Migration 2007 edition:11 location:Munich, Germany date:26-31 august 2007
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 FeS2 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 . 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.