Nanosized iron oxide colloids strongly enhance microbial iron reduction

Area of Science:

• Geomicrobiology
• Environmental Science
• Biogeochemistry

Background:

• Microbial iron reduction is a key subsurface process.
• Bioavailability of insoluble iron oxyhydroxides limits microbial reduction rates.
• Iron oxides exist as bulk minerals and nanosized colloids in subsurface environments.

Purpose of the Study:

• To investigate the role of colloidal iron oxides in microbial iron reduction.
• To compare the reduction rates of colloidal and bulk iron oxides by Geobacter sulfurreducens.
• To understand the factors influencing the enhanced reactivity of colloidal iron oxides.

Main Methods:

• Batch growth experiments using Geobacter sulfurreducens.
• Addition of colloidal iron oxides (ferrihydrite, hematite, goethite, akaganeite) as electron acceptors.
• Measurement of microbial iron reduction rates for colloidal and bulk iron phases.

Main Results:

• Colloidal iron oxides were reduced up to 2 orders of magnitude faster than bulk forms.
• Enhanced reactivity was attributed to both large surface area and higher per-unit-surface reactivity of colloids.
• Nanosized ferrihydrite aggregates significantly enhanced the reduction rates of bulk ferrihydrite.

Conclusions:

• Colloidal iron oxides are more bioavailable and reactive than bulk iron minerals.
• Nanosized iron oxides play a crucial role in accelerating microbial iron reduction in subsurface environments.
• Understanding colloidal iron oxide behavior is essential for predicting subsurface biogeochemical cycling.