The bioelectric well: a novel approach for in situ treatment of hydrocarbon-contaminated groundwater

Microbial electricity-driven anaerobic phenol degradation in bioelectrochemical systems

Microbial electrochemical technologies have been extensively employed for phenol removal. Yet, previous research has yielded inconsistent results, leaving uncertainties regarding the feasibility of phenol degradation under strictly anaerobic conditions using anodes as sole terminal electron acceptors. In this study, we employed high-performance liquid chromatography and gas chromatography-mass spectrometry to investigate the anaerobic phenol degradation pathway. Our findings provide robust evidence for the purely anaerobic degradation of phenol, as we identified benzoic acid, 4-hydroxybenzoic acid, glutaric acid, and other metabolites of this pathway. Notably, no typical intermediates of the aerobic phenol degradation pathway were detected. One-chamber reactors (+0.4 V vs. SHE) exhibited a phenol removal rate of 3.5 ± 0.2 mg L-1 d-1, while two-chamber reactors showed 3.6 ± 0.1 and 2.6 ± 0.9 mg L-1 d-1 at anode potentials of +0.4 and + 0.2 V, respectively. Our results also suggest that the reactor configuration certainly influenced the microbial community, presumably leading to different ratios of phenol consumers and microorganisms feeding on degradation products.

Toluene-driven anaerobic biodegradation of chloroform in a continuous-flow bioelectrochemical reactor

Subsurface co-contamination by multiple pollutants can be challenging for the design of bioremediation strategies since it may require promoting different and often antagonistic degradation pathways. Here, we investigated the simultaneous degradation of toluene and chloroform (CF) in a continuous-flow anaerobic bioelectrochemical reactor. As a result, 47 μmol L-1 d-1 of toluene and 60 μmol L-1 d-1 of CF were concurrently removed, when the anode was polarized at +0.4 V vs. Standard Hydrogen Electrode (SHE). Analysis of the microbial community structure and key functional genes allowed to identify the involved degradation pathways. Interestingly, when acetate was supplied along with toluene, to simulate the impact of a readily biodegradable substrate on process performance, toluene degradation was adversely affected, likely due to competitive inhibition effects. Overall, this study proved the efficacy of the developed bioelectrochemical system in simultaneously treating multiple groundwater contaminants, paving the way for the application in real-world scenarios.

A hybrid technique for sulfamethoxazole (SFM) removal using Enterobacter hormaechei HaG-7: Bio-electrokinetic degradation, pathway and toxicity

Prolonged exposure to antibiotics would likely favor the development of antibiotic resistance and their gene transfer among bacterial communities that are responsible for enriched antibiotic resistant microbes. Sulfamethoxazole (SFM) is a commonly used antibiotic that is released into the environment through human and animal wastes. Improper degradation of SFM poses severe threats to mankind and all life forms. The present study aims in analyzing the process and the probability of utilizing bio-electrokinetic degradation for elimination of SFM from artificially contaminated soil employing Enterobacter hormaechei HaG-7. The desired optimal conditions for SFM degradation (∼98%) were observed at SFM initial concentration (100 mg/L) with an inoculum dose (1% v/v) and applied potential voltage (1.5 V) at pH (7). The results indicated efficient and complete degradation of SFM when compared with the conventional biodegradation.

Bioelectrochemical degradation of petroleum hydrocarbons: A critical review and future perspectives

As typical pollutants, petroleum hydrocarbons that are widely present in various environmental media such as soil, water, sediments, and air, seriously endanger living organisms and human health. In the meantime, as a green environmental technology that integrates pollutant removal and resource recovery, bioelectrochemical systems (BESs) have been extensively applied to the removal of petroleum hydrocarbons from the environment. This review introduces working principles of BESs, following which it discusses the different reactor structures, application progresses, and key optimization factors when treating water, sewage sludges, sediments, and soil. Furthermore, bibliometrics was first used in this field to analyze the evolution of knowledge structure and forecast future hot topics. The research focus has shifted from the early generation of bioelectric energy to exploring mechanisms of soil remediation and microbial metabolisms, which will be closely integrated in the future. Finally, the future prospects of this field are proposed. This review focuses on the research status of bioelectrochemical degradation of petroleum hydrocarbons and provides a scientific reference for subsequent research.

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.

Soil microorganisms facilitated the electrode-driven trichloroethene dechlorination to ethene by Dehalococcoides species in a bioelectrochemical system

Bioelectrochemical dechlorination using organohalide-respiring bacteria (ORBs) is a promising technique for remediating contaminated groundwater. Generally, a longer enrichment period is required for selecting the ORB consortia to achieve bioelectrochemical dechlorination. However, the full dechloriantion is difficult to be achieved due to the absence of functional species (e.g. Dehalococcoides) in previously used enrich cultures. To overcome these challenges, bioelectrochemical dechlorination using a culture enriched with the pre-augmented Dehalococcoides was performed for the first time in this study. A two-chamber bioelectrochemical system (BES) inoculated with a pure Dehalococcoides culture and paddy soil with an applied voltage of -0.3 V (versus a standard hydrogen electrode) as the sole electron donor was used to achieve dechlorination. The ethene formation rate was 10-100 times higher than that in previous studies, indicating that inoculating the system with a pure Dehalococcoides culture and soil microorganisms gave effective full dechlorination performance. Microbial community analysis and bioelectrochemical analysis indicated that Desulfosporosinus species may have facilitated dechlorination through syntrophic interactions with Dehalococcoides. The results indicated that adding Dehalococcoides cells before operating a bioelectrochemical system is an effective way of achieving full dechlorination.

A Polyhydroxybutyrate (PHB)-Biochar Reactor for the Adsorption and Biodegradation of Trichloroethylene: Design and Startup Phase

In this work, polyhydroxy butyrate (PHB) and biochar from pine wood (PWB) are used in a mini-pilot scale biological reactor (11.3 L of geometric volume) for trichloroethylene (TCE) removal (80 mgTCE/day and 6 L/day of flow rate). The PHB-biochar reactor was realized with two sequential reactive areas to simulate a multi-reactive permeable barrier. The PHB acts as an electron donor source in the first “fermentative” area. First, the thermogravimetric (TGA) and differential scanning calorimetry (DSC) analyses were performed. The PHB-powder and pellets have different purity (96% and 93% w/w) and thermal properties. These characteristics may affect the biodegradability of the biopolymer. In the second reactive zone, the PWB works as a Dehalococcoides support and adsorption material since its affinity for chlorinated compounds and the positive effect of the “coupled adsorption and biodegradation” process has been already verified. A specific dechlorinating enriched culture has been inoculated in the PWB zone to realize a coupled adsorption and biodegradation process. Organic acids were revealed since the beginning of the test, and during the monitoring period the reductive dichlorination anaerobic pathway was observed in the first zone; no chlorinated compounds were detected in the effluent thanks to the PWB adsorption capacity.

Electrochemical water softening as pretreatment for nitrate electro bioremediation

Electro bioremediation is gaining interest as a sustainable treatment for contaminated groundwater. Nevertheless, the investigation is still at the laboratory level, and before their implementation is necessary to overcome important drawbacks. A prevalent issue is the high groundwater hardness that generates scale deposition on electrodes that irreversibly affects the treatment effectiveness and their lifetime. For this reason, the present study evaluated a novel and sustainable approach combining electrochemical water softening as a preliminary step for electro bioremediation of nitrate-contaminated groundwater. Batch mode tests were performed at mL-scale to determine the optimum reactor configuration (single- or two-chambers) and the suitable applied cathode potential for electrochemical softening. A single-chamber reactor working at a cathode potential of -1.2 V vs. Ag/AgCl was chosen. Continuous groundwater softening under this configuration achieved a hardness removal efficiency of 64 ± 4% at a rate of 305 ± 17 mg CaCO₃ m^-2_cathode h^-1. The saturation index at the effluent of the main minerals susceptible to precipitate (aragonite, calcite, and brucite) was reduced up to 90%. Softening activity plummeted after 13 days of operation due to precipitate deposition (mostly calcite) on the cathode surface. Polarity reversal periods were considered to detach the precipitated throughout the continuous operation. Their implementation every 3-4 days increased the softening lifetime by 48%, keeping a stable hardness removal efficiency. The nitrate content of softened groundwater was removed in an electro bioremediation system at a rate of 1269 ± 30 g NO₃^- m^-3_NCC d^-1 (97% nitrate removal efficiency). The energy consumption of the integrated system (1.4 kWh m^-3_treated) confirmed the competitiveness of the combined treatment and paves the ground for scaling up the process.

Evaluation of the Bioelectrochemical Approach and Different Electron Donors for Biological Trichloroethylene Reductive Dechlorination

Trichloroethylene (TCE) and more in general chlorinated aliphatic hydrocarbons (CAHs) can be removed from a contaminated matrix thanks to microorganisms able to perform the reductive dechlorination reaction (RD). Due to the lack of electron donors in the contaminated matrix, CAHs' reductive dechlorination can be stimulated by fermentable organic substrates, which slowly release molecular hydrogen through their fermentation. In this paper, three different electron donors constituted by lactate, hydrogen, and a biocathode of a bioelectrochemical cell have been studied in TCE dechlorination batch experiments. The batch reactors evaluated in terms of reductive dechlorination rate and utilization efficiency of the electron donor reported that the bio-electrochemical system (BES) showed a lower RD rate with respect of lactate reactor (51 ± 9 µeq/d compared to 98 ± 4 µeq/d), while the direct utilization of molecular hydrogen gave a significantly lower RD rate (19 ± 8 µeq/d), due to hydrogen low solubility in liquid media. The study also gives a comparative evaluation of the different electron donors showing the capability of the bioelectrochemical system to reach comparable efficiencies with a fermentable substrate without the use of other chemicals, 10.7 ± 3.3% for BES with respect of 3.5 ± 0.2% for the lactate-fed batch reactor. This study shows the BES capability of being an alternative at classic remediation approaches.

Metagenomic Analysis Reveals Microbial Interactions at the Biocathode of a Bioelectrochemical System Capable of Simultaneous Trichloroethylene and Cr(VI) Reduction

Bioelectrochemical systems (BES) are attractive and versatile options for the bioremediation of organic or inorganic pollutants, including trichloroethylene (TCE) and Cr(VI), often found as co-contaminants in the environment. The elucidation of the microbial players' role in the bioelectroremediation processes for treating multicontaminated groundwater is still a research need that attracts scientific interest. In this study, 16S rRNA gene amplicon sequencing and whole shotgun metagenomics revealed the leading microbial players and the primary metabolic interactions occurring in the biofilm growing at the biocathode where TCE reductive dechlorination (RD), hydrogenotrophic methanogenesis, and Cr(VI) reduction occurred. The presence of Cr(VI) did not negatively affect the TCE degradation, as evidenced by the RD rates estimated during the reactor operation with TCE (111±2 μeq/Ld) and TCE/Cr(VI) (146±2 μeq/Ld). Accordingly, Dehalococcoides mccartyi, the primary biomarker of the RD process, was found on the biocathode treating both TCE (7.82E+04±2.9E+04 16S rRNA gene copies g-1 graphite) and TCE/Cr(VI) (3.2E+07±2.37E+0716S rRNA gene copies g-1 graphite) contamination. The metagenomic analysis revealed a selected microbial consortium on the TCE/Cr(VI) biocathode. D. mccartyi was the sole dechlorinating microbe with H2 uptake as the only electron supply mechanism, suggesting that electroactivity is not a property of this microorganism. Methanobrevibacter arboriphilus and Methanobacterium formicicum also colonized the biocathode as H2 consumers for the CH4 production and cofactor suppliers for D. mccartyi cobalamin biosynthesis. Interestingly, M. formicicum also harbors gene complexes involved in the Cr(VI) reduction through extracellular and intracellular mechanisms.

Combined Strategies to Prompt the Biological Reduction of Chlorinated Aliphatic Hydrocarbons: New Sustainable Options for Bioremediation Application

Groundwater remediation is one of the main objectives to minimize environmental impacts and health risks. Chlorinated aliphatic hydrocarbons contamination is prevalent and presents particularly challenging scenarios to manage with a single strategy. Different technologies can manage contamination sources and plumes, although they are usually energy-intensive processes. Interesting alternatives involve in-situ bioremediation strategies, which allow the chlorinated contaminant to be converted into non-toxic compounds by indigenous microbial activity. Despite several advantages offered by the bioremediation approaches, some limitations, like the relatively low reaction rates and the difficulty in the management and control of the microbial activity, can affect the effectiveness of a bioremediation approach. However, those issues can be addressed through coupling different strategies to increase the efficiency of the bioremediation strategy. This mini review describes different strategies to induce the reduction dechlorination reaction by the utilization of innovative strategies, which include the increase or the reduction of contaminant mobility as well as the use of innovative strategies of the reductive power supply. Subsequently, three future approaches for a greener and more sustainable intervention are proposed. In particular, two bio-based materials from renewable resources are intended as alternative, long-lasting electron-donor sources (e.g., polyhydroxyalkanoates from mixed microbial cultures) and a low-cost adsorbent (e.g., biochar from bio-waste). Finally, attention is drawn to novel bio-electrochemical systems that use electric current to stimulate biological reactions.

New insights into the degradation of synthetic pollutants in contaminated environments

The environment is contaminated by synthetic contaminants owing to their extensive applications globally. Hence, the removal of synthetic pollutants (SPs) from the environment has received widespread attention. Different remediation technologies have been investigated for their abilities to eliminate SPs from the ecosystem; these include photocatalysis, sonochemical techniques, nanoremediation, and bioremediation. SPs, which can be organic or inorganic, can be degraded by microbial metabolism at contaminated sites. Owing to their diverse metabolisms, microbes can adapt to a wide variety of environments. Several microbial strains have been reported for their bioremediation potential concerning synthetic chemical compounds. The selection of potential strains for large-scale removal of organic pollutants is an important research priority. Additionally, novel microbial consortia have been found to be capable of efficient degradation owing to their combined and co-metabolic activities. Microbial engineering is one of the most prominent and promising techniques for providing new opportunities to develop proficient microorganisms for various biological processes; here, we have targeted the SP-degrading mechanisms of microorganisms. This review provides an in-depth discussion of microbial engineering techniques that are used to enhance the removal of both organic and inorganic pollutants from different contaminated environments and under different conditions. The degradation of these pollutants is investigated using abiotic and biotic approaches; interestingly, biotic approaches based on microbial methods are preferable owing to their high potential for pollutant removal and cost-effectiveness.

Effects of the Feeding Solution Composition on a Reductive/Oxidative Sequential Bioelectrochemical Process for Perchloroethylene Removal

Chlorinated aliphatic hydrocarbons (CAHs) are common groundwater contaminants due to their improper use in several industrial activities. Specialized microorganisms are able to perform the reductive dechlorination (RD) of high-chlorinated CAHs such as perchloroethylene (PCE), while the low-chlorinated ethenes such as vinyl chloride (VC) are more susceptible to oxidative mechanisms performed by aerobic dechlorinating microorganisms. Bioelectrochemical systems can be used as an effective strategy for the stimulation of both anaerobic and aerobic microbial dechlorination, i.e., a biocathode can be used as an electron donor to perform the RD, while a bioanode can provide the oxygen necessary for the aerobic dechlorination reaction. In this study, a sequential bioelectrochemical process constituted by two membrane-less microbial electrolysis cells connected in series has been, for the first time, operated with synthetic groundwater, also containing sulphate and nitrate, to simulate more realistic process conditions due to the possible establishment of competitive processes for the reducing power, with respect to previous research made with a PCE-contaminated mineral medium (with neither sulphate nor nitrate). The shift from mineral medium to synthetic groundwater showed the establishment of sulphate and nitrate reduction and caused the temporary decrease of the PCE removal efficiency from 100% to 85%. The analysis of the RD biomarkers (i.e., Dehalococcoides mccartyi 16S rRNA and tceA, bvcA, vcrA genes) confirmed the decrement of reductive dechlorination performances after the introduction of the synthetic groundwater, also characterized by a lower ionic strength and nutrients content. On the other hand, the system self-adapted the flowing current to the increased demand for the sulphate and nitrate reduction, so that reducing power was not in defect for the RD, although RD coulombic efficiency was less.

Bioelectrochemical degradation of monoaromatic compounds: Current advances and challenges

Monoaromatic compounds (MACs) are typical refractory organic pollutants which are existing widely in various environments. Biodegradation strategies are benign while the key issue is the sustainable supply of electron acceptors/donors. Bioelectrochemical system (BES) shows great potential in this field for providing continuous electrons for MACs degradation. Phenol and BTEX (Benzene, Toluene, Ethylbenzene and Xylenes) can utilize anode to enhance oxidative degradation, while chlorophenols, nitrobenzene and antibiotic chloramphenicol (CAP) can be efficiently reduced to less-toxic products by the cathode. However, there still have several aspects need to be improved including the scale, electricity output and MACs degradation efficiency of BES. This review provides a comprehensive summary on the BES degradation of MACs, and discusses the advantages, future challenges and perspectives for BES development. Instead of traditional expensive dual-chamber configurations for MACs degradation, new single-chamber membrane-less reactors are cost-effective and the hydrogen generated from cathodes may promote the anode degradation. Electrode materials are the key to improve BES performance, approaches to increase the biofilm enrichment and conductivity of materials have been discussed, including surface modification as well as composition of carbon and metal-based materials. Besides, the development and introduction of functional microbes and redox mediators, participation of sulfur/hydrogen cycling may further enhance the BES versatility. Some critical parameters, such as the applied voltage and conductivity, can also affect the BES performance, which shouldn't be overlooked. Moreover, sequential cathode-anode cascaded mode is a promising strategy for MACs complete mineralization.

Enhanced removal of bisphenol A from contaminated soil by coupling Bacillus subtilis HV-3 with electrochemical system

Exposure to endocrine disruptors interferes with the synthesis, release, transport and metabolic activities of hormones, thus impairing human health significantly. Bisphenol A (BpA), an endocrine disruptor, commonly released into the environment by industrial activities and needs immediate attention. This study aims at investigating the process and prospects of deploying bio-electrochemical systems (BES) for the removal of BpA from artificially contaminated soil using Bacillus subtilis HV-3. The BES was setup with desired operating conditions: initial concentration of BpA (80-150 mg/L), pH (3-11) and applied potential voltage (0.6-1.4 V). Under optimized conditions (initial BpA concentration, 100 mg/L; pH 7; and applied voltage 1.0 V), close to 98% degradation of BpA was achieved. The intermediates produced during degradation were analysed using High performance liquid chromatography-Mass spectrometry and the possible degradation pathway was elucidated. Phytotoxicity studies in the remediated soil with Phaseolus mungo confirmed the environmental applicability of the BES system.

In situ groundwater remediation with bioelectrochemical systems: A critical review and future perspectives

Groundwater contamination is an ever-growing environmental issue that has attracted much and undiminished attention for the past half century. Groundwater contamination may originate from both anthropogenic (e.g., hydrocarbons) and natural compounds (e.g., nitrate and arsenic); to tackle the removal of these contaminants, different technologies have been developed and implemented. Recently, bioelectrochemical systems (BES) have emerged as a potential treatment for groundwater contamination, with reported in situ applications that showed promising results. Nitrate and hydrocarbons (toluene, phenanthrene, benzene, BTEX and light PAHs) have been successfully removed, due to the interaction of microbial metabolism with poised electrodes, in addition to physical migration due to the electric field generated in a BES. The selection of proper BESs relies on several factors and problems, such as the complexity of groundwater and subsoil environment, scale-up issues, and energy requirements that need to be accounted for. Modeling efforts could help predict case scenarios and select a proper design and approach, while BES-based biosensing could help monitoring remediation processes. In this review, we critically analyze in situ BES applications for groundwater remediation, focusing in particular on different proposed setups, and we identify and discuss the existing research gaps in the field.

Structure and Functions of Hydrocarbon-Degrading Microbial Communities in Bioelectrochemical Systems

Bioelectrochemical systems (BESs) exploit the interaction between microbes and electrodes. A field of application thereof is bioelectrochemical remediation, an effective strategy in environments where the absence of suitable electron acceptors limits classic bioremediation approaches. Understanding the microbial community structure and genetic potential of anode biofilms is of great interest to interpret the mechanisms occurring in BESs. In this study, by using a whole metagenome sequencing approach, taxonomic and functional diversity patterns in the inoculum and on the anodes of three continuous-flow BES for the removal of phenol, toluene, and BTEX were obtained. The genus Geobacter was highly enriched on the anodes and two reconstructed genomes were taxonomically related to the Geobacteraceae family. To functionally characterize the microbial community, the genes coding for the anaerobic degradation of toluene, ethylbenzene, and phenol were selected as genetic markers for the anaerobic degradation of the pollutants. The genes related with direct extracellular electron transfer (EET) were also analyzed. The inoculum carried the genetic baggage for the degradation of aromatics but lacked the capacity of EET while anodic bacterial communities were able to pursue both processes. The metagenomic approach provided useful insights into the ecology and complex functions within hydrocarbon-degrading electrogenic biofilms.

Progress Towards Bioelectrochemical Remediation of Hexavalent Chromium

Chromium is one of the most frequently used metal contaminants. Its hexavalent form Cr(VI), which is exploited in many industrial activities, is highly toxic, is water-soluble in the full pH range, and is a major threat to groundwater resources. Alongside traditional approaches to Cr(VI) treatment based on physical-chemical methods, technologies exploiting the ability of several microorganisms to reduce toxic and mobile Cr(VI) to the less toxic and stable Cr(III) form have been developed to improve the cost-effectiveness and sustainability of remediating hexavalent chromium-contaminated groundwater. Bioelectrochemical systems (BESs), principally investigated for wastewater treatment, may represent an innovative option for groundwater remediation. By using electrodes as virtually inexhaustible electron donors and acceptors to promote microbial oxidation-reduction reactions, in in situ remediation, BESs may offer the advantage of limited energy and chemicals requirements in comparison to other bioremediation technologies, which rely on external supplies of limiting inorganic nutrients and electron acceptors or donors to ensure proper conditions for microbial activity. Electron transfer is continuously promoted/controlled in terms of current or voltage application between the electrodes, close to which electrochemically active microorganisms are located. Therefore, this enhances the options of process real-time monitoring and control, which are often limited in in situ treatment schemes. This paper reviews research with BESs for treating chromium-contaminated wastewater, by focusing on the perspectives for Cr(VI) bioelectrochemical remediation and open research issues.

Simulation tests of in situ groundwater denitrification with aquifer-buried biocathodes

Bioelectrochemical systems (BES) application was proposed for a variety of specific uses, due to these systems’ characteristics: electrodes can act as virtually inexhaustible electron acceptors/donors, offering a growth-support surface for microorganisms, and stimulating naturally-occurring microbial degradation activities. In situ, groundwater denitrification therefore seems to be a potential candidate for their use. In this study, buried biocathodes were operated in laboratory settings for the simulation of in situ groundwater denitrification. Two alternative configurations were tested: biocathode buried in sand, and biocathode buried in gravel. A control test with a biocathode in absence of sand/gravel was also performed. In all the cases, biocathodes were driven by power supply or potentiostat to guarantee a steady electron flux to the cathode. The presence of sand and gravel strongly influenced the denitrification process: in both configurations, accumulation of intermediate N-forms was detected, suggesting that the denitrification process was only partially achieved. In addition, a significant decrease (in the 20–36% range) in nitrate removal rates was measured in sand and gravel setups compared to the control reactor; this issue could be attributed to lack of recirculation that limited contact between substrate and electrode-adherent biofilm. Biocathodes buried in gravel obtained better results than those buried in sand due to the lower packing of the medium. The results of this study suggest that, in order to achieve successful in situ treatment, special design of submerged-biocathodic BESs is necessary.

Operation of a 2-Stage Bioelectrochemical System for Groundwater Denitrification

Nitrate groundwater contamination is an issue of global concern that has not been satisfactorily and efficiently addressed, yet. In this study, a 2-stage, sequential bioelectrochemical system (BES) was run to perform autotrophic denitrification of synthetic groundwater. The system was run at a 75.6 mgNO3−-N L−1NCC d−1 nitrate loading rate, achieving almost complete removal of nitrate (>93%) and Total Nitrogen (TN) (>93%). After treatment in the first stage reactor values of effluent nitrate compatible with the EU and USA limits for drinking water (<11.3 and 10 mgNO3−-N L−1, respectively) were achieved. Nitrite and nitrous oxide were observed in the first stage’s effluent, and were then successfully removed in the second stage. The observed nitrate removal rate was 73.4 ± 1.3 gNO3−-N m−3NCC d−1, while the total nitrogen removal rate was 73.1 ± 1.2 gN m−3NCC d−1. Specific energy consumptions of the system were 0.80 ± 0.00 kWh m−3, 18.80 ± 0.94 kWh kgNO3−-N−1 and 18.88 ± 0.95 kWh kgN−1. Combination of two denitrifying BES in series herein described proved to be effective.

Enhancement of gasworks groundwater remediation by coupling a bio-electrochemical and activated carbon system

Here, we show the electrical response, bacterial community, and remediation of hydrocarbon-contaminated groundwater from a gasworks site using a graphite-chambered bio-electrochemical system (BES) that utilizes granular activated carbon (GAC) as both sorption agent and high surface area anode. Our innovative concept is the design of a graphite electrode chamber system rather than a classic non-conductive BES chamber coupled with GAC as part of the BES. The GAC BES is a good candidate as a sustainable remediation technology that provides improved degradation over GAC, and near real-time observation of associated electrical output. The BES chambers were effectively colonized by the bacterial communities from the contaminated groundwater. Principal coordinate analysis (PCoA) of UniFrac Observed Taxonomic Units shows distinct grouping of microbial types that are associated with the presence of GAC, and grouping of microbial types associated with electroactivity. Bacterial community analysis showed that β-proteobacteria (particularly the PAH-degrading Pseudomonadaceae) dominate all the samples. Rhodocyclaceae- and Comamonadaceae-related OTU were observed to increase in BES cells. The GAC BES (99% removal) outperformed the control graphite GAC chamber, as well as a graphite BES and a control chamber both filled with glass beads.

Bioelectrochemical Systems for Removal of Selected Metals and Perchlorate from Groundwater: A Review

Groundwater contamination is a major issue for human health, due to its largely diffused exploitation for water supply. Several pollutants have been detected in groundwater; amongst them arsenic, cadmium, chromium, vanadium, and perchlorate. Various technologies have been applied for groundwater remediation, involving physical, chemical, and biological processes. Bioelectrochemical systems (BES) have emerged over the last 15 years as an alternative to conventional treatments for a wide variety of wastewater, and have been proposed as a feasible option for groundwater remediation due to the nature of the technology: the presence of two different redox environments, the use of electrodes as virtually inexhaustible electron acceptor/donor (anode and cathode, respectively), and the possibility of microbial catalysis enhance their possibility to achieve complete remediation of contaminants, even in combination. Arsenic and organic matter can be oxidized at the bioanode, while vanadium, perchlorate, chromium, and cadmium can be reduced at the cathode, which can be biotic or abiotic. Additionally, BES has been shown to produce bioenergy while performing organic contaminants removal, lowering the overall energy balance. This review examines the application of BES for groundwater remediation of arsenic, cadmium, chromium, vanadium, and perchlorate, focusing also on the perspectives of the technology in the groundwater treatment field.

Electron Fluxes in Biocathode Bioelectrochemical Systems Performing Dechlorination of Chlorinated Aliphatic Hydrocarbons

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 (E_24h > 99.0%) than the opened circuit (E_24h of 17.2–27.5%) and abiotic cathode (E_24h 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.

The bioelectric well: a novel approach for in situ treatment of hydrocarbon-contaminated groundwater

Groundwater contamination by petroleum hydrocarbons (PHs) is a widespread problem which poses serious environmental and health concerns. Recently, microbial electrochemical technologies (MET) have attracted considerable attention for remediation applications, having the potential to overcome some of the limiting factors of conventional in situ bioremediation systems. So far, field‐scale application of MET has been largely hindered by the limited availability of scalable system configurations. Here, we describe the ‘bioelectric well’ a bioelectrochemical reactor configuration, which can be installed directly within groundwater wells and can be applied for in situ treatment of organic contaminants, such as PHs. A laboratory‐scale prototype of the bioelectric well has been set up and operated in continuous‐flow regime with phenol as the model contaminant. The best performance was obtained when the system was inoculated with refinery sludge and the anode potentiostatically controlled at +0.2 V versus SHE. Under this condition, the influent phenol (25 mg l⁻¹) was nearly completely (99.5 ± 0.4%) removed, with an average degradation rate of 59 ± 3 mg l⁻¹ d and a coulombic efficiency of 104 ± 4%. Microbial community analysis revealed a remarkable enrichment of Geobacter species on the surface of the graphite anode, clearly pointing to a direct involvement of this electro‐active bacterium in the current‐generating and phenol‐oxidizing process.