Electroactive biofilm has a low tolerance for accidental shocks, such as extreme acid shock, which is a potential limitation for the application of bioelectrochemical systems (BESs), especially as a sensor for water quality monitoring. In this work, we encapsulated electroactive biofilms with biocompatible polydopamine (PDA) to protect against extreme acid shock. The bacterial cells were completely encapsulated in ∼50 nm films formed by PDA spheres, which protected their viability and current recoverability even after pH 0.5 and 1.5 shocks. The limiting current density of the PDA-encapsulated anode was 0.20 ± 0.05 A/m2, which was 1900% higher than that of the unprotected control (0.01 ± 0.01 A/m2) after strong acid shock (pH 0.5, 30 min). Without PDA encapsulation, the biofilm partly disintegrated with a thickness decreased by 68% from 72 to 23 μm, where 92% of the cells were dead. Our findings reported a novel and effective method for protecting electroactive biofilm under extreme conditions, which will greatly extend the use of BESs in the future.

•Gravity settling (GS) of bacteria to anodes reduces the startup time by 12%.•GS increases the current density by 29% to 8.41 ± 0.13 A m−2.•GS decreases the charge transfer resistance and increases anodic biomass.•GS also increases the electroactivity, especially at early stage biofilm formation.How to acclimate a highly electroactive biofilm in short time is the bottleneck to improve the power output of microbial fuel cells (MFCs). Here we demonstrated that a simple method, the gravity settling (GS) of planktonic bacteria, is cost effective to improve MFC performance instead of physical and chemical treatment of anodes. The startup time is 12% shorter, and the maximum current density increases by 29% to 8.41 ± 0.13 A m−2 than that of the control. Cyclic voltammetries at different growth stages show that GS has a remarkable improvement (66%) on limiting current at the lag stage than at exponential (32%) and mature stages (24%), which was due to the 73% decrease in charge transfer resistance. Biofilm analysis further reveals that the GS promotes biofilm electroactivity per protein in addition to the accumulation of more biomass by gravitational settling, especially at the very beginning of electroactive biofilm formation. Our findings provide new knowledge on MFC startup, which is also important to enhance power densities of large scale MFCs in the future.Download high-res image (106KB)Download full-size image

•Sediment extract (SE) is a better inoculum than soil and wastewater for biocathode.•The startup time is shortened by 17–25% with SE as the inoculum.•SE cathodes had a 150% and 54% higher limiting current than soil and wastewater.•Nitrospiraceae was the unique phylum selected in SE biocathode.•Unidentified Nitrospiraceae may play a key role on cathode electron transfer.Autotrophic bacteria are able to catalyze cathodic oxygen reduction as a renewable and sustainable inexpensive catalyst. However, the performance of biocathode varied over reactors, and we still not know how inoculums affect this system. Using three different inoculum of wastewater (WW), sediment extract (SE) and soil extract (SO) in parallel reactors, we found that SE achieved the shortest setup time (17–25% shorter) as well as the highest power density compared to those of SO and WW. Cyclic voltammetry (CV) further revealed that the current densities of SE biocathodes (100 ± 1 A/m3) was 150% and 67% higher than those of WW biocathodes (40 ± 1 A/m3) and SO biocathodes (65 ± 1 A/m3). Community analysis showed the selective pressure on biocathode facilitated the growth of Proteobacteria, Bacteroidetes, Firmicutes and Actinobacteria families. Different from WW and SO biocathodes, Nitrospirae was selectively enriched in SE biocathodes, corresponding to an obvious increase in Unidentified Nitrospiraceae population at genus level, which may play an important role on the cathodic electroactivity. These results confirmed that sediment extract is a better bacteria source than soil and wastewater for the acclimation of autotrophic electroactive bacteria, and the community comparison provided broader knowledge on biocathode microbiology.

•New resin-enhanced rolling activated carbon electrodes were developed.•The electrodes showed much higher performance and longer durability in capacitive deionization.•The electrodes showed better energy efficiency and lower energy consumption.Capacitive deionization (CDI) has emerged as an efficient process for low-salinity desalination, but electrode materials remain a major bottleneck. This study presents new hybrid CDI electrodes that for the first time directly incorporate ion exchange resins into activated carbon electrodes via a rolling press method. These thin and integrated electrodes showed superior performance over traditional membrane-electrode assemblies. When used in 2.0 g/L NaCl solution they increased desalination efficiency by 29–35% and 70–76% compared with activated carbon electrodes and carbon cloth electrode, respectively. The difference further increased to 41–47% and 121–131% when a lower concentration of 0.5 g/L NaCl was used. The resin-embedded carbon electrodes showed an electrosorption capacity of 12.7 and 18.3 mg NaCl/g electrode in 0.5 and 2.0 g/L NaCl solution, respectively. The charge efficiency ranged from 85–87%, and energy consumption was reduced by 25%. The high performance of the resin-enhanced activated carbon electrodes in CDI is attributed to pre-concentration of target ions and blockage of co-ions especially in low salinity conditions. This approach holds a good potential for CDI development, and further studies are needed for corrosion inhibition and capacity improvement.Download high-res image (136KB)Download full-size image

•Power densities decayed by 36% from 1286 to 822 mW/m2 in 6 months.•Local alkalinization and natural evaporation had an ignorable effect on performance.•Electric field induced salt precipitation contributes ∼53% of Rct at month 6.•Biofouling contributes ∼37% of Rct at month 6.•Electric field induced ion precipitation under biofilm clogged 37% of cathodic area.As a promising design for the real application of microbial fuel cells (MFCs) in wastewater treatment, activated carbon (AC) air-cathode is suffering from a serious power decay after long-term operation. However, the decay mechanism is still not clear because of the complex nature of contaminations. Different from previous reports, we found that local alkalinization and natural evaporation had an ignorable effect on cathode performance (∼2% decay on current densities), while electric field induced salt precipitation (∼53%) and biofouling (∼37%) were dominant according to the charge transfer resistance, which decreased power desities by 36% from 1286 ± 30 to 822 ± 23 mW m−2 in 6 months. Biofouling can be removed by scrapping, however, electric field induced salt precipitation under biofilm still clogged 37% of specific area in catalyst layer, which was even seen to penetrate through the gas diffusion layer. Our findings provided a new insight of AC air-cathode performance decay, providing important information for the improvement of cathodic longevity in the future.Download high-res image (214KB)Download full-size image

•Sub-MIC of tobramycin induced biofilm formation.•The anode biomass increased by 50% with sub-MIC of tobramycin.•The current density increased by 17% with sub-MIC of tobramycin.•Geobacter species was selectively enriched by sub-MIC of tobramycin.•Sub-MIC of tobramycin upregulated genes related to cytochromes and type IV pilus.Electroactive biofilms (EABs) generated from mixed inocula are attractive due to their unique direct extracellular electron transfer abilities and potential use in water pollution control. In this study, for the first time, we identified a chemical that can be used for EAB regulation (both inhibition and promotion). We confirmed that tobramycin, an antibiotic previously demonstrated to inhibit the activity of EABs, is an agonist of EAB formation at subminimal inhibitory concentrations (sub-MICs). Compared to the control, at tobramycin concentrations of 0.05 (1/80 MIC) and 0.1 mg/L (1/40 MIC), the time required to reach 3 A/m2 was shorter, and the limiting current densities increased by 17%. The enhanced EAB activity was primarily attributed to the 50% increase in biomass density from 289 ± 21 to 434 ± 12 μg protein/cm2 and the increased biofilm thickness from 28 ± 1 to 37 ± 0.5 μm. Geobacter species in the microbial communities were selectively increased from 76% to 82%, and their abundance was estimated to increase by 1.63-fold. The accelerated growth was further confirmed using the model strain G. sulfurreducens PCA. Transcriptomic analysis revealed that 0.05 mg/L of tobramycin led to a significant upregulation of genes related to cytochromes and the type IV pilus, suggesting a possible mechanism for the observed current enhancement. These findings extend our knowledge of the regulation of EAB formation by antibiotics and the selective enrichment of Geobacter from a mixed culture, with broader implications on the potential impact of trace antibiotics on the dissimilatory metal reduction process in water environment.Download high-res image (395KB)Download full-size image

•A hybrid air-cathode with the optimal carbon black/graphite ratio of 1:5 is made.•The maximum H2O2 yield is 11.9 mg L−1 h−1 cm−2.•Continuous flow without H2O2 accumulation increases current efficiency.•Oxygen for H2O2 synthesis is mainly contributed by air diffusion (66–94%).•The use of bioanode increases H2O2 yield and current efficiency.Carbon black and graphite hybrid air-cathode is proved to be effective for H2O2 production in bioelectrochemical systems. The optimal mass ratio of carbon black to graphite is 1:5 with the highest H2O2 yield of 11.9 mg L−1 h−1 cm−2 (12.3 mA cm−2). Continuous flow is found to improve the current efficiency due to the avoidance of H2O2 accumulation. In the biological system, the highest H2O2 yield reaches 3.29 mg L−1h−1 (0.079 kg m−3day−1) with a current efficiency of 72%, which is higher than the abiotic system at the same current density. H2O2 produced in this system is mainly from the oxygen diffused through this air-cathode (>66%), especially when a more negative cathode potential is applied (94% at −1.0 V). This hybrid air-cathode has advantages of high H2O2 yield, high current density and no need of aeration, which make the synthesis of H2O2 more efficient and economical.

•Addition of QAC (≤0.01 M) in anolyte reduced the start-up time by up to 29%.•QAC (≤0.01 M) in anolyte increased Coulombic efficiency by up to 95%.•High concentration of QAC (0.05 M) inhibited start-up of BES.•Peak current was increased by 23% when 0.01 M QAC was initially added.•QAC added during start-up imposed a selective pressure on microbial community.Quaternary ammonium has been demonstrated to be an efficient functional group for anodic material modification in bioelectrochemical systems (BESs). However, not all anode materials can be easily functionalized with quaternary ammonium. Here QAC of 0.05 M, 0.01 M and 0.001 M is directly added in anolyte instead of complex material functionalization. The startup time of 0.01 M (91 ± 0.5 h) and 0.001 M (101 ± 1 h) using real wastewater were 29% and 21% shorter than 128 ± 1.5 h of the no QAC control. Coulombic efficiency increased by 95% from 37 ± 1% (control) to 72 ± 2% (0.01 M), while the startup was obviously inhibited up to 0.05 M due to its biotoxicity. Cyclic voltammetry reveals that 0.01 M had a 23% higher peak current density than the control with a broader redox window observed. High-throughput sequencing confirmed that QAC imposed a selective stress on anodic microbial community. This provides a simple method to accelerate BES startup for anodes that not be easily modified.

Flocculants have been used to clarify water for thousands of years. However, the in situ evaluation of flocculant toxicity is difficult because flocculants usually exist as growing complex flocs which is hard to monitor. With alum (KAl(SO4)2·12H2O) as the typical flocculant, a bioelectrochemical sensor is designed to in situ detect its bacterial toxicity. The attenuation ratio of current densities linearly increased with alum concentration (R2 > 0.98) with a slope of 0.0054 A m–2 drop per mg L–1 of alum, indicating a typical toxic response of alum. Turnover and nonturnover cyclic voltammetries (CVs) revealed that the alum inhibited the electrochemical activity of bacteria rather than changing the electron transfer pathways. Alum also hindered the diffusion by flocculation at a concentration larger than 100 mg L–1, which was further confirmed by the linear decrease in viability when biofilm thickness increased. It was revealed that both inactivation of biofilm and influences on diffusion by alum can be detected by bioelectrochemical sensors, which provided a new platform to in situ investigate the biological toxicity of new flocculants.Keywords: alum; bioelectrochemical sensors; cyclic voltammetry; diffusion; toxic response

To enhance the performance of activated carbon air cathodes for microbial fuel cells (MFCs), scattered and dense air-cathodes are fabricated by the rolling-press method with the gas diffusion layer or the catalyst layer (CL) invaded to stainless steel meshes (SSM). The maximum power density with scattered cathodes (2503 ± 61 mW m−2) is 29% higher than those of dense cathodes because of the decreased internal resistance and increased oxygen reduction reaction activity. The decrease in the electrode volume density and invasion of CL into SSM has been demonstrated to be an optimal structure, with the highest exchange current density up to 1.26 A m−2 and the lowest charge transfer resistance as low as 1.5 Ω. The increase in performance is believed to result from the enhanced mass transport through extra pores and the contact of activated carbon to the current collector. This novel structure of an air-cathode has a promising future as an application in MFCs.

Bioelectrochemical remediation is an emerging technology for in situ removal of petroleum hydrocarbons in soil. Here we demonstrated that the remediation can be extended to a larger range by adding multilayer anodes in contaminated soils. Using a three anodes system with activated carbon as the cathodic catalyst, 918 C of charge transferred during 180 days in aged saline soil. The degradation of both polycyclic aromatic hydrocarbons (PAHs) and n-alkanes were accelerated in each layer compared to the disconnected control. The net degradation rates of total petroleum hydrocarbons, 16 priority PAHs and total n-alkanes (C8–C40) were 18%, 36% and 29%, respectively. Popular exoelectrogens (such as Geobacteraceae sp.) and Escherichia were identified, which possibly played an important role in this bioelectrochemical process.

Nano goethite with contents of 0, 2.5%, 5.0% and 7.5% (mass percentage) were added into activate carbon powder (AC) and rolled onto stainless steel mesh to fabricate new anodes for microbial fuel cells (MFCs). The maximum power density of MFC with 5.0% α-FeOOH (693 ± 20 mW m−2) is 36% higher than that of the AC control (508 ± 40 mW m−2). Based on the decrease of charge transfer resistance and the increases of both exchange current density and anodic peak current in electrochemical impedance and polarization tests, the addition of nano α-FeOOH kinetically promotes the extracellular electron transfer between bacteria and the electrode. The increase of constant-phase element and the decrease of Warburg element show that α-FeOOH enhances both the capacitance and diffusion condition on the surface of anode. However, the excess α-FeOOH of 7.5% incurs a negative effect on surface conductivity, capacitance and diffusion, probably due to the adsorption of biogenic Fe(II) on the anodic surface.Highlights► Anodes are made by rolling α-FeOOH into activated carbon on stainless steel mesh. ► The addition of α-FeOOH in anode increases the maximum power density by 36%. ► α-FeOOH in the anode accelerated the extracellular electron transfer. ► Excess α-FeOOH resulted in the decay of performance.

The Pt and Nafion free air-cathode is urgently needed for large scale membrane-less single chambered microbial fuel cells (MFCs). The activated carbon air-cathode (ACAC) consisted of a sintered catalyst layer (CL) and a gas diffusion layer (GDL) made by rolling method is such a promising cathode with a high performance. However, the microstructure of the CL in terms of three phase interface (TPI) needs to be further investigated. The maximum power density increases by 35% from 802 ± 20 mW m−2 (3.40 ± 0.03 A m−2) to 1086 ± 8 mW m−2 (2.80 ± 0.04 A m−2) when the CL is not sintered. The maximum Coulombic efficiency (CE) also increases from 34% to 40%, possibly due to the decrease of hydrophobicity and the total volume/area of the CL. The total pore area and the porosity of the CL decrease by 87% and 42% after sintering, indicating that sintering reduced those pores among activated carbon aggregates so that TPIs are reduced. However, the available gas channel is produced when the GDL abundant of polytetrafluoroethylene (PTFE) is sintered.Graphical abstractHighlights► The power density was increased by 35% when the catalyst layer was unsintered. ► The hydrophobicity of the catalyst layer was increased after sintering. ► The enhancement was due to the increase of pores in the unsintered catalyst layer. ► Three phase interface of catalyst layer could exist in 6 μm pores.

The microbial fuel cell (MFC), being an environment-friendly technology for wastewater treatment, is limited by low efficiency and high cost. Power output based on capital cost had been greatly increased in our previous work by introducing a novel activated carbon (AC) air-cathode (ACAC). The catalysis behavior of this ACAC was studied here based on catalysis kinetics and pore analysis of both carbon powders and catalyst layers (CLs). Plain AC (AC1#), ultracapacitor AC (AC2#), and non-AC (XC-72) powders were used as catalysts. The electron transfer number (n) of oxygen reduction reaction (ORR) with CLs increased by 5–23% compared to those n values of corresponding carbon powders before being rolled to CLs with PTFE, while the n value of Pt/C decreased by 38% when it was brushed with Nafion as the CL, indicating that rolling procedure with PTFE binder substantially increased the catalytic activity of carbon catalysts. Two–four times larger in micropore area of AC powders than non-AC powder resulted in 1.3–1.9 times increase in power density of MFCs. In addition, more uniform distribution of microporosity was found in AC1# than in AC2#, which could be the reason for the 25% increase in power density of ACAC1# (1355 ± 26 mW·m–2) compared to 1086 ± 8 mW·m–2 of ACAC2#.

Eight kinds of amino acids including L-Serine (Ser), L-Asparagine (Asn), L-Asparticacid (Asp), L-Glutamicacid (Glu), DL-Alanine (Ala), L-lysine (Lys), L-Histidine (His) and L-Arginine (Arg) are tested as substrates of single-chambered air–cathode microbial fuel cells (MFCs) with domestic wastewater as inoculation. Their total organic carbon (TOC) concentrations in solution are standardized as 720 mg L−1. Ser produces the highest power density of 768 mW m−2 and Ala produces the lowest of 556 mW m−2. The Coulombic efficiencies (CEs) vary from 13 ± 3% (obtained with Arg) to 30 ± 1% (obtained with Ala). The removal efficiencies of total nitrogen (TN) are from 55 ± 5% (Asn) to 94 ± 4% (Asp), which may be associated with CEs. Maximum voltage outputs and TOC concentrations of the substrates appear to satisfy the empirical Monod-type equation when the external resistance is 150 Ω. The performances of MFCs are considered to relate to the molecular weights and structures of eight amino acids.Highlights► We compare the performance of eight amino acids fed microbial fuel cells. ► Molecular weights and R-group structure of amino-acids effect power densities. ► The removal efficiencies of total nitrogen maybe associated with Coulombic efficiencies. ► N-contained wastewater is suitable substrate of MFCs.