Embedding electronic perpetual motion into single-atom catalysts for persistent Fenton-like reactions

Performance enhancement and mechanism of electroenhanced peroxymonosulfate activation by single-atom Fe catalyst modified electrodes

Peroxymonosulfate-based electrochemical advanced oxidation processes (PMS-EAOPs) have great potential for sustainable water purification, so an in-depth understanding of its catalytic mechanism is imperative to facilitate its practical application. Herein, the performance enhancement and mechanism of electroenhanced PMS activation by single-atom Fe catalyst modified carbon felt was investigated. Compared with the anode, the cathode exhibited faster bisphenol A degradation (kcathode = 0.073 vs. kanode = 0.015 min-1), increased PMS consumption (98.8 vs. 10.3%), and an order of magnitude reduction of Fe dissolution (0.068 vs. 0.787 mg L-1). Mass transfer is a key factor limiting PMS activation, while the electrostriction of water in the hydrophobic region caused by cathode electric field (CEF) significantly increased mass transfer coefficient (km, cathode = 1.49 × 10-4 vs. km, anode = 2.68 × 10-5 m s-1). The enhanced activation of PMS is a synergistic result between electroactivation and catalyst-activation, which is controlled by the applied current density. 1O2 and direct electron transfer are the main active species and activation pathway, which achieve high degradation efficiency over pH 3 to 10. Density functional theory calculations prove CEF increases the adsorption energy, lengthens the O-O bond in PMS, and promotes charge transfer. A flow-through convection unit achieves sustainable operation with high removal efficiency (99.5% to 97.5%), low electrical energy consumption (0.15 kWh log-1 m-3), and low Fe leaching (0.81% of the total single atom Fe). This work reveals the critical role of electric fields in modulating Fenton-like catalytic activity, which may advance the development of advanced oxidation processes and other electrocatalytic applications.

Pilot-scale and large-scale Fenton-like applications with nano-metal catalysts: From catalytic modules to scale-up applications

Recently, great efforts have been made to advance the pilot-scale and engineering-scale applications of Fenton-like processes using various nano-metal catalysts (including nanosized metal-based catalysts, smaller nanocluster catalysts, and single-atom catalysts, etc.). This step is essential to facilitate the practical applications of advanced oxidation processes (AOPs) for these highly active nano-metal catalysts. Before large-scale implementation, these nano-metal catalysts must be converted into the effective catalyst modules (such as catalytic membranes, fluidized beds, or polypropylene sphere suspension systems), as it is not feasible to use suspended powder catalysts for large-scale treatment. Therefore, the pilot-scale and engineering applications of nano-metal catalysts in Fenton-like systems in recent years is exciting. In addition, the combination of life cycle assessment (LCA) and techno-economic analysis (TEA) can provide a useful support tool for engineering scale Fenton-like applications. This paper summarizes the designs and fabrications of various advanced modules based on nano-metal catalysts, analyzes the advantages and disadvantages of these catalytic modules, and further discusses their Fenton-like pilot scale or engineering applications. Concepts of future Fenton-like engineering applications of nano-metal catalysts were also discussed. In addition, current challenges and future expectations in pilot-scale or engineering applications are assessed in conjunction with LCA and TEA. These challenges require further technological advances to enable larger scale engineering applications in the future. The aim of these efforts is to increase the potential of nanoscale AOPs for practical wastewater treatment.

Deciphering the Formation of Fe(IV) in the Fe(II)/Peroxydisulfate Process: The Critical Role of Sulfate Radical

This study delves into the formation of ferryl ions (Fe(IV)) within the Fe(II)/peroxydisulfate (PDS) process, a pivotal reaction in advanced oxidation processes (AOPs) aimed at organic pollutant removal. Our findings challenge the conventional view that Fe(IV) predominantly forms through oxygen transfer from PDS to Fe(II), revealing that sulfate radicals (SO4•-) play a crucial role in Fe(IV) generation. By employing competitive kinetics, the second-order rate constant for Fe(III) oxidation by SO4•- was quantified as 4.58 × 108 M-1 s-1. Factors such as the probe compound concentration, chloride presence, and iron species influence Fe(IV) generation, all of which were systematically evaluated. Additionally, the study explores Fe(IV) formation in various Fe(II)-activated AOPs, demonstrating that precursors like peroxymonosulfate and H2O2 can also directly oxidize Fe(II) to Fe(IV). Through experimental data, kinetic modeling, and oxygen-18 labeling experiments, this research offers a comprehensive understanding of the Fe(II)/PDS system, facilitating the optimization of AOPs for pollutant degradation. Finally, introducing HSO3- was proposed to shift the Fe(II)/PDS process from Fe(IV)-dominated to SO4•--dominated mechanisms, thereby enhancing pollutant removal efficiencies.

Selectively recovering rare earth elements with carboxyl immobilized metal-organic framework from ammonium-rich wastewater

The material with high adsorption capacity and selectivity is essential for recovering rare earth elements (REES) from ammonium (NH4+-N)-rich wastewater. Although the emerging metal-organic framework (MOF) has gained intensive attention in REES recovery, there are scientific difficulties unsolved regarding restricted adsorption capacity and selectivity, hindering its extensive engineering applications. In this work, a diethylenetriamine pentaacetic (DTPA)-modified MOF material (MIL-101(Cr)-NH-DTPA) was prepared through an amidation reaction. The MIL-101(Cr)-NH-DTPA showed enhanced adsorption capacity for La(III) (69.78 mg g-1), Eu(III) (103.01 mg g-1) and Er(III) (83.41 mg g-1). The adsorption isotherm and physical chemistry of materials indicated that the adsorption of REEs with MIL-101(Cr)-NH-DTPA was achieved via complexation instead of electrostatic adsorption. Such complexation reaction was principally governed by -COOH instead of -NH2 or -NO2. Meanwhile, the resulting material remained in its superior activity even after five cycles. Such a constructed adsorbent also exhibited excellent selective adsorption activity for La(III), Eu(III), and Er(III), with removal efficiency reaching 70% in NH4+-N concentrations ranging from 100 to 1500 mg L-1. This work offers underlying guidelines for exploitation an adsorbent for REEs recovery from wastewater.

Buffer-free ozone-ferrate(VI) systems for enhanced oxidation of electron-deficient contaminants: Synergistic enhancement effects, systematic toxicity assessment, and practical applications

The combination of ozone (O3) and ferrate (Fe(VI)) oxidation technology demonstrates substantial potential for practical applications, though it has been underreported, resulting in gaps in comprehensive activity assessments and thorough exploration of its mechanisms. This study reveals that the previous use of a borate buffer solution obscured certain synergistic reactions between O3 and Fe(VI), causing a reduction of activity by ∼40 % when oxidizing the electron-deficient pollutant atrazine. Consequently, we reassessed the activity and mechanisms using a buffer-salt-free O3/Fe(VI) system. Our findings showed that the hydroxyl radical (·OH) served as the predominant active species, responsible for an impressive 95.9 % of the oxidation activity against electron-deficient pollutants. Additional experiments demonstrated that the rapid production of neglected and really important superoxide radicals (·O2-) could facilitate the decomposition of O3 to generate ·OH and accelerate the reduction of Fe(VI) to Fe(V), reactivating O3 to produce ·OH anew. Intriguingly, as the reaction progressed, the initially depleted Fe(VI) was partially regenerated, stabilizing at over 50 %, highlighting the significant potential of this combined system. Moreover, this combined system could achieve a high mineralization efficiency of 80.4 % in treating actual coking wastewater, complemented by extensive toxicity assessments using Escherichia coli, wheat seeds, and zebrafish embryos, showcasing its robust application potential. This study revisits and amends previous research on the O3/Fe(VI) system, providing new insights into its activity and synergistic mechanisms. Such a combined technology has potential for the treatment of difficult-to-degrade industrial wastewater.

The mechanism differences between sulfadiazine degradation and antibiotic resistant bacteria inactivation by iron-based graphitic biochar and peroxydisulfate system

In this study, the activation of peroxydisulfate (PS) by K2FeO4-activation biochar (KFeB) and acid-picking K2FeO4-activation biochar (AKFeB) was investigated to reveal the mechanism differences between iron site and graphitic structure in sulfadiazine (SDZ) degradation and ARB inactivation, respectively. KFeB/PS and AKFeB/PS systems had similar degradation property towards SDZ, but only KFeB/PS system showed excellent bactericidal property. The mechanism study demonstrated that dissolved SDZ was degraded through electron transfer pathway mediated by graphitic structure, while suspended ARB was inactivated through free radicals generated by iron-activated PS, accompanied by excellent removal on antibiotic resistance genes (ARGs). The significant decrease in conjugative transfer frequency indicated the reduced horizontal gene transfer risk of ARGs after treatment with KFeB/PS system. Transcriptome data suggested that membrane protein channel disruption and adenosine triphosphate synthesis inhibition were key reasons for conjugative transfer frequency reduction. Continuous flow reactor of KFeB/PS system can efficiently remove antibiotics and ARB, implying the potential application in practical wastewater purification. In conclusion, this study provides novel insights for classified and collaborative control of antibiotics and ARB by carbon-based catalysts driven persulfate advanced oxidation technology.

Direct Electron Transfer-Driven Nontoxic Oligomeric Deposition of Sulfonamide Antibiotics onto Carbon Materials for In Situ Water Remediation

The rising in situ chemical oxidation (ISCO) technologies based on polymerization reactions have advanced the removal of emerging contaminants in the aquatic environment. However, despite their promise, uncertainties persist regarding their effectiveness in eliminating structurally complex contaminants, such as sulfonamide antibiotics (SAs). This study elucidated that oligomerization, rather than mineralization, predominantly governs the removal of SAs in the carbon materials/periodate system. The amine groups in SAs played a crucial role in forming organic radicals and subsequent coupling reactions due to their high f- index and low bond orders. Moreover, the study highlighted the robust adhesion of oligomers to the catalyst surface, facilitated by enhanced van der Waals forces and hydrophobic interactions. Importantly, plant and animal toxicity assessments confirmed the nontoxic nature of oligomers deposited on the carbon material surface, affirming the efficacy of carbon material-based ISCO in treating contaminated surface water and groundwater. Additionally, a novel classification approach, Δlog k, was proposed to differentiate SAs based on their kinetic control steps, providing deeper insights into the quantitative structure-activity relationship (QSAR) and facilitating the selection of optimal descriptors during the oligomerization processes. Overall, these insights significantly enhance our understanding of SAs removal via oligomerization and demonstrate the superiority of C-ISCO based on polymerization in water decontamination.

Size-Dependent Catalysis in Fenton-like Chemistry: From Nanoparticles to Single Atoms

State-of-the-art Fenton-like reactions are crucial in advanced oxidation processes (AOPs) for water purification. This review explores the latest advancements in heterogeneous metal-based catalysts within AOPs, covering nanoparticles (NPs), single-atom catalysts (SACs), and ultra-small atom clusters. A distinct connection between the physical properties of these catalysts, such as size, degree of unsaturation, electronic structure, and oxidation state, and their impacts on catalytic behavior and efficacy in Fenton-like reactions. In-depth comparative analysis of metal NPs and SACs is conducted focusing on how particle size variations and metal-support interactions affect oxidation species and pathways. The review highlights the cutting-edge characterization techniques and theoretical calculations, indispensable for deciphering the complex electronic and structural characteristics of active sites in downsized metal particles. Additionally, the review underscores innovative strategies for immobilizing these catalysts onto membrane surfaces, offering a solution to the inherent challenges of powdered catalysts. Recent advances in pilot-scale or engineering applications of Fenton-like-based devices are also summarized for the first time. The paper concludes by charting new research directions, emphasizing advanced catalyst design, precise identification of reactive oxygen species, and in-depth mechanistic studies. These efforts aim to enhance the application potential of nanotechnology-based AOPs in real-world wastewater treatment.

Oxygen-Centered Organic Radicals-Involved Unified Heterogeneous Self-Fenton Process for Stable Mineralization of Micropollutants in Water

Removing organic micropollutants from water through photocatalysis is hindered by catalyst instability and substantial residuals from incomplete mineralization. Here, a novel water treatment paradigm, the unified heterogeneous self-Fenton process (UHSFP), which achieved an impressive 32% photon utilization efficiency at 470 nm, and a significant 94% mineralization of organic micropollutants-all without the continual addition of oxidants and iron ions is presented. In UHSFP, the active species differs fundamentally from traditional photocatalytic processes. One electron acceptor unit of photocatalyst acquires only one photogenerated electron to convert into oxygen-centered organic radical (OCOR), then spontaneously completing subsequent processes, including pollutant degradation, hydrogen peroxide generation, activation, and mineralization of organic micropollutants. By bolstering electron-transfer capabilities and diminishing catalyst affinity for oxygen in the photocatalytic process, the generation of superoxide radicals is effectively suppressed, preventing detrimental attacks on the catalyst. This study introduces an innovative and cost-effective strategy for the efficient and stable mineralization of organic micropollutants, eliminating the necessity for continuous chemical inputs, providing a new perspective on water treatment technologies.

Circumventing bottlenecks in H2O2 photosynthesis over carbon nitride with iodine redox chemistry and electric field effects

Artificial photosynthesis using carbon nitride (g-C3N4) holds a great promise for sustainable and cost-effective H2O2 production, but the high carrier recombination rate impedes its efficiency. To tackle this challenge, we propose an innovative method involving multispecies iodine mediators (I-/I3-) intercalation through a pre-photo-oxidation process using potassium iodide (suspected deteriorated "KI") within the g-C3N4 framework. Moreover, we introduce an external electric field by incorporating cationic methyl viologen ions to establish an auxiliary electron transfer channel. Such a unique design drastically improves the separation of photo-generated carriers, achieving an impressive H2O2 production rate of 46.40 mmol g-1 h-1 under visible light irradiation, surpassing the most visible-light H2O2-producing systems. Combining various advanced characterization techniques elucidates the inner photocatalytic mechanism, and the application potential of this photocatalytic system is validated with various simulation scenarios. This work presents a significative strategy for preparing and applying highly efficient g-C3N4-based catalysts in photochemical H2O2 production.

2D/2D heterojunctions for rapid and self-cleaning removal of antibiotics via visible light-assisted peroxymonosulfate activation: Efficiency, synergistic effects, and applications

Developing eco-friendly and efficient technologies for treating antibiotic wastewater is crucial. Traditional methods face challenges in incomplete removal, high costs, and secondary pollution. Heterogeneous peroxymonosulfate (PMS) activation assisted by visible light shows promise, but suitable activators remain a huge challenge. Here, we synthesized cost-effective carbon nitride/bismuth bromide oxide (CN/BiOBr) heterojunctions. Such a heterojunction achieved rapid PMS activation, achieving over 90.00% tetracycline (TC) removal only within 1 min (kobs of 2.23 min-1), surpassing previous systems by nearly 1-2 orders of magnitude and even remarkably superior to the popular single-atom catalysts. The system exhibited self-cleaning properties, maintaining activity after 8 cycles and stability across a wide pH range (3.01 to 9.03). Quenching experiments and theoretical calculations elucidated the exclusive •O2- species involvement and removal pathways. Eco-toxicity assessment and total organic carbon results confirmed simultaneous degradation, detoxification, and mineralization. This system also showed excellent resistance to environmental factors, e.g., coexisting anions, varying pH, and water sources, and demonstrated potential in coking and medical wastewater purification. This study presents a novel technique for rapidly decontaminating antibiotic wastewater through visible light-assisted PMS activation and introduces innovative bionic catalytic oxidation combining light and darkness for practical applications.