Abstract
Bioelectrochemical systems (BESs) are regarded as a promising approach for the enhanced dechlorination of chlorinated aliphatic hydrocarbons (CAHs). However, the electron distribution and transfer considering dechlorination, methanogenesis, and other bioprocesses in these systems are little understood. This study investigated the electron fluxes in biocathode BES performing dechlorination of three typical CAHs, 1,1,2,2-tetrachloroethene (PCE), 1,1,2-trichloroethene (TCE) and 1,2-dichloroethane (1,2-DCA). Anaerobic sludge was inoculated to cathode and biocathode was acclimated by the direct acclimation and selection. The constructed biocathode at −0.26 V had significantly higher dechlorination efficiency (E24h > 99.0%) than the opened circuit (E24h of 17.2–27.5%) and abiotic cathode (E24h of 5.5–10.8%), respectively. Cyclic voltammetry analysis demonstrated the enhanced cathodic current and the positive shift of onset potential in the cathodic biofilm. Under autotrophic conditions with electrons from the cathode as sole energy source (columbic efficiencies of 80.4–90.0%) and bicarbonate as sole carbon source, CAHs dechlorination efficiencies were still maintained at 85.0 ± 2.0%, 91.4 ± 1.8%, and 84.9 ± 3.1% for PCE, TCE, and 1,2-DCA, respectively. Cis-1,2-dichloroethene was the final product for PCE and TCE, while 1,2-DCA went through a different dechlorination pathway with the non-toxic ethene as the final metabolite. Methane was the main by-product of the heterotrophic biocathode, and methane production could be enhanced to some extent by electrochemical stimulation. The various electron fluxes originating from the cathode and oxidation of organic substrates might be responsible for the enhanced CAHs dechlorination, while methane generation and bacterial growth would probably reduce the fraction of electrons provided for CAH dechlorination. The study deals with the dechlorination and competitive bioprocesses in CAH-dechlorinating biocathodes with a focus on electron fluxes.
Highlights
MATERIALS AND METHODSGroundwater contamination by chlorinated aliphatic hydrocarbons (CAHs), such as 1,1,2,2-tetrachloroethene (PCE), 1,1,2-trichloroethene (TCE) and 1,2-dichloroethane (1,2-DCA), have posed serious environmental and health concerns (Huang et al, 2014)
Several researches have recognized the role of electrochemical stimulation in regulating electron fluxes, the activity and the synergistic interactions of the microorganisms growing in the anode compartment of microbial fuel cells (Virdis et al, 2009; Wang et al, 2015b)
When biocathode was switched from heterotrophic to autotrophic (NaHCO3-fed) cultivation without external electron donor, E24h could still be maintained at 85.0 ± 2.0%, 91.4 ± 1.8% and 84.9 ± 3.1% for PCE, TCE, and 1,2-DCA, respectively
Summary
MATERIALS AND METHODSGroundwater contamination by chlorinated aliphatic hydrocarbons (CAHs), such as 1,1,2,2-tetrachloroethene (PCE), 1,1,2-trichloroethene (TCE) and 1,2-dichloroethane (1,2-DCA), have posed serious environmental and health concerns (Huang et al, 2014). For the three CAHs, the biocathode had 2.1–4.8 and 6.9–17.2 times higher dechlorination efficiency (E24h > 99.0%) than the opened circuit (E24h of 17.2–27.5%) and abiotic cathode (E24h of 5.5–10.8%), respectively, indicating the enhanced CAHs dechlorination performances with a biocatalyzed cathode (Figure 1). Based on the further negative scanning on cathode potential, about five times higher cathodic current area was observed in biocathode than that in the abiotic cathode, indicating the improved electrochemical activity for PCE reduction below −0.2 V (Figure 2).
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