The growing gap between global energy demand and the shortfall in energy production is becoming more critical because of the depletion of fossil fuel resources. However, these energy sources are not only limited, but they also pose serious threats to the environment and life on Earth. This has sparked a strong push to explore alternative energy carriers. This transition toward sustainability has focused on the potential of bioelectrochemical systems (BESs), notably microbial electrolysis cells (MECs), as a promising approach to the sustainable production of hydrogen, leveraging the dual merits of renewable energy generation and wastewater treatment. Due to their high organic load and availability, agro-industrial wastewater represents a suitable substrate for MEC-driven hydrogen production. Exploiting these wastes not only contributes to an abundant and cost-effective resource but also responds to critical environmental challenges associated with wastewater management. This review explores the intersection between bioelectrochemical hydrogen production and agro-industrial wastewater treatment, focusing on current advancements and developing patterns in this field. A comprehensive analysis of MEC technology is presented to investigate the effect of key progressions including reactor components and experimental parameters on system performance. Moreover, the role of microbial consortia and the influence of wastewater composition are critically analyzed. Furthermore, this review examines the current challenges including substrate variability, biofilm development, and scaling up from laboratory to industrial applications and outlines strategies to overcome them. By handling these delays, MEC technology could advance toward industrial implementation, paving the way for sustainable hydrogen production from agro-industrial wastewater streams.

Microbial fuel cells (MFCs) have emerged as a promising technology for simultaneous wastewater treatment and energy recovery; however, their performance is often limited by electrode properties and biofilm–electrode interactions. In this study, granular activated carbon (GAC) electrodes modified with Nb2O5 were evaluated in nitrifying and denitrifying MFCs treating vinasse, focusing on nitrogen conversion and bioelectrochemical performance. Nb2O5 incorporation increased the average pore diameter from 1008.0 to 1774.9 nm and was associated with substantial improvements in electrochemical behavior. Electrochemical impedance spectroscopy revealed reductions in charge transfer resistance of up to 86-fold compared with unmodified GAC, indicating more favorable interfacial electron-transfer conditions. These electrochemical improvements coincided with enhanced current generation, lower nitrite accumulation, and improved nitrate conversion, particularly under denitrifying conditions. Microbial community analyses revealed distinct biofilm compositions on modified electrodes, including the enrichment of taxa commonly reported in nitrifying, denitrifying, and bioelectrochemical environments. Although direct causal relationships could not be established, the combined structural, electrochemical, and microbial evidence suggests that Nb2O5 modification altered the biofilm–electrode interface and was associated with changes in nitrogen transformation pathways. These findings highlight the potential of Nb2O5-modified GAC as a promising strategy for improving bioelectrochemical system performance during the treatment of nitrogen-rich wastewaters.

The structure and electroactivity of electroactive biofilms (EABs) are strongly influenced by the composition and abundance of extracellular polymeric substances (EPS). In this study, a multi-cycle acetate feeding strategy was applied over 50 days of constant polarization at - 0.1 V/SCE, comprising six acetate addition (AA) cycles, to investigate the temporal evolution of EPS and its relationship with electrochemical performance, biofilm structure, and microbial dynamics. From AA3 to AA6, Geobacter progressively dominated the biofilms, with its abundance increasing from 59% to 85%, accompanied by a 2.1-fold increase in protein content. In mature EABs, protein content was strongly correlated with transferred charge, enabling sustained maximal current density and stable electroactivity after AA3. Polysaccharides in the loosely bound EPS was closely associated with biofilm thickness, reflecting its structural contribution, whereas polysaccharides in the tightly bound EPS acted as a carbon reservoir. Following AA3, seven days of acetate deprivation led to the consumption of polysaccharides in tightly bound EPS by the residual biofilm to sustain metabolic activity, resulting in a marked decrease in its content. Overall, these results highlight the central role of EPS dynamics in EAB maturation and long-term stability and functionality in bioelectrochemical systems.

Biohydrogen is a prodigious energy carrier, which emerged as one of the most practical solutions to combat global warming and climate change. In this regard, the emerging microbial electrolysis cell (MEC) technology could be utilized for green hydrogen production from a wide array of organic wastes. However, scaling-up of MECs is a significant barrier due to its architectural difficulties and increased internal resistance, resulting in the higher energy requirement and cost of the MEC at pragmatic scale. Thus, the present review elucidates the mechanism, different configurations and substrates, and scaling-up potential for biohydrogen production via MEC. Moreover, the techno-economic and environmental impact of biohydrogen production through MEC from different substrates is also presented. Furthermore, microbial dynamics that govern hydrogen production rate and commercialization potential are also reviewed critically, which makes this review article the first of its kind to the best of our knowledge.

Oxygen reduction reaction (ORR) is a key electrochemical process with significant implications at the industrial level from energy conversion/storage to corrosion protection and production of valuable chemicals. Oxygen is also critically involved in biological systems at the level of the respiratory chain, allowing the development of several biomimicking bioelectrochemical devices encompassing microbial fuel cells (MFCs) and enzymatic fuel cells (EFCs) exploited for power generation, organics transformation, water desalination, biosensing and other applications. Many studies on ORR mechanisms in near-neutral environments for potential integration with bioprocesses are currently ongoing with many phenomena still to be explained, especially concerning biotic and abiotic electrocatalysts. This comprehensive review aims at summarizing the state-of-the-art for each electrocatalyst category, namely: noble-metal/transition-metal-based/carbonaceous electrocatalysts, enzymes, and bacterial cells. In particular, the performances are compared based on their ORR mechanisms, quantitatively discussing the practical limitations, and addressing the technological challenges of their integration in sustainable electronics.

Hydrogenotrophic microorganisms reduce organic or inorganic molecules coupled with the oxidation of H2, for the production of valuable products such as carboxylic acids, methane, and ethanol, as well as the reduction of sulfates, nitrates or iron for remediation applications. H2 is usually dissolved by gas injection but the low gas-to-liquid mass transfer limits its biotic uptake. This study proposes water electroreduction (WE) as an alternative to gas injection (GI) for dissolving H2. First, WE and GI were compared in terms of H2 solubilization rate, efficiency, and maximum dissolved concentration in a 0.8 L potassium phosphate buffered medium (0.1 mol center dot L-1, pH = 7.2) used as a representative medium for the enrichment and study of hydrogenotrophic microorganisms. Although the H2 production rate of WE was around 18 times lower than the H2 supply rate used during GI, both techniques achieved similar H2 solubilization rate but different solubilization efficiencies. To reach a solubilization rate of 0.67 mmol center dot L-1 center dot h-1 by WE, 1.87 mmol center dot h-1 of H2 were produced, yielding a 35 % solubilization efficiency. In contrast, achieving a comparable solubilization rate of 0.79 mmol center dot L-1 center dot h-1 via GI required an injection of 42 mmol center dot h-1, corresponding to a solubilization efficiency of only 2 %. Moreover, the maximum concentration of dissolved H2 reached (0.62 mmol center dot L-1) by GI was limited by the saturation concentration at 1 bar (0.78 mmol center dot L-1), while by WE it reached 1.21 mmol center dot L-1. WE holds strong potential as a H2 source for biological processes involving hydrogenotrophs where high H2 concentrations are desirable to enhance metabolite production rates and selectivity.

Corrosion of steel in concrete is one of the major deterioration mechanisms for reinforced concrete (RC) structures such as floaters of floating offshore wind turbines (FOWTs). As these are vital components of FOWTs, addressing corrosion is critical to ensure their durability with minimal maintenance. The existing literature indicates that RC in the tidal zone can experience premature corrosion. To mitigate this, galvanic cathodic protection (CP) is a well-known approach for protecting RC structures. Therefore, a field experiment in the tidal zone was conducted to study the behaviour of aluminum anode CP for RC with CEM I and CEM V cement types across two concrete surface textures, smooth and rough. The half-cell potentials (HCP) (for specimens without CP), mixed potentials and protection current (for specimens with CP) were monitored continuously. Furthermore, the effect of water levels and biofilm on corrosion characteristics of steel in concrete and the efficiency of CP was assessed. The findings highlighted that the biofilm on the concrete surface acts as a physical barrier, limiting the diffusion of oxygen – affecting the corrosion characteristics of steel embedded in concrete. This influence was distinctly observed in both protected and non-protected categories. In the protected category, the average protection current was found to increase upon biofilm removal for CEM I concrete - indicating that the CP is efficient/or working with or without biofilm on the concrete surface. Finally, this paper highlights the importance of understanding how the presence of biofilm on concrete surfaces can affect the corrosion characteristics of steel embedded in concrete.

Thiabendazole (TBZ), a recalcitrant fungicide, is frequently applied in postharvest fruit treatment and generates significant volumes of industrial wastewater (WW) that conventional treatment plants cannot handle. This explores a bioelectrochemical system (BES) for TBZ degradation using Tunisian hypersaline sediments (THSs) as inoculum. Four sets of BES, along with biological controls, were tested using THS subjected to different levels of TBZ biostimulation. Sediments underwent one, two, or three biostimulation phases with increasing TBZ concentrations (0, 10, 100, and 300 mg kg−1). Potentiostatic control was applied to BES, polarized at 0.1 V vs. saturated calomel reference electrode (SCE), with a carbon felt working electrode (72 cm2 L−1) and maintained at 25°C. While current production was very low, sediments biostimulated with 100 mg kg−1 kg TBZ produced the highest current density (3.2 mA m−2), a 5-fold increase over untreated sediments (0.6 mA m−2). GC-FID analysis showed >99% TBZ degradation in all reactors. The TBZ half-elimination time from 27 days with biological treatments to 19 days in BES and further to 6 days following biostimulation. Bacterial analysis revealed a substantial microbial community shift after biostimulation, with a reduction in Bacillota (−64%) and an increase in Proteobacteria (+62%), dominated by Pseudomonas (45%) and Marinobacter (16%). These findings provide insight into the selective potential of biostimulation cycles to enhance microbial community composition and improve BES performance for TBZ wastewater treatment.

The durability of cover concrete in the submerged zone for floating offshore wind turbines is closely associated to the performance of the cement type used, which also plays a key role in the environmental impact of their construction and operation. A real-world study is conducted by simulating the concrete cover, submerging cementitious materials at a depth of 27 m in Banyuls-sur-Mer, located on the Mediterranean coast of France. The objective was to identify the short-term interactions between cementitious materials, biofilm developing at their surface, and seawater after 30- and 90 days exposure, with emphasis on, (i) the influence of cement type on the microbial composition of biofilm, and (ii) the influence of biofilm and seawater on the microstructural, chemical composition, and mineralogical changes within the cementitious matrix. After 30 days of exposure, scanning electron microscopy coupled to energy dispersive spectroscopy detected the formation of Mg- rich, S-rich, and Cl-rich zones in CEM I and CEM III concrete, while CEM V exhibited the same zonation after 90 days. These findings were corroborated by electron probe microanalysis. After 90 days of exposure, regardless of the cement type, calcium carbonate precipitated at the concrete-biofilm-seawater interface, predominantly in the form of aragonite crystals, as identified by X-ray diffraction analysis. In CEM I concrete, a brucite layer formed immediately beneath this CaCO3 deposit. The bacterial and eukaryotic diversity was identified using 16S rRNA and 18S rRNA sequencing, revealing diverse and dynamic communities over time. The macrofouling species, marine polychaetes (or serpulids), have been identified and considered to be biomineral in origin.

Due to the complex biogeochemistry of marine sedimentary systems, the burial depth of sedimentary electrodes significantly influences their bioelectrochemical activity. This study investigates the impact of burial conditions on sedimentary microbial electrodes in oxic and suboxic zones of reconstructed sedimentary systems was investigated. Carbon-felt electrodes were deployed in laboratory reactors filled with fresh marine sediments and seawater under three different exposure conditions. The results showed that the placement of electrodes, whether in sediments or in seawater, profoundly affected both the electrical current (anodic or cathodic) and the microbial communities colonizing the electrode biofilm. Electrodes placement in the transition zone between the oxic and suboxic zones led to the formation of microbial electrodes with hybrid bioelectrochemical properties. Analytical and numerical models were developed to calculate the ratio of anodic and cathodic surfaces operating at the scale of these sedimentary microbial electrodes.

Microbial electrolysis cells (MECs) have emerged as a promising technology for sustainable hydrogen production from wastewater treatment. An MEC consists of a microbial anode and a hydrogen evolution cathode, where microorganisms in the anode oxidize organic compounds, allowing the cathodic hydrogen production at lower potentials compared to abiotic electrolysis. The present study focuses on optimizing the catholyte composition and configuration in a MEC reactor to maximize hydrogen production rates while minimizing energy consumption. Indeed, buffer solutions of HCO3-, HPO42-, and H2PO4- at different concentrations and operation mode were tested as catholytes analysing hydrogen production rates and energy consumptions of the process. The results demonstrate the stability of the anodic electroactive biofilm over a 220-day period, achieving consistent COD removal and hydrogen production. The findings reveal that the catholyte composition and operating mode significantly affect the cathodic performances of the MEC. Indeed, catholytes with higher buffer concentrations allow for a limited catholyte alkalinisation improving hydrogen production rates while a low buffer solution promotes an increase in process energy consumption. Bicarbonate buffer solution utilized under batch operation mode showed the better performances for hydrogen production at the cathodic side of MECs showing a higher cathodic coulombic efficiency coupled with stable pH levels and cathodic potentials. Overall, this research demonstrates the potential of MECs for sustainable hydrogen production and highlights the importance of optimizing catholyte composition and operating mode to increase energy efficiency of process.

The management of corrosion in reinforced concrete (RC) structures is crucial for addressing the challenges posed by aging infrastructure, particularly in marine environments where the aggressiveness of seawater can severely impact durability. This study explores a novel approach known as BioGalvanic Cathodic Protection (BGCP), inspired by Benthic Microbial Fuel Cells, for the electrochemical maintenance of RC exposed to marine corrosion. BGCP utilizes electroactive microorganisms naturally present in marine sediments to form bioanodes on conductive materials, which provide protective electrical currents to partially submerged RC structures. A pilot study involved a 3-m-high concrete pier, which was partially immersed in natural seawater and sediments with embedded bioanodes. The current distribution from the BGCP system to the steel was monitored for over a year under various configurations, including changes in steel surface area and the number of bioanodes, while simulating tidal variations and monitoring ambient temperature. Results indicated that BGCP effectively demonstrated cathodic prevention for passive steel, with improved performance observed when multiple bioanodes were utilized. Indeed, the total current density received by the steel was in the range of [-0.2;-2 mA/m2] at all times. The current distribution varied with tidal changes, peaking at the air/water interface. A notable correlation emerged between temperature and current output, suggesting better performance at elevated temperatures. Although CP has not yet been achieved on actively corroding steel, BGCP offers significant potential for delaying corrosion initiation through the development of a selfsustaining and environmental-friendly technology.

Floating Offshore Wind Turbines (FOWTs) are designed to harness the energy produced by wind. Since these structures are in deep waters, the engineering and dynamics of steel and or concrete structural elements (floaters, chains, mooring, etc.) is important to ensure significant performance. The durability of concrete in submerged zones is affected by biological and chemical deterioration mechanisms, with the latter being controlled by transport of aggressive ions. The influence of concrete on biodiversity developing on its surface plays a significant role as it contributes to the overall environmental footprint of the structure. The aim of this experimental research was to identify the surface interactions between cementitious materials such as concrete, biofouling at their surface, and seawater on short-term exposure. CEM III concrete specimens were submerged at a depth of 27m at SOLA station, Banyuls-sur-mer, France. The microstructural and chemical changes in the cementitious material were analysed with scanning electron microscopy coupled to energy dispersive spectroscopy (SEM-EDS) and electron probe micro analysis (EPMA). The results of SEM-EDS analysis showed the formation of three zones, namely, magnesium-rich, sulfur-rich, and chloride-rich zones. Using Environmental DNA analysis, bacterial diversity was identified, revealing high abundances of alphaproteobacteria and gammaproteobacteria, and 18s rRNA sequencing unveiling a diverse eukaryotic community.

Most wastewater treatment plants involve activated sludge units, in which the organic matter to be removed is oxidised by aerobic microorganisms. These units are highly energy intensive because of the power consumed to force oxygen transfer to the wastewater by aeration. An innovative device is presented here; it is called the "electrochemical microbial tree (EMT)" and is based on the opposite strategy: the organic matter is drawn up from the wastewater towards the air phase by capillary action along a porous structure, which hosts the mi-croorganisms that oxidise the organic matter. The EMTs were made of carbon felt, with the bottom immersed in wastewater (14 cm), while the top emerged into the air at different heights (4, 8 or 12 cm). COD removal increased linearly with the height of the aerial section. The greatest height led to COD removal rates of 807 +/- 62 mg O2/L/h, i.e. 2.6 times those of the control experiments. During COD removal, the pH decreased from 7.6 to 7.4 +/- 0.2 with EMTs, while it increased to 7.9 +/- 0.1 in the controls. The biofilm on the immersed section developed as the height of the aerial section increased. Many electroactive species were identified in the microbial populations, belonging to the Bacteroidia, Gam-maproteobacteria and Clostridia classes. These observations revealed that electron transfer along the conductive felt contributed to organic matter oxidation, in parallel with mass transfer by capillarity. These pioneering results present the EMT as a promising new wastewater treatment disposal system that does not require any energy input.

AbstractThe decrease in the electrochemical activity of multi‐species microbial anodes in bioelectrochemical systems is the main bottleneck to overcome for bringing these technologies one‐step closer to the industrialization stage. In this study, microsized stainless steel electrodes were implemented to investigate the distinctive electrochemical behavior of salt marsh electroactive biofilms (EABs). Four main temporal stages of biocolonization and electrochemical activity were thoroughly described. Maximum biofilm growth rate, high viability and high extracellular protein matrix content favored the increasing electrochemical activity of the EAB up to its maximum current peak. Then, when gradual fall in current became irreversible, biofilm growth rate decreased together with dead cells accumulation and an increase for extracellular polysaccharides. In addition, analyses of microbial populations showed a shift from Marinobacterium spp. to Desulfuromonas spp. These findings suggest a chemical and microbial temporal evolution of the EAB, which can be directly correlated to the electrochemical performance of the bioanode.

Bioelectrochemical systems (BESs), rather than physicochemical processes, are used for wastewater remediation, electricity production, and zero carbon dioxide emission. Textile effluents contain organic and inorganic compounds that can fuel BESs. The main goal of this study was to understand the interplay between the anode material, its surface area, the potential applied to the working electrode (WE), and the concentration of the co-substrate, and how these factors lead to the formation of highly efficient thermohalophilic bioanodes (THB) retrieved from Chott El Djerid (SCD) hypersaline sediment for the treatment of synthetic textile wastewater. To this end, twenty-seven bioanode formation experiments were designed using a Box-Behnken matrix and response surface methodology to understand concomitant interactions. All experiments were conducted in electrochemical reactors of final volume 750 mL inoculated with 80% of enrichment medium containing three azo dyes at a concentration of 300 ppm and 20% of biocatalyst microbial SCD source, at 45 °C. The optimal levels were predicted using NemrodW software as carbon felt (CF) anode material, 6 cm2 anode surface, 7 g/L glucose concentration, and −0.1 V applied potential. These theoretical results were experimentally validated, using maximum current output of 5.23 ± 0.30 A/m2, decolorization rate of 100%, and a chemical oxygen demand (COD) removal rate of 96 ± 1%. Illumina Miseq results revealed that bacterial community harbored the bioanode was dominated at phylum level by Firmicutes (67.1%). At the species level, the biofilm was mainly colonized by Orenia metallireducens species (59.5%). Obtained findings show a promising application of THB in the degradation of recalcitrant molecules as well as for the energy recovery.

The treatment of textile wastewater (TWW) loaded with recalcitrant azo dyes in bioelectrochemical systems (BES) rather than in physicochemical processes is a low-cost and environmentally friendly process. The main objective of this study is to investigate the potential of different saline sediments collected from extreme Tunisian environments for the formation of bioanodes capable ofsimultaneous azo dyes degradation and electric current generation in synthetic (STWW) and real textile wastewaters (RTWW) characterized by a varied composition of azo dyes and a high salinity. The obtained bioanodes and anolytes were studied comparatively by electrochemical, microscopic, analytical, and molecular tools.Based on the UV–visible spectra analysis, the breakdown of the azo bond was confirmed. With RTWW, the BES achieved a chemical oxygen demand (COD) abatement rate of 85%with a current density of 2.5 A/m2. Microbial community analysis indicated that a diverse community of bacteria was active for effluent treatment coupled with energy production. At the phylum level, the electrodes were primarily colonized by proteobacteria and firmicutes, which are the two phyla most involved in bioremediation. The analysis of the microbial community also showed the abundance of Marinobacter hydrocarbonoclasticus and Marinobacter sp. species characterized by their high metabolic capacity, tolerance to extremophilic conditions, and role in hydrocarbon degradation.

Cathodic protection is an efficient solution that is implemented on reinforced concrete structures to address corrosion issues and prevent further deterioration. However, it has significant drawbacks in terms of sustainability. This paper explores a new biological CP technology for reinforced concrete structures in marine environments, where a bioanode is buried in marine sediments, and electrons are supplied through the oxidation of various substrates by electro-active micro-organisms. The issue was addressed both numerically and experimentally, with a first-of-a-kind experimental verification on a laboratory specimen. This new solution offers three fundamental advantages: autonomous operation, green solution, and free current supply.

Heterotrophic microbial sulfate reduction can occur in both natural and engineered systems. The process can influence the radionuclide speciation and mobility in deep repository of radioactive waste (DRRW). DRRW are characterised by significant masses of concrete, imposing alkaline pH in the waste cell. This paper aims to evaluate microbial sulfate reduction coupled with propionate oxidation influenced by the alkalinity of the environment: moderate alkaline pH close to 9.0 without cement paste, and moderate alkaline pH which was then moderately increased (pH 9.0 to 9.4) by the presence of pre-aged solid cement pastes via advanced and moderate ageing protocols, respectively. Regardless of the degree of ageing of the cement pastes, the sulfate reduction rate decreased by up to 84% when the pH increased from 9.0 to 9.4 in the presence of cement paste and by up to 90% in the absence of cement paste. No sulfate reduction or propionate oxidation was observed for pH > 9.5. Microbial metabolites, sulfide, CO2 and acetate, produced from the reduction of sulfate and oxidation of propionate, and the presence of the microorganisms attached to the cement pastes (composed of up to 80% of sulfate reducing bacteria) led to their biodeterioration. Sulfide enrichment, precipitation of secondary ettringite, and intensified decalcification were notably detected. Self-healing like phenomena were also observed: calcium leached from the cementitious phases reacted with carbonate produced by microorganisms to form of calcium carbonate relocated either in the micro-cracks or on the surface of the pre-aged solid cement pastes.