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

CHAPTER 11CONCLUSIONS AND FUTURE WORKSorption of trace elements onto iron oxide is an important process regulating contaminantmobility and bioavailability in natural systems. The purpose of this research is tounderstand the effect of iron oxides, as discrete particles and coatings, on contaminant<strong>adsorption</strong> and develop surface complexation models to predict heavy <strong>metal</strong> <strong>adsorption</strong>and competition.A model system for soils and sediments, iron oxide-coated silica, was synthesizedusing <strong>adsorption</strong> and precipitation methods. The degree of coating was highly sensitiveto the particle size of silica and ranged between 0.59 and 21.36 mg Fe g -1 solid. Theinteraction between goethite and silica potentially involves chemical forces. Iron oxidecoatings on the silica surface are non-uniform increasing surface area and introducingsmall pores. The pHpzc of coated silica is between that of pure silica and iron oxidesuggesting both silica and goethite surfaces are important for <strong>adsorption</strong>. Ni <strong>adsorption</strong>on the coated silica is greater than that for pure silica and the sub-micrometer goethitecoatings exhibit a larger capacity than discrete goethite impacting interactions at theaqueous interface.EXAFS analysis of Pb(II)-sorbed to freshly precipitated HFO suggest that Pb(II)ions form mononuclear bidentate edge-sharing surface complexes on FeO6 octahedra.This mechanism appears to be invariant of pH, ionic strength, Pb loading, and reactiontime. The internal sites on micropore walls appear to be no different than the externalones. Intraparticle surface diffusion modeling described the two-step Pb/HFO sorptionprocess reasonably well. The rate-limiting diffusion process may occur for decades156