Structural environment and oxidation state of Mn in goethite-groutite solid-solutions

Both X-ray absorption and diffraction techniques were used to study the str uctural environment and oxidation state of Mn in goethite-groutite solid so lutions, alpha -MnxFe1-xOOH, with x(Mn) less than or equal to 0.47. Rietvel d refinement of X-ray diffraction (XRD) data was employed to investigate th e statistical long-range structure. The results suggest that increasing x(M n) leads to a gradual elongation of Fe and Mn occupied octahedra which, in turn, causes a gradual increase of the lattice parameter a and a gradual de crease of b and c in line with Vegard's law. X-ray absorption fine structur e (XAFS) spec tra at the MnK alpha and FeK alpha edges revealed, however, t hat the local structure around Fe remains goethite-like for x(Mn) less than or equal to 0.47, while the local structure around Mn is goethite-like for x(Mn) less than or equal to 0.13, but groutite-like for higher x(Mn). The spectral observations were confirmed by XAFS-derived metal distances showin g smaller changes around Fe and larger changes around Mn as compared with t hose determined by XRD. Therefore, the XAFS results indicate formation of g routite-like clusters in the goethite host structure for x(Mn) > 0.13, whic h remain undetected by XRD. The first prominent resonance peak in the X-ray absorption near-edge spectra (XANES) of the Mn goethites was 17.2 to 17.8 eV above the Fermi level of Mn (6539 eV), in line with that of Mn3+ referen ce compounds, and well separated from that of Mn2+ and Mn4+ compounds. Ther efore, Mn in goethite is dominantly trivalent regardless of whether the sam ples were derived from Mn2+ or Mn3+ solutions. This may indicate a catalyti c oxidation of Mn2+ during goethite crystal growth similar to that found at the surface of Mn oxides.