Remediation of hexavalent chromium polluted soil in an industrial site by microbial fuel cells

The versatility of Microbial fuel cells (MFCs) has meant that the technology can now branch towards a growing variety of applications. One of the big challenges however, has not been the microbial community but the identification of materials that allow the organisms to thrive and ultimately produce power. Paper has recently been adopted as a viable material that functions both structurally and as the medium for proton exchange. To date; the most reported use for paper-based MFCs has been as a diagnostics tool, however it is highly desirable to develop paper MFCs as lightweight, portable power supplies. In a diagnostic role, the paper MFCs reported have been low-power and short-lived (lasting just minutes) which is not suitable if the role were as power supply. There have been reports of longer-term electrical outputs using paper-based MFCs (with air-cathodes) such as origami stacks1 and 3D-tetrahedron MFCs2. However, in both cases the method for feeding was via careful liquid injection. This is all very well in a lab-based environment but in a real-world scenario MFCs might need activating quickly and without delicate feeding requirements. Here it would be advantageous to simply distribute onto pools of liquid and have them passively intake the fuel. The goal of the current study therefore was to look at paper-based MFCs and investigate whether they might be able to passively intake nutrients from the surrounding environment. Two MFC designs were looked at; the first being flat 2D-MFCs with the anode on the underside of the paper, directly in contact with the liquid and the cathode on top, open to air. The second design was 3D-tetrahedron MFCs constructed from standard copier paper but with an absorbent cellulose material incorporated into the base. For all MFCs, air-cathodes were used without platinum or ferricyanide. For the 2D-flat MFCs, two methodologies were tested; the first was to print electrodes on either side of the paper and the second were hand-made, three-layered structures with conductive latex cathodes painted on one side and a sheet of carbon fibre adhered to the other. Each MFC was approximately 2cm x 4cm. In all experiments, there was no inoculation prior to the MFC being placed on puddles of enriched wastewater. The MFCs with printed electrodes reached peak OCV of 300mV which quickly dropped. In closed circuit the current peaked at 2.4 µA before rapidly declining which is clearly unsuitable for real-world use. The reason for the poor performance was the dissolution of the electrode as the liquid displaced the conductive elements as verified by the significant increase in resistance after use. Further work with printed MFCs will investigate incorporating a stabilising material to prevent the electrodes dissolving. The flat hand-made MFCs fared much better. Interestingly the conductive-latex cathodes were more resistive than the printed ones yet the MFCs were superior and more stable over time. These MFCs climbed and stabilised for 4 days at 11 µW (185mV). Different types of paper were trialled including baking, greaseproof and copier and all performed comparably. Further work will investigate stacking multiple flat-MFCs on single sheets of paper. The simple flat-MFCs are promising but for MFCs operating outside and tapping into nutrients in puddles they should ideally have enclosed chambers housing the anode. To trial this, 3D-tetrahedron MFCs with 15mL volume were set up to sit on pools of liquid with the only method of feeding via capillary motion of the absorbent base. The 3D-MFCs with absorbent bases were compared to MFCs with a waterproof coating over their base. Those with the absorbent bottoms immediately generated a current and continued increasing in power for over 2 weeks peaking and stabilising at 40µW (2.7 W/m3). In addition, when fresh nutrient was added to the reservoir (not directly to the MFCs), they responded almost immediately, a factor that could be advantageous if the role were biosensor. The MFCs with plastic coated bases showed no working voltage throughout the period. The output generated by the MFCs relying on a passive feeding mechanism is comparable to that produced by the same MFCs with sealed bases from a previous study (where injection feeding took place2). This output is sufficient to initiate a power management system and broadcast radio signals. These findings are an exciting development because lightweight paper-MFCs could potentially be dropped onto puddles of organic liquid, passively sucking up nutrients from the environment and subsequently broadcasting distress signals. References 1Fraiwan et al., (2016) Biosens Bioelectron 85: 190-197 2Winfield et al., (2015) J Mater Chem A 13: 7058-7065

Relic DNA does not obscure the microbial community of paddy soil microbial fuel cells

ABSTRACTThermophiles hydrolysis and acidogens fermentation were sequentially adopted to pretreat excess sludge for microbial fuel cell (MFC) electricity production. The results indicated that MFC fed with the thermophiles-acidogens pretreated sludge (MFC AB), reached a higher removal of ammonia nitrogen than the MFC fed with the heating hydrolysis and acidogens fermentation pretreated sludge (MFC NB). However, compared with the MFC AB, MFC NB presented a better performance for removal of soluble chemical oxygen demand (SCOD) (90.08%) and protein (82.42%). As for the electricity production, MFC NB obtained higher voltage of 0.632 V and maximum power density with 1.05 W/m3 while MFC AB reached maximum voltage of 0.373 V and maximum power density of 0.58 W/m3. Bacterial 16S rRNA-based molecular microbial techniques showed that microbial communities on both MFC anode biofilms was diverse and different. The cooperation of fermentation bacteria and electricigen Shewanella baltica in the MFC NB may have contributed towards the improvement of electricity generation.

Microbial fuel cell (MFC) is a sustainable technology to treat cattle manure slurry (CMS) for converting chemical energy to bioelectricity. In this work, two types of allochthonous inoculum including activated sludge (AS) and domestic sewage (DS) were added into the MFC systems to enhance anode biofilm formation and electricity generation. Results indicated that MFCs (AS + CMS) obtained the maximum electricity output with voltage approaching 577±7mV (~196h), followed by MFCs (DS + CMS) (520±21mV, ~236h) and then MFCs with autochthonous inoculum (429±62mV, ~263.5h). Though the raw cattle manure slurry (RCMS) could facilitate electricity production in MFCs, the addition of allochthonous inoculum (AS/DS) significantly reduced the startup time and enhanced the output voltage. Moreover, the maximum power (1.259±0.015W/m2) and the highest COD removal (84.72±0.48%) were obtained in MFCs (AS + CMS). With regard to microbial community, Illumina HiSeq of the 16S rRNA gene was employed in this work and the exoelectrogens (Geobacter and Shewanella) were identified as the dominant members on all anode biofilms in MFCs. For anode microbial diversity, the MFCs (AS + CMS) outperformed MFCs (DS + CMS) and MFCs (RCMS), allowing the occurrence of the fermentative (e.g., Bacteroides) and nitrogen fixation bacteria (e.g., Azoarcus and Sterolibacterium) which enabled the efficient degradation of the slurry. This study provided a feasible strategy to analyze the anode biofilm formation by adding allochthonous inoculum and some implications for quick startup of MFC reactors for CMS treatment.

SummaryIn microbial fuel cells (MFCs), microorganisms generate electrical current by oxidizing organic compounds. MFCs operated with different electron donors harbour different microbial communities, and it is unknown how that affects their response to starvation. We analysed the microbial communities in acetate‐ and glucose‐fed MFCs and compared their responses to 10 days starvation periods. Each starvation period resulted in a 4.2 ± 1.4% reduction in electrical current in the acetate‐fed MFCs and a 10.8 ± 3.9% reduction in the glucose‐fed MFCs. When feed was resumed, the acetate‐fed MFCs recovered immediately, whereas the glucose‐fed MFCs required 1 day to recover. The acetate‐fed bioanodes were dominated by Desulfuromonas spp. converting acetate into electrical current. The glucose‐fed bioanodes were dominated by Trichococcus sp., functioning as a fermenter, and a member of Desulfuromonadales, using the fermentation products to generate electrical current. Suspended biomass and biofilm growing on non‐conductive regions within the MFCs had different community composition than the bioanodes. However, null models showed that homogenizing dispersal of microorganisms within the MFCs affected the community composition, and in the glucose‐fed MFCs, the Trichococcus sp. was abundant in all locations. The different responses to starvation can be explained by the more complex pathway requiring microbial interactions to convert glucose into electrical current.

Microbial community and bioelectrochemical activities in MFC for degrading phenol and producing electricity: Microbial consortia could make differences

Recent advances in microbial fuel cell technology for energy generation from wastewater sources

Microbial community dynamic shifts associated with sulfamethoxazole degradation in microbial fuel cells

ABSTRACTThe chemical oxygen demand (COD) removal, electricity generation, and microbial communities were compared in 3 types of microbial fuel cells (MFCs) treating molasses wastewater. Single-chamber MFCs without and with a proton exchange membrane (PEM), and double-chamber MFC were constructed. A total of 10,000 mg L−1 COD of molasses wastewater was continuously fed. The COD removal, electricity generation, and microbial communities in the two types of single-chamber MFCs were similar, indicating that the PEM did not enhance the reactor performance. The COD removal in the single-chamber MFCs (89–90%) was higher than that in the double-chamber MFC (50%). However, electricity generation in the double-chamber MFC was higher than that in the single-chamber MFCs. The current density (80 mA m−2) and power density (17 mW m−2) in the double-chamber MFC were 1.4- and 2.2-times higher than those in the single-chamber MFCs, respectively. The bacterial community structures in single- and double-chamber MFCs were also distinguishable. The amount of Proteobacteria in the double-chamber MFC was 2–3 times higher than those in the single-chamber MFCs. For the archaeal community, Methanothrix (96.4%) was remarkably dominant in the single-chamber MFCs, but Methanobacterium (35.1%), Methanosarcina (28.3%), and Methanothrix (16.2%) were abundant in the double-chamber MFC.

Inocula selection in microbial fuel cells based on anodic biofilm abundance of Geobacter sulfurreducens

Many factors affect the performance of microbial fuel cells (MFCs). Considerable attention has been given to the impact of cell configuration and materials on MFC performance. Much less work has been done on the impact of the anode microbiota, particularly in the context of using complex substrates as fuel. One strategy to improve MFC performance on complex substrates such as wastewater, is to pre-enrich the anode with known, efficient electrogens, such as Geobacter spp. The implication of this strategy is that the electrogens are the limiting factor in MFCs fed complex substrates and the organisms feeding the electrogens through hydrolysis and fermentation are not limiting. We conducted a systematic test of this strategy and the assumptions associated with it. Microbial fuel cells were enriched using three different substrates (acetate, synthetic wastewater and real domestic wastewater) and three different inocula (Activated Sludge, Tyne River sediment, effluent from an MFC). Reactors were either enriched on complex substrates from the start or were initially fed acetate to enrich for Geobacter spp. before switching to synthetic or real wastewater. Pre-enrichment on acetate increased the relative abundance of Geobacter spp. in MFCs that were switched to complex substrates compared to MFCs that had been fed the complex substrates from the beginning of the experiment (wastewater-fed MFCs - 21.9 ± 1.7% Geobacter spp.; acetate-enriched MFCs, fed wastewater - 34.9 ± 6.7% Geobacter spp.; Synthetic wastewater fed MFCs – 42.5 ± 3.7% Geobacter spp.; acetate-enriched synthetic wastewater-fed MFCs - 47.3 ± 3.9% Geobacter spp.). However, acetate pre-enrichment did not translate into significant improvements in cell voltage, maximum current density, maximum power density or substrate removal efficiency. Nevertheless, coulombic efficiency (CE) was higher in MFCs pre-enriched on acetate when complex substrates were fed following acetate enrichment (wastewater-fed MFCs – CE = 22.0 ± 6.2%; acetate-enriched MFCs, fed wastewater – CE =58.5 ± 3.5%; Synthetic wastewater fed MFCs – CE = 22.0 ± 3.2%; acetate-enriched synthetic wastewater-fed MFCs – 28.7 ± 4.2%.) The relative abundance of Geobacter ssp. and CE represents the average of the nine replicate reactors inoculated with three different inocula for each substrate. Efforts to improve the performance of anodic microbial communities in MFCs utilizing complex organic substrates should therefore focus on enhancing the activity of organisms driving hydrolysis and fermentation rather the terminal-oxidizing electrogens.

Microbial fuel cells (MFC) are an environmentally friendly way of generating electricity, which is often accompanied by the decomposition of organic waste. A common problem for these devices is the low power generated by the electric current. In nature, the decomposition of organic waste, coupled with oxidation-reduction reactions, i.e. the transfer of electrons and protons formed during the decomposition is carried out by microbial communities, which, in their optimal state for this process, are called active sludge (AS). A mature active sludge consists of a so-called flocculium - round formations in the size from 30 to 100 and more microns, inside of which microorganisms carrying out various reactions of decomposition of organic substances are contained. It is easy to see that the MFC design is topologically similar to the active sludge flocculant. We tried to check how the presence of microorganisms in the water chamber affects the productivity of the MFC by filling the anode and cathode chambers with the same sludge mixture, and the test cells were shaken on a shaker to create favorable conditions for the formation of microbial communities. The active sludge cell in both chambers shaken on the shaker, over time, generated the highest voltage in the external circuit as compared to the control samples. The obtained data confirm the legitimacy of the assimilation of MFC flocculine AS. The evolution of the microbial community of the silt mixture in two directions - in the aerobic and anaerobic MFC chambers - apparently leads to the formation of two different communities mutually complementing each other as part of the MFC and improving the operation of the MFC, subject to additional dynamic provision (shaking). The effect of the addition of peptone to the anode chamber on the productivity of MFC was also investigated. The periodic addition of peptone significantly increased the output power of the MFC cell. Apparently, substrates of protein nature, representing a nutrient medium for electrogenic bacteria, can be used to stimulate the electrogenic activity of the microflora of late anaerobic silt.

Limitation of voltage reversal in the degradation of azo dye by a stacked double-anode microbial fuel cell and characterization of the microbial community structure

Microbial Fuel Cells (MFCs) are innovative environmental engineering systems that harness the metabolic activities of microbial communities to convert chemical energy in waste into electrical energy. However, MFC performance optimization remains challenging due to limited understanding of microbial metabolic mechanisms, particularly with complex substrates under realistic environmental conditions. This study investigated the effects of substrate complexity (acetate vs. starch) and varying mass transfer (stirred vs. non-stirred) on acclimatization rates, substrate degradation, and microbial community dynamics in air-cathode MFCs. Stirring was critical for acclimating to complex substrates, facilitating electrogenic biofilm formation in starch-fed MFCs, while non-stirred MFCs showed limited performance under these conditions. Non-stirred MFCs, however, outperformed stirred systems in current generation and coulombic efficiency (CE), especially with simple substrates (acetate), achieving 66% CE compared to 38% under stirred conditions, likely due to oxygen intrusion in the stirred systems. Starch-fed MFCs exhibited consistently low CE (19%) across all tested conditions due to electron diversion into volatile fatty acids (VFA). Microbial diversity was higher in acetate-fed MFCs but unaffected by stirring, while starch-fed MFCs developed smaller, more specialized communities. Kinetic analysis identified hydrolysis of complex substrates as the rate-limiting step, with rates an order of magnitude slower than acetate consumption. Combined hydrolysis-fermentation rates were unaffected by stirring, but stirring significantly impacted acetate consumption rates, likely due to oxygen-induced competition between facultative aerobes and electrogenic bacteria. These findings highlight the trade-offs between enhanced substrate availability and oxygen-driven competition in MFCs. For real-world applications, initiating reactors with dynamic stirring to accelerate acclimatization, followed by non-stirred operation, may optimize performance. Integrating MFCs with anaerobic digestion could overcome hydrolysis limitations, enhancing the degradation of complex substrates while improving energy recovery. This study introduces novel strategies to address key challenges in scaling up MFCs for wastewater treatment, bridging the gap between fundamental research and practical applications to advance environmental systems.

<p>The toxicity and persistence of chromium in soils challenge the ecosystem and human health. Various remediation strategies have been developed to eliminate soil Cr contamination, and the most popular one is chemical stabilization. <span><span>However, chemical stabilization only changes the form of Cr and does not change the concentration of Cr, so the long-term stability of Cr has been controversial. On the other hand, some researches found that the concentration of Cr(VI) in the stabilized soil after remediation has increased. </span></span>We collected Cr-contaminated soils and one-year-stabilized soils from four research sites in northern, central, and southwestern China, trying to understand the difference of Cr species and structure in soils with various soil properties. Results showed despite the different clay content and mineral composition, all contaminated and stabilized soils are alkaline (pH 7.36 ~ 10.5). In addition, there are differences in the pollution levels of Cr and Cr (VI) in soils. In northern China, Cr(VI) was the main state of Cr-contaminated soils; however, Cr is mainly present in Cr-contaminated soil in the form of Cr(III) in southern China. For chemical stabilized soils, Cr concentrations remained similar to Cr-contaminated soils (1500~9000 mg/kg), but the concentration of Cr(VI) (5~55 mg/kg) was reduced through commercial remediation materials. The speciation of Cr in Cr-contaminated soils transformed from exchangeable Cr and Cr bound to carbonates into Cr bound to Fe-oxides and residuals in stabilized soils. SEM-EDS, XAFS and μ-XRF results revealed the main forms and structure of Cr, and showed Cr unevenly distributed on the surface or edge of the mineral. The acid leaching test revealed that Cr(VI) could be released from Cr-contaminated soils by acid, and soils can release Cr(VI) under different acidity conditions. Cr(VI) from soils collected from northern and southern China was released from acidity of [H<sup>+</sup>]=0.1M and [H<sup>+</sup>]=0.5M, respectively. This was due to erosion of coating minerals or Cr(VI)-bearing minerals. Our study suggested that stabilization technology not only reduces the toxicity of chromium, improves the stability of chromium, but also partially recovers the physical and chemical properties of soil. Meanwhile, in future remediation projects, it is necessary to consider the existing forms of chromium in different soils to develop remediation strategies.</p>