2.4.7. X-ray photoelectron spectroscopy (XPS)
Photoelectron spectra of optimized adsorbent before and after
Pb(II) adsorption were recorded on a VG Escalab 200R electron
spectrometer equipped with a hemispherical electron analyzer,
using a MgKa (hm = 1253.6 eV, 1 eV = 1.603 1019 J) X-ray source.
After outgassing at 106 mbar, the samples were transferred to the
ion-pumped analysis chamber, in which the residual pressure was
kept below 4 109 mbar during data acquisition. The binding energy
(BE) of the Si 2p peak at 103.4 eV was taken as an internal standard.
The accuracy of the BE values was ± 0.1 eV. Peak intensities
were estimated by calculating the integral of each peak after subtracting
an S-shaped background and fitting the experimental peak
to a combination of Lorentzian/Gaussian lines (10L/90G). Atomic
surface contents was estimated from the areas of the peaks, corrected
using the corresponding sensitivity factors [41].
3. Result and discussion
3.1. Characterization of the adsorbents
Fig. 1 shows XRD patterns in low angles of the samples SBA15–
0.4 NH2, SBA15–0.3 NH2, SBA15–0.2 NH2 and SBA15. As seen in this
figure, all SBA-15-based samples show three well-resolved typical
diffraction peaks, which are associated with a bi-dimensional
p6mm hexagonal symmetry of the pores of SBA-15: one intense
reflection centered approximately at 2h 1.0 and two very lowintensity
peaks at about 2h 1.7 and 2.0, which can be indexed
as the (100), (110) and (200) hkl reflections, respectively [42].
As seen in this figure, all SBA15–xNH2 samples did not show displacement
in the position of these reflections with respect to pure
SBA-15 that indicated the absence of changes in the hexagonal
symmetry of the pores of the SBA-15 with the incorporation of –
NH2 groups.
The transmission electron micrograph (TEM) images further
verified the results of the low-angle XRD patterns. As example,
Fig. 2 shows the TEM images of the SBA15–0.3NH2 sample. A
well-ordered hexagonal array of mesopores can be seen when
the electron beam is parallel to the main axis of the cylindrical pores. When the electron beam is perpendicular to the main axis,
the presence of the parallel nanotubular pores of the parent SBA-
15 matrix is evidenced.