X-ray photoelectron and absorption spectroscopy investigation of Se-IV and Sb-V reduction by mackinawite

Both Se and Sb exist in nature in a wide range of oxidation states and can be potential hazardous contaminants depending on their speciation and reactivity. While Se typically occurs as oxyanions (SeO3-2 and SeO4-2) in oxic conditions, it can be reduced by FeII containing minerals such as mackinawite (FeS) to Se0 and Se–II [1, 2]. Similarly, Sb occurs in several oxidations states (-III, 0, III, V) and is shown to strongly adsorb to Fe oxide minerals as Sb(OH)6 - and subsequently reduced to the more mobile form Sb(OH)3(aq) [3]. In this study we employed cryogenic-XPS and XAS techniques in order to understand the redox processes involving SeIV and SbV at the surface of mackinawite.
Fe 2p XPS spectrum of pure mackinawite surface revealed presence of both FeII-S and FeIII-S species and the proportion of the latter increased when reacted to SeIV suggesting oxidation of surface FeII. In addition, presence of elemental S at the surface of the reacted sample suggested oxidation of sulfur as well. Corresponding Se K-edge XANES spectra of the reacted sample confirmed reduction of Se with the formation of FeSe. These results suggest that Se reduction is coupled to both S-II/S0 and FeII/FeIII redox half reactions.
Sb 3d spectrum of the reacted sample revealed that SbV was completely reduced to SbIII at the surface of mackinawite.
However, the S 2p spectrum of the reacted sample remained largely unchanged except for a slight increase in surface monosulfide content. This suggests that in contrast to the SeIVmackinawite system, S did not take part in the redox reaction involving SbV. The Fe 2p spectrum, however, showed a distinct shoulder of FeIII-species indicating oxidation of surface FeII. Corresponding Sb K-edge EXAFS of the reacted sample confirmed that the SbIII was coordinated to three sulfur atoms at a distance of 2.5 Å as in Sb2S3, which most likely explains the slight increase in the surface monosulfide content in the S 2p spectrum.
These results demonstrate the importance of surface mediated redox reactions in controlling the fate of toxic
contaminats such as Se and Sb in soil and groundwater.
[1] Charlet et al. (2007) GCA 71, 5731-5749. [2] Scheinost & Charlet (2008) ES&T, online. [3] Scheinost et al. (2006) GCA 70, 3299-3312.