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

33existing in almost all soils and other surface formations (e.g. lakes, streams), goethite ispredominant under cool to temperate, humid climates and associated with hematite inwarmer regions. Soil goethite can be formed in two ways (Cornell and Schwertmann,1996): Ferrous iron can be released from iron silicates, carbonates, and sulfides, forexample, or reduced through microbial respiration. The ferrous ion is then oxidizedunder aerobic conditions and precipitates via a nucleation-crystal growth process.Alternatively, ferrihydrite precipitation undergoes dissolution/reprecipitation andconverts to goethite. The transformation is slow at neutral pH (months to years), but canbe greatly accelerated (in days) by organic reducing agents such as cysteine(Schwertmann and Cornell, 1991). In the laboratory, goethite formation is favored byraising or lowering the pH away from the PZC of ferrihydrite (pH 7-8, Schwertmann andCornell, 1991) which promotes dissolution of ferrihydrite (Schwertmann and Murad,1983).Because its surface chemistry and crystal morphology are well characterized andbecause its widespread in natural environments, goethite has been extensively used as amodel component for soils and sediments in a variety of investigations (e.g. Tiller et al.1984; Hayes and Leckie, 1987; Crawford et al. 1993; Bargar et al. 1998; Sahai et al.2000; Trivedi et al. 2001b; Templeton et al. 2003). For the same reasons, <strong>adsorption</strong> togoethite will be the focus of this research and goethite will be synthesized throughferrihydrite transformation under basic conditions (pH>12). Considering that the pH fornatural aquatic environments ranges between six and eight and becomes acidic with forexample the introduction of acid mine drainage or historically poor waste managementpractices, <strong>adsorption</strong> studies will be investigated covering an environmentally relevant