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

16released from Zn sorption to HFO; the authors explained their results as formation ofmultinuclear species. Although these approaches are usually successful in modeling, thesurface species may not be unique or physically meaningful.2.2.2 Adsorption IsothermIsotherms exhibit several types of shapes depending on <strong>metal</strong> concentration and/orsurface coverage (Hayes and Katz, 1996). At low concentration and low surfacecoverage, isotherms are often linear, the slope of which is referred to as distributioncoefficient (K,1). Kd has been used extensively to describe the partitioning of ions at thesolid/liquid interface and is often found to be a function of pH, solution composition, andsolid/liquid ratio (Jackson and Inch, 1989; Kooner, 1993; Lee et al. 1996; Larsen andPostma, 1997). Therefore, one condition may not be applicable to another. When anisotherm is conducted over a wider concentration range, it often becomes highlynonlinear and may level off at relatively high <strong>metal</strong> concentration. This type of isothermcan be described by the Langmuir model using two parameters, K for the equilibriumconstant and C t for site density. The Langmuir isotherm has been applied in a variety of<strong>adsorption</strong> studies (Pierce and Moore, 1980; Kanungo, 1994; Lee et al. 1996; Gupta,1998; Small et al. 2001; Christophi and Axe, 2000; Trivedi et al. 2001b; Dong et al.2001; Vaishya and Gupta, 2002, 2004; Delolme et al. 2004; Juang and Chung, 2004).The Freundlich isotherm has been used often for <strong>metal</strong> <strong>adsorption</strong> on oxides or soilsamples (Barrow et al. 1989; Gupta, 1998; Sen et al. 2002; Vaishya and Gupta, 2002,2004), and suggests the existence of more than one type of site.Isotherms may be more complicated than discussed above. Robertson and Leckie(1998) observed three regions in the isotherm for Cu <strong>adsorption</strong> to goethite: below —10-5