A Gibbs Free Energy-Based Assessment of Microbial Electrocatalysis.

The use of microbial catalysts for electrode reactions enables novel bioremediation and bioproduction processes. To understand the electrochemical performance of the electrode reactions, knowledge of their thermodynamics is essential. We elaborate here on the Growth Reference System (GRS), simplifying thermodynamic calculations in the aforementioned context to, for example, demonstrate that cathodic bioprocesses generally suffer from higher overpotentials than do anodic processes. Abiotic hydrogen production cannot be thermodynamically excluded for any of the cathodic microbial electrosynthesis processes described thus far. Predictions for maximum biomass production correlated to electron flow are in line with experimental observations. We include a comprehensive set of thermodynamic and electrochemical data to support calculations relevant to the field of microbial electrocatalysis.

A case in support of implementing innovative bio-processes in the metal mining industry.

The metal mining industry faces many large challenges in future years, among which is the increasing need to process low-grade ores as accessible higher grade ores become depleted. This is against a backdrop of increasing global demands for base and precious metals, and rare earth elements. Typically about 99% of solid material hauled to, and ground at, the land surface currently ends up as waste (rock dumps and mineral tailings). Exposure of these to air and water frequently leads to the formation of acidic, metal-contaminated run-off waters, referred to as acid mine drainage, which constitutes a severe threat to the environment. Formation of acid drainage is a natural phenomenon involving various species of lithotrophic (literally 'rock-eating') bacteria and archaea, which oxidize reduced forms of iron and/or sulfur. However, other microorganisms that reduce inorganic sulfur compounds can essentially reverse this process. These microorganisms can be applied on industrial scale to precipitate metals from industrial mineral leachates and acid mine drainage streams, resulting in a net improvement in metal recovery, while minimizing the amounts of leachable metals to the tailings storage dams. Here, we advocate that more extensive exploitation of microorganisms in metal mining operations could be an important way to green up the industry, reducing environmental risks and improving the efficiency and the economy of metal recovery.

In-situ caustic generation from sewage: the impact of caustic strength and sewage composition.

Periodic caustic dosage is a commonly used method by the water industry to elevate pH levels and deactivate sewer biofilms responsible for hydrogen sulfide generation. Caustic (NaOH) can be generated in-situ from sewage using a divided electrochemical cell, which avoids the need for transport, handling and storage of concentrated caustic solutions. In this study, we investigated the impact of caustic strength in the cathode compartment and the impact of sodium concentration in sewage on the Coulombic efficiency (CE) for caustic generation. The CE was found to be independent of the caustic strength produced in the range of up to ~3 wt%. Results showed that a caustic solution of ~3 wt% could be produced directly from sewage at a CE of up to 75 ± 0.5%. The sodium concentration in sewage had a significant impact on the CE for caustic generation as well as on the energy requirements of the system, with a higher sodium concentration leading to a higher CE and lower energy consumption. The proton, calcium, magnesium and ammonium concentrations in sewage affected the CE for caustic generation, especially at low sodium concentrations. Economical assessment based on the experimental results indicated that sulfide control in sewers using electrochemically-generated caustic from sewage is an economically attractive strategy.

Dynamically adaptive control system for bioanodes in serially stacked bioelectrochemical systems.

Microbial bioelectrochemical systems (BESs) use microorganisms as catalysts for electrode reactions. They have emerging applications in bioenergy, bioproduction, and bioremediation. BESs can be scaled up as a linked series of units or cells; however, this may lead to so-called cell reversal. Here, we demonstrate a cell balance system (CBS) that controls individual BES cells connected electrically in series by dynamically adapting the applied potential in the kilohertz frequency range relative to the performance of the bioanode. The CBS maintains the cell voltage of individual BES cells at or below a maximum set point by bypassing a portion of applied current with a high-frequency metal oxide semiconductor field-effect transistor switch control system. We demonstrate (i) multiple serially connected BES cells started simultaneously and rapidly from a single power source, as the CBS imparts no current limitation, (ii) continuous, stable, and independent performance of each stacked BES cell, and (iii) stable BES cell and stack performance under excessive applied currents. This control system has applications for not only serially stacked BESs in scaled-up stacks but also rapidly starting individual- and/or lab-scale BESs.

Long-term field test of an electrochemical method for sulfide removal from sewage.

Corrosion caused by hydrogen sulfide leads to significant costs for the rehabilitation or replacement of corroded sewer pipes. Conventional methods to prevent sewer corrosion normally involve the dosing of significant amounts of chemicals with the associated transport and storage costs as well as considerable maintenance and control requirement. Recently, a novel chemical free method for sulfide abatement based on electrochemical sulfide oxidation was shown to be highly effective for the removal of sulfide from synthetic and real sewage. Here, we report on the electrochemical removal of sulfide using Ta/Ir and Pt/Ir coated titanium electrodes under simulated sewer conditions during field trials. The results showed that sulfide can successfully be removed to levels below the normal target value at the end of a simulated rising main (i.e. <1mg/L). A coulombic efficiency for dissolved oxygen generation of ≈ 60% was obtained and was independent of the current density. Scaling of the electrode and the membrane was observed in the cathode compartment and as a result the cell potentials increased over time. The cathode potentials returned to their original potential after switching the polarity every two days, but a more frequent switching would be needed to reduce the energy requirements of the system. Accelerated lifetime experiments indicated that a lifetime of 6.0 ± 1.9 years can be expected under polarity switching conditions at a pH of 14 and significantly longer at lower pH values. As operating the system without switching simplifies construction as well as operation, the choice whether to switch or not will in practice depend on operational cost (higher/lower energy) versus capital cost (reactor and peripherals). Irrespective of the approach, our study demonstrates that electrochemical sulfide control in sewer systems may be an attractive new option.

Electrochemical sulfide oxidation from domestic wastewater using mixed metal-coated titanium electrodes.

Hydrogen sulfide generation is a major issue in sewer management. A novel method based on electrochemical sulfide oxidation was recently shown to be highly effective for sulfide removal from synthetic and real sewage. Here, we compare the performance of five different mixed metal oxide (MMO) coated titanium electrode materials for the electrochemical removal of sulfide from domestic wastewater. All electrode materials performed similarly in terms of sulfide removal, removing 78±5%, 77±1%, 85±4%, 84±1%, and 83±2% at a current density of 10 mA/cm(2) using Ta/Ir, Ru/Ir, Pt/Ir, SnO(2) and PbO(2), respectively. Elevated chloride concentrations, often observed in coastal areas, did not entail any significant difference in performance. Independent of the electrode material used, sulfide oxidation by in situ generated oxygen was the predominant reaction mechanism. Passivation of the electrode surface by deposition of elemental sulfur did not occur. However, scaling was observed in the cathode compartment. This study shows that all the MMO coated titanium electrode materials studied are suitable anodic materials for sulfide removal from wastewater. Ta/Ir and Pt/Ir coated titanium electrodes seem the most suitable electrodes since they possess the lowest overpotential for oxygen evolution, are stable at low chloride concentration and are already used in full scale applications.

Electrochemical oxidation of reverse osmosis concentrate on mixed metal oxide (MMO) titanium coated electrodes.

Reverse osmosis (RO) membranes have been successfully applied around the world for wastewater reuse applications. However, RO is a physical separation process, and besides the clean water stream (permeate) a reverse osmosis concentrate (ROC) is produced, usually representing 15-25% of the feed water flow and containing the organic and inorganic contaminants at higher concentrations. In this study, electrochemical oxidation was investigated for the treatment of ROC generated during the reclamation of municipal wastewater effluent. Using laboratory-scale two-compartment electrochemical systems, five electrode materials (i.e. titanium coated with IrO2-Ta2O5, RuO2-IrO2, Pt-IrO2, PbO2, and SnO2-Sb) were tested as anodes in batch mode experiments, using ROC from an advanced water treatment plant. The best oxidation performance was observed for Ti/Pt-IrO2 anodes, followed by the Ti/SnO2-Sb and Ti/PbO2 anodes. The effectiveness of the treatment appears to correlate with the formation of oxidants such as active chlorine (i.e. Cl2/HClO/ClO-). As a result, electro-generated chlorine led to the abundant formation of harmful by-products such as trihalomethanes (THMs) and haloacetic acids (HAAs), particularly at Ti/SnO2-Sb and Ti/Pt-IrO2 anodes. The highest concentration of total HAAs (i.e. 2.7 mg L(-1)) was measured for the Ti/SnO2-Sb electrode, after 0.55 Ah L(-1) of supplied specific electrical charge. Irrespective of the used material, electrochemical oxidation of ROC needs to be complemented by a polishing treatment to alleviate the release of halogenated by-products.

Redistribution of wastewater alkalinity with a microbial fuel cell to support nitrification of reject water.

In wastewater treatment plants, the reject water from the sludge treatment processes typically contains high ammonium concentrations, which constitute a significant internal nitrogen load in the plant. Often, a separate nitrification reactor is used to treat the reject water before it is fed back into the plant. The nitrification reaction consumes alkalinity, which has to be replenished by dosing e.g. NaOH or Ca(OH)(2). In this study, we investigated the use of a two-compartment microbial fuel cell (MFC) to redistribute alkalinity from influent wastewater to support nitrification of reject water. In an MFC, alkalinity is consumed in the anode compartment and produced in the cathode compartment. We use this phenomenon and the fact that the influent wastewater flow is many times larger than the reject water flow to transfer alkalinity from the influent wastewater to the reject water. In a laboratory-scale system, ammonium oxidation of synthetic reject water passed through the cathode chamber of an MFC, increased from 73.8 ± 8.9 mgN/L under open-circuit conditions to 160.1 ± 4.8 mgN/L when a current of 1.96 ± 0.37 mA (15.1 mA/L total MFC liquid volume) was flowing through the MFC. These results demonstrated the positive effect of an MFC on ammonium oxidation of alkalinity-limited reject water.

Electrochemical sulfide removal from synthetic and real domestic wastewater at high current densities.

Hydrogen sulfide generation is the key cause of sewer pipe corrosion, one of the major issues in water infrastructure. Current abatement strategies typically involve addition of various types of chemicals to the wastewater, which incurs large operational costs. The transport, storage and application of these chemicals also constitute occupational and safety hazards. In this study, we investigated high rate electrochemical oxidation of sulfide at Ir/Ta mixed metal oxide (MMO) coated titanium electrodes as a means to remove sulfide from wastewater. Both synthetic and real wastewaters were used in the experiments. Electrochemical sulfide oxidation by means of indirect oxidation with in-situ produced oxygen appeared to be the main reaction mechanism at Ir/Ta MMO coated titanium electrodes. The maximum obtained sulfide removal rate was 11.8 ± 1.7 g S m(-2) projected anode surface h(-1) using domestic wastewater at sulfide concentrations of ≥ 30 mg L(-1) or higher. The final products of the oxidation were sulfate, thiosulfate and elemental sulfur. Chloride and acetate concentrations did not entail differences in sulfide removal, nor were the latter two components affected by the electrochemical oxidation. Hence, the use of electrodes to generate oxygen in sewer systems may constitute a promising method for reagent-free removal of sulfide from wastewater.

Biofilm stratification during simultaneous nitrification and denitrification (SND) at a biocathode.

The aeration of the cathode compartment of bioelectrochemical systems (BESs) was recently shown to promote simultaneous nitrification and denitrification (SND). This study investigates the cathodic metabolism under different operating conditions as well as the structural organization of the cathodic biofilm during SND. Results show that a maximal nitrogen removal efficiency of 86.9 ± 0.5%, and a removal rate of 3.39 ± 0.08 mg NL(-1)h(-1) could be achieved at a dissolved oxygen (DO) level of 5.73 ± 0.03 mg L(-1) in the catholyte. The DO levels used in this study are higher than the thresholds previously reported as detrimental for denitrification. Analysis of the cathodic half-cell potential during batch tests suggested the existence of an oxygen gradient within the biofilm while performing SND. FISH analysis corroborated this finding revealing that the structure of the biofilm included an outer layer occupied by putative nitrifying organisms, and an inner layer where putative denitrifying organisms were most dominant. To our best knowledge this is the first time that nitrifying and denitrifying microorganisms are simultaneously observed in a cathodic biofilm.

Electrochemical oxidation of trace organic contaminants in reverse osmosis concentrate using RuO2/IrO2-coated titanium anodes.

During membrane treatment of secondary effluent from wastewater treatment plants, a reverse osmosis concentrate (ROC) containing trace organic contaminants is generated. As the latter are of concern, effective and economic treatment methods are required. Here, we investigated electrochemical oxidation of ROC using Ti/Ru(0.7)Ir(0.3)O(2) electrodes, focussing on the removal of dissolved organic carbon (DOC), specific ultra-violet absorbance at 254 nm (SUVA(254)), and 28 pharmaceuticals and pesticides frequently encountered in secondary treated effluents. The experiments were conducted in a continuously fed reactor at current densities (J) ranging from 1 to 250 A m(-2) anode, and a batch reactor at J = 250 A m(-2). Higher mineralization efficiency was observed during batch oxidation (e.g. 25.1 ± 2.7% DOC removal vs 0% removal in the continuous reactor after applying specific electrical charge, Q = 437.0 A h m(-3) ROC), indicating that DOC removal is depending on indirect oxidation by electrogenerated oxidants that accumulate in the bulk liquid. An initial increase and subsequent slow decrease in SUVA(254) during batch mode suggests the introduction of auxochrome substituents (e.g. -Cl, NH(2)Cl, -Br, and -OH) into the aromatic compounds. Contrarily, in the continuous reactor ring-cleaving oxidation products were generated, and SUVA(254) removal correlated with applied charge. Furthermore, 20 of the target pharmaceuticals and pesticides completely disappeared in both the continuous and batch experiments when applying J ≥ 150 A m(-2) (i.e. Q ≥ 461.5 A h m(-3)) and 437.0 A h m(-3) (J = 250 A m(-2)), respectively. Compounds that were more persistent during continuous oxidation were characterized by the presence of electrophilic groups on the aromatic ring (e.g. triclopyr) or by the absence of stronger nucleophilic substituents (e.g. ibuprofen). These pollutants were oxidized when applying higher specific electrical charge in batch mode (i.e. 1.45 kA h m(-3) ROC). However, baseline toxicity as determined by Vibrio fischeri bioluminescence inhibition tests (Microtox) was increasing with higher applied charge during batch and continuous oxidation, indicating the formation of toxic oxidation products, possibly chlorinated and brominated organic compounds.

Dehalogenation of iodinated X-ray contrast media in a bioelectrochemical system.

Iodinated X-ray contrast media (ICM) are only to a limited extent removed from conventional wastewater treatment plants, due to their high recalcitrance. This work reports on the cathodic dehalogenation of the ICM iopromide in a bioelectrochemical system (BES), fed with acetate at the anode and iopromide at the cathode. When the granular graphite cathode potential was decreased from -500 to -850 mV vs standard hydrogen electrode (SHE), the iopromide removal and the iodide release rates increased from 0 to 4.62 ± 0.01 mmol m(-3) TCC d(-1) and 0 to 13.4 ± 0.16 mmol m(-3) TCC d(-1) (Total Cathodic Compartment, TCC) respectively. Correspondingly, the power consumption increased from 0.4 ± 1 to 20.5 ± 3.3 W m(-3) TCC. The Coulombic efficiency of the iopromide dehalogenation at the cathode was less than 1%, while the Coulombic efficiency of the acetate oxidation at the anode was lower than 50% at various granular graphite cathode potentials. The results suggest that iopromide could be completely dehalogenated in BESs when the granular graphite cathode potential was controlled at -800 mV vs SHE or lower. This finding was further confirmed using mass spectrometry to identify the dehalogenated intermediates and products of iopromide in BESs. Kinetic analysis indicates that iopromide dehalogenation in batch experiments can be described by a first-order model at various cathode potentials. This work demonstrates that the BESs have a potential for efficient dehalogenation of ICM from wastewater or environmental streams.

Butler-Volmer-Monod model for describing bio-anode polarization curves.

A kinetic model of the bio-anode was developed based on a simple representation of the underlying biochemical conversions as described by enzyme kinetics, and electron transfer reactions as described by the Butler-Volmer electron transfer kinetics. This Butler-Volmer-Monod model was well able to describe the measured bio-anode polarization curves. The Butler-Volmer-Monod model was compared to the Nernst-Monod model described the experimental data significantly better. The Butler-Volmer-Monod model has the Nernst-Monod model as its full electrochemically reversible limit. Contrary to the Nernst-Monod model, the Butler-Volmer-Monod model predicts zero current at equilibrium potential. Besides, the Butler-Volmer-Monod model predicts that the apparent Monod constant is dependent on anode potential, which was supported by experimental results.

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.

High current generation coupled to caustic production using a lamellar bioelectrochemical system.

Recently, bioelectrochemical systems (BESs) have emerged as a promising technology for energy and product recovery from wastewaters. To become economically viable, BESs need to (i) reach sufficient turnover rates at scale and (ii) generate a product that offsets the investment costs within a reasonable time frame. Here we used a liter scale, lamellar BES to produce a caustic solution at the cathode. The reactor was operated as a three-electrode system, in which the anode potential was fixed and power was supplied over the reactor to allow spontaneous anodic current generation. In laboratory conditions, with acetate as electron donor in the anode, the system generated up to 1.05 A (at 1.77 V applied cell voltage, 1015 A m(-3) anode volume), and allowed for the production of caustic to 3.4 wt %, at an acetate to caustic efficiency of 61%. The reactor was subsequently operated on a brewery site, directly using effluent from the brewing process. Currents of up to 0.38 A were achieved within a six-week time frame. Considerable fluctuations over weekly periods were observed, due to operational parameter changes. This study is the first to demonstrate effective production of caustic at liter scale, using BESs both in laboratory and field conditions. It also shows that input of power can easily be justified by product value.

Life cycle assessment of high-rate anaerobic treatment, microbial fuel cells, and microbial electrolysis cells.

Existing wastewater treatment options are generally perceived as energy intensive and environmentally unfriendly. Much attention has been focused on two new approaches in the past years, (i) microbial fuel cells and (ii) microbial electrolysis cells, which directly generate electrical current or chemical products, respectively, during wastewater treatment. These systems are commonly denominated as bioelectrochemical systems, and a multitude of claims have been made in the past regarding the environmental impact of these treatment options. However, an in-depth study backing these claims has not been performed. Here, we have conducted a life cycle assessment (LCA) to compare the environmental impact of three industrial wastewater treatment options, (i) anaerobic treatment with biogas generation, (ii) a microbial fuel cell treatment, with direct electricity generation, and (iii) a microbial electrolysis cell, with hydrogen peroxide production. Our analysis showed that a microbial fuel cell does not provide a significant environmental benefit relative to the "conventional" anaerobic treatment option. However, a microbial electrolysis cell provides significant environmental benefits through the displacement of chemical production by conventional means. Provided that the target conversion level of 1000 A.m(-3) can be met, the decrease in greenhouse gas emissions and other environmentally harmful emissions (e.g., aromatic hydrocarbons) of the microbial electrolysis cell will be a key driver for the development of an industrial standard for this technology. Evidently, this assessment is highly dependent on the underlying assumptions, such as the used reactor materials and target performance. This provides a challenge and an opportunity for researchers in the field to select and develop appropriate and environmentally benign materials of construction, as well as demonstrate the required 1000 A.m(-3) performance at pilot and full scale.

Simultaneous nitrification, denitrification and carbon removal in microbial fuel cells.

Microbial fuel cells (MFCs) can use nitrate as a cathodic electron acceptor, allowing for simultaneous removal of carbon (at the anode) and nitrogen (at the cathode). In this study, we supplemented the cathodic process with in situ nitrification through specific aeration, and thus obtained simultaneous nitrification and denitrification (SND) in the one half-cell. Synthetic wastewater containing acetate and ammonium was supplied to the anode; the effluent was subsequently directed to the cathode. The influence of oxygen levels and carbon/nitrogen concentrations and ratios on the system performances was investigated. Denitrification occurred simultaneously with nitrification at the cathode, producing an effluent with levels of nitrate and ammonium as low as 1.0+/-0.5 mg N L(-1) and 2.13+/-0.05 mg N L(-1), respectively, resulting in a nitrogen removal efficiency of 94.1+/-0.9%. The integration of the nitrification process into the cathode solves the drawback of ammonium losses due to diffusion between compartments in the MFC, as previously reported in a system operating with external nitrification stage. This work represents the first successful attempt to combine SND and organics oxidation while producing electricity in an MFC.

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.

Decolorization of azo dyes in bioelectrochemical systems.

Azo dyes are ubiquitously used in the textile industry. These dyes need to be removed from the effluent prior to discharge to sewage due to their intense color and toxicity. In this study we investigated the use of a bioelectrochemical system (BES) to abioticlly cathodic decolorization of a model azo dye, Acid Orange 7 (AO7), where the process was driven by microbial oxidation of acetate atthe anode. Effective decolorization of AO7 at rates up to 264 +/- 0.03 mol m(-3) NCC d(-1) (net cathodic compartment, NCC) was achieved at the cathode, with concomitant energy recovery. The AO7 decolorization rate was significantly enhanced when the BES was supplied with power, reaching 13.18 +/- 0.05 mol m(-3) NCC d(-1) at an energy consumption 0.012 +/- 0.001 kWh mol(-1) AO7 (at a controlled cathode potential of -400 mV vs SHE). Compared with conventional anaerobic biological methods, the required dosage of organic cosubstrate was significantly reduced in the BES. A possible cathodic reaction mechanism for the decolorization of AO7 is suggested based on the decolorization products identified: the azo bond of AO7 was cleaved at the cathode, resulting in the formation of the colorless sulfanilic acid and 1-amino-2-naphthol.