Adaptively Evolving Bacterial Communities for Complete and Selective Reduction of Cr(VI), Cu(II), and Cd(II) in Biocathode Bioelectrochemical Systems

Enhanced chromium and nitrogen removal by constructing a biofilm reaction system based on denitrifying bacteria preferential colonization theory

Understanding the developmental characteristics of microbial communities in biofilms is crucial for designing targeted functional microbial enhancements for the remediation of complex contamination scenarios. The strong prioritization effect of microorganisms confers the ability to colonize strains that arrive first dominantly. In this study, the auto-aggregating denitrifying bacterial Pseudomonas stutzeri strain YC-34, which has both nitrogen and chromium removal characteristics, was used as a biological material to form a stable biofilm system based on the principle of dominant colonization and biofortification. The effect of the biofilm system on nitrogen and chromium removal was characterized by measuring the changes in the quality of influent and effluent water. The pattern of biofilm changes was analyzed by measuring biofilm content and thickness and characterizing extracellular polymer substances (EPS). Further analysis of the biofilm microbiota characteristics and potential functions revealed the mechanism of strain YC-34 biofortified biofilm. The results revealed that the biofilm system formed could achieve 90.56% nitrate-nitrogen removal with an average initial nitrate-nitrogen concentration of 51.9 mg/L and 40% chromium removal with an average initial hexavalent chromium Cr(VI) concentration of 7.12 mg/L. The biofilm properties of the system were comparatively analyzed during the biofilm formation period, the fluctuation period of Cr(VI)-stressed water quality, and the stabilization period of Cr(VI)-stressed water quality. The biofilm system may be able to increase the structure of hydrogen bonds, the type of protein secondary structure, and the abundance of amino acid-like components in the EPS, which may confer biofilm tolerance to Cr(VI) stress and allow the system to maintain a stable biofilm structure. Furthermore, microbial characterization indicated an increase in microbial diversity in the face of chromium stress, with an increase in the abundance of nitrogen removal-associated functional microbiota and an increasing trend in the abundance of nitrogen transfer pathways. These results demonstrate that the biofilm system is stable in nitrogen and chromium removal. This bioaugmentation method may provide a new way for the remediation of heavy metal-polluted water bodies and also provides theoretical and application parameters for the popularization and application of biofilm systems.

DIET-like and MIET-like mutualism of S. oneidensis MR-1 and metal-reducing function microflora boosts Cr(VI) reduction

Microbial treatment of Cr(VI) is an environmentally friendly and low-cost approach. However, the mechanism of mutualism and the role of interspecies electron transfer in Cr(VI) reducing microflora are unclear. Herein, we constructed an intersymbiotic microbial association flora to augment interspecies electron transfer via functionalizing electroactive Shewanella oneidensis MR-1 with metal-reducing microflora, and thus the efficiency of Cr(VI) reduction. The findings suggest that the metal-reducing active microflora could converts glucose into lactic acid and riboflavin for S. oneidensis MR-1 to act as a carbon source and electron mediator. Thus, when adding initial 25 mg/L Cr (VI), this microflora exhibited an outstanding Cr (VI) removal efficiency (100%) at 12 h and elevated Cr (III) immobilization efficiency (80%) at 60 h with the assistance of 25 mg/L Cu(II). A series of electrochemical experiments proved this remarkable removal efficiency were ascribed to the improved interspecies electron transfer efficiency through direct interspecies electron transfer and riboflavin through mediated interspecies electron transfer. Furthermore, the metagenomic analysis revealed the expression level of the electron transport pathway was promoted. Intriguing high abundance of genes participating in the bio-reduction and biotransformation of Cr(VI) was also observed in functional microflora. These outcomes give a novel strategy for enhancing the reduction and fixation of harmful heavy metals by coculturing function microflora with electrogenic microorganisms.

Utilization of nickel-graphite electrode as an electron donor for high-efficient microbial removal of solved U(VI) mediated by Leifsonia sp

Enzymatically catalyzed reduction of metals by bacteria has potential application value to uranium-mine wastewater. However, its practical implementation has long been restricted by its intrinsic drawbacks such as low efficiency and long treatment-time. This study aims to explore the effect of electrodes on U (VI) removal efficiency by a purified indigenous bacteria isolated from a uranium mining waste pile in China. The effects of current intensity, pH, initial U (Ⅵ) concentration, initial dosage of bacteria and contact time on U (Ⅵ) removal efficiency were investigated via static experiments. The results show that U(VI) removal rate was stabilized above 90% and the contact time sharply shortened within 1 h when utilized nickel-graphite electrode as an electron donor. Over the treatment ranges investigated maximum removal of U (Ⅵ) was 96.04% when the direct current was 10 mA, pH was 5, initial U (Ⅵ) concentration was 10 mg/L, and dosage of Leifsonia sp. was 0.25 g/L. In addition, it is demonstrated that U (VI) adsorption by Leifsonia sp. is mainly chemisorption and/or reduction as the quasi-secondary kinetics is more suitable for fitting the process. FTIR results indicated that amino, amide, aldehyde and phosphate -containing groups played a role in the immobilization of U (VI) more or less. SEM and EDS measurements revealed that U appeared to be more obviously aggregated on the surface of cells. A plausible explanation for this, supported by XPS, is that U (VI) was partially reduced to U (IV) by direct current then precipitated on the cells surface. These observations reveal that Nickel-graphite electrode exhibited good electro-chemical properties and synergistic capacity with Leifsonia sp. which potentially provides a new avenue for uranium enhanced removal/immobilization by indigenous bacteria.

Performance and mechanism of a bioelectrochemical system for reduction of heavy metal cadmium ions

This study explores the removal of Cd(ii) from wastewater using a microbial electrolysis cell (MEC) to investigate the electrochemical performance and removal kinetics of an anodic polarity reversal biocathode and the mechanism of action of electrochemically active bacteria. Comparative electrochemical methods showed that using an anodic polarity reversal biocathode resulted in greater than 90% removal of different concentrations of Cd(ii) within three days, which may be related to the catalytic effect of anodic electrochemically active bacteria. However, due to the ability of bacteria to regulate, up to nearly 2 mg L-1 of Cd(ii) ions will remain in solution. As shown by the linear fitting relationship between scanning speed and peak current, the removal process was dominated by adsorption control for 20-80 mg L-1 Cd(ii) and diffusion control for 100 mg L-1 Cd(ii). The analysis of raw sludge and sludge containing Cd(ii) showed that Arcobacter and Pseudomonas were the primary cadmium-tolerant bacteria, and that the ability to remove Cd(ii) was the result of a synergistic collaboration between autotrophic and heterotrophic Gram-negative bacteria.

Electrodeposited copper enhanced removal of 2,4-dichlorophenol in batch and flow reaction in Cu@CC-PS-MFC system

Combination of microbial fuel cell (MFC) and advanced oxidation process (AOP) is promising for pollutant removal. In this paper, Cu0-loaded carbon cloth cathode by electrodeposition (Cu@CC-PS-MFC) was applied to enhance 2,4-dichlorophenol (2,4-DCP) degradation based on persulfate (PS) activation in microbial fuel cell. Cu0 exhibited a typical structure of face-centered cubic metal polyhedron on carbon cloth. The removal of 2,4-DCP by Cu@CC-PS-MFC (75.6%) was enhanced by more than 50% compared to CC-PS-MFC (49.2%) after 1 h of reaction. 30 mg/L 2,4-DCP in Cu@CC-PS-MFC was completely removed and achieved a high mineralization (80.6%) after 9 h of reaction under optimized condition with low dissolved copper ion concentration (0.615 mg/L). Meanwhile, more than 90% removal of 2,4-DCP was stably achieved with flow operation condition (hydraulic residence time of 7.2 h). The change of copper valent state Cu0/Cu2O/CuO was the main mechanism of PS activation with main reactive species of O•H and O21. The bioanode of MFC enhanced the in-situ regeneration of ≡Cu+ and ≡Cu0 on the catalyst surface by transporting electrons, which was believed to contribute to good catalyst lifetime and excellent 2,4-DCP removal. Electrodeposited copper contributes to the enhanced degradation of 2,4-DCP with energy recovery at the same time which can further broaden the application MFC.

Mechanistic insights into the response of electroactive biofilms to Cd2+ shock: bacterial viability and electron transfer behavior at the cellular and community levels

Electroactive biofilms (EABs) play a crucial role in environmental bioremediation due to their excellent extracellular electron transfer (EET) capabilities. However, Cd2+ can have toxic effects on the electrochemical performance of EABs, and the comprehensive inhibition mechanism of EABs in response to Cd2+ shock remains elusive. This study indicated that Cd2+ shock significantly reduced biomass and increased oxidative stress in EABs at the cellular level. The bacterial viability of EABs in phase III under 0.5 mM Cd2+ shock (EABCd2+-III0.5) decreased by 16.31% compared to EABCK-III. Moreover, intracellular NADH, c-Cyts, and the abundance of electroactive species were essential indicators to evaluate EET behavior of EABs. In EABCd2+-III0.5, these indicators decreased by 26.32%, 33.40%, and 20.65%, respectively. Structural equation modeling analysis established quantitative correlations between core components and electrochemical activity at cellular and community levels. The correlation analysis revealed that the growth and electron transfer functions of EABs were predictive indicators for their electrochemical performance, with standardized path coefficients of 0.407 and 0.358, respectively. These findings enhance our understanding of EABs' response to Cd2+ shock and provide insights for improving their performance in heavy metal wastewater.

Research progress on using biological cathodes in microbial fuel cells for the treatment of wastewater containing heavy metals

Various types of electroactive microorganisms can be enriched to form biocathodes that reduce charge-transfer resistance, thereby accelerating electron transfer to heavy metal ions with high redox potentials in microbial fuel cells. Microorganisms acting as biocatalysts on a biocathode can reduce the energy required for heavy metal reduction, thereby enabling the biocathode to achieve a lower reduction onset potential. Thus, when such heavy metals replace oxygen as the electron acceptor, the valence state and morphology of the heavy metals change under the reduction effect of the biocathode, realizing the high-efficiency treatment of heavy metal wastewater. This study reviews the mechanisms, primary influencing factors (e.g., electrode material, initial concentration of heavy metals, pH, and electrode potential), and characteristics of the microbial community of biocathodes and discusses the electron distribution and competition between microbial electrodes and heavy metals (electron acceptors) in biocathodes. Biocathodes reduce the electrochemical overpotential in heavy metal reduction, permitting more electrons to be used. Our study will advance the scientific understanding of the electron transport mechanism of biocathodes and provide theoretical support for the use of biocathodes to purify heavy metal wastewater.

Critical Review on the Mechanisms of Fe2+ Regeneration in the Electro-Fenton Process: Fundamentals and Boosting Strategies

This review presents an exhaustive overview on the mechanisms of Fe³⁺ cathodic reduction within the context of the electro-Fenton (EF) process. Different strategies developed to improve the reduction rate are discussed, dividing them into two categories that regard the mechanistic feature that is promoted: electron transfer control and mass transport control. Boosting the Fe³⁺ conversion to Fe²⁺ via electron transfer control includes: (i) the formation of a series of active sites in both carbon- and metal-based materials and (ii) the use of other emerging strategies such as single-atom catalysis or confinement effects. Concerning the enhancement of Fe²⁺ regeneration by mass transport control, the main routes involve the application of magnetic fields, pulse electrolysis, interfacial Joule heating effects, and photoirradiation. Finally, challenges are singled out, and future prospects are described. This review aims to clarify the Fe³⁺/Fe²⁺ cycling process in the EF process, eventually providing essential ideas for smart design of highly effective systems for wastewater treatment and valorization at an industrial scale.

Improved performance of Cr(vi)-reducing microbial fuel cells by nano-FeS hybridized biocathodes

Biocathode microbial fuel cells (MFCs) show promise for Cr(vi)-contaminated wastewater treatment. However, biocathode deactivation and passivation caused by highly toxic Cr(vi) and nonconductive Cr(iii) deposition limit the development of this technology. A nano-FeS hybridized electrode biofilm was fabricated by simultaneously feeding Fe and S sources into the MFC anode. This bioanode was then reversed as the biocathode to treat Cr(vi)-containing wastewater in a MFC. The MFC obtained the highest power density (40.75 ± 0.73 mW m^-2) and Cr(vi) removal rate (3.99 ± 0.08 mg L^-1 h^-1), which were 1.31 and 2.00 times those of the control, respectively. The MFC also maintained high stability for Cr(vi) removal in three consecutive cycles. These improvements were due to synergistic effects of nano-FeS with excellent properties and microorganisms in the biocathode. The mechanisms were: (1) the accelerated electron transfer mediated by nano-FeS 'electron bridges' strengthened bioelectrochemical reactions, firstly realizing deep reduction of Cr(vi) to Cr(0) and thus effectively alleviating cathode passivation; (2) nano-FeS as 'armor' layers improved cellular viability and extracellular polymeric substance secretion; (3) the biofilm selectively enriched a diversity of bifunctional bacteria for electrochemical activity and Cr(vi) removal. This study provides a new strategy to obtain electrode biofilms for sustainable treatment of heavy metal wastewater.

Synergistic Cr(VI) reduction and adsorption of Cu(II), Co(II) and Ni(II) by zerovalent iron-loaded hydroxyapatite

Multi-metal contaminated soil, such as Cr(VI), Cu(II), and Co(II), still challenge the environmental remediation. In this work, zerovalent iron-loaded hydroxyapatite (ZVI/HAP) was first applied to simultaneously adsorb multi-metal in contaminated soil. During the remediation, the co-existing Cu(II), Ni(II), and Co(II) were adsorbed and precipitated onto ZVI/HAP. This "spontaneous deposition" simultaneously achieved the adsorption of the cationic metals and improved the isoelectric point of ZVI/HAP to 4.83 from 1.59, thus significantly alleviating the electronegativity to enhance the capture and reduction efficiency of Cr(VI). The application of ZVI/HAP resulted in the reduction of more than 99% of total Cr(VI) in contaminated soil, and the almost complete adsorption of water-soluble and DTPA-extractable Cu, Ni and Co within 20 d. Based on the sequential extraction and risk reduction assessment, soil Cr, Cu, Ni, and Co speciation was transformed from an unstable state (exchangeable and carbonate-bound fractions) to a relatively stable state, reducing the risk of heavy metals in contaminated soil significantly. This study developed an efficient strategy for the remediation of multi-metal contaminated soil.

SRB-based bioelectrochemical system: A potential multipollutant combatant for enhanced landfill waste stabilization

Due to the lower proportion of organic matter and higher toxicity of the aged landfill, most of the advanced treatment technologies are not effective from economic, environmental, and social perspectives. This study evaluates the potential of sulfate-reducing bacteria (SRB) based bioelectrochemical-system (BES) in the decontamination of landfill wastes by reducing GHGs emissions and levels of soluble pollutants. The landfill waste (solid/leachate) collected from the Pirana Landfill site was assessed for economical long-term treatment and scaling up the feasibility of the designed system. The present system demonstrated significant improvement in volumetric hydrogen production of 3.1:1 (H2:CH4) by suppressing methanogenesis with a significant reduction in heavy metals concentration and other organic components. Despite being amended with 0.1 N ammonia, the treated leachate level of NO3 (2.350 ± 1.077 mg/L) was reduced by 5.3 times, hence reducing further groundwater pollution from landfill leaching. The BES-treated solid waste was more stabilized as shown by a fivefold increase in surface area and can be potentially applied for leachate immobilization and bio-fortification of agricultural fields. The vector arrangement and magnitude with differences in magnitudes for both leachate and solid waste supported the on-site applicability of BES treatment. Concerning the affinity in various treatment systems, the dendrogram clearly showed Ca and Fe placed far from each other (3506.08), in comparison to Fe and Mg (1186.6), followed by Fe and Cu (1544.6). Voltammograms proved the efficacy of the enriched electrochemically active bacteria (EAB), to support the treatment of landfill solid waste and leachate sustainably.

Weak electro-stimulation promotes microbial uranium removal: Efficacy and mechanisms

Removal and recovery of uranium from uranium-mine wastewater is beneficial to environmental protection and resource preservation. Reduction of soluble hexavalent U (U(VI)) to insoluble tetravalent uranium (U(IV)) by microbes is a plausible approach for this purpose, but its practical implementation has long been restricted by its intrinsic drawbacks. The electro-stimulated microbial process offers promise in overcoming these drawbacks. However, its applicability in real wastewater has not been evaluated yet, and its U(VI) removal mechanisms remain poorly understood. Herein, we report that introducing a weak electro-stimulation considerably boosted microbial U(VI) removal activities in both synthetic and real wastewater. The U(VI) removal has proceeded via U(VI)-to-U(IV) reduction in the biocathode, and the electrochemical characterization demonstrates the crucial role of the electroactive biofilm. Microbial community analysis shows that the broad biodiversity of the cathode biofilm is capable of U(VI) reduction, and the molecular ecological network indicates that synthetic metabolisms among electroactive and metal-reducing bacteria play major roles in electro-microbial-mediated uranium removal. Metagenomic sequencing elucidates that the electro-stimulated U(VI) bioreduction may proceed via e-pili, extracellular electron shuttles, periplasmic and outer membrane cytochrome, and thioredoxin pathways. These findings reveal the potential and mechanism of the electro-stimulated U(VI) bioreduction system for the treatment of U-bearing wastewater.

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.

Efficient recovery of dissolved Fe(II) from near neutral pH Fenton via microbial electrolysis

Fe(II) regeneration from ferric sludge via a biocathode and citrate system has recently been proposed to avoid iron-sludge accumulation and iron consumption in homogeneous Fenton treatments. However, poor regeneration rate of Fe(II) from ferric sludge at a near-neutral pH, without an iron-complexing agent, limited its wider practical application. Here, a biocathode augmented with Geobacter sulfurreducens hosted by a microbial electrolysis cell was developed to efficiently regenerate dissolved Fe(II) from ferric sludge at near-neutral pH levels, without using iron-complexing agents. In the Geobacter sulfurreducens-rich biocathode without complexing agents, the regeneration rate of dissolved Fe(II) increased three-fold compared with the biocathode before inoculating Geobacter sulfurreducens. The highest concentration of dissolved Fe(II) increased from 45 mg Fe/L to 199 mg Fe/L at pH 6 when 0.5 V of voltage was applied. Furthermore, 84 mg Fe/L of dissolved Fe(II) was successfully regenerated from ferric sludge during the 123 days' operation of flow-through biocathode. Finally, the regenerated Fe(II) solution without organic matters was successfully applied in a near-neutral pH Fenton treatment to remove recalcitrant pollutants. This Geobacter sulfurreducens-rich biocathode, with its low chemical consumption, high regeneration rate and feasibility for continuous flow operation, offers a more efficient method to realize iron-free in homogeneous Fenton treatments.

Catalysing electrowinning of copper from E-waste: A critical review

Smart technologies and digitalisation have increased the consumption of scarce metals that threaten the sustainability of intricated industries. Additionally, the growing streams of waste electrical and electronic equipment (e-waste) are significant hazards to public health and the environment. Thus, there is an escalating need to recover metals from e-waste for sustainable management of metal resources. Hydrometallurgical processing of e-waste, involving copper (Cu) leaching and its subsequent recovery from pregnant leach solution (PLS) via electrowinning, has emerged as an efficient strategy to close the recycling loop. Electrowinning from PLS demonstrated higher Cu recovery efficiency and operational feasibility with a lower reagent use and lesser waste generation. Nevertheless, multiple issues challenged its practical implementation, including selective recovery of Cu from PLS containing mixed metals and high energy consumption. This review (1) identifies the factors affecting Cu electrowinning from PLS; (2) evaluates the composition of lixiviants influencing Cu electrowinning; (3) appraises various catalysts developed for enhancing Cu electrodeposition; and (4) reviews coupled systems that minimised process energy consumption. From the literature review, electrocatalysts are prospective candidates for improving Cu electrowinning as they reduced the cathodic reduction overpotentials, enhanced surface reaction kinetics and increased current efficiency. Other catalysts, including bioelectrocatalysts and photoelectrocatalysts, are applicable for dilute electrolytes with further investigations required to validate their feasibility. The coupled systems, including slurry electrolysis, bioelectrochemical systems and coupled redox fuel cells, minimise process energy requirements by systematically coupling the cathodic reduction reaction with suitable anodic oxidation reactions having thermodynamically low overpotentials. Among these systems, slurry electrolysis utilising a single-step processing of e-waste is feasible for commericalisation though operational challenges must be addressed to improve its sustainability. The other systems require further studies to improve their scalability. It provides an important direction for energy-efficient Cu electrowinning from PLS, ultimately promoting a circular economy for the scarce metal resources.

A sodium hyposulfite fuel cell for efficient Cr(VI) removal

This work shows a strategy of reducing hexavalent chromium (Cr(VI)) by sodium hyposulfite (Na₂S₂O₃) with self-generated electricity via a dual-chamber non-biological fuel cell (D-nBFC). Therein, Na₂S₂O₃ was electro-oxidized on graphite felt (GF) at anode and Cr(VI) in strong acidic solution was electro-reduced at GF/CCP cathode (GF decorated with conductive carbon paint (CCP)). Additionally, an agar salt bridge, consisting of saturated KCl solution, was introduced to form complete circuit by offering ions. The results showed that Cr(VI) was reduced to trivalent chromium (Cr(III)) and the D-nBFC system could produce electricity in this process. This system could obtain a high Cr(VI) removal efficiency (97.0%), 110 μA maximum current, and 13.4 mW m^-2 maximum power density in 4 h. In addition, the proposed system had high reusability after five cycles and the relative standard deviation was only 3.4% (n = 5). Thus, this D-nBFC system provides a promising and eco-friendly method for treatment of Cr(VI) pollution and generating electricity simultaneously, and also has potential application value for other heavy metals remediation.

Electrified bioreactors: the next power-up for biometallurgical wastewater treatment

Over the past decades, biological treatment of metallurgical wastewaters has become commonplace. Passive systems require intensive land use due to their slow treatment rates, do not recover embedded resources and are poorly controllable. Active systems however require the addition of chemicals, increasing operational costs and possibly negatively affecting safety and the environment. Electrification of biological systems can reduce the use of chemicals, operational costs, surface footprint and environmental impact when compared to passive and active technologies whilst increasing the recovery of resources and the extraction of products. Electrification of low rate applications has resulted in the development of bioelectrochemical systems (BES), but electrification of high rate systems has been lagging behind due to the limited mass transfer, electron transfer and biomass density in BES. We postulate that for high rate applications, the electrification of bioreactors, for example, through the use of electrolyzers, may herald a new generation of electrified biological systems (EBS). In this review, we evaluate the latest trends in the field of biometallurgical and microbial-electrochemical wastewater treatment and discuss the advantages and challenges of these existing treatment technologies. We advocate for future research to focus on the development of electrified bioreactors, exploring the boundaries and limitations of these systems, and their validity upon treating industrial wastewaters.

Physiological metabolism of electrochemically active bacteria directed by combined acetate and Cd(II) in single-chamber microbial electrolysis cells

It is of great interest to explore physiological metabolism of electrochemically active bacteria (EAB) for combined organics and heavy metals in single-chamber microbial electrolysis cells (MECs). Four pure culture EAB varying degrees responded to the combined acetate (1.0-5.0 g/L) and Cd(II) (20-150 mg/L) at different initial concentrations in the single-chamber MECs, shown as significant relevance of Cd(II) removal (2.57-7.35 mg/L/h) and H₂ production (0-0.0011 m³/m³/h) instead of acetate removal (73-130 mg/L/h), to these EAB species at initial Cd(II) below 120 mg/L and initial acetate below 3.0 g/L. A high initial acetate (5.0 g/L) compensated the Cd(II) inhibition and broadened the removal of Cd(II) to 150 mg/L. These EAB physiologically released variable amounts of extracellular polymeric substances with a compositional diversity in response to the changes of initial Cd(II) and circuital current whereas the activities of typical intracellular enzymes were more apparently altered by the initial Cd(II) than the circuital current. These results provide experimental validation of the presence, the metabolic plasticity and the physiological response of these EAB directed by the changes of initial Cd(II) and acetate concentrations in the single-chamber MECs, deepening our understanding of EAB physiological coping strategies in metallurgical microbial electro-ecological cycles.

"In-situ synthesized" iron-based bimetal promotes efficient removal of Cr(VI) in by zero-valent iron-loaded hydroxyapatite

Anionic Cr(VI) and cationic heavy metals generally co-exist in industrial effluents and threaten the public health. Zero-valent iron (ZVI) particles tent to passivate rapidly, which results in a gradual drop in its reactivity. In this work, a strategy of "in-situ synthesized" iron-based bimetal was first developed to stimulate the self-activation of passivated ZVI. During this process, ZVI-loaded hydroxyapatite (ZVI/HAP) was prepared to enhance the affinity for co-existing Cu²⁺, which promoted the in-situ Cu⁰ deposition on ZVI/HAP to form a Fe-Cu bimetal. The deposited Cu⁰ significantly decreased the activation energy (E_a) of Cr(VI) reduction by 24.9%, and its corresponding Cr(VI) removal (96.53%) was much higher that of single Cr(VI) system (68.67%) within 9 h. More importantly, the removal of Cr(VI) and Cu²⁺ were synchronously achieved. Systematical electrochemical characterizations were first introduced to explore the galvanic behaviors of iron-based bimetal. The charge transfer resistance and the negative open circuit potential of ZVI/HAP significantly decreased with the Cu⁰ deposition, thereby accelerating the electron transfer from Fe⁰ to Cu²⁺. The enhanced electron transfer further facilitated the Fe(II) release to promote Cr(VI) reduction. This "in-situ synthesized" iron-based bimetal strategy provides a novel pattern for ZVI activation and exhibits practical application in remediation of combined contaminant.

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.

Regeneration of Fe(II) from Fenton-derived ferric sludge using a novel biocathode

Fenton reactions are widely applied when degrading recalcitrant pollutants, but reusing the resulting ferric sludge remains a challenge. A novel concept for regenerating Fe(II) solution at pH 6 based on ferric sludge from neutral Fenton was herein proposed. The microbial fuel cell (MFC) with biocathode and citric acid was used for the first time to promote the regenerated rate of Fe(II) from ferric sludge. The concentration of dissolved Fe(II) reached 120 mg/L in biocathode, which was much higher than that obtained in abiotic cathode (<1 mg/L). The main chemical cost of regenerating Fe(II) was only 3.3% of the commercial Fe(II). Subsequently, the regenerated Fe(II) solution was used to activate H₂O₂, to remove pharmaceuticals from the municipal wastewater effluent. A wide range of pharmaceuticals was successfully removed at neutral pH in 60 min, and the efficiency of the treatment was similar to when the same dosage of commercial Fe(II) was applied.

Intermittent electro field regulated mutualistic interspecies electron transfer away from the electrodes for bioenergy recovery from wastewater

Lately, extracellular electron transfer (EET) is widely disclosed on the surface of the bioelectrodes, and conductive (bio)carriers involved in anaerobic biodegradation/biosynthesis. By electrostimulation, microbial consortia colonize the electrodes and accelerate substrate (waste/wastewater) metabolization on the bioanode or biosynthesize value-added products (methane, acetate, etc.) on the biocathode. However, the connections and contributions of planktonic microbial communities have not been effectually understood. Herein, electromethanogenesis were comprehensively investigated in response to different driving-force modes: intermittent electric field applied by manual on-off or natural solar power and continuous electrical field. Intermittent modes implied preferable electron transfer efficiency, higher methane yield and energy recovery efficiencies from wastewater by the microbes in the bulk solutions. Microbial community analysis revealed that less electroactive microorganisms and acetotrophic methanogens in the bulk solutions were accommodated under the intermittent modes than the continuous electrical field, whereas more fermentative bacteria and hydrogenotrophic methanogens evolved in the intermittent driving modes, implying that the interspecies electron transfer both on and away from the electrodes were favorably regulated. Redundancy and network analysis proved that more complicated ecological interactions were shown in the bulk solutions with the periodic on/off of electrical field. These results hinted that the electrostimulation effectively regulated EET bacteria, even in the bulk solutions, while more efficient electron flow to methane through interspecies electron transfer was developed during the intermittent driving regulation.

Removal mechanisms of Cr(VI) and Cr(III) by biochar supported nanosized zero-valent iron: Synergy of adsorption, reduction and transformation

In this study, sludge-derived biochar was prepared and utilized to support nano-zero-valent iron (NZVI-SDBC) for removing Cr(VI) and Cr(III) from aqueous solution with the aim of investigating their removal and transformation. Under the conditions of initial pH of 4, dosage of 1 g/L, temperature of 25 °C, and rotational speed of 160 rpm, 64.13% Cr species could be removed by NZVI-SDBC from Cr(VI) solution and 28.89% from Cr(III) solution. Coexisting ions experiments showed that Cu(II) and humic acids dramatically affected the removal of Cr(VI) and Cr(III), while the effect of Na(I) and Ca(II) was almost negligible. Based on this, through the coexistence and pre-loaded Cr(III) experiments, the conversion from Cr(VI) to Cr(III) was demonstrated to enhance the further attraction on Cr(VI) and promote the subsequent removal of Cr(VI). The SDBC of NZVI-SDBC could serve as electron shuttle mediator to facilitate the electron transfer between adsorbed Cr(VI) and NZVI for ortho-reduction. The transformation and removal mechanisms were further discussed by various characterizations. The kinetics of Cr(VI) removal suggested that the removal process of Cr(VI) could be divided into three phases dominated by different mechanisms (adsorption, direct/ortho reduction, electrostatic attraction), in which Cr(VI) and Cr(III) showed different behaviors of interaction. The removal of Cr(III) mainly depended on sufficient adsorption sites and the direct complexation with Fe(II). Finally, the reusability of NZVI-SDBC was assessed by adsorption/desorption recycling test. These results provided new insights into the removal and transformation mechanisms of Cr(VI) and Cr(III) by biochar-based nanocomposites.

Multi-component design and in-situ synthesis of visible-light-driven SnO2/g-C3N4/diatomite composite for high-efficient photoreduction of Cr(VI) with the aid of citric acid

A novel ternary SnO₂/g-C₃N₄/diatomite (SCN/DE) nanocomposite was rationally designed and successfully synthesized via a two-step method with in-situ polymerization and self-assembling. Under visible light illumination, the resulting SCN/DE composite exhibited superior photocatalytic performance and good reusability for the photoreduction of Cr(VI) to Cr(III) in the presence of citric acid, the apparent rate constant of SCN/DE composite was up to around 22.68 times, 13.53 times and 8.65 times as much as those of g-C₃N₄ (CN), g-C₃N₄/diatomite (CN/DE) and SnO₂/g-C₃N₄ (SCN) composites, respectively. The citric acid is a co-catalyst (chelating agent) rather than a reactant during the reactive process. Besides, the intimate interface contact and ternary heterogeneous structure were established among the SnO₂, g-C₃N₄ and diatomite. The induced positive charged surface of diatomite should be the key factor in enhancing photoactivity of the resultant SCN/DE composite, which significantly accelerated the charge separation of photogenerated electron-hole pairs as well as improved the adsorption performance towards Cr (VI). In particular, a possible reduction pathway of Cr(VI) to Cr(III) by SCN/DE composite with the assistance of citric acid was first investigated and proposed. This work provides a novel strategy for synthesizing highly efficient mineral-based photocatalysts with great promising application foreground for Cr(VI)-containing wastewater treatment.

Quantitative evaluation of effects of different cathode materials on performance in Cd(II)-reduced microbial electrolysis cells

Three materials including stainless steel woven mesh (SSM), nickel foam (NF) and carbon cloth (CC) were conducted as cathode in Cd(II)-reduced microbial electrolysis cells (MECs), respectively. By using electrode potential slope (EPS) method, the experimental open circuit potentials of three cathodes were similar, while the SSM cathode showed the smallest resistance (6 ± 1 mΩ m²), following by NF cathode (18 ± 2 mΩ m²) and CC cathode (32 ± 5 mΩ m²). These values were analyzed to predicte higher current density and more positive cathode potential in the MEC with SSM cathode under subsequent operating conditions. Electrochemical performance was more likely to be limited by current density than cathode potential. Accordingly, the MEC with SSM cathode obtained better system performance than that with other cathodes. This study further expands the application of EPS method that quantitatively evaluating and effectively selecting cathode materials for better system performance in Cd(II)-reduced MECs.

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

A facultative exoelectrogen strain Lsc-8 belonging to the Cellulomonas genus with the ability to degrade carboxymethyl cellulose (CMC) coupled with the reduction of Cr(VI), was successfully isolated from rumen content. The maximum output power density of the microbial fuel cells (MFCs) inoculated strain Lsc-8 was 9.56 ± 0.37 mW·m^-2 with CMC as the sole carbon source. From the biomass analysis it can be seen that the electricity generation of the MFCs was primarily attributed to the planktonic cells of strain Lsc-8 rather than the biofilm attached on the electrode, which was different from Geobacter sulfurreducens. Especially, during electricity generation of the MFCs using CMC as carbon source in the anode chamber, the Cr(VI) reduction were simultaneously realized. And it is also found that the Cr(VI) reduction ratio by strain Lsc-8 is directly related to the initial Cr(VI) concentration, and it increased with the increase of initial Cr(VI) concentration at first, then started to decrease when the Cr(VI) concentration was above 21 mg ·L^-1. Meanwhile, the highest output power density of 3.47 ± 0.28 mW·m^-2 was observed coupling with 95.22 ± 2.72 % of Cr(VI) reduction. These data suggested that the strain Lsc-8 could reduce high toxicity Cr(VI) to low toxicity Cr(III) coupled with electricity generation in MFCs with CMC as the carbon source. Our results also suggested that this study will provide a possibility to simultaneously degrade Cr(VI) and generate electricity by using cellulose as the carbon source via MFCs.

Bifunctional cathode using a biofilm and Pt/C catalyst for simultaneous electricity generation and nitrification in microbial fuel cells

Biofouling frequently causes catalyst deterioration at the cathode of microbial fuel cells (MFCs). A biofilm-covered Pt/C cathode (BPC) was fabricated via in situ cultivation of a biofilm on a Pt/C cathode (PC) in a dual-chambered MFC, which enables effective removal of NH4+-N and copious generation of electricity. Experimental results show 99% NH4+-N removal by the nitrifying bacteria that constitute 35.7% of all microorganisms on the BPC and a maximum BPC-MFC power density of 0.97 W/m2, which is comparable to that of PC-MFCs (0.99 W/m2). BPC biofilm size is restricted by the limited amount of organic material in the cathode chamber, which constrains the biomass to less than 0.3 g protein /m2. The bifunctional-cathode equipped MFC shows great promise as an energy-saving technology for wastewater treatment in the future.

In situ groundwater remediation with bioelectrochemical systems: A critical review and future perspectives

Groundwater contamination is an ever-growing environmental issue that has attracted much and undiminished attention for the past half century. Groundwater contamination may originate from both anthropogenic (e.g., hydrocarbons) and natural compounds (e.g., nitrate and arsenic); to tackle the removal of these contaminants, different technologies have been developed and implemented. Recently, bioelectrochemical systems (BES) have emerged as a potential treatment for groundwater contamination, with reported in situ applications that showed promising results. Nitrate and hydrocarbons (toluene, phenanthrene, benzene, BTEX and light PAHs) have been successfully removed, due to the interaction of microbial metabolism with poised electrodes, in addition to physical migration due to the electric field generated in a BES. The selection of proper BESs relies on several factors and problems, such as the complexity of groundwater and subsoil environment, scale-up issues, and energy requirements that need to be accounted for. Modeling efforts could help predict case scenarios and select a proper design and approach, while BES-based biosensing could help monitoring remediation processes. In this review, we critically analyze in situ BES applications for groundwater remediation, focusing in particular on different proposed setups, and we identify and discuss the existing research gaps in the field.

Understanding the interdependence of strain of electrotroph, cathode potential and initial Cu(II) concentration for simultaneous Cu(II) removal and acetate production in microbial electrosynthesis systems

Metallurgical microbial electrosynthesis systems (MES) are holding great promise for simultaneous heavy metal removal and acetate production from heavy metal-contaminated and organics-barren waters. How critical parameters of strain of electrotroph, cathode potential and initial heavy metal concentration affect MES performance, however, is not yet fully understood. Heavy metal of Cu(II) and four Cu(II)-tolerant electrotrophs (Stenotrophomonas maltophilia JY1, Citrobacter sp. JY3, Pseudomonas aeruginosa JY5 and Stenotrophomonas sp. JY6) were employed to evaluate MES performance at various cathode potentials (-900 or -600 mV vs. standard hydrogen electrode) and initial Cu(II) concentrations (60-120 mg L^-1). Each electrotrophs exhibited incremental Cu(II) removals with increased Cu(II) at -900 mV, higher than at -600 mV or in the abiotic controls. Acetate production by JY1 and JY6 decreased with the increase in initial Cu(II), compared to an initial increase and a decrease thereafter for JY3 and JY5. For each electrotrophs, the biofilms than the planktonic cells released more amounts of extracellular polymeric substances (EPS) with a compositional diversity and stronger Cu(II) complexation at -900 mV. These were higher than at -600 mV, or in the controls either under open circuit conditions or in the absence of Cu(II). This work demonstrates the interdependence of strain of electrotroph, cathode potential and initial Cu(II) on simultaneous Cu(II) removal and acetate production through the release of different amounts of EPS with diverse composites, contributing to enhancing the controlled MES for efficient recovery of value-added products from Cu(II)-contaminated and organics-barren waters.

Microbial Assisted Hexavalent Chromium Removal in Bioelectrochemical Systems

Groundwater is the environmental matrix that is most frequently affected by anthropogenic hexavalent chromium contamination. Due to its carcinogenicity, Cr(VI) has to be removed, using environmental-friendly and economically sustainable remediation technologies. BioElectrochemical Systems (BESs), applied to bioremediation, thereby offering a promising alternative to traditional bioremediation techniques, without affecting the natural groundwater conditions. Some bacterial families are capable of oxidizing and/or reducing a solid electrode obtaining an energetic advantage for their own growth. In the present study, we assessed the possibility of stimulating bioelectrochemical reduction of Cr(VI) in a dual-chamber polarized system using an electrode as the sole energy source. To develop an electroactive microbial community three electrodes were, at first, inserted into the anodic compartment of a dual-chamber microbial fuel cell, and inoculated with sludge from an anaerobic digester. After a period of acclimation, one electrode was transferred into a polarized system and it was fixed at −0.3 V (versus standard hydrogen electrode, SHE), to promote the reduction of 1000 µg Cr(VI) L−1. A second electrode, served for the set-up of an open circuit control, operated in parallel. Cr(VI) dissolved concentration was analysed at the initial, during the experiment and final time by spectrophotometric method. Initial and final microbial characterization of the communities enriched in polarized system and open circuit control was performed by 16S rRNA gene sequencing. The bioelectrode set at −0.3 V showed high Cr(VI) removal efficiency (up to 93%) and about 150 µg L−1 day−1 removal rate. Similar efficiency was observed in the open circuit (OC) even at about half rate. Whereas, purely electrochemical reduction, limited to 35%, due to neutral operating conditions. These results suggest that bioelectrochemical Cr(VI) removal by polarized electrode offers a promising new and sustainable approach to the treatment of groundwater Cr(VI) plumes, deserving further research.

Volatile organic compounds in the salt-lake sediments of the Tibet Plateau influence prokaryotic diversity and community assembly

Volatile organic compounds (VOCs) are important environmental factors because they supply nutrients for microbial cells and mediate intercellular interactions. However, few studies have focused on the effects of VOCs on prokaryotic diversity and community composition. In this study, we examined the relationship between prokaryotic diversity and community composition and the content of VOCs in salt-lake sediments from the Tibet Plateau using amplicon sequencing of the 16S rRNA gene. Results showed that the alpha-diversity indices (Chao1, Shannon, and Simpson) were generally negatively correlated with the content of 36 VOCs (P < 0.05). The prokaryotic communities were significantly driven by multiple VOCs at the lineage-dependent pattern (P < 0.05). Further analysis indicated that VOCs, including 3-methylpyruvate, biuret, isocitric acid, and stearic acid, jointly explained 37.3% of the variations in prokaryotic communities. Supplemental VOCs-pyruvate, biuret, alanine, and aspartic acid-notably decreased the Chao1 and Shannon indices and significantly assembled co-occurrence networks for the bacterial communities in the saline sediments. Together, these results demonstrated that VOCs play a critical role in the regulation of the diversity, compositions, and network structures of prokaryotic communities in saline sediments.

Recent advances in the recovery of metals from waste through biological processes

Wastes containing critical metals are generated in various fields, such as energy and computer manufacturing. Metal-bearing wastes are considered as secondary sources of critical metals. The conventional physicochemical methods of metals recovery are energy-intensive and cause further pollution. Low-cost and eco-friendly technologies including biosorbents, bioelectrochemical systems (BESs), bioleaching, and biomineralization, have become alternatives in the recovery of critical metals. However, a relatively low recovery rate and selectivity severely hinder their large-scale applications. Researchers have expanded their focus to exploit novel strain resources and strategies to improve the biorecovery efficiency. The mechanisms and potential applicability of modified biological techniques for improving the recovery of critical metals need more attention. Hence, this review summarize and compare the strategies that have been developed for critical metals recovery, and provides useful insights for energy-efficient recovery of critical metals in future industrial applications.

Progress Towards Bioelectrochemical Remediation of Hexavalent Chromium

Chromium is one of the most frequently used metal contaminants. Its hexavalent form Cr(VI), which is exploited in many industrial activities, is highly toxic, is water-soluble in the full pH range, and is a major threat to groundwater resources. Alongside traditional approaches to Cr(VI) treatment based on physical-chemical methods, technologies exploiting the ability of several microorganisms to reduce toxic and mobile Cr(VI) to the less toxic and stable Cr(III) form have been developed to improve the cost-effectiveness and sustainability of remediating hexavalent chromium-contaminated groundwater. Bioelectrochemical systems (BESs), principally investigated for wastewater treatment, may represent an innovative option for groundwater remediation. By using electrodes as virtually inexhaustible electron donors and acceptors to promote microbial oxidation-reduction reactions, in in situ remediation, BESs may offer the advantage of limited energy and chemicals requirements in comparison to other bioremediation technologies, which rely on external supplies of limiting inorganic nutrients and electron acceptors or donors to ensure proper conditions for microbial activity. Electron transfer is continuously promoted/controlled in terms of current or voltage application between the electrodes, close to which electrochemically active microorganisms are located. Therefore, this enhances the options of process real-time monitoring and control, which are often limited in in situ treatment schemes. This paper reviews research with BESs for treating chromium-contaminated wastewater, by focusing on the perspectives for Cr(VI) bioelectrochemical remediation and open research issues.

Improved chromium reduction and removal from wastewater in continuous flow bioelectrochemical systems

Bioelectrochemical systems (BESs) including microbial electrolysis cells (MECs) and microbial fuel cells (MFCs) are promising for hexavalent chromium [Cr(VI)] reduction and total chromium (Cr) removal from wastewater. This study assessed the performance of simple, inexpensive, and continuous flow BESs with neither cathode catalyst nor proton exchange membrane for Cr(VI) reduction and total Cr removal. The effect of bioreactor configuration and wastewater feed mode on the performance of the BESs was investigated. Biological Cr(VI) reduction in the MEC followed a first-order kinetics with a rate constant of 0.103 d^-1, significantly higher than that of the control (0.033 d^-1). For comparison, the first-order reduction rate constants in the MFCs with the Cr(VI) fed to the anodic and the cathodic zones were 0.072 and 0.064 d^-1, respectively. The BESs improved total Cr removal through coprecipitating Cr(III) and phosphors as evidenced from the scanning electron microscopy energy-dispersive X-ray spectroscopy analysis. The total Cr removal efficiencies in the control, MFCs, and MEC were 26.1%, 56.7%, and 66.2%, respectively. Only 25.1% to 26.7% of total Cr was present intracellularly in the BESs (both MFCs and MEC), whereas 31.8% ± 1.4% and 38.0% ± 0.9% of total Cr in the anodic and cathodic zones of the control were present intracellularly. Overall, the BESs demonstrated a great potential to reduce Cr(VI) and remove total Cr with the MEC having the fastest Cr(VI) reduction and most efficient total Cr removal. Furthermore, the BESs significantly reduced the intracellular total Cr content.

Bioelectrochemical assisted dechlorination of tetrachloroethylene and 1,2-dichloroethane by acclimation of anaerobic sludge

Volatile chlorinated hydrocarbons (VCHs) are often found as a type of persistent and ubiquitous contaminant in groundwater. The feasibility, characteristics and microbial mechanism of acclimation of biodiversity-rich inoculation source for bioelectrochemical stimulated VCH dechlorination remain poorly understood. Here, the superior bioelectrochemical catalytic activities were observed for tetrachloroethylene (0.26 mM d^-1) and 1,2-dichloroethane (2.20 mM d^-1) dechlorination in anaerobic sludge-acclimated biocathodes with an optimal potential of -0.5 V, averaging 1.60-2.71 times higher than those reported in previous works on biocathodes. When the cathode was applied as the sole electron donor for dechlorination, columbic efficiencies reached the values greater than 80%. Tetrachloroethylene dechlorination showed a metabolic pathway with cis-1,2-dichloroethene as the main product, whereas 1,2-dichloroethane was dechlorinated entirely to the nontoxic ethene. The cathodic biofilms were highly abundant with the dechlorination and electro-active genera, while significant bacterial consortium variation was observed in response to the different VCH types and changes in cathodic potential. Bacillus, Pseudomonas and Lactococcus were mostly enriched for tetrachloroethylene dechlorination, and pceA, tceA and omcX were highly expressed. Geobacter was the most predominant during 1,2-dichloroethane dechlorination with rdhA, tceA and omcX highly expressed. In addition, although the impact of cathodic potentials was weaker than that of VCH types, the lower cathodic potentials, the more abundant of the electrode respiring populations and the higher expression of extracellular electron transfer related gene. This study demonstrated the great potential of acclimation of anaerobic sludge by electrical stimulation for accelerating VCH remediations and gave insights into its working molecular mechanisms.

Electrosynthesis of acetate from inorganic carbon (HCO3-) with simultaneous hydrogen production and Cd(II) removal in multifunctional microbial electrosynthesis systems (MES)

The simultaneous production of acetate from bicarbonate (from CO₂ sequestration) and hydrogen gas, with concomitant removal of Cd(II) heavy metal in water is demonstrated in multifunctional metallurgical microbial electrosynthesis systems (MES) incorporating Cd(II) tolerant electrochemically active bacteria (EAB) (Ochrobactrum sp. X1, Pseudomonas sp. X3, Pseudomonas delhiensis X5, and Ochrobactrum anthropi X7). Strain X5 favored the production of acetate, while X7 preferred the production of hydrogen. The rate of Cd(II) removal by all EAB (1.20-1.32 mg/L/h), and the rates of acetate production by X5 (29.4 mg/L/d) and hydrogen evolution by X7 (0.0187 m³/m³/d) increased in the presence of a circuital current. The production of acetate and hydrogen was regulated by the release of extracellular polymeric substances (EPS), which also exhibited invariable catalytic activity toward the reduction of Cd(II) to Cd(0). The intracellular activities of glutathione (GSH), catalase (CAT), superoxide dismutase (SOD) and dehydrogenase were altered by the circuital current and Cd(II) concentration, and these regulated the products distribution. Such understanding enables the targeted manipulation of the MES operational conditions that favor the production of acetate from CO₂ sequestration with simultaneous hydrogen production and removal/recovery of Cd(II) from metal-contaminated and organics-barren waters.

Reduction of Cu(II) and simultaneous production of acetate from inorganic carbon by Serratia Marcescens biofilms and plankton cells in microbial electrosynthesis systems

Simultaneous Cu(II) reduction (6.42 ± 0.02 mg/L/h), acetate production (1.13 ± 0.02 mg/L/h) from inorganic carbon (i.e., CO₂ sequestration), and hydrogen evolution (0.0315 ± 0.0005 m³/m³/d) were achieved in a Serratia marcescens Q1 catalyzed microbial electrosynthesis system (MES). The biofilms released increasing amounts of extracellular polymeric substances (EPS) with a higher compositional diversity and stronger Cu(II) complexation, compared to the plankton cells, at higher Cu(II) concentrations (up to 80 mg/L) and circuital currents (cathodic potential of -900 mV vs. standard hydrogen electrode (SHE)). Moreover, the biofilms reduced Cu(II) to Cu(0) more effectively than the plankton cells. At Cu(II) concentrations below 80 mg/L, the dehydrogenase activity in the biofilms was higher than in the plankton cells, and increased with circuital current, which was converse to the lower activities of catalase (CAT), superoxide dismutase (SOD) and antioxidative glutathione (GSH) in the biofilms than the plankton cells, although all these physiological activities were positively correlated with the concentration of Cu(II). This is the first study that evaluates the EPS constituents and the physiological activities of the biofilms and the plankton cells in the MESs, that favors the production of acetate from CO₂ sequestration and the simultaneous reduction of Cu(II) from organics-barren waters contaminated with heavy metals.

Response of antibiotic resistance to the co-existence of chloramphenicol and copper during bio-electrochemical treatment of antibiotic-containing wastewater

Concerns have been raised regarding co-selection for antibiotic resistance among microorganisms exposed to antibiotics and metals. As a promising approach for treating antibiotics and heavy metal-containing wastewater, a bio-electrochemical system (BES) can be used for antibiotic and heavy metal removal. This study determined the fate of antibiotic resistance genes (ARGs) in a BES when exposed to chloramphenicol (CAP) and Cu²⁺. The ARGs encoding the efflux pump (cmlA, floR, and tetC), the class 1 integron integrase-encoding gene, and the sul1 gene were analyzed. The results indicated that the co-existence of CAP and different concentrations of Cu²⁺ had significant influence on the relative abundances of the ARGs. The changes in the bacterial community structure and the results of a quantitative correlation analysis between the bacterial community and the ARGs confirmed that the shift in the potential hosts was the key reason for the changes of the ARGs. This study sheds new light on the mechanisms of ARGs variations in BES under the co-selection pressure of antibiotics and heavy metals.

A loop of catholyte effluent feeding to bioanodes for complete recovery of Sn, Fe, and Cu with simultaneous treatment of the co-present organics in microbial fuel cells

A loop of catholyte effluent feeding to the bioanodes of air-cathode microbial fuel cells (MFCs) achieved complete recovery of mixed Sn(II), Fe(II) and Cu(II), with simultaneous treatment of the co-present organics in synthetic wastewater of printed circuit boards (PrCBs). This in-situ utilization of caustic in the cathodes and the neutralization of acid in the anodes achieved superior metal recovery performance at an optimal hydraulic retention time (HRT) of 24 h. Cathode chambers primarily removed Sn of 91 ± 4% (bottom: 74 ± 3%; electrode: 17 ± 1%), Fe of 89 ± 8% (bottom: 64 ± 4%; electrode: 25 ± 2%), and Cu of 92 ± 7% (electrode: 63 ± 5%; bottom: 29 ± 1%), compared to Sn of 9 ± 3% (electrode: 7 ± 1%; bottom: 2 ± 1%), Fe of 9 ± 3% (electrode: 8 ± 3%; bottom: 1 ± 0%), and Cu of 7 ± 3% (electrode: 4 ± 1%; bottom: 3 ± 1%) in the bioanodes. Bacterial communities on the anodes were well evolutionarily developed after the feeding of catholyte effluent, with the increase in abundance of Rhodopseudomonas and Geobacter, and the shift from Thiobacillus and Acinetobacter to Pseudomonas, Comamonas, Aeromonas and Azospira. This loop of cathodic effluent feeding to the bioanodes of MFCs may represent a unique method for complete metal recovery with simultaneous extraction of renewable electrical energy from the co-present organics. This study also offers new insights into the development of compact microbial electro-metallurgical processes for simultaneous recovery of value-added products from PrCBs processing wastewaters and accomplishing the national wastewater discharge standard for both metals and organics.

A novel combination of bioelectrochemical system with peroxymonosulfate oxidation for enhanced azo dye degradation and MnFe2O4 catalyst regeneration

Advanced oxidation process (AOP) based on peroxymonosulfate (PMS) activation was established in microbial fuel cell (MFC) system with MnFe₂O₄ cathode (MFC-MnFe₂O₄/PMS) aimed to enhance azo dye degradation and catalyst regeneration. The effects of loading amount of MnFe₂O₄ catalyst, applied voltage, catholyte pH and PMS dosage on the degradation of Orange II were investigated. The stability of the MnFe₂O₄ cathode for successive PMS activation was also evaluated. The degradation of Orange was accelerated in the MFC-MnFe₂O₄/PMS with apparent degradation rate constant increased to 1.8 times of that in the MnFe₂O₄/PMS control. A nearly complete removal of Orange II (100 mg L^-1) was attained in the MFC-MnFe₂O₄/PMS under the optimum conditions of 2 mM PMS, 10 mg cm^-2 MnFe₂O₄ loading, pH 7-8 and 480 min reaction time. MFC driven also extended the longevity of the MnFe₂O₄ catalyst for PMS activation due to the in-situ regeneration of ≡Mn²⁺ and ≡Fe²⁺ through accepting electrons from the cathode, and over 80% of Orange II was still removed in the 7^th run. Additionally, the MFC-MnFe₂O₄/PMS system could recover electricity during Orange II degradation with a maximum power density of 206.2 ± 3.1 mW m^-2. PMS activation by MnFe₂O₄ was the primary pathway for SO₄^- generation, and SO₄^- based oxidation was the primary mechanism for Orange II degradation. MFCs driven coupled with PMS activated AOP systems provides a novel strategy for efficient and persistent azo dye degradation.

Inhibitory effect of cadmium(II) ion on anodic electrochemically active biofilms performance in bioelectrochemical systems

Cadmium(II) ion can affect the anode performance of bioelectrochemical systems (BES); however, how the presence of Cd²⁺ affect the extracellular electron transfer of anodic electrochemically active biofilms (EABs), the microbial viability and species composition of microorganism on the anode remain poorly understood. Here, we investigated the inhibitory effect of Cd²⁺ at different concentrations on the electrochemical performance and the biofilm community in mixed-culture enriched BES. The electrochemical performance of the BES was not inhibited at 2 mg L^-1 Cd²⁺, while higher concentrations of 5-20 mg L^-1 resulted in the decrease in the maximum power density, with 0.34 ± 0.01 W m^-2 at 5 mg L^-1, 0.28 ± 0.01 W m^-2 at 10 mg L^-1, and 0.17 ± 0 W m^-2 at 20 mg L^-1, respectively. When adding 30 mg/L Cd²⁺, there was almost no power output. The decline of the power output was possibly ascribed to the suppressed viability and the change of species richness as evident from confocal laser scanning microscopy and microbial community analysis. Cyclic voltammogram and electrochemical impedance spectroscopy revealed that high concentration of Cd²⁺ exceeding 5 mg L^-1 can inhibit the secretion of outer membrane cytochromes, thus reducing the electron transfer between the EABs and the anode surface. Analysis of bacterial structures showed a decrease in Geobacter accompanied by an increase in Stenotrophomonas and Azospira in response to Cd²⁺ at 10 and 20 mg L^-1. This study added to the in-depth analysis of the inhibition of Cd²⁺ on EABs, and provided new insights into the removing Cd²⁺ and organics simultaneously in BES.

Prospects in bioelectrochemical technologies for wastewater treatment

Bioelectrochemical technologies are emerging as innovative solutions for waste treatment, offering flexible platforms for both oxidation and reduction reaction processes. A great variety of applications have been developed by utilizing the energy produced in bioelectrochemical systems, such as direct electric power generation, chemical production or water desalination. This manuscript provides a literature review on the prospects in bioelectrochemical technologies for wastewater treatment, including organic, nutrients and metals removal, production of chemical compounds and desalination. The challenges and perspectives for scale-up were discussed. A technological strategy to improve the process monitoring and control based on big data platforms is also presented. To translate the viability of wastewater treatment based on bioelectrochemical technologies into commercial application, it is necessary to exploit interdisciplinary areas by combining the water/wastewater sector, energy and data analytics technologies.

Bioelectrochemical Systems for Removal of Selected Metals and Perchlorate from Groundwater: A Review

Groundwater contamination is a major issue for human health, due to its largely diffused exploitation for water supply. Several pollutants have been detected in groundwater; amongst them arsenic, cadmium, chromium, vanadium, and perchlorate. Various technologies have been applied for groundwater remediation, involving physical, chemical, and biological processes. Bioelectrochemical systems (BES) have emerged over the last 15 years as an alternative to conventional treatments for a wide variety of wastewater, and have been proposed as a feasible option for groundwater remediation due to the nature of the technology: the presence of two different redox environments, the use of electrodes as virtually inexhaustible electron acceptor/donor (anode and cathode, respectively), and the possibility of microbial catalysis enhance their possibility to achieve complete remediation of contaminants, even in combination. Arsenic and organic matter can be oxidized at the bioanode, while vanadium, perchlorate, chromium, and cadmium can be reduced at the cathode, which can be biotic or abiotic. Additionally, BES has been shown to produce bioenergy while performing organic contaminants removal, lowering the overall energy balance. This review examines the application of BES for groundwater remediation of arsenic, cadmium, chromium, vanadium, and perchlorate, focusing also on the perspectives of the technology in the groundwater treatment field.

Amendment soil with biochar to control antibiotic resistance genes under unconventional water resources irrigation: Proceed with caution

The spread of antibiotic resistance genes (ARGs) has become a cause for serious concern because of its potential risk to public health. The use of unconventional water resources (e.g., reclaimed water or piggery wastewater) in agriculture to relieve groundwater shortages may result in an accumulation of ARGs in soil. Biochar addition has been proven to be a beneficial method to alleviate the pollution of ARGs in manure-amended soil. However, the role of biochar on ARGs in soil-plant systems repeatedly irrigated with unconventional water resources is unknown. Under reclaimed water or piggery wastewater irrigation, rhizobox experiments using maize plants in soil amended with biochar were conducted to investigate the variation of typical ARGs (tet and sul genes) in soil-plant systems during a 60-day cultivation, and ARGs was characterized by high-throughput qPCR with a 48 (assays) × 108 (samples) array. Only piggery wastewater irrigation significantly increased the abundance of ARGs in rhizosphere and bulk soils and root endophytes. Following 30-day cultivation, the abundance of ARGs in soil was significantly lower due to biochar addition. However, by day 60, the abundance of ARGs in soil supplemented with biochar was significantly higher than in the control soils. Antibiotics, bio-available heavy metals, nutrients, bacterial community, and mobile gene elements (MGEs) were detected and analyzed to find factors shaping ARGs dynamics. The behavior of ARGs were associated with antibiotics but not with bio-available heavy metals. The correlation between ARGs and available phosphorus was stronger than that of ARGs with total phosphorus. MGEs had good relationship with ARGs, and MGEs shifts contributed most to ARGs variation in soil and root samples. In summary, this study provides insights into potential options for biochar use in agricultural activities.