Activation of Peroxydisulfate on Carbon Nanotubes: Electron-Transfer Mechanism

Comparative study on the activation of peroxymonosulfate and peroxydisulfate by Ar plasma-etching CNTs for sulfamethoxazole degradation: Efficiency and mechanisms

Sulfamethoxazole (SMX), a widely utilized antibiotic, was continually detected in the environment, causing serious risks to aquatic ecology and water security. In this study, carbon nanotubes (CNTs) with abundant defects were developed by argon plasma-etching technology to enhance the activation of persulfate (PS, including peroxymonosulfate (PMS) and peroxydisulfate (PDS)) for SMX degradation while reducing environmental toxicity. Obviously, the increase of ID/IG value from 0.980 to 1.333 indicated that Ar plasma-etching successfully introduced rich defects into CNTs. Of note, Ar-90-CNT, whose Ar plasma-etching time was 90 min with optimum catalytic performance, exhibited a significant discrepancy between PMS activation and PDS activation. Interestingly, though the Ar-90-CNT/PDS system (kobs = 0.0332 min-1) was more efficient in SMX elimination than the Ar-90-CNT/PMS system (kobs = 0.0190 min-1), Ar plasma-etching treatment had no discernible enhancement in the catalytic efficiency of MWCNT for PDS activation. Then the discrepancy on activation mechanism between PMS and PDS was methodically investigated through quenching experiments, electron spin resonance (ESR), chemical probes, electrochemical measurements and theoretical calculations, and the findings unraveled that the created vacancy defects were the ruling active sites for the production of dominated singlet oxygen (1O2) in the Ar-90-CNT/PMS system to degrade SMX, while the electron transfer pathway (ETP), originated from PDS activation by the inherent edge defects, was the central pathway for SMX removal in the Ar-90-CNT/PDS system. Based on the toxicity test of Microcystis aeruginosa, the Ar-90-CNT/PDS system was more effective in alleviating environmental toxicity during SMX degradation. These findings not only provide insights into the discrepancy between PMS activation and PDS activation via carbon-based materials with controlled defects regulated by the plasma-etching strategy, but also efficiently degrade sulfonamide antibiotics and reduce the toxicity of their products.

Promoting oxygen vacancies utility for tetracycline degradation via peroxymonosulfate activation by reduced Mg-doped Co3O4: Kinetics and key role of electron transfer pathway

Developing cobalt-based catalysts with a high abundance of oxygen vacancies (Vo) and exceptional Vo utility efficiency for the prompt removal of stubborn contaminants through peroxymonosulfate (PMS) activation poses a significant challenge. Herein, we reported the synthesis of the reduced Mg-doped Co3O4 nanosheets, i.e. Mg-doped Co3O4-r, via Mg doping and followed by NaBH4 reduction, aiming to degrade tetracycline (TC). Various characterization results illustrated that NaBH4 reduction imparted higher Vo utility efficiency to Mg-doped Co3O4-r, along with an ample presence of reduced Co2+ species and an increased surface area, thereby substantially elevating PMS activation capability. Notably, Mg-doped Co3O4-r achieved more than 97.9% degradation of 20 mg/L TC within 10 min, showing an over 8-fold increase in reaction rate relative to the Mg-doped Co3O4 (kobs: 0.3285 min-1 vs 0.0399 min-1). The high removal efficiency of TC was sustained across a broad pH range of 3-11, even in the presence of common anions and humic acid. Radical quenching trials, EPR outcomes, and electrochemical analysis indicated that neither radicals nor 1O2 were the primary active species. Instead, electron transfer pathway played a dominant role in TC degradation. The Mg-doped Co3O4-r displayed excellent recyclability and versatility. Even after the fifth cycle, it maintained an impressive 83.0% removal of TC. Furthermore, it exhibited rapid degradation capabilities for various pollutants, including levofloxacin, pefloxacin, ciprofloxacin, malachite green, and rhodamine B. The TC degradation pathway was proposed based on LC-MS determination of its degradation intermediates. This study showcases an innovative strategy for the rational design of an efficient cobalt-based activator, leveraging electron transfer pathways through PMS activation to degrade antibiotics effectively.

Were Persulfate-Based Advanced Oxidation Processes Really Understood? Basic Concepts, Cognitive Biases, and Experimental Details

Persulfate (PS)-based advanced oxidation processes (AOPs) for pollutant removal have attracted extensive interest, but some controversies about the identification of reactive species were usually observed. This critical review aims to comprehensively introduce basic concepts and rectify cognitive biases and appeals to pay more attention to experimental details in PS-AOPs, so as to accurately explore reaction mechanisms. The review scientifically summarizes the character, generation, and identification of different reactive species. It then highlights the complexities about the analysis of electron paramagnetic resonance, the uncertainties about the use of probes and scavengers, and the necessities about the determination of scavenger concentration. The importance of the choice of buffer solution, operating mode, terminator, and filter membrane is also emphasized. Finally, we discuss current challenges and future perspectives to alleviate the misinterpretations toward reactive species and reaction mechanisms in PS-AOPs.

Mechanism of directed activation of peroxymonosulfate by Fe-N/O unsymmetrical coordination-modulated polarized electric field

Iron-nitrogen co-doped carbon materials as heterogeneous catalysts have attracted much attention in advanced oxidation processes involving peroxymonosulfate (PMS) due to their unique structure and enormous catalytic potential. However, there is limited research on the influence of different coordination structures on the central iron atoms. Through simple pyrolysis, we introduced oxygen atoms into the Fe-N coordination structure, constructing Fe-N/O@C catalysts with Fe-N2O2 coordination structure, and achieved efficient degradation of bisphenol A (BPA). Quenching experiments, electron paramagnetic resonance, and electrochemical analysis indicate that compared to the free radical activation pathway of Fe-N@C, high-valent iron-oxo species (≡Fe(Ⅳ) = O) are the main reactive oxygen species (ROS) in the Fe-N/O@C/PMS system. Meanwhile, we compared the differences in the oxidation states of Fe atoms and electron density in different coordination structures, revealing the formation of high-valent iron-oxo species and the mechanism of interfacial electron transfer. Therefore, this study provides new insights into the design and development of Fe-N co-doped catalysts for resource-efficient and environmentally friendly catalytic oxidation systems.

Prehydrated Electrons Activated by Continuous Electron Transfer Stemmed from Peracetic Acid Homolysis Mediated by Diamond Surface Defects for Enhanced PFOA Destruction

Research on the use of peracetic acid (PAA) activated by nonmetal solid catalysts for the removal of dissolved refractory organic compounds has gained attention recently due to its improved efficiency and suitability for advanced water treatment (AWT). Among these catalysts, nanocarbon (NC) stands out as an exceptional example. In the NC-based peroxide AWT studies, the focus on the mechanism involving multimedia coordination on the NC surface (reactive species (RS) path, electron reduction non-RS pathway, and singlet oxygen non-RS path) has been confined to the one-step electron reaction, leaving the mechanisms of multichannel or continuous electron transfer paths unexplored. Moreover, there are very few studies that have identified the nonfree radical pathway initiated by electron transfer within PAA AWT. In this study, the complete decomposition (kobs = 0.1995) and significant defluorination of perfluorooctanoic acid (PFOA, deF% = 72%) through PAA/NC has been confirmed. Through the use of multiple electrochemical monitors and the exploration of current diffusion effects, the process of electron reception and conduction stimulated by PAA activation was examined, leading to the discovery of the dynamic process from the PAA molecule → NC solid surface → target object. The vital role of prehydrated electrons (epre-) before the entry of resolvable electrons into the aqueous phase was also detailed. To the best of our knowledge, this is the first instance of identifying the nonradical mechanism of continuous electron transfer in PAA-based AWT, which deviates from the previously identified mechanisms of singlet oxygen, single-electron, or double-electron single-path transfer. The pathway, along with the strong reducibility of epre- initiated by this pathway, has been proven to be essential in reducing the need for catalysts and chemicals in AWT.

pH-modulated oxidation of organic pollutants for water decontamination: A deep insight into reactivity and oxidation pathway

Solution pH is one of the primary factors affecting the efficiency of water decontamination. Although the influence of pH on oxidants activation, catalyst activity, and reactive oxygen species have been widely explored, there is still a scarcity of systemic studies on the changes in the oxidation behavior of organic pollutants at different pH levels. Herein, we report the influence laws of pH on the forms, reactivities, active sites, degradation pathways, and products toxicities of organic pollutants. Changes in pH cause the protonation or deprotonation of organic pollutants and further affect their forms and chemistry (e.g., electrostatic force, hydrophobicity, and oxidation potential). The oxidation potential of organic pollutants follows the order: protonated form > pristine form > deprotonated form. Moreover, protonation or deprotonation can modify the active sites and degradation pathways of organic pollutants, wherein deprotonation renders them more susceptible to electrophilic attack, while protonation reduces their activity against electrophilic and nucleophilic attacks. Additionally, pH adjustments can modify the degradation pathway and the toxicity of transformation products. Overall, pH changes can affect the oxidation fate of organic pollutants by altering their structure, which distinguishes it from the effect of pH on oxidants or oxidant activation processes.

Oxidative polymerization versus degradation of organic pollutants in heterogeneous catalytic persulfate chemistry

Catalytic polymerization pathways in advanced oxidation processes (AOPs) have recently drawn much attention for organic pollutant elimination owing to the rapid removal kinetics, high selectivity, and recovery of organic carbon from wastewater. This work presents a review on the polymerization regimes in AOPs and their applications in wastewater decontamination. The review mainly highlights three critical issues in polymerization reactions induced by persulfate activation (Poly-PS-AOPs), including heterogeneous catalysts, persulfate activation pathways, and properties of organic substrates. The dominant influencing factors on the selection of catalysts, activation regimes of reactive oxygen species, and polymerization processes of organic substrates are discussed in detail. Moreover, we systematically demonstrate the merits and challenges of Poly-PS-AOPs upon pollutant degradation and polymer synthesis. We particularly highlight that Poly-PS-AOPs technology could be promising in the treatment of industrial wastewater containing heterocyclic organics and the synthesis of polymers and polymer-functionalized materials for advanced environmental and energy applications.

Mechanically treated Mn2O3 triggers peracetic acid activation for superior non-radical oxidation of micropollutants: Identification of reactive complexes

This study used a simple mechanical ball milling strategy to significantly improve the ability of Mn2O3 to activate peracetic acid (PAA) for sustainable and efficient degradation of organic micropollutant (like bisphenol A, BPA). BPA was successfully removed and detoxified via PAA activation by the bm-Mn2O3 within 30 min under neutral environment, with the BPA degradation kinetic rate improved by 3.4 times. Satisfactory BPA removal efficiency can still be achieved over a wide pH range, in actual water and after reuse of bm-Mn2O3 for four cycles. The change in hydrophilicity of Mn2O3 after ball milling evidently elevated the affinity of Mn2O3 for binding to PAA, while the reduction in particle size exposed more active sites contributing partially to catalytic oxidation. Further analysis revealed that BPA oxidation in the ball mill-treated Mn2O3 (bm-Mn2O3)/PAA process mainly depends on the bm-Mn2O3-PAA complex (i.e., Mn(III)-OO(O)CCH3) mediated non-radical pathway rather than R-O• and Mn(IV). Especially, the existence of the Mn(III)-PAA complex was definitely verified by in situ Raman spectroscopy and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Simultaneously, density functional theory calculations determined that PAA adsorbs readily on manganese sites thereby favoring the formation of Mn(III)-OO(O)CCH3 complexes. This study advances an in-depth understanding of the underlying mechanisms involved in the manganese oxide-catalyzed activation of PAA for superior non-radical oxidation of micropollutants.

Phosphate-mediated degradation of organic pollutants in water with peroxymonosulfate revisited: Radical or non-radical oxidation?

Whilst it is generally recognized that phosphate enables to promote the removal of some organic pollutants with peroxymonosulfate (PMS) oxidation, however, there is an ongoing debate as to whether free radicals are involved. By integrating different methodologies, here we provide new insights into the reaction mechanism of the binary mixture of phosphates (i.e., NaH2PO4, Na2HPO3, and NaH2PO2) with peroxymonosulfate (PMS) or hydrogen peroxide (H2O2). Enhanced degradation of organic pollutants and observation of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) adducts (i.e. DMPOOH and 5,5-dimethyl-2-oxopyrroline-1-oxyl (DMPOX)) with electron paramagnetic resonance (EPR) in most phosphates/PMS system seemly support a radical-dominant mechanism. However, fluorescence probe experiments confirm that no significant amount of hydroxyl radicals (•OH) are produced in such reaction systems. PMS in the phosphate solutions (without any organics) remains relatively stable, but is only consumed while organic substrates are present, which is distinct from a typical radical-dominant Co2+/PMS system where PMS is continuously decomposed. Through density functional theory (DFT) calculation, the energy barriers of the phosphates/PMS reaction processes are greatly decreased when non-radical mechanism dominates. Complementary evidence suggests that the reactive intermediates of PMS-phosphate complex, rather than the free radicals, are capable of oxidizing electron-rich substrates such as DMPO and organic pollutants. Taking the case of phosphate/PMS system as an example, this study demonstrates the necessity of acquisition of lines of evidence for resolving paradoxes in identifying EPR adducts.

Unraveling the Role of Humic Acid in the Oxidation of Phenolic Contaminants by Soluble Manganese Oxo-Anions

Humic acid (HA) is ubiquitous in natural aquatic environments and effectively accelerates decontamination by permanganate (Mn(VII)). However, the detailed mechanism remains uncertain. Herein, the intrinsic mechanisms of HA's impact on phenolics oxidation by Mn(VII) and its intermediate manganese oxo-anions were systematically studied. Results suggested that HA facilitated the transfer of a single electron from Mn(VII), resulting in the sequential formation of Mn(VI) and Mn(V). The formed Mn(V) was further reduced to Mn(III) through a double electron transfer process by HA. Mn(III) was responsible for the HA-boosted oxidation as the active species attacking pollutants, while Mn(VI) and Mn(V) tended to act as intermediate species due to their own instability. In addition, HA could serve as a stabilizer to form a complex with produced Mn(III) and retard the disproportionation of Mn(III). Notably, manganese oxo-anions did not mineralize HA but essentially changed its composition. According to the results of Fourier-transform ion cyclotron resonance mass spectrometry and the second derivative analysis of Fourier-transform infrared spectroscopy, we found that manganese oxo-anions triggered the decomposition of C-H bonds on HA and subsequently produced oxygen-containing functional groups (i.e., C-O). This study might shed new light on the HA/manganese oxo-anion process.

Dual Nonradical Catalytic Pathways Mediated by Nanodiamond-Derived sp2/sp3 Hybrids for Sustainable Peracetic Acid Activation and Water Decontamination

Peracetic acid (PAA) oxidation catalyzed by metal-free carbons is promising for advanced water decontamination. Nevertheless, developing reaction-oriented and high-performance carbocatalysts has been limited by the ambiguous understanding of the intrinsic relationship between carbon chemical/molecular structure and PAA transformation behavior. Herein, we comprehensively investigated the PAA activation using a family of well-defined sp2/sp3 carbon hybrids from annealed nanodiamonds (ANDs). The activity of ANDs displays a volcano-type trend, with respect to the sp2/sp3 ratio. Intriguingly, sp3-C-enriched AND exhibits the best catalytic activity for PAA activation and phenolic oxidation, which is different from persulfate chemistry in which the sp2 network normally outperforms sp3 hybridization. At the electron-rich sp2-C site, PAA undergoes a reduction reaction to generate a reactive complex (AND-PAA*) and induces an electron-transfer oxidation pathway. At the sp3-C site adjacent to C═O, PAA is oxidized to surface-confined OH* and O* successively, which ultimately evolves into singlet oxygen (1O2) as the primary reactive species. Benefiting from the dual nonradical regimes on sp2/sp3 hybrids, AND mediates a sustainable redox recycle with PAA to continuously generate reactive species to attack water contaminants, meanwhile maintaining structural/chemical integrity and exceptional reusability in cyclic runs.

Peroxydisulfate-Driven Reductive Dechlorination as Affected by Soil Constituents: Free Radical Formation and Conversion

We report a previously unrecognized but efficient reductive degradation pathway in peroxydisulfate (PDS)-driven soil remediation. With supplements of naturally occurring low-molecular-weight organic acids (LMWOAs) in anaerobic biochar-activated PDS systems, degradation rates of 12 γ-hexachlorocyclohexanes (HCH)-spiked soils boosted from 40% without LMWOAs to a maximum of 99% with 1 mM malic acid. Structural analysis revealed that an increase in α-hydroxyl groups and a diminution in pKa1 values of LMWOAs facilitated the formation of reductive carboxyl anion radicals (COO•-) via electrophilic attack by SO4•-/•OH. Furthermore, degradation kinetics were strongly correlated with soil organic matter (SOM) contents than iron minerals. Combining a newly developed in situ fluorescence detector of reductive radicals with quenching experiments, we showed that for soils with high, medium, and low SOM contents, dominant reactive species switched from singlet oxygen/semiquinone radicals to SO4•-/•OH and then to COO•- (contribution increased from 30.8 to 66.7%), yielding superior HCH degradation. Validation experiments using SOM model compounds highlighted critical roles of redox-active moieties, such as phenolic - OH and quinones, in radical formation and conversion. Our study provides insights into environmental behaviors related to radical activation of persulfate in a broader soil horizon and inspiration for more advanced reduction technologies.

Atomic-Level Engineered Cobalt Catalysts for Fenton-Like Reactions: Synergy of Single Atom Metal Sites and Nonmetal-Bonded Functionalities

Single atom catalysts (SACs) are atomic-level-engineered materials with high intrinsic activity. Catalytic centers of SACs are typically the transition metal (TM)-nonmetal coordination sites, while the functions of coexisting non-TM-bonded functionalities are usually overlooked in catalysis. Herein, the scalable preparation of carbon-supported cobalt-anchored SACs (CoCN) with controlled Co─N sites and free functional N species is reported. The role of metal- and nonmetal-bonded functionalities in the SACs for peroxymonosulfate (PMS)-driven Fenton-like reactions is first systematically studied, revealing their contribution to performance improvement and pathway steering. Experiments and computations demonstrate that the Co─N3C coordination plays a vital role in the formation of a surface-confined PMS* complex to trigger the electron transfer pathway and promote kinetics because of the optimized electronic state of Co centers, while the nonmetal-coordinated graphitic N sites act as preferable pollutant adsorption sites and additional PMS activation sites to accelerate electron transfer. Synergistically, CoCN exhibits ultrahigh activity in PMS activation for p-hydroxybenzoic acid oxidation, achieving complete degradation within 10 min with an ultrahigh turnover frequency of 0.38 min-1, surpassing most reported materials. These findings offer new insights into the versatile functions of N species in SACs and inspire rational design of high-performance catalysts in complicated heterogeneous systems.

From nano- to macroarchitectures: designing and constructing MOF-derived porous materials for persulfate-based advanced oxidation processes

Persulfate-based advanced oxidation processes (PS-AOPs) have gained significant attention as an effective approach for the elimination of emerging organic contaminants (EOCs) in water treatment. Metal-organic frameworks (MOFs) and their derivatives are regarded as promising catalysts for activating peroxydisulfate (PDS) and peroxymonosulfate (PMS) due to their tunable and diverse structure and composition. By the rational nanoarchitectured design of MOF-derived nanomaterials, the excellent performance and customized functions can be achieved. However, the intrinsic fine powder form and agglomeration ability of MOF-derived nanomaterials have limited their practical engineering application. Recently, a great deal of effort has been put into shaping MOFs into macroscopic objects without sacrificing the performance. This review presents recent advances in the design and synthetic strategies of MOF-derived nano- and macroarchitectures for PS-AOPs to degrade EOCs. Firstly, the strategies of preparing MOF-derived diverse nanoarchitectures including hierarchically porous, hollow, yolk-shell, and multi-shell structures are comprehensively summarized. Subsequently, the approaches of manufacturing MOF-based macroarchitectures are introduced in detail. Moreover, the PS-AOP application and mechanisms of MOF-derived nano- and macromaterials as catalysts to eliminate EOCs are discussed. Finally, the prospects and challenges of MOF-derived materials in PS-AOPs are discussed. This work will hopefully guide the design and development of MOF-derived porous materials in SR-AOPs.

Removal of sulfonamide antibiotics by non-free radical dominated peroxymonosulfate oxidation catalyzed by cobalt-doped sulfur-containing biochar from sludge

The reuse of activated sludge as a solid waste is severely underutilized due to the limitations of traditional treatment and disposal methods. Given that, the sulfur-containing activated sludge catalyst doped with cobalt (SK-Co(1.0)) was successfully prepared by one-step pyrolysis and calcinated at 850 ℃. The generation of CoSx was successfully characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), indicating that the sulfur inside the sludge was the anchoring site for the externally doped cobalt. Cobalt (Ⅱ) (Co2+), as the main adsorption site for peroxymonosulfate(PMS), formed a complex (SK-Co(1.0)-PMS* ) and created the conditions for the generation of surface radicals. The SK-Co(1.0)/PMS system showed high degradation efficiency and apparent rate constants for Sulfamethoxazole (SMX) (91.56% and 0.187 min-1) and Sulfadiazine (SDZ) (90.73% and 0.047 min-1) within 10 min and 30 min, respectively. Three sites of generation of 1O2, which played a dominant role in the degradation of SMX and SDZ in the SK-Co(1.0)/PMS system, were summarized as：sulfur vacancies (SVs), the Co3+/Co2+ cycles promoted by sulfur(S) species, oxygen-containing functional groups (C-O). The degradation mechanisms and pathways had been thoroughly investigated using DFT calculations. In view of this, a new idea for the resource utilization of activated sludge solid waste was provided and a new strategy for wastewater remediation was proposed.

Origins of Selective Oxidation in Carbon-Based Nonradical Oxidation Processes toward Organic Pollutants: Quantitative Structure-Activity Relationships (QSARs)

Metal-free carbon material-mediated nonradical oxidation processes (C-NOPs) have emerged as a research hotspot due to their excellent performance in selectively eliminating organic pollutants in aqueous environments. However, the selective oxidation mechanisms of C-NOPs remain obscure due to the diversity of organic pollutants and nonradical active species. Herein, quantitative structure-activity relationship (QSAR) models were employed to unveil the origins of C-NOP selectivity toward organic pollutants in different oxidant systems. QSAR analysis based on adsorption and oxidation descriptors revealed that C-NOP selectivity depends on the oxidation potentials of organic pollutants rather than on adsorption interactions. However, the dominance of electronic effects in selective oxidation decreases with increasing structural complexity of organic pollutants. Moreover, the oxidation threshold solely depends on the inherent electronic nature of organic pollutants and not on the reactivity of nonradical active species. Notably, the accuracy of substituent descriptors (Hammett constants) and theoretical descriptors (e.g., highest occupied molecular orbital energy, ionization potential, and single-electron oxidation potential) is significantly influenced by the complexity and molecular state of organic pollutants. Overall, the study findings reveal the origins of organic pollutant-oriented selective oxidation and provide insight into the application of descriptors in QSAR analysis.

Defluorination of perfluorooctanoic acid and perfluorooctane sulfonic acid by heterogeneous catalytic system of Fe-Al2O3/O3: Synergistic oxidation effects and defluorination mechanism

In this study, catalytic ozonation by Fe-Al2O3 was used to investigate the defluorination of PFOA and PFOS, assessing the effects of different experimental conditions on the defluorination efficiency of the system. The oxidation mechanism of the Fe-Al2O3/O3 system and the specific degradation and defluorination mechanisms for PFOA and PFOS were determined. Results showed that compared to the single O3 system, the defluorination rates of PFOA and PFOS increased by 2.32- and 5.92-fold using the Fe-Al2O3/O3 system under optimal experimental conditions. Mechanistic analysis indicated that in Fe-Al2O3, the variable valence iron (Fe) and functional groups containing C and O served as important reaction sites during the catalytic process. The co-existence of 1O2, OH, O2- and high-valence Fe(IV) constituted a synergistic oxidation system consisting of free radicals and non-radicals, promoting the degradation and defluorination of PFOA and PFOS. DFT theoretical calculations and the analysis of intermediate degradation products suggested that the degradation pathways of PFOA and PFOS involved Kolbe decarboxylation, desulfonation, alcoholization and intramolecular cyclization reactions. The degradation and defluorination pathways of PFOA and PFOS consisted of the stepwise removal of -CF2-, with PFOS exhibiting a higher defluorination rate than PFOA due to its susceptibility to electrophilic attack. This study provides a theoretical basis for the development of heterogeneous catalytic ozonation systems for PFOA and PFOS treatment.

Enhanced activation of persulfate by modified red mud biochar for degradation of dye pollutant: Resource utilization and non-radical activation

The substantial development of the dyeing and printing industry has resulted in an increased discharge of dye wastewater containing a large amount of recalcitrant organic pollutants. Furthermore, the landfill disposal of red mud has led to significant environmental pollution such as soil erosion and groundwater contamination. Therefore, this study aimed to promote the resource utilization of red mud by preparing advanced oxidation catalyst, resulting in effective treatment of dye wastewater, and the primary reaction mechanism was revealed. In this study, biochar-loading red mud (RBC) was applied to activate persulfate (PDS) for the degradation of acid orange 7 (AO7) with the initial concentration of 50 mg L-1. The maximum removal rate of 2.45 mg L·min-1 was achieved in 20 min and corresponding with the removal ratio of 98.0% under the PDS concentration of 20 mM (4.76 g L-1). Eventually, the removal ratio of 99.2% was attained within 60 min. The high catalytic efficiency was probably ascribed to the singlet oxygen (1O2) dominant non-radical pathway and RBC-mediated electron transfer mechanism. It was found that Fe(II), specific surface areas and functional groups on the catalyst were highly related to its catalytic efficiency and passivation. RBC had better reusability due to the loading of biochar and the reduction of zero-valent iron. The non-radical pathway mechanism and electron transfer mechanism were proposed for the activation of PDS, and non-radical pathway played a dominant role. Besides, the degradation pathways and toxicity assessment were analyzed. This research proposed a new electron transfer mechanism for activation process of PDS, which can provide a theoretical support for further studies. Overall, this study demonstrated that catalysts synthesized from red mud and biomass exhibit highly efficient activation in degrading the model pollutant AO7 through PDS activation. The catalyst displayed promising reusability and practical applicability, offering potential advancements in both the resource utilization and reduction of red mud.

New Insights into the Role of Humic Acid in Permanganate Oxidation of Diclofenac: A Novel Electron Transfer Mechanism

Humic acid (HA) ubiquitously existing in aquatic environments has been reported to significantly impact permanganate (KMnO4) decontamination processes. However, the underlying mechanism of the KMnO4/HA system remained elusive. In this study, an enhancing effect of HA on the KMnO4 oxidation of diclofenac (DCF) was observed over a wide solution pH range of 5-9. Surprisingly, the mechanism of HA-induced enhancement varied with solution pH. Quenching and chemical probing experiments revealed that manganese intermediates (Mn(III)-HA and MnO2) were responsible for the enhancement under acidic conditions but not under neutral and alkaline conditions. By combining KMnO4 decomposition, galvanic oxidation process experiments, electrochemical tests, and FTIR and XPS analysis, it was interestingly found that HA could effectively mediate the electron transfer from DCF to KMnO4 in neutral and alkaline solutions, which was reported for the first time. The formation of an organic-catalyst complex (i.e., HA-DCF) with lower reduction potential than the parent DCF was proposed to be responsible for the accelerated electron transfer from DCF to KMnO4. This electron transfer likely occurred within the complex molecule formed through the interaction between HA-DCF and KMnO4 (i.e., HA-DCF-KMnO4). These results will help us gain a more comprehensive understanding of the role of HA in the KMnO4 oxidation processes.

Novel Magnetic MnFe2O4-Decorated Graphite-Like Porous Biochar as a Heterogeneous Catalyst for Activation of Peroxydisulfate Toward Degradation of Rhodamine B

A magnetic MnFe2O4-modified graphite-like porous biochar composite (MnFe2O4/KFS800) was synthesized by the hydrothermal method, and its catalytic activity was evaluated in the activation of peroxydisulfate toward degradation of Rhodamine B. After characterization by SEM, XRD, and the BET method, the specific surface area and total pore volume of the MnFe2O4/KFS800 catalyst reached 121 m2/g and 0.263 m3/g, and exhibited plate-like morphology with good crystallinity. The degradation rate of Rhodamine B by the obtained composite was more than 91.1% when the initial concentration of RhB was 10 mg/L, the dosage of MnFe2O4/KFS800 was 0.2 g/L, and the initial pH was 6.7. Then the anti-interference ability of the obtained composite was studied, and it was found that there was a little effect on the degradation of Rhodamine B with the presence of humic acid. Finally, quenching test, EPR research, and XPS analysis were conducted to reveal the catalytic mechanism, and possible mechanism was a synergistic behavior of free radicals (SO4•-, •OH, O2•-) and nonfree radicals (1O2), and trace amounts of uncarbonized bagasse was also involved in the formation of free radicals.

Water hyacinth derived hierarchical porous biochar absorbent: Ideal peroxydisulfate activator for efficient phenol degradation via an electron-transfer pathway

In this paper, a facile hydrothermal pretreatment and molten salt activation route was presented for preparing a self-doped porous biochar (HMBC) from a nitrogenous biomass precursor of water hyacinth. With an ultrahigh specific surface area (2240 m2 g-1), well-developed hierarchical porous structure, created internal structural defects and doped surface functionalities, HMBC exhibited an excellent adsorption performance and catalytic activity for phenol removal via peroxydisulfate (PDS) activation. Specifically, the porous structure promoted the adsorption of PDS on HMBC, forming a highly active HMBC/PDS* complex and thereby increasing the oxidation potential of the system. Meanwhile, the carbon defective structure, graphitic N and CO groups enhanced the electron transfer process, favoring the HMBC/PDS system to catalyze phenol oxidation via an electron transfer dominated pathway. Thus, the system degraded phenol effectively with an ultralow activation energy of 4.9 kJ mol-1 and a remarkable oxidant utilization efficiency of 8.2 mol mol-oxidant-1 h-1 g-1. More importantly, the system exhibited excellent resistance to water quality and good adaptability for decontaminating different organic pollutants with satisfactory mineralization efficiency. This study offers valuable insights into the rational designing of a low-cost biochar catalyst for efficient PDS activation towards organic wastewater remediation.

Mineralization versus polymerization pathways in heterogeneous Fenton-like reactions

Fenton reaction has been widespread application in water purification due to the excellent oxidation performances. However, the poor cycle efficiency of Fe(III)/Fe(II) is one of the biggest bottlenecks. In this study, graphite (GP) was used as a green carbon catalyst to accelerate Fenton-like (H2O2/Fe3+ and persulfate/Fe3+) reactions by promoting ferric ion reduction and intensifying diverse peroxide activation pathways. Significantly, the carboxyl group on GP anchors iron ions to form GP-COOFe(III) which promote persulfate adsorption to form surface complexes and induce an electron transfer pathway (ETP). While the electron-rich hydroxyl and carbonyl groups will combine to from GP-COFe(II), a reductive intermediate to activate peroxide to generate free radicals (from H2O2 and PDS) or high-value iron [Fe(IV)] (from PMS). Consequently, different pathways lead to distinct degree of oxidation: i) radicals in H2O2/Fe3+/GP prefer to mineralize bisphenol A (BPA) with no selectivity; ii) Fe(IV) in PMS/Fe3+/GP partially oxidizes BPA but cannot open the aromatic ring; iii) ETP in PMS/ or PDS/Fe3+/GP drives coupling reactions to form polymeric products covered on catalyst surface. Thus, rational engineering surface functionality of graphite and selecting proper peroxides can realize on-demand selectivity and oxidation capacity in Fenton-like systems.

Fenton-like activity and pathway modulation via single-atom sites and pollutants comediates the electron transfer process

The studies on the origin of versatile oxidation pathways toward targeted pollutants in the single-atom catalysts (SACs)/peroxymonosulfate (PMS) systems were always associated with the coordination structures rather than the perspective of pollutant characteristics, and the analysis of mechanism commonality is lacking. In this work, a variety of single-atom catalysts (M-SACs, M: Fe, Co, and Cu) were fabricated via a pyrolysis process using lignin as the complexation agent and substrate precursor. Sixteen kinds of commonly detected pollutants in various references were selected, and their lnkobs values in M-SACs/PMS systems correlated well (R2 = 0.832 to 0.883) with their electrophilic indexes (reflecting the electron accepting/donating ability of the pollutants) as well as the energy gap (R2 = 0.801 to 0.840) between the pollutants and M-SACs/PMS complexes. Both the electron transfer process (ETP) and radical pathways can be significantly enhanced in the M-SACs/PMS systems, while radical oxidation was overwhelmed by the ETP oxidation toward the pollutants with lower electrophilic indexes. In contrast, pollutants with higher electrophilic indexes represented the weaker electron-donating capacity to the M-SACs/PMS complexes, which resulted in the weaker ETP oxidation accompanied with noticeable radical oxidation. In addition, the ETP oxidation in different M-SACs/PMS systems can be regulated via the energy gaps between the M-SACs/PMS complexes and pollutants. As a result, the Fenton-like activities in the M-SACs/PMS systems could be well modulated by the reaction pathways, which were determined by both electrophilic indexes of pollutants and single-atom sites. This work provided a strategy to establish PMS-based AOP systems with tunable oxidation capacities and pathways for high-efficiency organic decontamination.

Critical Functions of Soil Components for In Situ Persulfate Oxidation of Sulfamethoxazole: Inherent Fe(II) Minerals-Coordinated Nonradical Pathway

Naturally occurring iron (Fe) minerals have been proved to activate persulfate (PS) to generate reactive species, but the role of soil-inherent Fe minerals in activating PS as well as the underlying mechanisms remains poorly understood. Here, we investigated sulfamethoxazole (SMX) degradation by PS in two Fe-rich soils and one Fe-poor soil. Unlike with the radical-dominant oxidation processes in Fe-poor soil, PS was effectively activated through nonradical pathways (i.e., surface electron-transfer) in Fe-rich soils, accounting for 68.4%-85.5% of SMX degradation. The nonradical mechanism was evidenced by multiple methods, including electrochemical, in situ Raman, and competition kinetics tests. Inherent Fe-based minerals, especially those containing Fe(II) were the crucial activators of PS in Fe-rich soils. Compared to Fe(III) minerals, Fe(II) minerals (e.g., ilmenite) were more liable to form Fe(II) mineral-PS* complexes to initiate the nonradical pathways, oxidizing adjacent SMX via electron transfer. Furthermore, mineral structural Fe(II) was the dominant component to coordinate such a direct oxidation process. After PS oxidation, low-crystalline Fe minerals in soils were transformed into high-crystalline Fe phases. Collectively, our study shows that soil-inherent Fe minerals can effectively activate PS in Fe-rich soils, so the addition of exogenous iron might not be required for PS-based in situ chemical oxidation. Outcomes also provide new insights into the activation mechanisms when persulfate is used for the remediation of contaminated soils.

New insights into Co3O4-carbon nanotube membrane for enhanced water purification: Regulated peroxymonosulfate activation mechanism via nanoconfinement

Co-based peroxymonosulfate (PMS) activation system with fascinating catalytic performance has become a promising technology for water purification, but it always suffers from insufficient mass transfer, less exposed active sites and toxic metal leaching. In this work, a carbon nanotube membrane confining Co3O4 inside (Co3O4-in-CNT) was prepared and was coupled with PMS activation (catalytic membrane process) for sulfamethoxazole (SMX) removal. Compared with counterpart with surface-loaded Co3O4 (Co3O4-out-CNT), the Co3O4-in-CNT catalytic membrane process exhibited enhanced SMX removal (99.5% vs. 89.1%) within residence time of 2.89 s, reduced Co leaching (20 vs. 147 μg L-1) and more interestingly, the nonradical-to-radical mechanism transformation (from 1O2 and electron transfer to SO4•- and •OH). These phenomena were ascribed to the nanoconfinement effect in CNT, which enhanced mass transfer (2.80 × 10-4 vs. 5.98 × 10-5 m s-1), accelerated Co3+/Co2+ cycling (73.4% vs. 65.0%) and showed higher adsorption energy for PMS (cleavage of O-O bond). Finally, based on the generated abundant reactive oxygen species (ROS), the seven degradation pathways of SMX were formed in system.

Using waste to treat waste: facile synthesis of hollow carbon nanospheres from lignin for water decontamination

Lignin, the most abundant natural material, is considered as a low-value commercial biomass waste from paper mills and wineries. In an effort to turn biomass waste into a highly valuable material, herein, a new-type of hollow carbon nanospheres (HCNs) is designed and synthesized by pyrolysis of biomass dealkali lignin, as an efficient nanocatalyst for the elimination of antibiotics in complex water matrices. Detailed characterization shows that HCNs possess a hollow nanosphere structure, with abundant graphitic C/N and surface N and O-containing functional groups favorable for peroxydisulfate (PDS) activation. Among them, HCN-500 provides the maximum degradation rate (95.0%) and mineralization efficiency (74.4%) surpassing those of most metal-based advanced oxidation processes (AOPs) in the elimination of oxytetracycline (OTC). Density functional theory (DFT) calculations and high-resolution mass spectroscopy (HR-MS) were employed to reveal the possible degradation pathway of OTC elimination. In addition, the HCN-500/PDS system is also successfully applied to real antibiotics removal in complex water matrices (e.g. river water and tap water), with excellent catalytic performances.

Active Center Size-Dependent Fenton-Like Chemistry for Sustainable Water Decontamination

Accurately controlling catalytic activity and mechanism as well as identifying structure-activity-selectivity correlations in Fenton-like chemistry is essential for designing high-performance catalysts for sustainable water decontamination. Herein, active center size-dependent catalysts with single cobalt atoms (CoSA), atomic clusters (CoAC), and nanoparticles (CoNP) were fabricated to realize the changeover of catalytic activity and mechanism in peroxymonosulfate (PMS)-based Fenton-like chemistry. Catalytic activity and durability vary with the change in metal active center sizes. Besides, reducing the metal size from nanoparticles to single atoms significantly modulates contributions of radical and nonradical mechanisms, thus achieving selective/nonselective degradation. Density functional theory calculations reveal evolutions in catalytic mechanisms of size-dependent catalytic systems over different Gibbs free energies for reactive oxygen species generation. Single-atom site contact with PMS is preferred to induce nonradical mechanisms, while PMS dissociates and generates radicals on clusters and nanoparticles. Differences originating from reaction mechanisms endow developed systems with size-dependent selectivity and mineralization for treating actual hospital wastewater in column reactors. This work brings an in-depth understanding of metal size effects in Fenton-like chemistry and guides the design of intelligent catalysts to fulfill the demand of specific scenes for water purification.

Mechanism insights into photo-assisted peroxymonosulfate activation on oxygen vacancy-enriched nolanites via an electron transfer regime

The utilization of photo-assisted persulfate activation for the removal of organic contaminants in water has garnered significant research interest in recent times. However, there remains a lack of clarity regarding specific contributions of light irradiation and catalyst structure in this process. Herein, a photo-assisted peroxymonosulfate (PMS) activation system is designed for the highly efficient degradation of organic contaminants on oxygen vacancy-enriched nolanites (Vo-FVO). Results suggest that the degradation of bisphenol A (BPA) in this system is a nonradical-dominated process via an electron transfer regime, in which VO improves the local electron density and thus facilitates the electron shuttling between BPA and PMS. During BPA degradation, PMS adsorbed at the surface of FVO-180 withdraws electrons near VO and forms FVO-PMS* complexes. Upon light irradiation, photoelectrons effectively restore the electron density around VO, thereby enabling a sustainable electron transfer for the highly efficient degradation of BPA. Overall, this work provides new insights into the mechanism of persulfate activation based on defects engineering in nolanite minerals.

O2 plasma-modified carbon nanotube for sulfamethoxazole degradation via peroxymonosulfate activation: Synergism of radical and non-radical pathways boosting water decontamination and detoxification

Sulfamethoxazole (SMX), a widely used antibiotic, has triggered increasing attention due to its extensive detection in wastewater effluent, causing serious ecological threats. Herein, a carbon-based heterogeneous catalyst was developed by the O2 plasma-etching process, regulating oxygen-containing functional groups (OFGs) and defects of carbon nanotubes (O-CNT) to activate peroxymonosulfate (PMS) for highly efficient SMX abatement. Through adjusting the etching time, the desired active sites (i.e., C=O and defects) could be rationally created. Experiments collectively suggested that the degradation of SMX was owing to the contribution of synergism by radical (•OH (17.3%) and SO4•- (39.3%)) and non-radical pathways (1O2, 43.4%), which originated from PMS catalyzed by C=O and defects. In addition, the possible degradation products and transformation pathways of SMX in the system were inferred by combining the Fukui function calculations and the LC-MS/MS analysis. And the possible degradation pathway was effective in reducing the environmental toxicity of SMX, as evidenced by the T.E.S.T. software and the micronucleus experiment on Vicia faba root tip. Also, the catalytic system exhibited excellent performance for different antibiotics removal, such as amoxicillin (AMX), carbamazepine (CBZ) and isopropylphenazone (PRP). This study is expected to provide an alternative strategy for antibiotics removal in water decontamination and detoxification.

Singlet oxygen in the removal of organic pollutants: An updated review on the degradation pathways based on mass spectrometry and DFT calculations

The degradation of pollutants by a non-radical pathway involving singlet oxygen (1O2) is highly relevant in advanced oxidation processes. Photosensitizers, modified photocatalysts, and activated persulfates can generate highly selective 1O2 in the medium. The selective reaction of 1O2 with organic pollutants results in the evolution of different intermediate products. While these products can be identified using mass spectrometry (MS) techniques, predicting a proper degradation mechanism in a 1O2-based process is still challenging. Earlier studies utilized MS techniques in the identification of intermediate products and the mechanism was proposed with the support of theoretical calculations. Although some reviews have been reported on the generation of 1O2 and its environmental applications, a proper review of the degradation mechanism by 1O2 is not yet available. Hence, we reviewed the possible degradation pathways of organic contaminants in 1O2-mediated oxidation with the support of density functional theory (DFT). The Fukui function (FF, f-, f+, and f0), HOMO-LUMO energies, and Gibbs free energies obtained using DFT were used to identify the active site in the molecule and the degradation mechanism, respectively. Electrophilic addition, outer sphere type single electron transfer (SET), and addition to the hetero atoms are the key mechanisms involved in the degradation of organic contaminants by 1O2. Since environmental matrices contain several contaminants, it is difficult to experiment with all contaminants to identify their intermediate products. Therefore, the DFT studies are useful for predicting the intermediate compounds during the oxidative removal of the contaminants, especially for complex composition wastewater.

Ultrafast and energy-saving microwave-assisted conversion of inert carbon nanomaterials to highly efficient Fenton-like metal-free catalysts for pollutants degradation

Carbon-driven persulfate (PDS)-based Fenton-like reactions have been widely viewed as prospective strategies to cope with the water pollution. However, high cost, harsh condition and complex modification processes are usually required to boost the catalytic activities of carbocatalysts. Herein, we proposed an ultrafast, energy-efficient, and convenient approach to convert various low-performance carbon materials into highly efficient catalysts by microwave treatment in just 1 min without any other tedious treatment. This process only requires 57 kJ/g energy input, 5 orders of magnitude lower than the traditional calcination process. The catalytic performance of microwave-treated materials could increase by more than 380 times, which is even better than those of the single-atom catalysts. Moreover, DFT calculations and QSARs analyses reveal that the negatively charged carboxyl group is not conducive to the adsorption of PDS (S2O82-) due to electrostatic repulsion, and also increases the work function of the carbocatalysts, which hinders the electron transfer process.

Electron Delocalization Realizes Speedy Fenton-Like Catalysis over a High-Loading and Low-Valence Zinc Single-Atom Catalyst

A zinc (Zn)-based single-atom catalyst (SAC) is recently reported as an active Fenton-like catalyst; however, the low Zn loading greatly restricts its catalytic activity. Herein, a molecule-confined pyrolysis method is demonstrated to evidently increase the Zn loading to 11.54 wt.% for a Zn SAC (ZnSA -N-C) containing a mixture of Zn-N4 and Zn-N3 coordination structures. The latter unsaturated Zn-N3 sites promote electron delocalization to lower the average valence state of Zn in the mix-coordinated Zn-Nx moiety conducive to interaction of ZnSA -N-C with peroxydisulfate (PDS). A speedy Fenton-like catalysis is thus realized by the high-loading and low-valence ZnSA -N-C for PDS activation with a specific activity up to 0.11 min L-1 m-2 , outstripping most Fenton-like SACs. Experimental results reveal that the formation of ZnSA -N-C-PDS* complex owing to the strong affinity of ZnSA -N-C to PDS empowers intense direct electron transfer from the electron-rich pollutant toward this complex, dominating the rapid bisphenol A (BPA) elimination. The electron transfer pathway benefits the desirable environmental robustness of the ZnSA -N-C/PDS system for actual water decontamination. This work represents a new class of efficient and durable Fenton-like SACs for potential practical environmental applications.

Thermodynamic and Kinetic Behaviors of Persulfate-Based Electron-Transfer Regime in Carbocatalysis

A carbon-based advanced oxidation process is featured for the nonradical electron-transfer pathway (ETP) from electron-donating organic compounds to activated persulfate complexes, enabling it as a green technology for the selective oxidation of organic pollutants in complex water environments. However, the thermodynamic and kinetic behaviors of the nonradical electron-transfer regime had been ambiguous due to a neglect of the influence of pH on the mechanisms. In this study, three kinds of organic pollutants were divided in the carbon-based ETP regime: (i) physio-adsorption, (ii) adsorption-dominated ETP (oxidation rate slightly surpasses adsorption rate), and (iii) oxidation-dominated ETP (oxidation rate outpaces the adsorption rate). The differential kinetic behaviors were attributed to the physicochemical properties of the organic pollutants. For example, the hydrophobicity, molecular radius, and positive electrostatic potential controlled the mass-transfer process of the adsorption stage of the reactants (peroxydisulfate (PDS) and organics). Meanwhile, other descriptors, including the Fukui index, oxidation potential, and electron cloud density regulated the electron-transfer processes and thus the kinetics of oxidation. Most importantly, the oxidation pathways of these organic pollutants could be altered by adjusting the water chemistry. This study reveals the principles for developing efficient nonradical systems to selectively remove and recycle organic pollutants in wastewater.

Carbocatalysts for Enhancing Permanganate Oxidation of Sulfisoxazole

Permanganate (Mn(VII)) is extensively applied in water purification due to its stability and ease of handling, but it is a mild oxidant for trace organic contaminants (TrOCs). Hence, there is significant interest in strategies for enhancing reaction kinetics, especially in combination with efficient and economical carbocatalysts. This study compared the performance of four carbocatalysts (graphite, graphene oxide (GO), reduced-GO (rGO), and nitrogen-doped rGO (N-rGO)) in accelerating sulfisoxazole (SSX) oxidation by Mn(VII) and found that GO exhibited the greatest catalytic performance. Besides, the Mn(VII)/GO system shows desirable capacities to remove a broad spectrum of TrOCs. We proposed that the degradation of SSX in Mn(VII)-GO suspensions follows two routes: (i) direct oxidation of SSX by Mn species [both Mn(VII) and in situ formed MnO2(s)] and (ii) a carbocatalyst route, where GO acts as an electron mediator, accepting electrons from SSX and transferring them to Mn(VII). We developed a mathematical model to show the contribution of each parallel pathway and found one-electron transfer is primarily responsible for accelerating SSX removal in the Mn(VII)/GO system. Findings in this study showed that GO provides a simple and effective strategy for enhancing the reactivity of Mn(VII) and provided mechanistic insights into the GO-catalyzed redox reaction between SSX and Mn(VII).

A novel copper oxide/titanium membrane integrated with peroxymonosulfate activation for efficient phenolic pollutants degradation

Herein, a novel CuO catalyst functionalized Ti-based catalytic membrane (FCTM) was prepared via the regulated electro-deposition technique followed with low-temperature calcination. The morphology of CuO catalyst and oxygen vacancy (OV) content can be controlled by adjusting the preparation conditions, under optimal condition (400 °C, electrolyte as sulfuric acid), the fern-shaped CuO catalyst was formed and the OV content was up to its highest level. Under the optimal treatment condition, the 4-chlorophenol (4-CP) removal of the membrane filtration combined with peroxymonosulfate (PMS) activation (MFPA) process was up to 98.2% (TOC removal of 88.2%). Mechanism studying showed that the enhanced performance in this system was mainly due to the increased production of singlet oxygen (1O2) via the co-effect of fern-shaped CuO (increased specific surface area) and its fine-tuned OV (precursor of 1O2), which not only synergistically enhanced adsorption ability but also offered more active sites for PMS activation. Theoretical calculations showed that the OV-rich CuO displayed high adsorption energy for PMS molecule, leading to the change in OO and OH bond (tend to 1O2) of the PMS molecule. Finally, the possible three degradation pathways of 4-CP were formed by the electrophilic attacking of 1O2.

Polar electric field-modulated peroxymonosulfate selective activation for removal of organic contaminants via non-radical electron transfer process

Nonradical electron transfer process (ETP) in peroxomonosulfate (PMS) based advanced oxidation processes (AOPs) is regarded promising for selective degradation of organic contaminants in water, however, the subjective modulation strategy and the definitive mechanistic elucidation of ETP are still lacking. Herein, we proposed a heretofore unreported yet efficient ETP indution approach by construction of polar electrical field on biochar via nonmetallic elements co-doping. Physicochemical characterizations and density functional theory (DFT) calculations verified the electronegativity difference among boron, nitrogen, and sulfur elements bestowed robust local electric fields on biochar surface (BC-BNS), which effectively enhanced the adsorption complexation and charge transfer between biochar and PMS. Compared to the other single-doped or co-doped biochar, BC-BNS exhibited superior catalytic performance of PMS activation for degradation of atrazine (ATZ) (kobs=0.036 min-1), as well as various kinds of electron-rich organics. The remarkable catalytic degradation capacity was further verified in various aqueous matrices and background factors, representing the excellent selectivity. Analysis of contribution from reactive oxygen species and electrochemical testing together substantiated the role of polar electric fields in facilitating the modulation from singlet oxygen (1O2) to ETP as a prevailing mechanism. DFT calculations and apparent interactions revealed the dissociation of S-O bond was thermodynamically favored within this potent localized electric field, which further induced the cleavage of OO bond and ultimately promoted the dual electron transfer between ATZ and PMS. The superiority of BC-BNS/PMS system was further validated with the low ecotoxicity caused by enhanced dechlorination, the low energy consumption, and the long-term effectiveness. The novel modulation principle and atomic-level mechanism exploration gave suggestions for advancing ETP-dominated AOP to remove recalcitrant contaminants during water treatment and restoration.

Insight into the selection of oxidant in persulfate activation system: The effect of the target pollutant properties

As a rising branch of advanced oxidation processes, persulfate activation has attracted growing attention. Unlike catalysts that have been widely studied, the selection of persulfate is previously overlooked. In this study, the affecting factors of persulfates were studied. The effect of target pollutant properties on superior persulfate species (the species with a higher degradation efficiency) was investigated by multiwalled carbon nanotube (MWCNT)/persulfate catalytic systems. Innovatively, the EHOMO (or vertical ionization potential (VIP)) value of the target pollutant was proposed to be an index to judge the superior persulfate species, and the threshold is VIP= 6.397-6.674 eV, EHOMO= -8.035∼- 7.810 eV, respectively. To be specific, when the VIP of phenolic compounds is higher (or EHOMO of phenolic compounds is lower) than the threshold, the catalytic performance of peroxymonosulfate would be higher than that of peroxydisulfate. Moreover, the effects of coexisting cations on peroxydisulfate superior species were further investigated. It was illustrated that the hydrated cation radius of coexisting cations would influence the pollutant degradation efficiency under some circumstances. This study provides a new approach to improve the cost of persulfate activation systems and promotes the underlying downstream application of persulfate activation systems.

Modulation of radical and nonradical pathways via modified carbon nanotubes toward efficient oxidation of binary pollutants in water

In order to minimize the knowledge gap between single and binary pollutants degradation by persulfate-based advanced oxidation processes (PS-AOPs), iron-loaded N-doped carbon nanotubes (Fe-NCNT) and its acid-washing sample (Fe-NCNT-W) were synthesized as peroxymonosulfate (PMS) activator for simultaneous oxidation of acid orange 7 (AO7) and electron-rich (phenol/ibuprofen) or electron-deficient pollutants (nitrobenzene/benzoic acid). Mechanistic studies revealed that both radical (HO•, SO4•-) and nonradical (electron-transfer, high-valent iron) pathways involved for organic oxidation in Fe-NCNT/PMS system, while electron-transfer pathway (ETP) and high-valent iron-oxo species accounted for pollutant degradation at the surface and inner space of Fe-NCNT-W, respectively. The oxidation performances in single or binary systems were systematically investigated. In comparison to benchmark radical-based (Fe2+/PMS), nonradical ETP (NCNT/PMS) and mixed (Fe-NCNT/PMS) systems, Fe-NCNT-W/PMS outperformed superior performance toward oxidation of binary pollutants with little inference from solution pH or background substances, which could also be fabricated into membrane reactor for actual dyeing sewage treatment. Such superiorities should be mainly ascribed to the particular selectivity and intensive treatment of nonradical pathways in Fe-NCNT-W/PMS system with nanoconfinement effect. This work affords novel insights into the treatment of combined pollution via PMS activation by engineered nanomaterials.

Insights into the electron transfer regime of permanganate activation on carbon nanomaterial reduced from carbon dioxide

Simultaneously eliminating novel contaminants in the water environment while also achieving high-value utilization of CO2 poses a significant challenge in water purification. Herein, a CO2-reduced carbon catalyst (CRC) was synthesized via the chemical vapor deposition method for permanganate (PM) activation, fulfilling the ultra-efficient removal of bisphenol A (BPA). The primary mechanism responsible for the BPA degradation in the CRC/PM process is electron transfer. Hydroxyl groups and defect structures on CRC act as electron mediators, facilitating the transfer of electrons from contaminants to PM. On the basis of the quantitative structure-activity relationship, the elimination performance of the CRC/PM process exhibited variability in accordance with the inherent characteristics of pollutants. In addition, the yield of manganese intermediates was also observed in the CRC/PM process, which only serve as redox intermediates rather than active species attacking organics. Ascribed to nonradical mechanisms, the CRC/PM system exhibited remarkable stability and demonstrated significant resistance to the presence of background substances. Moreover, BPA degradation pathways were clarified via mass spectrometry analysis and density functional theory calculations, with intermediate products exhibiting lower toxicity. This study provided new insights into the employment of carbon catalysts derived from CO2 for PM nonradical activation to degrade contaminants in various water matrices.

Enhanced ferrate(VI) oxidation of organic pollutants through direct electron transfer

Fe(VI) is a versatile agent for water purification, and various strategies have been developed to improve its pollutant removal efficiency. Herein, it was found that in addition to intermediate iron species [Fe(IV)/Fe(V)], direct electron transfer (DET) played a significant role in the abatement of organic pollutants in Fe(VI)/carbon quantum dots (CQDs) system. Around 86, 83, 73, 64, 52, 45 and 17% of BPA, DCF, SMX, 4-CP, phenol, p-HBA, and IBP (6 μM) could be oxidized by 30 μM of Fe(VI), whereas with the addition of CQDs (4 mg/L), the oxidation ratio of these pollutants increased to 98, 99, 80, 88, 87, 66 and 57%, respectively. The negative impact induced by solution pH and background constituents on Fe(VI) abatement of pollutants could be alleviated by CQDs, and CQDs acted as catalysts for mediating DET from organic pollutants to Fe(VI). Theoretical calculation revealed that iron species [Fe(VI)/Fe(V)/Fe(IV)] was responsible for the oxidation of 36% of phenol, while DET contributed to the oxidation of 64% of phenol in the Fe(VI)/CQDs system. Compared with iron species oxidation, the CQDs mediated DET from pollutants to Fe(VI) was more efficient for utilizing the oxidation capacity of Fe(VI). The DET mechanism presented in the study provides a prospective strategy for improving the pollution control potential of Fe(VI).

Activation of peroxymonosulfate by Fe-Mn-modified MWCNTs for selective decontamination: Formation of high-valent metal-oxo species and superoxide anion radicals

The extensive presence of organic micropollutants in complex water matrices requires the development of selective oxidation technologies. In this study, a novel selective oxidation process was developed via the conjunction of FeMn/CNTs with peroxymonosulfate and successfully used to remove micropollutants such as sulfamethoxazole (SMX) and bisphenol A from aqueous solutions. FeMn/CNTs were prepared using a facile co-precipitation method, characterized using a series of surface characterization techniques, and then tested for pollutant removal. The results showed that the FeMn/CNTs had much greater reactivity than CNTs, manganese oxide, and iron oxide. The pseudo-first-order rate constant with FeMn/CNTs was more than 2.9-5.7 times that of the other tested materials. The FeMn/CNTs had great reactivity in a wide range of pH values from 3.0 to 9.0, with the best reactivity found at pH values of 5.0 and 7.0. High-valent metal-oxo species such as Fe(IV)O and Mn(IV)O and superoxide anion radicals were determined to be the reactive species and were responsible for the oxidation of SMX. These reactive species were selective; therefore, the overall removal performance of SMX was not obviously influenced by high levels of water components including chloride ions, bicarbonates, and natural organic matters. The results from this study may promote the design and application of selective oxidation technologies for micropollutant abatement.

Unexpectedly Enhanced Organics Removal in Persulfate Oxidation with High Concentration of Sulfate: The Origin and the Selectivity

Massive anions in high saline wastewater are primary factors that restricted the efficiency of pollutant degradation in advanced oxidation processes (AOPs). Herein, we reported the influence laws of different anions at high concentration on the electron-transfer process in the activation of persulfate, and especially, the sulfate anion exhibited the excellent promotion effect. Depending on the ionic charge, polarizability, and size, the anions exerted diverse effects on the dispersed phase and zeta potential of carbonaceous catalysts, which further embodied in the removal of pollutants. Based on the differences of reaction rate constant in water solution and high saline solution, the order was ClO4- < NO3- < Cl- < SO42- < CO32-, obeying the Hofmeister series. The enhancement of the sulfate anion was widely confirmed with different carbonaceous catalysts and pollutants with various structures. It could be attributed to the higher oxidation capacity, the faster interfacial electron transfer, and the better catalyst dispersion in the high sulfate environment. On the other hand, the decrease of zeta potential of the catalyst induced by sulfate reinforced the electrostatic attraction or repulsion with pollutants, which caused the selectivity of the sulfate promotion effect. Overall, this study provides new insights into the mechanism of influence of anions on AOPs, which refreshed the cognition of the role of sulfate on pollutant degradation, and helps guide the treatment design of high salinity wastewater.

Selective Removal of Organic Pollutants in Groundwater and Surface Water by Persulfate-Assisted Advanced Oxidation: The Role of Electron-Donating Capacity

The efficiency of persulfate-assisted advanced oxidation processes (PS-AOPs) in degrading organic pollutants is affected by the electron-donating capability of organic substances present in the water source. In this study, we systematically investigate the electron-donating capacity (EDC) difference between groundwater and surface water and demonstrate the dependence of removal efficiency on the EDC of target water by PS-AOPs with carbon nanotubes (CNTs) as a catalyst. Laboratory analyses and field experiments reveal that the CNT/PS system exhibits higher performance in organic pollutant removal in groundwater with a high concentration of phenols, compared to surface water, which is rich in quinones. We attribute this disparity to the selective electron transfer pathway induced by potential difference between PS-CNT and organic substance-CNT intermediates, which preferentially degrade organic substances with stronger electron-donating capability. This study provides valuable insights into the inherent selective removal mechanism and application scenarios of electron transfer process-dominated PS-AOPs for water treatment based on the electron-donating capacity of organic pollutants.

Insights into the carbon nanotubes-mediated activation of permanganate for decontamination under high salinity

Radical-based advanced oxidation process (AOPs) has attracted great interests in wastewater treatment field. However, by the traditional radical-based method, the degradation of organic pollution is greatly suppressed when radicals react with the co-existing anions in the solution. Herein, an efficient method for degrading of contaminant under high salinity conditions is discussed through a non-radical pathway. Carbon nanotubes (CNTs) was employed as an electron transfer medium to facilitate the electron conversion from contaminants to potassium permanganate (PM). Based the results of quenching experiments, probe experiments, and galvanic oxidation process experiments, the degradation mechanism of CNTs/PM process was demonstrated to be electron transfer, rather than reactive intermediate Mn species. As a result, typical influencing factors including salt concentration, cations, and humic acid have less of an impact on degradation during CNTs/PM processes. In addition, the CNTs/PM system exhibits superior reusability and universality of pollutants, which has the potential to be applied as a non-radical pathway for the purification of contaminant in the large-scale high salinity wastewater treatment.

MOF-Derived Ni/ZIF-8/ZnO Arrays on Carbon Fiber Cloth for Efficient Adsorption-Catalytic Oxidation

The catalytic oxidation of toxic organic pollutants in water requires enhanced efficiency for commercial applications. A ZnO nanorod array grown on a carbon fiber cloth (CFC) serves as the zinc source to ensure that the Ni/ZIF-8/ZnO nanoreactor is constructed. The Ni/ZIF-8/ZnO/CFC nanoreactor efficiently activates peroxymonosulfate (PMS) for bisphenol A (BPA) degradation owing to its high density of active sites, high adsorbability, and dispersibility structure, which concentrates catalytic and adsorptive sites within a confined space. Experimental and theoretical calculations clearly show that the introduction of Ni is beneficial for improving the adsorption of BPA and the activation of PMS. The synergistic mechanism of BPA adsorption-PMS activation is also investigated, and the degradation pathway of BPA is examined. Moreover, a filter catalytic unit is constructed using Ni/ZIF-8/ZnO/CFC to achieve a continuous zero discharge of BPA, which is convenient for nanocatalyst recycling. This study aims to develop a new strategy for the removal of emerging organic pollutants from water using a system with strong adsorption and catalytic capabilities.

Metal Borides as Excellent Co-Catalysts for Boosted and Long-Lasting Fenton-like Reaction: Dual Co-Catalytic Centers of Metal and Boron

The continuous electron supply for oxidant decomposition-induced reactive oxygen species (ROS) generation is the main contributor for the long-standing micropollutant oxidation in the iron-based advanced oxidation processes (AOPs). Herein, as a new class of co-catalysts, metal borides with dual active sites and preeminent conductive performance can effectively overcome the inherent drawback of Fenton-like reactions by steadily donating electrons to inactive Fe(III). Among the metal borides, tungsten boride (WB) exhibits a significant co-catalytic performance run ahead of common heterogeneous co-catalysts and exceptionally high stability. Based on qualitative and semi-quantitative tests, the hydroxyl radical, sulfate radical, and iron(IV)-oxo complex are all produced in the WB/Fe(III)/PDS system and Fe(IV)-induced methyl phenyl sulfoxide decomposition is up to 72%. Moreover, the production efficiency of ROS and relative proportions of radical and nonradical pathways change with various experimental conditions (dosages of PDS, WB, and solution pH) and water matrices. The rate-determining step of Fe(II) regeneration is greatly accelerated resulting from the synergetic effect between exposed metallic reactive sites and nonmetallic boron with reductive properties of WB. In addition, the self-dissolution of surface tungsten oxide and boron oxide leads to a renovated surface for sustainable Fe(III) reduction in long-term operations. Our discovery provides an efficient and sustainable strategy in the field of enhanced AOPs for water remediation.

Merits and Limitations of Radical vs. Nonradical Pathways in Persulfate-Based Advanced Oxidation Processes

Urbanization and industrialization have exerted significant adverse effects on water quality, resulting in a growing need for reliable and eco-friendly treatment technologies. Persulfate (PS)-based advanced oxidation processes (AOPs) are emerging as viable technologies to treat challenging industrial wastewaters or remediate groundwater impacted by hazardous wastes. While the generated reactive species can degrade a variety of priority organic contaminants through radical and nonradical pathways, there is a lack of systematic and in-depth comparison of these pathways for practical implementation in different treatment scenarios. Our comparative analysis of reaction rate constants for radical vs. nonradical species indicates that radical-based AOPs may achieve high removal efficiency of organic contaminants with relatively short contact time. Nonradical AOPs feature advantages with minimal water matrix interference for complex wastewater treatments. Nonradical species (e.g., singlet oxygen, high-valent metals, and surface activated PS) preferentially react with contaminants bearing electron-donating groups, allowing enhancement of degradation efficiency of known target contaminants. For byproduct formation, analytical limitations and computational chemistry applications are also considered. Finally, we propose a holistically estimated electrical energy per order of reaction (EE/O) parameter and show significantly higher energy requirements for the nonradical pathways. Overall, these critical comparisons help prioritize basic research on PS-based AOPs and inform the merits and limitations of system-specific applications.

Key role of Fe(VI)-activated Bi2WO6 in the photocatalytic oxidation of sulfonamides: Mediated electron transfer mechanism

The widespread use of sulfonamides (SAs) in animals and human infections has raised significant concerns regarding their presence in ambient waterways and potential for inducing antimicrobial resistance. Herein, we report on the capacity of ferrate (VI) (Fe^VIO₄^2-, Fe(VI)) to facilitate the photocatalytic degradation of sulfamethazine (SMT) via bismuth tungstate (Bi₂WO₆, BWO) under blue LED light (Vis/BWO/Fe(VI)) exposure, at rates that were 45-fold faster than BWO photocatalysis. Both the stepwise and time-series addition of Fe(VI) contributed to the degradation. Multiple lines of evidence confirmed that the common reactive species (RSs) in BWO-based photocatalytic systems and Fe(VI)-involved systems (e.g., •OH/h⁺, O₂^•-, ¹O₂ and Fe(V)/Fe(IV)) played subtle roles in our study system. Herein, for the first time, it was discovered that the precursor complex (BWO-Fe(V)/Fe(IV)* )) was the main contributor to induce electron transfer of SAs through the "conductive bridge" effect of BWO. The studied system was able to effectively degrade SMT in synthetic hydrolyzed urine (SHU) with low interference from background substances in water. This work not only offers a novel facilitation strategy for BWO, but also holds a great application prospect for contamination remediation in urine.

Enhanced degradation of pharmaceuticals in wastewater by coupled radical and non-radical pathways: Further unravelling kinetics and mechanism

Advanced oxidation processes based on radicals and/or non-radical catalysis are emerging as promising technologies for eliminating pharmaceuticals (PhACs) from wastewater. However, the respective contributions of different removal pathways (radicals or non-radical) for PhAC degradation still lacks quantitative investigation. Zero-valent iron and carbon nanotubes are frequently used to generate both radicals and non-radical species via the activation of persulfate (Fe⁰/SWCNT/PDS). Herein, the removal kinetics of 1 μM PhACs are depicted, and the corresponding synergistic mechanism of the Fe⁰/SWCNT/PDS process is discussed. Coupled removal pathways showed the higher degradation of PhACs than the individual pathways. Radicals quenching studies combined with electron spin resonance characterisation suggested that the radical-based removal pathway tends to attack electron-deficient organics, whereas its counterpart is more likely to work on electron-rich organics. From the perspectives of the contribution rate, the redox cycles of conjugated Fe species play a more important role in the generation of radicals than free Fe species, and the faster electron transfer in the conductive bridge offered by SWCNT is responsible for the effective corrosion of Fe⁰ and the decomposition of PDS. Six real wastewater samples were used to prove the generality of the above removal contribution, regardless of the wastewater samples, and the results suggested that identical attack patterns were obtained in all real wastewater samples, although coexistence matrix slightly suppressed PhAC removal. This work provides a deeper insight into the high-performance working mechanism on synergistic interactions and contaminant removal in a combined catalysis system.

Chitin-derived biochar with nitrogen doping to activate persulfate for phenol degradation: Application potential and electron transfer pathway in system

The fast and efficient removal of organic pollutants (e.g., phenolics) remains one of the focus problems in environment pollution. Thus, a chitin-derived biochar with nitrogen doping (N-BC) was successfully prepared at a lower calcination temperature of 600 °C, which is environmentally friendly and energy saving. The N-BC was analyzed by SEM, FTIR, BET, XRD, XPS and Raman spectroscopy to confirm that the doping of nitrogen element provided sufficient defect sites to promote the activation of persulfate (PDS). Quenching experiments and EPR results revealed the presence of •OH and •O₂^- contributed to phenol degradation in N-BC 600/PDS system. In addition, the linear sweep voltammogram experiments also demonstrated the existence of electron transfer pathway. The electrons were donated from phenol and shifted to PDS through N-BC. The graphitic N and carbon defects in N-BC served as the active sites of the reaction and involved absorption and transfer of electrons as the key character. Moreover, the removal rates of phenol and TOC reached 98.8% and 58.2% within 2 h, indicating that N-BC effectively activated the persulfate to degrade phenol. This study provides the theoretical support and potential applications for the activation of persulfate by nitrogen-doped biochar to degrade other phenolic compounds.