Zn <strong>adsorption</strong> isotherms reveal more than one sorption mechanism on silica andcoated silica. Although GACS is composed of two types of surfaces, only one type of<strong>adsorption</strong> site was reflected in Ni and Zn isotherms up to 10 .6 M. Polymer formationmay be needed for modeling silica and GACS system. Depicting accurate <strong>adsorption</strong>mechanisms under various conditions need to be addressed using techniques includingspectroscopic ones.Therefore, future work associated with this research should include addressing<strong>adsorption</strong> mechanisms on silica and iron oxide-coated silica under various conditionsusing techniques including XAS. Amorphous silica can be used in this regard because ofits high surface area. Speciation results can then be used to constrain surfacecomplexation modeling for <strong>adsorption</strong> to coated silica. Additionally, heavy <strong>metal</strong>distribution is regulated by microorganisms in the environment as well. Metal ions maybe involved in metabolic activities and/or sorbed to biotic coatings. This interaction isnot well understood yet and needs to be addressed for better description of naturalsystems. Models developed for biotically and/or abiotically coated heterogeneoussystems can then be incorporated into environmental management programs to predictthe bioavailability of heavy <strong>metal</strong>s and evaluate the potential ecological risk.