Sustainable remediation: electrochemically assisted microbial dechlorination of tetrachloroethene-contaminated groundwater

Abstract

Microbial electric systems (MESs) hold significant promise for the sustainable remediation of chlorinated solvents such as tetrachlorethene (perchloroethylene, PCE). Although the bio-electrochemical potential of some specific bacterial species such as Dehalcoccoides and Geobacteraceae have been exploited, this ability in other undefined microorganisms has not been extensively assessed. Hence, the focus of this study was to investigate indigenous and potentially bio-electrochemically active microorganisms in PCE-contaminated groundwater. Lab-scale MESs were fed with acetate and carbon electrode/PCE as electron donors and acceptors, respectively, under biostimulation (BS) and BS-bioaugmentation (BS-BA) regimes. Molecular analysis of the indigenous groundwater community identified mainly Spirochaetes, Firmicutes, Bacteroidetes, and γ and δ-Proteobacteria. Environmental scanning electron photomicrographs of the anode surfaces showed extensive indigenous microbial colonization under both regimes. This colonization and BS resulted in 100% dechlorination in both treatments with complete dechlorination occurring 4 weeks earlier in BS-BA samples and up to 11.5 μA of current being generated. The indigenous non-Dehalococcoides community was found to contribute significantly to electron transfer with ∼61% of the current generated due to their activities. This study therefore shows the potential of the indigenous non-Dehalococcoides bacterial community in bio-electrochemically reducing PCE that could prove to be a cost-effective and sustainable bioremediation practice.

Fig. 1

(A) Components of two-chamber NCBE-type MES used in this study (Bennetto, 1990); (B) schematics explaining mechanism of MES.

Fig. 2

MES-assisted reductive dechlorination of PCE and simultaneous current production in (A) MES 1 and 2 with BS and (B) MES 3 and 4 with BS-BA treatments. Values are average of duplicate cultures. The arrows show when fresh catholyte and electron donors were added.

Fig. 3

Microbial community fingerprint from MES analysed using DGGE. MES 1 and 2 represent BS only (lanes 2–5); MES 3 and 4 with BS-BA treatment (lanes 7–10), while lane 12 indicates control MES. Lanes 1, 6 and 11 represent marker. Band numbers designate dominant bands excised from DGGE gel for sequence analysis that correspond to the band numbers in Table 2.

Fig. 4

Environmental scanning electron photomicrographs of bacterial biofilms grown on carbon fibre anode surfaces during PCE dechlorination within (A) MES with BS only (B) MES with BS-BA and (C) control 1–3 MES at the end of experiment. No biofilm was observed in controls 1–3 MES; however, thick biofilm was noticeable in MES run with BS and BS-BA treatments.