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

22sorbed to 2-line ferrihydrite, this structure did not change as a function of time (up to 2months), ferrihydrite morphology (with and without freeze-drying), and competing ions(Cu2±). In their analysis, Cu and Pb were expected to bind to the same types of sites.However, competition was not observed. Again as discussed above, Trivedi et al. (2003)observed inner-sphere bidentate edge- and monodentate corner-sharing Pb(II) complexes,at constant pH, the configuration of the sorption complex was observed to be independentof the adsorbate concentration thus suggesting one average type of mechanism.Ni has also been studied extensively using EXAFS. Much of the existing workhas focused on its sorption to aluminum oxides, silica, or related minerals (Clause et al.1992; Paulhiac and Clause, 1993; Scheidegger et al. 1996, 1998; Ford et al. 1999;Roberts et al. 1999; Nachtegaal and Sparks, 2003; Voegelin and Kretzschmar, 2005),where the formation of layered double hydroxide (LDH) precipitates were observed.Elzinga and Sparks (1999) reported that <strong>adsorption</strong> and surface precipitation areconsecutive mechanisms involving nonspecific or specific <strong>adsorption</strong> followed bydissolution of Al and nucleation of a mixed Ni/Al phase. In the process, Al dissolutionhas been found to be the rate limiting mechanism based on rates of Ni uptake and Aldissolution (Scheidegger et al. 1998). EXAFS analysis reveals that formation of Ni-AlLDH is characterized by increased second shell contributions as the structure grows(Scheidegger et al. 1998). Also, as pH decreased from 7.5 to 6, the rate of formationdecreased from 15 min to 72 hours (Roberts et al. 1999). Recently, however, Strathmannand Myneni (2005) found that Ni(II) forms inner-sphere mononuclear bidentatecomplexes with aluminol groups on boehmite (y-A1OOH), where contact times rangedfrom 1 to 31 days with loadings between 7.0x10 -6 and 4.11x10 -5 mol g-1 . The total Ni(II)