Microbial and mineral evolution in zero valent iron-based permeable reactive barriers during long-term operations

Area of Science:

• Environmental Science
• Geochemistry
• Microbiology

Background:

• Zero valent iron (Fe(0)) permeable reactive barriers (PRBs) are crucial for subsurface contaminant remediation.
• Long-term performance of Fe(0) PRBs is influenced by subsurface biogeochemical processes.
• Understanding microbial impacts on Fe(0) degradation and mineral formation is vital for PRB longevity.

Purpose of the Study:

• To evaluate the biogeochemical impacts on the long-term performance of Fe(0)-based PRBs.
• To investigate the influence of microbial activity on mineral phase evolution in Fe(0) PRBs.
• To compare mineral formation in biotic and abiotic conditions.

Main Methods:

• Laboratory flow-through column experiments were conducted for 210 days.
• Two particle sizes of Fe(0) were used, with biotic (added carbon) and abiotic (gamma radiation) conditions.
• Mineral phases were characterized using XRD, SEM-EDX, and micro-XRF; microbial communities were identified via 16S rRNA gene sequencing.

Main Results:

• Heavy metals (Zn, As) were efficiently removed in all columns without clogging or capacity exhaustion.
• Microbial communities shifted from Acidithiobacillus ferrooxidans to Desulfosporosinus species (sulfate-reducing bacteria) in biotic columns.
• Biotic columns showed mackinawite (FeS) and sulfate green rust, while abiotic columns predominantly formed magnetite/maghemite phases.

Conclusions:

• Biogeochemical processes significantly alter Fe(0) mineralogy over time.
• Microbial activity promotes the formation of specific iron sulfide and green rust phases, distinct from abiotic conditions.
• These findings are critical for predicting and enhancing the long-term efficacy of Fe(0) PRBs in environmental remediation.