Microbiome Composition and Dynamics of a Reductive/Oxidative Bioelectrochemical System for Perchloroethylene Removal: Effect of the Feeding Composition.

Chlorinated solvents still represent an environmental concern that requires sustainable and innovative bioremediation strategies. This study describes the microbiome composition of a novel bioelectrochemical system (BES) based on sequential reductive/oxidative dechlorination for complete perchloroethylene (PCE) removal occurring in two separate but sequential chambers. The BES has been tested under various feeding compositions [i.e., anaerobic mineral medium (MM), synthetic groundwater (SG), and real groundwater (RG)] differing in presence of sulfate, nitrate, and iron (III). In addition, the main biomarkers of the dechlorination process have been monitored in the system under various conditions. Among them, Dehalococcoides mccartyi 16S rRNA and reductive dehalogenase genes (tceA, bvcA, and vcrA) involved in anaerobic dechlorination have been quantified. The etnE and etnC genes involved in aerobic dechlorination have also been quantified. The feeding composition affected the microbiome, in particular when the BES was fed with RG. Sulfuricurvum, enriched in the reductive compartment, operated with MM and SG, suggesting complex interactions in the sulfur cycle mostly including sulfur oxidation occurring at the anodic counter electrode (MM) or coupled to nitrate reduction (SG). Moreover, the known Mycobacterium responsible for natural attenuation of VC by aerobic degradation was found abundant in the oxidative compartment fed with RG, which was in line with the high VC removal observed (92 ± 2%). D. mccartyi was observed in all the tested conditions ranging from 8.78E + 06 (with RG) to 2.35E + 07 (with MM) 16S rRNA gene copies/L. tceA was found as the most abundant reductive dehalogenase gene in all the conditions explored (up to 2.46 E + 07 gene copies/L in MM). The microbiome dynamics and the occurrence of biomarkers of dechlorination, along with the kinetic performance of the system under various feeding conditions, suggested promising implications for the scale-up of the BES, which couples reductive with oxidative dechlorination to ensure the complete removal of highly chlorinated ethylene and mobile low-chlorinated by-products.

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

[This corrects the article DOI: 10.3389/fmicb.2021.747670.].

Autotrophic Acetate Production under Hydrogenophilic and Bioelectrochemical Conditions with a Thermally Treated Mixed Culture.

Bioelectrochemical systems are emerging technologies for the reduction in CO2 in fuels and chemicals, in which anaerobic chemoautotrophic microorganisms such as methanogens and acetogens are typically used as biocatalysts. The anaerobic digestion digestate represents an abundant source of methanogens and acetogens microorganisms. In a mixed culture environment, methanogen's inhibition is necessary to avoid acetate consumption by the presence of acetoclastic methanogens. In this study, a methanogenesis inhibition approach based on the thermal treatment of mixed cultures was adopted and evaluated in terms of acetate production under different tests consisting of hydrogenophilic and bioelectrochemical experiments. Batch experiments were carried out under hydrogenophilic and bioelectrochemical conditions, demonstrating the effectiveness of the thermal treatment and showing a 30 times higher acetate production with respect to the raw anaerobic digestate. Moreover, a continuous flow bioelectrochemical reactor equipped with an anion exchange membrane (AEM) successfully overcomes the methanogens reactivation, allowing for a continuous acetate production. The AEM membrane guaranteed the migration of the acetate from the biological compartment and its concentration in the abiotic chamber avoiding its consumption by acetoclastic methanogenesis. The system allowed an acetate concentration of 1745 ± 30 mg/L in the abiotic chamber, nearly five times the concentration measured in the cathodic chamber.

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.

Control of Sulfate and Nitrate Reduction by Setting Hydraulic Retention Time and Applied Potential on a Membraneless Microbial Electrolysis Cell for Perchloroethylene Removal.

A membraneless microbial electrolysis cell (MEC) has been developed for perchloroethylene (PCE) removal through the reductive dechlorination reaction. The MEC consists of a tubular reactor of 8.24 L equipped with a graphite-granule working electrode which stimulates dechlorinating microorganisms while a graphite-granule cylindrical envelopment contained in a plastic mesh constituted the counter electrode of the MEC. Synthetic PCE-contaminated groundwater has been used as the feeding solution to test the nitrate and sulfate reduction reactions on the MEC performance at different hydraulic retention times (HRTs) (4.1, 1.8, and 1.2) and different cathodic potentials [-350, -450, and -650 mV vs standard hydrogen electrode (SHE)]. The HRT decrease from 4.1 to 1.8 d promoted a considerable increase in sulfate removal from 38 ± 11 to 113 ± 26 mg/Ld with a consequent current increase, while a shorter HRT of 1.2 d caused a partial inhibition of sulfate reduction with a consequent current decrease from -99 ± 3 to -52 ± 6 mA. Similarly, the cathodic potential investigation showed a direct correlation of current generation and sulfate removal in which the utilization of a cathodic potential of -350 mV versus SHE allowed for an 80% decrease in the sulfate removal rate with a consequent current decrease from -163 ± 7 to 41 ± 5 mA. The study showed the possibility to mitigate the energy consumption of the process by avoiding side reactions and current generation, through the selection of an appropriate HRT and applied cathodic potential.

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.

Chromate fate and effect in bioelectrochemical systems for remediation of chlorinated solvents.

A continuous-flow bioelectrochemical reactor was developed in a previous study to address the bioremediation of groundwater contaminated by trichloroethene (TCE). The present report investigated the applicability of the same system in the presence of Cr(VI) and its possible inhibitory effect on dehalorespiring bacterial populations. Preliminary batch tests were performed at the optimal cathodic reducing potential for the reductive dechlorination (RD) of TCE (-0.65 V vs. the standard hydrogen electrode) with two different dechlorinating microorganism consortia. The results demonstrated that Cr(VI) removal efficacy was increased by microorganisms that had been previously acclimatised to Cr(VI). Specifically, Cr(VI) was completely reduced only in the presence of acclimated microorganisms. The presence of chromate negatively affected RD performance, by either (i) limiting the TCE transformation to cis-dichloroethene at lower concentrations, or (ii) completely inhibiting RD at higher concentrations. In contrast, after the acclimation period, RD was extended down to vinyl chloride, which is the main TCE daughter product. Finally, the continuous flow reactor was fed by synthetic groundwater contaminated with TCE (50 µM) and Cr(VI) (45 µM), and the experimental results showed that Cr(VI) was completely reduced under RD conditions. Moreover, TCE removal was complete, with vinyl chloride and ethene as the main intermediates, thus indicating that chromate inhibition was decreased by Cr(VI) removal.

Ammonium Recovery and Biogas Upgrading in a Tubular Micro-Pilot Microbial Electrolysis Cell (MEC).

Here, a 12-liter tubular microbial electrolysis cell (MEC) was developed as a post treatment unit for simultaneous biogas upgrading and ammonium recovery from the liquid effluent of an anaerobic digestion process. The MEC configuration adopted a cation exchange membrane to separate the inner anodic chamber and the external cathodic chamber, which were filled with graphite granules. The cathodic chamber performed the CO2 removal through the bioelectromethanogenesis reaction and alkalinity generation while the anodic oxidation of a synthetic fermentate partially sustained the energy demand of the process. Three different nitrogen load rates (73, 365, and 2229 mg N/Ld) were applied to the inner anodic chamber to test the performances of the whole process in terms of COD (Chemical Oxygen Demand) removal, CO2 removal, and nitrogen recovery. By maintaining the organic load rate at 2.55 g COD/Ld and the anodic chamber polarization at +0.2 V vs. SHE (Standard Hydrogen Electrode), the increase of the nitrogen load rate promoted the ammonium migration and recovery, i.e., the percentage of current counterbalanced by the ammonium migration increased from 1% to 100% by increasing the nitrogen load rate by 30-fold. The CO2 removal slightly increased during the three periods, and permitted the removal of 65% of the influent CO2, which corresponded to an average removal of 2.2 g CO2/Ld. During the operation with the higher nitrogen load rate, the MEC energy consumption, which was simultaneously used for the different operations, was lower than the selected benchmark technologies, i.e., 0.47 kW/N·m3 for CO2 removal and 0.88 kW·h/kg COD for COD oxidation were consumed by the MEC while the ammonium nitrogen recovery consumed 2.3 kW·h/kg N.

Electro-fermentation and redox mediators enhance glucose conversion into butyric acid with mixed microbial cultures.

Electro-fermentation (EF) is an emerging and promising technology consisting in the use of a polarized electrode to control the spectrum of products deriving from anaerobic bioprocesses. Here, the effect of electrode polarization on the fermentation of glucose has been studied with two mixed microbial cultures, both in the absence and in the presence of exogenous redox mediators, to verify the viability of the proposed approach under a broader and previously unexplored range of operating conditions. In unmediated experiments, EF (with the cathode polarized at -700 mV vs. SHE, Standard Hydrogen Electrode) caused an increase in the yield of butyric acid production provided that glucose was consumed along with its own fermentation products (i.e. acetic acid and ethanol). The maximum obtained yield accounted for 0.60 mol mol-1. Mediated experiments were performed with Neutral Red or AQDS at a concentration of 500 µM both in the absence and in the presence of the electrode polarized at -700 mV or -300 mV vs. SHE, respectively. Mediators showed a high selectivity towards the generation of n-butyric acid isomer from the condensation of acetate and ethanol, hence suggesting that they provided microbial cells with the required reducing power otherwise deriving from glucose in unmediated experiments.

Influence of the set anode potential on the performance and internal energy losses of a methane-producing microbial electrolysis cell.

The effect of the set anode potential (between + 200 mV and - 200 mV vs. SHE, standard hydrogen electrode) on the performance and distribution of internal potential losses has been analyzed in a continuous-flow methane-producing microbial electrolysis cell (MEC).Both acetate removal rate (at the anode) and methane generation rate (at the cathode) were higher (1 gCOD/L day and 0.30 m(3)/m(3) day, respectively) when the anode potential was controlled at + 200 mV. However, both the yields of acetate conversion into current and current conversion into methane were very high (72-90%) under all the tested conditions. Moreover, the sum of internal potential losses decreased from 1.46 V to 0.69 V as the anode potential was decreased from + 200 mV to - 200 mV, with cathode overpotentials always representing the main potential losses. This was likely to be due to the high energy barrier which has to be overcome in order to activate the cathode reaction. Finally, the energy efficiency correspondingly increased reaching 120% when the anode was controlled at - 200 mV.

Effect of the anode feeding composition on the performance of a continuous-flow methane-producing microbial electrolysis cell.

A methane-producing microbial electrolysis cell (MEC) was continuously fed at the anode with a synthetic solution of soluble organic compounds simulating the composition of the soluble fraction of a municipal wastewater. The MEC performance was assessed at different anode potentials in terms of chemical oxygen demand (COD) removal efficiency, methane production, and energy efficiency. As a main result, about 72-80% of the removed substrate was converted into current at the anode, and about 84-86% of the current was converted into methane at the cathode. Moreover, even though both COD removed and methane production slightly decreased as the applied anode potential decreased, the energy efficiency (i.e., the energy recovered as methane with respect to the energy input into the system) increased from 54 to 63%. Denaturing gradient gel electrophoresis (DGGE) analyses revealed a high diversity in the anodic bacterial community with the presence of both fermentative (Proteiniphilum acetatigenes and Petrimonas sulphurifila) and aerobic (Rhodococcus qingshengii) microorganisms, whereas only two microorganisms (Methanobrevibacter arboriphilus and Methanosarcina mazei), both assignable to methanogens, were observed in the cathodic community.

Electrochemical stimulation of microbial cis-dichloroethene (cis-DCE) oxidation by an ethene-assimilating culture.

This study investigated the feasibility of using a polarized graphite electrode as direct or indirect (via electrolytic oxygen generation) electron acceptor to stimulate the microbial oxidation of cis-dichloroethene (cis-DCE) in contaminated groundwater. A microbial culture was enriched in the anode chamber of a bioelectrochemical cell using a mixture of cis-DCE and ethene as substrates. The bioelectrochemical cell was operated by controlling the anode potential at +1.0 V or +1.5 V versus the standard hydrogen electrode (SHE). Enhanced cometabolic removal of cis-DCE, with ethene serving as the growth substrate, was observed in batch tests with the anode polarized at +1.5 V versus SHE. At this potential, (chloro)ethenes removal was probably sustained by molecular oxygen generated at the anode from water oxidation. Conversely, negligible anaerobic degradation was observed at +1.0 V versus SHE (a potential which does not allow oxygen generation), hence suggesting that molecular oxygen is needed to initiate (chloro)ethene degradation. PCR-DGGE analysis of the microbial culture followed by band sequencing and phylogenetic analysis evidenced the selective enrichment of a Bacillus species, providing a strong indication that this microorganism was responsible for cis-DCE and ethene degradation.