Role of TEMPO in Enhancing Permanganate Oxidation toward Organic Contaminants

Overlooked interaction between redox-mediator and bisphenol-A in permanganate oxidation

Research efforts on permanganate (Mn(VII)) combined with redox-mediator (RM), have received increasing attention due to their significant performance for bisphenol-A (BPA) removal. However, the mechanisms underpinning BPA degradation remain underexplored. Here we show the overlooked interactions between RM and BPA during permanganate oxidation by introducing an RM-N-hydroxyphthalimide (NHPI). We discovered that the concurrent generation of MnO2 and phthalimide-N-oxyl (PINO) radical significantly enhances BPA oxidation within the pH range of 5.0-6.0. The detection of radical cross-coupling products between PINO radicals and BPA or its derivatives corroborates the pivotal role of radical cross-coupling in BPA oxidation. Intriguingly, we observed the formation of an NHPI-BPA complex, which undergoes preferential oxidation by Mn(VII), marked by the emergence of an electron-rich domain in NHPI. These findings unveil the underlying mechanisms in the Mn(VII)/RM system and bridge the knowledge gap concerning BPA transformation via complexation. This research paves the way for further exploration into optimizing complexation sites and RM dosage, significantly enhancing the system's efficiency in water treatment applications.

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.

Role of bipyridyl in enhancing ferrate oxidation toward micropollutants

Enhancing Fe(VI) oxidation ability by generating high-valent iron-oxo species (Fe(IV)/Fe(V)) has attracted continuous interest. This work for the first time reports the efficient activation of Fe(VI) by a well-known aza-aromatic chelating agent 2,2'-bipyridyl (BPY) for micropollutant degradation. The presence of BPY increased the degradation constants of six model compounds (i.e., sulfamethoxazole (SMX), diclofenac (DCF), atenolol (ATL), flumequine (FLU), 4-chlorophenol (4-CP), carbamazepine (CBZ)) with Fe(VI) by 2 - 6 folds compared to those by Fe(VI) alone at pH 8.0. Lines of evidence indicated the dominant role of Fe(IV)/Fe(V) intermediates. Density functional theory calculations suggested that the binding of Fe(III) to one or two BPY molecules initiated the oxidation of Fe(III) to Fe(IV) by Fe(VI), while Fe(VI) was reduced to Fe(V). The increased exposures of Fe(IV)/Fe(V) were experimentally verified by the pre-generated Fe(III) complex with BPY and using methyl phenyl sulfoxide as the probe compound. The presence of chloride and bicarbonate slightly affected model compound degradation by Fe(VI) in the presence of BPY, while a negative effect of humic acid was obtained under the same conditions. This work demonstrates the potential of N-donor heterocyclic ligand to activate Fe(VI) for micropollutant degradation, which is instructive for the Fe(VI)-based oxidation processes.

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.

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.

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).

Pathways in permanganate oxidation of mandelic acid: reactivity and selectivity of intermediate manganese species

We report a comprehensive kinetic and product study of the oxidation of mandelic acid (MA) by permanganate in the pH range of 1-13, including a full account of total oxidizing equivalents (five and three-electron change in acidic and basic media, respectively). In the entire pH range, the reaction shows a primary kinetic deuterium isotope effect (kH/kD ≥8-9), indicating rate-limiting hydride transfer. The deuterium label in α-deutero-mandelic acid is retained in benzaldehyde. Benzaldehyde (BZ) is formed in post-rate limiting steps due to reactions involving manganese intermediates. In alkaline pH (≥13), in the presence of barium acetate, Mn(VI) is removed as insoluble blue barium manganate; the stoichiometry of the first step of reduction was found to be: MA + 2Mn(VII) → PGA + 2Mn(VI). Manganate, MnO42-, is directly reduced to MnO2 giving an additional mole of phenylglyoxylic acid (PGA). The experimentally observed ratio of benzaldehyde to phenylglyoxylic (BZ/PGA) provides a basis for discrimination between mechanistic choices that include direct reduction of Mn(V) to Mn(III) (in an acidic medium), disproportionation to Mn(IV) and Mn(VI) or oxidation to Mn(VI) by a second mole of permanganate. Interestingly, at pH 4, a stoichiometric, soluble Mn(IV) is observed for the first time for hydroxy-acid oxidation, reminiscent of the Guyard reaction. Because of the widespread use of permanganate as an environmentally green oxidant, results from mandelic acid oxidation have implications for the remediation of dissolved organic matter (DOM) including hydrocarbons and nitroaromatics in waste and groundwater.

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.

PTIO as a redox mediator to enhance organic contaminants oxidation by permanganate

Although permanganate (Mn(VII)) is extensively utilized as a strong oxidizer for the purification of water, the direct reaction rates between some refractory pollutants and Mn(VII) are moderate or relatively low. In this study, we found that 2-phenyl-4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl (PTIO), could act as a redox mediator to enhance bisphenol A (BPA) degradation by Mn(VII) at pH 5.0 - 9.0, with a removal higher than 80% over 5 min. Moreover, the Mn(VII)/PTIO system is highly efficient toward a broad spectrum of contaminants. Mechanism was elucidated as following: PTIO was oxidized by Mn(VII) to PTIO+, an oxoammonium cation. As a newly generated reactive species, PTIO+ could oxidize organics and be reduced to PTIOH (PTIO hydroxylamine) or PTIO simultaneously. The redox cycle of PTIO in consecutive runs as an electron shuttle proved its stability and reusability in Mn(VII) oxidation. In addition to being an electron shuttle, PTIO also acts as an activator of Mn(VII) to promote the production of MnO2, which plays a vital role in enhancing BPA abatement at the acidic condition. For the purpose of further understanding the interaction between PTIO and target contaminants, three corresponding degradation pathways for BPA were proposed. Notably, the transformation products of BPA coupling with PTIO were detected, indicating PTIO inhibited the self-coupling of BPA and facilitated the ring-opening pathway. In addition, the ubiquitous humic acid has a positive effect on the Mn(VII)/PTIO system, suggesting a high promise of this system for practical application.

Trade-off effect of dissolved organic matter on degradation and transformation of micropollutants: A review in water decontamination

The degradation of micropollutants by various treatments is commonly affected by the ubiquitous dissolved organic matter (DOM) in the water environment. To optimize the operating conditions and decomposition efficiency, it is necessary to consider the impacts of DOM. DOM exhibits varied behaviors in diverse treatments, including permanganate oxidation, solar/ultraviolet photolysis, advanced oxidation processes, advanced reduction process, and enzyme biological treatments. Besides, the different sources (i.e., terrestrial and aquatic, etc) of DOM, and operational circumstances (i.e., concentration and pH) fluctuate different transformation efficiency of micropollutants in water. However, so far, systematic explanations and summaries of relevant research and mechanism are rare. This paper reviewed the "trade-off" performances and the corresponding mechanisms of DOM in the elimination of micropollutants, and summarized the similarities and differences for the dual roles of DOM in each of the aforementioned treatments. Inhibition mechanisms typically include radical scavenging, UV attenuation, competition effect, enzyme inactivation, reaction between DOM and micropollutants, and intermediates reduction. Facilitation mechanisms include the generation of reactive species, complexation/stabilization, cross-coupling with pollutants, and electron shuttle. Moreover, electron-drawing groups (i.e., quinones, ketones functional groups) and electron-supplying groups (i.e., phenols) in the DOM are the main contributors to its trade-off effect.

Studies of Dopamine Oxidation Process by Atmospheric Pressure Glow Discharge Mass Spectrometry

An atmospheric pressure glow discharge ionisation source was constructed and utilized to study the dopamine (DA) oxidation process coupling with mass spectrometry. During the DA oxidation process catalysed by polyphenol oxidase (PPO), six cationic intermediates were directly detected by the atmospheric pressure glow discharge mass spectrometry (APGD-MS). Combined with tandem mass spectrometry, the structures of the dopamine o-semiquinone radical (DASQ) and leukodopaminochrome radical (LDAC^●) intermediates and structures of the isomers of dopaminochrome (DAC) and 5,6-dihydroxyindole (DHI) were further characterised with the introduction of 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) and deuterium oxide (D₂O) to APGD-MS. Meanwhile, UV-Vis studies confirmed the important role of PPO in catalyzing the DA oxidation reaction. Based on APGD-MS studies, a possible mechanism could be proposed for DA oxidation catalysed by PPO. Furthermore, APGD-MS could provide possibilities for the effective detection and characterisation of short-lived intermediates, even in complicated systems.

Quantitative evaluation of the actual hydrogen atom donating activities of O-H bonds in phenols: structure-activity relationship

The H-donating activity of phenol and the H-abstraction activity of phenol radicals have been extensively studied. In this article, the second-order rate constants of 25 hydrogen atom transfer (HAT) reactions between phenols and PINO and DPPH radicals in acetonitrile at 298 K were studied. Thermo-kinetic parameters ΔG ^≠o(XH) were obtained using a kinetic equation [ΔG ^≠ _XH/Y = ΔG ^≠o(XH) + ΔG ^≠o(Y)]. Bond dissociation free energies ΔG ^o(XH) were calculated by the iBonD HM method, whose details are available at https://pka.luoszgroup.com/bde_prediction. Intrinsic resistance energies ΔG ^≠ _XH/X and ΔG ^≠o(X) were determined as ΔG ^≠o(XH) and ΔG ^o(XH) were available. ΔG ^o(XH), ΔG ^≠ _XH/X, ΔG ^≠o(XH) and ΔG ^≠o(X) were used to assess the H-donating abilities of the studied phenols and the H-abstraction abilities of phenol radicals in thermodynamics, kinetics and actual HAT reactions. The effect of structures on these four parameters was discussed. The reliabilities of ΔG ^≠o(XH) and ΔG ^≠o(X) were examined. The difference between the method of determining ΔG ^≠ _XH/X mentioned in this study and the dynamic nuclear magnetic method mentioned in the literature was studied. Via this study, not only ΔG ^o(XH), ΔG ^≠ _XH/X, ΔG ^≠o(XH) and ΔG ^≠o(X) of phenols could be quantitatively evaluated, but also the structure-activity relationship of phenols is clearly demonstrated. Moreover, it lays the foundation for designing and synthesizing more antioxidants and radicals.

ABNO-Functionalized Silica as an Efficient Catalyst for Enhancing Permanganate Oxidation of Emerging Contaminants

Recent reports indicate that some soluble electron-shuttling compounds (or organic mediators) can accelerate reactions between permanganate (Mn(VII)) and contaminants of emerging concern. However, practical application is limited to homogeneous electron-shuttling compounds. This study reports on the development and application of a heterogeneous electron-shuttling catalyst for Mn(VII) reactions with bisphenol A (BPA). First, we screened a series of poly/monocyclic nitroxides, finding that 9-azabicyclo[3.3.1]nonane N-oxyl (ABNO) provides the most significant enhancement of Mn(VII)/BPA reaction kinetics, where Mn(VII) oxidizes ABNO to BPA-reactive ABNO⁺. Next, we immobilized ABNO onto silica (SiO₂) by covalent bonding of 9-azabicyclo[3,3,1]nonan-3-one-9-oxyl (keto-ABNO) via a 3-aminopropyltriethoxysilane bridge to yield an ABNO@SiO₂ heterogeneous catalyst. The performance of ABNO@SiO₂ in catalyzing Mn(VII)/BPA reactions is demonstrated, with BPA reaction kinetics being highly dependent on catalyst dosage and pH conditions. The stability of ABNO@SiO₂ was retained at pH 5.0 and decreased slightly at pH 7.0 over five successive Mn(VII)/BPA reaction cycles. Kinetics modeling shows that BPA reacts with immobilized ABNO⁺, Mn(VII), and in situ formed MnO₂. Moreover, ABNO⁺ can form via ABNO reactions with both Mn(VII) and the in situ formed MnO₂. These results indicate a promising strategy for developing practical heterogeneous catalysts for enhancing Mn(VII) reactivity and treatment applications.

Role of trace TEMPO as electron shuttle in enhancing chloroquine phosphate elimination in UV-LED-driven persulfate activation process

Chloroquine Phosphate (CP) is an antiviral drug used for treatment of COVID-19. It is released into wastewater and eventually contaminates natural water. This study reports an effective homogeneous catalysis way for CP degradation by the 2,2,6,6-Tetramethylpiperidine-N-oxyl (TEMPO) enhanced persulfate (PDS) activation under UVB-LEDs irradiation at 305 nm. TEMPO at a low concentration (0.1 μM) enhanced CP degradation in UV₃₀₅/PDS process in deionized water at different pHs, in different anions and different molecular weight dissolved organic matter solutions and in real surface water. The enhancement was verified to be attributed to the electron shuttle role of TEMPO, which promoted the yield of SO₄ ^•- by enhancing electron donating capacity of the reacting system. The degradation products of CP and their acute toxicities suggested that UV₃₀₅/PDS/TEMPO process has better performance on CP detoxification than UV₃₀₅/PDS process. This study provides a new way to tackle the challenge of pharmaceutical pollutions in homogeneous photocatalysis process for natural water and sewage restoration.

The role of trace N-Oxyl compounds as redox mediator in enhancing antiviral ribavirin elimination in UV/Chlorine process

Ribavirin (RBV) is an antiviral drug used for treating COVID-19 infection. Its release into natural waters would threaten the health of aquatic ecosystem. This study reports an effective approach to degrade RBV by the trace N-oxyl compounds (2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) and N-Hydroxyphthalimide (NHPI)) enhanced UV activated free chlorine (UV/Chlorine) process. The results indicated that TEMPO and NHPI at low concentrations (0.1 μM and 1 μM, respectively) could strongly enhance RBV degradation in both deionized water with different pHs and practical surface water. The enhancement was verified to be attributed to the transformation of TEMPO and NHPI into their reactive forms (i.e., TEMPO⁺ and PINO), which generations deeply relied on radicals. The two N-oxyl compounds inhibit ClO• yield by hindering the reaction of free chlorine vs. HO• and Cl•. The analyses on acute toxicities of RBV degradation products indicate that UV/Chlorine/N-oxyl compounds process can detoxify RBV more efficiently than UV/Chlorine process.

Enhanced coagulation and oxidation by the Mn(VII)-Fe(III)/peroxymonosulfate process: Performance and mechanisms

To improve the performance of the conventional coagulation process, a permanganate (Mn(VII)) pre-oxidation combined with Fe(III)/peroxymonosulfate (PMS) coagulation process (Mn(VII)-Fe(III)/PMS) that can significantly improve the removal of dissolved organic carbon (DOC), turbidity, and micropollutants is proposed in this study. Compared with conventional Fe(III) coagulation, the Mn(VII)-Fe(III)/PMS process can also significantly enhance the removal of iohexol and sulfamethoxazole in raw water. During this process, the primary reduction product, Mn(IV), after Mn(VII) pre-oxidation was adsorbed on the floc surfaces and involved in the Fe(III)/PMS process. The natural organic matter (NOM) in raw water mediated the redox cycle of iron. The synergistic effect of NOM, Fe, and Mn facilitated the redox cycle of Mn(III)/Mn(IV) and Fe(III)/Fe(II) to promote the activation of PMS. The sulfate radical (SO₄^•-) played an important role in the degradation of micropollutants. The formation potential of the detected volatile disinfection by-product (DBP) during the subsequent chlorination was reduced by 21.9% after the Mn(VII)-Fe(III)/PMS process. This study demonstrated the promising application of the Mn(VII)-Fe(III)/PMS process for coagulation and micropollutant control and illustrated the reaction mechanism. This study provides guidance for improving conventional drinking water treatment processes.

Enhanced oxidation of organic contaminants by Mn(VII) in water

Studies that promote chemical oxidation by permanganate (MnO₄^-; Mn(VII)) as a viable technology for water treatment and environmental purification have been quickly accumulating over the past decades. Various methods to activate Mn(VII) have been proposed and their efficacy in destructing a wide range of emerging organic contaminants has been demonstrated. This article aims to present a state-of-art review on the development of Mn(VII) activation methods, including photoactivation, electrical activation, the addition of redox mediators, carbonaceous materials, and other chemical agents, with a particular focus on the potential activation mechanism and critical influencing factors. Different reaction mechanisms are involved in activated Mn(VII) oxidation processes, including the generation of reactive intermediates derived from Mn(VII) (e.g., Mn(III), Mn(V), and Mn(VI)) or activators (e.g., intermediates of redox mediators and Ru catalysts), reactive oxygen species (ROS) (e.g., •OH, O₂^•-, and ¹O₂), as well as electron transfer from organics to Mn(VII) via catalysts as the electron mediator. Except •OH that is generated as one of co-oxidants in UV/Mn(VII) process, other reactive species are relatively mild oxidants, which are more selective toward organic substrates and highly tolerant toward various water matrices (e.g., inorganic ions and natural organic matter) compared to strongly oxidizing radical species. Therefore, activated Mn(VII) oxidation processes show a good prospect for efficient removal of target contaminants in natural and complex environmental matrices. However, there are some disputes about the dominant reactive species generated in these processes, and their identification methods may be not appropriate, causing serious confusion in the mechanistic understanding. So, further efforts are still needed to fill the knowledge gap and also to address the application challenges of these technologies.

New Insights into the Actual H-Abstraction Activities of Important Oxygen and Nitrogen Free Radicals: Thermodynamics and Kinetics in Acetonitrile

The H-abstraction activity of a free radical is a research hotspot and has been extensively studied. In this article, the second-order rate constants of 21 HAT reactions in acetonitrile at 298 K were chosen from several published literature. A kinetic study on the H-abstraction reaction from TEMPOH by a DPPH^• radical was carried out. This reaction was researched as an insertion point. By combining this reaction with the 21 HAT reactions in this paper, the thermokinetic parameters of 28 free radicals X and their corresponding antioxidants XH were obtained by the cross-HAT reaction method. The scales of the H-abstraction activities of these 28 oxygen and nitrogen free radicals were determined by using the thermokinetic parameters ΔG ^≠o(X). Applications of the thermokinetic parameter ΔG ^≠o(X) in assessing the actual H-abstraction activity of a free radical quantitatively and selecting a suitable free radical in scientific research and chemical production were discussed. Predictions of the rate constants by using thermokinetic parameters of reactants were researched, and the reliabilities of the predicted activation free energies of XH/Y reactions were also examined.

Abatement of Organic Contaminants by Mn(VII)/TEMPOs: Effects of TEMPOs Structure, Organic Contaminant Speciation, and Active Oxidizing Species

In this study, a representative redox mediator, 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO), and its para-substituted derivatives (TEMPOs: 4-hydroxyl-TEMPO, 4-acetylamino-TEMPO, and 4-amino-TEMPO) significantly accelerated the abatement of trace organic contaminants (TrOCs, i.e., bisphenol-A (BPA), phenol, amines, and phenylbutazone) by Mn(VII) over a wide pH range of 4.0-9.0. The addition of substituents at para to the > N-O^• moiety significantly influenced the degradation kinetics of TrOCs by changing the reduction potentials of TEMPOs and the corresponding oxoammonium cations (TEMPOs⁺); a linear relationship was observed between the substituents' para Hammett sigma constants and the reduction potentials of TEMPOs and TEMPOs⁺. Pseudo-first-order reaction rate constants (k_obs, min^-1) of TrOC degradation by Mn(VII)/TEMPOs were also affected by the pK_a of the TrOCs. Generally, the highest k_obs values for individual TrOCs were observed at pH near the pK_a even for TEMPOs⁺ with relatively pH-invariant reduction potentials. Overall, TrOC abatement kinetics were related to a combination of reactive species (Mn(VII), in situ formed MnO₂, and TEMPOs⁺). For BPA, the relative contributions (R) of reactive species ranked as R(TEMPOs⁺) > R(Mn(VII)) > R(in situ formed MnO₂) at pH 4.0-8.0, whereas R(Mn(VII)) > R(TEMPOs⁺) at pH 9.0 mainly owing to a change in BPA speciation as the pH approached the pK_a1 value for BPA. The results of this study are useful for the development of heterogeneous TEMPO-based redox mediators and future applications of TEMPO-mediated oxidation systems for accelerated abatement of TrOCs in water.

Enhanced degradation of emerging contaminants by permanganate/quinone process: Case study with bisphenol A

Permanganate (Mn(VII)) is widely used as a mild oxidant in water treatment. However, the reaction rates of some emerging contaminants with Mn(VII) are extremely low. In this study, benzoquinone (BQ), a redox mediator with the important component in dissolved organic matter (DOM), enhanced the oxidation of bisphenol A (BPA) by Mn(VII) in a wide pH range of 4.0-10.0. The redox cycle of BQ would produce semiquinone radicals, which could act as ligands to stabilize the formed Mn(III) in the system to promote the oxidation of BPA. Notably, the presence of BQ might promote the formation of MnO₂. A novel mechanism was proposed that singlet oxygen (¹O₂), Mn(III)-ligands (Mn(III)-L) and in-situ formed MnO₂ were the main contributors to accelerate BPA degradation in the Mn(VII)/BQ system. Under acidic conditions, the in-situ formed MnO₂ involved in the redox reaction and part of the Mn(IV) was reduced to Mn(III), indicating that the electron transfer of BQ promoted the formation of active Mn species and enhanced the Mn(VII) oxidation performance. Semiquinone radicals generated by BQ transformation would couple with the hydrogen substitution products of BPA to inhibit BPA self-coupling and promote the ring-opening reactions of BPA. Mn(VII)/BQ had better effect in raw water than in pure water, indicating that the Mn(VII)/BQ system has high potential for practical application. This study provided insights into the role of DOM in enhancing the Mn(VII) oxidation in water treatment.