Abstract:
X-ray absorption fine structure (XAFS) spectroscopic analysis at the As, Se, and Mn K-edges was used to study arsenate [As(V)O-4(3-)] and selenite [Se(IV)O-3(2-)] sorption complexes on the synthetic hydrous manganese oxides (HMOs) vernadite (delta-MnO2) and K-birnessite (nominal composition: K4Mn14O27.9H(2)O). No significant changes were observed in sorption complex structure as a function of sorbent, pH (5 to 8), surface coverage (0.04 to 0.73 mumol/m(2)), or reaction time (5 to 22 h) in the arsenate or selenite systems. In the arsenate/HMO system, extended XAFS parameters indicate an average second-neighbor As(V) coordination of 2.0 +/- 0.4 Mn at an average distance of 3.16 +/- 0.01 Angstrom, which is consistent with formation of AS(V)O-4 sorption complexes sharing corners with two adjacent Mn(IV)O-6 surface species (i.e., bidentate, binuclear). In the selenite/HMO system, selenite surface complexes are surrounded by two shells of Mn atoms, which could represent two different adsorption complexes or a precipitate. The first shell consists of 1.6 +/- 0.4 Mn at 3.07 +/- 0.01 Angstrom, which is consistent with the selenite anion forming bidentate (mononuclear) edge-sharing complexes with Mn(II)O-6 or Mn(III)O-6 octahedra. The second shell consists of 1.4 +/- 0.4 Mn at 3.49 +/- 0.03 Angstrom, consistent with selenite forming monodentate, corner-sharing complexes with Mn(II)O-6 or Mn(III)O-6 octahedra. Pauling bond valence analysis that uses the extended XAFS-derived bond lengths for As(V)-O, Se(IV)-O, and Mn-O bonds indicates that the proposed surface complexes of selenite and arsenate on HMOs should be stable. Although a nearly identical Se(IV) coordination environment is found in a crystalline Mn(II)-Se(IV) precipitate (which has a structure similar to that of MnSeO3 . H2O), there are significant differences in the X-ray absorption near-edge structure and extended XAFS spectra of this precipitate and the selenite/HMO sorption samples. These differences coupled with transmission electron microscopy results suggest that if a precipitate is present it lacks long-range order characteristic of crystalline MnSeO3 . H2O.
