Biocatalysed sulphate removal in a BES cathode

Biogenic sulfur recovery from sulfate-laden antibiotic production wastewater using a single-chamber up-flow bioelectrochemical reactor

The high-concentration sulfate (SO42-) in the antibiotic production wastewater hinders the anerobic methanogenic process and also proposes possible environmental risk. In this study, a novel single-chamber up-flow anaerobic bioelectrochemical reactor (UBER) was designed to realize simultaneous SO42- removal and elemental sulfur (S0) recovery. With the carbon felt, the cathode was installed underneath and the anode above to meet the different biological niches for sulfate reducing bacteria (SRB) and sulfur oxidizing bacteria (SOB). The bio-anode UBER (B-UBER) demonstrated a much higher average SO42- removal rate (SRR) of 113.2 ± 5.7 mg SO42--S L-1 d-1 coupled with a S0 production rate (SPR) of 54.4 ± 5.8 mg S0-S L-1 d-1 at the optimal voltage of 0.8 V than that in the abio-anode UBER (control reactor) (SRR = 86.6 ± 13.4 mg SO42--S L-1 d-1; SPR = 25.5 ± 9.7 mg S0-S L-1 d-1) under long-term operation. A large amount of biogenic S0 (about 72.2 mg g-1 VSS) was recovered in the B-UBER. The bio-anode, dominated by Thiovirga (SOB genus) and Acinetobacter (electrochemically active bacteria genus), exhibited a higher current density, lower overpotential, and lower internal resistance. C-type cytochromes mainly served as the crucial electron transfer mediator for both direct and indirect electron transfer, so that significantly increasing electron transfer capacity and biogenic S0 recovery. The reaction pathways of the sulfur transformation in the B-UBER were hypothesized that SRB utilized acetate as the main electron donor for SO42- reduction in the cathode zone and SOB transferred electrons to the anode or oxygen to produce biogenic S0 in the anode zone. This study proved a new pathway for biogenic S0 recovery and sulfate removal from sulfate-laden antibiotic production wastewater using a well-designed single-chamber bioelectrochemical reactor.

Improving the performance of bioelectrochemical sulfate removal by applying flow mode

Treatment of wastewater contaminated with high sulfate concentrations is an environmental imperative lacking a sustainable and environmental friendly technological solution. Microbial electrochemical technology (MET) represents a promising approach for sulfate reduction. In MET, a cathode is introduced as inexhaustible electron source for promoting sulfate reduction via direct or mediated electron transfer. So far, this is mainly studied in batch mode representing straightforward and easy-to-use systems, but their practical implementation seems unlikely, as treatment capacities are limited. Here, we investigated bioelectrochemical sulfate reduction in flow mode and achieved removal efficiencies (E_sulfate , 89.2 ± 0.4%) being comparable to batch experiments, while sulfate removal rates (R_sulfate , 3.1 ± 0.2 mmol L^-1 ) and Coulombic efficiencies (CE, 85.2 ± 17.7%) were significantly increased. Different temperatures and hydraulic retention times (HRT) were applied and the best performance was achieved at HRT 3.5 days and 30°C. Microbial community analysis based on amplicon sequencing demonstrated that sulfate reduction was mainly performed by prokaryotes belonging to the genera Desulfomicrobium, Desulfovibrio, and Desulfococcus, indicating that hydrogenotrophic and heterotrophic sulfate reduction occurred by utilizing cathodically produced H₂ or acetate produced by homoacetogens (Acetobacterium). The advantage of flow operation for bioelectrochemical sulfate reduction is likely based on higher absolute biomass, stable pH, and selection of sulfate reducers with a higher sulfide tolerance, and improved ratio between sulfate-reducing prokaryotes and homoacetogens.

Promoted Sb removal with hydrogen production in microbial electrolysis cell by ZIF-67-derived modified sulfate-reducing bacteria bio-cathode

Bio-cathode Microbial electrolysis cell (MEC) has been widely discovered for heavy metals removal and hydrogen production. However, low electron transfer efficiency and heavy metal toxicity limit MEC treatment efficiency. In this study, ZIF-67 was introduced to modify Sulfate-reducing bacteria (SRB) bio-cathode to enhance the bioreduction of sulfate and Antimony (Sb) with hydrogen production in the MEC. ZIF-67 modified bio-cathode was developed from a bio-anode microbial fuel cell (MFC) by operating with an applied voltage of 0.8 V to reverse the polarity. Cyclic voltammetry, linear sweep voltammetry and electrochemical impedance were done to confirm the performance of the ZIF-67 modified SRB bio-cathode. The synergy reduction of sulfate and Sb was accomplished by sulfide metal precipitation reaction from SRB itself. Maximum sulfate reduction rate approached 93.37 % and Sb removal efficiency could reach 92 %, which relies on the amount of sulfide concentration generated by sulfate reduction reaction, with 0.923 ± 0.04 m³ H₂/m³ of hydrogen before adding Sb and 0.857 m³ H₂/m³ of hydrogen after adding Sb. The hydrogen was mainly produced in this system and the result of gas chromatography (GC) indicated that 73.27 % of hydrogen was produced. Meanwhile the precipitates were analyzed by X-ray diffraction and X-ray photoelectron spectroscopy to confirm Sb₂S₃ was generated from Sb (V).

Insight of the bio-cathode biofilm construction in microbial electrolysis cell dealing with sulfate-containing wastewater

Signaling molecules are useful in biofilm formation, but the mechanism for biofilm construction still needs to be explored. In this study, a signaling molecule, N-butyryl-l-Homoserine lactone (C₄-HSL), was supplied to enhance the construction of the sulfate-reducing bacteria (SRB) bio-cathode biofilm in microbial electrolysis cell (MEC). The sulfate reduction efficiency was more than 90% in less time under the system with C₄-HSL addition. The analysis of SRB bio-cathode biofilms indicated that the activity, distribution, microbial population, and secretion of extracellular polymers prompted by C₄-HSL, which accelerate the sulfate reduction, in particular for the assimilatory sulfate reduction pathway. Specifically, the relative abundance of acidogenic fermentation bacteria increased, and Desulfovibrio was co-metabolized with acidogenic fermentation bacteria. This knowledge will help to reveal the potential of signaling molecules to enhance the SRB bio-cathode biofilm MEC construction and improve the performance of treating sulfate-containing wastewater.

Bioelectrochemical systems in aid of sustainable biorefineries for the production of value-added products and resource recovery from wastewater: A critical review and future perspectives

Bioelectrochemical systems (BES) have the potential to be used in a variety of applications such as waste biorefinery, pollutants removal, CO₂ capture, and the electrosynthesis of clean and renewable biofuels or byproducts, among others. In contrast, many technical challenges need to be addressed before BES can be scaled up and put into real-world applications. Utilizing BES, this review article presents a state-of-the-art overall view of crucial concepts and the most recent innovative results and achievements acquired from the BES system. Special attention is placed on a hybrid approach for product recovery and wastewater treatment. There is also a comprehensive overview of waste biorefinery designs that are included. In conclusion, the significant obstacles and technical concerns found throughout the BES studies are discussed, and suggestions and future requirements for the virtual usage of the BES concept in actual waste treatment are outlined.

Enhanced sulfur recovery and sulfate reduction using single-chamber bioelectrochemical system

The aim of this study was to investigate the feasibility of sulfate removal and elemental sulfur (S⁰) recovery in the single-chamber bioelectrochemical system (S-BES). The performance of S-BES was compared with that of dual-chamber bioelectrochemical system (D-BES). The S-BES was constructed with graphite felt as the anode and graphite brush as the cathode. The D-BES was constructed with proton exchange membrane as the separator between anode and cathode chambers. With an applied voltage of 1.0 V and 1 g/L acetate as the substrate, the S-BES and D-BES were tested by feeding with 480 mg/L SO₄^2- in the phosphate buffer. Results showed that the maximum current density of 37.6 ± 4.5 mA/m³ was reached in the S-BES, which was higher than that in the D-BES (i.e., 22.2 ± 2.6 mA/m³). The SO₄^2- removal was much higher in the S-BES than in the D-BES (99.5% vs. 57.2%). In the effluent and the electrodes of S-BES, S⁰ was identified with Raman and X- Ray diffraction analyses. The S⁰ recovery on the anode was 13.7 times of that on the cathode of S-BES, indicating that S⁰ was mainly produced on the anode. The measured total S⁰ recovery reached 67.5% in the S-BES. High relative abundance of Desulfurella (47.1%) and Geobacter (26.1%) dominated the community in the anode biofilm of S-BES. The excellent performance of S-BES may be attributed to the neutral pH in the solution and the synergistic reaction between the anode and cathode. Results from this study should be useful to enhance the S-BES applications in treating wastewater containing sulfate.

Efficient methane production from waste activated sludge and Fenton-like pretreated rice straw in an integrated bio-electrochemical system

Integrated microbial electrolysis cell-anaerobic digestion (MEC-AD) systems have demonstrated potential advantages for methane production in the presence of small amounts of residual inhibitors. In this study, a series of tests were conducted to analyse the acidification and methanogenesis performance of pretreated rice straw (RS) in anaerobic digestion (AD) and MEC-AD systems after the addition of Fenton-like reagents. The results indicated that the short-chain acids (SCFAs) accumulations reached 2284.64 ± 21.57 mg COD/L with a dosage ratio of 1/4 (g RS/g VSS sludge) in the MEC-AD system and that methane production increased by 63.8% compared with that of an individual AD system. In the interim, the net energy output reached 1.09 × 10³ J/g TCOD, which was 1.23 times higher than that of the AD system. The residual Fe³⁺/Fe²⁺ in the pretreatment reagent was capable of promoting acidification and methanogenesis in sludge and RS fermentation. The RS hydrolysis products could constrain methanogenesis, which can be mitigated by introducing an MEC. The microbiological analyses revealed that the MEC strongly increased the enrichment of hydrogenotrophic methanogens, especially Methanobacterium (61.16%). Meanwhile, the Syntrophomonas and Acetobacterium abundances increased to 2.81% and 2.65%, respectively, which suggested the reinforcement of acetogenesis and methanogenesis. Therefore, the enhanced hydrogenotrophic methanogens might have served as the key for enhancing the efficiency of methanogenesis due to the introduction of an MEC.

Microbial electrolysis: a promising approach for treatment and resource recovery from industrial wastewater

Wastewater is one of the most common by-products of almost every industrial process. Treatment of wastewater alone, before disposal, necessitates an excess of energy. Environmental concerns over the use of fossil fuels as a source of energy have prompted a surge in demand for alternative energy sources and the development of sophisticated procedures to extract energy from unconventional sources. Treatment of municipal and industrial wastewater alone accounts for about 3% of global electricity use while the amount of energy embedded in the waste is at least 2–4 times greater than the energy required to treat the same effluent. The microbial electrolysis cell (MEC) is one of the most efficient technologies for waste-to-product conversion that uses electrochemically active bacteria to convert organic matter into hydrogen or a variety of by-products without polluting the environment. This paper highlights existing obstacles and future potential in the integration of Microbial Electrolysis Cell with other processes like anaerobic digestion coupled system, anaerobic membrane bioreactor and thermoelectric micro converter.

Antimony reduction by a non-conventional sulfate reducer with simultaneous bioenergy production in microbial fuel cells

Environmental toxicity of antimony (Sb) is significantly increased through the widespread industrial application. The extended release of Sb above the regulatory level became a risk to humans habituated in the ecosystem. Conventional methods to remediate Sb demand high energy or resource input, which further leads to secondary pollution. The bio-electrochemical system offers a promising bioremediation strategy to remove or reduce toxic heavy metals. Thus, this research explores the possibilities of simultaneous metal sulfide (MeS) precipitation and electricity production using a full biological Microbial fuel cell (MFC). A non-conventional sulfate-reducing bacteria (SRB) Citrobacter freundii SR10 was used for this investigation, where the MFC was operated for lactate utilization in the bio-anode and Sb reduction at the bio-cathode. This study observed 81% of coulombic efficiency (bio-anode) and 97% of sulfate reduction with 99.3% Sb (V) reduction (bio-cathode), and it was concluded that the MeS precipitation entirely depends on sulfide concentration via SR10 sulfate reduction. The MFC-SR10 offers a maximum power density of 1652.9 ± 32.1 mW/m^3, and their performance was depicted using cyclic voltammetry and electrochemical impedance spectroscopy. The Sb reduction was evaluated through fluorescence spectroscopy, and the Sb (V) MeS precipitation was confirmed as stibnite (Sb₂S₃) by Raman spectroscopy and X-ray photoelectron spectroscopy. Furthermore, the matured anodic and cathodic biofilm formation was confirmed by Scanning electron microscopy with Energy-dispersive X-ray spectroscopy. Thus the MFC with SRB bio-cathode can be used as an alternative to simultaneously remove sulfate and Sb from the wastewater with electricity production.

Electrified bioreactors: the next power-up for biometallurgical wastewater treatment

Over the past decades, biological treatment of metallurgical wastewaters has become commonplace. Passive systems require intensive land use due to their slow treatment rates, do not recover embedded resources and are poorly controllable. Active systems however require the addition of chemicals, increasing operational costs and possibly negatively affecting safety and the environment. Electrification of biological systems can reduce the use of chemicals, operational costs, surface footprint and environmental impact when compared to passive and active technologies whilst increasing the recovery of resources and the extraction of products. Electrification of low rate applications has resulted in the development of bioelectrochemical systems (BES), but electrification of high rate systems has been lagging behind due to the limited mass transfer, electron transfer and biomass density in BES. We postulate that for high rate applications, the electrification of bioreactors, for example, through the use of electrolyzers, may herald a new generation of electrified biological systems (EBS). In this review, we evaluate the latest trends in the field of biometallurgical and microbial‐electrochemical wastewater treatment and discuss the advantages and challenges of these existing treatment technologies. We advocate for future research to focus on the development of electrified bioreactors, exploring the boundaries and limitations of these systems, and their validity upon treating industrial wastewaters.

Energetics, electron uptake mechanisms and limitations of electroautotrophs growing on biocathodes - A review

Electroautotrophs are microorganisms that can take the electrons needed for energy generation, CO₂ fixation and other metabolic reactions from a polarized electrode. They have been the focus of intense research for its application in wastewater treatment, bioelectrosynthetic processes and hydrogen generation. As a general trend, current densities produced by the electron uptake of these microorganisms are low, limiting their applicability at large scale. In this work, the electron uptake mechanisms that may operate in electroautotrophs are reviewed, aiming at finding possible causes for this low performance. Biomass yields, growth rates and electron uptake rates observed when these microorganisms use chemical electron donors are compared with those typically obtained with electrodes, to explore limitations and advantages inherent to the electroautotrophic metabolism. Also, the factors affecting biofilm development are analysed to show how interfacial interactions condition bacterial adhesion, biofilm growth and electrons uptake. Finally, possible strategies to overcome these limitations are described.

Mixotrophic bacteria for environmental detoxification of contaminated waste and wastewater

Mixotrophic bacteria provide a desirable alternative to the use of classical heterotrophic or chemolithoautotrophic bacteria in environmental technology, particularly under limiting nutrients conditions. Their bi-modal ability of adapting to inorganic or organic carbon feed and sulfur, nitrogen, or even heavy metal stress conditions are attractive features to achieve efficient bacterial activity and favorable operation conditions for the environmental detoxification or remediation of contaminated waste and wastewater. This review provides an overview on the state of the art and summarizes the metabolic traits of the most promising and emerging non-model mixotrophic bacteria for the environmental detoxification of contaminated wastewater and waste containing excess amounts of limiting nutrients. Although mixotrophic bacteria usually function with low organic carbon sources, the unusual capabilities of mixotrophic electroactive exoelectrogens and electrotrophs in bioelectrochemical systems and in microbial electrosynthesis for accelerating simultaneous metabolism of inorganic or organic C and N, S or heavy metals are reviewed. The identification of the mixotrophic properties of electroactive bacteria and their capability to drive mono- or bidirectional electron transfer processes are highly exciting and promising aspects. These aspects provide an appealing potential for unearthing new mixotrophic exoelectrogens and electrotrophs, and thus inspire the next generation of microbial electrochemical technology and mixotrophic bacterial metabolic engineering. KEY POINTS: • Mixotrophic bacteria efficiently and simultaneously remove C and N, S or heavy metals. • Exoelectrogens and electrotrophs accelerate metabolism of C and N, S or heavy metals. • New mixotrophic exoelectrogens and electrotrophs should be discovered and exploited.

Efficient reduction of antimony by sulfate-reducer enriched bio-cathode with hydrogen production in a microbial electrolysis cell

Bio-cathode Microbial electrolysis cell (MEC) is a promising and eco-friendly technology for concurrent hydrogen production and heavy metal reduction. However, the bioreduction of Antimony (Sb) in a bio-electrochemical system with H₂ production is not explored. In this study, two efficient sulfate-reducing bacterial (SRB) strains were used to investigate the enhanced bioreduction of sulfate and Sb with H₂ production in the MEC. SRB Bio-cathode MEC was developed from the microbial fuel cell (MFC) and operated with an applied voltage of 0.8 V. The performance of the SRB bio-cathode was confirmed by cyclic voltammetry, linear sweep voltammetry and electrochemical impedance spectroscopy. SRB strains of BY7 and SR10 supported the synergy reduction of sulfate and Sb by sulfide metal precipitation reaction. Hydrogen gas was the main product of SRB bio-cathode, with 86.9%, and 83.6% of H₂ is produced by SR10 and BY7, respectively. Sb removal efficiency reached up to 88.2% in BY7 and 96.3% in SR10 with a sulfate reduction rate of 92.3 ± 2.6 and 98.4 ± 1.6 gm^-3d^-1 in BY7 and SR10, respectively. The conversion efficiency of Sb (V) to Sb (III) reached up to 70.1% in BY7 and 89.2% in SR10. It was concluded that the total removal efficiency of Sb relies on the amount of sulfide concentration produced by the sulfate reduction reaction. The hydrogen production rate was increased up to 1.25 ± 0.06 (BY7) and 1.36 ± 0.02 m³ H₂/(m³·d) (SR10) before addition of Sb and produced up to 0.893 ± 0.03 and 0.981 ± 0.02 m³H₂/(m³·d) after addition of Sb. The precipitates were characterized by X-ray diffraction and X-ray photoelectron spectroscopy, which confirmed Sb (V) was reduced to Sb₂S₃.

Implementation of a Sulfide-Air Fuel Cell Coupled to a Sulfate-Reducing Biocathode for Elemental Sulfur Recovery

Bio-electrochemical systems (BES) are a flexible biotechnological platform that can be employed to treat several types of wastewaters and recover valuable products concomitantly. Sulfate-rich wastewaters usually lack an electron donor; for this reason, implementing BES to treat the sulfate and the possibility of recovering the elemental sulfur (S⁰) offers a solution to this kind of wastewater. This study proposes a novel BES configuration that combines bio-electrochemical sulfate reduction in a biocathode with a sulfide–air fuel cell (FC) to recover S⁰. The proposed system achieved high elemental sulfur production rates (up to 386 mg S⁰-S L⁻¹ d⁻¹) with 65% of the sulfate removed recovered as S⁰ and a 12% lower energy consumption per kg of S⁰ produced (16.50 ± 0.19 kWh kg⁻¹ S⁰-S) than a conventional electrochemical S⁰ recovery system.

Optimisation of the operational parameters for a comprehensive bioelectrochemical treatment of acid mine drainage

Bioelectrochemical systems provide a promising tool for the treatment of acid mine drainage (AMD). Biological sulphate reduction powered with electrical energy consumes acidity and produces sulphide, which can precipitate metals. However, the produced sulphide and the changes in pH resulting from the biological processes affect the efficiency and the environmental impacts of this treatment significantly. In this work, the effects of pH and sulphur speciation on the sulphate reduction rate (SRR) and comprehensive AMD treatment were evaluated in two-chamber microbial electrolysis cells at a cathode potential of -0.8 V vs. NHE. The increase of initial sulphate concentration from below 1000 mg to above 1500 mg S-SO₄^2-/L increased SRR from 121 ± 25 to 177 ± 19 mg S-SO₄^2-/L/d. SRR further increased to 347 mg S-SO₄^2-/L/d when the operation mode was changed from batch to periodical addition of sulphate and acidity (363 mg S-SO₄^2-/L/d and 22.6 mmol H⁺/L/d, respectively). The average SRR remained above 150 mg S-SO₄^2-/L/d even at pH above 8.5 and with the total dissolved sulphide concentration increasing above 1300 mg S-TDSu/L. Operation at pH above 8 enabled the recovery of over 90% of the sulphur as dissolved sulphide and thus assisted in minimising the formation and release of toxic H₂S.

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.

Enhanced methane production by alleviating sulfide inhibition with a microbial electrolysis coupled anaerobic digestion reactor

Anaerobic digestion (AD) of organics is a challenging task under high-strength sulfate (SO₄^2-) conditions. The generation of toxic sulfides by SO₄^2--reducing bacteria (SRB) causes low methane (CH₄) production. This study investigated the feasibility of alleviating sulfide inhibition and enhancing CH₄ production by using an anaerobic reactor with built-in microbial electrolysis cell (MEC), namely ME-AD reactor. Compared to AD reactor, unionized H₂S in the ME-AD reactor was sufficiently converted into ionized HS^- due to the weak alkaline condition created via cathodic H₂ production, which relieved the toxicity of unionized H₂S to methanogenesis. Correspondingly, the CH₄ production in the ME-AD system was 1.56 times higher than that in the AD reactor with alkaline-pH control and 3.03 times higher than that in the AD reactors (no external voltage and no electrodes) without alkaline-pH control. MEC increased the amount of substrates available for CH₄-producing bacteria (MPB) to generate more CH₄. Microbial community analysis indicated that hydrogentrophic MPB (e.g. Methanosphaera) and acetotrophic MPB (e.g. Methanosaeta) participated in the two major pathways of CH₄ formation were successfully enriched in the cathode biofilm and suspended sludge of the ME-AD system. Economic revenue from increased CH₄ production totally covered the cost of input electricity. Integration of MEC with AD could be an attractive technology to alleviate sulfide inhibition and enhance CH₄ production from AD of organics under SO₄^2--rich condition.

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.

Enhanced sulfate reduction accompanied with electrically-conductive pili production in graphene oxide modified biocathodes

This study aimed to investigate the graphene oxide (GO) conversion by the sulfate-reducing biocathode and its modified effects on performance of the microbial electrolysis cell (MEC). Biocathodes were acclimated with autotrophic sulfate-reducing cultures using medium containing 500 mg L^-1 sulfate. Sulfate reductive rate in the MEC was 230 and 135 g m^-3 d^-1, respectively, with and without 30 mg L^-1 GO addition. Raman measurements showed that GO was efficiently reduced to graphene by the biocathode within 24 h. Higher electrochemical activity and smaller charge transfer resistance were detected on biofilm with GO affected. With high electrical conductivity of 307 ± 36 μS cm^-1, pili substance were observed on GO affected biofilm. As dominated by Desulfovibrio sp., the biocathode could use GO as the sole electron acceptor and maintained high activity. The results from this study should provide useful information for applications of nanomaterials in the biocathode MEC.

How to tackle the stringent sulfate removal requirements in mine water treatment-A review of potential methods

Sulfate (SO₄^2-) is a ubiquitous anion in natural waters. It is not considered toxic, but it may be detrimental to freshwater species at elevated concentrations. Mining activities are one significant source of anthropogenic sulfate into natural waters, mainly due to the exposure of sulfide mineral ores to weathering. There are several strategies for mitigating sulfate release, starting from preventing sulfate formation in the first place and ending at several end-of-pipe treatment options. Currently, the most widely used sulfate-removal process is precipitation as gypsum (CaSO₄·2H₂O). However, the lowest reachable concentration is theoretically 1500 mg L^-1 SO₄^2- due to gypsum's solubility. At the same time, several mines worldwide have significantly more stringent sulfate discharge limits. The purpose of this review is to examine the process options to reach low sulfate levels (< 1500 mg L^-1) in mine effluents. Examples of such processes include alternative chemical precipitation methods, membrane technology, biological treatment, ion exchange, and adsorption. In addition, aqueous chemistry and current effluent standards concerning sulfate together with concentrate treatment and sulfur recovery are discussed.

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.

Microbial Electrochemical Technologies for Wastewater Treatment: Principles and Evolution from Microbial Fuel Cells to Bioelectrochemical-Based Constructed Wetlands

Microbial electrochemical technologies (MET) rely on the presence of the metabolic activity of electroactive bacteria for the use of solid-state electrodes for oxidizing different kinds of compound that can lead to the synthesis of chemicals, bioremediation of polluted matrices, the treatment of contaminants of interest, as well as the recovery of energy. Keeping these possibilities in mind, there has been growing interest in the use of electrochemical technologies for wastewater treatment, if possible with simultaneous power generation, since the beginning of the present century. In the last few years, there has been growing interest in exploring the possibility of merging MET with constructed wetlands offering a new option of an intensified wetland system that could maintain a high performance with a lower footprint. Based on that interest, this paper explains the general principles of MET, and the different known extracellular electron transfer mechanisms ruling the interaction between electroactive bacteria and potential solid-state electron acceptors. It also looks at the adoption of those principles for the development of MET set-ups for simultaneous wastewater treatment and power generation, and the challenges that the technology faces. Ultimately, the most recent developments in setups that merge MET with constructed wetlands are presented and discussed.

Electron transfer mechanism of biocathode in a bioelectrochemical system coupled with chemical absorption for NO removal

A biocathode with the function of Fe(III)EDTA and Fe(II)EDTA-NO reduction was applied in a microbial electrolysis cell coupled with chemical absorption for NO removal from flue gas. As the mediated electron transfer was excluded by the same electrochemical characterizations of the biocathodes before and after a 48 h continuous operation, the profiles of reduction experiments indicated that direct electron transfer was the main mechanism of Fe(III)EDTA reduction, while Fe(III)EDTA-NO was mainly reduced via Fe(II)-assisted autotrophic denitrification. The microscopy of the biocathode confirmed the existence of pili, which was supposed to be bacterial nanowires for electron transfer. The analysis of microbial community revealed that iron-reducing bacteria, including Escherichia coli, had the possibility of electron uptake from electrode via physical contact. These results first time gave us in-depth understanding of the electron transfer in the multifunctional biocathode and mechanism for further enhancement of the bioreduction processes.

A novel bioelectrochemical system for chemical-free permanent treatment of acid mine drainage

The mining sector is currently under unprecedented pressure due to stringent environmental regulations. As a consequence, a permanent acid mine drainage (AMD) treatment is increasingly being regarded as a desirable target with direct benefits for the environment and the operational and economic viability of the resources sector. In this study we demonstrate that a novel bioelectrochemical system (BES) can deliver permanent treatment of acid mine drainage without chemical dosing. The technology consists of a two-cell bioelectrochemical setup to enable the removal of sulfate from the ongoing reduction-oxidation sulfur cycle to less than 550 mg L^-1 (85 ± 2% removal from a real AMD of an abandoned silver mine), thereby also reducing salinity at an electrical energy requirement of 10 ± 0.3 kWh kg^-1 of SO₄^2--S removed. In addition, the BES operation drove the removal and recovery of the main cations Al, Fe, Mg, Zn at rates of 151 ± 0 g Al m^-3 d^-1, 179 ± 1 g Fe m^-3 d^-1, 172 ± 1 g Mg m^-3 d^-1 and 46 ± 0 g Zn m^-3 d^-1 into a concentrate stream containing 263 ± 2 mg Al, 279 ± 2 mg Fe, 152 ± 0 mg Mg and 90 ± 0 mg Zn per gram of solid precipitated after BES fed-rate control treatment. The solid metal-sludge was twice less voluminous and 9 times more readily settleable than metal-sludge precipitated using NaOH. The continuous BES treatment also demonstrated the concomitant precipitation of rare earth elements together with yttrium (REY), with up to 498 ± 70 μg Y, 166 ± 27 μg Nd, 155 ± 14 μg Gd per gram of solid, among other high-value metals. The high-REY precipitates could be used to offset the treatment costs.

Evaluation of key parameters on simultaneous sulfate reduction and sulfide oxidation in an autotrophic biocathode

Bioelectrochemical systems (BESs) are being studied as an alternative technology for the treatment of several kinds of wastewaters with a lack of electron donor such as high-strength sulfate wastewaters. This study evaluates different parameters that influence the simultaneous sulfate reduction and sulfide oxidation in an autotrophic biocathode: ion-exchange membrane (IEM), cathodic pH and cathode potential. Two different membranes were studied to evaluate sulfate and sulfide adsorption and diffusion from the cathode to the anode, observing that a cation-exchange membrane (CEM) widely decreased these effects. Three different cathode pH (5.5, 7 and 8.5) were studied in a long-term operation observing that pH = 7 was the optimal for sulfate removal, achieving reduction rates around 150 mg S-SO₄^2- L^-1 d^-1. Microbial community analysis of the cathode biofilm demonstrated a high abundance of sulfate-reducing bacteria (SRB, 67% at pH 7, 60% at pH 8.5 and 42% at pH 5.5), mainly Desulfovibrio sp. at pH 5.5 and 7 and Desulfonatronum sp. at pH 8.5. The cathode potential also was studied from -0.7 to -1.2 V vs. SHE achieving sulfate removal rates higher than 700 mg S-SO₄^2- L^-1 d^-1 at cathode potentials from -1.0 to -1.2 V vs. SHE. Also, the highest cathodic recovery and the highest sulfur species imbalance were observed at a cathode potential of -1.0 V vs. SHE, which indicated a higher elemental sulfur production.

Opportunities for groundwater microbial electro-remediation

Groundwater pollution is a serious worldwide concern. Aromatic compounds, chlorinated hydrocarbons, metals and nutrients among others can be widely found in different aquifers all over the world. However, there is a lack of sustainable technologies able to treat these kinds of compounds. Microbial electro‐remediation, by the means of microbial electrochemical technologies (MET), can become a promising alternative in the near future. MET can be applied for groundwater treatment in situ or ex situ, as well as for monitoring the chemical state or the microbiological activity. This document reviews the current knowledge achieved on microbial electro‐remediation of groundwater and its applications.

Bioanode as a limiting factor to biocathode performance in microbial electrolysis cells

The bioanode is important for a microbial electrolysis cell (MEC) and its robustness to maintain its catalytic activity affects the performance of the whole system. Bioanodes enriched at a potential of +0.2V (vs. standard hydrogen electrode) were able to sustain their oxidation activity when the anode potential was varied from -0.3 up to +1.0V. Chronoamperometric test revealed that the bioanode produced peak current density of 0.36A/m² and 0.37A/m² at applied potential 0 and +0.6V, respectively. Meanwhile hydrogen production at the biocathode was proportional to the applied potential, in the range from -0.5 to -1.0V. The highest production rate was 7.4L H₂/(m² cathode area)/day at -1.0V cathode potential. A limited current output at the bioanode could halt the biocathode capability to generate hydrogen. Therefore maximum applied potential that can be applied to the biocathode was calculated as -0.84V without overloading the bioanode.

Selective cathodic microbial biofilm retention allows a high current-to-sulfide efficiency in sulfate-reducing microbial electrolysis cells

Selective microbial retention is of paramount importance for the long-term performance of cathodic sulfate reduction in microbial electrolysis cells (MECs) due to the slow growth rate of autotrophic sulfate-reducing bacteria. In this work, we investigate the biofilm retention and current-to-sulfide conversion efficiency using carbon granules (CG) or multi-wall carbon nanotubes deposited on reticulated vitreous carbon (MWCNT-RVC) as electrode materials. For ~2months, the MECs were operated at sulfate loading rates of 21 to 309gSO₄ -S/m²/d. Although MWCNT-RVC achieved a current density of 57±11A/m², greater than the 32±9A/m² observed using CG, both materials exhibited similar sulfate reduction rates (SRR), with MWCNT-RVC reaching 104±16gSO₄ -S/m²/d while 110±13gSO₄ -S/m²/d were achieved with CG. Pyrosequencing analysis of the 16S rRNA at the end of experimentation revealed a core community dominated by Desulfovibrio (28%), Methanobacterium (19%) and Desulfomicrobium (14%), on the MWCNT-RVC electrodes. While a similar Desulfovibrio relative abundance of 29% was found in CG-biofilms, Desulfomicrobium was found to be significantly less abundant (4%) and Methanobacterium practically absent (0.2%) on CG electrodes. Surprisingly, our results show that CG can achieve higher current-to-sulfide efficiencies at lower power consumption than the nano-modified three-dimensional MWCNT-RVC.

High-efficient acetate production from carbon dioxide using a bioanode microbial electrosynthesis system with bipolar membrane

The aim of this study was to develop an efficient bioanode microbial electrosynthesis system (MES) to convert carbon dioxide into acetate using bioenergy from the wastewater. The bioanode MESs were constructed using proton exchange membrane (PEM) and bipolar membrane (BPM) as separator, respectively, and operated under different voltages (i.e., 0.8, 1.0, 1.2, and 1.4V). Since BPM could dissociate H₂O into H⁺ and OH^- in situ to buffer the pH change in the chambers, the BPM-MES achieved 238% improvement in cathodic acetate production rate, 45% increase in anodic substrate removal efficiency, and more than five times enhancement in current output, as compared to the PEM-MES. The biomass on the surface of anode and cathode, and the relative abundance of Acetobacterium in the cathode of BPM-MES was higher than that in PEM-MES. Bioanode MES with BPM should be a useful microbial electrosynthesis strategy for acetate production using bioenergy from wastewater treatment.

Treatment of high-strength sulfate wastewater using an autotrophic biocathode in view of elemental sulfur recovery

Treatment of high-strength sulfate wastewaters is becoming a research issue not only for its optimal management but also for the possibility of recovering elemental sulfur. Moreover, sulfate-rich wastewater production is expected to grow due to the increased SO₂ emission contained in flue gases which are treated by chemical absorption in water. Bioelectrochemical systems (BESs) are a promising alternative for sulfate reduction with a lack of electron donor, since hydrogen can be generated in situ from electricity. However, complete sulfate reduction leads to hydrogen sulfide as final sulfur compound. This work is the first to demonstrate that, in addition to an efficient sulfate-rich wastewater treatment, elemental sulfur could be recovered in a biocathode of a BES under oxygen limiting conditions. The key of the process is the biological oxidation of sulfide to elemental sulfur simultaneously to the sulfate reduction in the cathode using the oxygen produced in the anode that diffuses through the membrane. High sulfate reduction rates (up to 388 mg S-SO₄^2- L^-1 d^-1) were observed linked to a low production of sulfide. Accumulation of elemental sulfur over graphite fibers of the biocathode was demonstrated by energy dispersive spectrometry, discarding the presence of metal sulfides. Microbial community analysis of the cathode biofilm demonstrated the presence of sulfate-reducing bacteria (mainly Desulfovibrio sp.) and sulfide-oxidizing bacteria (mainly Sulfuricurvum sp.). Hence, this biocathode allows simultaneous biological sulfate reduction and biological sulfide oxidation to elemental sulfur, opening up a novel process for recovering sulfur from sulfate-rich wastewaters.

Efficient azo dye decolorization in a continuous stirred tank reactor (CSTR) with built-in bioelectrochemical system

A continuous stirred tank reactor with built-in bioelectrochemical system (CSTR-BES) was developed for azo dye Alizarin Yellow R (AYR) containing wastewater treatment. The decolorization efficiency (DE) of the CSTR-BES was 97.04±0.06% for 7h with sludge concentration of 3000mg/L and initial AYR concentration of 100mg/L, which was superior to that of the sole CSTR mode (open circuit: 54.87±4.34%) and the sole BES mode (without sludge addition: 91.37±0.44%). The effects of sludge concentration and sodium acetate (NaAc) concentration on azo dye decolorization were investigated. The highest DE of CSTR-BES for 4h was 87.66±2.93% with sludge concentration of 12,000mg/L, NaAc concentration of 2000mg/L and initial AYR concentration of 100mg/L. The results in this study indicated that CSTR-BES could be a practical strategy for upgrading conventional anaerobic facilities against refractory wastewater treatment.

Effect of the cathode potential and sulfate ions on nitrate reduction in a microbial electrochemical denitrification system

Recently, bioelectrochemical systems have been demonstrated as advantageous for denitrification. Here, we investigated the nitrate reduction rate and bacterial community on cathodes at different cathode potentials [−300, −500, −700, and −900 mV vs. standard hydrogen electrode (SHE)] in a two-chamber microbial electrochemical denitrification system and effects of sulfate, a common nitrate co-contaminant, on denitrification efficiency. The results indicated that the highest nitrate reduction rates (3.5 mg L−1 days−1) were obtained at a cathode potential of −700 mV, regardless of sulfate presence, while a lower rate was observed at a more negative cathode potential (−900 mV). Notably, although sulfate ions generally inhibited nitrate reduction, this effect was absent at a cathode potential of −700 mV. Polymerase chain reaction–denaturing gradient gel electrophoresis revealed that bacterial communities on the graphite-felt cathode were significantly affected by the cathode potential change and sulfate presence. Shinella-like and Alicycliphilus-like bacterial species were exclusively observed on cathodes in reactors without sulfate. Ochrobactrum-like and Sinorhizobium-like bacterial species, which persisted at different cathode potentials irrespective of sulfate presence, were shown to contribute to bioelectrochemical denitrification. This study suggested that a cathode potential of around −700 mV versus SHE would ensure optimal nitrate reduction rate and counteract inhibitory effects of sulfate. Additionally, sulfate presence considerably affects denitrification efficiency and microbial community of microbial electrochemical denitrification systems.

Simultaneous sulfate and zinc removal from acid wastewater using an acidophilic and autotrophic biocathode

The aim of this study was to develop microbial electrolysis cell (MEC) with a novel acidophilic and autotrophic biocathode for treatment of acid wastewater. A biocathode was developed using acidophilic sulfate-reducing bacteria as the catalyst. Artificial wastewater with 200mgL(-1) sulfate and different Zn concentrations (0, 15, 25, and 40 mg L(-1)) was used as the MEC catholyte. The acidophilic biocathode dominated by Desulfovibrio sp. with an abundance of 66% (with 82% of Desulfovibrio sequences similar to Desulfovibrio simplex) and achieved a considerable sulfate reductive rate of 32 gm(-3)d(-1). With 15 mg L(-1) Zn added, the sulfate reductive rate of MEC improved by 16%. The formation of ZnS alleviated the inhibition from sulfide and sped the sulfate reduction. With 15 and 25 mgL(-1) Zn added, more than 99% of Zn was removed from the wastewater. Dissolved Zn ions in the catholyte were converted into insoluble Zn compounds, such as zinc sulfide and zinc hydroxide, due to the sulfide and elevated pH produced by sulfate reduction. The MEC with acidophilic and autotrophic biocathode can be used as an alternative to simultaneously remove sulfate and metals from acid wastewaters, such as acid mine drainage.

Microbiome characterization of MFCs used for the treatment of swine manure

Conventional swine manure treatment is performed by anaerobic digestion, but nitrogen is not treated. Microbial Fuel Cells (MFCs) allow organic matter and nitrogen removal with concomitant electricity production. MFC microbiomes treating industrial wastewaters as swine manure have not been characterized. In this study, a multidisciplinary approach allowed microbiome relation with nutrient removal capacity and electricity production. Two different MFC configurations (C-1 and C-2) were used to treat swine manure. In C-1, the nitrification and denitrification processes took place in different compartments, while in C-2, simultaneous nitrification-denitrification occurred in the cathode. Clostridium disporicum and Geobacter sulfurreducens were identified in the anode compartments of both systems. C. disporicum was related to the degradation of complex organic matter compounds and G. sulfurreducens to electricity production. Different nitrifying bacteria populations were identified in both systems because of the different operational conditions. The highest microbial diversity was detected in cathode compartments of both configurations, including members of Bacteroidetes, Chloroflexiaceae and Proteobacteria. These communities allowed similar removal rates of organic matter (2.02-2.09 kg COD m(-3)d(-1)) and nitrogen (0.11-0.16 kg Nm(-3)d(-1)) in both systems. However, they differed in the generation of electric energy (20 and 2 mW m(-3) in C-1 and C-2, respectively).

Influence of nitrate and sulfate reduction in the bioelectrochemically assisted dechlorination of cis-DCE

This paper investigated the reductive dechlorination (RD) of cis-dichloroethylene (cis-DCE) (average influent 14.2±0.7 μM) by a bioelectrochemical system (BES), in the presence of real contaminated groundwater containing high levels of nitrate and sulfate. The BES enhanced both the RD and competing reactions, such as nitrate and sulfate reductions, which occurred with neither an external organic carbon source nor any inoculum other than the indigenous microbial consortia in the real groundwater. In preliminary batch tests, RD and full nitrate removal occurred after a short lag phase, whereas sulfate reduction occurred slowly and alongside the RD. Under continuous flow conditions (hydraulic retention time, HRT, 1.4 d), the competition of different electron acceptors was strongly affected by the cathodic potential in the range -550 to -750 mV vs. standard hydrogen electrode (SHE). Nitrate reduction was driven to completion at all tested cathodic potentials, whereas sulfate reduction and the RD rate increased as the cathodic potential became more negative. At -750 mV vs. SHE, strong methanogenesis was also observed and became the most important sink of electrons. The overall coulombic efficiency decreased while the potential became more negative. The RD contribution was always less than 1%. Hence, greater energy consumption was required to obtain higher RD rate and better conversion. Anodic oxidation was only observed at -750 mV vs. SHE where almost 39% of residual vinyl chloride (VC) was oxidized and the sulfate was formed back from sulfide (further contributing to electric waste).

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.

Methanobacterium enables high rate electricity-driven autotrophic sulfate reduction

The autotrophic reduction of sulfate can be sustained with a cathode as the only electron donor in bioelectrochemical systems (BES).