Self-regenerable bio-hybrid with biogenic ferrous sulfide nanoparticles for treating high-concentration chromium-containing wastewater

Enhanced bioelectroremediation of heavy metal contaminated groundwater through advancing a self-standing cathode

Hexavalent chromium (Cr(VI)) contamination in groundwater poses a substantial global challenge due to its high toxicity and extensive industrial applications. While the bioelectroremediation of Cr(VI) has attracted huge attention for its eco-friendly attributes, its practical application remains constrained by the hydrogeochemical conditions of groundwater (mainly pH), low electron transfer efficiency, limitations in electrocatalyst synthesis and electrode fabrication. In this study, we developed and investigated the use of N, S co-doped carbon nanofibers (CNFs) integrated on a graphite felt (GF) as a self-standing cathode (NS/CNF-GF) for the comprehensive reduction of Cr(VI) from real contaminated groundwater. The binder free cathode, prepared through electro-polymerization, was employed in a dual-chamber microbial fuel cell (MFC) for the treatment of Cr (VI)-laden real groundwater (40 mg/L) with a pH of 7.4. The electrochemical characterization of the prepared cathode revealed a distinct electroactive surface area, more wettability, facilitating enhanced adsorption and rapid electron transfer, resulting in a commendable Cr(VI) reduction rate of 0.83 mg/L/h. The MFC equipped with NS/CNF-GF demonstrated the lowest charge transfer resistance (Rct) and generated the highest power density (155 ± 0.3 mW/m2) compared to control systems. The favorable electrokinetics for modified cathode led to swift substrate consumption in the anode, releasing more electrons and protons, thereby accelerating Cr(VI) reduction to achieve the highest cathodic coulombic efficiency (C.Eca)of80 ± 1.3 %. A similar temporal trend observed between Cr(VI) removal efficiency, COD removal efficiency, and C.Eca, underscores the effective performance of the modified electrode. The reusability of the binder free cathode, exemption from catholyte preparation and the absence of pH regulation requirements highlighted the potential scalability and applicability of our findings on a larger scale.

Biological Self-Assembled Transmembrane Electron Conduits for High-Efficiency Ammonia Production in Microbial Electrosynthesis

Usually, CymA is irreplaceable as the electron transport hub in Shewanella oneidensis MR-1 bidirectional electron transfer. In this work, biologically self-assembled FeS nanoparticles construct an artificial electron transfer route and implement electron transfer from extracellular into periplasmic space without CymA involvement, which present similar properties to type IV pili. Bacteria are wired up into a network, and more electron transfer conduits are activated by self-assembled transmembrane FeS nanoparticles (electron conduits), thereby substantially enhancing the ammonia production. In this study, we achieved an average NH4+-N production rate of 391.8 μg·h-1·L reactor-1 with the selectivity of 98.0% and cathode efficiency of 65.4%. Additionally, the amide group in the protein-like substances located in the outer membrane was first found to be able to transfer electrons from extracellular into intracellular with c-type cytochromes. Our work provides a new viewpoint that contributes to a better understanding of the interconnections between semiconductor materials and bacteria and inspires the exploration of new electron transfer chain components.

Microbial innovations in chromium remediation: mechanistic insights and diverse applications

The ubiquity of hexavalent chromium (Cr(VI)) from industrial activities poses a critical environmental threat due to its persistence, toxicity and mutagenic potential. Traditional physico-chemical methods for its removal often entail significant environmental drawbacks. Recent advancements in remediation strategies have emphasized nano and bioremediation techniques as promising avenues for cost-effective and efficient Cr(VI) mitigation. Bioremediation harnesses the capabilities of biological agents like microorganisms, and algae to mitigate heavy metal contamination, while nano-remediation employs nanoparticles for adsorption purposes. Various microorganisms, including E. coli, Byssochlamys sp., Pannonibacter phragmitetus, Bacillus, Aspergillus, Trichoderma, Fusarium, and Chlorella utilize bioreduction, biotransformation, biosorption and bioaccumulation mechanisms to convert Cr(VI) to Cr(III). Their adaptability to different environments and integration with nanomaterials enhance microbial activity, offering eco-friendly solutions. The study provides a brief overview of metabolic pathways involved in Cr(VI) bioreduction facilitated by diverse microbial species. Nitroreductase and chromate reductase enzymes play key roles in nitrogen and chromium removal, with nitroreductase requiring nitrate and NADPH/NADH, while the chromium reductase pathway relies solely on NADPH/NADH. This review investigates the various anthropogenic activities contributing to Cr(VI) emissions and evaluates the efficacy of conventional, nano-remediation, and bioremediation approaches in curbing Cr(VI) concentrations. Additionally, it scrutinizes the mechanisms underlying nano-remediation techniques for a deeper understanding of the remediation process. It identifies research gaps and offers insights into future directions aimed at enhancing the real-time applicability of bioremediation methods for mitigating with Cr(VI) pollution and pave the way for sustainable remediation solutions.

Ferrous sulfide nanoparticles can be biosynthesized by sulfate-reducing bacteria: Synthesis, characterization and removal of heavy metals from acid mine drainage

Ferrous sulfide nanoparticles (nFeS) have proven to be effective in removing heavy metals (HMs) from wastewater. One such approach, which has garnered much attention as a sustainable technology, is via the in situ microbial synthesis of nFeS. Here, a sulfate-reducing bacteria (SRB) strain, Geobacter sulfurreducens, was used to initially biosynthesize ferrous sulfide nanoparticles (SRB-nFeS) and thereafter remove HMs from acid mine drainage (AMD). SRB-nFeS was characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) coupled to an energy dispersive spectrometer (EDS), three-dimensional excitation-emission matrix (3D-EEM) spectroscopy, Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). Such characterization showed that SRB mediated the reduction of SO42- to S2- to form nFeS, where the metabolized substances functioned as complexing agents which coordinated with nFeS to form biofunctional SRB-nFeS with improved stability. One advantage of this synthetic route was that the attachment of nFeS to the bacterial surface protected SRB cells from HM toxicity. Furthermore, due to a synergistic effect between nFeS and SRB, HM removal from both solution and AMD by SRB-nFeS was enhanced relative to the constituent components. Thus, after 5 consecutive cycles of HM removal, SRB-nFeS removed, Pb(Ⅱ) (92.6%), Cd(Ⅱ) (78.7%), Cu(Ⅱ) (76.0%), Ni(Ⅱ) (62.5%), Mn(Ⅱ) (62.2%), and Zn(Ⅱ) (88.5%) from AMD This study thus provides new insights into the biosynthesis of SRB-nFeS and its subsequent practical application in the removal of HMs from AMD.

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.

Minicells as an Escherichia coli mechanism for the accumulation and disposal of fluorescent cadmium sulphide nanoparticles

BACKGROUND

Bacterial biosynthesis of fluorescent nanoparticles or quantum dots (QDs) has emerged as a unique mechanism for heavy metal tolerance. However, the physiological pathways governing the removal of QDs from bacterial cells remains elusive. This study investigates the role of minicells, previously identified as a means of eliminating damaged proteins and enhancing bacterial resistance to stress. Building on our prior work, which unveiled the formation of minicells during cadmium QDs biosynthesis in Escherichia coli, we hypothesize that minicells serve as a mechanism for the accumulation and detoxification of QDs in bacterial cells.

RESULTS

Intracellular biosynthesis of CdS QDs was performed in E. coli mutants ΔminC and ΔminCDE, known for their minicell-producing capabilities. Fluorescence microscopy analysis demonstrated that the generated minicells exhibited fluorescence emission, indicative of QD loading. Transmission electron microscopy (TEM) confirmed the presence of nanoparticles in minicells, while energy dispersive spectroscopy (EDS) revealed the coexistence of cadmium and sulfur. Cadmium quantification through flame atomic absorption spectrometry (FAAS) demonstrated that minicells accumulated a higher cadmium content compared to rod cells. Moreover, fluorescence intensity analysis suggested that minicells accumulated a greater quantity of fluorescent nanoparticles, underscoring their efficacy in QD removal. Biosynthesis dynamics in minicell-producing strains indicated that biosynthesized QDs maintained high fluorescence intensity even during prolonged biosynthesis times, suggesting continuous QD clearance in minicells.

CONCLUSIONS

These findings support a model wherein E. coli utilizes minicells for the accumulation and removal of nanoparticles, highlighting their physiological role in eliminating harmful elements and maintaining cellular fitness. Additionally, this biosynthesis system presents an opportunity for generating minicell-coated nanoparticles with enhanced biocompatibility for diverse applications.

Biological upcycling of nickel and sulfate as electrocatalyst from electroplating wastewater

Upcycling nickel (Ni) to useful catalyst is an appealing route to realize low-carbon treatment of electroplating wastewater and simultaneously recovering Ni resource, but has been restricted by the needs for costly membranes or consumption of large amount of chemicals in the existing upcycling processes. Herein, a biological upcycling route for synchronous recovery of Ni and sulfate as electrocatalysts, with certain amount of ferric salt (Fe3+) added to tune the product composition, is proposed. Efficient biosynthesis of bio-NiFeS nanoparticles from electroplating wastewater was achieved by harnessing the sulfate reduction and metal detoxification ability of Desulfovibrio vulgaris. The optimal bio-NiFeS, after further annealing at 300 °C, served as an efficient oxygen evolution electrocatalyst, achieving a current density of 10 mA·cm-1 at an overpotential of 247 mV and a Tafel slope of 60.2 mV·dec-1. It exhibited comparable electrocatalytic activity with the chemically-synthesized counterparts and outperformed the commercial RuO2. The feasibility of the biological upcycling approach for treating real Ni-containing electroplating wastewater was also demonstrated, achieving 99.5 % Ni2+removal and 41.0 % SO42- removal and enabling low-cost fabrication of electrocatalyst. Our work paves a new path for sustainable treatment of Ni-containing wastewater and may inspire technology innovations in recycling/ removal of various metal ions.

Enhancing Commercially Iron Powder Electron Transport by Surface Biosulfuration to Achieve Uranium Extraction from Uranium Ore Wastewater

The zero-valent iron (ZVI) has attracted increasing attention due to the enhanced reactivity of ZVI to uranium wastewater. However, ZVI practical application is hampered due to its susceptibility to oxidation and the formation of passivation layers during storage and in situ restoration. To address these issues, we used a biosulfuration approach to modify ZVI for application in uranium ore wastewater treatment. A series of physicochemical characterization tools and photoelectronic analyses showed that BS-ZVI considerably increased carrier separation efficiency and visible light absorption capacity, resulting in a significant photoassisted enhancement effect on uranium extraction. Accordingly, the uranium removal efficiency of BS-ZVI reached 91% within 60 min, and its maximum adsorption capacity was 336.3 mg/g. By analyzing the mechanism, the improved U(VI) removal performance was mostly responsible on the dissolution of the passivation layer on the surface of ZVI, the generation of Fe(II) and FeS, and the important role of Shewanella putrefaciens extracellular polymers (EPS). Overall, the BS-ZVI biohybrid merges with the high activity of ZVI, bio-FeS, and self-regeneration ability of bacteria, expanding a promising new approach for sustainable treatment of uranium mine wastewater.

Nanoarmor: cytoprotection for single living cells

Single cell modification or hybridization technology has become a popular direction in bioengineering in recent years, with applications in clean energy, environmental stewardship, and sustainable human development. Here, we draw attention to nanoarmor, a representative achievement of cytoprotection and functionalization technology. The fundamental principles of nanoarmor need to be studied with input from multiple disciplines, including biology, chemistry, and material science. In this review, we explain the role of nanoarmor and review progress in its applications. We also discuss three main challenges associated with its development: self-driving ability, heterojunction characteristics, and mineralization formation. Finally, we propose a preliminary classification system for nanoarmor.

Removal of toxic metals using iron sulfide particles: A brief overview of modifications and mechanisms

Growing mechanization has released higher concentrations of toxic metals in water and sediment, which is a critical concern for the environment and human health. Recent studies show that naturally occurring and synthetic iron sulfide particles are efficient at removing these hazardous pollutants. This review seeks to provide a concise summary of the evolution in the production of iron sulfide particles, specifically nanoparticles, through the years. This review presents an outline of the synthesis process for the most dominant forms of iron sulfide: mackinawite (FeS), pyrite (FeS2), pyrrhotite (Fe1-x S), and greigite (Fe3S4). The review confirms that both natural forms of iron sulfide and modified forms of iron sulfide are highly effective at removing different heavy metals and metalloids from water. Concurrently, this review reveals the interaction mechanism between toxic metals and iron sulfide, along with the impact of conditions for remedy and rectification. None the less, modifications and future investigations into the synthesis of novel iron sulfides, their use to adsorb diverse environmental pollutants, and their fate after injection into polluted aquifers, remain crucial to maximizing pollution control.

Extracellular polymeric substances sustain photoreduction of Cr(VI) by Shewanella oneidensis-CdS biohybrid system

Photosensitized biohybrid system (PBS) enables bacteria to exploit light energy harvested by semiconductors for rapid pollutants transformation, possessing a promising future for water reclamation. Maintaining a biocompatible environment under photocatalytic conditions is the key to developing PBS-based treatment technologies. Natural microbial cells are surrounded by extracellular polymeric substances (EPS) that either be tightly bound to the cell wall (i.e., tightly bound EPS, tbEPS) or loosely associated with cell surface (i.e., loosely bound EPS, lbEPS), which provide protection from unfavorable environment. We hypothesized that providing EPS fractions can enhance bacterial viability under adverse environment created by photocatalytic reactions. We constructed a model PBS consisting of Shewanella oneidensis and CdS using Cr(VI) as the target pollutant. Results showed complete removal of 25 mg/L Cr(VI) within 90 min without an electron donor, which may mainly rely on the synergistic effect of CdS and bacteria on photoelectron transfer. Long-term cycling experiment of pristine PBS and PBS with extra EPS fractions (including lbEPS and tbEPS) for Cr(VI) treatment showed that PBS with extra lbEPS achieved efficient Cr(VI) removal within five consecutive batch treatment cycles, compared to the three cycles both in pristine PBS and PBS with tbEPS. After addition of lbEPS, the accumulation of reactive oxygen species (ROS) was greatly reduced via the EPS-capping effect and quenching effect, and the toxic metal internalization potential was lowered by complexation with Cd and Cr, resulting in enhanced bacterial viability during photocatalysis. This facile and efficient cytoprotective method helps the rational design of PBS for environmental remediation.

Shewanella oneidensis MR-1 and oxalic acid mediated vanadium reduction and redistribution in vanadium-containing tailings

The microbially- and chemically-mediated redox process is critical in controlling the fate of vanadium (V) in tailing environment. Although the microbial reduction of V has been widely studied, the coupled biotic reduction mediated by beneficiation reagents and the underlying mechanism remain unclear. Herein, the reduction and redistribution of V in V-containing tailings and Fe/Mn oxide aggregates mediated by Shewanella oneidensis MR-1 and oxalic acid were explored. The dissolution of Fe-(hydr)oxides by oxalic acid promoted the microbe-mediated V release from solid-phase. After 48-day of reaction, the dissolved V concentrations in the bio-oxalic acid treatment reached maximum values of 1.72 ± 0.36 mg L^-1 and 0.42 ± 0.15 mg L^-1 in the tailing system and the aggregate system, respectively, significantly higher than those in control (0.63 ± 0.14 mg L^-1 and 0.08 ± 0.02 mg L^-1). As the electron donor, oxalic acid enhanced the electron transfer process of S. oneidensis MR-1 for V(V) reduction. The mineralogical characterization of final products indicates that S. oneidensis MR-1 and oxalic acid promoted solid-state conversion from V₂O₅ to NaV₆O₁₅. Collectively, this study demonstrates that microbe-mediated V release and redistribution in solid-phase were promoted by oxalic acid, suggesting that the role of organic agents for the V biogeochemical cycle in natural systems deserves greater attention.

Heavy-metal resistant bio-hybrid with biogenic ferrous sulfide nanoparticles: pH-regulated self-assembly and wastewater treatment application

Self-assembled bio-hybrids with biogenic ferrous sulfide nanoparticles (bio-FeS) on the cell surface are attractive for reduction of toxic heavy metals due to higher activity than bare bacteria, but they still suffer from slow synthesis and regeneration of bio-FeS and bacterial activity decay for removal of high-concentration heavy metals. A further optimization of the bio-FeS synthesis process and properties is of vital importance to address this challenge. Herein, we present a simple pH-regulation strategy to enhance bio-FeS synthesis and elucidated the underlying regulatory mechanisms. Slightly raising the pH from 7.4 to 8.3 led to 1.5-fold higher sulfide generation rate due to upregulated expression of thiosulfate reduction-related genes, and triggered the formation of fine-sized bio-FeS (29.4 ± 6.1 nm). The resulting bio-hybrid exhibited significantly improved extracellular reduction activity and was successfully used for treatment of high-concentration chromium -containing wastewater (Cr(VI), 80 mg/L) at satisfactory efficiency and stability. Its feasibility for bio-augmented treatment of real Cr(VI)-rich electroplating wastewater was also demonstrated, showing no obvious activity decline during 7-day operation. Overall, our work provides new insights into the environmental-responses of bio-hybrid self-assembly process, and may have important implications for optimized application of bio-hybrid for wastewater treatment and environmental remediation.

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.

A bioinspired polydopamine-FeS nanocomposite with high antimicrobial efficiency via NIR-mediated Fenton reaction

Ferrous and sulfur ions are essential elements for the human body, which play an active role in maintaining the body's normal physiology. Meanwhile, mussel-inspired polydopamine (PDA) possesses good hydrophilicity and biocompatibility. In the present work, ferrous sulfide embedded into polydopamine nanoparticles (PDA@FeS NPs) was designed and synthesized via a simple predoping polymerization-coprecipitation strategy and the intelligent PDA matrix successfully prevented the oxidation and agglomeration of FeS nanoparticles. Importantly, there was an obvious synergistic enhancement of the photothermal effect between polydopamine and ferrous sulfide. The PDA@FeS NPs exhibited excellent photothermal antibacterial effects against both E. coli and S. aureus. The near-infrared (NIR) light-mediated release of ferrous ions could reach about 26.5% under weakly acidic conditions, further triggering the Fenton reaction to produce toxic hydroxyl radicals (·OH) in the presence of hydrogen peroxide. The antibacterial mechanism could be attributed to cell membrane damage and cellular content leakage with the synergistic effect of PTT and CDT. This study highlighted the germicidal efficacy of PDA@FeS NPs and provided a new strategy for designing and developing next-generation antibacterial platforms.

Versatile mechanisms and enhanced strategies of pollutants removal mediated by Shewanella oneidensis: A review

The removal of environmental pollutants is important for a sustainable ecosystem and human health. Shewanella oneidensis (S. oneidensis) has diverse electron transfer pathways and can use a variety of contaminants as electron acceptors or electron donors. This paper reviews S. oneidensis's function in removing environmental pollutants, including heavy metals, inorganic non-metallic ions (INMIs), and toxic organic pollutants. S. oneidensis can mineralize o-xylene (OX), phenanthrene (PHE), and pyridine (Py) as electron donors, and also reduce azo dyes, nitro aromatic compounds (NACs), heavy metals, and iodate by extracellular electron transfer (EET). For azo dyes, NACs, Cr(VI), nitrite, nitrate, thiosulfate, and sulfite that can cross the membrane, S. oneidensis transfers electrons to intracellular reductases to catalyze their reduction. However, most organic pollutants cannot be directly degraded by S. oneidensis, but S. oneidensis can remove these pollutants by self-synthesizing catalysts or photocatalysts, constructing bio-photocatalytic systems, driving Fenton reactions, forming microbial consortia, and genetic engineering. However, the industrial-scale application of S. oneidensis is insufficient. Future research on the metabolism of S. oneidensis and interfacial reactions with other materials needs to be deepened, and large-scale reactors should be developed that can be used for practical engineering applications.

Comparison of Cr(VI) adsorption and photocatalytic reduction efficiency using leonardite powder

It is very important to treat Cr(VI) from the aquatic environment due to its toxic and harmful effects. Conventional treatment methodology involving biological pathways is generally ineffective for wastewater containing Cr(VI). Therefore, it is necessary to develop environmentally friendly and economical methods to remove Cr(VI) from the aquatic environment. In this study, leonardite, which is a natural mineral that has no harmful effects on the environment, was used for Cr(VI) removal. Leonardite was used in both adsorption and photocatalytic treatment systems by only pulverizing without any chemical treatment. Characterizations of leonardite were obtained using X-ray fluorescence (XRF), fouirer transform infrared spektrofotometre (FTIR), scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDX) analyses methods. The effects of solution pH (2-10), particle size (45-300 μm), adsorbent dose (0.25-3 g/L), and initial concentration (10-30 mg/L) on Cr(VI) removal efficiency were investigated in both adsorption and photocatalytic experiments. In the adsorption process, a complete removal efficiency (100%) was obtained for 3 g/L of adsorbent dose with an initial Cr(VI) concentration of 10 mg/L at pH 2 for 2 h. In the photocatalytic process, 100% removal efficiency of Cr(VI) was obtained when four times less adsorbent dosage was used under the same conditions. In addition, the reuse of leonardite powder was also investigated under optimum experimental conditions. Leonardite powder preserved approximately 70% of its activity in the photocatalytic process while it lost 50% of its activity after 5 reuses in adsorption process.

Electroactive Fe-biochar for redox-related remediation of arsenic and chromium: Distinct redox nature with varying iron/carbon speciation

Electroactive Fe-biochar has attracted significant attention for As(III)/Cr(VI) immobilization through redox reactions, and its performance essentially lies in the regulation of various Fe/C moieties for desired redox performance. Here, a series of Fe-biochar with distinct Fe/C speciation were rationally produced via two-step pyrolysis of iron minerals and biomass waste at 400-850 °C (BCX-Fe-Y, X and Y represented the first- and second-step pyrolysis temperature, respectively). The redox transformation of Cr(VI) and As(III) by Fe-biochar was evaluated in simulated wastewater under oxic or anoxic conditions. Results showed that more effective Cr(VI) reduction could be achieved by BCX-Fe-400, while a higher amount of As (III) was oxidized by BCX-Fe-850 under the anoxic environment. Besides, BCX-Fe-400 could generate more reactive oxygen species (e.g.,^•OH) by reducing the O₂, which enhanced the redox-related transformation of pollutants under the oxic situation. The evolving redox performance of Fe-biochar was governed by the transition of the redox state from reductive to oxidative related to the Fe/C speciation. The small-sized amorphous/low-crystalline ferrous minerals contributed to a higher electron-donating capacity (0.43-1.28 mmol g^-1) of BCX-Fe-400. In contrast, the oxidative surface oxygen-functionalities (i.e., carboxyl and quinoid) on BCX-Fe-850 endowed a stronger electron-accepting capacity (0.71-1.39 mmol g^-1). Moreover, the graphitic crystallites with edge-type defects and porous structure facilitated the electron transfer, leading to a higher electron efficiency of BCX-Fe-850. Overall, we unveiled the roles of both Fe and C speciation in maneuvering the redox reactivity of Fe-biochar, which can advance our rational design of electroactive Fe-biochar for redox-related environmental remediation.

Sustainable and efficient technologies for removal and recovery of toxic and valuable metals from wastewater: Recent progress, challenges, and future perspectives

Due to their numerous effects on human health and the natural environment, water contamination with heavy metals and metalloids, caused by their extensive use in various technologies and industrial applications, continues to be a huge ecological issue that needs to be urgently tackled. Additionally, within the circular economy management framework, the recovery and recycling of metals-based waste as high value-added products (VAPs) is of great interest, owing to their high cost and the continuous depletion of their reserves and natural sources. This paper reviews the state-of-the-art technologies developed for the removal and recovery of metal pollutants from wastewater by providing an in-depth understanding of their remediation mechanisms, while analyzing and critically discussing the recent key advances regarding these treatment methods, their practical implementation and integration, as well as evaluating their advantages and remaining limitations. Herein, various treatment techniques are covered, including adsorption, reduction/oxidation, ion exchange, membrane separation technologies, solvents extraction, chemical precipitation/co-precipitation, coagulation-flocculation, flotation, and bioremediation. A particular emphasis is placed on full recovery of the captured metal pollutants in various reusable forms as metal-based VAPs, mainly as solid precipitates, which is a powerful tool that offers substantial enhancement of the remediation processes' sustainability and cost-effectiveness. At the end, we have identified some prospective research directions for future work on this topic, while presenting some recommendations that can promote sustainability and economic feasibility of the existing treatment technologies.