XPS and XAS investigation of Sb(V) reduction on mackinawite: Effect of pH and surface loading

Sb exists in nature in a wide range of oxidation states and can be a potential hazardous contaminant depending on its speciation and reactivity. In this study we employed cryogenic-XPS and XAS techniques in order to understand the reduction of SbV at the surface of mackinawite (FeS) as a function of pH and surface loading.
Experimental Conditions
Nanoparticulate, Mackinawite samples were prepared and stored as suspensions under strictly anoxic conditions (~ 1 ppmv O2) in a Jacomex glovebox. Sorption experiments at pH 5 and pH 8 with increasing Sb concentrations (0.1, 0.3, 0.6, 0.8 mM) were also conducted in the same glovebox under anoxic conditions [1, 2]. X-ray Photoelectron Spectroscopy (XPS) and X-ray Absorption Spectroscopy (XAS) measurements were conducted on frozen wet pastes of reacted mackinawite samples.
Results and Discussion
Sb3d XPS spectra of reacted sample series at pH 5 revealed presence of increasing amounts of SbIII at the surface suggesting that the rate of reduction of SbV was directly proportional to surface loading. Corresponding Fe 2p ans S 2p spectra of the same samples showed significant increase in the proportion of FeIII species and presence of S0 at the surface. However, the proportion of S0 remained constant suggesting that Fe is the dominant redox partner in this sytem. At pH 8, presence of SbIII was only observed at higher surface loadings indicating that adsorption and redox processes both occur at much slower rate compared to that at pH 5. In addition, increased contribution of surface monosulfide group at pH 5 suggest that Sb is most likely bound to S atoms at the surface, which was confirmed by Sb K-edge EXAFS spectra. In contrast, at pH 8, such increase in surface monosulfide contributions are absent, suggesting Sb is most likely bound to O atoms at the surface.