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

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

Oxidation processes and me.

This publication summarizes my journey in the field of chemical oxidation processes for water treatment over the last 30+ years. Initially, the efficiency of the application of chemical oxidants for micropollutant abatement was assessed by the abatement of the target compounds only. This is controlled by reaction kinetics and therefore, second-order rate constant for these reactions are the pre-requisite to assess the efficiency and feasibility of such processes. Due to the tremendous efforts in this area, we currently have a good experimental data base for second-order rate constants for many chemical oxidants, including radicals. Based on this, predictions can be made for compounds without experimental data with Quantitative Structure Activity Relationships with Hammet/Taft constants or energies of highest occupied molecular orbitals from quantum chemical computations. Chemical oxidation in water treatment has to be economically feasible and therefore, the extent of transformation of micropollutants is often limited and mineralization of target compounds cannot be achieved under realistic conditions. The formation of transformation products from the reactions of the target compounds with chemical oxidants is inherent to oxidation processes and the following questions have evolved over the years: Are the formed transformation products biologically less active than the target compounds? Is there a new toxicity associated with transformation products? Are transformation products more biodegradable than the corresponding target compounds? In addition to the positive effects on water quality related to abatement of micropollutants, chemical oxidants react mainly with water matrix components such as the dissolved organic matter (DOM), bromide and iodide. As a matter of fact, the fraction of oxidants consumed by the DOM is typically > 99%, which makes such processes inherently inefficient. The consequences are loss of oxidation capacity and the formation of organic and inorganic disinfection byproducts also involving bromide and iodide, which can be oxidized to reactive bromine and iodine with their ensuing reactions with DOM. Overall, it has turned out in the last three decades, that chemical oxidation processes are complex to understand and to manage. However, the tremendous research efforts have led to a good understanding of the underlying processes and allow a widespread and optimized application of such processes in water treatment practice such as drinking water, municipal and industrial wastewater and water reuse systems.

Degradation of triclosan by peroxydisulfate/peroxomonosulfate binary oxidants activation under thermal conditions: Efficiency and mechanism.

Peroxydisulfate (PDS) and peroxymonosulfate (PMS) could be efficiently activated by heat to generate reactive oxygen species (ROS) for the degradation of organic contaminants. However, defects including the inefficiency treatment and pH dependence of monooxidant process are prominent. In this study, synergy of heat and the PDS-PMS binary oxidant was studied for efficient triclosan (TCS) degradation and apply in rubber wastewater. Under different pH values, the degradation of TCS followed pseudo-first-order kinetics, the reaction rate constant (kobs) value of TCS in heat/PDS/PMS system increased from 1.8 to 4.4 fold and 6.8-49.1 fold when compared to heat/PDS system and heat/PMS system, respectively. Hydroxyl radicals (·OH), sulfate radicals (SO4·-) and singlet oxygen (1O2) were the major ROS for the degradation of TCS in heat/PDS/PMS system. In addition, the steady-state concentrations of ·OH/1O2 and SO4·-/·OH/1O2 increased under acidic conditions and alkaline conditions, respectively. It was concluded that the pH regulated the ROS for degradation of TCS in heat/PDS/PMS system significantly. Based on the analysis of degradation byproducts, it was inferred that the dechlorination, hydroxylation and ether bond breaking reactions occurred during the degradation of TCS. Moreover, the biological toxicity of the ten byproducts was lower than that of TCS was determined. Furthermore, the heat/PDS/PMS system is resistant to the influence of water substrates and can effectively improve the water quality of rubber wastewater. This study provides a novel perspective for efficient degradation of TCS independent of pH in the heat/PDS/PMS system and its application of rubber wastewater.

Comparison of photoinduced and electrochemically induced degradation of venlafaxine.

The European Union requires environmental monitoring of the antidepressant drug venlafaxine. Advanced oxidation processes provide a remedy against the spread of micropollutants. In this study, the photoinduced and electrochemical decompositions of venlafaxine were investigated in terms of mechanism and efficacy using high-performance liquid chromatography coupled to high-resolution multifragmentation mass spectrometry. Kinetic analysis, structure elucidation, matrix variation, and radical scavenging indicated the dominance of a hydroxyl-mediated indirect mechanism during photodegradation and hydroxyl and direct electrochemical oxidation for electrochemical degradation. Oxidants, sulfate, and chloride ions acted as accelerants, which reduced venlafaxine half-lives from 62 to 25 min. Humic acid decelerated degradation during ultra-violet irradiation up to 50%, but accelerated during electrochemical oxidation up to 56%. In silico quantitative structure activity relationship analysis predicted decreased environmental hazard after advanced oxidation process treatment. In general, photoirradiation proved more efficient due to faster decomposition and slightly less toxic transformation products. Yet, matrix effects would have to be carefully evaluated when potential applications as a fourth purification stage were to be considered.

Influence of pesticide mixture on their heterogeneous atmospheric degradation by ozone and OH radicals.

Pesticides in the atmosphere can exist in both gaseous and particulate phases due to their semi-volatile properties. They can undergo degradation when exposed to atmospheric oxidants like ozone and hydroxyl radicals. The majority of studies on the atmospheric reactivity of pesticides study them in combination, without considering potential mixture effects that could induce uncertainties in the results. Therefore, this study aims to address this gap, through laboratory studies using a flow reactor, and by evaluating the degradation kinetics of pendimethalin mixed with folpet, tebuconazole, and S-metolachlor, which were simultaneously adsorbed on hydrophobic silica particles that mimic atmospheric aerosols. The comparison with other mixtures, including pendimethalin, from the literature has shown similar reactivity with ozone and hydroxyl radicals, indicating that the degradation kinetics of pesticides is independent of the mixture. Moreover, the degradation rates of the four pesticides under study indicate that they are not or slightly degraded by ozone, with half-lives ranging from 29 days to over 800 days. In contrast, when exposed to hydroxyl radicals, tebuconazole exhibited the fastest reactivity, with a half-life of 4 days, while pendimethalin had a half-life of 17 days.

Effects of water quality on bacterial inactivation by ferrate(VI).

Ferrate (Fe(VI)) is an emerging green oxidant which has great potential and prospect in water disinfection. However, the effects of water quality on Fe(VI) disinfection remain unclear. This study systematically investigated the effects of pH, organic matters and inorganic ions on Fe(VI) inactivation of Escherichia coli (E. coli). Results showed that pH was the dominant influencing factor and the inactivation efficiency as well as inactivation rate constant was negatively correlated with pH (6.8-8.4). HFeO4- was found to be the critical Fe(VI) species contributing to the inactivation. As for organic matters (0-5 mg C/L), protein and humic acid significantly accelerated the decay of Fe(VI) and had negative effects on the inactivation efficiency, while polysaccharide slightly inhibited the inactivation due to the low reactivity with Fe(VI). As for inorganic ions, bicarbonate (0-2 mM) could stabilize Fe(VI) and decreased the inactivation rate constant, while ammonium (0-1 mM) had little effect on the inactivation of E. coli. In addition, the comprehensive effects of water quality on Fe(VI) disinfection in actual reclaimed water were also evaluated. The inactivation of E. coli in secondary effluent and denitrifying effluent was found to be inhibited compared to that in phosphate buffer. Overall, this study is believed to provide valuable information on Fe(VI) disinfection for water and wastewater treatment practices.

Integrated Fenton-like and ozonation based advanced oxidation processes for treatment of real-time textile effluent containing azo reactive dyes.

The treatment of real-time textile effluent, collected from the Common Effluent Treatment Plant (CETP) of Kerala Industrial Infrastructure Development Corporation (KINFRA) at Kannur (District), Kerala (State), India, have been studied by utilizing the Fenton-like and ozone (O3) based advanced oxidation processes (AOPs). The Fenton-like AOP has been conducted as the pre-treatment of textile effluent involving the activation of persulfate (PS) and hydrogen peroxide (H2O2) as a single and the mixed oxidants by using the Flyash (FA)-Pd composite particles as the activator. The maximum chemical oxygen demand (COD) removal of 84% has been observed for a stand-alone O3 based treatment at an O3 flow rate of 5-6 g h-1. By conducting the pre-treatment of textile effluent with the PS, H2O2, and mixed oxidants (PS and H2O2) based Fenton-like AOPs, the COD removal after an O3 based post-treatment has been observed to be 83, 87, and 93% respectively at an O3 flow rate of 2, 3, and 5 g h-1. Hence, the Fenton-like pre-treatment involving the activation of mixed oxidants has been determined to be the best method for the highest COD removal of real-time textile effluent. The optimum values of initial oxidant-ratio (initial [H2O2]:initial [PS]), initial oxidant-dosage, and ozonation time, for the mixed oxidants based Fenton-like pre-treatment, have been determined to be 3 wt% mM-1, 6:2 wt% mM-1, and 60 min respectively. Under the most optimum conditions, the COD removal has been attributed to the combination of O2•- (for the pre-treatment) and •OOH (for the post-treatment) which possess relatively lower oxidation potential values.

From Theory to Practice: Leveraging Chemical Principles To Improve the Performance of Peroxydisulfate-Based In Situ Chemical Oxidation of Organic Contaminants.

In situ chemical oxidation (ISCO) using peroxydisulfate has become more popular in the remediation of soils and shallow groundwater contaminated with organic chemicals. Researchers have studied the chemistry of peroxydisulfate and the oxidative species produced upon its decomposition (i.e., sulfate radical and hydroxyl radical) for over five decades, describing reaction kinetics, mechanisms, and product formation in great detail. However, if this information is to be useful to practitioners seeking to optimize the use of peroxydisulfate in the remediation of hazardous waste sites, the relevant conditions of high oxidant concentrations and the presence of minerals and solutes that affect radical chain reactions must be considered. The objectives of this Review are to provide insights into the chemistry of peroxydisulfate-based ISCO that can enable more efficient operation of these systems and to identify research needed to improve understanding of system performance. By gaining a deeper understanding of the underlying chemistry of these complex systems, it may be possible to improve the design and operation of peroxydisulfate-based ISCO remediation systems.

The synergistic effect of oxidant-peroxide coupling systems for water and wastewater treatments.

With the growing complexity and severity of water pollution, it has become increasingly challenging to effectively remove contaminants or inactivate microorganisms just by traditional chemical oxidants such as O3, chlorine, Fe(VI) and Mn(VII). Up till now, numerous studies have indicated that these oxidants in combination with peroxides (i.e., hydrogen peroxide (H2O2), peroxymonosulfate (PMS), peracetic acid (PAA) and periodate (PI)) exhibited excellent synergistic oxidation. This paper provided a comprehensive review on the combination of aforementioned oxidant-peroxide applied in water and wastewater treatments. From one aspect, the paper thoroughly elucidated the synergy mechanism of each oxidant-peroxide combination in turn. Among these combinations, H2O2 or PMS generally performed as the activator of four traditional oxidants above to accelerate reactive species generation and therein various reaction mechanisms, including electron transfer, O atom abstraction and oxo ligand substitution, were involved. In addition, although neither PAA nor PI was able to directly activate Fe(VI) and Mn(VII), they could act as the stabilizer of intermediate reactive iron/manganese species to improve the latter utilization efficiency. From another aspect, this paper summarized the influence of water quality parameters, such as pH, inorganic ions and natural organic matter (NOM), on the oxidation performance of most combined systems. Finally, this paper highlighted knowledge gaps and identified areas that require further research.

[Remediation of Three Oxidants on Polycyclic Aromatic Hydrocarbons in Coking Contaminated Soil and Its Response to Indigenous Microorganisms].

To explore the influences of chemical oxidation on the physiological and ecological functions of indigenous microorganisms during contaminated soil remediation, three oxidants, including KMnO4, Na2S2O8, and O3, were selected to investigate their remediation effects on PAHs and the responses to indigenous microorganisms under different liquid-solid ratios, in this study. The results showed that:when the ΣPAHs concentration was 679.1 mg·kg-1 and the dosage of KMnO4 and Na2S2O8 was 1%, the removal efficiency of ΣPAHs reached up to 96.9% and 95.7% under the liquid-solid ratio of 6:1; for the O3 treatment, the removal efficiency of ΣPAHs was the highest(82.3%) at the O3 dosage and the liquid-solid ratio of 72 mg·min-1 and 8:1, respectively. The removal efficiency of low ring(3-4 rings) PAHs was higher than that of high ring(5-6 rings) PAHs under different liquid-solid ratios. The highest removal efficiencies were observed for phenanthrene and acenaphthene, whereas for benzo[a]pyrene, only the KMnO4treatment provided an effective performance, showing the highest removal efficiency of 97.4%. The microbial quantity analysis indicated that the quantity of soil microorganisms in the soil dropped sharply after being treated with KMnO4, decreasing from 108 copies·g-1 to 105 copies·g-1, whereas it changed only slightly after being treated with Na2S2O8 and O3. The community structure analysis showed that Proteobacteria were predominant in the contaminated soil, with the relative abundance of 99.5%. The addition of KMnO4 and Na2S2O8 significantly increased the microbial diversity; in particular, the relative abundance of a variety of microorganisms(such as Ralstonia and Acinetobacter) that can degrade PAHs was remarkably increased. The analysis of microbial metabolic function pathways revealed that chemical oxidation could simultaneously increase the relative abundance of PAHs-degrading bacteria and improve the ability of organic metabolism. Overall, the KMnO4 treatment greatly altered the quantity of microorganisms and the structure of the microbial community and the relative abundance of PAHs-degrading microorganisms at the liquid-solid ratio of 6:1.

Dioxygen Reduction and Bioinspired Oxidations by Non-heme Iron(II)-α-Hydroxy Acid Complexes.

ConspectusAerobic organisms involve dioxygen-activating iron enzymes to perform various metabolically relevant chemical transformations. Among these enzymes, mononuclear non-heme iron enzymes reductively activate dioxygen to catalyze diverse biological oxidations, including oxygenation of C-H and C═C bonds and C-C bond cleavage with amazing selectivity. Several non-heme enzymes utilize organic cofactors as electron sources for dioxygen reduction, leading to the generation of iron-oxygen intermediates that act as active oxidants in the catalytic cycle. These unique enzymatic reactions influence the design of small molecule synthetic compounds to emulate enzyme functions and to develop bioinspired catalysts for performing selective oxidation of organic substrates with dioxygen. Selective electron transfer during dioxygen reduction on iron centers of synthetic models by a sacrificial reductant requires appropriate design strategies. Taking lessons from the role of enzyme-cofactor complexes in the selective electron transfer process, our group utilized ternary iron(II)-α-hydroxy acid complexes supported by polydentate ligands for dioxygen reduction and bioinspired oxidations. This Account focuses on the role of coordinated sacrificial reductants in the selective electron transfer for dioxygen reduction by iron complexes and highlights the versatility of iron(II)-α-hydroxy acid complexes in affecting dioxygen-dependent oxidation/oxygenation reactions. The iron(II)-coordinated α-hydroxy acid anions undergo two-electron oxidative decarboxylation concomitant with the generation of reactive iron-oxygen oxidants. A nucleophilic iron(II)-hydroperoxo species was intercepted in the decarboxylation pathway. In the presence of a Lewis acid, the O-O bond of the nucleophilic oxidant is heterolytically cleaved to generate an electrophilic iron(IV)-oxo-hydroxo oxidant. Most importantly, the oxidants generated with or without Lewis acid can carry out cis-dihydroxylation of alkenes. Furthermore, the electrophilic iron-oxygen oxidant selectively hydroxylates strong C-H bonds. Another electrophilic iron(IV)-oxo oxidant, generated from the iron(II)-α-hydroxy acid complexes in the presence of a protic acid, carries out C-H bond halogenation by using a halide anion.Thus, different metal-oxygen intermediates could be generated from dioxygen using a single reductant, and the reactivity of the ternary complexes can be tuned using external additives (Lewis/protic acid). The catalytic potential of the iron(II)-α-hydroxy complexes in performing O2-dependent oxygenations has been demonstrated. Different factors that govern the reactivity of iron-oxygen oxidants from ternary iron(II) complexes are presented. The versatile reactivity of the oxidants provides useful insights into developing catalytic methods for the selective incorporation of oxidized functionalities under environmentally benign conditions using aerial oxygen as the terminal oxidant.

Unveiling the treatment-driven secondary risks of pharmaceuticals: New lessons from calcium channel blocker transformation by multiple oxidants.

The worldwide detection of numerous pharmaceuticals and their transformation products (TPs) in different environmental matrices has gained considerable concern about their potential ecological hazards. Increasing evidence suggested that calcium channel blockers (CCBs) are ubiquitous pharmaceutical pollutants in natural waters. However, their TPs, reaction pathways, and secondary risks have been limitedly known during oxidative water treatment. This study systematically assessed the TP formation and transformation mechanisms of two typical CCBs (i.e., amlodipine, AML; verapamil, VER) oxidized by ferrate(VI), permanganate, and ozone, as well as the in silico prediction on the TPs' properties. The high-resolution mass spectrometer analysis suggested a total of 16 TPs of AML and 8 TPs of VER identified for these reaction systems. Transformation of AML mainly proceeded through hydroxylation of the aromatic ring, ether bond cleavage, NH2 substitution by a hydroxyl group, and H-abstraction, while VER was oxidized via hydroxylation/opening of the aromatic ring and cleavage of the CN bond. Notably, certain TPs of both CCBs were estimated with low biodegradation, multi-endpoint toxicity, and high persistence and bioaccumulation, suggesting their severe risks to aquatic ecosystems. This study has implications for understanding the environmental behaviors, fate, and secondary risks of the globally prevalent and concerned CCBs under oxidative water treatment scenarios.

The ferryl generation by fenton reaction driven by catechol.

The Fenton and Fenton-like reactions are based on the decomposition of hydrogen peroxide catalyzed by Fe(II), primarily producing highly oxidizing hydroxyl radicals (HO∙). While HO∙ is the main oxidizing species in these reactions, Fe(IV) (FeO2+) generation has been reported as one of the primary oxidants. FeO2+ has a longer lifetime than HO∙ and can remove two electrons from a substrate, making it a critical oxidant that may be more efficient than HO∙. It is widely accepted that the preferential generation of HO∙ or FeO2+ in the Fenton reaction depends on factors such as pH and Fe: H2O2 ratio. Reaction mechanisms have been proposed to generate FeO2+, which mainly depend on the radicals generated in the coordination sphere and the HO∙ radicals that diffuse out of the coordination sphere and react with Fe(III). As a result, some mechanisms are dependent on prior HO∙ radical production. Catechol-type ligands can induce and amplify the Fenton reaction by increasing the generation of oxidizing species. Previous studies have focused on the generation of HO∙ radicals in these systems, whereas this study investigates the generation of FeO2+ (using xylidine as a selective substrate). The findings revealed that FeO2+ production is increased compared to the classical Fenton reaction and that FeO2+ generation is mainly due to the reactivity of Fe(III) with HO∙ from outside the coordination sphere. It is proposed that the inhibition of FeO2+ generation via HO∙ generated from inside the coordination sphere is caused by the preferential reaction of HO∙ with semiquinone in the coordination sphere, favoring the formation of quinone and Fe(III) and inhibiting the generation of FeO2+ through this pathway.

Selective Hydroxylation of C(sp3 )-H Bonds in Steroids.

Steroids are highly prevalent structures in small-molecule therapeutics, with the level of oxidation being key to their biological activity and physicochemical properties. These C(sp3 )-rich tetracycles contain many stereocentres, which are important for creating specific vectors and protein binding orientations. Therefore, the ability to hydroxylate steroids with a high degree of regio-, chemo- and stereoselectivity is essential for researchers working in this field. This review will cover three main methods for the hydroxylation of steroidal C(sp3 )-H bonds: biocatalysis, metal-catalysed C-H hydroxylation and organic oxidants, such as dioxiranes and oxaziridines.

Structural insights into the action mechanisms of artificial electron acceptors in photosystem II.

Photosystem II (PSII) utilizes light energy to split water, and the electrons extracted from water are transferred to QB, a plastoquinone molecule bound to the D1 subunit of PSII. Many artificial electron acceptors (AEAs) with molecular structures similar to that of plastoquinone can accept electrons from PSII. However, the molecular mechanism by which AEAs act on PSII is unclear. Here, we solved the crystal structure of PSII treated with three different AEAs, 2,5-dibromo-1,4-benzoquinone, 2,6-dichloro-1,4-benzoquinone, and 2-phenyl-1,4-benzoquinone, at 1.95 to 2.10 Å resolution. Our results show that all AEAs substitute for QB and are bound to the QB-binding site (QB site) to receive electrons, but their binding strengths are different, resulting in differences in their efficiencies to accept electrons. The acceptor 2-phenyl-1,4-benzoquinone binds most weakly to the QB site and showed the highest oxygen-evolving activity, implying a reverse relationship between the binding strength and oxygen-evolving activity. In addition, a novel quinone-binding site, designated the QD site, was discovered, which is located in the vicinity of QB site and close to QC site, a binding site reported previously. This QD site is expected to play a role as a channel or a storage site for quinones to be transported to the QB site. These results provide the structural basis for elucidating the actions of AEAs and exchange mechanism of QB in PSII and also provide information for the design of more efficient electron acceptors.

Comparison of effects of multiple oxidants with an ultrasonic system under unified system conditions for bisphenol A degradation.

Bisphenol A (BPA) as a trace contaminant has been reported, due to widespread use in the plastics industry. This study applied the 35 kHz ultrasound (US) to activate four different common oxidants (H2O2, HSO5-, S2O82-, and IO4-) for BPA degradation. With increasing initial concentration of oxidants, the degradation rate of BPA increased. The synergy index confirmed that a synergistic relationship between US and oxidants. This study also examined the impact of pH and temperature. The results showed that the kinetic constants of US, US-H2O2, US-HSO5- and US-IO4-decreased when the pH increased from 6 to 11. The optimal pH for US-S2O82- was 8. Notably, increasing temperature decreased the performance of US, US-H2O2, and US-IO4- systems, while it could increase the degradation of BPA in US-S2O82- and US-HSO5-. The activation energy for BPA decomposition using the US-IO4- system was the lowest, at 0.453nullkJnullmol-1, and the synergy index was the highest at 2.22. Additionally, the ΔG# value was found to be 2.11 + 0.29T when the temperature ranged from 25 °C to 45 °C. The main oxidation contribution is achieved by hydroxyl radicals in scavenger test. The mechanism of activation of US-oxidant is heat and electron transfer. In the case of the US-IO4- system, the economic analysis yielded 271 kwh m-3, which was approximately 2.4 times lower than that of the US process.

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.

Substrate Conformation Regulates Aromatic C-H Vs C-F Bond Activation in Heme-Dependent Tyrosine Hydroxylase.

A recently discovered heme-dependent enzyme tyrosine hydroxylase (TyrH) offers a green approach for functionalizing the high-strength C-H and C-F bonds in aromatic compounds. However, there is ambiguity regarding the nature of the oxidant (compound 0 or compound I) involved in activating these bonds. Herein, using comprehensive molecular dynamics (MD) simulations and hybrid quantum mechanical/molecular mechanical calculations, we reveal that it is compound I (Cpd I) that acts as the primary oxidant involved in the functionalization of both C-F and C-H bonds. The energy barrier for C-H and C-F activation using compound 0 (Cpd 0) as an oxidant was very high, indicating that Cpd 0 cannot be an oxidant. Consistent with the previous experimental finding, our simulation shows two different conformations of the substrate, where one orientation favors the C-H activation, while the other conformation prefers the C-F activation. As such, our mechanistic study shows that nature utilizes just one oxidant, that is, Cpd I, but it is the active site conformation that decides whether it selects C-F or C-H functionalization which may resemble involvement of two different oxidants.

Ultraviolet-based synergistic processes for wastewater disinfection: A review.

Ultraviolet (UV) irradiation is widely used for wastewater disinfection but suffers from low inactivation rates and can cause photoreactivation of microorganisms. Synergistic disinfection with UV and oxidants is promising for enhancing the inactivation performance. This review summarizes the inactivation effects on representative microorganisms by UV/hydrogen peroxide (H2O2), UV/ozone (O3), UV/persulfate (PS), UV/chlorine, and UV/chlorine dioxide (ClO2). UV synergistic processes perform better than UV or an oxidant alone. UV mainly attacks the DNA or RNA in microorganisms; the oxidants H2O2 and O3 mainly attack the cell walls, cell membranes, and other external structures; and HOCl and ClO2 enter cells and oxidize proteins and enzymes. Free radicals can have strong oxidation effects on cell walls, cell membranes, proteins, enzymes, and even DNA. At similar UV doses, the inactivation rates of Escherichia coli with UV alone, UV/H2O2, UV/O3, UV/PS (peroxydisulfate or peroxymonosulfate), and UV/chlorinated oxidant (chlorine, ClO2, and NH2Cl) range from 2.03 to 3.84 log, 2.62-4.30 log, 4.02-6.08 log, 2.93-5.07 log, and 3.78-6.55 log, respectively. The E. coli inactivation rates are in the order of UV/O3 ≈ UV/Cl2 > UV/PS > UV/H2O2. This order is closely related to the redox potentials of the oxidants and quantum yields of the radicals. UV synergistic disinfection processes inhibit photoreactivation of E. coli in the order of UV/O3 > UV/PS > UV/H2O2. The activation mechanisms and formation pathways of free radicals with different UV-based synergistic processes are presented. In addition to generating HO·, O3 can reduce the turbidity and chroma of wastewater to increase UV penetration, which improves the disinfection performance of UV/O3. This knowledge will be useful for further development of the UV-based synergistic disinfection processes.

Methylmercury Degradation by Trivalent Manganese.

Methylmercury (MeHg) is a potent neurotoxin and has great adverse health impacts on humans. Organisms and sunlight-mediated demethylation are well-known detoxification pathways of MeHg, yet whether abiotic environmental components contribute to MeHg degradation remains poorly known. Here, we report that MeHg can be degraded by trivalent manganese (Mn(III)), a naturally occurring and widespread oxidant. We found that 28 ± 4% MeHg could be degraded by Mn(III) located on synthesized Mn dioxide (MnO2-x) surfaces during the reaction of 0.91 µg·L-1 MeHg and 5 g·L-1 mineral at an initial pH of 6.0 for 12 h in 10 mM NaNO3 at 25 °C. The presence of low-molecular-weight organic acids (e.g., oxalate and citrate) substantially enhances MeHg degradation by MnO2-x via the formation of soluble Mn(III)-ligand complexes, leading to the cleavage of the carbon-Hg bond. MeHg can also be degraded by reactions with Mn(III)-pyrophosphate complexes, with apparent degradation rate constants comparable to those by biotic and photolytic degradation. Thiol ligands (cysteine and glutathione) show negligible effects on MeHg demethylation by Mn(III). This research demonstrates potential roles of Mn(III) in degrading MeHg in natural environments, which may be further explored for remediating heavily polluted soils and engineered systems containing MeHg.