Sulfate Local Coordination Environment in Schwertmannite

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.