Electrokinetic-Enhanced Bioremediation of Trichloroethylene-Contaminated Low-Permeability Soils: Mechanistic Insight from Spatio-Temporal Variations of Indigenous Microbial Community and Biodehalogenation Activity

Electrokinetic-enhanced bioremediation (EK-Bio), particularly bioaugmentation with injection of biodehalogenation functional microbes such as Dehalococcoides, has been documented to be effective in treating a low-permeability subsurface matrix contaminated with chlorinated ethenes. However, the spatio-temporal variations of indigenous microbial community and biodehalogenation activity of the background matrix, a fundamental aspect for understanding EK-Bio, remain unclear. To fill this gap, we investigated the variation of trichloroethylene (TCE) biodehalogenation activity in response to indigenous microbial community succession in EK-Bio by both column and batch experiments. For a 195 day EK-Bio column (∼1 V/cm, electrolyte circulation, lactate addition), biodehalogenation activity occurred first near the cathode (<60 days) and then spread to the anode (>90 days), which was controlled by electron acceptor (i.e., Fe(III)) competition and microbe succession. Amplicon sequencing and metagenome analysis revealed that iron-reducing bacteria (Geobacter, Anaeromyxobacter, Geothrix) were enriched within initial 60 d and were gradually replaced by organohalide-respiring bacteria (versatile Geobacter and obligate Dehalobacter) afterward. Iron-reducing bacteria required an initial long time to consume the competitive electron acceptors so that an appropriate reductive condition could be developed for the enrichment of organohalide-respiring bacteria and the enhancement of TCE biodehalogenation activity.