Microbial activity influences electrical conductivity of biofilm anode

Self-assembled electrochemically active biofilms doped with carbon nanotubes: Electron exchange efficiency and cytotoxicity evaluation

Thick electrochemically active biofilms (EABs) will lead to insufficient extracellular electron transfer (EET) rate because of the limitation of both substrate diffusion and electron exchange. Herein, carbon nanotubes (CNTs)-doped EABs are developed through self-assembly. The highly conductive biofilms (internal resistance of ∼211 Ω) are efficiently enriched at CNTs dosage of 1 g L-1, with the stable power output of 0.568 W m-2 over three months. The embedded CNTs can act as electron tunnel to accelerate the EET rate in thick biofilm. Self-charging/discharging experiments and Nernst-Monod model stimulation demonstrate a higher net charge storage capacity (0.15 C m-2) and more negative half-saturation potential (-0.401 V) for the hybrid biofilms than that of the control (0.09 C m-2, and -0.378 V). Enzyme activity tests and the observation of confocal laser scanning microscopy by live/dead staining show a nearly negligible cytotoxicity of CNTs, and non-targeted metabonomics analysis reveals fourteen differential metabolites that do not play key roles in microbial central metabolic pathways according to KEGG compound database. The abundance of typical exoelectrogens Geobacter sp. is 2-fold of the control, resulting in a better bioelectrocatalytic activity. These finding provide a possible approach to prolong electron exchange and power output by developing a hybrid EABs doped with conductive material.

The restricted mass transfer inside the anode pore channel affects the electroactive biofilms formation, community composition and the power production in microbial electrochemical systems

Porous anodes improve system performance in microbial electrochemical systems by increasing the specific surface area for electroactive bacteria. In this study, multilayer anodes with different pore diameters were constructed to assess the impact of pore size and depth on anode performance. This layered structure makes detecting electroactive biofilms more accessible layer by layer, which is the first study to examine electroactive biofilms' molecular biology and electrochemical properties at different depths in pores with varied pore sizes. The millimeter-scale pores inside the bioanode have a limited effect in increasing power. The larger the pore diameter, the higher the maximum power density (Pmax) obtained. The Pmax of anodes with 4 mm pore (1.91 ± 0.15 W m-2) was 1.4 times higher than that of the non-perforated (1.37 ± 0.07 W m-2) and 0.5 mm pore anodes (1.39 ± 0.04 W m-2). Electricigens can colonize into pore channels for at least 10 mm with a pore diameter ≥3 mm and current densities >0.05 A m-2. However, in the pores channel with 0.5 mm diameter, electricigens can only colonize to a depth of 2 mm. The biofilm thickness, electricity output, metabolic activity, and biocommunity changed with pore depth and were restricted by the limited mass transfer. The Geobacter sp. was the dominant species in inter-pore biofilms, with 43.8 %-78.6 % in abundance and decreased in quantity as pore depth increased. The inter-pore biofilms on the outer layer contributed a current density of 0.17 ± 0.003 A m-2, while that of the inner layer was only 0.02 ± 0.01 A m-2. Further studies found that the pore edge mass transfer effect can contribute up to 75 % of the current. The mass transfer process at the pore edge region could be a multidirectional mass transfer rather than a pore channel mass transfer.

Microbe-Anode Interactions: Comparing the impact of genetic and material engineering approaches to improve the performance of microbial electrochemical systems (MES)

Microbial electrochemical systems (MESs) are a highly versatile platform technology with a particular focus on power or energy production. Often, they are used in combination with substrate conversion (e.g., wastewater treatment) and production of value-added compounds via electrode-assisted fermentation. This rapidly evolving field has seen great improvements both technically and biologically, but this interdisciplinarity sometimes hampers overseeing strategies to increase process efficiency. In this review, we first briefly summarize the terminology of the technology and outline the biological background that is essential for understanding and thus improving MES technology. Thereafter, recent research on improvements at the biofilm-electrode interface will be summarized and discussed, distinguishing between biotic and abiotic approaches. The two approaches are then compared, and resulting future directions are discussed. This mini-review therefore provides basic knowledge of MES technology and the underlying microbiology in general and reviews recent improvements at the bacteria-electrode interface.

In Situ Probing the Mass Transport Property Inside an Imitated Three-Dimensional Porous Bioelectrode

Three-dimensional porous materials have been demonstrated as the most successful bioelectrodes in bioelectrochemical systems due to their high specific surface area and abundant adhesion regions for electroactive bacteria. However, the pore clogging potentially limits the mass transfer process inside the electrode due to the unreasonable structure design and long-term operation. The investigation of mass transport behavior in the porous scaffolds is of great significance for designing the electrode structure and optimizing bioelectrochemical system performance. To in situ characterize the mass transport behavior in the orderly pore structure, model electrodes with 100 copper wires (10 × 10) are constructed to imitate a three-dimensional porous structure (pore size: ∼150 μm) commonly employed in bioelectrodes. The poor proton effective diffusion coefficient solidly demonstrates that the mass transport inside the three-dimensional porous electrode is critically inhibited, leading not only to a progressive change and sparse biomass in the biofilm development process but also to biofilm acidification due to serious proton accumulation. It finally results in sluggish bacterial metabolic activity and a decreased electrocatalytic capacity. The interior space of porous electrodes cannot be adequately utilized, resulting in the inability to fully exploit the advantages of their abundant surface area. Consequently, the construction of gradient porous electrodes with small inner and large outer pores to enhance mass transport is a feasible proposal for enhancing performance. The proposed methodology of establishing model electrodes combined with the in situ detection technique within porous electrodes is crucial for acquiring various types of physicochemical information inside the bioelectrode, such as biofilm growth situation, biochemical reaction conditions, as well as mass transfer characteristics. More importantly, the work provides a fundamental basis for designing highly efficient bioelectrodes.

A comparison of reactor configuration using a fruit waste fed two-stage anaerobic up-flow leachate reactor microbial fuel cell and a single-stage microbial fuel cell

Food waste generation and its consequent environmental impacts are increasing due to rapid urbanization, the global population, and associated food demand. Microbial fuel cells (MFCs) are a sustainable technology through which this food waste can be treated and used to produce bioelectricity. This study used two MFC configurations, a two-stage anaerobic up-flow leachate reactor MFC and a single-stage MFC, comparing the potential to treat solid fruit waste and fruit waste leachate. The two-stage MFC showed a higher potential to remove substrate at a shorter time compared to single-stage MFC. In 30 days, the two-stage anaerobic up-flow leachate reactor had a power density of 221 mW/m². It was able to remove more total solids (by 95 %), volatile solids (by 70 %), total chemical oxygen demand (by 83 %), soluble chemical oxygen demand (by 87 %), and carbohydrates (by 33 %) compared to the single-stage MFC. However, the single-stage MFC showed higher coulombic efficiency (by 86.7 %) compared to the two-stage MFC. The efficiency of single-stage MFC improved by adding buffer and maintaining a neutral pH level of the substrate. The results of this study emphasize the importance of reactor design and demonstrate that MFC can be a viable technology to generate bioenergy from food waste.

Evaluation of the effect of biofilm formation on the reductive transformation of triclosan in cathode-modified electrolytic systems

The performance of electrochemical reduction is often enhanced by electrode modification techniques. However, there is a risk of microbial colonization on the electrode surface to form biofilms in the treatment of actual wastewater with modified electrodes. In this work, the effects of biofilm formation on modified electrodes with reduced graphene oxide (rGO), platinum/carbon (Pt/C), and carbon nanotube (CNT) were investigated in triclosan (TCS) degradation. With biofilm formation, the TCS degradation efficiencies of carbon cloth (CC), rGO@CC, Pt/C@CC, and CNT@CC decayed to 54.53 %, 59.77 %, 69.19 %, and 53.97 %, respectively, compared to the raw electrodes. Confocal laser scanning microscopy and microbial community analysis revealed that the difference in biofilm thickness and activity were the major influencing factors on the discrepant TCS degradation rather than the microbial community structure. The electrochemical performance tests showed that the biofilm formation increased the ohmic resistance by an order of magnitude in rGO@CC, Pt/C@CC, and CNT@CC, and the charge transfer resistance was increased by 2.45, 3.78, and 7.75 times, respectively. The dechlorination and hydrolysis governed the TCS degradation pathway in all electrolysis systems, and the toxicity of electrochemical reductive products was significantly decreased according to the Toxicity Estimation Software Tool analysis. This study presented a systematic assessment of the biofilm formation on modified electrodes in TCS reduction, and the undisputed experimental outcomes were obtained to enrich the knowledge of implementing modified electrodes for practical applications.

Extracellular electron transfer and the conductivity in microbial aggregates during biochemical wastewater treatment: A bottom-up analysis of existing knowledge

Microbial extracellular electron transfer (EET) plays a crucial role in bioenergy production and resource recovery from wastewater. Interdisciplinary efforts have been made to unveil EET processes at various spatial scales, from nanowires to microbial aggregates. Electrical conductivity has been frequently measured as an indicator of EET efficiency. In this review, the conductivity of nanowires, biofilms, and granular sludge was summarized, and factors including subjects, measurement methods, and conducting conditions that affect the conductivity difference were discussed in detail. The high conductivity of nanowires does not necessarily result in efficient EET in microbial aggregates due to the existence of non-conductive substances and contact resistance. Improving the conductivity measurement of microbial aggregates is important because it enables the calculation of an EET flux from conductivity and a comparison of the flux with mass transfer coefficients. This review provides new insight into the significance, characterization, and optimization of EET in microbial aggregates during a wastewater treatment process.

Optimization of biofilm conductance measurement with two-electrode microbial electrochemical cells (MECs)

This study was conducted to develop a standardized and consistent method for biofilm conductance measurement for an improved comprehension of extracellular electron transfer. Biofilm conductance (2.12 ± 0.25 × 10^-4 S) with and without a fixed anode potential did not show significant difference. The conductance showed a sigmoidal relationship with anode potential. The current-voltage profile of the tested biofilm at applied voltage larger than 100 mV showed deviation from Ohm's law. Up to 69% decrease in biofilm conductance and deviation from Ohm's law were observed in the current-voltage profile when the measurement time increased. By choosing the voltage range (0- 100 mV) and step (25 mV), measurement time (100-s at each voltage step), and anode control mode, these operation settings were found more suitable for consistent and accurate biofilm conductance measurement in the 2-Au MEC system. This represents the first study that comprehensively evaluated the environmental and instrumental parameters for biofilm conductance measurement.

Microbial electrolysis cells for the production of biohydrogen in dark fermentation - A review

To assess biohydrogen for future green energy, this review revisited dark fermentation and microbial electrolysis cells (MECs). Hydrogen evolution rate in mesophilic dark fermentation is as high as 192 m³ H₂/m³-d, however hydrogen yield is limited. MECs are ideal for improving hydrogen yield from carboxylate accumulated from dark fermentation, whereas hydrogen production rate is too slow in MECs. Hence, improving anode kinetic is very important for realizing MEC biohydrogen. Intracellular electron transfer (IET) and extracellular electron transfer (EET) can limit current density in MECs, which is proportional to hydrogen evolution rate. EET does not limit current density once electrically conductive biofilms are formed on anodes, potentially producing 300 A/m². Hence, IET kinetics mainly govern current density in MECs. Among parameters associated with IET kinetic, population of anode-respiring bacteria in anode biofilms, biofilm density of active microorganisms, biofilm thickness, and alkalinity are critical for current density.

Biohydrogen production and purification: Focusing on bioelectrochemical systems

Innovative technologies on green hydrogen production become significant as the hydrogen economy has grown globally. Biohydrogen is one of green hydrogen production methods, and microbial electrochemical cells (MECs) can be key to biohydrogen provision. However, MECs are immature for biohydrogen technology due to several limitations including extracellular electron transfer (EET) engineering. Fundamental understanding of EET also needs more works to accelerate MEC commercialization. Interestingly, studies on biohydrogen gas purification are limited although biohydrogen gas mixture requires complex purification for use. To facilitate an MEC-based biohydrogen technology as the green hydrogen supply this review discussed EET kinetics, engineering of EET and direct interspecies electron transfer associated with hydrogen yield and the application of advanced molecular biology for improving EET kinetics. Finally, this article reviewed biohydrogen purification technologies to better understand purification and use appropriate for biohydrogen, focusing on membrane separation.

Molecular biology and modeling analysis reveal functional roles of propionate to acetate ratios on microbial syntrophy and competition in electro-assisted anaerobic digestion

This study examined the significance of propionate to acetate (HPr/HAc) ratios on microbial syntrophy and competition in microbial electrolysis cell-assisted anaerobic digestion (MEC-AD). In addition to molecular biology and phylogenetic analysis, a numerical MEC-AD model was developed by modifying Anaerobic Digestion Model No.1 to predict the effects of different HPr/HAc ratios (0.5, 1.5, 2.5, and 5). The HPr/HAc ratios of 0.5 and 1.5 maintained efficient syntrophy among electroactive bacteria, hydrogenotrophic methanogens, and homoacetogens, leading to higher methane yields. In contrast, higher HPr/HAc ratios of 2.5 and 5 were detrimental to methanogenesis. Both microbial community analysis and numerical modeling results suggested that higher propionate levels could promote the enrichment of H₂-utilizing acetogens, thereby triggering their competition with hydrogenotrophic methanogens. Moreover, protein fraction in extracellular polymeric substances and the relative expression of genes associated with extracellular electron transfer in both anode and cathode biofilms were markedly decreased with increasing HPr/HAc ratios, indicating partial inhibition of microbial electroactivity. Overall, these results illuminate deep insight into anaerobic syntrophy, contributing to the process kinetics and methane yields in MEC-AD systems. Furthermore, from a practical viewpoint, the results can also be helpful in effective control of MEC-AD operation without propionate accumulation.

A Review of Stand-Alone and Hybrid Microbial Electrochemical Systems for Antibiotics Removal from Wastewater

The growing concern about residual antibiotics in the water environment pushes for innovative and cost-effective technologies for antibiotics removal from wastewater. In this context, various microbial electrochemical systems have been investigated as an alternative to conventional wastewater technologies that are usually ineffective for the adequate removal of antibiotics. This review article details the development of stand-alone and hybrid or integrated microbial electrochemical systems for antibiotics removal from wastewater. First, technical features, antibiotics removal efficiencies, process optimization, and technological bottlenecks of these systems are discussed. Second, a comparative summary based on the existing reports was established to provide insights into the selection between stand-alone and hybrid systems. Finally, research gaps, the relevance of recent progress in complementary areas, and future research needs have been discussed.

Economic affordable carbonized phenolic foam anode with controlled structure for microbial fuel cells

In microbial fuel cells (MFCs), the anode electrode is a core structure as the catalytic area of exoelectrogens. The anode material for large-scale MFCs needs excellent bioelectrochemical performance and low fabrication costs. Herein, carbonized phenolic foam with controllable porous structures was developed as the bio-capacitor of MFCs. The proportion of sodium dodecylbenzene sulfonate (SDBS), which improved mixing and dissolution between the resin liquid and the foaming agent, was adjusted to form open pores on the foam film and skeletons, which promoted both the capacitance and biocompatibility of the anode. Within SDBS proportion from 0 to 1.2 wt%, the anode SPF-9 (0.9 wt%) obtained the best capacitance (37 ± 0.13 F g^-1), electrochemical active surface area (87 ± 0.38 cm²) and hydrophilia (contact angle 79 ± 0.2°). The MFCs with SPF-9 obtained the highest power density of 3980 ± 178 mW m^-2, while those of carbon-cloth anodes were 1600 ± 28 mW m^-2. The biofilm of SPF-9 also demonstrated higher activity and obtained larger abundance of exoelectrogens (68 ± 0.38%). The increased capacitance and biocompatibility mainly resulted in the good performance of SPF-9. The carbonized phenolic foam anode material was worth considering for the future application of MFCs due to its superior electrochemical performance and large-quantity fabrication capability.

Butyrate production and purification by combining dry fermentation of food waste with a microbial fuel cell

This study developed and evaluated a high-purity butyrate producing bioprocess from food waste by combining dry fermentation (DF) with a microbial fuel cell (MFC). Acclimatization of a DF reactor with an enrichment culture resulted in high food waste degradation (VS removed, %) and butyrate production. A high VS degradation of 81%, butyrate concentration of up to 24 gCOD_butyrate/L and butyrate yields of 497 gCOD_butyrate/kg VS_added was obtained in the DF reactor. As a result, butyrate comprised 83% of all short chain fatty acids (SCFA) in the DF broth. Acetate (10%) and propionate (7%) comprised the rest of the SCFA. The butyrate composition was further purified by feeding the DF broth to a multi-electrode MFC enriched with anode respiring bacteria (ARB) such as Geobacter sp. (>55%). The ARB in the MFC removed acetate and propionate while purified butyrate was recovered in the MFC effluent. Butyrate purity in the MFC effluent reached as high as 99% at hydraulic retention time of 72 h. Along with butyrate purification, the MFC produced electric power in a range of 0.1-0.6 Wh/gCODbutyrate_recovered (or 0.01-7.85 kWh/ton of food waste), demonstrating that MFCs can be an energy-positive butyrate purification bioprocess.

A multi-perspective review on microbial electrochemical technologies for food waste valorization

The worldwide generation of food waste (FW) has been increasing enormously due to the growing food industry and population. However, FW contains a large amount of biodegradable organics that can be converted to clean energy, which can potentially minimize the utilization of fossil fuels. Conventional biowaste valorization technologies, such as anaerobic digestion and composting, have been adopted for FW management for recovering useful biogas and compost. However, they are often limited by high capital and operation costs, low recovery efficiency, slow process kinetics, and system instability. On the other hand, microbial electrochemical technologies (METs) have been highly promising for efficiently harvesting bioenergy and high value-added products from FW. Hence, this article critically reviews up-to-date studies on applying various METs regarding their value-added products recovery efficiencies from FW. Moreover, this review lists existing challenges, ways to optimize the system performance and provides perspectives on future research needs.

External resistance acclimation regulates bio-anode: new perspective from biofilm structure and its correlation with anode performance

External resistance is important for the anode and cell performance. However, little attentions were paid on the effect of external resistance on the variation of biofilm structure. Here, we used external resistance ranged from 4000 to 500 Ω for anodic acclimation to investigate the correlation between anode performance and biofilm structure. With the reduce of external resistance, the maximum current density of anode increased from 1.0 to 3.4 A/m², which was resulted from a comprehensive effect of reduced charge transfer resistance and increased diffusion resistance. Biological analysis showed that with the reduce of external resistance, biomass and extracellular polymeric substances content increased by 109 and 286%, cell viability increased by 22.7%, which contributed to the reduced charge transfer resistance. But the porosity of anodic biofilm decreased by 27.8%, which led to an increased diffusion resistance of H⁺. This work provided a clear correlation between the electrochemical performance and biofilm structure.

Buffer capacity regulates the stratification of anode-respiring biofilm during brewery wastewater treatment

Improving the buffer capacity of the electrolyte can enhance the anode performance in bioelectrochemical systems (BESs). To elucidate the mechanism underlying the facilitated BESs performance, this study used three different anode biofilms cultured with different concentrations of phosphate buffer (5, 50 and 100 mM) to investigate the biofilm response, in terms of the spatial structure of metabolic activity and microbial community, to different buffer capacities. Results showed that the electrochemical activities of the anode biofilms positively correlated with the buffer concentration. The spatial stratification of metabolic activity and microbial community of the anode biofilms were regulated by the buffer capacity, and the spatial microbial heterogeneity of the anode biofilm decreased as the buffer concentration increased. With increasing buffer capacity, Geobacter spp. were enriched in both the inner and outer layers of the biofilm, and the inhibition of methanogens growth improved the COD removal attributed to anode respiration. Additionally, the stimulation of EPS production in biofilms played a role in increasing the electrochemical performance of biofilms by buffer improvement. This study first revealed the regulation of buffer capacity on the stratification of anode biofilm during brewery wastewater treatment, which provided a deep insight into the relation of biofilm structure to its electrochemical properties.

Anode biofilm influence on the toxic response of microbial fuel cells under different operating conditions

The response of microorganisms in microbial fuel cells (MFCs) to toxic compounds under different operating conditions, such as flow rate and culture time, was investigated herein. While it has been reported that MFCs can detect some toxic substances, it is unclear if operating conditions affect MFCs toxicity response. In this study, the toxic response time of MFCs decreased when the flow rate increased from 0.5 mL/min to 2 mL/min and then increased with 5 mL/min. The inhibition rates at 0.5 mL/min, 2 mL/min, and 5 mL/min were 8.4% ± 1.6%, 45.1% ± 5.3%, and 4.9% ± 0.3%, respectively. With the increase of culture time from 7 days to 90 days, the toxic response time of MFCs gradually increased. The inhibition rates at culture times of 7 days, 45 days, and 90 days were 45.1% ± 5.3%, 32.6% ± 6.6%, and 23.2% ± 1.3%, respectively. Increasing the culture time will reduce the sensitivity of MFC. The results showed that MFCs can respond quickly at a flow rate of 2 mL/min after cultivation for 7 days. Under these conditions, the power density can reach 1137.0 ± 65.5 mW/m², the relative content of Geobacter sp. is 57%, and the ORP of the multilayers changed from -159.2 ± 1.6 mV to -269.9 ± 1.7 mV within 200 μm biofilm thickness. These findings show that increasing the flow rate and shortening the culture time are conducive for the toxicity response of MFCs, which will increase the sensitivity of MFCs in practical applications.

The evaluation of long term performance of microbial fuel cell based Pb toxicity shock sensor

Microbial fuel cell (MFC) sensor exhibits attractive prospects for online monitoring of water toxicity as an early warning device. However, the accumulation of dead cells in anode biofilm might decrease the sensing sensitivity of MFC during long term operation. In addition, with repeated exposure to toxins, the microbial community of anode biofilm would also adjust to build up higher endurance to environmental toxicity. In this study, the long term sensing sensitivity of MFC sensor and the microbial community changes were characterized with Pb²⁺ as the target toxin. The results show that newly formed biofilm with higher live/dead cell ratio exhibited higher sensitivity than mature biofilm. Modification of anodic biofilm via high current stimulation was applied to increase the ratio of live cells, which led to enhanced sensing sensitivity of MFC with mature anode biofilm. However, the enhancement was relatively limited for biofilm that was previously exposed to repeated Pb²⁺ shocks. Microbial community analysis revealed that the proportions of microbial species possessing higher environmental robustness, such as Hyphomicrobiaceae and Cloacibacillus, significantly increased in the anode biofilm after long term repeated Pb²⁺ shocks.

A modelling study of the spatially heterogeneous mutualism between electroactive biofilm and planktonic bacteria

Microbial cooperation widely exists in anaerobic reactors degrading complex pollutants, conventionally studied separately inside the biofilm or the planktonic community. Recent experiments discovered the mutualism between the planktonic bacteria and electroactive biofilm treating propionate, an end-product usually accumulated in anaerobic digesters. Here, a one-dimensional multispecies model found the preference on acetate-based pathway over the hydrogen-based in such community, evidenced by the fact that acetate-originated current takes 66% of the total value and acetate-consuming anode-respiring bacteria takes over 80% of the biofilm. Acetate-based anodic respiration most apparently influences biofilm function while propionate fermentation is the dominant planktonic bio-reaction. Additionally, initial planktonic propionate level shows the ability of coordinating the balance between these two extracellular electron transfer pathways. Increasing the propionate concentration from 2 to 50 mM would increase the steady hydrogen-originated current by 210% but decrease the acetate-originated by 26%, suggesting a vital influence of the planktonic microbial process to the metabolic balance in biofilm. Best strategy to promote the biofilm activity is to increase the biomass density and biofilm conductivity simultaneously, which would increase the current density by 875% without thickening the biofilm thickness or prolonging the growth apparently.

Physiological potential of extracellular polysaccharide in promoting Geobacter biofilm formation and extracellular electron transfer

Geobacter sulfurreducens biofilms have promising applications in renewable energy, pollutant bioremediation, and bioelectronic applications. Genetically manipulating G. sulfurreducens biofilms is an effective strategy to improve the capacity of extracellular electron transfer (EET). Extracellular polysaccharide, a sticky component surrounding microbes, plays an important role in EET. Herein, we constructed a mutant of G. sulfurreducens strain PCA overexpressing the gene GSU1501 (part of the ATP-dependent exporter of the polysaccharide biosynthesis gene operon), designated strain PCA-1501, to increase EET capacity. Experimental results showed that the overexpression of GSU1501 increased extracellular polysaccharide secretion by 25.5%, which promoted the formation of biofilm with higher thickness and viability, as well as the content of extracellular c-type cytochromes. Compared with the control strain, the mutant showed a higher capacity of Fe(III) oxide reduction and current generation (increased by 20.4% and 22.2%, respectively). Interestingly, the overexpression of GSU1501 hindered the pili formation by reducing the transcription level of pilA; a compensatory relationship between extracellular polysaccharide and pili in promoting biofilm formation deserves further investigation. This study provides a feasible method to promote the EET capacity of G. sulfurreducens biofilms, which benefit their bioelectrochemical applications.

Electroactive biofilms on surface functionalized anodes: The anode respiring behavior of a novel electroactive bacterium, Desulfuromonas acetexigens

Surface chemistry is known to influence the formation, composition, and electroactivity of electron-conducting biofilms. However, understanding of the evolution of microbial composition during biofilm development and its impact on the electrochemical response is limited. Here we present voltammetric, microscopic and microbial community analysis of biofilms formed under fixed applied potential for modified graphite electrodes during early (90 h) and mature (340 h) growth phases. Electrodes modified to introduce hydrophilic groups (-NH₂, -COOH and -OH) enhance early-stage biofilm formation compared to unmodified or electrodes modified with hydrophobic groups (-C₂H₅). In addition, early-stage films formed on hydrophilic electrodes are dominated by the gram-negative sulfur-reducing bacterium Desulfuromonas acetexigens while Geobacter sp. dominates on -C₂H₅ and unmodified electrodes. As biofilms mature, current generation becomes similar, and D. acetexigens dominates in all biofilms irrespective of surface chemistry. Electrochemistry of pure culture D. acetexigens biofilms reveal that this microbe is capable of forming electroactive biofilms producing considerable current density of > 9 A/m² in a short period of potential-induced growth (~19 h following inoculation) using acetate as an electron donor. The inability of D. acetexigens biofilms to use H₂ as a sole source electron donor for current generation shows promise for maximizing H₂ recovery in single-chambered microbial electrolysis cell systems treating wastewaters.

Microbial electrochemical systems for hydrogen peroxide synthesis: Critical review of process optimization, prospective environmental applications, and challenges

Hydrogen peroxide (H₂O₂) is an industrial chemical that has been widely adopted for various industrial applications, including water and wastewater treatment. Currently, the majority of H₂O₂ is being produced through the anthraquinone oxidation process, which is disadvantageous due to the requirement of toxic raw materials and high energy input. Recently, microbial electrochemical cells (MXCs), such as microbial fuel cells and microbial electrolysis cells, have demonstrated great potential for effective H₂O₂ production via cathodic oxygen-reduction reaction (ORR). Previous studies have specified key operational parameters for scaling-up of H₂O₂-producing MXCs, where improvements in production rate, conversion efficiency, product concentration and stability are attainable. Moreover, various systems have demonstrated their value proposition in the contaminant removal aspects through direct removal of various environmental pollutants, water disinfection, and many more. This review is intended to highlight promising ways of H₂O₂ production with MXCs and on-site environmental applications of bioelectrochemically-produced H₂O₂.

Changes in syntrophic microbial communities, EPS matrix, and gene-expression patterns in biofilm anode in response to silver nanoparticles exposure

Understanding the toxic effect of silver nanoparticles (AgNPs) on various biological wastewater treatment systems is of significant interest to researchers. In recent years, microbial electrochemical technologies have opened up new opportunities for bioenergy and chemicals production from organic wastewater. However, the effects of AgNPs on microbial electrochemical systems are yet to be understood fully. Notably, no studies have investigated the impact of AgNPs on a microbial electrochemical system fed with a complex fermentable substrate. Here, we investigated the impact of AgNPs (50 mg/L) exposure to a biofilm anode in a microbial electrolysis cell (MEC) fed with glucose. The volumetric current density was 29 ± 2.0 A/m³ before the AgNPs exposure, which decreased to 20 ± 2.2 A/m³ after AgNPs exposure. The biofilms produced more extracellular polymeric substances (EPS) to cope with the AgNPs exposure, while carbohydrate to protein ratio in EPS considerably increased from 0.4 to 0.7. Scanning electron microscope (SEM) imaging also confirmed the marked excretion of EPS, forming a thick layer covering the anode biofilms after AgNPs injection. Transmission electron microscope (TEM) imaging showed that AgNPs still penetrated some microbial cells, which could explain the deterioration of MEC performance after AgNPs exposure. The relative expression level of the quorum signalling gene (LuxR) increased by 30%. Microbial community analyses suggested that various fermentative bacterial species (e.g., Bacteroides, Synergistaceae_vadinCA02, Dysgonomonas, etc.) were susceptible to AgNPs toxicity, which led to the disruption of their syntrophic partnership with electroactive bacteria. The abundance of some specific electroactive bacteria (e.g., Geobacter species) also decreased. Moreover, decreased relative expressions of various extracellular electron transfer associated genes (omcB, omcC, omcE, omcZ, omcS, and pilA) were observed. However, the members of family Enterobacteriaceae, known to perform a dual function of fermentation and anodic respiration, became dominant after biofilm anode exposed to AgNPs. Thus, EPS extraction provided partial protection against AgNPs exposure.

Energy recovery and carbon/nitrogen removal from sewage and contaminated groundwater in a coupled hydrolytic-acidogenic sequencing batch reactor and denitrifying biocathode microbial fuel cell

Developing cost-effective technology for treatment of sewage and nitrogen-containing groundwater is one of the crucial challenges of global water industries. Microbial fuel cells (MFCs) oxidize organics from sewage by exoelectrogens on anode to produce electricity while denitrifiers on cathode utilize the generated electricity to reduce nitrogen from contaminated groundwater. As the exoelectrogens are incapable of oxidizing insoluble, polymeric, and complex organics, a novel integration of an anaerobic sequencing batch reactor (ASBR) prior to the MFC simultaneously achieve hydrolytic-acidogenic conversion of complex organics, boost power recovery, and remove Carbon/Nitrogen (C/N) from the sewage and groundwater. The results obtained revealed increases in the fractions of soluble organics and volatile fatty acids in pretreated sewage by 52 ± 19% and 120 ± 40%, respectively. The optimum power and current generation with the pretreated sewage were 7.1 W m^-3 and 45.88 A m^-3, respectively, corresponding to 8% and 10% improvements compared to untreated sewage. Moreover, the integration of the ASBR with the biocathode MFC led to 217% higher carbon and 136% higher nitrogen removal efficiencies compared to the similar system without ASBR. The outcomes of the present study represent the promising prospects of using ASBR pretreatment and successive utilization of solubilized organics in denitrifying biocathode MFCs for simultaneous energy recovery and C/N removal from both sewage and nitrate nitrogen-contaminated groundwater.

The Limits of Three-Dimensionality: Systematic Assessment of Effective Anode Macrostructure Dimensions for Mixed-Culture Electroactive Biofilms

This study analyzes the biofilm growth and long-term current production of mixed-culture, electrochemically active biofilms (EABs) on macrostructured electrodes under low-shear-force conditions. The channel dimensions were altered systematically in the range 400 μm to 2 mm, and the channel heights were varied between 1 and 4 mm to simulate macrostructures of different scales. Electrodes with finer-structured surfaces produced higher current densities in the short term owing to their large surface area but were outperformed in the long term because the accumulation of biomass led to limitations of mass transfer into the structures. The best long-term performance was observed for electrodes with channel dimensions of 1×4 mm, which showed no significant decrease in performance in the long term. Channels with a diameter of 400 μm were overgrown by the biofilm, which led to a transition from 3 D to 2 D behavior, indicating that structures of this scale might not be suitable for long-term operation under low-shear-stress conditions.

Shift of biofilm and suspended bacterial communities with changes in anode potential in a microbial electrolysis cell treating primary sludge

This study, for the first time, documented microbial community shifts in response to the changes in anode potential in a microbial electrolysis cell (MEC) operated with primary sludge. At an anode potential of -0.4 V vs. Ag/AgCl, the MEC showed COD and VSS removal efficiencies of 73 ± 1% and 75 ± 2%, respectively. The volumetric current density and specific hydrogen production rate were 23 ± 1.2 A/m³, and 145 ± 4.1 L/m³-d, respectively. The anodic microbial community was consisted of various fermentative/hydrolytic bacteria (e.g., Bacteroides and Dysgonomonas) and anode-respiring bacteria (Geobacter), while different hydrolytic/fermentative bacteria were abundant in suspension. The MEC showed substantially inferior performance along with a higher accumulation of various volatile fatty acids when the anode potential was switched to more positive values (0 V and +0.4 V). Both biofilms and suspended communities were also shifted when the anode potential was changed. Notably, at +0.4 V, Geobacter genus entirely disappeared from the biofilms, while Paludibacter species (known fermentative bacteria) were selectively enriched in biofilms. Also, the relative abundance of genus Bacteroides (known hydrolytic bacteria) substantially decreased in both biofilms and suspension, which was correlated with the inferior hydrolysis of VSS. Quantitative comparison of biofilms and suspended microbial communities at different anode potentials revealed a sharp decrease in bacterial cell numbers in anode biofilms after changing anode potential from -0.4 V to +0.4 V. By contrast, bacterial cell numbers in suspension were slightly decreased. Collectively, these results provide new insights into the role of anode potential in shaping key microbial players associated with hydrolysis/fermentation and anodic respiration processes when MECs are operated with real biowastes.

Shear Stress Affects Biofilm Structure and Consequently Current Generation of Bioanode in Microbial Electrochemical Systems (MESs)

Shear stress is an important factor that affects the formation and structure of anode biofilms, which are strongly related to the extracellular electron transfer phenomena and bioelectric performance of bioanodes. Here, we show that using nitrogen sparging to induce shear stress during anode biofilm formation increases the linear sweep voltammetry peak current density of the mature anode biofilm from 2.37 ± 0.15 to 4.05 ± 0.25 A/m². Electrochemical impedance spectroscopy results revealed that the shear-stress-enriched anode biofilm had a low charge transfer resistance of 46.34 Ω compared to that of the unperturbed enriched anode biofilm (72.2 Ω). Confocal laser scanning microscopy observations showed that the shear-stress-enriched biofilms were entirely viable, whereas the unperturbed enriched anode biofilm consisted of a live outer layer covering a dead inner-core layer. Based on biomass and community analyses, the shear-stress-enriched biofilm had four times the biofilm density (136.0 vs. 27.50 μg DNA/cm³) and twice the relative abundance of Geobacteraceae (over 80 vs. 40%) in comparison with those of the unperturbed enriched anode biofilm. These results show that applying high shear stress during anode biofilm enrichment can result in an entirely viable and dense biofilm with a high relative abundance of exoelectrogens and, consequently, better performance.

Impact of antimicrobial silver nanoparticles on anode respiring bacteria in a microbial electrolysis cell

This study assessed the impact of antimicrobial AgNPs (50 mg L^-1, 30-50 nm) on the electrocatalytic activity of a mixed-culture anode biofilm enriched with Geobacter species. The current densities and electrochemical kinetics were maintained after exposure to AgNPs in consecutive fed-batch cycles, despite significant changes in morphological structures and bacterial communities. Bacterial community analysis showed a substantial increase in the Geobacter population in response to AgNPs exposure, indicating their higher tolerance to AgNPs. In contrast, the population of other anode respiring bacteria (ARB) belongs to Acinetobacter, Dysgonomonas, and Cloacibacillus genera appeared to be very sensitive to AgNPs toxicity as their relative abundance significantly decreased. Microscopic imaging showed that AgNPs were accumulated within anode biofilm matrix without penetration inside the cells. Moreover, the anode biofilm became denser because of enhanced extracellular polymeric substances (EPSs) production by ARB after exposure of AgNPs, implying that EPS could protect ARB against AgNPs toxicity.

Enhanced methanogenic co-degradation of propionate and butyrate by anaerobic microbiome enriched on conductive carbon fibers

Recent studies have shown that the addition of conductive materials can promote direct interspecies electron transfer (DIET) between bacteria and methanoarchaea. This study demonstrated that carbon fibers could significantly stimulate methanogenic conversion of propionate and butyrate as co-substrate, while only butyrate was completely degraded in the unamended control bioreactor. In the carbon fibers-amended bioreactor, specific methane production (mL-CH₄/g COD_Initial) and methanogenesis rate (d^-1) increased by around 2.4 and 6.7 times, respectively. Various electroactive bacteria were abundant in the carbon fibers-amended bioreactor, whereas different known fermentative bacteria were abundant in the control. Moreover, carbon fibers substantially increased the abundance of Methanosaeta species. These results suggest that electroactive bacteria could be involved in DIET with Methanosaeta species enabling co-degradation of propionate and butyrate. Additionally, electrical conductivities of the biomass were comparable in both configurations, indicating that carbon fibers were the primary route for DIET.

One-year operation of 1000-L modularized microbial fuel cell for municipal wastewater treatment

This study constructed a 1000 L modularized MFC system, the largest volume so far, to treat practical municipal wastewater. This MFC system was operated under two different water flow connections in two municipal wastewater treatment plants (MWTP) for more than one year to test their treating abilities for wastewater with both low (average 80 mg L^-1) and high initial COD concentration (average 250 mg L^-1). The COD concentration in the effluent from the MFC system remained below 50 mg L^-1 with a removal rate of 70-90%, which stably met the level A of the first class in discharge standard of pollutants for MWTP of China. A maximum power density of 125 W m^-3 (7.58 W m^-2) was generated when the MFC system was fed with artificial wastewater, while it lay in a range of 7-60 W m^-3 (0.42-3.64 W m^-2) when treating municipal wastewater. The energy recovery of 0.033 ± 0.005 kWh per m³ of municipal wastewater was achieved, with a hydraulic retention time (HRT) of 2 h.

mcrA sequencing reveals the role of basophilic methanogens in a cathodic methanogenic community

Cathodic methanogenesis is a promising method for accelerating and stabilising bioenergy recovery in anaerobic processes. The change in composition of microbial (especially methanogenic) communities in response to an applied potential-and especially the associated pH gradient-is critical for achieving this goal, but is not well understood in cathodic biofilms. We found here that the pH-polarised region in the 2 mm surrounding the cathode ranged from 6.9 to 10.1, as determined using a pH microsensor; this substantially affected methane production rate as well as microbial community structure. Miseq sequencing data of a highly conserved region of the mcrA gene revealed a dramatic variation in alpha diversity of methanogens concentrated in electrode biofilms under the applied potential, and confirmed that the dominant microbes at the cathode were hydrogenotrophic methanogens (mostly basophilic Methanobacterium alcaliphilum). These results indicate that regional pH variation in the microenvironment surrounding the electrode is an ecological niche enriched with Methanobacterium.

Electrokinetic analyses in biofilm anodes: Ohmic conduction of extracellular electron transfer

This review explores electron transfer kinetics from an electron donor to the anode in electrically conductive biofilm anodes. Intracellular electron transfer (IET) from the donor to the anode is well described with the Monod equation. In comparison, mechanisms of extracellular electron transfer (EET) conduction are unclear yet, complicating EET kinetics. However, in biofilm anodes where potential gradient to saturated current density is less than ∼300 mV, Ohmic conduction successfully describe conductive EET mainly with biofilm conductivity (K_bio) and biofilm thickness (L_f). High K_bio essential for production of high current density is found in Geobacter pure or enriched biofilm anodes, but other exoelectrogens could make biofilms electrically conductive. IET is rate-limiting for current density in conductive biofilms, and biofilm density of active exoelectrogens and L_f are operating parameters that can be optimized further to improve current density.