Electroactive bacteria (EAB) are natural microorganisms (mainly Bacteria and Archaea) living in various habitats (e.g., water, soil, sediment), including extreme ones, which can interact electrically each other and/or with their extracellular environments. There has been an increased interest in recent years in EAB because they can generate an electrical current in microbial fuel cells (MFCs). MFCs rely on microorganisms able to oxidize organic matter and transfer electrons to an anode. The latter electrons flow, through an external circuit, to a cathode where they react with protons and oxygen. Any source of biodegradable organic matter can be used by EAB for power generation. The plasticity of electroactive bacteria in exploiting different carbon sources makes MFCs a green technology for renewable bioelectricity generation from wastewater rich in organic carbon. This paper reports the most recent applications of this promising technology for water, wastewater, soil, and sediment recovery. The performance of MFCs in terms of electrical measurements (e.g., electric power), the extracellular electron transfer mechanisms by EAB, and MFC studies aimed at heavy metal and organic contaminant bioremediationF are all described and discussed.

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Electroactive Bacteria in Natural Ecosystems and Their Applications in Microbial Fuel Cells for Bioremediation: A Review

This study evaluated the effects of electrode arrangement and aquatic plant selection on the performance of single-chamber constructed wetland-microbial fuel cell (CW-MFC) in the treatment of wastewater containing Cr(VI). The CW-MFC with the electrode distance of 10 cm obtained the highest Cr(VI) removal efficiency of 98.8 % at 9 h and the largest power density of 37.8 mW/m 2 . Short electrode distance would destroy anodic anoxic atmosphere by oxygen secretion of plant roots, while large electrode distance would increase the internal resistance of the system. In the three sets of CW-MFCs with different cathode areas of 25 cm 2 , 75 cm 2 and 100 cm 2 , the system with 75 cm 2 cathode area achieved the highest maximum power density of 39.9 mW/m 2 and corresponding Cr(VI) removal efficiency of 90.9 %. Cr(VI) removal by the CW-MFC conformed to first-order kinetics, and the main Cr(VI) removal mechanisms involved cathodic reduction of Cr(VI) to Cr(III), Cr(III) deposition and plant uptake. High-throughput analysis indicates that Lactococcus and Streptococcus were key electroactive bacteria for bioelectricity generation, while Rhodobacter and Hydrogenophaga were responsible for cathodic Cr(VI) reduction. To select optimal wetland plant with the characteristics of both high Cr enrichment capacity and high bioenergy output performance, six CW-MFCs with different plant species were established. Scirpus Validus CW-MFC exhibited better comprehensive performance with maximum power density of 40.6 mW/m 2 and Cr(VI) removal rate constant of 0.346 h −1 . For ensuring high electricity yield and efficient Cr(VI) removal, Scirpus validus can be selected as the optimal aquatic plant of the CW-MFC. • Appropriate electrode distance favored electricity generation and Cr(VI) removal. • Moderate cathode surface area was better for CW-MFC treating Cr(VI) wastewater. • CW-MFC achieved high first-order Cr(VI) removal rate constant of 0.471 h −1 . • Scirpus validus exhibited both higher bioenergy and Cr(VI) removal capacities. • Cr(VI) removal involved the cathodic reduction of Cr(VI) and Cr(III) deposition. 

Wetland plants selection and electrode optimization for constructed wetland-microbial fuel cell treatment of Cr(VI)-containing wastewater

This research provided an excellent novel hexavalent chromium (Cr(VI))-removal bacterium, Bacillus paramycoides Cr6, and investigated its removal mechanism from the perspective of molecular biology. Cr6 could resist up to 2500 mg/L Cr(VI), and the removal rate of 2000 mg/L Cr(VI) reached 67.3% under the optimal culture conditions of 220 r/min, pH 8 and 31 ℃. When the initial concentration of Cr(VI) was 200 mg/L, Cr6 had a removal rate of 100% within 18 h. The differential transcriptome analysis identified two key structural genes named bcr005 and bcb765 of Cr6, which were upregulated by Cr(VI). Their functions were predicted and further confirmed by bioinformatic analyses and in vitro experiments. bcr005 encodes Cr(VI)-reductase BCR005, and bcb765 encodes Cr(VI)-binding protein BCB765. Real-time fluorescent quantitative PCRs were performed, and the data illustrated a parallel pathway (one is Cr(VI) reduction, and the other is Cr(VI) immobilisation) of Cr6 to remove Cr(VI), which relies on the synergistic expression of the genes bcr005 and bcb765 induced by different concentrations of Cr(VI). In summary, a deeper molecular mechanism of Cr(VI) microorganism removal was elaborated; Bacillus paramycoides Cr6 was an exceptional novel Cr(VI)-removed bacterial resource, while BCR005 and BCB765 were two new-found efficient enzymes that have potential practical applications for sustainable microbial remediation of Cr-contaminated water.

Cr(VI)-bioremediation mechanism of a novel strain Bacillus paramycoides Cr6 with the powerful ability to remove Cr(VI) from contaminated water.

Constructed wetland (CW), a green wastewater treatment technology, is extensively used in the world owing to its advantages of less investment, low energy consumption, simple operation and maintenance, and satisfactory purification effect. Currently, with the promotion of CW application, a variety of problems have been gradually exposed. This paper summarizes the common problems of CWs, including poor wastewater purification capacity in cold climate, vulnerability of aquatic plants, clogging risk, greenhouse gases emissions, large land area occupation and inadequate management. In view of the above existing problems, optimized schemes are put forward. At cold temperature, proper thermal insulation, additional aeration, hydraulic condition optimization, and effluent recirculation are recommended to promote pollutants removal by CWs. To strengthen the function of phytoremediation in CWs, it is necessary to choose suitable plant species and reasonable planting patterns. A variety of methods (e.g., solid sampling, hydraulic conductivity test, tracer experiments, bio-electrochemical sensor, geophysical methods) have been developed for monitoring and evaluation of CW clogging. Anti-clogging measures mainly include wastewater pretreatment, replacement of matrix, addition of chemical oxidants, biosurfactants and enzymes, introduction of degrading bacteria or earthworms etc. CWs often emit greenhouse gases (e.g., CH4 and N2O) during wastewater treatment, which can be controlled by conventional methods (e.g., flow patterns, vegetation types, electron receptor supplementation, special filler utilization) and bio-electrochemical method. CW database construction and utilization, appropriate CW type design (e.g., hybrid CW), and strict technical management and supervision are recommended to realize the standardized and efficient operation of CWs. 

Current problems and countermeasures of constructed wetland for wastewater treatment: A review

Cr(VI) is present naturally as Cr2O72- and its negative charge repels electrons (e-), preventing direct reduction to Cr(III). The Cr(VI) reduction process consumes H+ , suggesting that conventional Cr(VI) reduction techniques a strictly require a low pH, increasing their cost. Therefore, the development of a process that enables Cr(VI) reduction at neutral pH conditions is of interest. In this work, Cu(II) was used as an electron shuttle medium in a microbial fuel cell (MFC). The effects of MFC operating conditions, such as Cu(II)/Cr(VI) ratio, substrate (sodium acetate) concentration and external resistance on the power density (PD) and Cr(VI) removal efficiency were investigated using the response surface methodology (RSM). The maximum Cr(VI) removal efficiency (73%) and power production capacity (30.57 mW/m2) were achieved under pH-neutral conditions with a Cu(II)/Cr(VI) ratio of 1.65, a substrate concentration of 1.36 g/L, and an external resistance of 1360 Ω. The Cr(VI) removal efficiency of the MFC system without added Cu(II) was only 43%, and the power production capacity (15.85 mW/m2) was only half of that under optimal conditions. The removal efficiency of Cu(II) was 100% after seven days. Chromium and copper deposits were observed on the cathode surface, indicating that Cu(II) was deposited on the cathode surface following Cr(VI) removal, eliminating the secondary contamination that is caused by the addition of excess Cu(II). An RSM-optimized MFC system exhibits improved chemical oxygen demand removal, Coulombic efficiency and normalized energy recovery (NER). Its NER is at least 1.5 times higher than that of an unoptimized MFC system, indicating that the MFC most efficiently converts substrates into electrons when the operating conditions are optimized. This work demonstrates that Cu(II) plays a critical role as an electron-shuttle mediator in the removal of Cr(VI). The electron transfer function of Cu(II) increases the efficiency of the reduction that involves Cr(VI), electrons and H+, and eliminates the cost of adding acidic agents, as required in the conventional method in wastewater treatment. The obtained Cu(II)/Cr(VI) ratio is used to determine the maximum amount of Cr(VI) that can be removed by a scaled-up MFC system. 

Optimizing the parameters of microbial fuel cells using response surface methodology to increase Cr(VI) removal efficiency and power production

Due to chemical agents consumption and energy demand during traditional Cr(VI)-containing wastewater treatments, it is essential to explore a cost-effective and sustainable technique to remove Cr(VI) from wastewater. Combining of phytoremediation and bio-electrochemical technology, an Acorus calamus L. constructed wetland-microbial fuel cell (CW-MFC) was ingeniously designed to purify Cr(VI)-containing wastewater and harvest bioelectricity, and possible Cr(VI) removal routes were explored. Acorus calamus L. increased the CW-MFC voltage by approximately 69.75%, and maximum power density reached 36.43 mW m−2. After 9 h, the planted CW-MFC achieved Cr(VI) removal efficiency of 98.92% at an influent Cr(VI) concentration of 12.07 mg/L, while only 76.43% of the Cr(VI) was eliminated in the unplanted CW-MFC. The Cr(VI) removal was achieved mainly through bio-electrochemical reduction and plant uptake, which accounted for 53–67% and 22–33%, respectively. The addition of low concentrations of Cr(VI) improved the CW-MFC power generation, while high levels of Cr(VI) resulted in power decline. The highest maximum power density of 46.63 mW m−2 was observed when influent Cr(VI) concentration was 6.69 mg/L. The CW-MFC's Cr(VI) removal conformed to the first-order reaction kinetics, and a high rate constant of 0.2887 h−1 was achieved even at a Cr(VI) concentration of 18.21 mg/L. The main removal pathway of Cr was associated with cathodic bio-electrochemical reduction of Cr(VI) to Cr(III) and Cr(OH)3 precipitation, and the final fate of Cr was comprehensively analyzed. The obtained research results provide an important reference for the practical application of CW-MFC in Cr(VI)-containing wastewater treatment. 

Acorus calamus L. constructed wetland-microbial fuel cell for Cr(VI)-containing wastewater treatment and bioelectricity production

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Wetland plants selection and electrode optimization for constructed wetland-microbial fuel cell treatment of Cr(VI)-containing wastewater

Acorus calamus L. constructed wetland-microbial fuel cell for Cr(VI)-containing wastewater treatment and bioelectricity production