Role of sulfur during acetate oxidation in biological anodes

Reduction of S0 deposited on electroactive biofilm under an oxidative potential

The inevitable deposition of S⁰ on the electroactive biofilm (EAB) via anodic sulfide oxidation affects the stability of bioelectrochemical systems (BESs) when an accidental discharge of sulfide occurred, leading to the inhibition of electroacitivity, because the potential of anode (e.g., 0 V versus Ag/AgCl) is ~500 mV more positive than the redox potential of S^2-/S⁰. Here we found that S⁰ deposited on the EAB can be spontaneously reduced under this oxidative potential independent of microbial community variation, leading to a self-recovery of electroactivity (> 100 % in current density) with biofilm thickening (~210 μm). Transcriptomics of pure culture indicated that Geobacter highly expressed genes involving in S⁰ metabolism, which had an additional benefit to improve the viability (25 % - 36 %) of bacterial cells in biofilm distant from the anode and the cellular metabolic activity via electron shuttle pair of S⁰/S^2-(S_x^2-). Our findings highlighted the importance of spatially heterogeneous metabolism to its stability when EABs encountered with the problem of S⁰ deposition, and that in turn improved the electroactivity of EABs.

Efficient sulfur cycling improved the performance of flowback water treatment in a microbial fuel cell

The sulfate-reducing mediate microbial fuel cell (MFC) shows advantages in treating recalcitrant flowback water (FW) from shale gas extraction, but the stability under fluctuant concentrations of sulfate in FW remains unknown. Herein, we investigated the impact of fluctuant sulfate concentrations on the performance of FW treatment in MFCs. Sulfate concentration showed a significant role in the MFC treating FW, with a COD removal of 69.8 ± 9.7% and a peak power density of 2164 ± 396 mW/m³ under 247.5 mg/L sulfate, but only 39.1% and 1216 mW/m³ under 50 mg/L sulfate. The fluctuation of sulfate in a short time allowed to a stable performance, but a longtime intermittent decrease of feeding sulfate concentration significantly inhibited power generation to no more than 512 mW/m³. The sulfur cycling between sulfate and sulfide existed in the system, but the cycling rate became much lower after the longtime intermittent decrease, with resulting to the decreased power generation. Abundant sulfur-oxidizing bacteria (SOB) of Desulfuromonadaceae and Helicobacteraceae in the MFC stably feeding with 247.5 mg/L sulfate supported a high sulfur cycling rate. With the cooperation of abundant sulfate-reducing bacteria (SRB) of Desulfovibrionaceae (capable of producing electricity) on the anode and Desulfobacteraceae in anolyte, this sulfur cycling endowed the MFC with high sulfate tolerance and critically contributed to recalcitrant organics removal and power generation. However, much less SOB of Helicobacteraceae and Campylobacteraceae on the anode with high S⁰ accumulation on the surface after the longtime intermittent decrease of sulfate likely led to the low sulfur cycling rate. With also less SRB of Marinilabiaceae (capable of producing electricity) and Synergistaceae in the system, this low sulfur cycling rate thus hampered power generation. This research provides an important reference for the bioelectrochemical treatment of wastewater containing recalcitrant organics and sulfate.

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.

Role of vitamins and minerals as immunity boosters in COVID-19

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) known as coronavirus disease (COVID-19), emerged in Wuhan, China, in December 2019. On March 11, 2020, it was declared a global pandemic. As the world grapples with COVID-19 and the paucity of clinically meaningful therapies, attention has been shifted to modalities that may aid in immune system strengthening. Taking into consideration that the COVID-19 infection strongly affects the immune system via multiple inflammatory responses, pharmaceutical companies are working to develop targeted drugs and vaccines against SARS-CoV-2 COVID-19. A balanced nutritional diet may play an essential role in maintaining general wellbeing by controlling chronic infectious diseases. A balanced diet including vitamin A, B, C, D, E, and K, and some micronutrients such as zinc, sodium, potassium, calcium, chloride, and phosphorus may be beneficial in various infectious diseases. This study aimed to discuss and present recent data regarding the role of vitamins and minerals in the treatment of COVID-19. A deficiency of these vitamins and minerals in the plasma concentration may lead to a reduction in the good performance of the immune system, which is one of the constituents that lead to a poor immune state. This is a narrative review concerning the features of the COVID-19 and data related to the usage of vitamins and minerals as preventive measures to decrease the morbidity and mortality rate in patients with COVID-19.

Comparative insights into genome signatures of ferric iron oxide- and anode-stimulated Desulfuromonas spp. strains

BACKGROUND

Halotolerant Fe (III) oxide reducers affiliated in the family Desulfuromonadaceae are ubiquitous and drive the carbon, nitrogen, sulfur and metal cycles in marine subsurface sediment. Due to their possible application in bioremediation and bioelectrochemical engineering, some of phylogenetically close Desulfuromonas spp. strains have been isolated through enrichment with crystalline Fe (III) oxide and anode. The strains isolated using electron acceptors with distinct redox potentials may have different abilities, for instance, of extracellular electron transport, surface recognition and colonization. The objective of this study was to identify the different genomic signatures between the crystalline Fe (III) oxide-stimulated strain AOP6 and the anode-stimulated strains WTL and DDH964 by comparative genome analysis.

RESULTS

The AOP6 genome possessed the flagellar biosynthesis gene cluster, as well as diverse and abundant genes involved in chemotaxis sensory systems and c-type cytochromes capable of reduction of electron acceptors with low redox potentials. The WTL and DDH964 genomes lacked the flagellar biosynthesis cluster and exhibited a massive expansion of transposable gene elements that might mediate genome rearrangement, while they were deficient in some of the chemotaxis and cytochrome genes and included the genes for oxygen resistance.

CONCLUSIONS

Our results revealed the genomic signatures distinctive for the ferric iron oxide- and anode-stimulated Desulfuromonas spp. strains. These findings highlighted the different metabolic abilities, such as extracellular electron transfer and environmental stress resistance, of these phylogenetically close bacterial strains, casting light on genome evolution of the subsurface Fe (III) oxide reducers.

Overview of recent developments of resource recovery from wastewater via electrochemistry-based technologies

As the rapid increase of the worldwide population, recovering valuable resources from wastewater have attracted more and more attention by governments and academia. Electrochemical technologies have been extensively investigated over the past three decades to purify wastewater. However, the application of these technologies for resource recovery from wastewater has just attracted limited attention. In this review, the recent (2010-2020) electrochemical technologies for resource recovery from wastewater are summarized and discussed for the first time. Fundamentals of typical electrochemical technologies are firstly summarized and analyzed, followed by the specific examples of electrochemical resource recovery technologies for different purposes. Based on the fundamentals of electrochemical reactions and without the addition of chemical agents, metallic ions, nutrients, sulfur, hydrogen and chemical compounds can be effectively recovered by means of electrochemical reduction, electrochemical induced precipitation, electrochemical stripping, electrochemical oxidation and membrane-based electrochemical processes, etc. Pros and cons of each electrochemical technology in practical applications are discussed and analyzed. Single-step electrochemical process seems ineffectively to recover valuable resources from the wastewater with complicated constituents. Multiple-step processes or integrated with biological and membrane-based technologies are essential to improve the performance and purity of products. Consequently, this review attempts to offer in-depth insights into the developments of next-generation of electrochemical technologies to minimize energy consumption, boost recovery efficiency and realize the commercial application.

Enhanced Hydrocarbons Biodegradation at Deep-Sea Hydrostatic Pressure with Microbial Electrochemical Snorkels

In anaerobic sediments, microbial degradation of petroleum hydrocarbons is limited by the rapid depletion of electron acceptors (e.g., ferric oxide, sulfate) and accumulation of toxic metabolites (e.g., sulfide, following sulfate reduction). Deep-sea sediments are increasingly impacted by oil contamination, and the elevated hydrostatic pressure (HP) they are subjected to represents an additional limitation for microbial metabolism. While the use of electrodes to support electrobioremediation in oil-contaminated sediments has been described, there is no evidence on their applicability for deep-sea sediments. Here, we tested a passive bioelectrochemical system named ”oil-spill snorkel” with two crude oils carrying different alkane contents (4 vs. 15%), at increased or ambient HP (10 vs. 0.1 MPa). Snorkels enhanced alkanes biodegradation at both 10 and 0.1 MPa within only seven weeks, as compared to nonconductive glass controls. Microprofiles in anaerobic, contaminated sediments indicated that snorkels kept sulfide concentration to low titers. Bulk-sediment analysis confirmed that sulfide oxidation by snorkels largely regenerated sulfate. Hence, the sole application of snorkels could eliminate a toxicity factor and replenish a spent electron acceptor at increased HP. Both aspects are crucial for petroleum decontamination of the deep sea, a remote environment featured by low metabolic activity.

The anodic potential shaped a cryptic sulfur cycling with forming thiosulfate in a microbial fuel cell treating hydraulic fracturing flowback water

The flowback water (FW) from shale gas exploitation can be effectively treated by bioelectrochemical technology, but sulfide overproduction remains to be addressed. Herein, sulfate-reducing bacteria (SRB) meditated microbial fuel cells (MFCs) with anodic potential control were used. COD removal gradually increased to 67.4 ± 5.1% in electrode-potential-control (EPC) MFCs and 78.9 ± 2.4% in the MFC with open circuit (OC-MFC). However, in EPC MFCs sulfate removal stabilized at much lower levels (no more than 19.9 ± 1.9%) along with much lower sulfide concentrations, but in OC-MFC it increased and finally stabilized at 59.9 ± 0.1%. Partial sulfur reuse in EPC MFCs was indicated by the current production. Notably, thiosulfate was specially detected under low potentials and effectively oxidized in EPC MFCs, especially under -0.1 V vs. SHE, which probably related to the sulfur reuse. Metagenomics analysis showed that the anode with -0.1 and -0.2 V likely shunted electrons from cytochromes that used for reducing DsrC-S⁰ trisulfide and thus contributed to producing thiosulfate and decreasing sulfide production. Meanwhile, the anode with -0.1 V specially accumulated sulfur-oxidizing system (Sox) genes regarding thiosulfate and sulfite oxidation to sulfate, which concurred to the effective thiosulfate oxidation and also indicated the possible direct sulfite oxidation to sulfate during the sulfur cycling. But the anode of -0.2 V highly accumulated genes for thiosulfate and sulfite reduction. Both anodes also distinctly accumulated genes regarding thiosulfate oxidation to tetrathionate and sulfide oxidation to sulfur or polysulfide. Further, sulfur-oxidizing bacteria were specially enriched in EPC MFCs and likely contributed to thiosulfate and sulfite oxidation. Thus, we suggested that the higher electrode potential (e.g. -0.1 V) can shape a cryptic sulfur cycling, in which sulfate was first reduced to sulfite, and then reoxidized to sulfate by forming thiosulfate as an important intermediate or by direct sulfite oxidation. The results provide new sights on the bioelectrochemical treatment of wastewater containing complex organics and sulfate.

Understanding the complexity of wastewater: The combined impacts of carbohydrates and sulphate on the performance of bioelectrochemical systems

Bioelectrochemical systems (BES) have long been viewed as a promising wastewater treatment technology. However, in reality, the performance of bioelectrochemical systems fed with real (and therefore complex) wastewaters is often disappointing. We have sought to investigate the combined impacts of complex substrates and presence of electron acceptors. In particular, this study illustrates and systematically evaluates the disparity in performance between a BES acclimatised with acetate and those acclimatised with more complex carbohydrates (glucose, sucrose or starch) and in the presence and absence of sulphate. Relative to acetate only, operating with complex carbohydrates reduced current by 73%-87% and coulombic efficiency by 4%-50%. Acclimation with complex carbohydrates seriously impeded the colonisation anode by Geobacteraceae, resulting in substantially reduced capacity to produce current (60.2% on average). Combined acclimation with sulphate further reduced current by 35% on average, and resulted in a total reduction of 83%-93% relative to the acetate control. However, the presence of an electrogenic sulphide-sulphur shuttle meant sulphate had little effect on the coulombic efficiency of the BES. The results indicate that a reduction in current and coulombic efficiency is, at present, an unavoidable consequence of operating a BES fed with complex wastewater. Researchers, designers and policy makers should incorporate such losses in both their plans and their prognostications.

Progress Towards Bioelectrochemical Remediation of Hexavalent Chromium

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

Recovery of elemental sulfur with a novel integrated bioelectrochemical system with an electrochemical cell

Several industrial activities produce wastewater with high sulfate content that can cause significant environmental issues. Although bioelectrochemical systems (BESs) have recently been studied for the treatment of sulfate contained in this wastewater, the recovery of elemental sulfur with BESs is still in its beginnings. This work proposes a new reactor configuration named BES-EC, consisting of the coupling of a BES with an electrochemical cell (EC), to treat this type of wastewater and recover elemental sulfur. The reactor consisted of four electrodes: i) an abiotic anode, ii) a biocathode for the autotrophic sulfate reduction, iii) an anode of an electrochemical cell (EC) for the partial oxidation of sulfide to elemental sulfur (the biocathode and the EC anode were placed in the same chamber) and iv) an abiotic EC cathode. Several cathode potentials and sulfate loads were tested, obtaining high sulfate removal rates (up to 888 mg SO₄^2--S L^-1 d^-1 at -0.9 V vs. SHE with a specific energy consumption of 9.18 ± 0.80 kWh kg^-1 SO₄^2--S). Exceptionally high theoretical elemental sulfur production rates (up to 498 mg S⁰-S L^-1 d^-1) were achieved with the EC controlled at a current density of 2.5 A m^-2. Electron recovery around 80% was observed throughout most of the operation of the integrated system. In addition, short experiments were performed at different current densities, observing that sulfate removal did not increase proportionally to the higher applied current density. However, when the BES was controlled at 30 A m^-2 and the EC at 7.5 A m^-2, the proportion of elemental sulfur produced corresponded to 92.9 ± 1.9% of all sulfate removed.

Factors affecting the efficiency of a bioelectrochemical system: a review

The great potential of bioelectrochemical systems (BESs) in pollution control combined with energy recovery has attracted increasing attention.

Simultaneous removal of organic matter and iron from hydraulic fracturing flowback water through sulfur cycling in a microbial fuel cell

The high volume of flowback water (FW) generated during shale gas exploitation is highly saline, and contains complex organics, iron, heavy metals, and sulfate, thereby posing a significant challenge for the environmental management of the unconventional natural gas industry. Herein, the treatment of FW in a sulfur-cycle-mediated microbial fuel cell (MFC) is reported. Simultaneous removal efficiency for chemical oxygen demand (COD) and total iron from a synthetic FW was achieved, at 72 ± 7% and 90.6 ± 8.7%, respectively, with power generation of 2667 ± 529 mW/m³ in a closed-circuit MFC (CC-MFC). However, much lower iron removal (38.5 ± 4.5%) occurred in the open-circuit MFC (OC-MFC), where the generated FeS fine did not precipitate because of sulfide supersaturation. Enrichment of both sulfur-oxidizing bacteria (SOB), namely Helicobacteraceae in the anolyte and the electricity-producing bacteria, namely Desulfuromonadales on the anode likely accelerated the sulfur cycle through the biological and bioelectrochemical oxidation of sulfide in the anodic chamber, and effectively increased the molar ratio of total iron to sulfide, thus alleviating sulfide supersaturation in the closed circuitry. Enrichment of SOB in the anolyte might be attributed to the formation of FeS electricity wire and likely contributed to the stable high power generation. Bacteroidetes, Firmicutes, Proteobacteria, and Chloroflexi enriched in the anodic chamber were responsible for degrading complex organics in the FW. The treatment of real FW in the sulfur-cycle-mediated MFC also achieved high efficiency. This research provides a promising approach for the treatment of wastewater containing organic matters, heavy metals, and sulfate by using a sulfur-cycle-mediated MFC.

Anode potential selection for sulfide removal in contaminated marine sediments

Sulfate reducing microorganisms are typically involved in hydrocarbon biodegradation in the sea sediment, with their metabolism resulting in the by-production of toxic sulfide. In this context, it is of utmost importance identifying the optimal value for anodic potential which ensures efficient toxic sulfide removal. Along this line, in this study the (bio)electrochemical removal of sulfide was tested at anodic potentials of -205 mV, +195 mV and +300 mV (vs Ag/AgCl), also in the presence of a pure culture of the sulfur-oxidizing bacterium Desulfobulbus propionicus. Current production, sulfide concentration and sulfate concentration were monitored over time. At the end of the experiment sulfur deposition on the electrodes and the microbial communities were characterized by SEM-EDS and by next generation sequencing of the 16S rRNA gene respectively. Results confirmed that current production was linked to sulfide removal and D. propionicus promoted back oxidation of deposited sulfur to sulfate. The highest electron recovery was observed at +195 mV vs Ag/AgCl, and the lowest sulfur deposition was obtained at -205 mV vs Ag/AgCl anode polarization.

Bio-electrochemical system (BES) as an innovative approach for sustainable waste management in petroleum industry

Petroleum industry is one of the largest and fast growing industries due to the ever increasing global energy demands. Petroleum refinery produces huge quantities of wastes like oily sludge, wastewater, volatile organic compounds, waste catalyst, heavy metals, etc., because of its high capacity and continuous operation of many units. Major challenge to this industry is to manage the huge quantities of waste generated from different processes due to the complexity of waste as well as changing stringent environmental regulations. To decrease the energy loss for treatment and also to conserve the energy stored in the chemical bonds of these waste organics, bio-electrochemical system (BES) may be an efficient tool that reduce the economics of waste disposal by transforming the waste into energy pool. The present review discusses about the feasibility of using BES as a potential option for harnessing energy from different waste generated from petroleum refineries.

Effects of wastewater constituents and operational conditions on the composition and dynamics of anodic microbial communities in bioelectrochemical systems

Over the last decade, there has been an ever-growing interest in bioelectrochemical systems (BES) as a sustainable technology enabling simultaneous wastewater treatment and biological production of, e.g. electricity, hydrogen, and further commodities. A key component of any BES degrading organic matter is the anode where electric current is biologically generated from the oxidation of organic compounds. The performance of BES depends on the interactions of the anodic microbial communities. To optimize the operational parameters and process design of BES a better comprehension of the microbial community dynamics and interactions at the anode is required. This paper reviews the abundance of different microorganisms in anodic biofilms and discusses their roles and possible side reactions with respect to their implications on the performance of BES utilizing wastewaters. The most important operational parameters affecting anodic microbial communities grown with wastewaters are highlighted and guidelines for controlling the composition of microbial communities are given.

Anoxic biodegradation of triclosan and the removal of its antimicrobial effect in microbial fuel cells

Triclosan (TCS) is an emerging organic contaminant in the environment. Here, the anoxic bio-degradation of TCS in microbial fuel cells (MFCs) was explored. It was found that anoxic biodegradation of TCS could be achieved in MFC, and the removal rate of TCS was accelerated after reactor acclimation. After 7 months of operation, 10mg/L TCS could be removed within 8days in MFCs. Fluorescence microscopy results revealed that the microbe cells in the reactors were intact, and the microbes were in active state. Flow cytometry test showed that the proliferation of inoculated microbe was higher in MFC effluent than that in TCS solution. These data indicate that the biotoxicity of TCS has been largely eliminated after the treatment. The microbial community shift during the TCS degradation process was investigated as well. Species such as Geothrix, Corynebacterium, Sulfobacillus, GOUTA19, Geobacter, Acidithiobacillus and Acinetobacter, which were capable for the degradation of benzene-related and dechlorination of chlorine-containing chemicals, were flourished in the electrode biofilm. They may participate in the biodegradation of TCS. This work provides a new perspective for the anoxic biodegradation of recalcitrant organics, and can be useful for the in-situ bioremediation of environmental pollutants with the removal of their biotoxicity.

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.

Electro-biocatalytic treatment of petroleum refinery wastewater using microbial fuel cell (MFC) in continuous mode operation

Refinery wastewater (RW) treatment in microbial fuel cell (MFC) was studied in batch mode operation followed by continuous mode operation with 8h and 16h hydraulic retention time (HRT). The MFC performance was evaluated in terms of power density, organics removal, specific contaminants (oil & grease, phenol and sulfide) removal and energy conversion efficiency with respect to operation mode. Higher power density of 225±1.4mW/m² was observed during continuous mode operation with 16h HRT along with a substrate degradation of 84.4±0.8% including the 95±0.6 of oil content. The columbic efficiency during this operation was about 2±0.8% and the projected power yield was 340±20kWh/kg COD_R/day. Batch mode operation also showed good substrate degradation (81±1.8%) but took longer HRT which resulted in significantly low substrate degradation rate (0.036±0.002kgCOD_R/m³-day) over continuous mode operation (1.05±0.01kgCOD_R/m³-day). Overall, current study depicted the possibility of utilizing RW as substrate in MFC for power generation along with its treatment.

Process and kinetics of azo dye decolourization in bioelectrochemical systems: effect of several key factors

This study explored the influence of several key factors on the process and kinetics of azo dye decolourization in bioelectrochemical systems (BESs), including cathode potential, dissolved oxygen (DO) concentration of catholyte and biofilm formed on the cathode. The results show that azo dye methyl orange (MO) decolourization in the BES could be well described with the pseudo first-order kinetics. The MO decolourization efficiency increased from 0 to 94.90 ± 0.01% and correspondingly the reaction rate constant increased from 0 to 0.503 ± 0.001 h⁻¹ with the decrease in cathodic electrode potential from −0.2 to −0.8 V vs Ag/AgCl. On the contrary, DO concentration of the catholyte had a negative impact on MO decolourization in the BES. When DO concentration increased from zero to 5.80 mg L⁻¹, the MO decolourization efficiency decreased from 87.19 ± 4.73% to 27.77 ± 0.06% and correspondingly the reaction rate constant reduced from 0.207 ± 0.042 to 0.033 ± 0.007 h⁻¹. Additionally, the results suggest that the biofilm formed on the cathode could led to an adverse rather than a positive effect on azo dye decolourization in the BES in terms of efficiency and kinetics.

Anodes Stimulate Anaerobic Toluene Degradation via Sulfur Cycling in Marine Sediments

Hydrocarbons released during oil spills are persistent in marine sediments due to the absence of suitable electron acceptors below the oxic zone. Here, we investigated an alternative bioremediation strategy to remove toluene, a model monoaromatic hydrocarbon, using a bioanode. Bioelectrochemical reactors were inoculated with sediment collected from a hydrocarbon-contaminated marine site, and anodes were polarized at 0 mV and +300 mV (versus an Ag/AgCl [3 M KCl] reference electrode). The degradation of toluene was directly linked to current generation of up to 301 mA m(-2) and 431 mA m(-2) for the bioanodes polarized at 0 mV and +300 mV, respectively. Peak currents decreased over time even after periodic spiking with toluene. The monitoring of sulfate concentrations during bioelectrochemical experiments suggested that sulfur metabolism was involved in toluene degradation at bioanodes. 16S rRNA gene-based Illumina sequencing of the bulk anolyte and anode samples revealed enrichment with electrocatalytically active microorganisms, toluene degraders, and sulfate-reducing microorganisms. Quantitative PCR targeting the α-subunit of the dissimilatory sulfite reductase (encoded by dsrA) and the α-subunit of the benzylsuccinate synthase (encoded by bssA) confirmed these findings. In particular, members of the family Desulfobulbaceae were enriched concomitantly with current production and toluene degradation. Based on these observations, we propose two mechanisms for bioelectrochemical toluene degradation: (i) direct electron transfer to the anode and/or (ii) sulfide-mediated electron transfer.

Enriching distinctive microbial communities from marine sediments via an electrochemical-sulfide-oxidizing process on carbon electrodes

Sulfide is a common product of marine anaerobic respiration, and a potent reactant biologically and geochemically. Here we demonstrate the impact on microbial communities with the removal of sulfide via electrochemical methods. The use of differential pulse voltammetry revealed that the oxidation of soluble sulfide was seen at +30 mV (vs. SHE) at all pH ranges tested (from pH = 4 to 8), while non-ionized sulfide, which dominated at pH = 4 was poorly oxidized via this process. Two mixed cultures (CAT and LA) were enriched from two different marine sediments (from Catalina Island, CAT; from the Port of Los Angeles, LA) in serum bottles using a seawater medium supplemented with lactate, sulfate, and yeast extract, to obtain abundant biomass. Both CAT and LA cultures were inoculated in electrochemical cells (using yeast-extract-free seawater medium as an electrolyte) equipped with carbon-felt electrodes. In both cases, when potentials of +630 or +130 mV (vs. SHE) were applied, currents were consistently higher at +630 then at +130 mV, indicating more sulfide being oxidized at the higher potential. In addition, higher organic-acid and sulfate conversion rates were found at +630 mV with CAT, while no significant differences were found with LA at different potentials. The results of microbial-community analyses revealed a decrease in diversity for both CAT and LA after electrochemical incubation. In addition, some bacteria (e.g., Clostridium and Arcobacter) not well-known to be capable of extracellular electron transfer, were found to be dominant in the electrochemical cells. Thus, even though the different mixed cultures have different tolerances for sulfide, electrochemical-sulfide removal can lead to major population changes.

Electrochemical nutrient recovery enables ammonia toxicity control and biogas desulfurization in anaerobic digestion

Organic waste streams can be valorized and reduced in volume with anaerobic digestion (AD). An often-encountered key issue however is the high ammonium (NH4(+)) content of certain waste streams. Ammonia (NH3), in equilibrium with NH4(+), is a toxic compound to the methanogenic community, which limits the organic loading rate and endangers process stability. An electrochemical system (ES) linked to a digester could, besides recovering this nutrient, decrease NH3 toxicity through electrochemical extraction. Therefore, two digesters with and without ES attached in the recirculation loop were operated to test whether the ES could control NH3 toxicity. During periods of high ammonium loading rates, the methane (CH4) production of the ES-coupled reactor was up to 4.5 times higher compared to the control, which could be explained through simultaneous NH4(+) extraction and electrochemical pH control. A nitrogen flux of 47 g N m(–2) membrane d(–1) could be obtained in the ES-coupled reactor, resulting in a current and removal efficiency of 38 ± 5% and 28 ± 2%, respectively, at an electrochemical power input of 17 ± 2 kWh kg(–1) N. The anode also oxidized sulfide, resulting in a significantly lower H2S emission via the biogas. Lastly, limited methanogenic community dynamics pointed to a nonselective influence of the different operational conditions.

Deciphering the electron transfer mechanisms for biogas upgrading to biomethane within a mixed culture biocathode

This study describes the electron transfer mechanism of a BES fed with the effluent from water scrubbing to improve biogas upgrading.

Biomass retention on electrodes rather than electrical current enhances stability in anaerobic digestion

Anaerobic digestion (AD) is a well-established technology for energy recovery from organic waste streams. Several studies noted that inserting a bioelectrochemical system (BES) inside an anaerobic digester can increase biogas output, however the mechanism behind this was not explored and primary controls were not executed. Here, we evaluated whether a BES could stabilize AD of molasses. Lab-scale digesters were operated in the presence or absence of electrodes, in open (no applied potential) and closed circuit conditions. In the control reactors without electrodes methane production decreased to 50% of the initial rate, while it remained stable in the reactors with electrodes, indicating a stabilizing effect. After 91 days of operation, the now colonized electrodes were introduced in the failing AD reactors to evaluate their remediating capacity. This resulted in an immediate increase in CH4 production and VFA removal. Although a current was generated in the BES operated in closed circuit, no direct effect of applied potential nor current was observed. A high abundance of Methanosaeta was detected on the electrodes, however irrespective of the applied cell potential. This study demonstrated that, in addition to other studies reporting only an increase in methane production, a BES can also remediate AD systems that exhibited process failure. However, the lack of difference between current driven and open circuit systems indicates that the key impact is through biomass retention, rather than electrochemical interaction with the electrodes.

Anoxic bio-electrochemical system for treatment of complex chemical wastewater with simultaneous bioelectricity generation

Bioelectrochemical treatment system (BET) with anoxic anodic microenvironment was studied with chemical wastewater (CW) in comparison with anoxic treatment (AxT, sequencing batch reactor (SBR)) with same parent anaerobic consortia. BET system documented relatively higher treatment efficiency at higher organic load (5.0 kg COD/m(3)) accounting for COD removal efficiency of (90%) along with nitrate (48%), phosphate (51%), sulphates (68%), colour (63%) and turbidity (90%) removal, compared to AxT operation (COD, 47%; nitrate, 36%; phosphate, 32%; sulphate, 35%; colour, 45% and turbidity, 54%). The self-induced bio-potential developed due to the electrode assembly in BET resulted in effective treatment with simultaneous bioelectricity generation (631 mA/m(2)). AxT operation showed persistent reduction behaviour, while simultaneous redox behaviour was observed with BET indicating balanced electron transfer. BET operation illustrated higher wastewater toxicity reduction compared to the AxT system which documents the variation in bio-electrocatalytic behaviour of same consortia under different microenvironment.

Anodic reactivity of ferrous sulfide precipitates changing over time due to particulate speciation

The disposal of ferric phosphate (FePO4) sludge, routinely generated in wastewater and drinking water treatment, has a major impact on the overall treatment cost. Iron sulfide (FeSx) precipitation via sulfide addition to ferric phosphate (FePO4) sludge has been proven to be an effective method for phosphate recovery. Electrochemical oxidation of FeSx can then be utilized to recover ferric iron for reuse back in the phosphate removal process. In this study, the reactivity of FeSx particles for anodic oxidation at pH 4 was studied as a function of time after FeSx precipitate generation at a S/Fe molar ratio of 1.75. Cyclic voltammetry showed high reactivity for fresh FeSx particles, but the reactivity diminished significantly over a period of 1 month. X-ray absorption spectroscopy (XAS) revealed that this reduced reactivity with time is a consequence of the transformation of the FeSx particles in suspension from mackinawite (FeS) to pyrite (FeS2).

A new biological phosphorus removal process in association with sulfur cycle

Hong Kong has practiced seawater toilet flushing since 1958, saving 750,000 m(3) freshwater every day. A high sulfate-to-COD ratio (>1.25 mg SO4/mg COD) in the saline sewage resulting from this practice has enabled us to develop the Sulfate reduction Autotrophic denitrification and Nitrification Integrated (SANI(®)) process with minimal sludge production. This study seeks to expand the SANI process into an enhanced biological phosphorus removal (EBPR) process. A sulfur cycle associated EBPR was explored in an alternating anaerobic/oxygen-limited aerobic sequencing batch reactor with acetate fed as sole electron donor and sulfate as sulfur source at a total organic carbon to sulfur ratio of 1.1-3.1 (mg C/mg S). Phosphate uptake and polyphosphate formation was observed in this reactor that sustained high phosphate removal (20 mg P/L removed with 320 mg COD/L). This new EBPR process was supported by six observations: 1) anaerobic phosphate release associated with acetate uptake, poly-phosphate hydrolysis, poly-hydroxyalkanoate (PHA) (and poly-S(2-)/S(0)) formation and an "aerobic" phosphate uptake associated with PHA (and poly-S(2-)/S(0)) degradation, and polyphosphate formation; 2) a high P/VSS ratio (>0.16 mg P/mg VSS) and an associated low VSS/TSS ratio (0.75) characteristic of conventional PAOs; 3) a lack of P-release and P-uptake with formaldehyde inactivation and autoclaved sterilized biomass; 4) an absence of chemical precipitated P crystals as determined by XRD analysis; 5) a sludge P of more than 90% polyphosphate as determined by sequential P extraction; and 6) microscopically, observed PHA, poly-P and S globules in the biomass.

Biocatalysed sulphate removal in a BES cathode

Sulphate reduction in a biological cathode and physically separated from biological organic matter oxidation has been studied in this paper. The bioelectrochemical system was operated as microbial fuel cell (for bioelectricity production) to microbial electrolysis cell (with applied voltage). Sulphate reduction was not observed without applied voltage and only resulted when the cathodic potential was poised at -0.26V vs. SHE, with a minimum energy requirement of 0.7V, while maximum removal occurred at 1.4V applied. The reduction of sulphate led to sulphide production, which was entrapped in the ionic form thanks to the high biocathode pH (i.e. pH of 10) obtained during the process.

Biocatalyst behavior under self-induced electrogenic microenvironment in comparison with anaerobic treatment: evaluation with pharmaceutical wastewater for multi-pollutant removal

Biocatalyst behavior was comparatively evaluated under diverse microenvironments viz., self-induced electrogenic (bioelectrochemical treatment, BET) and anaerobic treatment (AnT) microenvironments, with real-field pharmaceutical wastewater. Relatively higher treatment efficiency was observed with BET (COD removal, 78.70%) over AnT (32%) along with the power output. Voltammetric profiles of AnT showed persistent reduction behavior, while BET depicted simultaneous redox behavior. BET operation documented significantly higher bio-electrocatalytic activity (kapp, 245.22 s(-1)) than AnT (kapp, 7.35 s(-1)). The electron accepting conditions due to the presence of electrode in the BET might contributed to higher electrogenesis leading to enhanced substrate degradation along with the removal of multiple pollutants accounting for the effective reduction of toxicity levels of wastewater. Even at higher organic loads, BET operation showed good treatment efficiency without process inhibition. Introduction of electrode-membrane assembly in anaerobic microenvironment showed significant change in the electrocatalytic behavior of biocatalyst resulting in enhanced treatment of complex wastewater.

Removal of copper from aqueous solution by electrodeposition in cathode chamber of microbial fuel cell

Based on energetic analysis, a novel approach for copper electrodeposition via cathodic reduction in microbial fuel cells (MFCs) was proposed for the removal of copper and recovery of copper solids as metal copper and/or Cu(2)O in a cathode with simultaneous electricity generation with organic matter. This was examined by using dual-chamber MFCs (chamber volume, 1L) with different concentrations of CuSO(4) solution (50.3 ± 5.8, 183.3 ± 0.4, 482.4 ± 9.6, 1007.9 ± 52.0 and 6412.5 ± 26.7 mg Cu(2+)/L) as catholyte at pH 4.7, and different resistors (0, 15, 390 and 1000 Ω) as external load. With glucose as a substrate and anaerobic sludge as an inoculum, the maximum power density generated was 339 mW/m(3) at an initial 6412.5 ± 26.7 mg Cu(2+)/L concentration. High Cu(2+) removal efficiency (>99%) and final Cu(2+) concentration below the USA EPA maximum contaminant level (MCL) for drinking water (1.3mg/L) was observed at an initial 196.2 ± 0.4 mg Cu(2+)/L concentration with an external resistor of 15 Ω, or without an external resistor. X-ray diffraction analysis confirmed that Cu(2+) was reduced to cuprous oxide (Cu(2)O) and metal copper (Cu) on the cathodes. Non-reduced brochantite precipitates were observed as major copper precipitates in the MFC with a high initial Cu(2+) concentration (0.1M) but not in the others. The sustainability of high Cu(2+) removal (>96%) by MFC was further examined by fed-batch mode for eight cycles.

Microbial electrosynthesis - revisiting the electrical route for microbial production

Microbial electrocatalysis relies on microorganisms as catalysts for reactions occurring at electrodes. Microbial fuel cells and microbial electrolysis cells are well known in this context; both use microorganisms to oxidize organic or inorganic matter at an anode to generate electrical power or H(2), respectively. The discovery that electrical current can also drive microbial metabolism has recently lead to a plethora of other applications in bioremediation and in the production of fuels and chemicals. Notably, the microbial production of chemicals, called microbial electrosynthesis, provides a highly attractive, novel route for the generation of valuable products from electricity or even wastewater. This Review addresses the principles, challenges and opportunities of microbial electrosynthesis, an exciting new discipline at the nexus of microbiology and electrochemistry.

Electrochemical sulfide removal and recovery from paper mill anaerobic treatment effluent

Sulfide can be removed from wastewater and recovered as elemental sulfur using an electrochemical process. Recently, we demonstrated this principle of product recovery on synthetic feeds. Here, we present a lab scale electrochemical reactor continuously removing sulfide from the effluent of an anaerobic treatment process operated on paper mill wastewater. The effluent contained 44+/-7 mg of sulfide-S L(-1). Sulfide was reduced to 8+/-2 mg-S L(-1), at a removal rate of 0.845+/-0.133 kg-S m(-3) of total anodic compartment (TAC) d(-1). The removed sulfide was recovered (75+/-4% recovery) as pure concentrated alkaline sulfide/polysulfide solution, from which solid elemental sulfur was obtained. The electrochemical sulfide removal was not affected by different soluble constituents or particulate materials present in the wastewater. However, over time sulfide removal decreased due to biological sulfur reduction using the organics present in the wastewater. Therefore, a periodic switching strategy between anode and cathode was developed. Biofilm formation was avoided as the pH of the cathode solution increased to inhibitory levels during cathodic operation, while still allowing full recovery of the sulfur as end product.

Nitrobenzene removal in bioelectrochemical systems

Nitrobenzene occurs as a pollutant in wastewaters originating from numerous industrial and agricultural activities. It needs to be removed prior to discharge to sewage treatment works because of its high toxicity and persistence. In this study, we investigated the use of a bioelectrochemical system (BES) to remove nitrobenzene at a cathode coupled to microbial oxidation of acetate at an anode. Effective removal of nitrobenzene at rates up to 1.29 +/- 0.04 mol m(-3) TCC d(-1) (total cathodic compartment, TCC) was achieved with concomitant energy recovery. Correspondingly, the formation rate for the reduction product aniline was 1.14 +/- 0.03 mol m(-3) TCC d(-1). Nitrobenzene removal and aniline formation rates were significantly enhanced when the BES was supplied with power, reaching 8.57 +/- 0.03 and 6.68 +/- 0.03 mol m(-3) TCC d(-1), respectively, at an energy consumption of 17.06 +/- 0.16 W m(-3) TCC (current density at 59.5 A m(-3) TCC). Compared to those of conventional anaerobic biological methods for nitrobenzene removal, the required dosage of organic cosubstrate was significantly reduced in this system. Although aniline was always identified as the major product of nitrobenzene reduction at the cathode of BES in this study, the Coulombic efficiencies of nitrobenzene removal and aniline formation were dependent on the current density of the BES.