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

125This reaction is also consistent with the reported proton release for Zn <strong>adsorption</strong> ongoethite (Forbes et al. 1976; Kooner, 1993; Trainor et al. 2000). Although the systemswere open to the atmosphere, there was no evidence of structural contributions fromcarbonate in XAS analyses (Chapter 8; Trivedi et al. 2001a). Carbonate species have notbeen reported to affect <strong>adsorption</strong> of Zn, Ni, Cd, and Pb under atmospheric conditions(Balistrieri and Murray, 1982; Lützenkirchen and Behra, 1994; Green-Pedersen et al.1997; Villalobos et al. 2001; Strathmann and Myneni, 2005).9.1 Potentiometric Titration DataTitration results (Figure 9.1) reveal that the point of zero charge (PZC) for goethite is atpH 8.8 ± 0.97 (pHpzc). This value is consistent with reported ones, which range from 7.5to 9.5 (Lumsdon and Evans, 1994; Robertson and Leckie, 1998; Peacock and Sherman,2004). The variation in the reactivity of goethite is caused by the synthetic conditionssuch as total base added, rate of addition, the presence of sorbing ions, and aging time(Schwertmann et al. 1985; Villalobos et al. 2003). Each crystal plane has a distinct sitedensity (Hiemstra et al. 1989); the pHpzc may also be a function of the particle shape,size, and surface area. Sverjensky (2005) reported that pHpzc may be lower withoutsufficient purging to remove carbon dioxide compared to otherwise obtained values.Previous studies (Evans et al. 1979; Zeltner and Anderson, 1988) have shown that whenthe goethite suspension was purged for 2 months, the PZC increased from 8.1 to 9.1; intheir interpretation, changes in PZC were due to CO2 <strong>adsorption</strong> and desorption.However, aging also affects crystallinity, which would impact the PZC as well.