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					                     XANES-EXAFS ANALYSIS OF SE(+IV) SOLID PHASE
                        REACTION PRODUCTS WITH FES2 AND FES

                                C. Bruggeman1, †; A. Maes1; E. Breynaert1
1
 Laboratory for Colloid Chemistry, Katholieke Universiteit Leuven, Kasteelpark Arenberg 23, B-3001
                                           Leuven, Belgium
                †
                  Current affiliation: SCK•CEN, Boeretang 200, B-2400 Mol, Belgium

Only a very limited amount of information is available concerning the interaction between Se
oxyanions and sulphidic mineral phases such as pyrite, troilite and mackinawite. However, it is a well-
known fact that in sediments Se is closely correlated with pyrite [1, 2], where it can substitute for
sulphur and thus form FeSe, FeSe2 or mixed FeSxSey phases which control its solubility [3, 4].
In the present study, the solid phase reaction products of selenite oxyanions with FeS and FeS 2 at pH
7-8 are investigated using X-ray Absorption Near-Edge Spectroscopy and Extended X-ray Absorption
Fine Structure (XANES-EXAFS) to elucidate the selenium speciation and, thus, the underlying
geochemical reaction mechanisms.
Reference spectra were collected from Se(IV) solution species and reduced Se solid phases such as
amorphous and crystalline elemental selenium, and FeSe. It was shown that the energy position of the
XANES absorption edge is indicative for the Se valence state, as observed before [5]. Also, the
XANES region can be used to identify reduced Se solid phases. Se K-edge EXAFS spectra and
Fourier-transformed Radial Structure Functions (RSFs) could be fitted very well using two
coordination shells only.
Comparing spectra taken from the solid phase of experiments in which FeS2 was contacted with SeO32,
with the reference spectra, the presence of amorphous elemental selenium was reveiled, providing
direct evidence that FeS2 acts as a redox mediator for oxidised Se species and thus inevitably controls
the redox speciation of Se under geochemical conditions revelant for geological disposal of high-level
nuclear waste. A similar sample in which FeS was contacted with SeO32-, showed the formation of
FeSe as end product. Sulphide minerals subjected to HCl-based pretreatment did not show any
difference with respect to the spectra obtained after equilibration with SeO32-, but an influence on the
reaction kinetics was noted.

[1] Elson C.M., MacDonald A.S, Analytica Chimica Acta, 110, 153 (1979)
[2] Velinsky D.J., Cutter G.A. Analytica Chimica Acta, 235, 419 (1990)
[3] Masscheleyn P.H., Delaune R.D., Patrick W.H., J. Env. Sci. Health A - Environmental science and
engineering & Toxic and hazardous substance control, 26, 555 (1991)
[4] Olin A., Noläng B., Osadchii E.G., Öhman L.-O., Rosén E., Chemical Thermodynamics Series
Volume 7: Chemical thermodynamics of selenium, OECD Nuclear Energy Agency, Data Bank, Issy-
les-Moulineaux, France (2005)
[5] Oger P.M., Daniel I., Cournoyer B., Simionovici A. Spectrochimica Acta Part B, 59, 1681 (2004)

				
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posted:6/12/2011
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