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

Coupling of bioelectrochemical toluene oxidation and trichloroethene reductive dechlorination for single-stage treatment of groundwater containing multiple contaminants

Bioremediation of groundwater contaminated by a mixture of aromatic hydrocarbons and chlorinated solvents is typically challenged because these contaminants are degraded via distinctive oxidative and reductive pathways, thus requiring different amendments and redox conditions. Here, we provided the proof-of-concept of a single-stage treatment of synthetic groundwater containing toluene and trichloroethene (TCE) in a tubular bioelectrochemical reactor, known as a “bioelectric well”. Toluene was degraded by a microbial bioanode (up to 150 μmol L−1 d−1) with a polarized graphite anode (+0.2 V vs. SHE) serving as the terminal electron acceptor. The electric current deriving from microbially-driven toluene oxidation resulted in (abiotic) hydrogen production (at a stainless-steel cathode), which sustained the reductive dechlorination of TCE to less-chlorinated intermediates (i.e., cis-DCE, VC, and ETH), at a maximum rate of 500 μeq L−1 d−1, in the bulk of the reactor. A phylogenetic and functional gene-based analysis of the “bioelectric well” confirmed the establishment of a microbiome harboring the metabolic potential for anaerobic toluene oxidation and TCE reductive dechlorination. However, Toluene degradation and current generation were found to be rate-limited by external mass transport phenomena, thus indicating the existing potential for further process optimization

Syntrophy drives the microbial electrochemical oxidation of toluene in a continuous-flow "bioelectric well"

Microbial electrochemical technologies (MET) are promising for the remediation of groundwater pollutants such as petroleum hydrocarbons (PH). Indeed, MET can provide virtually inexhaustible electron donors or acceptors directly in the subsurface environment. However, the degradation mechanisms linking contaminants removal to electric current flow are still largely unknown, hindering the development of robust design criteria. Here, we analysed the degradation of toluene, a model PH, in a bioelectrochemical reactor known as "bioelectric well"operated in continuous-flow mode at various influent toluene concentrations. With increasing concentration of toluene, the removal rate increased while the current tended to a plateau, hence the columbic efficiency decreased. Operation at open circuit confirmed that the bioelectrochemical degradation of toluene proceeded via a syntrophic pathway involving cooperation between different microbial populations. First of all, hydrocarbon degraders quickly converted toluene into metabolic intermediates probably by breaking the aromatic ring upon fumarate addition. Subsequently, fermentative bacteria converted these intermediates into volatile fatty acids (VFA) and likely also H2, which were then used as substrates by electroactive microorganisms forming the anodic biofilm. As toluene degradation is faster than subsequent conversion steps, the increase in intermediate concentration could not result in a current increase. This work provides valuable insights on the syntrophic degradation of BTEX, which are essential for the application of microbial electrochemical system to groundwater remediation of petroleum hydrocarbons

A microcosm treatability study for evaluating wood mulch-based amendments as electron donors for trichloroethene (Tce) reductive dechlorination

In this study, wood mulch-based amendments were tested in a bench-scale microcosm experiment in order to assess the treatability of saturated soils and groundwater from an industrial site contaminated by chlorinated ethenes. Wood mulch was tested alone as the only electron donor in order to assess its potential for stimulating the biological reductive dechlorination. It was also tested in combination with millimetric iron filings in order to assess the ability of the additive to accelerate/improve the bioremediation process. The efficacy of the selected amendments was compared with that of unamended control microcosms. The results demonstrated that wood mulch is an effective natural and low-cost electron donor to stimulate the complete reductive dechlorination of chlorinated solvents to ethene. Being a side-product of the wood industry, mulch can be used in environmental remediation, an approach which perfectly fits the principles of circular economy and addresses the compelling needs of a sustainable and low environmental impact remediation. The efficacy of mulch was further improved by the co-presence of iron filings, which accelerated the conversion of vinyl chloride into the ethene by increasing the H2 availability rather than by catalyzing the direct abiotic dechlorination of contaminants. Chemical analyses were corroborated by biomolecular assays, which confirmed the stimulatory effect of the selected amendments on the abundance of Dehalococcoides mccartyi and related reductive dehalogenase genes. Overall, this paper further highlights the application potential and environmental sustainability of wood mulch-based amendments as low-cost electron donors for the biological treatment of chlorinated ethenes

Passive electrobioremediation approaches for enhancing hydrocarbons biodegradation in contaminated soils

Electrobioremediation technologies hold considerable potential for the treatment of soils contaminated by petroleum hydrocarbons (PH), since they allow stimulating biodegradation processes with no need for subsurface chemicals injection and with little to no energy consumption. Here, a microbial electrochemical snorkel (MES) was applied for the treatment of a soil contaminated by hydrocarbons. The MES consists of direct coupling of a microbial anode with a cathode, being a single conductive, non-polarized material positioned suitably to create an electrochemical connection between the anoxic zone (the contaminated soil) and the oxic zone (the overlying oxygenated water). Soil was also supplemented with electrically conductive particles of biochar as a strategy to construct a conductive network with microbes in the soil matrix, thus extending the radius of influence of the snorkel. The results of a comprehensive suite of chemical, microbiological and ecotoxicological analyses evidenced that biochar addition, rather than the presence of a snorkel, was the determining factor in accelerating PH removal from contaminated soils, possibly accelerating syntrophic and/or cooperative metabolisms involved in the degradation of PH. The enhancement of biodegradation was mirrored by an increased abundance of anaerobic and aerobic microorganisms known to be involved in the degradation of PH and related functional genes. Plant ecotoxicity assays confirmed a reduction of soils toxicity in treatments receiving electrically conductive biochar

Simultaneous removal of hydrocarbons and sulfate from groundwater using a “bioelectric well”

Petroleum hydrocarbons (PHs) are often found in groundwater due to human activities like accidental spills, causing health and environmental risks, and requiring remediation. Microbial Electrochemical Technologies (METs) have emerged as a promising alternative to conventional bioremediation techniques for the treatment of PH-contaminated groundwater. However, the field-application of these promising sustainable as well as cost-effective technologies is still scarce. One major reason is the lack of scalable reactor configurations. Herein, an upgraded version of the “bioelectric well”, a novel tubular bioelectrochemical reactor that can be installed directly within a groundwater well, was tested for the simultaneous removal of oxidableoxidizable (i.e., toluene and other PH) and reducible (i.e., sulfate) compounds from a real contaminated groundwater. After a proof-of-concept study in batch mode, the system was operated in continuous-flow mode for 48 days with the anode polarized at 0.2 V vs. SHE and a hydraulic retention time of 11 h. In these conditions, a steady-state removal rate of toluene as high as 31 ± 2 mg L−1 d−1 was achieved, which was more than double the value observed with the open circuit potential (OCP) control and one of the highest reported in literature. The electrode polarization went along with a higher abundance of key-functional genes involved in toluene degradation. This was not only showing its clear functional connection to the microbial metabolism, but further allowed to identify the involved electrogenic biodegradation pathway. In addition, the system simultaneously removed sulfate (30 ± 1 mg L−1 d−1), with bacteria likely using the H2 generated at the cathode as electron donor. Nevertheless, the apparent sulfate removal rate in the polarized and in the OCP runs was similar. The analysis of the microbial communities evidenced a high abundance of the genus Chlorobium in the effluent of the polarized run. These microorganisms were probably responsible for the continuous oxidative regeneration of sulfate from the sulfide produced at the cathode by sulfate-reducing bacteria. This phenomenon probably hindered the overall removal of sulfate by the bioelectrochemical system