Membrane-Less Bioelectrochemical Reactor for the Treatment of Groundwater Contaminated by Toluene and Trichloroethene

To address the ever-growing environmental problem of groundwater contamination, microbial electrochemical technologies (METs) are being studied as promising substitutes for traditional remediation techniques. Among their many advantages, they possess the capability of providing a virtually inexhaustible electron acceptor (or donor) directly in the aquifer without addition of air, oxygen or other chemicals. In this way, they can promote microbially-driven oxidation and/or reduction of contaminants in-situ, in a more sustainable and cost-effective way. In this work, a tubular membrane-less bioelectrochemical reactor known as the “bioelectric well” was tested at the laboratory-scale for the simultaneous removal of toluene and trichloroethene (TCE), two ubiquitous and harmful groundwater pollutants. The reactor housed a cylindrical graphite anode and a concentric stainless-steel mesh cathode, which were kept physically separated by a polyethylene mesh, which still ensured hydraulic connection between the electrodes. The anode potential was set at +0.2 V vs. the standard hydrogen electrode (SHE) by means of a potentiostat. The reactor was packed with sand in order to favor the retention of slow-growing anaerobic dechlorinating bacteria. Throughout the study, the reactor was operated in continuous-flow mode (average hydraulic retention time 12 hours), and was fed with artificial groundwater spiked with toluene and TCE. The performance of the system was evaluated primarily in terms of the electric current generation, toluene oxidation, TCE removal, reductive dechlorination (RD) products formation, and coulombic efficiency. When the system was polarized, a steady increase in the removal of contaminants was observed, together with an increase in the formation of RD products and methane. At the end of the polarized run, the removal rates of toluene and TCE were as high as 0.76 ± 0.23 and 0.35 ± 0.05 µmol L-1 d-1, corresponding to 38% and 89% of the contaminant load present in the influent for toluene and TCE, respectively. Overall, the obtained results convincingly demonstrate that anodic and cathodic processes can be simultaneously exploited within an ad hoc designed bioelectrochemical reactor for the treatment of problematic groundwater containing a mixture of oxidizable and reducible contaminants.