Activation of Peroxydisulfate on Carbon Nanotubes: Electron-Transfer Mechanism

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.

MgxCu-biochar activated peroxydisulfate triggers reductive species for the reduction and enhanced electron-transfer degradation of electron-deficient aromatic pollutants

In wastewater treatment by persulfate-based advanced oxidation processes (PS-AOPs), electron-deficient aromatic pollutants (EDAPs) are refractory to nonradical pathway. To explore an efficient degradation pathway for EDAPs, Mg_xCu-biochar (BC) (x = 0.5, 1, 1.5) activated peroxydisulfate (PDS) was developed, which could trigger reductive species (•H) to reduce EDAPs first, and subsequently facilitate electron-transfer degradation of reduced intermediates. The roles of Mg-doping in Mg_xCu-BC to promote PDS activation and 2,4-dibromophenol (DBP) degradation were investigated. The mechanisms were then explored via electron paramagnetic resonance (EPR), chemical probes and Density Functional Theory (DFT) calculations. The results showed that Mg-doping improved metal-support interactions (MSIs) of Mg_xCu-BC, inducing •H formation via electron transfer from Cu atoms during PDS activation, which was thermodynamically favorable. The degradation rate of DBP (k_obs, 0.0494 min^-1) and Br^- release (5.35 mg L^-1) in Mg₁Cu-BC systems were more 31 and 33 times than that in Cu-BC/PDS system, respectively. The degradation mechanism of •H-enhanced electron transfer processes was that •H attacked one Br group of DBP, and then debrominated intermediates were mineralized by electron transfer processes in the Mg₁Cu-BC/PDS system. Overall, this study reports a novel pathway in PS-AOPs for selective degradation of EDAPs in wastewaters.

Dynamic active-site induced by host-guest interactions boost the Fenton-like reaction for organic wastewater treatment

In heterogeneous catalysis, uncovering the dynamic evolution of active sites in the working conditions is crucial to realizing increased activity and enhanced stability of catalyst in Fenton-like activation. Herein, we capture the dynamic changes in the unit cell of Co/La-SrTiO₃ catalyst during the exemplary peroxymonosulfate activation process using X-ray absorption spectroscopy and in situ Raman spectroscopy, revealing the substrate tuned its structural evolution, which is the reversible stretching vibration of O-Sr-O and Co/Ti-O bonds in different orientations. This process effectively promotes the generation of key SO₅* intermediates, which is beneficial to the formation of ¹O₂ and SO₄^•- from persulfate on the Co active site. Density functional theory and X-ray absorption spectroscopy show that the optimized structural distortion enhanced the metal-oxygen bond strength by tuning the e_g orbitals and increased the number of transferred electrons to peroxymonosulfate by about 3-fold, achieving excellent efficiency and stability in removing organic pollutants.

Strong coupling of super-hydrophilic and vacancy-rich g-C3N4 and LDH heterostructure for wastewater purification: Adsorption-driven oxidation

Adsorption and wettability are crucial components of catalytic oxidation. To increase the reactive oxygen species (ROS) generation/utilization efficiency of peroxymonosulfate (PMS) activators, defect engineering and 2D nanosheet characteristics were used to regulate electronic structures and expose more active sites. Two-dimensional (2D) super-hydrophilic heterostructure by connecting cobalt species modified nitrogen vacancy-rich g-C₃N₄ (V_n-CN) and LDH (V_n-CN/Co/LDH) with high-density active sites and multi-vacancies, as well as high conductivity and adsorbability, to expedite ROS generation. The degradation rate constant of ofloxacin (OFX) was 0.441 min^-1 via the V_n-CN/Co/LDH/PMS system, which was 1-2 orders greater than in the previous studies. Confirmation of the contribution ratios of various reactive oxygen species (ROS), SO₄^·- and ¹O₂ in bulk solution, O₂^·- on the catalyst surface was the most abundant ROS. The catalytic membrane was constructed utilizing Vn-CN/Co/LDH as the assembly element. The 2D membrane achieved the continuous effective discharge of OFX in the simulated water after 80 h/4 cycles of continuous flowing-through filtration-catalysis. This study provides fresh insights into designing a PMS activator for environmental remediation activated on demand.

Unraveling the role of multi-walled carbon nanotubes in a corn-soil system: Plant growth, oxidative stress and heavy metal(loid)s behavior

In the age of nanotechnological advancement, carbon nanotubes (CNTs) are drawing global attention. However, few studies have been published on the crop growth responses to CNTs in heavy metal(loid)s contaminated environments. A pot experiment was conducted to assess the effect of multi-walled carbon nanotubes (MWCNTs) on plant development, oxidative stress, and heavy metal(loid)s behavior in a corn-soil system. Corn (Zea mays L.) seedlings were cultivated in soil containing Cadmium (Cd) and Arsenic (As) that had been primed with 0, 100, 500, and 1000 mg kg^-1 MWCNTs. The application of 100 and 500 mg kg^-1 MWCNTs improved shoot length by 6.45% and 9.21% after 45 days, respectively. Total plant dry biomass increased by 14.71% when treated with 500 mg kg^-1 MWCNTs but decreased by 9.26% when exposed to 1000 mg kg^-1 MWCNTs. MWCNTs treatment did not affect Cd accumulation in plants. On the other hand, the bio-concentration factor of As was inversely associated with plant growth (p < 0.05), which was declined in MWCNTs treatments. Oxidative stress was aggravated when plants were exposed to MWCNTs, thus activating the antioxidant enzymes system in the corn. In contrast, TCLP-extractable Cd and As in soil significantly decreased than in the control. Additionally, the soil nutrients were changed under MWCNTs treatments. Our findings also revealed that a particular concentration of MWCNTs can mitigate the toxicity of Cd and As in corn seedlings. Therefore, these results suggest the prospective application of CNTs in agricultural production, ensuring environmental and soil sustainability.

Persulfate promoted visible photocatalytic elimination of bisphenol A by g-C3N4-CeO2 S-scheme heterojunction: The dominant role of photo-induced holes

In the last few years, coupling heterogeneous photocatalysis with persulfate (PDS) activation process is an efficient approach to generate abundant reactive oxidative species towards organic contaminant removal in water, however, the key role of PDS in photocatalytic process remains ambiguous. Herein, a novel g-C₃N₄-CeO₂ (CN-CeO₂) step-scheme (S-scheme) composite was constructed to photo-degrade bisphenol A (BPA) with the presence of PDS under visible irradiation. At 2.0 mM PDS, 0.7 g/L CN-CeO₂ and natural pH 6.2, 94.2% of BPA could be eliminated in 60 min under visible light (Vis) illumination. Aside from the previous view of free radical generation, it tends to assume that most of PDS molecules acted as electron sacrificial agents for capturing photo-induced e^- to form sulfate ions, greatly improving the charge carrier separation and thus enhancing the oxidation capacity of nonradical hole (h⁺) for the removal of BPA. Good correlations are further found between the rate constant and descriptor variables (i.e., Hammett constant σ^-/σ⁺ and half-wave potential E_1/2), exhibiting selective oxidation for organic pollutants in the Vis/CN-CeO₂/PDS system. The study brings more insights into mechanistic understanding of persulfate-enhanced photocatalytic process for addressing water decontamination.

Probing the activation mechanism of nitrogen-doped carbonaceous materials for persulfates: Based on the differences between peroxymonosulfate and peroxydisulfate

The activation processes of persulfates by metal-free nitrogen-doped carbonaceous material (NCM) remain unclear due to their complex structures and heterogeneous nature. On the other hand, from the perspective of persulfates, it is possible to clarify the reaction between persulfates and NCM by considering the differences in activation behaviors between peroxymonosulfate (PMS) and peroxydisulfate (PDS). Our study aims to compare the differences between NCM-PDS and NCM-PMS using a fully metal-free NCM as a model catalyst. Firstly, NCM-PDS was more efficient than NCM-PMS in degrading phenolic compounds (PCs). Secondly, the stoichiometric ratio between consumed persulfates and DCP removed in the NCM-PDS (0.73) is lower than in the NCM-PMS (1.08). Thirdly, PMS and PDS adsorb on NCM in different ways, suggesting that the peak O-O bond in PDS has blue shifted from 814 cm^-1 to 805 cm^-1, while that of O-O bond in PMS has shifted from 889 cm^-1 to 834 cm^-1. Additionally, the hydrogen bond between the phenolic group and oxidants plays a critical role in PCs degradation by NCM-PDS, exhibiting a stronger pH effect and higher kinetic isotope effects (KIEs) than NCM-PMS. A proton-coupled electron transfer process has been proposed for PCs degradation using NCM-PDS, and a scheme of reaction pathways has been provided for the NCM-PMS/PDS-PCs system. The study results provide a deeper understanding of the activation of persulfates by NCM, as well as a strategy for selecting oxidants.

Electron-transfer pathways insights into contaminants oxidized by Cu-OOSO3- intermediate: Effects of oxidation states of Cu and solution pH values

The copper-peroxy complex (Cu-OOSO₃^-) metastable intermediate has been confirmed to oxidize contaminants via a single-electron-transfer pathway or an oxygen-atom-transfer pathway. And the effects of Cu oxidation states and reaction pH conditions on the intermediate properties have not been explored in depth. Here, copper oxide (CuO_x) catalysts with different Cu oxidation states were synthesized by a simple precipitation method by controlling the reaction temperature from 0 to 45 °C. CuO_x displayed a strong catalytic dependence on the Cu oxidation state, and CuO_x-30 with Cu average valence on the catalyst surface of 1.61 was more reactive for catalytic degradation of bisphenol A with peroxymonosulfate (PMS). Notably, CuO_x-30, with the best electron-accepting ability, was easier to bonding with PMS to form the Cu-OOSO₃^- reactive complex, and the generated intermediate exhibited the strongest capacity to obtain electrons from contaminants. Moreover, the electron-transfer pathways were closely related to the average valence of Cu, and the contribution of the oxygen-atom-transfer pathway changed volcanic with increasing Cu valence. Meanwhile, the reaction predominantly involved the oxygen-atom-transfer pathway under acidic conditions (pH=3), while the contribution of the single-electron-transfer pathway raised with increasing pH values. Hence, this work was devoted to providing new insights into the CuO_x-inducing PMS activation and vital supplementary to the properties of the Cu-OOSO₃^- intermediate.

Radical-/non-radical-mediated catalyst activation of peroxymonosulfate for efficient atrazine degradation

Efficient degradation technologies are urgent to be developed to avoid the ecological and healthy hazards brought from atrazine (ATZ). LaCoO_3-δ/peroxymonosulfate (PMS) system was proved to have strong degradation capabilities to contaminants. In this work, we intended to investigate the effect of the synthesis method on LaCoO_3-δ. However, the hydrothermal method yielded a new material (H-Co) with better catalytic performance than LaCoO_3-δ, which showed stable catalytic ability at pH 3.0-9.0 and 5 consecutive cycles. The coexistence of inorganic Cl^-, SO₄^2-, NO₃^-, H₂PO₄^-, HCO₃^- and organic humic acids exerted little influences on the H-Co/PMS system. In addition, the actual livestock and poultry breeding wastewater could be well degraded and mineralized by the H-Co/PMS system. Free radical burst experiments and EPR characterization were performed to verify the synergistic effects of free radicals and non-free radicals during ATZ degradation. Based on SEM, XRD, O₂-TPD, FTIR, XPS, and electrochemistry characterizations, the efficient catalytic ability of H-Co could be attributed to the abundant oxygen vacancies, surface hydroxyl groups, zero-valent cobalt sites and high electronic conductivity. The degradation pathways were proposed based on the detection of degradation intermediates of ATZ by UPLC-MS. Moreover, the toxic of ATZ during the oxidation was evaluated by TEXT and E. coli inhibition assay. This work comprehensively analyzed the catalytic reaction mechanism of the H-Co/PMS system and provided a feasible pathway for the treatment of the actual livestock and poultry breeding wastewater.

A review of advanced oxidation process towards organic pollutants and its potential application in fracturing flowback fluid

Fracturing flowback fluid (FFF) including various kinds of organic pollutants that do harms to people and new treatments are urgently needed. Advanced oxidation processes (AOPs) are suitable methods in consideration with molecular weight, removal cost and efficiency. Here, we summarize the recent studies about AOP treatments towards organic pollutants and discuss the application prospects in treatment of FFF. Immobilization and loading methods of catalysts, evaluation method of degradation of FFF, and continuous treatment process flow are discussed in this review. In conclusion, further studies are urgently needed in aspects of catalyst loading methods, macromolecule organic evaluation methods, industrial process, and pathways of macromolecule organics' decomposition.

Exploring degradation properties and mechanisms of emerging contaminants via enhanced directional electron transfer by polarized electric fields regulation in Fe-N4-Cx

Heterogeneous catalysis offers an opportunity to overcome the low efficiency and secondary pollution limitations of emerging contaminants (ECs) purification technologies, but it is still challenging to regulate electron directed transport for achieving high catalysis efficiency and selectivity due to insufficient understanding of the electron transfer pathways and behavioral mechanisms during its catalysis. Here, by tuning the defects of the C-N coordination of the support, the polarized electric field (PEF) characteristics are changed, which in turn affects the electron transport behavior. The results show that the charge offset on Fe-N₄-C_x forms a PEF, which will induce directional electron transport. After the quantitative structure-activity relationship (QSAR) fitting analysis, the greater the degree of C-N defects, the higher the intensity of the PEF, which in turn enhances the electron transport and promotes the catalytic behavior. In addition, the surface pyrrole N site can adsorb enrofloxacin (ENR) and enrich it on the surface. This can reduce the transport distance of reactive oxygen species (ROS) to synergize catalysis and adsorption, resulting in rapid degradation of ECs. Combined with liquid chromatograph mass spectrometer (LC-MS) results and theoretical calculations, five degradation pathways of ENR were speculated, mainly including the oxidation of piperazine and the cleavage of the quinolone ring. This work proposes a novel PEF regulation strategy and explores its mechanism for safe treatment of ECs.

Density Functional Theory and Machine Learning-Based Quantitative Structure-Activity Relationship Models Enabling Prediction of Contaminant Degradation Performance with Heterogeneous Peroxymonosulfate Treatments

Heterogeneous peroxymonosulfate (PMS) treatment is recognized as an effective advanced oxidation process (AOP) for the treatment of organic contaminants. Quantitative structure-activity relationship (QSAR) models have been applied to predict the oxidation reaction rates of contaminants in homogeneous PMS treatment systems but are seldom applied in heterogeneous treatment systems. Herein, we established QSAR models updated with density functional theory (DFT) and machine learning approaches to predict the degradation performance for a series of contaminants in heterogeneous PMS systems. We imported the characteristics of organic molecules calculated using constrained DFT as input descriptors and predicted the apparent degradation rate constants of contaminants as the output. The genetic algorithm and deep neural networks were used to improve the predictive accuracy. The qualitative and quantitative results from the QSAR model for the degradation of contaminants can be used to select the most appropriate treatment system. A strategy for selection of the optimum catalyst for PMS treatment of specific contaminants was also established according to the QSAR models. This work not only increases our understanding of contaminant degradation in PMS treatment systems but also highlights a novel QSAR model to predict the degradation performance in complicated heterogeneous AOPs.

Carbonyl and defect of metal-free char trigger electron transfer and O2- in persulfate activation for Aniline aerofloat degradation

Residual flotation reagents in mineral processing wastewater can trigger severe ecological threats to the local groundwater if they are discharged without treatment. Metal-free biochar-induced persulfate-advanced oxidation processes (KCBC/PS) were used in this study to elucidate the degradation of aniline aerofloat (AAF) - a typical flotation reagent. In KCBC/PS system, AAF can be removed at low doses of catalyst (KCBC, 0.05 g/L) and oxidant (PS, 0.3 mM) additions with high efficiency. The analysis revealed the dominance of O₂^•- among the identified reactive oxygen species (ROS), which achieved deeper mineralization for the AAF degradation in the KCBC/PS system. The role of the electron transfer mechanism was equally important; the importance was corroborated by the chemical quenching experiments, electron spin resonance (ESR) detection, probe experiments, and electrochemical analysis. It benefited from the electron transfer mechanism in the KCBC/PS system and exhibited a wide pH adaptation (3.5-11) and high resistance to inorganic anions for real mining wastewater treatment. Combined with theoretical calculations and other analyses, the carbonyl group was deemed to be the active site of the non-radical pathway of biochar, while the site of the conversion of SO₄^•- to superoxide radicals by biochar activation represented a defect. These findings revealed a synergistic effect of multiple active sites on PS activation in biochar-based materials. Moreover, the intermediate degradation products of AAF from mass spectrometry indicated a possible pathway through the density functional theory (DFT) method, which was effective in reducing the environmental toxicity of pollutants for the first time according to the T.E.S.T software and seed germination experiments. Overall, our study proposed a novel modification strategy for cost-effective and environmentally friendly biochar-based catalysts, while also deepening our understanding of the mechanism of activation of persulfate by metal-free carbon-based materials.

Generation of FeIV =O and its Contribution to Fenton-Like Reactions on a Single-Atom Iron-N-C Catalyst

Generating Fe^IV =O on single-atom catalysts by Fenton-like reaction has been established for water treatment; however, the Fe^IV =O generation pathway and oxidation behavior remain obscure. Employing an Fe-N-C catalyst with a typical Fe-N₄ moiety to activate peroxymonosulfate (PMS), we demonstrate that generating Fe^IV =O is mediated by an Fe-N-C-PMS* complex-a well-recognized nonradical species for induction of electron-transfer oxidation-and we determined that adjacent Fe sites with a specific Fe₁ -Fe₁ distance are required. After the Fe atoms with an Fe₁ -Fe₁ distance <4 Å are PMS-saturated, Fe-N-C-PMS* formed on Fe sites with an Fe₁ -Fe₁ distance of 4-5 Å can coordinate with the adjacent Fe^II -N₄ , forming an inter-complex with enhanced charge transfer to produce Fe^IV =O. Fe^IV =O enables the Fenton-like system to efficiently oxidize various pollutants in a substrate-specific, pH-tolerant, and sustainable manner, where its prominent contribution manifests for pollutants with higher one-electron oxidation potential.

Robust polystyrene resin-supported nano-CoFe2O4 mediated peroxymonosulfate activation for efficient oxidation of 1-hydroxyethane 1,1-diphosphonic acid

Nanosized spinel cobalt ferrite (CoFe₂O₄) shows high performance in peroxymonosulfate (PMS) activation for decontamination in water, but is yet challenged by the easily leached Co(II) with high toxicity. Herein, macroporous polystyrene resin is used as the support to improve the stability of CoFe₂O₄ nanoparticles during PMS activation. CoFe₂O₄@S201 exerted high catalytic activity toward PMS activation for oxidation of 1-hydroxyethane 1,1-diphosphonic acid (HEDP), with the apparent rate normalized by Co content 38.2 times higher than that of the unsupported CoFe₂O₄. Meanwhile, one order of magnitude lower Co leaching (< 2.1 μg L^-1) was detected during the catalytic oxidation. The Co(II)-PMS complex was the primary oxidant responsible for the oxidation of HEDP. The catalytic durability and stability of CoFe₂O₄@S201 for degradation of HEDP in actual wastewater were systematically evaluated in both batch and continuous-flow mode. It is found that the organic resin, which is often considered to be intolerant to oxidation, is rather stable during the non-radical process. The total cobalt leaching of the fresh CoFe₂O₄@S201 cannot be ignored in the 100-h continuous-flow run. In contrast, much lower cobalt leaching and slightly higher oxidation efficiency were observed for the regenerated CoFe₂O₄@S201, which might be due to the removal of unreactive and unstable Co sites on the surface in the first trial. The findings shed light on the potential of organic supports for improving the stability and activity of nanosized CoFe₂O₄ and other nano-catalysts toward practical application.

Enhanced non-radical activation of persulfate with pompon-like NiO microspheres for removing sulfamethoxazole in water

The development of efficient heterogeneous catalyst for non-radical activation of persulfate (PS) is highly desired for removing organic pollutants in water. Herein, four NiO samples were prepared by different methods, and their performance for PS activation was investigated using sulfamethoxazole (SMX) as the target pollutant. The structure, surface chemical state, and redox ability of these samples were measured by various characterization techniques, and the key property affecting PS activation efficiency was explored. The results showed that the degradation of SMX by these samples all followed the non-radical mechanism, and the activated PS was the dominant active species. Among them, pompon-like NiO microspheres exhibited the highest activity due to its large surface area and especially high oxidation ability. Catalyst with high oxidation ability or reducing ability should facilitate the non-radical or radical activation of PS, respectively. SMX was completely removed by pompon-like NiO microspheres within 10 min, and the reaction rate constant was calculated to be 0.4199 min-1. An adsorption-degradation experiment was designed to verify the high stability and oxidation potential of the adsorbed PS on NiO surface. Pompon-like NiO microspheres exhibited good reusability, and its performance was barely affected by water quality, demonstrating its potential application in water treatment.

Environmental-friendly hydrochar-montmorillonite composite for efficient catalytic degradation of dicamba and alleviating its damage to crops

In recent years, carbon-based materials catalyzing peroxymonosulfate (PMS) for green degradation of persistent organic pollutants have attracted increasing attention. However, PMS activation by hydrochar composite (e.g. hydrochar-montomorillonite) has rarely been investigated. Herein, a simple preparation, low-cost and eco-friendly catalyst of hydrochar-montmorillonite composite (HC-Mt) was prepared to firstly catalyze PMS for the degradation of dicamba (DIC). The as-prepared HC-Mt showed a remarkably better catalyzing performance for PMS than pure hydrochar (HC) due to its good physicochemical characteristics and abundant oxygen-containing groups. Furthermore, the electron spin resonance (ESR) and quenching tests revealed that active species such as SO₄^-, OH and O₂^- all participated in the degradation process. DIC sites on C6, Cl 10, and O15 exhibited higher reactivity according to the density functional theory (DFT) calculation, which were easily attacked by active species. The DIC degradation mainly occurred via hydroxyl substitution, decarboxylation, oxidation and ring-cleavage and finally most of the intermediates were mineralized into CO₂ and H₂O. Finally, the phytotoxicity assessment was measured by the germination growth situation of tobacco and mung beans in the presence of DIC (with or without treatment by HC-Mt/PMS). The result showed that HC-Mt/PMS could significantly reduce the phytotoxicity of DIC to crops, suggesting that catalyzing PMS using HC-Mt was environmentally friendly. Therefore, this work did not only provide a novel catalyzing PMS strategy using hydrochar composite for wastewater treatment, but also give a new idea for herbicide phytotoxicity management.

Electron-transfer-based peroxymonosulfate activation on defect-rich carbon nanotubes: Understanding the substituent effect on the selective oxidation of phenols

Nanocarbon-based persulfate oxidation technologies are promising for green elimination of phenolic pollutants. Previous studies revealed the electron transfer via defective carbon nanotube (CNTs) for selective oxidation of various phenols. However, an underlying relationship between the molecular structure of phenols and the selectivity of electron transfer-induced oxidation has not been well understood. Herein, we report that defect-rich CNTs could initiate electron-transfer regime from phenols to peroxymonosulfate (PMS), resulting in the efficient degradation of phenols. Further studies uncover a distinctive substituent group-dependent selective oxidation of phenols via the CNT-mediated electron transfer process. Specifically, the degradation rate of para-substituted phenols with electron-donating groups (e.g., -NH₂ and -OCH₃) is faster than those with electron-withdrawing groups (e.g., -NO₂ and -COOH). For a kind of substituted phenols, the substituent position has a great influence on the phenols degradation and their degradation rates follow this sequence: para > ortho > meta -position. Besides, increasing the number of the substituent group can accelerate the degradation of substituted phenols. This study elucidates the substituent effect on the electron transfer-dominated selective oxidation of phenols for the first time, which guides the application of carbon/persulfate system for the targeted remediation of phenols-polluted wastewater.

Insights into the mechanism of carbocatalysis for peracetic acid activation: Kinetic discernment and active site identification

Peracetic-acid-based advanced oxidation processes (PAA-AOPs) on metal-free catalysts have emerged as charming strategies for water contaminant removal. However, the involved reactive species and their corresponding active sites are ambiguous. Herein, using carbon nanotube (CNT) as a model carbocatalyst, we demonstrated that, under neutral conditions, the CNT-PAA* complex was the dominant reactive species to oxidize phenolic compounds via electron-transfer process (ETP), whereas the surface-bound hydroxyl radicals (^·OH_surface) played a minor role on the basis of quenching and electrochemical tests as well as Raman spectroscopy. More importantly, the experimental and density functional theory (DFT) calculation results collaboratively proved that the active site for ETP was the sp²-hybridized carbon on the CNT bulk, while that for radical generation was the edge-located hydroxyl group (C-OH), which lowered the energy barrier for cleaving the O-O bond in CNT-PAA* complex. We further discerned the oxidation kinetic constants (k_oxid) of different pollutants from the apparent kinetic constants in CNT/PAA system. The significant negative linear correlation between lnk_oxid and half-wave potential of phenolic compounds suggests that the pollutants with a lower one-electron oxidation potential (i.e., stronger electron-donating ability) are more easily oxidized. Overall, this study scrutinizes the hybrid radical and non-radical mechanism and the corresponding active sites of the CNT/PAA system, providing insights into the application of PAA-AOPs and the development of ETP in the remediation of emerging organic pollutants.

Tailoring the Biochar Physicochemical Properties Using a Friendly Eco-Method and Its Application on the Oxidation of the Drug Losartan through Persulfate Activation

In this study, spent malt rootlet-derived biochar was modified by a friendly eco-method using a low temperature (100 °C) and dilute acid, base, or water. The modification significantly enhanced the surface area from 100 to 308–428 m2g−1 and changed the morphology and the carbon phase. In addition, the mineral’s percentage and zero-point charge were significantly affected. Among the examined materials, the acid-treated biochar exhibited higher degradation of the drug losartan in the presence of persulfate. Interestingly, the biochar acted as an adsorbent at pH 3, whereas at pH = 5.6 and 10, the apparent kinetic constant’s ratio koxidation/kadsorption was 3.73 ± 0.03, demonstrating losartan oxidation. Scavenging experiments indirectly demonstrated that the role of the non-radical mechanism (singlet oxygen) was crucial; however, sulfate and hydroxyl radicals also significantly participated in the oxidation of losartan. Experiments in secondary effluent resulted in decreased efficiency in comparison to pure water; this is ascribed to the competition between the actual water matrix constituents and the target compound for the active biochar sites and reactive species.

Elimination of bisphenol A with visible light-enhanced peroxydisulfate activation process mediated by Fe3+-nitrilotriacetic acid complex

In recent years, visible light assisted advanced oxidation processes (AOPs) are appealing in the elimination of pollutants. Herein, an innovative and eco-friendly visible light enhanced Fe³⁺-nitrilotriacetic acid system for the activation of peroxydisulfate (Vis/Fe³⁺-NTA/PDS) was proposed for the removal of bisphenol A (BPA). Fe³⁺-NTA could be dissociated through ligand-to-metal charge transfer (LMCT) to realize the generation of Fe²⁺ for the continuous activation of PDS to remove BPA. The use of 0.10 mM Fe³⁺, 0.10 mM NTA and 1.00 mM PDS led to 97.5% decay of 0.05 mM BPA and 66.3% of TOC removal in 30 min with the illumination of visible light at initial pH 3.0. The sulfate and hydroxyl radicals were proved to be the dominant species leading to BPA removal by means of radical scavenging experiments, radical probes and electron paramagnetic resonance (EPR) technique. The effects of various operating parameters, natural water constituents as well as different water matrices on BPA abatement were explored. The intermediate products of BPA degradation were identified and a possible transformation pathway was proposed. Briefly, this research provides an attractive strategy for the remediation of refractory wastewater using NTA assisted with visible light in the homogeneous Fe³⁺/PDS system.

Insight into the roles of microenvironment and active site on the mechanism regulation in metal-free persulfate activation process coupling with an electric field

The regulation of the persulfate activation mechanism is highly desirable and meaningful for the treatment of different wastewaters. The role of active sites for mechanism regulation in carbon-driven persulfate activation is still ambiguous due to the complex and easily neglected microenvironment (concentration distributions of organics and oxidants) nearby carbon catalyst. This work aims to reveal the critical roles of active site and microenvironment on the activation mechanism through N-doped modification and application of an electric field (AC/PS/EC). Several N-doped activated carbon catalysts were prepared by activating for different times to adjust the surface active center and adsorption selectivity under an electric field. The contribution ratio of radical pathway and non-radical pathway for organic elimination significantly varied with the concentration distribution of organics and oxidants nearby the microelectrodes. The increased electro-adsorption of persulfate anion was found to be the primary promoting factor for the radical pathway for organic oxidation, resulting in a synergistic increase in degradation rate in the AC/PS/EC system. The quantitative structure-activity relationships analysis also revealed that the electro-adsorption selectivity was dominated by the surface graphitic N and pore structure of catalyst. This study sheds new light on the oxidative pathway regulation to deal with complex wastewater in a flexible and efficient manner.

Understanding the nonradical activation of peroxymonosulfate by different crystallographic MnO2: The pivotal role of MnIII content on the surface

Manganese oxide-activated persulfate plays a critical role in water purification and in situ chemical oxidation processes, but the underlying mechanism needs to be further revealed. Herein, the detailed mechanism of MnO₂ with various crystallographic structures (α-, β-, γ-, and δ-MnO₂) towards peroxymonosulfate (PMS) activation was investigated. PMS activated by tunnel structured α-, β-, and γ-MnO₂ showed higher acetaminophen (ACE) removal than layer structured δ-MnO₂ with the removal efficiency following an order of α-MnO₂ (85%) ≈ γ-MnO₂ (84%) > β-MnO₂ (65%) > δ-MnO₂ (31%). Integrated with chemical quenching experiments, electron paramagnetic resonance, Raman spectra, X-ray photoelectron spectroscopy, and Langmuir-Hinshelwood model on kinetic data, both surface-bound PMS complexes and direct oxidation by surface manganese species (Mn^{(Ⅳ, Ⅲ)}_(s)) were disclosed as the dominant oxidation mechanism for ACE degradation in α-, β-, and γ-MnO₂/PMS, which were rarely observed in previous reports. Moreover, the catalytic activity of α-, β-, and γ-MnO₂ was positively correlated to the Mn^III_(s) content on the catalyst surface. Higher content of Mn^III_(s) would stimulate the generation of more oxygen vacancies, which was conducive to the adsorption of PMS and the formation of reactive complexes. Overall, this study might provide deeper insight into the nonradical activation mechanism of PMS over different crystallographic MnO₂.

Electro-Induced Carbon Nanotube Discrete Electrodes for Sustainable Persulfate Activation

In electrochemical advanced oxidation processes (EAOPs), the rate-limiting step is the mass transfer of pollutants to the electrodes due to the limited active surface areas. To this end, we established a three-dimensional (3D) EAOP system by coupling conventional graphite electrodes with dispersed carbon nanotubes (CNTs). The electrodes (particularly the anode) induced electric field spontaneously polarized CNTs into dispersed reactive particle electrodes (CNT-PEs) in the solution, which remarkably promoted electrochemical activation of peroxydisulfate (PDS) to generate surface CNT-PDS* complexes and surface-bound radicals (SBRs). Based on the excited potential (E_CNT-PEs) at different positions in the 3D electric field, CNT-PEs were activated into three states. (i) E_CNT-PEs < E_organic, CNT-PEs are chemically inert toward DCP oxidation; (ii) E_organic < E_CNT-PEs < E_water, CNT-PEs will oxidize DCP via an electron-transfer process (ETP); (iii) E_CNT-PEs > E_water, both CNT-PDS* complexes and the anode will oxidize water to produce SBRs. Thus, DCP could be oxidized by CNT-PDS* complexes via ETP to form polychlorophenols on the CNT surface, causing rapid deactivation of the micro-electrodes. In contrast, SBRs attack DCP directly into chloride ions and hydroxylated products, maintaining the surface cleanliness and activity of CNT-PEs for long-term operations.

Removal of Gaseous Volatile Organic Compounds by a Multiwalled Carbon Nanotubes/Peroxymonosulfate Wet Scrubber

In this study, a wet scrubber coupled with a persulfate-based advanced oxidation process [carbocatalysts/peroxymonosulfate (PMS)] was demonstrated to efficiently remove gaseous volatile organic compounds (VOCs). The removal efficiency of a representative VOC, styrene, was stable at above 98%, and an average mineralization rate was achieved at 76% during 2 h. The removal efficiency of the carbocatalysts/PMS wet scrubber for styrene was much higher than that of pure water, carbocatalysts/water, or PMS/water systems. Quenching experiments, electron spin resonance spectroscopy, in-situ Raman spectroscopy and density functional theory (DFT) calculations indicated that singlet oxygen (¹O₂) and oxidative complexes are the main reactive oxygen species and that both contributed to styrene removal. In particular, carbonyl groups (C═O) in the carbocatalyst were found to be the active sites for activating PMS during styrene oxidation. The role of ¹O₂ was discovered to be benzene ring breaking and a possible non-radical oxidation pathway of styrene was proposed based on time-of-flight mass spectroscopy which was further verified by DFT calculations. In particular, the electron transfer process of multi world carbon nanotubes-PMS* in styrene oxidation was further studied in-depth by experiments and DFT calculations. The unstable vinyl on styrene was simultaneously degraded by the oxidative complexes and ¹O₂ into benzene, and finally oxidized by ¹O₂ into H₂O and CO₂. This study provides an effective method for VOC removal and clearly illustrates the complete degradation mechanism of styrene in a nonradical PMS-based process by a wet scrubber.