Anaerobic treatment of groundwater co-contaminated by toluene and copper in a single chamber bioelectrochemical system.

Addressing the simultaneous removal of multiple coexisting groundwater contaminants poses a significant challenge, primarily because of their different physicochemical properties. Indeed, different chemical compounds may necessitate establishing distinct, and sometimes conflicting, (bio)degradation and/or removal pathways. In this work, we investigated the concomitant anaerobic treatment of toluene and copper in a single-chamber bioelectrochemical cell with a potential difference of 1 V applied between the anode and the cathode. As a result, the electric current generated by the bioelectrocatalytic oxidation of toluene at the anode caused the abiotic reduction and precipitation of copper at the cathode, until the complete removal of both contaminants was achieved. Open circuit potential (OCP) experiments confirmed that the removal of copper and toluene was primarily associated with polarization. Analogously, abiotic experiments, at an applied potential of 1 V, confirmed that neither toluene was oxidized nor copper was reduced in the absence of microbial activity. At the end of each experiment, both electrodes were characterized by means of a comprehensive suite of chemical and microbiological analyses, evidencing a highly selected microbial community competent in the biodegradation of toluene in the anodic biofilm, and a uniform electrodeposition of spherical Cu2O nanoparticles over the cathode surface.

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.

Enhancing the Anaerobic Biodegradation of Petroleum Hydrocarbons in Soils with Electrically Conductive Materials.

Anaerobic bioremediation is a relevant process in the management of sites contaminated by petroleum hydrocarbons. Recently, interspecies electron transfer processes mediated by conductive minerals or particles have been proposed as mechanisms through which microbial species within a community share reducing equivalents to drive the syntrophic degradation of organic substrates, including hydrocarbons. Here, a microcosm study was set up to investigate the effect of different electrically conductive materials (ECMs) in enhancing the anaerobic biodegradation of hydrocarbons in historically contaminated soil. The results of a comprehensive suite of chemical and microbiological analyses evidenced that supplementing the soil with (5% w/w) magnetite nanoparticles or biochar particles is an effective strategy to accelerate the removal of selected hydrocarbons. In particular, in microcosms supplemented with ECMs, the removal of total petroleum hydrocarbons was enhanced by up to 50% relative to unamended controls. However, chemical analyses suggested that only a partial bioconversion of contaminants occurred and that longer treatment times would have probably been required to drive the biodegradation process to completion. On the other hand, biomolecular analyses confirmed the presence of several microorganisms and functional genes likely involved in hydrocarbon degradation. Furthermore, the selective enrichment of known electroactive bacteria (i.e., Geobacter and Geothrix) in microcosms amended with ECMs, clearly pointed to a possible role of DIET (Diet Interspecies Electron Transfer) processes in the observed removal of contaminants.

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.

Insights into the syntrophic microbial electrochemical oxidation of toluene: a combined chemical, electrochemical, taxonomical, functional gene-based, and metaproteomic approach.

Biodegradation of aromatic hydrocarbons in anoxic contaminated environments is typically limited by the lack of bioavailable electron acceptors. Microbial electrochemical technologies (METs) are able to provide a virtually inexhaustible electron acceptor in the form of a solid electrode. Recently, we provided first experimental evidence for the syntrophic degradation of toluene in a continuous-flow bioelectrochemical reactor known as the "bioelectric well". Herein, we further analyzed the structure and function of the electroactive toluene-degrading microbiome using a suite of chemical, electrochemical, phylogenetic, proteomic, and functional gene-based analyses. The bioelectric well removed 83 ± 7 % of the toluene from the influent with a coulombic efficiency of 84 %. Cyclic voltammetry allowed to identify the formal potentials of four putative electron transfer sites, which ranged from -0.2 V to +0.1 V vs. SHE, consistent with outer membrane c-type cytochromes and pili of electroactive Geobacter species. The biofilm colonizing the surface of the anode was indeed highly enriched in Geobacter species. On the other hand, the planktonic communities thriving in the bulk of the reactor harbored aromatic hydrocarbons degraders and fermentative propionate-producing microorganisms, as revealed by phylogenetic and proteomic analyses. Most likely, propionate, acetate or other VFAs produced in the bulk liquid from the degradation of toluene were utilized as substrates by the electroactive biofilm. Interestingly, key-functional genes related to the degradation of toluene were found both in the biofilm and in the planktonic communities. Taken as a whole, the herein reported results highlight the importance of applying a comprehensive suite of techniques to unravel the complex cooperative metabolisms occurring in METs.

Passive electrobioremediation approaches for enhancing hydrocarbons biodegradation in contaminated soils.

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

An underappreciated DIET for anaerobic petroleum hydrocarbon-degrading microbial communities.

Direct interspecies electron transfer (DIET) via electrically conductive minerals can play a role in the anaerobic oxidation of petroleum hydrocarbons in contaminated sites and can be exploited for the development of new, more effective bioremediation approaches.

Hyperthermiphile biofilms of Thermotoga neapolitana on different materials and electrostimulated: SEM micrographs and chemical data of the glucose fermentation in electrochemical reactors.

Hyperthermophile bacteria were seldom investigated in bioelectrochemical systems although they allow more effective control of the inoculum in comparison with mesophilic bacteria. Biofilm formed in hyperthermophilic conditions (>60 °C) also rarely was documented (d'Ippolito et al., 2020; Belkin et al., 1986, Pysz et al., 2004). Scanning Electron Microscopy (SEM) micrographs documenting biofilms formed by the Hyperthermophile bacterium Thermotoga neapolitana on different solid materials (ceramic carrier, stainless steel mesh, carbon felt, carbon paper, expanse graphite, and carbon cloth) are shown in this report. Also, micrographs of the biofilm formed on electrodes of carbon cloth under a dynamic polarization oscillating around ±1 V (±0.8 V and ±1.2 V) are reported. Two procedures of sample preparation for SEM analyses are described and used: 1) a fast drying of samples, which is enough to underline the biofilm shape that covers solids, and 2) a chemical treating of the samples with glutaraldehyde, which better preserves the shape of bacterial cell components in the biofilm, although this treatment might cause the detachment of pieces of the biofilm. The different effect of potentiostatic and potentiodynamic polarizations on the glucose metabolism of T. neapolitana has been screened and discussed in the associated article [1]. Here, data of Optical Densities (O.D.) of culture media are provided, indicating the presence or absence of bacteria growth in the bulk of the media. Data have been collected every 24 h from the differently polarized bioreactors. The electrodes set-up of small bioreactors is also illustrated. Chemical data, optical data and SEM images, accordingly, document a retard in the glucose fermentation process due to a settlement of T. Neapolitana in a stationary phase. The polarization of electrodes can modify the stationary condition, inducing a possible change of the bacteria metabolism.

A Storable Mediatorless Electrochemical Biosensor for Herbicide Detection.

A novel mediatorless photo-bioelectrochemical sensor operated with a biofilm of the cyanobacterium Synechocystis PCC6803 wt. for herbicide detection with long term stability (>20 days) was successfully developed and tested. Photoanodic current generation was obtained in the absence of artificial mediators. The inhibitory effect on photocurrent of three commonly used herbicides (i.e., atrazine, diuron, and paraquat) was used as a means of measuring their concentrations in aqueous solution. The injection of atrazine and diuron into the algal medium caused an immediate photocurrent drop due to the inhibition of photosynthetic electron transport. The detected concentrations were suitable for environmental analysis, as revealed by a comparison with the freshwater quality benchmarks set by the Environmental Protection Agency of the United States (US EPA). In contrast, paraquat caused an initial increase (~2 h) of the photocurrent effect of about 200%, as this compound can act as a redox mediator between the cells and the anode. A relatively long-term stability of the biosensor was demonstrated, by keeping anodes colonized with cyanobacterial biofilm in the dark at 4 °C. After 22 days of storage, the performance in terms of the photocurrent was comparable with the freshly prepared biosensor. This result was confirmed by the measurement of chlorophyll content, which demonstrated preservation of the cyanobacterial biofilm. The capacity of this biosensor to recover after a cold season or other prolonged environmental stresses could be a key advantage in field applications, such as in water bodies and agriculture. This study is a step forward in the biotechnological development and implementation of storable mediatorless electrochemical biosensors for herbicide detection.

Microbial recycling cells (MRCs): A new platform of microbial electrochemical technologies based on biocompatible materials, aimed at cycling carbon and nutrients in agro-food systems.

This article reviews the mechanisms that drive nutrients and carbon sequestration from wastewaters by microbial electrochemical technologies (METs). In this framework, a new generation of METs is also presented (to be called microbial recycling cells, MRCs), based on 100%-recyclable materials (biomass-derived char coal, clay, terracotta, paper, ligno-cellulosic plant materials, etc.), which can act as bio-electrodes, separators and structural frames. In traditional METs architectures (based on technological materials such as carbon cloths, plastic panels, membranes, binders), inorganic salts precipitation and adsorption, as well as biofouling due to organic-matter deposition, are considered as main drawbacks that clog and hinder the systems over relatively short periods. In MRCs, these mechanisms should be maximized, instead of being avoided. In this perspective, both inorganic and organic forms of the main nutrients are sequestered from wastewater and deposited on METs modules. Once the systems become saturated, they can entirely be recycled as agricultural soil conditioners or as base for organic-mineral fertilizers.