title: Sulfate Local Coordination Environment in Schwertmannite.
abstract: Schwertmannite, a nanocrystalline ferric oxyhydroxy-sulfate mineral, plays an important role in many environmental geochemical processes in acidic sulfate-rich environments. The sulfate coordination environment in schwertmannite, however, remains unclear, hindering our understanding of the structure, formation, and environmental behavior of the mineral. In this study, sulfur K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopic analyses in combination with infrared spectroscopy were used to determine the sulfate local atomic environment in wet and air-dried schwertmannite samples after incubation at various pHs and ionic strengths. Results indicate that sulfate exists as both inner- and outer-sphere complexes in schwertmannite. Regardless of the sample preparation conditions, the EXAFS-determined S-Fe interatomic distances are 3.22-3.26 Å, indicative of bidentate-binuclear sulfate inner-sphere complexes. XANES spectroscopy shows that the proportion of the inner-sphere complexes decreases with increasing pH for both wet and dried samples and that the dried samples contain much more inner-sphere complexes than the wet ones at any given pH. Assuming that schwertmannite is a distorted akaganéite-like structure, the sulfate inner-sphere complexation suggests that, the double chains of the edge-sharing Fe octahedra, enclosing the tunnel, must contain defects, on which reactive singly-Fe coordinated hydroxyl functional groups form for ligand exchange with sulfate. The drying effect suggests that the tunnel contains readily exchangeable H2O molecules in addition to sulfate ions.
authors.affiliation: Department of Ecosystem Science and Management, University of Wyoming , Laramie, Wyoming 82071, United States.|Key Laboratory of Arable Land Conservation (Middle and Lower Reaches of Yangtze River), Ministry of Agriculture, College of Resources and Environment, Huazhong Agricultural University , Wuhan 430070, China.
authors.forename: Xiaoming
authors.initials: X
authors.lastname: Wang
authors.affiliation: Department of Ecosystem Science and Management, University of Wyoming , Laramie, Wyoming 82071, United States.
authors.forename: Chunhao
authors.initials: C
authors.lastname: Gu
authors.affiliation: Key Laboratory of Arable Land Conservation (Middle and Lower Reaches of Yangtze River), Ministry of Agriculture, College of Resources and Environment, Huazhong Agricultural University , Wuhan 430070, China.
authors.forename: Xionghan
authors.initials: X
authors.lastname: Feng
authors.affiliation: Department of Ecosystem Science and Management, University of Wyoming , Laramie, Wyoming 82071, United States.
authors.forename: Mengqiang
authors.initials: M
authors.lastname: Zhu
mesh_terms: D006863:Hydrogen-Ion Concentration; D058085:Iron Compounds / Q000737:chemistry*; D012996:Solutions; D017550:Spectroscopy, Fourier Transform Infrared; D013431:Sulfates / Q000737:chemistry*; D056928:X-Ray Absorption Spectroscopy; D014961:X-Ray Diffraction
chemical_list: D058085:Iron Compounds; D012996:Solutions; D013431:Sulfates; C511969:schwertmannite