Phosphate-Imposed Constraints on Schwertmannite Stability under Reducing Conditions.
Valerie A Schoepfer1, Edward D Burton1, Scott G Johnston1
1Southern Cross GeoScience, Southern Cross University , PO Box 157, Lismore, New South Wales 2480, Australia.
Environmental Science & Technology
|August 3, 2017
Summary
Phosphate influences schwertmannite stability in acid sulfate systems. Intermediate phosphate levels enhance schwertmannite dissolution, while high levels stabilize it against reductive conditions.
Area of Science:
- Environmental Geochemistry
- Mineralogy
- Biogeochemistry
Background:
- Schwertmannite, a ferric oxyhydroxysulfate mineral, is prevalent in acid sulfate environments.
- Phosphate (PO4^3-) availability is crucial in these systems and may influence schwertmannite's fate.
- Understanding phosphate's role is key to predicting nutrient cycling in acid sulfate soils.
Purpose of the Study:
- To investigate the impact of varying phosphate concentrations on schwertmannite stability.
- To examine schwertmannite's response to reductive conditions influenced by phosphate.
- To elucidate the mechanisms controlling schwertmannite dissolution and transformation.
Main Methods:
- Schwertmannite suspensions were treated with four phosphate loadings (0, 80, 400, 800 μmoles g^-1).
- Suspensions were inoculated with wetland sediment and incubated under anaerobic (N2-purged) conditions for 41 days.
- pH, Fe(II) concentrations, and mineral transformation were monitored throughout the experiment.
Main Results:
- Dissimilatory Fe(III)-reduction led to Fe(II) accumulation, peaking at intermediate phosphate loadings.
- Schwertmannite transformation to goethite occurred in the absence and at low phosphate concentrations.
- Higher phosphate loadings significantly inhibited schwertmannite transformation to goethite.
Conclusions:
- Intermediate phosphate concentrations optimize reductive dissolution of schwertmannite.
- Low phosphate limits reductive dissolution by promoting rapid schwertmannite to goethite transformation.
- High phosphate concentrations stabilize schwertmannite, likely through surface complexation, hindering dissolution.
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