One Electron Oxidation Induced Dimerization of 5-Hydroxytryptophol: Role of 5-Hydroxy Substitution

Where should Fenton go for the degradation of refractory organic contaminants in wastewater?

Fenton process has become a research hotspot due to the nonselective and efficient degradation of dissolved organic matter (DOM) by ·OH. However, there are still many challenges and bottlenecks for conventional Fenton (CF). This study provides the first comprehensive insight into the mechanisms of DOM degradation by the Fenton process, including the various subcategories of humic substances, emerging trace contaminants, including persistent organic pollutants, endocrine disrupting chemicals, and pharmaceuticals and personal care products, and the interference of humus and low molecular weight organic acids on the removal of trace contaminants. In addition, a statistical comparison of the economics of CF and three types of Fenton-like technologies (Photo-Fenton, Electro-Fenton, and Ultrasonic-Fenton) is conducted based on existing studies, which can be used as a reference for engineering applications. Moreover, a brief overview of the categories and characteristics of heterogeneous Fenton, which have been extensively studied in recent years, and a comparison of their catalysts are presented. In the end, the paper advances a possible future research direction.

Understanding the Importance of Periodate Species in the pH-Dependent Degradation of Organic Contaminants in the H2O2/Periodate Process

Although periodate-based advanced oxidation processes have been proven to be efficient in abating organic contaminants, the activation properties of different periodate species remain largely unclear. Herein, by highlighting the role of H₄IO₆^-, we reinvestigated the pH effect on the decontamination performance of the H₂O₂/periodate process. Results revealed that elevating pH from 2.0 to 10.0 could markedly accelerate the rates of organic contaminant decay but decrease the amounts of organic contaminant removal. This pH-dependent trend of organic contaminant degradation corresponded well with the HO^· yield and the variation of periodate species. Specifically, although ¹O₂ could be detected at pH 9.0, HO^· was determined to be the major reactive oxidizing species in the H₂O₂/periodate process under all the tested pH levels. Furthermore, it was suggested that only H₄IO₆^- and H₂I₂O₁₀^4- could serve as the precursors of HO^·. The second-order rate constant for the reaction of H₂I₂O₁₀^4- species with H₂O₂ was determined to be ∼1199.5 M^-1 s^-1 at pH 9.0, which was two orders of magnitude greater than that of H₄IO₆^- (∼2.2 M^-1 s^-1 at pH 3.0). Taken together, the reaction pathways of H₂O₂ with different periodate species were proposed. These fundamental findings could improve our understanding of the periodate-based advanced oxidation processes.

Removal of phthalates from aqueous solution by semiconductor photocatalysis: A review

While phthalate esters are commonly used as plasticizers to improve the flexibility and workability of polymeric materials, their presence and detection in various environments has become a significant concern. Phthalate esters are known to have endocrine-disrupting effects, which affects reproductive health and physical development. As a result, there is now increased focus and urgency to develop effective and energy efficient technologies capable of removing these harmful compounds from the environment. This review explores the use of semiconductor photocatalysis as an efficient and promising solution towards achieving removal and degradation of phthalate esters. A comprehensive review of photocatalysts reported in the literature demonstrates the range of materials including commercial TiO₂, solar activated catalysts and composite materials capable of enhancing adsorption and degradation. The degradation pathways and kinetics are also considered to provide the reader with an insight into the photocatalytic mechanism of removal. In addition, through the use of two key platforms (the technology readiness level scale and electrical energy per order), the crucial parameters associated with advancing photocatalysis for phthalate ester removal are discussed. These include enhanced surface interaction, catalyst platform development, improved light delivery systems and overall system energy requirements with a view towards pilot scale and industrial deployment.

Mechanistic insight into degradation of endocrine disrupting chemical by hydroxyl radical: An experimental and theoretical approach

Advanced oxidation processes (AOPs) based on formation of free radicals at ambient temperature and pressure are effective for treating endocrine disrupting chemicals (EDCs) in waters. In this study, we systematically investigated the degradation kinetics of bisphenol A (BPA), a representative EDC by hydroxyl radical (OH) with a combination of experimental and theoretical approaches. The second-order rate constant (k) of BPA with OH was experimentally determined to be 7.2 ± 0.34 × 10⁹ M^-1 s^-1 at pH 7.55. We also calculated the thermodynamic and kinetic behaviors for the bimolecular reactions by density functional theory (DFT) using the M05-2X method with 6-311++G** basis set and solvation model based on density (SMD). The results revealed that H-abstraction on the phenol group is the most favorable pathway for OH. The theoretical k value corrected by the Collins-Kimball approach was determined to be 1.03 × 10¹⁰ M^-1 s^-1, which is in reasonable agreement with the experimental observation. These results are of fundamental and practical importance in understanding the chemical interactions between OH and BPA, and aid further AOPs design in treating EDCs during wastewater treatment processes.

First assessment on degradability of sodium p-perfluorous nonenoxybenzene sulfonate (OBS), a high volume alternative to perfluorooctane sulfonate in fire-fighting foams and oil production agents in China

First report on biotic and abiotic degradability assessment of the fluorinated surfactants OBS.

Kinetics and mechanism of (•)OH mediated degradation of dimethyl phthalate in aqueous solution: experimental and theoretical studies

The hydroxyl radical ((•)OH) is one of the main oxidative species in aqueous phase advanced oxidation processes, and its initial reactions with organic pollutants are important to understand the transformation and fate of organics in water environments. Insights into the kinetics and mechanism of (•)OH mediated degradation of the model environmental endocrine disruptor, dimethyl phthalate (DMP), have been obtained using radiolysis experiments and computational methods. The bimolecular rate constant for the (•)OH reaction with DMP was determined to be (3.2 ± 0.1) × 10(9) M(-1)s(-1). The possible reaction mechanisms of radical adduct formation (RAF), hydrogen atom transfer (HAT), and single electron transfer (SET) were considered. By comparing the experimental absorption spectra with the computational results, it was concluded that the RAF and HAT were the dominant reaction pathways, and OH-adducts ((•)DMPOH1, (•)DMPOH2) and methyl type radicals (•)DMP(-H)α were identified as dominated intermediates. Computational results confirmed the identification of transient species with maximum absorption around 260 nm as (•)DMPOH1 and (•)DMP(-H)α, and these radical intermediates then converted to monohydroxylated dimethyl phthalates and monomethyl phthalates. Experimental and computational analyses which elucidated the mechanism of (•)OH-mediated degradation of DMP are discussed in detail.

Radiation and quantum chemical studies of chalcone derivatives

The reactions of oxidizing radicals ((*)OH, Br(2)(*-), and SO(4)(*-)) with -OH-, -CH(3)-, or -NH(2)-substituted indole chalcones and hydroxy benzenoid chalcones were studied by radiation and quantum chemical methods. The (*)OH radical was found to react by addition at diffusion-controlled rates (k = 1.1-1.7 x 10(10) dm(3) mol(-1) s(-1)), but Br(2)(*-) radical reacted by 2 orders of magnitude lower. Quantum chemical calculations at the B3LYP/6-31+G(d,p) level of theory have shown that the (C2-OH)(*), (C11-OH)(*), and (C10-OH)(*) adducts of the indole chalcones and the (C7-OH)(*) and (C8-OH)(*) adducts of the hydroxy benzenoid chalcones are more stable with DeltaH = -39 to -28 kcal mol(-1) and DeltaG = -32 to -19 kcal mol(-1). This suggests that (*)OH addition to the alpha,beta-unsaturated bond is a major reaction channel in both types of chalcones and is barrierless. The stability and lack of dehydration of the (*)OH adducts arise from two factors: strong frontier orbital interaction due to the low energy gap between interacting orbitals and the negligible Coulombic repulsion due to small absolute values of Mulliken charges. The transient absorption spectrum measured in the (*)OH radical reaction with all the indole chalcone derivatives exhibited a maximum at 390 nm, which is in excellent agreement with the computed value (394 nm). The formation of three phenolic products under steady-state radiolysis is in line with the three stable (*)OH adducts predicted by theory. Independent of the substituent, identical spectra (lambda(max) = 330-360 and approximately 580 nm) were obtained on one-electron oxidation of the three indole chalcones. MO calculations predict the deprotonation from the -NH group is more efficient than from the substituent due to the larger electron density on the N1 atom forming the chalcone indolyl radical. Its reduction potential was determined to be 0.56 V from the ABTS(*-)/ABTS(2-) couple. In benzenoid chalcones, the (*)OH adduct spectrum is characterized by a peak at 270 nm and a broad maximum centered in the range 430-450 nm with an intense bleaching at 340 nm. The spectrum formed by electron transfer in these derivatives with lambda(max) = 280 and 380 nm (epsilon(280) = 5000 dm(3) mol(-1) cm(-1) and epsilon(380) = 700 dm(3) mol(-1) cm(-1)) was assigned to its phenoxyl radical. Our pulse radiolysis experiments in combination with quantum chemical calculations demonstrate that chalcones are efficient scavengers of damaging oxyl radicals.

Oxidation of tryptamine and 5-hydroxytryptamine: a pulse radiolysis and quantum chemical study

The reactions of oxidizing radicals (*)OH, N(3)(*), Br(2)(*-), and NO(2)(*) with tryptamine (Tpe) and 5-hydroxytryptamine (HTpe) were studied by pulse radiolysis and analyzed by quantum chemical calculations. Barring NO(2)(*) radical, the rate constants for their reaction with Tpe and HTpe were found to be diffusion controlled and the rates in the NO(2)(*) radical reaction with HTpe are lower by 2 orders of magnitude with k approximately 1 x 10(7) dm(3) mol(-1) s(-1). The transient spectra formed on oxidation of Tpe and HTpe exhibited peaks at 330 and 530 nm (indolyl radical) and 420 nm (indoloxyl radical), respectively, and the latter is in reasonable agreement with the calculated value (407 nm). Both radicals decay through direct recombination, but only the indoloxyl radical was observed to react with the parent molecule to give a (HTpe-Ind)(*) radical adduct for [HTpe] > or = 50 x 10(-6) mol dm(-3). The calculated optimized geometries in water revealed the formation of two distinct types of radical adducts, one through the H-O bond and the other by C-C linkage. The H-O bonded radical adduct was found to be exothermic with a reaction enthalpy of -4 kcal mol(-1) and bond length 0.1819 nm and the C-C bonded radical adducts are endothermic and rate determining but are finally driven by exothermic processes involving intermolecular H transfer followed by intramolecular reorganization through H shift resulting in stable C4-C4' and C2-C4' dimers with reaction enthalpies of -39 and -44 kcal mol(-1), respectively, and this process was found to be thermodynamically as efficient as direct recombination of indoloxyl radicals. The formation of the two dimer products was also seen in steady-state radiolysis. The lack of adduct formation in the case of indolyl radical with Tpe is due to the positive free energy change (DeltaG = 10 kcal mol(-1)). The energetics for the (*)OH addition have shown dependence on the site of activation with (HTpe-OH)(*) adducts at C2 and C4 and the (Tpe-OH)(*) adduct at C2 being more thermodynamically stable and the water elimination to give the indoloxyl radical proceeds fast from (HTpe-OH)(*) adduct at C4 due to favorable geometry.

Effect of pH on One-Electron Oxidation Chemistry of Organoselenium Compounds in Aqueous Solutions

Pulse radiolysis coupled with absorption detection has been employed to study one-electron oxidation of selenomethionine (SeM), selenocystine (SeCys), methyl selenocysteine (MeSeCys), and selenourea (SeU) in aqueous solutions. Hydroxyl radicals (*OH) in the pH range from 1 to 7 and specific one-electron oxidants Cl2*- (pH 1) and Br2*- (pH 7) have been used to carry out the oxidation reactions. The bimolecular rate constants for these reactions were reported to be in the range of 2 x 10(9) to 10 x 10(9) M(-1) s(-1). Reactions of oxidizing radicals with all these compounds produced selenium-centered radical cations. The structure and stability of the radical cation were found to depend mainly on the substituent and pH. SeM, at pH 7, produced a monomer radical cation (lambdamax approximately 380 nm), while at pH 1, a dimer radical cation was formed by the interaction between oxidized and parent SeM (lambdamax approximately 480 nm). Similarly, SeCys, at pH 7, on one-electron oxidation, produced a monomer radical cation (lambdamax approximately 460 nm), while at pH 1, the reaction produced a transient species with (lambdamax approximately 560 nm), which is also a monomer radical cation. MeSeCys on one-electron oxidation in the pH range from 1 to 7 produced monomer radical cations (lambdamax approximately 350 nm), while at pH < 0, the reaction produced dimer radical cations (lambdamax approximately 460 nm). SeU at all the pH ranges produced dimer radical cations (lambdamax approximately 410 nm). The association constants of the dimer radical cations of SeM, MeSeCys, and SeU were determined by following absorption changes at lambdamax as a function of concentration. From these studies it is concluded that formation of monomer and dimer radical cations mainly depends on the substitution, pH, and the heteroatoms like N and O. The availability of a lone pair on an N or O atom at the beta or gamma position results in monomer radical cations having intramolecular stabilization. When such a lone pair is not available, the monomer radical cation is converted into a dimer radical cation which acquires intermolecular stabilization by the other selenium atom. The pH dependency confirms the role of protonation on stabilization. The oxidation chemistry of these selenium compounds is compared with that of their sulfur analogues.