Electricity production of microbial fuel cells by degrading cellulose coupling with Cr(VI) removal

Mechanism of Cr(VI) bioreduction by Clostridium sp. LQ25 under Fe(III) reducing conditions

The Cr(VI) bioreduction has attracted widespread attention in the field of Cr(VI) pollution remediation due to its environmental friendliness. Further in-depth research on the reduction mechanisms is necessary to enhance the efficiency of Cr(VI) bioreduction. However, the limited research on Cr(VI) bioreduction mechanisms remains a bottleneck for the practical application of Cr(VI) reduction. In this study, The Cr(VI) reduction of strain LQ25 was significantly improved when Fe(III) was used as an electron acceptor, which increased by 1.6-fold maximum within the set Cr(VI) concentration range. Based on this, the electron transfer process of Cr(VI) reduction was analyzed using strain LQ25. Based on genomic data, flavin proteins were found to interact closely with electron transfer-related proteins using protein-protein interaction (PPi) analysis. Transcriptome analysis revealed that flavin synthesis genes (ribE, ribBA, and ribH) and electron transfer flavoprotein genes (fixA, etfA, fixB, and etfB) were significantly upregulated when Fe(III) was used as the electron acceptor. These results indicate that the fermentative dissimilatory Fe(III)-reducing bacterial strain LQ25 mainly uses flavin as an electron shuttle for electron transfer, which differs from the common use of cytochrome c in respiratory bacteria. These findings on the mechanism of Cr(VI) bioreduction provide technical support for improving the efficiency of Cr(VI) reduction which promote the practical application of Cr(VI) bioreduction in the field of Cr(VI) pollution remediation.

Research progress of novel bio-denitrification technology in deep wastewater treatment

Excessive nitrogen emissions are a major contributor to water pollution, posing a threat not only to the environment but also to human health. Therefore, achieving deep denitrification of wastewater is of significant importance. Traditional biological denitrification methods have some drawbacks, including long processing times, substantial land requirements, high energy consumption, and high investment and operational costs. In contrast, the novel bio-denitrification technology reduces the traditional processing time and lowers operational and maintenance costs while improving denitrification efficiency. This technology falls within the category of environmentally friendly, low-energy deep denitrification methods. This paper introduces several innovative bio-denitrification technologies and their combinations, conducts a comparative analysis of their denitrification efficiency across various wastewater types, and concludes by outlining the future prospects for the development of these novel bio-denitrification technologies.

Physiological and transcriptional studies reveal Cr(VI) reduction mechanisms in the exoelectrogen Cellulomonas fimi Clb-11

A facultative exoelectrogen, Cellulomonas fimi strain Clb-11, was isolated from polluted river water. This strain could generate electricity in microbial fuel cells (MFCs) with carboxymethyl cellulose (CMC) as the carbon source, and the maximum output power density was 12.17 ± 2.74 mW·m^-2. In addition, Clb-11 could secrete extracellular chromate reductase or extracellular electron mediator to reduce Cr(VI) to Cr(III). When the Cr(VI) concentration was less than 0.5 mM in Luria-Bertani (LB) medium, Cr(VI) could be completely reduced by Clb-11. However, the Clb-11 cells swelled significantly in the presence of Cr(VI). We employed transcriptome sequencing analysis to identify genes involved in different Cr(VI) stress responses in Clb-11. The results indicate that 99 genes were continuously upregulated while 78 genes were continuously downregulated as the Cr(VI) concentration increased in the growth medium. These genes were mostly associated with DNA replication and repair, biosynthesis of secondary metabolites, ABC transporters, amino sugar and nucleotide sugar metabolism, and carbon metabolism. The swelling of Clb-11 cells might have been related to the upregulation of the genes atoB, INO1, dhaM, dhal, dhak, and bccA, which encode acetyl-CoA C-acetyltransferase, myo-inositol-1-phosphate synthase, phosphoenolpyruvate-glycerone phosphotransferase, and acetyl-CoA/propionyl-CoA carboxylase, respectively. Interestingly, the genes cydA and cydB related to electron transport were continuously downregulated as the Cr(VI) concentration increased. Our results provide clues to the molecular mechanism of Cr(VI) reduction by microorganisms in MFCs systems.

Comparative Studies on Electrodes for Rumen Bacteria Microbial Fuel Cells

Microbial fuel cells (MFCs) using rumen bacteria have been proposed as a power source for running devices inside cattle. In this study, we explored the key parameters of the conventional bamboo charcoal electrode in an attempt to improve the amount of electrical power generated by the microbial fuel cell. We evaluated the effects of the electrode's surface area, thickness, and rumen content on power generation and determined that only the electrode's surface area affects power generation levels. Furthermore, our observations and bacterial count on the electrode revealed that rumen bacteria concentrated on the surface of the bamboo charcoal electrode and did not penetrate the interior, explaining why only the electrode's surface area affected power generation levels. A Copper (Cu) plate and Cu paper electrodes were also used to evaluate the effect of different electrodes on measuring the rumen bacteria MFC's power potential, which had a temporarily higher maximum power point (MPP) compared to the bamboo charcoal electrode. However, the open circuit voltage and MPP decreased significantly over time due to the corrosion of the Cu electrodes. The MPP for the Cu plate electrode was 775 mW/m² and the MPP for the Cu paper electrode was 1240 mW/m², while the MPP for bamboo charcoal electrodes was only 18.7 mW/m². In the future, rumen bacteria MFCs are expected to be used as the power supply of rumen sensors.

Analysis and enhancement of the energy utilization efficiency of corn stover using strain Lsc-8 in a bioelectrochemical system

The strain Lsc-8 can produce a current density of 33.08 µA cm^-2 using carboxymethylcellulose (CMC) as a carbon source in a three-electrode configuration. A co-culture system of strain Lsc-8 and Geobacter sulfurreducens PCA was used to efficiently convert cellulose into electricity to improve the electricity generation capability of microbial fuel cells (MFCs). The maximum current density achieved by the co-culture with CMC was 559 μA cm^-2, which was much higher than that of strain Lsc-8 using CMC as the carbon source. The maximum power density reached 492.05 ± 52.63 mW cm^-2, which is much higher than that previously reported. Interaction mechanism studies showed that strain Lsc-8 had the ability to secrete riboflavin and convert cellulose into acetic acid, which might be the reason for the high electrical production performance of the co-culture system. In addition, to the best of our knowledge, a co-culture or single bacteria system using agricultural straw as the carbon source to generate electricity has not been reported. In this study, the maximum current density of the three-electrode system inoculated with strain Lsc-8 was 14.56 μA cm^-2 with raw corn stover as the sole carbon source. Raw corn stover as a carbon source was also investigated for use in a co-culture system. The maximum current density achieved by the co-culture was 592 μA cm^-2. The co-culture system showed a similar electricity generation capability when using raw corn stover and when using CMC. This research shows for the first time that a co-culture or single bacteria system can realize both waste biomass treatment and waste power generation.

Bioelectrochemical systems-based metal removal and recovery from wastewater and polluted soil: Key factors, development, and perspective

Bioelectrochemical systems (BES) are considered efficient and sustainable technologies for bioenergy generation and simultaneously removal/recovery metal (loid)s from soil and wastewater. However, several current challenges of BES-based metal removal and recovery, especially concentrating target metals from complex contaminated wastewater or soil and their economic feasibility of engineering applications. This review summarized the applications of BES-based metal removal and recovery systems from wastewater and contaminated soil and evaluated their performances on electricity generation and metal removal/recovery efficiency. In addition, an in depth review of several key parameters (BES configurations, electrodes, catalysts, metal concentration, pH value, substrate categories, etc.) of BES-based metal removal and recovery was carried out to facilitate a deep understanding of their development and to suggest strategies for scaling up their specific application fields. Finally, the future intervention on multifunctional BES to improve their performances of mental removal and recovery were revealed.

Recovery of heavy metals from industrial wastewater using bioelectrochemical system inoculated with novel Castellaniella species

Water pollution is a global issue that has drastically increased in recent years due to rapid industrial development. Different technologies have been designed for the removal of pollutants from wastewater. However, most of these techniques are expensive, generate new waste, and focus solely on metal removal instead of metal recovery. In this study, novel facultative exoelectrogenic strains designated Castellaniella sp. A5, Castellaniella sp. B3, and Castellaniella sp. A3 were isolated from a microbial fuel cell (MFC). These isolates were utilized as pure and mixed culture inoculums in a bioelectrochemical system (BES) to produce bioelectricity and treat simulated industrial wastewater. A single-chamber MFC inoculated with the mixed culture attained the highest electricity generation (i.e., 320 mW/m² power density and 3.19 A/m² current density), chemical oxygen demand removal efficiency (91.15 ± 0.05%), and coulombic efficiency (54.81 ± 4.18%). In addition, the BES containing biofilms of the mixed culture achieved the highest Cu, Cr, and Cd removal efficiencies of 99.89 ± 0.07%, 99.59 ± 0.53%, and 99.91 ± 0.04%, respectively. The Cr⁶⁺ and Cu²⁺ in the simulated industrial wastewater were recovered via microbial electrochemical reduction as Cr³⁺ and Cu⁰, respectively. However, Cd²⁺ precipitated as Cd (OH)₂ or CdCO₃ on the surface of the cathodes. These results suggest that a mixed culture inoculum of Castellaniella sp. A5, Castellaniella sp. B3, and Castellaniella sp. A3 has great potential as a biocatalyst in BES for heavy metals recovery from industrial wastewater.

Synchronous Cr(VI) Remediation and Energy Production Using Microbial Fuel Cell from a Subsurface Environment: A Review

Applying microbial fuel cell (MFC) technology for eco-remediation of Cr(VI) pollution from a subsurface environment has great scientific value and practical significance due to its promising advantages of pollutant remediation and renewable energy generation. The aim of the current review is to summarize the migration characteristics of Cr(VI) in a subsurface soil/water environment and investigate the factors affecting the MFC performance for synchronous Cr(VI) remediation and power generation, and sequentially highlight diverse challenges of MFC technology for in situ remediation of subsurface groundwater and soils. The critical review put forward that Cr(VI) removal efficiency and energy production of MFC can be improved by enhancing the adjustability of cathode pH, setting potential, modifying electrode, and incorporating other technologies into MFC. It was recommended that designing typical large-scale, long-term continuous flow MFC systems, adding electron shuttle media or constructing artificial electron according to actual groundwater/soil and Cr(VI) pollution characteristics, site geology, and the hydrogeology condition (hydrochemical conditions, colloid type, and medium) are essential to overcome the limitations of the small size of the laboratory experiments and improve the application of technology to in situ Cr(VI) remediation. This review provided reference and ideas for future research of MFC-mediated onsite Cr(VI) remediation.

Microbial recovery of critical metals from secondary sources

The continuous development of technologies involving critical metals, both in Europe and over the world, and geopolitical challenges in areas rich in critical metal sources, imposed increased research efforts to recover them from secondary sources, by eco-efficient processes. Yet, microbes-metal interactions are not sufficiently exploited to recover metals from secondary sources, although they are already used in ore extraction. This review examines and compare strategies and processes involving microorganisms for critical metals recovery, since conventional physico-chemical methods are energy-intensive and often polluting. Two groups of microbial assisted recovery processes are discussed: metal mobilization from metal bearing waste, and selective metal separation from leaching solutions by immobilization on microbial biomass. Because most of the identified microbial technologies are developed on laboratory scale, the increase of biorecovery efficiency is compulsory for enhancing scaling-up potential. Future developments focused on novel microorganisms and high-performance strategies for critical metal recovery by microbial processes are considered.

Chemical-Assisted Microbially Mediated Chromium (Cr) (VI) Reduction Under the Influence of Various Electron Donors, Redox Mediators, and Other Additives: An Outlook on Enhanced Cr(VI) Removal

Chromium (Cr) (VI) is a well-known toxin to all types of biological organisms. Over the past few decades, many investigators have employed numerous bioprocesses to neutralize the toxic effects of Cr(VI). One of the main process for its treatment is bioreduction into Cr(III). Key to this process is the ability of microbial enzymes, which facilitate the transfer of electrons into the high valence state of the metal that acts as an electron acceptor. Many underlying previous efforts have stressed on the use of different external organic and inorganic substances as electron donors to promote Cr(VI) reduction process by different microorganisms. The use of various redox mediators enabled electron transport facility for extracellular Cr(VI) reduction and accelerated the reaction. Also, many chemicals have employed diverse roles to improve the Cr(VI) reduction process in different microorganisms. The application of aforementioned materials at the contaminated systems has offered a variety of influence on Cr(VI) bioremediation by altering microbial community structures and functions and redox environment. The collective insights suggest that the knowledge of appropriate implementation of suitable nutrients can strongly inspire the Cr(VI) reduction rate and efficiency. However, a comprehensive information on such substances and their roles and biochemical pathways in different microorganisms remains elusive. In this regard, our review sheds light on the contributions of various chemicals as electron donors, redox mediators, cofactors, etc., on microbial Cr(VI) reduction for enhanced treatment practices.

Microbial Cd(II) and Cr(VI) resistance mechanisms and application in bioremediation

The heavy metals cadmium (Cd) and chromium (Cr) are extensively used in industry and result in water and soil contamination. The highly toxic Cd(II) and Cr(VI) are the most common soluble forms of Cd and Cr, respectively. They enter the human body through the food chain and drinking water and then cause serious illnesses. Microorganisms can adsorb metals or transform Cd(II) and Cr(VI) into insoluble or less bioavailable forms, and such strategies are applicable in Cd and Cr bioremediation. This review focuses on the highlighting of novel achievements on microbial Cd(II) and Cr(VI) resistance mechanisms and their bioremediation applications. In addition, the knowledge gaps and research perspectives are also discussed in order to build a bridge between the theoretical breakthrough and the resolution of Cd(II) and Cr(VI) contamination problems.