Microbial reductive dechlorination of trichloroethene to ethene with electrodes serving as electron donors without the external addition of redox mediators

Chemical-physical parameters and microbial community changes induced by electrodes polarization inhibit PCB dechlorination in a marine sediment

Microbial reductive dechlorination of organohalogenated pollutants is often limited by the scarcity of electron donors, that can be overcome with microbial electrochemical technologies (METs). In this study, polarized electrodes buried in marine sediment microcosms were investigated to stimulate PCB reductive dechlorination under potentiostatic (-0.7 V vs Ag/AgCl) and galvanostatic conditions (0.025 mA·cm-2-0.05 mA·cm-2), using graphite rod as cathode and iron plate as sacrificial anode. A single circuit and a novel two antiparallel circuits configuration (2AP) were investigated. Single circuit polarization impacted the sediment pH and redox potential (ORP) proportionally to the intensity of the electrical input and inhibited PCB reductive dechlorination. The effects on the sediment's pH and ORP, along with the inhibition of PCB reductive dechlorination, were mitigated in the 2AP system. Electrodes polarization stimulated sulfate-reduction and promoted the enrichment of bacterial clades potentially involved in sulfate-reduction as well as in sulfur oxidation. This suggested the electrons provided were consumed by competitors of organohalide respiring bacteria and specifically sequestered by sulfur cycling, which may represent the main factor limiting the applicability of METs for stimulating PCB reductive dechlorination in marine sediments.

Tetrachloroethane (TeCA) removal through sequential graphite-mixed metal oxide electrodes in a bioelectrochemical reactor

Electro-bioremediation offers a promising approach for eliminating persistent pollutants from groundwater since allows the stimulation of biological dechlorinating activity, utilizing renewable electricity for process operation and avoiding the injection of chemicals into aquifers. In this study, a two-chamber microbial electrolysis cell has been utilized to achieve both reductive and oxidative degradation of tetrachloroethane (TeCA). By polarizing the graphite granules cathodic chamber at -650 mV vs the standard hydrogen electrode and employing a mixed metal oxide (MMO) counter electrode for oxygen production, the reductive and oxidative environment necessary for TeCA removal has been established. Continuous experiments were conducted using two feeding solutions: an optimized mineral medium for dechlorinating microorganisms, and synthetic groundwater containing sulphate and nitrate anions to investigate potential side reactions. The bioelectrochemical process efficiently reduced TeCA to a mixture of trans-dichloroethylene, vinyl chloride, and ethylene, which were subsequently oxidized in the anodic chamber with removal efficiencies of 37 ± 2%, 100 ± 4%, and 100 ± 5%, respectively. The introduction of synthetic groundwater with nitrate and sulphate stimulated reductions in these ions in the cathodic chamber, leading to a 17% decrease in the reductive dechlorination rate and the appearance of other chlorinated by-products, including cis-dichloroethylene and 1,2-dichloroethane (1,2-DCA), in the cathode effluent. Notably, despite the lower reductive dechlorination rate during synthetic groundwater operation, aerobic dechlorinating microorganisms within the anodic chamber completely removed VC and 1,2-DCA. This study represents the first demonstration of a sequential reductive and oxidative bioelectrochemical process for TeCA mineralization in a synthetic solution simulating contaminated groundwater.

Bioelectrochemical system accelerates reductive dechlorination through extracellular electron transfer networks

Bioelectrochemical system is considered as a promising approach for enhanced bio-dechlorination. However, the mechanism of extracellular electron transfer in the dechlorinating consortium is still a controversial issue. In this study, bioelectrochemical systems were established with cathode potential settings at -0.30 V (vs. SHE) for trichloroethylene reduction. The average dechlorination rate (102.0 μM Cl·d-1) of biocathode was 1.36 times higher than that of open circuit (74.7 μM Cl·d-1). Electrochemical characterization via cyclic voltammetry illustrated that electrostimulation promoted electrochemical activity for redox reactions. Moreover, bacterial community structure analyses indicated electrical stimulation facilitated the enrichment of electroactive and dechlorinating populations on cathode. Metagenomic and quantitative polymerase chain reaction (qPCR) analyses revealed that direct electron transfer (via electrically conductive pili, multi-heme c-type cytochromes) between Axonexus and Desulfovibrio/cathode and indirect electron transfer (via riboflavin) for Dehalococcoides enhanced dechlorination process in BES. Overall, this study verifies the effectiveness of electrostimulated bio-dechlorination and provides novel insights into the mechanisms of dechlorination process enhancement in bioelectrochemical systems through electron transfer networks.

Electrokinetic-Enhanced Bioremediation of Trichloroethylene-Contaminated Low-Permeability Soils: Mechanistic Insight from Spatio-Temporal Variations of Indigenous Microbial Community and Biodehalogenation Activity

Electrokinetic-enhanced bioremediation (EK-Bio), particularly bioaugmentation with injection of biodehalogenation functional microbes such as Dehalococcoides, has been documented to be effective in treating a low-permeability subsurface matrix contaminated with chlorinated ethenes. However, the spatio-temporal variations of indigenous microbial community and biodehalogenation activity of the background matrix, a fundamental aspect for understanding EK-Bio, remain unclear. To fill this gap, we investigated the variation of trichloroethylene (TCE) biodehalogenation activity in response to indigenous microbial community succession in EK-Bio by both column and batch experiments. For a 195 day EK-Bio column (∼1 V/cm, electrolyte circulation, lactate addition), biodehalogenation activity occurred first near the cathode (<60 days) and then spread to the anode (>90 days), which was controlled by electron acceptor (i.e., Fe(III)) competition and microbe succession. Amplicon sequencing and metagenome analysis revealed that iron-reducing bacteria (Geobacter, Anaeromyxobacter, Geothrix) were enriched within initial 60 d and were gradually replaced by organohalide-respiring bacteria (versatile Geobacter and obligate Dehalobacter) afterward. Iron-reducing bacteria required an initial long time to consume the competitive electron acceptors so that an appropriate reductive condition could be developed for the enrichment of organohalide-respiring bacteria and the enhancement of TCE biodehalogenation activity.

Humin and biochar accelerated microbial reductive dechlorination of 2,4,6-trichlorophenol under weak electrical stimulation

The extracellular electron transfer (EET) is regarded as one of the crucial factors that limit the application of the bioelectrochemical system (BES). In this study, two different solid-phase redox mediators (RMs), biochar (1.2 g/L, T-B) and humin (1.2 g/L, T-H) were used for boosting the microorganisms accessing the electrons required for 2,4,6-TCP dechlorination under weak electrical stimulation (-0.278 V vs. Standard hydrogen electrode). BES with dissolved RM anthraquinone-2,6-disulfonate (AQDS 0.5 mmol/L, T-A) was used as a comparison. The results showed that dechlorination of 2,4,6-TCP could be greatly accelerated by biochar (1.78 d^-1) and humin (1.50 d^-1) than AQDS (0.24 d^-1) and no RM control (T-M, 0.27 d^-1). Moreover, phenol became the predominant dechlorination product in T-H (78.5 %) and T-B (63.0 %) instead of 4-CP in T-M (67.1 %) and T-A (89.8 %). Pseudomonas, Sulfurospirillum, Desulfuromonas, Dehalobacter, Anaeromyxobacter, and Dechloromonas belonging to Proteobacteria or Firmicutes rather than Chloroflexi might be responsible for the dechlorination activity. Notably, different RMs tended to stimulate distinct electroactive bacteria. Pseudomonas was the most abundant microorganism in T-M (41.92 %) and T-A (17.24 %), while Rhodobacter was most prevalent in T-H (20.04 %) and Azonexus was predominant in T-B (48.48 %). This study is essential in advancing the understanding of EET in BES for microbial degradation of organohalide contaminants under weak electrical stimulation.

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.

Response of 2,4,6-trichlorophenol-reducing biocathode to burial depth in constructed wetland sediments

Biocathode systems could be used for in-situ bioremediation of chlorophenols (CPs) in constructed wetland (CW) sediments. However, little is known regarding whether or how cathode burial depths affect the dechlorination of CPs in sediments. Here, 2,4,6-trichlorophenol (2,4,6-TCP)-dechlorinating biocathode systems were constructed under a cathode potential of - 0.7 V (vs. a saturated calomel electrode, SCE) at three different cathode burial depths (5, 10, and 15 cm). The 2,4,6-TCP removal efficiency and average transformation rate with the biocathode increased by 21.46-36.86% and 14.63-34.88% compared to those in the non-electrode groups. Deeper cathode burial depths enhanced the 2,4,6-TCP dechlorination performance. Furthermore, the oxidation-reduction potential (ORP) of the sediment decreased with sediment depth and the applied potential created a more favorable redox environment for the enrichment of functional bacteria. Deeper cathode burial depths also promoted the selective enrichment of electro-active and dechlorinating bacteria (e.g., Bacillus and Dehalobacter, respectively). The biocathode thus served as the carrier, electron source, and regulator of functional bacteria to accelerate the transformation of 2,4,6-TCP (2,4,6-TCP → 2,4-dichlorophenol → 4-chlorophenol → phenol) in sediments. These results offer insights into the effects of cathode burial depth on 2,4,6-TCP dechlorination in sediments from a redox environment and microbial community structure standpoint.

Bioelectrochemical system for dehalogenation: A review

Halogenated organic compounds are persistent pollutants, whose persistent contamination and rapid spread seriously threaten human health and the safety of ecosystems. It is difficult to remove them completely by traditional physicochemical techniques. In-situ remediation utilizing bioelectrochemical technology represents a promising strategy for degradation of halogenated organic compounds, which can be achieved through potential modulation. In this review, we summarize the reactor configuration of microbial electrochemical dehalogenation systems and relevant organohalide-respiring bacteria. We also highlight the mechanisms of electrode potential regulation of microbial dehalogenation and the role of extracellular electron transfer in dehalogenation process, and further discuss the application of bioelectrochemical technology in bioremediation of halogenated organic compounds. Therefore, this review summarizes the status of research on microbial electrochemical dehalogenation systems from macroscopic to microscopic levels, providing theoretical support for the development of rapid and efficient in situ bioremediation technologies for halogenated organic compounds contaminated sites, as well as insights for the removal of refractory fluorides.

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.

Microbial Chain Elongation and Subsequent Fermentation of Elongated Carboxylates as H2-Producing Processes for Sustained Reductive Dechlorination of Chlorinated Ethenes

In situ anaerobic groundwater bioremediation of trichloroethene (TCE) to nontoxic ethene is contingent on organohalide-respiring Dehalococcoidia, the most common strictly hydrogenotrophic Dehalococcoides mccartyi (D. mccartyi). The H₂ requirement for D. mccartyi is fulfilled by adding various organic substrates (e.g., lactate, emulsified vegetable oil, and glucose/molasses), which require fermenting microorganisms to convert them to H₂. The net flux of H₂ is a crucial controlling parameter in the efficacy of bioremediation. H₂ consumption by competing microorganisms (e.g., methanogens and homoacetogens) can diminish the rates of reductive dechlorination or stall the process altogether. Furthermore, some fermentation pathways do not produce H₂ or having H₂ as a product is not always thermodynamically favorable under environmental conditions. Here, we report on a novel application of microbial chain elongation as a H₂-producing process for reductive dechlorination. In soil microcosms bioaugmented with dechlorinating and chain-elongating enrichment cultures, near stoichiometric conversion of TCE (0.07 ± 0.01, 0.60 ± 0.03, and 1.50 ± 0.20 mmol L^-1 added sequentially) to ethene was achieved when initially stimulated by chain elongation of acetate and ethanol. Chain elongation initiated reductive dechlorination by liberating H₂ in the conversion of acetate and ethanol to butyrate and caproate. Syntrophic fermentation of butyrate, a chain-elongation product, to H₂ and acetate further sustained the reductive dechlorination activity. Methanogenesis was limited during TCE dechlorination in soil microcosms and absent in transfer cultures fed with chain-elongation substrates. This study provides critical fundamental knowledge toward the feasibility of chlorinated solvent bioremediation based on microbial chain elongation.

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.

Biotechnology and nanotechnology for remediation of chlorinated volatile organic compounds: current perspectives

Chlorinated volatile organic compounds (CVOCs) are persistent organic pollutants which are harmful to public health and the environment. Many CVOCs occur in substantial quantities in groundwater and soil, even though their use has been more carefully managed and restricted in recent years. This review summarizes recent data on several innovative treatment solutions for CVOC-affected media including bioremediation, phytoremediation, nanoscale zero-valent iron (nZVI)-based reductive dehalogenation, and photooxidation. There is no optimally developed single technology; therefore, the possibility of using combined technologies for CVOC remediation, for example bioremediation integrated with reduction by nZVI, is presented. Some methods are still in the development stage. Advantages and disadvantages of each treatment strategy are provided. It is hoped that this paper can provide a basic framework for selection of successful CVOC remediation strategies.

Sustainability in ElectroKinetic Remediation Processes: A Critical Analysis

In recent years, the development of suitable technologies for the remediation of environmental contaminations has attracted considerable attention. Among these, electrochemical approaches have gained prominence thanks to the many possible applications and their proven effectiveness. This is particularly evident in the case of inorganic/ionic contaminants, which are not subject to natural attenuation (biological degradation) and are difficult to treat adequately with conventional methods. The purpose of this contribution is to present a critical overview of electrokinetic remediation with particular attention on the sustainability of the various applications. The basis of technology will be briefly mentioned, together with the phenomena that occur in the soil and how that will allow its effectiveness. The main critical issues related to this approach will then be presented, highlighting the problems in terms of sustainability, and discussing some possible solutions to reduce the environmental impact and increase the cost-effectiveness and sustainability of this promising technology.

Microbial extracellular electron transfer and strategies for engineering electroactive microorganisms

Electroactive microorganisms (EAMs) are ubiquitous in nature and have attracted considerable attention as they can be used for energy recovery and environmental remediation via their extracellular electron transfer (EET) capabilities. Although the EET mechanisms of Shewanella and Geobacter have been rigorously investigated and are well characterized, much less is known about the EET mechanisms of other microorganisms. For EAMs, efficient EET is crucial for the sustainable economic development of bioelectrochemical systems (BESs). Currently, the low efficiency of EET remains a key factor in limiting the development of BESs. In this review, we focus on the EET mechanisms of different microorganisms, (i.e., bacteria, fungi, and archaea). In addition, we describe in detail three engineering strategies for improving the EET ability of EAMs: (1) enhancing transmembrane electron transport via cytochrome protein channels; (2) accelerating electron transport via electron shuttle synthesis and transmission; and (3) promoting the microbe-electrode interface reaction via regulating biofilm formation. At the end of this review, we look to the future, with an emphasis on the cross-disciplinary integration of systems biology and synthetic biology to build high-performance EAM systems.

Bioremediation of typical chlorinated hydrocarbons by microbial reductive dechlorination and its key players: A review

Chlorinated hydrocarbon contamination in soils and groundwater has a severe negative impact on the human health. Microbial reductive dechlorination is a major degradation pathway of chlorinated hydrocarbon in anaerobic subsurface environments, has been extensively studied. Recent progress on the diversity of the reductive dechlorinators and the key enzymes of chlororespiration has been well reviewed. Here, we present a thorough overview of the studies related to bioremediation of chloroethenes and polychlorinated biphenyls based on enhanced in situ reductive dechlorination. The major part of this review is to provide an up-to-date summary of functional microorganisms which are either detected during in situ biostimulation or applied in bioaugmentation strategies. The applied biostimulants and corresponding reductive dechlorination products are also summarized and the future research needs are finally discussed.

Transformation of tetrachloroethylene in a flow-through electrochemical reactor

Electrochemical transformation of harmful tetrachloroethylene (PCE) is evaluated as a method for management of groundwater plumes to protect the drinking water resource, its consumers and the environment. In contrast to previous work that reported transformation of trichloroethylene, a byproduct of PCE, this work focuses on transformation of PCE in a saturated porous matrix and the influence of design parameters on the removal performance. Design parameters investigated were electrode configuration, catalyst load, electrode spacing, current intensity, orientation of reactor and flow through a porous matrix. A removal of 86% was reached in the fully liquid-filled, horizontally oriented reactor at a current of 120 mA across a cathode → bipolar electrode → anode arrangement with a Darcy velocity of 0.03 cm/min (150 m/yr). The palladium load on the cathode significantly influenced the removal. Enhanced removal was observed with increased electrode spacing. Presence of an inert porous matrix improved PCE removal by 9%-point compared to a completely liquid-filled reactor. Normalization of the data indicated, that a higher charge transfer per contaminant mass is required for removal of low PCE concentrations. No chlorinated intermediates were formed. The results suggest, that PCE can be electrochemically transformed in reactor designs replicating that of a potential field-implementation. Further work is required to better understand the reduction and oxidation processes established and the parameters influencing such. This knowledge is essential for optimization towards testing in complex conditions and variations of contaminated sites.

Electrochemical transformation of an aged tetrachloroethylene contamination in realistic aquifer settings

Electrochemical removal of chlorinated ethenes in groundwater plumes may potentially overcome some of the challenges faced by current remediation technologies. So far, studies have been conducted in simplified settings of synthetic groundwater and inert porous matrices. This study is a stepwise investigation of the influence of field-extracted groundwater, sandy sediment and groundwater aquifer temperatures on the removal of an aged partially degraded contamination of tetrachloroethylene (PCE) at a typical groundwater flow rate. The aim is to assess the potential for applying electrochemistry at contaminated sites. At a constant current of 120 mA, pH and conductivity were unaffected downgradient the electrochemical zone. Major groundwater species were reduced and oxidized. Some minerals deposited, others dissolved. Hydrogen peroxide, a strong oxidant, was formed in levels up to 5 mg L^-1 with a limited distribution into the sandy sediment. Trichloromethane was formed, supposedly by oxidation of organic matter in the sandy sediment in the presence of chloride. The more realistic the settings, the higher the PCE removal, bringing concentrations down to 7.8 ± 2.3 μg L^-1. A complete removal of trichloroethylene and cis-1,2-dichloroethylene was obtained. The results suggest that competing reactions related to the natural complex hydrogeochemistry are insignificant in terms of affecting the electrochemical degradation of PCE and chlorinated intermediates.

Reductive/Oxidative Sequential Bioelectrochemical Process for Perchloroethylene Removal

An innovative bioelectrochemical reductive/oxidative sequential process was developed and tested on a laboratory scale to obtain the complete mineralization of perchloroethylene (PCE) in a synthetic medium. The sequential bioelectrochemical process consisted of two separate tubular bioelectrochemical reactors that adopted a novel reactor configuration, avoiding the use of an ion exchange membrane to separate the anodic and cathodic chamber and reducing the cost of the reactor. In the reductive reactor, a dechlorinating mixed inoculum received reducing power to perform the reductive dechlorination of perchloroethylene (PCE) through a cathode chamber, while the less chlorinated daughter products were removed in the oxidative reactor, which supported an aerobic dechlorinating culture through in situ electrochemical oxygen evolution. Preliminary fluid dynamics and electrochemical tests were performed to characterize both the reductive and oxidative reactors, which were electrically independent of each other, with each having its own counterelectrode. The first continuous-flow potentiostatic run with the reductive reactor (polarized at −450 mV vs SHE) resulted in obtaining 100% ± 1% removal efficiency of the influent PCE, while the oxidative reactor (polarized at +1.4 V vs SHE) oxidized the vinyl chloride and ethylene from the reductive reactor, with removal efficiencies of 100% ± 2% and 92% ± 1%, respectively.

1,4-Dioxane-contaminated groundwater remediation in the anode chamber of a microbial fuel cell

A two-chambered microbial fuel cell (MFC) was used for the first time for the remediation of an emerging contaminant-1,4-dioxane in its anode chamber. Groundwater historically detected 1,4-dioxane contamination was sampled from a Superfund site. Comparative study was carried out between metabolic (i.e., 1,4-dioxane as sole carbon source) and cometabolic (i.e., 1,4-dioxane and methanol as carbon sources) anodic degradations. It was found that cometabolic degradation increased 1,4-dioxane removal by 10%-52% after 7 days and increased maximum power production of the MFC by 18% to 88.9 mW/m³ . Oxalic acid was detected as a main metabolic degradation product. Beside oxalic acid, acetic acid and isopropanol were also detected as main products for cometabolic degradation. The presence of a biofilm for 1,4-dioxane anodic degradation was observed by a scanning electron microscopy. Phyla of Bacteroidetes, Firmicutes, and Proteobacteria, as well as a variety of species, were identified for the first time-especially Rikenella sp. and Solitalea canadensis, whose relative abundances were the highest of 18.8% and 24.0% for metabolic and cometabolic degradation, respectively. This study provided an innovative and sustainable approach for 1,4-dioxane anodic biodegradation, which would be potentially utilized for remediation of groundwater contaminated by 1,4-dioxane. PRACTITIONER POINTS: Groundwater contaminated with 1,4-dioxane was remediated in the anode chamber of a two-chambered microbial fuel cell. Cometabolic pathway increased 1,4-dioxane removal and power production of the MFC compared to metabolic pathway. The presence of a biofilm for 1,4-dioxane anodic degradation was observed, and oxalic acid was a main degradation product. This study would be potentially utilized for 1,4-dioxane-contaminated groundwater remediation with simultaneous energy production. External voltage supply for bioelectrochemical remediation of groundwater would potentially be reduced when treating chlorinated hydrocarbons co-occurred with 1,4-dioxane.

Combining electrokinetic transport and bioremediation for enhanced removal of crude oil from contaminated marine sediments: Results of a long-term, mesocosm-scale experiment

Marine sediments represent an important sink of harmful petroleum hydrocarbons after an accidental oil spill. Electrobioremediation techniques, which combine electrokinetic transport and biodegradation processes, represent an emerging technological platform for a sustainable remediation of contaminated sediments. Here, we describe the results of a long-term mesocosm-scale electrobioremediation experiment for the treatment of marine sediments contaminated by crude oil. A dimensionally stable anode and a stainless-steel mesh cathode were employed to drive seawater electrolysis at a fixed current density of 11 A/m². This approach allowed establishing conditions conducive to contaminants biodegradation, as confirmed by the enrichment of Alcanivorax borkumensis cells harboring the alkB-gene and other aerobic hydrocarbonoclastic bacteria. Oil chemistry analyses indicated that aromatic hydrocarbons were primarily removed from the sediment via electroosmosis and low molecular weight alkanes (nC₆ to nC₁₀) via biodegradation.

Potential electron donor for nanoiron supported hydrogenotrophic denitrification: H2 gas, Fe0, ferrous oxides, Fe2+(aq), or Fe2+(ad)?

The mechanism of electron transmission in combined nanoiron-bacteria denitrification cannot be explained by the classic model, in which an Fe⁰H₂-nitrate transferring chain is proposed. In this study, we used characteristic techniques and electrochemical analysis to investigate the necessity of molecular hydrogen for the combined denitrifying system using commercial nanoiron with Alcaligenes eutrophus, and to analyze its potential electron donor. Based on our results, nitrate removal and its by-products (NO₂^- and NH₄⁺) generation was not significantly affected by residual H₂ gas, indicating that H₂ was not necessary for hydrogenotrophic denitrification. As to the potential electron donor analysis, nanoscale zero-valent iron did not appear to be the electron donor due to its high level of toxicity (83% mortality using nanoiron versus 36% in the control cells). In addition, when iron oxides (Fe₃O₄, Fe₂O₃ and FeOOH on the nanoiron surface) and free ferrous ions [Fe²⁺(aq)] were present alone, they were not utilized by the bacteria to degrade nitrate. According to the results of electrochemical analysis, adsorbed ferrous iron [Fe²⁺(ad)] on ferric oxides might be the electron donor in this kind of nitrate removal.

Time-dependent bacterial community and electrochemical characterizations of cathodic biofilms in the surfactant-amended sediment-based bioelectrochemical reactor with enhanced 2,3,4,5-tetrachlorobiphenyl dechlorination

Applying an electric field to stimulate the microbial reductive dechlorination of polychlorinated biphenyls (PCBs) represents a promising approach for bioremediation of PCB-contaminated sites. This study aimed to demonstrate the biocathodic film-facilitated reduction of PCB 61 in a sediment-based bioelectrochemical reactor (BER) and, more importantly, the characterizations of electrode-microbe interaction from microbial and electrochemical perspectives particularly in a time-dependent manner. The application of a cathodic potential (-0.45 V vs. SHE) significantly improved the rate and extent of PCB 61 dechlorination compared to the open-circuit scenario (without electrical stimulation), and the addition of an external surfactant further increased the dechlorination, with Tween 80 exerting more pronounced effects than rhamnolipid. The bacterial composition of the biofilms and the bioelectrochemical kinetics of the BERs were found to be time-dependent and to vary considerably with the incubation time and slightly with the coexistence of an external surfactant. Excellent correlations were observed between the dechlorination rate and the relative abundance of Dehalogenimonas, Dechloromonas, and Geobacter, the dechlorination rate and the cathodic current density recorded from the chronoamperometry tests, and the dechlorination rate and the charge transfer resistance derived from the electrochemical impedance tests, with respect to the 120 day-operation. After day 120, PCB 61 was resistant to further appreciable reduction, but substantial hydrogen production was detected, and the bacterial community and electrochemical parameters observed on day 180 were not distinctly different from those on day 120.

The fate of antibiotic resistance genes and their potential hosts during bio-electrochemical treatment of high-salinity pharmaceutical wastewater

Pharmaceutical wastewaters containing antibiotics and high salinity can damage traditional biological treatment and result in the proliferation of antibiotic resistance genes (ARGs). Bioelectrochemical system (BES) is a promising approach for treating pharmaceutical wastewater. However, the fate of ARGs in BES and their correlations with microbial communities and horizontal genes transfer are unknown. In this study, we investigated the response of ARGs to bio-electrochemical treatment of chloramphenicol wastewater and their potential hosts under different salinities. Three ARGs encoding efflux pump (cmlA, floR and tetC), one class 1 integron integrase encoding gene (intI1), and sul1 gene (associate with intI1) were analyzed. Correlation analysis between microbial community and ARGs revealed that the abundances of potential hosts of ARGs were strongly affected by salinity, which further determined the alteration in ARGs abundances under different salinities. There were no significant correlations between ARGs and intI1, indicating that horizontal gene transfer was not related to the important changes in ARGs. Moreover, the chloramphenicol removal efficiency was enhanced under a moderate salinity, attributed to the altered microbial community driven by salinity. Therefore, microbial community shift is the major factor for the changes of ARGs and chloramphenicol removal efficiency in BES under different salinities. This study provides new insights on the mechanisms underlying the alteration of ARGs in BES treating high-salinity pharmaceutical wastewater.

Electrochemical hydrodechlorination of perchloroethylene in groundwater on a Ni-doped graphene composite cathode driven by a microbial fuel cell

Enhancing the activity of the cathode and reducing the voltage for electrochemical hydrodechlorination of chlorohydrocarbon were always the challenges in the area of electrochemical remediation. In this study, a novel cathode material of Ni-doped graphene generated by Ni nanoparticles dispersed evenly on graphene was prepared to electrochemically dechlorinate PCE in groundwater. The reduction potential of Ni-doped graphene for PCE electrochemical hydrodechlorination was −0.24 V (vs. Ag/AgCl) determined by cyclic voltammetry. A single MFC with a voltage of 0.389–0.460 V and a current of 0.221–0.257 mA could drive electrochemical hydrodechlorination of PCE effectively with Ni-doped graphene as the cathode catalyst, and the removal rate of PCE was significantly higher than that with single Ni or graphene as the cathode catalyst. Moreover, neutral conditions were more suitable for Ni-doped graphene to electrochemically hydrodechlorinate PCE in groundwater and no byproduct was accumulated.

Ni-doped graphene was prepared to electrochemically dechlorinate PCE driven by a microbial fuel cell. Dechlorination efficiency and reduction potential were significantly higher than for bare Ni or graphene.

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.

Effect of bio-electrochemical system on the fate and proliferation of chloramphenicol resistance genes during the treatment of chloramphenicol wastewater

Bioelectrochemical systems can effectively degrade antibiotics, but there is the need to better understand the fate of antibiotic resistance bacteria and antibiotic resistance genes during the bioelectrochemical degradation of antibiotics. In this study, a BES was developed as a platform to investigate the fate of chloramphenicol resistance bacteria (CRB) and the expression of chloramphenicol resistance genes (CRGs) under different operating conditions during chloramphenicol biodegradation. The results indicated that chloramphenicol was effectively removed and chloramphenicol removal efficiency could be improved under less chloramphenicol concentration and more negative cathode potential. Higher chloramphenicol concentration enhanced the enrichment of CRB and expression of CRGs. Furthermore, the abundances of CRB were enhanced under more negative cathode potential, the expression of CRGs under less negative cathode potential were induced. However, both the enrichment of CRB and expression of CRGs could be moderated under a medium cathode potential. This result could provide the scientific reference for research about the fate of antibiotic resistance genes in bioelectrochemical systems.

Electrobioremediation of oil spills

Annually, thousands of oil spills occur across the globe. As a result, petroleum substances and petrochemical compounds are widespread contaminants causing concern due to their toxicity and recalcitrance. Many remediation strategies have been developed using both physicochemical and biological approaches. Biological strategies are most benign, aiming to enhance microbial metabolic activities by supplying limiting inorganic nutrients, electron acceptors or donors, thus stimulating oxidation or reduction of contaminants. A key issue is controlling the supply of electron donors/acceptors. Bioelectrochemical systems (BES) have emerged, in which an electrical current serves as either electron donor or acceptor for oil spill bioremediation. BES are highly controllable and can possibly also serve as biosensors for real time monitoring of the degradation process. Despite being promising, multiple aspects need to be considered to make BES suitable for field applications including system design, electrode materials, operational parameters, mode of action and radius of influence. The microbiological processes, involved in bioelectrochemical contaminant degradation, are currently not fully understood, particularly in relation to electron transfer mechanisms. Especially in sulfate rich environments, the sulfur cycle appears pivotal during hydrocarbon oxidation. This review provides a comprehensive analysis of the research on bioelectrochemical remediation of oil spills and of the key parameters involved in the process.

An Overview of Electron Acceptors in Microbial Fuel Cells

Microbial fuel cells (MFC) have recently received increasing attention due to their promising potential in sustainable wastewater treatment and contaminant removal. In general, contaminants can be removed either as an electron donor via microbial catalyzed oxidization at the anode or removed at the cathode as electron acceptors through reduction. Some contaminants can also function as electron mediators at the anode or cathode. While previous studies have done a thorough assessment of electron donors, cathodic electron acceptors and mediators have not been as well described. Oxygen is widely used as an electron acceptor due to its high oxidation potential and ready availability. Recent studies, however, have begun to assess the use of different electron acceptors because of the (1) diversity of redox potential, (2) needs of alternative and more efficient cathode reaction, and (3) expanding of MFC based technologies in different areas. The aim of this review was to evaluate the performance and applicability of various electron acceptors and mediators used in MFCs. This review also evaluated the corresponding performance, advantages and disadvantages, and future potential applications of select electron acceptors (e.g., nitrate, iron, copper, perchlorate) and mediators.

Microbial polychlorinated biphenyl dechlorination in sediments by electrical stimulation: The effect of adding acetate and nonionic surfactant

The necessity for developing an efficient and cost-effective in situ bioremediation technology for sediments contaminated with polychlorinated biphenyls (PCBs) has prompted the application of low-voltage electrical fields to anaerobic digestion systems. Here we show that the use of a sediment-based bio-electrochemical reactor (BER) poised at a potential of -0.50V (vs. a standard calomel electrode, SCE) substantially enhanced the reduction of 2,3,4,5-tetrachlorobiphenyl (PCB 61) when acetate was added as a carbon source. The addition of surfactant Tween 80 to the BER further accelerated the PCB 61 transformation. The comparative study of closed- and open-circuit reactors demonstrated the enrichment conditions affecting the bacterial community structure, the dominant dechlorination metabolisms, and thus the extent, the rate and the products of the reduction of PCBs. The dominant bacterial dechlorinators detected in the BERs in the presence of acetate and Tween 80 are Dehalogenimonas, Dehalobacter, Sulfuricurvum, Dechloromonas and Geobacter, which should be responsible for PCB dechlorination. This study improves understanding of the key factors influencing dechlorination activity in sediment-based BERs polarized at a low potential, as well as the metabolic mechanisms dominating in the PCB dechlorination process.

Bioelectrochemical approach for reductive and oxidative dechlorination of chlorinated aliphatic hydrocarbons (CAHs)

A sequential reductive-oxidative treatment was developed in this study in a continuous-flow bioelectrochemical reactor to address bioremediation of groundwater contaminated by trichloroethene (TCE) and less-chlorinated but still harmful intermediates, such as vinyl chloride. In order to optimize the anodic compartment, whereby the oxygen-driven microbial oxidation of TCE-daughter products occurs, abiotic batch experiments were performed with various anode materials poised at +1.20 V vs. SHE (i.e., graphite rods and titanium mesh anode coated with mixed metal oxides (MMO)) and setups (i.e., electrodes embedded within a bed of silica beads or graphite granule). The MMO anode displayed higher efficiency (>90%) for oxygen generation compared to the graphite electrodes. Additionally, the graphite bed presence adversely affects oxygen generation, likely due to the oxygen scavenging. This effect was completely eliminated by replacing the graphite granules with silica beads. The anodic setups were thereafter verified in a mentioned reactor at an applied TCE loading rate of approximately 20 μM d^-1 and a hydraulic retention time of 1.4 d in each compartment. The cathode consisted of a bed of graphite granules and was potentiostatically controlled at -0.65 V vs. SHE. The best reactor performance in terms of removal efficiency (i.e., >97%), removal rate (i.e., 121.8 ± 2.7 μeq L^-1 d^-1), and the residual concentration (i.e., 5.03 ± 0.63 μeq L^-1) of chlorinated contaminants was achieved with the MMO anode placed in a silica bed. Ecotoxicity tests performed with algae confirmed these results by showing progressive toxicity reduction from inlet to cathodic and anodic effluent using this reactor configuration.

Addition of iron oxides in sediments enhances 2,3,4,5-tetrachlorobiphenyl (PCB 61) dechlorination by low-voltage electric fields

The presence of iron oxides in sediments significantly improves anaerobic dechlorination of PCB (i.e., PCB 61) in bioelectrochemical reactors.

Review of chemical and electrokinetic remediation of PCBs contaminated soils and sediments

Polychlorinated biphenyls (PCBs) are manmade organic compounds, and pollution due to PCBs has been a global environmental problem because of their persistence, long-range atmospheric transport and bioaccumulation. Many physical, chemical and biological technologies have been utilized to remediate PCBs contaminated soils and sediments, and there are some emerging new technologies and combined methods that may provide cost-effective alternatives to the existing remediation practice. This review provides a general overview on the recent developments in chemical treatment and electrokinetic remediation (EK) technologies related to PCBs remediation. In particular, four technologies including photocatalytic degradation of PCBs combined with soil washing, Fe-based reductive dechlorination, advanced oxidation process, and EK/integrated EK technology (e.g., EK coupled with chemical oxidation, nanotechnology and bioremediation) are reviewed in detail. We focus on the fundamental principles and governing factors of chemical technologies, and EK/integrated EK technologies. Comparative analysis of these technologies including their major advantages and disadvantages is summarized. The existing problems and future prospects of these technologies regarding PCBs remediation are further highlighted.

PCE dechlorination by non-Dehalococcoides in a microbial electrochemical system

The bioremediation of tetrachloroethene (perchloroethene; PCE) contaminated sites generally requires a supply of some fermentable organic substrates as an electron donor. On the other hand, organic substrates can induce the massive growth of microorganisms around the injection wells, which can foul the contaminated subsurface environment. In this study, PCE dechlorination to ethene was performed in a microbial electrochemical system (MES) using the electrode (a cathode polarized at −500 mV vs. standard hydrogen electrode) as the electron donor. Denaturing gel gradient electrophoresis and pyrosequencing revealed a variety of non-Dehalococcoides bacteria dominant in MES, such as Acinetobacter sp. (25.7 % for AS1 in suspension of M3), Rhodopseudomonas sp. (10.5 % for AE1 and 10.1 % for AE2 in anodic biofilm of M3), Pseudomonas aeruginosa (22.4 % for BS1 in suspension of M4), and Enterobacter sp. (21.7 % for BE1 in anodic biofilm of M4) which are capable of electron transfer, hydrogen production and dechlorination. The Dehalococcoides group, however, was not detected in this system. Therefore, these results suggest that a range of bacterial species outside the Dehalococcoides can play an important role in the microbial electrochemical dechlorination process, which may lead to innovative bioremediation technology.

Microbial selenite reduction with organic carbon and electrode as sole electron donor by a bacterium isolated from domestic wastewater

Selenium is said to be multifaceted element because it is essential at a low concentration but very toxic at an elevated level. For the purpose of screening a potential microorganism for selenite bioremediation, we isolated a bacterium, named strain THL1, which could perform both heterotrophic selenite reduction, using organic carbons such as acetate, lactate, propionate, and butyrate as electron donors under microaerobic condition, and electrotrophic selenite reduction, using an electrode polarized at -0.3V (vs. standard hydrogen electrode) as the sole electron donor under anaerobic condition. This bacterium determined to be a new strain of the genus Cronobacter, could remove selenite with an efficiency of up to 100%. This study is the first demonstration on a pure culture could take up electrons from an electrode to perform selenite reduction. The selenium nanoparticles produced by microbial selenite reduction might be considered for recovery and use in the nanotechnology industry.

In-Situ Remediation Approaches for the Management of Contaminated Sites: A Comprehensive Overview

Though several in-situ treatment methods exist to remediate polluted sites, selecting an appropriate site-specific remediation technology is challenging and is critical for successful clean up of polluted sites. Hence, a comprehensive overview of all the available remediation technologies to date is necessary to choose the right technology for an anticipated pollutant. This review has critically evaluated the (i) technological profile of existing in-situ remediation approaches for priority and emerging pollutants, (ii) recent innovative technologies for on-site pollutant remediation, and (iii) current challenges as well as future prospects for developing innovative approaches to enhance the efficacy of remediation at contaminated sites.

Bioelectrochemically-assisted reductive dechlorination of 1,2-dichloroethane by a Dehalococcoides-enriched microbial culture

The aim of this study was to verify the possibility to use a polarized graphite electrode as an electron donor for the reductive dechlorination of 1,2-dichloroethane, an ubiquitous groundwater contaminant. The rate of 1,2-DCA dechlorination almost linearly increased by decreasing the set cathode potential over a broad range of set cathode potentials (i.e., from -300 mV to -900 mV vs. the standard hydrogen electrode). This process was primarily dependent on electrolytic H2 generation. On the other hand, reductive dechlorination proceeded (although quite slowly) with a very high Coulombic efficiency (near 70%) at a set cathode potential of -300 mV, where no H2 production occurred. Under this condition, reductive dechlorination was likely driven by direct electron uptake from the surface of the polarized electrode. Taken as a whole, this study further extends the range of chlorinated contaminants which can be treated with bioelectrochemical systems.

Relative contribution of set cathode potential and external mass transport on TCE dechlorination in a continuous-flow bioelectrochemical reactor

Microbial bioelectrochemical systems, which use solid-state cathodes to drive the reductive degradation of contaminants such as the chlorinated hydrocarbons, are recently attracting considerable attention for bioremediation applications. So far, most of the published research has focused on analyzing the influence of key (bio)electrochemical factors influencing contaminant degradation, such as the cathode potential, whereas only few studies have examined the potential impact of mass transport phenomena on process performance. Here we analyzed the performance of a flow-through bioelectrochemical reactor, continuously fed with a synthetic groundwater containing trichloroethene at three different linear fluid velocities (from 0.3 m d(-1) to 1.7 m d(-1)) and three different set cathode potentials (from -250 mV to -450 mV vs. the standard hydrogen electrode). The obtained results demonstrated that, in the range of fluid velocities which are characteristics for natural groundwater systems, mass transport phenomena may strongly influence the rate and extent of reductive dechlorination. Nonetheless, the relative importance of mass transport largely depends on the applied cathode potential which, in turn, controls the intrinsic kinetics of biological reactions and the underlying electron transfer mechanisms.

Innovative biological approaches for monitoring and improving water quality

Water quality is largely influenced by the abundance and diversity of indigenous microbes present within an aquatic environment. Physical, chemical and biological contaminants from anthropogenic activities can accumulate in aquatic systems causing detrimental ecological consequences. Approaches exploiting microbial processes are now being utilized for the detection, and removal or reduction of contaminants. Contaminants can be identified and quantified in situ using microbial whole-cell biosensors, negating the need for water samples to be tested off-site. Similarly, the innate biodegradative processes can be enhanced through manipulation of the composition and/or function of the indigenous microbial communities present within the contaminated environments. Biological contaminants, such as detrimental/pathogenic bacteria, can be specifically targeted and reduced in number using bacteriophages. This mini-review discusses the potential application of whole-cell microbial biosensors for the detection of contaminants, the exploitation of microbial biodegradative processes for environmental restoration and the manipulation of microbial communities using phages.

Enhanced Alcaligenes faecalis Denitrification Rate with Electrodes as the Electron Donor

The utilization by Alcaligenes faecalis of electrodes as the electron donor for denitrification was investigated in this study. The denitrification rate of A. faecalis with a poised potential was greatly enhanced compared with that of the controls without poised potentials. For nitrate reduction, although A. faecalis could not reduce nitrate, at three poised potentials of +0.06, -0.06, and -0.15 V (versus normal hydrogen electrode [NHE]), the nitrate was partially reduced with -0.15- and -0.06-V potentials at rates of 17.3 and 28.5 mg/liter/day, respectively. The percentages of reduction for -0.15 and -0.06 V were 52.4 and 30.4%, respectively. Meanwhile, for nitrite reduction, the poised potentials greatly enhanced the nitrite reduction. The nitrite reduction rates for three poised potentials (-0.06, -0.15, and -0.30 V) were 1.98, 4.37, and 3.91 mg/liter/h, respectively. When the potentials were cut off, the nitrite reduction rate was maintained for 1.5 h (from 2.3 to 2.25 mg/liter/h) and then greatly decreased, and the reduction rate (0.38 mg/liter/h) was about 1/6 compared with the rate (2.3 mg/liter/h) when potential was on. Then the potentials resumed, but the reduction rate did not resume and was only 2 times higher than the rate when the potential was off.

Electro-bioremediation of contaminated sediment by electrode enhanced capping

In-situ capping often eliminates or slows natural degradation of hydrocarbon due to the reducing conditions in the sediments. The purpose of this research was to demonstrate a reactive capping technique, an electrode enhanced cap, to produce favorable conditions for hydrocarbon degradation and evaluate this reactive capping technique for contaminated sediment remediation. Two graphite electrodes were placed horizontally at different layers in a cap and connected to external power of 2 V. Redox potentials increased and pH decreased around the anode. Phenanthrene concentration decreased and PAH degradation genes increased in the vicinity of the anode. Phenanthrene concentrations at 0-1 cm sediment beneath the anode decreased to ∼50% of initial concentration over ∼70 days, while phenanthrene levels in control reactor kept unchanged. A degradation model of electrode enhanced capping was developed to simulate reaction-diffusion processes, and model results show that a reaction-dominated region was created in the vicinity of the anode. Although the degradation dominated region was thin, transport processes in a sediment cap environment are typically sufficiently slow to allow this layer to serve as a permeable reactive barrier for hydrocarbon decontamination.

Anaerobic arsenite oxidation with an electrode serving as the sole electron acceptor: a novel approach to the bioremediation of arsenic-polluted groundwater

Arsenic contamination of soil and groundwater is a serious problem worldwide. Here we show that anaerobic oxidation of As(III) to As(V), a form which is more extensively and stably adsorbed onto metal-oxides, can be achieved by using a polarized (+497 mV vs. SHE) graphite anode serving as terminal electron acceptor in the microbial metabolism. The characterization of the microbial populations at the electrode, by using in situ detection methods, revealed the predominance of gammaproteobacteria. In principle, the proposed bioelectrochemical oxidation process would make it possible to provide As(III)-oxidizing microorganisms with a virtually unlimited, low-cost and low-maintenance electron acceptor as well as with a physical support for microbial attachment.