Phosphate loading alters schwertmannite transformation rates and pathways during microbial reduction

Area of Science:

• Environmental Geochemistry
• Microbial Ecology
• Mineralogy

Background:

• Acid sulfate systems contain schwertmannite, a metastable ferric oxyhydroxysulfate.
• Phosphate (PO43-) is a nutrient causing eutrophication and can interact with schwertmannite.
• Reducing conditions in these systems destabilize schwertmannite, influencing Fe(III) and SO42- reduction.

Purpose of the Study:

• To investigate the influence of phosphate (PO43-) on microbial Fe(III) and SO42- reduction.
• To determine the effect of phosphate on mineralogical transformations in schwertmannite systems under reducing conditions.

Main Methods:

• Microbially-inoculated schwertmannite suspensions were prepared with varied phosphate (PO43-) loadings.
• Suspensions were incubated under anoxic conditions for 82 days.
• Mineralogical evolution and reduction processes were monitored.

Main Results:

• Increased phosphate (PO43-) attenuated the onset of microbial Fe(III) reduction, delaying pH increases.
• In phosphate-free systems, goethite formed first, followed by mackinawite and siderite.
• In higher phosphate treatments, vivianite and/or sulfate green rust formed, replacing goethite and mackinawite.

Conclusions:

• Phosphate loading alters the rates and onset of microbial Fe(III) and SO42- reduction.
• Phosphate influences the formation of secondary iron-bearing minerals.
• Schwertmannite reduction sequesters significant phosphate as vivianite and green rusts under anoxic conditions.