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the polyhedron around Fe [2] and in particular when Fe +3 ions occupy tetrahedral sites fitting with one Lerentzian is used.
The Lorentzian is centered ~7113.0 eV, i.e. Fe is trivalent, occupies tetrahedral sites and its valence is independent of the
EAFD glasses concentration
Intensity (arb. units)
0.3
0.2
0.1
10 wt%
EAFD
15 wt%
EAFD
20 wt%
EAFD
25 wt%
EAFD
30 wt%
EAFD
(a)
7108 7110 7112 7114 7116 7118
FeO 6
(at%)
Area (a.u.)
70
60
50
40
30
20
10
0.40
0.38
0.36
0.34
0.32
0.30
0.28
10 15 20 25 30
Energy (eV) EAFD concentration (wt%)
Figure 2: (a) The pre-edge peak (raw and fitted data) of the Fe-K-NEXAFS
spectra of the studied glasses. (b) The dependence of the area under the preedge
peak and the percentage of the FeO 6 octahedra on the EAFD content.
(b)
Table 1: Fe-K-NEXAFS results of the
glasses and references Fe 2 O 3 and Fe 3 O 4 .
Sample
Pre-edge
peak (eV)
(±0.2)
Area
under
pre-edge
(a.u.)
Fe 2 O 3
Fe 3 O 4
7114.6 0.43±0.02
7113.1
7116.5
7112.7 0.63±0.03
7111.1
7114.6
10
wt% 7113.0 0.31±0.01
EAFD
15
wt% 7113.1 0.32±0.01
EAFD
20
wt%
EAFD
7113.0 0.33±0.01
Another interesting result relates to the changes of the area under the pre-edge peak as a function of the EAFD
concentration. As shown in Fig 2(b), the increase in the EAFD content results in a systematic increase of the area under the
pre-edge peak. This change can be explained in terms of the different coordination environment of Fe in the vitreous matrix.
Indeed, according to the Fe-K-EXAFS results [3], the increase of the EAFD concentration in the glasses results to a
systematic decrease in the number of the O atoms that are tetrahedrally coordinated with Fe. Hence, it can be stated that the
area under the pre-edge peak in glasses is a fingerprint of the coordination of Fe: the increase in the area indicates undercoordination
of Fe in the vitreous matrix. It has been previously reported that in crystalline Fe complexes [2], the distortion
in the FeO x polyhedron leads to an alteration in the area under the pre-edge peak. This is attributed to mixing of the 3d metal
orbitals with the 2p orbitals of the oxygen atoms which is enhanced along the distortion axis and thus the intensity of the 1s
→ 3d transition increases. Furthermore, the use of the mixed model in the Fe-K-EXAFS analysis [3] revealed that Fe
occupies both tetrahedral and octahedral sites into the vitreous matrix. The combined EXAFS and NEXAFS results of the
studied samples shown in Fig. 2(b) demonstrate that the increase of the area under the pre-edge peak is correlated to the
percentage of the FeO 6 octahedra: the area increases when the percentage of the FeO 6 octahedra decreases due to the low
intensity of the dipole mechanism in octahedral environments of the Fe ions.
Finally, structureless Zn-K-NEXAFS spectra suggest that the samples are amorphous. Furthermore, the Zn absorption
edge is located at 9662.7eV, an energy which is characteristic of tetrahedral coordination of Zn +2 [4]. Thus, the NEXAFS
results demonstrate that the bonding environment of Zn is not affected by the increase in the EAFD concentration and the
Zn +2 ions forms ZnO 4 into the vitreous matrix.
4. Conclusions
The application of NEXAFS spectroscopy in a series of EAFD-rich vitrified industrial wastes was implemented so as to
determine the valance and bonding geometry of both Zn and Fe ions. It is disclosed that the bonding environment of Fe
changes as the EAFD concentration increases. In particular, it is demonstrated that Fe +3 preferentially occupies noncentrosymmetric
sites, such as tetrahedral, with increasing EAFD content, i.e. the glass forming ability of Fe +3 becomes
dominant. On the contrary, Zn is always divalent and occupies tetrahedral sites in the glass, thus acts as a glass former.
Acknowledgement: The experimental work was realized with financial support from the EU BESSY-ID.2006.2.60326 and the
"PYTHAGORAS II" programs. One of the authors, F. Pinakidou, acknowledges financial support from the Greek Foundation
of Research Scholarships (IKY).
References
[1] M. Pellino., A. Karamanov, P. Pisciella, S. Criscussi and D. Zonetti, Waste Manag., 22 (2002) 945.
[2] T. Westre et al., J. Am. Chem. Soc., 119 (1997) 6297.
[3] F. Pinakidou et al,, ibid.
[4] F. Pinakidou et al. J. Haz. Mater., 142 (2007) 297
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