Biogeochemical processes in ethanol stimulated uranium-contaminated subsurface sediments

Area of Science:

• Environmental Microbiology
• Geochemistry
• Bioremediation

Background:

• Subsurface sediments can be contaminated with uranium.
• Ethanol amendment is a potential strategy to stimulate microbial processes for contaminant remediation.
• Understanding microbial community and geochemical responses is crucial for effective bioremediation.

Purpose of the Study:

• To investigate the geochemical and microbial community response to ethanol amendment in uranium-contaminated subsurface sediment.
• To assess the role of terminal electron-accepting processes (TEAPs) in uranium reduction.
• To identify key microbial players involved in sediment biogeochemistry and uranium transformation.

Main Methods:

• Laboratory incubation experiment with uranium-contaminated sediment.
• Addition of 13C-labeled ethanol as an electron donor.
• Monitoring of geochemical parameters (e.g., redox processes).
• Analysis of microbial community structure using reverse transcribed 16S rRNA gene sequencing.
• Stable isotope probing using phospholipid fatty acids (PLFAs) to track carbon assimilation.

Main Results:

• A sequential pattern of TEAPs (NO3- reduction, Fe(III) reduction, SO4(2-) reduction, CH4 production) was observed in ethanol-amended slurries.
• Approximately 60% of U(VI) was reduced during the Fe(III) reduction phase; no further U(VI) reduction occurred in later stages.
• Ethanol amendment led to significant increases in microbial groups related to Dechloromonas, Geobacter, and Herbaspirillum, with Geobacter incorporating ethanol.
• Sulfate-reducing bacteria also showed increased abundance and incorporated ethanol during their activity phase.

Conclusions:

• Ethanol amendment effectively stimulates microbial activity and uranium reduction in contaminated sediments, largely following established TEAP models.
• Uranium reduction is primarily linked to Fe(III) reduction, suggesting specific microbial pathways are responsible.
• The complex redox speciation of uranium cannot be fully explained by simplified thermodynamic models alone.