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

43Lin-edge, from 8,133 to 8,884 eV over the Ni K-edge, and from 6,912 to 7,857 eV overthe Fe K-edge. Pb, Ni, and Fe foils were used as references for energy calibration. Atleast five scans for Fe and typically —20 scans for Pb and Ni were collected. The analysiswas carried out on the averaged data to improve signal statistics. All EXAFS data werecollected at room temperature. XAS data for reference compounds were collected intransmission mode. Both transmission and fluorescence data were collected for thehydrated ion in a <strong>metal</strong> nitrate solution in order to address self-absorption (Tröger et al.1992).The XAS data were analyzed with WinXAS 2.3 (Ressler, 1998) and followedstandard procedures (Bunker and Sayers, 1988). For each scan, the background wasfitted with a linear polynomial through the pre-edge region and subtracted. The edgejump of the background-corrected XAS spectra was then normalized to one by refining100-200 eV above the edge with a zero order polynomial and normalizing the spectra tothe ordinate value of this polynomial. The energy threshold (E0) of the absorptionspectrum was determined from the first inflection point of the edge region and was usedto convert the absorption spectrum from energy to k space. A cubic spline function wasapplied to account for the isolated atomic absorption μ0(E). EXAFS function (x(k)) wasobtained after subtracting ,u0(E) from the absorption of the atom in condensed matter.The x(k) data were weighted by k 3 to enhance the higher k-space data and then Fouriertransformed using the Bessel window function to produce the radial structural function(RSF). The RSF was not corrected for phase shift. EXAFS fitting of the RSF wasconducted using theoretical phase shift and amplitude functions calculated by FEFF 7