The speciation of antimony is strongly influenced by its oxidation state (V, III, 0, —III). Redox processes under anaerobic groundwater conditions may therefore greatly alter the environmental behaviour of Sb. Employing X-ray absorption and photoelectron spectroscopy, we show here that Sb(V) is reduced to Sb(III) by magnetite and mackinawite, two ubiquitous Fe(II)-containing minerals, while Sb(III) is not reduced further. At the surface of magnetite, Sb(III) forms a highly symmetrical sorption complex at the position otherwise occupied by tetrahedral Fe(III). The Sb(V) reduction increases with pH, and at pH values >6.5 Sb(V) is completely reduced to Sb(III) within 30 days. In contrast, at the mackinawite surface, Sb(V) is completely reduced across a wide pH range and within 1 h. The Sb(V) reduction proceeds solely by oxidation of surface Fe(II), while the oxidation state of sulphide is conserved. Independent of whether Sb(V) or Sb(III) was added, an amorphous or nano-particulate SbS3-like solid formed.

Mackinawite, tetragonal FeS, has been synthesised by reacting iron with Na2S solutions. A Rietveld structure refinement of X-ray powder diffraction data, recorded using X-rays monochromated from synchrotron radiation with a wavelength of 0.6023 Å, has been performed. The structure has been refined in the tetragonal space group, P4/nmm, and has the following cell parameters: a = 3.6735(4), c = 5.0328(7) Å, V = 67.914(24) Å3. Our refinement shows that the FeS4 tetrahedron in mackinawite is almost perfectly regular, with a much smaller distortion than has been previously reported. An improved X-ray diffraction data set is provided.