Sulfate
Local Coordination Environment in Schwertmannite
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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 H<sub>2</sub>O molecules in addition to sulfate ions