ABTS as Both Activator and Electron Shuttle to Activate Persulfate for Diclofenac Degradation: Formation and Contributions of ABTS•+, SO4•-, and •OH

Novel role of Silver(I) as electron shuttle for polymerization of chlorophenols by permanganate oxidation

Permanganate (Mn(VII)) oxidation is attracting increasing attention in the field of water treatment, however, it exhibits limited chemical oxygen demand (COD) removal due to its inability to completely destroy the structure of pollutants. This paper discovered the novel role of ionic silver as electron shuttle for regulation of chlorophenols (CPs) polymerization during Mn(VII) oxidation. The Mn(VII)-Ag(I) system displayed remarkable removal of CPs in real water bodies and facilitated polymerization to at least hexamerization for improved COD removal. The ring-closure reaction was proposed for the first time, potentially stabilizing chained oligomers and reducing their migration toxicity. Ag(I) plays a dual role to create the electron-deficient state of Mn(VII) and enhance the oxidation susceptibility of 2,4-DCP via complexation, which mediates the electron transfer to generate abundant phenoxyl radicals to initiate polymerization for the formation of filterable and settleable oligomers. Findings of this work would provide new inspirations for the development of highly-efficient, cost-effective and environment-friendly Mn(VII) oxidation technologies in removal of CPs-like contaminants.

d-Orbital Single Electron Filling O─O π* Bonds on WO3S1 Sites for Highly Selective Generation of Hydroxyl Radicals

Hydroxyl radical (•OH) stemming from dissolved oxygen (O2) via photocatalysis is very attractive, but its poor selectivity and generation efficiency greatly limit its application. Herein, a kind of tungsten single site co-coordinated with O and S atoms (WO3S1) is established on ZnIn2S4 (W-ZIS). The strong interactions in WO3S1 shift the d-band center toward the Fermi level, enhancing the adsorption of O2. These interactions improve the accumulation of photo-generated electrons on WO3S1, facilitating the dissociation of O─O bonds in crucial intermediates and promoting the selective conversion from O2 into •OH. This brings a state-of-the-art selectivity (40.2%) and generation efficiency (1668.90 mmol. g-1. L-1. h-1) of •OH production. Experimental results and theoretical simulations have elucidated that O2 can be reduced by d-orbitals single electron (↑, _, _, _, _, _) of WO3S1 transfer to 2p-orbital O─O pi anti-bonding (π*: px and py), initially activating O2. Additionally, WO3S1 sites facilitate the cleavage of H2O, optimizing proton adsorption through W─O orbital coupling in WO3S1 and promoting the transformation of oxygen-containing intermediates. More importantly, d-orbitals single electron can fill O─O π* bond in •OOH intermediate, weakening the covalency of the O─O bond, mitigating the formation of H2O2 and shortening the pathway for •OH generation.).

Removal of Sulfonamide Antibiotics in Peracetic Acid-Mediated Natural Polyphenol Systems via an Overlooked Polymerization Pathway: Role of ortho-Quinones

Natural polyphenols can be oxidized into reactive quinones, which might play a key role in the removal of specific contaminants in natural polyphenol-related advanced oxidation processes (AOPs). In this study, peracetic acid (PAA) was employed in combination with natural protocatechuic acid (PCA) to remove sulfonamide antibiotics (SAs) from water. More than 95% removal of sulfamethoxazole (SMX) and other SAs was observed in the PCA/PAA system, and neutral pH conditions (5.0-8.0) were more conducive to the removal of SMX. The PCA/PAA system exhibited a great anti-interference ability against complex water matrices. ortho-Quinone, generated from the oxidation of PCA by PAA, played a dominant role in the SMX removal. Electrons tended to transfer from SMX to the generated ortho-quinones and form covalent bonds, resulting in the production of less toxic oligomers via the overlooked polymerization pathway. A reduction in the toxicity of the SMX solution was found following treatment with the PCA/PAA system. More interestingly, several polyphenols structurally related to PCA could also facilitate SMX removal using PAA as the oxidant. Overall, this study proposes a novel strategy for developing reactive quinones dominated AOPs with robust anti-interference performance, as well as enhances the understanding of contaminant removal via an overlooked polymerization pathway in natural polyphenol-related AOPs.

Coordination engineering of heterogeneous high-valent Fe(IV)-oxo for safe removal of pollutants via powerful Fenton-like reactions

Coordination engineering of high-valent Fe(IV)-oxo (FeIV=O) is expected to break the activity-selectivity trade-off of traditional reactive oxygen species, while attempts to regulate the oxidation behaviors of heterogeneous FeIV=O remain unexplored. Here, by coordination engineering of Fe-Nx single-atom catalysts (Fe-Nx SACs), we propose a feasible approach to regulate the oxidation behaviors of heterogeneous FeIV=O. The developed Fe-N2 SACs/peroxymonosulfate (PMS) system delivers boosted performance for FeIV=O generation, and thereby can selectively remove a range of pollutants within tens of seconds. In-situ spectra and theoretical simulations suggest that low-coordination Fe-Nx SACs favor the generation of FeIV=O via PMS activation as providing more electrons to facilitate the desorption of the key *SO4H intermediate. Due to their disparate attacking sites to sulfamethoxazole (SMX) molecules, Fe-N2 SACs mediated FeIV=O (FeIVN2=O) oxidize SMX to small molecules with less toxicity, while FeIVN4=O produces series of more toxic azo compounds through N-N coupling with more complex oxidation pathways.

A Two-Phase Hydrogenation Membrane for Contaminants Reduction at High Hydrogen Reagent Utilization Efficiency

Heterogeneous hydrogenation is surging as a promising strategy for selective removal of water pollutants, yet numerous efforts rely on catalyst design to advance catalytic activity. Herein, we enhanced the mass transfer and the utilization of hydrogen reagent through construction of a two-phase flow-through membrane reaction device (Pd/SiC-MR). Pd/SiC-MR displays high efficiency and selectivity toward removal of multiple pollutants. For instance, rapid (∼0.35 s) and exclusive hydrogenation (>99%) of carbon-chlorine bond in organohalogens were realized at high water flux (220 L/m2/h). More importantly, the two-phase Pd/SiC-MR reaction system achieved 31.4% utilization of hydrogen reagent, 1-3 orders of magnitude higher than those by classical slurry or fixed-bed reactor. The high hydrogenation performance is attributed to the close proximity of the hydrogen source, reactive hydrogen atom, and pollutant under high molecular collision frequency in membrane pores. Our study opens an approach for improved hydrogen reagent utilization while reserving the high pollutant removal efficiency through altering operating conditions, beyond complex material design limitations in hydrogenation water purification.

Efficient low-strength diclofenac elimination via adsorption-concentration and peroxydisulfate activation mineralization by distinct pretreated biocarbon composites

Sodium diclofenac (DCF) widely exists in actual water matrices, which can negatively impact ecosystems and aquatic environments even at low-strength. Herein, the adsorption-concentration-mineralization process was innovatively constructed for low-strength DCF elimination by freeze-dried biocarbon and oven-dried biocarbon coupled with cobalt oxide composites derived from the same waste biomass. Surprisingly, low-strength DCF of 0.5 mg/L was adsorbed rapidly and enriched to high-strength DCF under light with a concentration efficiency of 99.67 % by freeze-dried biocarbon. Subsequently, the concentrated DCF was economically mineralized by bifunctional oven-dried biocarbon coupled with cobalt oxide composites for peroxydisulfate (PDS) activation with full PDS activation and 76.11 % mineralization efficiency. Compared with direct low-strength DCF oxidation, adsorption-concentration-mineralization consumed less energy and none PDS residues. Mechanisms confirmed that DCF was adsorbed by freeze-dried biocarbon through hydrogen bonds and π-π stacking interactions, which were switched on due to electron-induced effect by light in DCF desorption-concentration. Furthermore, nonradical pathway (electron transfer) and radical pathway (SO4•-) were involved in efficient PDS activation by oven-dried biocarbon coupled with cobalt oxide composites for concentrated DCF mineralization, and the former was more prominent, in which graphitic carbon, cobalt redox cycle and carboxy groups were the main active sites. Overall, an energy-efficient strategy was proposed for elimination of low-strength DCF in real water matrices.

Enhanced Degradation of Micropollutants in a Peracetic Acid/Mn(II) System with EDDS: An Investigation of the Role of Mn Species

Extensive research has been conducted on the utilization of a metal-based catalyst to activate peracetic acid (PAA) for the degradation of micropollutants (MPs) in water. Mn(II) is a commonly employed catalyst for homogeneous advanced oxidation processes (AOPs), but its catalytic performance with PAA is poor. This study showed that the environmentally friendly chelator ethylenediamine-N,N'-disuccinic acid (EDDS) could greatly facilitate the activation of Mn(II) in PAA for complete atrazine (ATZ) degradation. In this process, the EDDS enhanced the catalytic activity of manganese (Mn) and prevented disproportionation of transient Mn species, thus facilitating the decay of PAA and mineralization of ATZ. By employing electron spin resonance detection, quenching and probe tests, and 18O isotope-tracing experiments, the significance of high-valent Mn-oxo species (Mn(V)) in the Mn(II)-EDDS/PAA system was revealed. In particular, the involvement of the Mn(III) species was essential for the formation of Mn(V). Mn(III) species, along with singlet oxygen (1O2) and acetyl(per)oxyl radicals (CH3C(O)O•/CH3C(O)OO•), also contributed partially to ATZ degradation. Mass spectrometry and density functional theory methods were used to study the transformation pathway and mechanism of ATZ. The toxicity assessment of the oxidative products indicated that the toxicity of ATZ decreased after the degradation reaction. Moreover, the system exhibited excellent interference resistance toward various anions and humid acid (HA), and it could selectively degrade multiple MPs.

Unlocking high-efficiency decontamination by building a novel heterogeneous catalytic reduction system of thiourea dioxide/biochar

Herein, we reported a new contaminant purification paradigm, which enabled highly efficient reductive denitration and dechlorination using a green, stable reducing agent thiourea dioxide (TDO) coupled with biochar (BC) over a wide pH range under anoxic conditions. Specifically, BC acted as both activators and electron shuttles for TDO decomposition to achieve complete anoxic degradation of p-nitrophenol (PNP), p-nitroaniline, 4-chlorophenol and 2,4-dichlorophenol within 2 h. During this process, multiple strongly reducing species (i.e., SO22-, SO2•- and e-/H•) were generated in BC/TDO systems, accounting for 13.3%, 9.7% and 75.5% of PNP removal, respectively. While electron transfer between TDO and H+ or contaminants mediated by BC led to H• generation and contaminant reduction. These processes depended on the electron-accepting capacity and electron-conducting domains of biochar. Significantly, the BC/TDO systems were highly efficient at a pH of 2.0-8.0, especially under acidic conditions, which performed robustly in common natural water constituents.

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.

Enhanced chalcopyrite-catalyzed heterogeneous Fenton oxidation of diclofenac by ABTS

The widely used 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) has gained growing attention in advanced oxidation processes (AOPs), whereas there was limited knowledge regarding the feasibility of ABTS in enhancing heterogeneous Fenton oxidation so far. Hereof, ABTS was introduced into the chalcopyrite (CuFeS2)- catalyzed heterogeneous Fenton oxidation process to degrade diclofenac (DCF), and the degradation efficiency was enhanced by 25.5% compared with CuFeS2/H2O2 process. The available reactive oxygen species (ROS) and the enhanced mechanism were elaborated. Experimental results uncovered that •OH was the dominant reactive species responsible for the DCF degradation in the CuFeS2/H2O2/ABTS process, and ABTS•+ was derived from both •OH and Fe(IV). The presence of ABTS contributed significantly to the redox cycle of surface Fe of CuFeS2, and the roles of reductive sulfur species and surface Cu(I) in promoting surface Fe cycling also could not be neglected. In addition, the effects of several influencing factors were considered, and the potential practicability of this oxidation process was examined. The results demonstrate that the CuFeS2/H2O2/ABTS process would be a promising approach for water purification. This study will contribute to the development of enhancing strategies using ABTS as a redox mediator for heterogeneous Fenton oxidation of pharmaceuticals.

Activation of periodate by ABTS as an electron shuttle for degradation of aqueous organic pollutants and enhancement effect of phosphate

Periodate (PI) based advanced oxidation processes (AOPs) have recently attracted much attention due to their high application potential in water purification through production of reactive species. In the study, 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) was used as a representative electron shuttle, and its reaction with PI was investigated in detail. It was found that PI can be activated by ABTS via one-electron transfer to produce ABTS•+ and IO3•, cooperatively promoting oxidation of organic contaminants such as bisphenol A (BPA). Their contribution in BPA oxidation at pH 7 was estimated as 81.9% and 18.1%, respectively. With phosphate, BPA oxidation rate in the PI/ABTS process increased linearly with raised phosphate concentrations from 0 to 10 mM. The enhancement effect of phosphate is attributed to formation of PI-phosphate complexes, which facilitate PI activation by ABTS, and production of more ABTS•+ and IO3•, and additional phosphate radicals. Accordingly, the contribution of IO3• and phosphate radicals in BPA oxidation raised to 57.7% in the process with 4 mM phosphate, while that of ABTS•+ decreased to 42.3%. The reaction stoichiometry ratio of ABTS to PI was measured as 1.1 at pH 7, suggesting the little involvement of IO3• and phosphate radicals in production of ABTS•+ due to their high self-quenching. The PI/ABTS process exhibited excellent anti-interference capacity towards water matrix components (e.g. Cl-, HCO3- and natural organic matters). Moreover, an immobilized ABTS (ABTS/ZnAl-LDH) was successfully developed as a heterogeneous electron shuttle for PI oxidation, which resultantly exhibited the good catalytic activity and stability in degradation of BPA, further improving feasibility of the process in treatment of actual water. This work advances understanding on reaction of PI with ABTS from stoichiometric and kinetic aspects, and provides a high performance AOP for selective oxidation of trace organic contaminants.

In Situ Monitoring of the Polymerization Kinetics of Organic Pollutants during Persulfate-Based Advanced Oxidation Processes Using Plasmonic Colorimetry

Using sulfate radicals to initiate polymer production in persulfate-based advanced oxidation processes (AOPs) is an emerging strategy for organics removal. However, our understanding of this process remains limited due to a dearth of efficient methods for in situ and real time monitoring of polymerization kinetics. This study leverages plasmonic colorimetry to monitor the polymerization kinetics of an array of aromatic pollutants in the presence of sulfate radicals. We observed that the formation of polymer shells on the surfaces of gold nanoparticles (AuNPs) led to an increase and red shift in their localized surface plasmon resonance (LSPR) band as a result of an increased refractive index surrounding the AuNP surfaces. This observation aligns with Mie theory simulations and transmission electron microscopy-electron energy loss spectroscopy characterizations. Our study demonstrated that the polymerization kinetics exhibits a significant reliance on the electrophilicity and quantity of benzene rings, the concentration of aromatic pollutants, and the dosage of oxidants. In addition, we found that changes in LSPR band wavelength fit well into a pseudo-first-order kinetic model, providing a comprehensive and quantitative insight into the polymerization kinetics involving diverse organic compounds. This technique holds the potential for optimizing AOP-based water treatment by facilitating the polymerization of aromatic pollutants.

Enhanced chlorination of diclofenac using ABTS as electron shuttle: Performance, mechanism and applicability

Chlorination, one of the most common oxidation strategies, performed limited degradation capacity towards many emerging organic contaminants under neutral pH conditions. In this study, 2,2'-azinobis(3-ethylbenzothiazoline)-6-sulfonate (ABTS) was discovered to possess an outstanding activation property towards free available chlorine (FAC) during the chlorination of diclofenac (DCF) among pH 6.0-9.5. ABTS radical (ABTS•+) primarily accounted for the elimination of DCF in the ABTS/FAC system, although hydroxyl radicals, reactive chlorine species, and singlet oxygen were also generated via the self-decomposition of FAC. ABTS acted as the electron shuttle to degrade DCF in the ABTS/FAC system, where ABTS was firstly oxidized by FAC to ABTS•+ via single electron transfer, and followed by the elimination of DCF with the generated ABTS•+. Eight DCF degradation intermediates were identified by LC/Q-TOF/MS, and four DCF degradation pathways were proposed. Real water bodies, humic acid, and the coexistent anions of Cl-, HCO3-, NO3-, and SO42- performed negligible influence on DCF removal in ABTS/FAC system. ABTS/FAC system was much superior to sole chlorination in terms of toxicity reduction and anti-interference capacity. Overall, this study innovatively introduced ABTS as the electron shuttle to enhance the oxidative capacity of FAC under neutral pH conditions and provided a new insight that the ABTS-like organic/synthetic components might play an important role in degrading emerging organic contaminants by chlorination in water treatment.

Development of a Deep Eutectic Solvent-Assisted Kaempferol Hydrogel: A Promising Therapeutic Approach for Psoriasis-like Skin Inflammation

Psoriasis is an incurable inflammatory skin disease that is mediated by the immune system. Although kaempferol has been known for its anti-inflammatory, antioxidant, and anticancer properties, its therapeutic effectiveness is often limited due to its poor water solubility and low bioavailability. To address these challenges, we developed a promising kaempferol hydrogel (DK-pGEL) using Pluronic F127 and a deep eutectic solvent (DES) with varying concentrations of kaempferol. In this study, we first evaluated the rheological properties and viscosity of the DK-pGEL hydrogel. The G' of DK-pGEL (∼14 kPa) hydrogels was significantly lower than the control group (∼30 kPa) at 37 °C. The DK-pGEL hydrogel exhibited ideal fluidity and viscosity at 37 °C, as demonstrated by its shear-thinning behavior. Moreover, the DK-pGEL hydrogel showed controlled release characteristics with a drug release of 97.43 ± 5.37 μg/mL over 60 h. Furthermore, in vitro antioxidant experiments revealed that DK-pGEL exhibited significant radical scavenging ability against the DPPH-radical (96.27 ± 0.37%), ABTS-radical (98.11 ± 0.79%), hydroxyl-radical (66.36 ± 1.01%), and superoxide-radical (90.52 ± 0.79%) at a concentration of 250 μg/mL kaempferol. Additionally, DK-pGEL exhibited notable cellular antioxidant effects by inhibiting reactive oxygen species generation. Cell viability assays (CCK8) and live/dead cell assays were conducted to assess the cytotoxicity of DK-pGEL. The results showed that DK-pGEL could effectively inhibit HaCaT cell proliferation without causing significant cytotoxicity. To evaluate the therapeutic potential of DK-pGEL, an imiquimod (IMQ)-induced mouse model of psoriasis-like lesions was employed. Remarkably, the DK-pGEL hydrogel could significantly reduce the psoriasis area and severity index score, improve the histopathology induced by IMQ, and downregulate the expression of pro-inflammatory cytokines (TNF-α, IL-6, and IL-17A) in the skin tissue. These findings demonstrate that the DES-assisted kaempferol hydrogel holds promise as a topical drug delivery system for psoriasis treatment.

Plasma-microbubble treatment and sustainable agriculture application of diclofenac-contaminated wastewater

The demand for efficient wastewater treatment is becoming increasingly urgent due to the rising threat of pharmaceutical residues in water. As a sustainable advanced oxidation process, cold plasma technology is a promising approach for water treatment. However, the adoption of the technology encounters several challenges, including the low treatment efficiency and the potentially unknown environmental impact. Here, microbubble generation was integrated with cold plasma system to enhance treatment of wastewater contaminated with diclofenac (DCF). The degradation efficiency depended on the discharge voltage, gas flow, initial concentration, and pH value. The best degradation efficiency was 90.9% after 45 min plasma-bubble treatment under the optimum process parameters. The hybrid plasma-bubble system exhibited strongly synergistic performance heralded by up to seven-times higher DCF removal rates than the two systems operated separately. The plasma-bubble treatment remains effective even after addition of SO₄^2-, Cl^-, CO₃^2-, HCO₃^-, and humic acid (HA) as interfering background substances. The contributions of •O₂^-, O₃, •OH, and H₂O₂ reactive species to the DCF degradation process were specified. The synergistic mechanisms for DCF degradation were deduced through the analysis of the degradation intermediates. Further, the plasma-bubble treated water was proven safe and effective to stimulate seed germination and plant growth for sustainable agriculture applications. Overall, these findings provide new insights and a feasible approach with a highly synergistic removal effect for the plasma-enhanced microbubble wastewater treatment, without generating secondary contaminants.

Rapid Detection of Cd2+ Ions in the Aqueous Medium Using a Highly Sensitive and Selective Turn-On Fluorescent Chemosensor

Herein, a novel optical chemosensor, (CM1 = 2, 6-di((E)-benzylidene)-4-methylcyclohexan-1-one), was designed/synthesized and characterized by ¹H-NMR and FT-IR spectroscopy. The experimental observations indicated that CM1 is an efficient and selective chemosensor towards Cd²⁺, even in the presence of other metal ions, such as Mn²⁺, Cu²⁺, Co^2+, Ce³⁺, K⁺, Hg²⁺,^, and Zn²⁺ in the aqueous medium. The newly synthesized chemosensor, CM1, showed a significant change in the fluorescence emission spectrum upon coordination with Cd²⁺. The formation of the Cd²⁺ complex with CM1 was confirmed from the fluorometric response. The 1:2 combination of Cd²⁺ with CM1 was found optimum for the desired optical properties, which was confirmed through fluorescent titration, Job's plot, and DFT calculation. Moreover, CM1 showed high sensitivity towards Cd²⁺ with a very low detection limit (19.25 nM). Additionally, the CM1 was recovered and recycled by the addition of EDTA solution that combines with Cd²⁺ ion and, hence, frees up the chemosensor.

Single-Atom MnN5 Catalytic Sites Enable Efficient Peroxymonosulfate Activation by Forming Highly Reactive Mn(IV)-Oxo Species

Four-nitrogen-coordinated transitional metal (MN₄) configurations in single-atom catalysts (SACs) are broadly recognized as the most efficient active sites in peroxymonosulfate (PMS)-based advanced oxidation processes. However, SACs with a coordination number higher than four are rarely explored, which represents a fundamental missed opportunity for coordination chemistry to boost PMS activation and degradation of recalcitrant organic pollutants. We experimentally and theoretically demonstrate here that five-nitrogen-coordinated Mn (MnN₅) sites more effectively activate PMS than MnN₄ sites, by facilitating the cleavage of the O-O bond into high-valent Mn(IV)-oxo species with nearly 100% selectivity. The high activity of MnN₅ was discerned to be due to the formation of higher-spin-state N₅Mn(IV)═O species, which enable efficient two-electron transfer from organics to Mn sites through a lower-energy-barrier pathway. Overall, this work demonstrates the importance of high coordination numbers in SACs for efficient PMS activation and informs the design of next-generation environmental catalysts.