Liquid-gas phase transition enables microbial electrolysis and H2-based membrane biofilm hybrid system to degrade organic pollution and achieve effective hydrogenotrophic denitrification of groundwater.

Electron-donor Lacking was the limiting factor for the denitrification of oligotrophic groundwater and hydrogenotrophic denitrification provided an efficient approach without secondary pollution. In this study, a hybrid system with microbial electrolysis cell (MEC) assisted hydrogen-based membrane biofilm reactor (MBfR) was established for advanced groundwater denitrification. The liquid-gas phase transition prevented the potential pollution from organic wastes in MEC to groundwater, while the bubble-free diffusion of MBfR promoted hydrogen utilization efficiency. The negative-pressure extraction from MEC and the positive pressure for gas supply into MBfR increased the hydrogen proportion and current density of MEC, and improved the kinetic constant K of the denitrification reaction in MBfR. With actual groundwater, the MEC-MBfR hybrid system achieved a nitrate reduction of 97.8% with an effluent NO3--N of 2.2 ± 1.0 mg L-1. The hydrogenotrophic denitrifiers of Thauera, Pannonibacter, and Azonexus, dominated the denitrification biofilm on the membrane and elastic filler in MBfR.

Graphite accelerate dissimilatory iron reduction and vivianite crystal enlargement.

Biomineralized vivianite induced by dissimilatory iron reduction bacteria (DIRB) has received increasing attention because it alleviates phosphorus crisis and phosphorus pollution simultaneously. However, the relatively small crystal size and low Fe(III) reduction rate restrict the separation and recovery of vivianite. In this study, graphite was selected as additive to enhance vivianite biomineralization with soluble ferric citrate and insoluble hematite as two representative electron acceptors. As soluble ferric citrate provided abundant accessible electron acceptors, relatively inconspicuous increase (lower than 7%) was observed for graphite on vivianite formation while inoculated with raw sewage or DIRB. In contrast, graphite considerably increased vivianite formation efficiency by 23% in insoluble hematite inoculated with raw sewage. The graphite promotion on vivianite formation in hematite batch was magnified to 70% by DIRB. Dosing hematite inhibited the supply of electron acceptors, while conductive graphite promoted the electrical connection between minerals and DIRB, thus improved the Fe(III) reduction rate and efficiency. In addition, secondary minerals in hematite exhibited a larger aspect ratio and tended to aggregate on graphite. Graphite enlarged the vivianite size in hematite from 10 µm to 90 µm due to aggregation. Enhancing dissimilatory iron reduction (DIR) rate of iron oxides and enlarging crystal size provide new insights for vivianite formation and separation during wastewater treatment.

Geobacter Autogenically Secretes Fulvic Acid to Facilitate the Dissimilated Iron Reduction and Vivianite Recovery.

Biosynthetic organic matters, such as humus, play important roles in iron and phosphorus cycling in soil and aquatic systems. As an important member of humus, fulvic acid (FA) is ubiquitous in different environmental media, such as water, soil, and sediments. In this study, we fabricated the network among phosphate supply, metabolism pathway of FA, iron reduction, and vivianite recovery at the batch scale. Both the vivianite recovery performance and the content of biosynthetic FA were positively related to the phosphorus dosage. The highest vivianite formation efficiency of 53% was obtained in the Fe/P = 1 batch, accompanied with the maximal iron reduction rate of 2.29 mM·day-1, which was 2.66 times higher than that of the Fe/P = 3 batch. Simultaneously, the highest content of FA was detected in extracellular polymeric substances (EPS) of the Fe/P = 1 batch. Metabolome analysis revealed that FA biosynthesis was mainly relevant to tricarboxylic acid (TCA) cycle, amino acid metabolism, and purine metabolism, with glutamate and aspartate as the precursors. Sufficient phosphate stimulated the FA biosynthesis by modulating the biosynthesis and transformation of glutamate and aspartate. After adding 10 mg L-1 FA in Fe/P = 1 batch, the maximal iron reduction rate increase by 35%, as well as 12% improvement of the vivianite formation efficiency. Transcriptome revealed that FA promotes iron reduction and vivianite recovery by upregulating the expression of metal ion binding-, flagella-, and electron transfer activity-related genes.

Unignorable toxicity of formaldehyde on electroactive bacteria in bioelectrochemical systems.

Formaldehyde poses significant threats to the ecosystem and is widely used as a toxicity indicator to obtain electrical signal feedback in electroactive biofilm (EAB)-based sensors. Although many optimizations have been adopted to improve the performance of EAB to formaldehyde, nearly no studies have discussed the toxicity of formaldehyde to EAB. Here, EABs were acclimated with a stable current density (8.9 ± 0.2 A/m2) and then injected with formaldehyde. The current density decreased by 27% and 98% after the injection of 1 and 10 ppm formaldehyde, respectively, compared with that in the control. The ecotoxicity of formaldehyde caused the irreversible loss of current with 3% (1 ppm) and 81% (10 ppm). Confocal laser scanning microscopy and scanning electron microscopy results showed that the redox activity was inhibited by formaldehyde, and the number of dead/broken cells increased from 2% to 40% (1 ppm) and 91% (10 ppm). The contents of the total protein and extracellular polymer substances decreased by more than 28% (1 ppm) and 75% (10 ppm) because of the cleavage reaction caused by formaldehyde. Bacterial community analysis showed that the proportion of Geobacter decreased from 81% to 53% (1 ppm) and 24% (10 ppm). As a result, the current production was significantly impaired, and the irreversible loss increased. Toxicological analysis demonstrated that formaldehyde disturbed the physiological indices of cells, thereby inducing apoptosis. These findings fill the gap of ecotoxicology of toxicants to EAB in a bioelectrochemical system.

Highly efficient electro-generation of H2O2 by adjusting liquid-gas-solid three phase interfaces of porous carbonaceous cathode during oxygen reduction reaction.

Equilibrium of three reactants (oxygen, proton and electron) in oxygen reduction reaction at large current flux is necessary for highly efficient electro-generation of H2O2. In this work, we investigated reactants equilibrium and H2O2 electrochemical production in liquid-gas-solid three phase interfaces on rolling cathodes with high electroactive area. Electrocatalytic reaction accelerated the electrolyte intrusion into hydrophobic porous catalyst layer for higher electroactive surface area, resulting in a 21% increase of H2O2 yield at 15 mA cm-2. Air aerated cathode submerged in air/O2 aeration solution was unable to produce H2O2 efficiently due to the lack of O2 in three phase interfaces (TPIs), especially at current density > 2.5 mA cm-2. For air breathing cathode, stable TPIs inside the active sites was created by addition of gas diffusion layer, to increase H2O2 production from 11 ±â€¯2 to 172 ±â€¯11 mg L-1 h-1 at 15 mA cm-2. Pressurized air flow application enhanced both oxygen supply and H2O2 departure transfer to obtain a high H2O2 production of 461 ±â€¯11 mg L-1 h-1 with CE of 89 ±â€¯2% at 35 mA cm-2, 45% higher than passive gas transfer systems. Our findings provided a new insight of carbonaceous air cathode performance in producing H2O2, providing important information for the practical application and amplification of cathodes in the future.

Heterotopic formaldehyde biodegradation through UV/H2 O2 system with biosynthetic H2 O2.

Biodegradation was regarded an environmentally benign and cost-effective technology for formaldehyde (CH2 O) removal. However, the biotoxicity of CH2 O inhibited microbial activity and decreased removal performance. We developed a novel heterotopic CH2 O biodegradation process that combined bioelectrochemical system (BES) and UV/H2 O2 . Instead of exogenous addition, H2 O2 was biosynthesized with electron transferred from electrochemically active bacteria. Heterotopic biodegradation of CH2 O was more efficient and faster than in situ biodegradation, as confirmed by 69%-308% higher removal efficiency and 98% shorter degradation time. Operated under optimal conditions for 30 min, which are optical distance of 2 cm, initial H2 O2 concentration of 102 mg/L, and pH 3, heterotopic biodegradation removed 78%, 73%, 49%, and 30% of CH2 O with 6, 8, 10, and 20 mg/L initial concentration. Mild formation of hydroxyl radicals from UV/H2 O2 is beneficial to sustainable CH2 O degradation and efficient H2 O2 utilization. Heterotopic biodegradation is a promising technology for efficient degradation of other organic compounds with biological toxicity. PRACTITIONER POINTS: H2 O2 biosynthesis through electrochemically active bacteria (EAB) served as source of ·OH for CH2 O removal in UV/H2 O2 . Heterotopic CH2 O biodegradation avoided the biotoxicity of CH2 O. Heterotopic biodegradation of CH2 O saved 98% time than in-situ biodegradation. Heterotopic CH2 O biodegradation improved 69%-308% efficiency than in-situ.

Optimal set of electrode potential enhances the toxicity response of biocathode to formaldehyde.

The autotrophic biocathode was promising as a broad spectrum, rapid-responding and sensitive sensing element for the early warning of toxicants in water. However, we found that the baseline current and the responsivity strongly relied on the cathode potential. Here we poised cathode potentials at 0, -0.2 and -0.4 V to investigate the effect of electrode potential on the sensor responsivity. With formaldehyde as the tested toxicant, the biocathode poised at -0.2 V had the highest baseline current (118.2 ±â€¯10.7 A m-2) and the lowest toxicity response concentration (0.00148%), which exhibited a 6-64 times higher response ratio (1.4 × 104 A%-1 m-3) than those controlled at 0 V (2.3 × 103 A%-1 m-3) and -0.4 V (2.2 × 102 A%-1 m-3). First derivative of cyclic voltammetries revealed that the biocathode acclimated at -0.2 V had a highest main peak centered at 0.301 ±â€¯0.006 V and several minor peaks between -0.2 to 0.2 V. Bacterial community analysis showed that Proteobacteria and Bacteroidetes families closely related to the sensing performance. Interestingly, Nitrospirae was obviously acclimated at -0.2 V, indicating that bacteria belonging to this phylum possibly contributed to the highest responsivity as well. Our findings revealed that the optimal set of electrode potential was critical to promote the toxicity responses of biocathode to the formaldehyde, and the differences were mainly from the microbial communities selected by different cathode potentials.