Effects of exogenic chloride on oxidative degradation of chlorinated azo dye by UV-activated peroxodisulfate

Recently, the degradation of organic compounds in saline dye wastewater by sulfate radicals (SO₄⁻)-based advanced oxidation processes (AOPs) have attracted much attention. However, previous studies on these systems have selected non-chlorinated dyes as model compounds, and little is known about the transformation of chlorinated dyes in such systems. In this study, acid yellow 17 (AY-17) was selected as a model of chlorinated contaminants, and the degradation kinetics and evolution of oxidation byproducts were investigated in the UV/PDS system. AY-17 can be efficiently degraded (over 98% decolorization) under 90 min irradiation at pH 2.0–3.0, and the reaction follows pseudo-first order kinetics. Cl^‒ accelerated the degradation of AY-17, but simultaneously led to an undesirable increase of absorbable organic halogen (AOX). Several chlorinated byproducts were identified by liquid chromatography-mass spectrometry (LC–MS/MS) in the UV/PDS system. It indicates that endogenic chlorine and exogenic Cl^– reacted with SO₄⁻ to form chloride radicals, which are involved in the dechlorination and rechlorination of AY-17 and intermediates. The possible degradation mechanisms of AY-17 photooxidative degradation are proposed. This work provides valuable information for further studies on the role of exogenic chloride in the degradation of chlorinated azo dyes and the kinetic parameters in the PDS-based oxidation process.     

Photochemical properties of peroxo complexes of titanium,intermediate products in the photocatalytic liquid-phase oxidation of alcohols

A study has been made of the photochemical reactions of titanium(IV) peroxo complexes formed in the reaction of titanium tetrachloride with hydrogen peroxide in alcoholic solutions and which are characterized by intense charge transfer bands at 360–425 nm. Irradiation of the solutions with light of = 254 nm leads to the decomposition of the titanium(IV) peroxo complexes, the formation of titanium(III) compounds, as well as the oxidation of the alcohol to aldehyde. In the irradiated frozen solutions associated complexes of titanium(III), organic free radicals formed from the alcohol molecule, as well as peroxy radicals have been identified by EPR. Irradiation with light corresponding to the longwave band of the peroxo complexes leads to their decomposition but titanium(III) compounds and alcohol oxidation products are not formed in this case. In irradiated frozen solutions the formation of paramagnetic titanium(IV) complexes containing the fragment Ti...O ₂ has been established, as well as other paramagnetic particles identified tentatively as coordinated Ti...O ₂ ^· radicals or radical pairs. It is shown that the decomposition of hydrogen peroxide in the presence of titanium(IV) compounds is photocatalytic.Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 24, No. 1, pp. 67–74, January–February, 1988.     

Vitamin B3 as a high acid-alkali tolerant peroxidase mimic for colorimetric detection of hydrogen peroxide and glutathione

Small-molecule enzyme mimics as biocatalysts have been extensively applied in diverse colorimetric sensors fabrication. However, excavating potential organic enzyme mimics with high catalytic activity still remains challenging. In this study, the peroxidase mimicking activity of nicotinic acid (VB3) was demonstrated for the first time through chromogenic substrate 3, 3′, 5, 5′-tetramethylbenzidine (TMB) at the existence of hydrogen peroxide (H₂O₂). The catalytic activity of VB3 kept more than 80% of its optimum activity in a broad pH range of 3.0–9.0. In addition, the kinetic parameter (Michaelis constant, K_m = 0.037 mM) of VB3 catalysis to H₂O₂ is smaller than natural horseradish peroxidase (HRP) and previously reported peroxidase mimics. The catalytic mechanism of VB3 is mainly attributed to the active species of hydroxyl radical (OH) and partially attributed to the superoxide free radicals (O₂^?). A convenient and sensitive colorimetric method based on VB3-H₂O₂-TMB chromogenic system for H₂O₂ and glutathione detection was fabricated with the linear ranges of 5.0–100.0 μM and 5.0–50.0 μM, respectively. In short, this work will not only bring new enlightenment on the physiological functions and practical applications in the analytical field of VB3, but also provide a new type of structural reference for small-molecule enzyme mimics.     

Metal-free visible-light-promoted thiocyanation/cyclization cascade for the synthesis of thiocyanato-containing isoquinolinediones

A visible-light induced metal-free thiocyanate radical addition/intramolecular cyclization cascade reaction for the synthesis of thiocyanato-containing isoquinolinediones from N-alkyl-N-methacryloylbenzamides is described. The organic dye 9-mesityl-10-methylacridinium perchlorate (Acr⁺-Mes ClO₄⁻) is used as a photocatalyst, and cheap and readily available ammonium thiocyanate is used to provide thiocyanate radical by single-electron transfer pathway. The reaction completes the synthesis of CS and CC bonds in one pot with abundant molecular oxygen as the sole sacrificial reagent. The method is easy to implement, and 25 new compounds have been prepared in moderate to good yields under mild conditions. This is the first time that a thiocyanate group has been introduced into isoquinoline-1,3(2H,4H)-diones to construct highly functional drug-like molecules.     

Effective removal of chlorinated organic pollutants by bimetallic iron-nickel sulfide activation of peroxydisulfate

Chlorinated organic pollutants(COPs) have caused serious contaminants in soil and groundwater,hence developing methods to remove these pollutants is necessary and urgent.By a simple hydrothermal method,we synthesized the bimetallic iron-nickel sulfide(FeNiS) particles which exhibited excellent catalytic property of COPs removal.FeNiS was chosen as the peroxydisulfate(PDS) activator to removal COPs including 4-chlorophenol(4-CP),1,4-dichlorophenol(1,4-DCP) and 2,4,6-trichlorophenol(2,4,6-TCP).The results show that FeNiS can efficiently activate PDS to produce sulfate radical(SO_4~(·-)) which plays major role in the oxidative dechlorination and degradation due to its strong oxidizing property and the ability of producing hydroxyl radicals(~·OH) in the alkaline condition.Meanwhile,the Cl-abscised from COPs during the dechlorination can turn into the chlorine radicals and enhance the degradation and cause further mineralization of intermediate products.This bimetallic FeNiS catalyst is a promising PDS activator for removal of chlorinated organics.     

Oxygen dependent oxidation of trimethoprim by sulfate radical: Kinetic and mechanistic investigations

Trimethoprim (TMP) is a typical antibiotic to treat infectious disease, which is among the most commonly detected antibacterial agents in natural waters and municipal wastewaters. In the present study, the impacts of dissolved oxygen (DO) on the oxidation efficiency and pathways of TMP by reaction with sulfate radicals (SO₄⁻) were investigated. Our results revealed that the presence of DO was favourable for TMP degradation. Specifically, TMP would react initially with SO₄⁻ via electron-transfer process to form a carbon-centered radical. In the absence of oxygen, the carbon-centered radical could undergo hydrolysis to produce α-hydroxytrimethoprim (TMP−OH), followed by the further oxidation which generated α-ketotrimethoprim (TMP=O). However, in the presence of oxygen, the carbon-centered radical would alternatively combine with oxygen, leading to a sequential reaction in which peroxyl radical and a tetroxide were formed, and finally generated TMP−OH and TMP=O simultaneously. The proposed pathways were further confirmed by density functional theory (DFT) calculations. The results obtained in this study would emphasize the significance of DO on the oxidation of organic micro-pollutants by SO₄⁻.     

High active amorphous Co(OH)2 nanocages as peroxymonosulfate activator for boosting acetaminophen degradation and DFT calculation

Acetaminophen (ACE) is commonly used in analgesic and antipyretic drug, which is hardly removed by traditional wastewater treatment processes. Herein, amorphous Co(OH)₂ nanocages were explored as peroxymonosulfate (PMS) activator for efficient degradation of ACE. In the presence of amorphous Co(OH)₂ nanocages, 100% of ACE removal was reached within 2 min with a reaction rate constant k₁ = 3.68 min^?1 at optimum pH 5, which was much better than that of crystalline β-Co(OH)₂ and Co₃O₄. Amorphous materials (disorder atom arrangement) with hollow structures possess large specific surface area, more reactive sites, and abundant vacancies structures, which could efficiently facilitate the catalytic redox reactions. The radicals quenching experiment demonstrated that SO₄^? radicals dominated the ACE degradation rather than OH radicals. The mechanism of ACE degradation was elucidated by the analysis of degradation intermediates and theoretical calculation, indicating that the electrophilic SO₄^? and OH tend to attack the atoms of ACE with high Fukui index (f ^?). Our finding highlights the remarkable advantages of amorphous materials as heterogeneous catalysts in sulfate radicals-based AOPs and sheds new lights on water treatment for the degradation of emerging organic contaminants.     

Graphdiyne as a novel nonactive anode for wastewater treatment: A theoretical study

Electrochemical oxidation of water to produce highly reactive hydroxyl radicals (OH) is the dominant factor that accounts for the organic compounds removal efficiency in water treatment. As an emerging carbon-based material, the investigation of electrocatalytic of water to produce OH on Graphdiyne (GDY) anode is firstly evaluated by using first-principles calculations. The theoretical calculation results demonstrated that the GDY anode owns a large oxygen evolution reaction (OER) overpotential (η^OER = 1.95 V) and a weak sorptive ability towards oxygen evolution intermediates (HO*, not OH). The high Gibbs energy change of HO* (3.18 eV) on GDY anode makes the selective production of OH (ΔG = 2.4 eV) thermodynamically favorable. The investigation comprises the understanding of the relationship between OER to electrochemical advanced oxidation process (EAOP), and give a proof-of-concept of finding the novel and robust environmental EAOP anode at quantum chemistry level.     

Free-radical generation mechanism and antimalarial activity of cyclohexyl endoperoxides

A kinetic analysis has been carried out for a cascade of intramolecular oxidation reactions of free radicals generated in the redox reactions of substituted cyclohexyl endoperoxides (15 compounds) with the Fe²⁺ ion. Each radical conversion reaction has been characterized by its enthalpy, activation energy, and rate constant. Kinetic characteristics have been calculated by the intersecting parabolas method. Depending on their structure, cyclohexyl endoperoxides generate one to three radicals. There is a linear empirical correlation between the number of radicals generated by a peroxide and its molar antimalarial activity (IC ₅₀/M, where M is the molar mass of the peroxide). The peroxides that generate no more than one radical show no antimalarial activity.     

Metabolomics techniques applied in the investigation of phenolic acids from the agro-industrial by-product of Carapa guianensis Aubl

Processing of Carapa guianensis seeds to obtain oil on an industrial scale generates a significant amount of by-product, approximately 66% w/w, which is called cake and is a potential source of biomolecules, including simple phenolic structures. For this reason, studies were carried out on the chemical profiles of hydrolyzed extract from this agro-industrial by-product through High Performance Thin-Layer Chromatography (HPTLC) and Gas Chromatography coupled to Mass Spectrometry (GC–MS). These techniques were used to detect metabolic classes and/or groups, and to identify, for the first time, thirteen simple phenolic acids in this by-product. The sample antioxidant capacity was determined by methods of 2,2-diphenyl-1-picrylhydrazyl (DPPH)and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS⁺) radicals direct sequestration. The hydrolyzed fraction showed a total of 63.47% in the relative abundance of the total of compounds, standing out: p-hydroxybenzoic acid (39.19%) and protocatechuic acid (3,4-dihydroxybenzoic acid) (5.62%), both from hydroxybenzoic acids and 3-(3,4-dihydroxyphenyl)lactic acid, (7.76%) hydroxycinnamic acids derivatives. In these results, the fraction rich in simple phenolic acids was obtained, attributing the prominent behavior of this matrix antioxidant activity, expressed by (IC₅₀: of 16.42 µg/mL and 6.52 µg/mL for DPPH and ABTS⁺ radicals, respectively). The research demonstrated an alternative to applicability that involves sustainability from agro-industrial. These techniques were used to detect metabolic classes and/or groups, and to identify, for the first time, thirteen simple phenolic acids in this by-product, generating a process capable of converting biomass into a bioproduct, consisting of bioactive compounds, in addition to adding value to the industrial chain.     

C–H functionalization reactions enabled by hydrogen atom transfer to carbon-centered radicals

Selective functionalization of ubiquitous unactivated C–H bonds is a continuous quest for synthetic organic chemists. In addition to transition metal catalysis, which typically operates under a two-electron manifold, a recent renaissance in the radical approach relying on the hydrogen atom transfer (HAT) process has led to tremendous growth in the area. Despite several challenges, protocols proceeding via HAT are highly sought after as they allow for relatively easy activation of inert C–H bonds under mild conditions leading to a broader scope and higher functional group tolerance and sometimes complementary reactivity over methods relying on traditional transition metal catalysis. A number of methods operating via heteroatom-based HAT have been extensively reported over the past few years, while methods employing more challenging carbon analogues have been less explored. Recent developments of mild methodologies for generation of various carbon-centered radical species enabled their utilization in the HAT process, which, in turn, led to the development of remote C(sp³)–H functionalization reactions of alcohols, amines, amides and related compounds. This review covers mostly recent advances in C–H functionalization reactions involving the HAT step to carbon-centered radicals.

Intramolecular and intermolecular HAT to C-centered radicals enables selective C–H functionalization of organic molecules.     

Effect of isotopic composition on free-radical reactions in polyolefins

On substitution of hydrogen by deuterium the radiation yield of free radicals in polyolefins decreases ca. 2-fold. Under ultraviolet illumination (γ < 300 nm) in deuterated polyethylene (DPE), allyl radicals are converted into alkyl radicals (hfs constants a = 3.2 Oe, a = 4.4 Oe); in deuterated polypropylene (DPP), allyl and peroxide radicals are converted into alkyl radicals (a = 3.7 Oe). At 77°K under ultraviolet light in γ-irradiated polyethylene (PE) and DPE, triene, tetraene, dienyl, trienyl, and tetraenyl radicals are formed. Deuterium substitution has no effect on absorption spectra of polyenyl radicals and polyenes. In polyolefins the quantum yields of photochemical reactions of allyl and peroxide radicals are 0.1–1.0. Photochemical reactions of peroxide radicals result in a radical concentration increase of ca. 3-fold. In reactions of peroxide radicals in the dark, a kinetic isotope effect has been observed. Activation energies of substitution reactions of peroxide radicals are in PE, 9.3 ± 0.3 kcal/mole; in DPE, 10.2 ± 0.3 kcal/mole; in PP, 12.7 ± 0.4 kcal/mole; in DPP, 14 ± 0.5 kcal/mole. Possible mechanisms of the effect of polyolefin isotope composition on radical formation by high energy irradiation as well as on photochemical and dark reactions of free radicals are discussed. The effect of the energy released from phototransformations and radiationless deactivation of macroradicals on the migration of free valence is also considered.     

Copper-catalyzed synthesis of oxime ethers from iminoxy radical (CN–O) and maleimides via radical addition

An efficient Cu(II)-catalyzed radical addition of maleimides has been achieved. The identified copper catalyst enables the formation of oxime radicals (N–O) by cleaving the O–H bond in ketoximes, followed by the radical addition to N-substituted maleimides. The oxime radicals (N–O) were detected and confirmed by EPR spectroscopy and variable-temperature ¹H NMR. The simple one-pot reaction realizes the facile preparation of a variety of oxime ether adduct products in moderate to good yields.     

The electrochemical advanced oxidation processes coupling of oxidants for organic pollutants degradation: A mini-review

The electrochemical advanced oxidation processes(EAOPs) have been extensively applied in the treatment of organic pollutants degradation.Herein,the mini review provides the coupling systems about EAOPs and different oxidants(e.g.,persulfate(PS),peroxymonosulfate(PMS),and ozone(O_3)),including EAOPs-PS systems,EAOPs-PMS systems,EAOPs-peroxone systems,and photoelectro-oxidants systems,for the organic compounds degradation.The coupling system of EAOPs with oxidants is an effective way to improve the generated free radicals(e.g.,HO~·and SO_4~(·-)) concentration and to accelerate pollutant degradation.In this review,we make a summary of the homogeneous and heterogeneous EAOPs-oxidant processes.The reaction mechanisms of EAOPs combined with different oxidants are elucidated in detail,as well as the synergistic effect for improving the degradation and mineralization efficiency.     

Tuning the stability of organic radicals: from covalent approaches to non-covalent approaches

Organic radicals are important species with single electrons. Because of their open-shell structure, they are widely used in functional materials, such as spin probes, magnetic materials and optoelectronic materials. Owing to the high reactivity of single electrons, they often serve as a key intermediate in organic synthesis. Therefore, tuning the stability of radicals is crucial for their functions. Herein, we summarize covalent and non-covalent approaches to tune the stability of organic radicals through steric effects and tuning the delocalization of spin density. Covalent approaches can tune the stability of radicals effectively and non-covalent approaches benefit from dynamicity and reversibility. It is anticipated that the further development of covalent and non-covalent approaches, as well as the interplay between them, may push the fields forward by enriching new radical materials and radical mediated reactions.

Covalent and non-covalent approaches to tune the stability of organic radicals through steric effects and the delocalization of spin density.     

Covalent organic frameworks functionalized carbon fiber paper for the capture and detection of hydroxyl radical in the atmosphere

Developing a fast, sensitive and convenient method for the detection of hydroxyl radicals (OH) in the atmosphere could help us know the precursor levels of atmospheric species and control air pollution. In this work, the carbon fiber paper (CFP) functionalizing with a kind of covalent organic frameworks (COFs), formed from 1,3,5-triformylphloroglucinol (Tp) and benzidine (BD) (COF(TpBD)), was firstly used a new platform for OH trapping and detection. The COF(TpBD) modified CFP was acted as a filter to impregnate salicylic acid (SA) and a detector to detect 2,5-dihydroxybenzoic acid (2,5-DHBA) which was produced from the reaction between the impregnated SA and OH in the atmosphere. This method provided a linearity for 2,5-DHBA from 5.0 × 10⁻¹⁴ mol/L 1.0 × 10⁻⁹ mol/L with a detection limit of 6.9 × 10⁻¹⁵ mol/L, which is corresponding to the amount of OH from 3.0 × 10⁷ to 6.0 × 10¹¹ molecules/cm³ with the detection limit of 4.1 × 10⁶ molecules/cm³. This COF(TpBD)-CFP platform has been successfully applied for the detection of OH concentration under different conditions of Yangzhou when the sampling time was shortened to 30 min. This work has provided a new method for atmospheric OH detection with excellent sensitivity, simplicity, and high speed.     

Visible Light-Promoted Ring-Opening Alkenylation of Cyclic Hemiacetals through β-Scission of Alkoxy Radicals

The chemistry of alkoxy radicals was extensively explored during the period of 1960s to 1990s, but it has remained dormant for the past few decades. Recently, alkoxy radicals attract the attentions again, because new methods for generating alkoxy radical species have emerged. These newly developed methods are mainly based on the photolysis by visible light under mild conditions, thus allowing for new transformations of the carbon-centered radical species that are generated from the β-scission or hydrogen abstraction of the alkoxy radicals. Herein, we demonstrate that the alkoxy radicals derived from cyclic hemiacetals can be generated through visible-light-induced electron transfer with sodium iodide and triphenylphosphine as the catalyst. The alkoxy radicals subsequently undergo β-scission to generate carbon-centered radicals, which are trapped by cinnamic acids, aryl alkenes, vinylboronic acid and silyl enol ether to deliver the corresponding C—C bond forming products. This catalytic method for ring-opening alkenylation reaction of cyclic hemiacetal derivatives under visible-light irradiation conditions demonstrates the compatibility of the visible light-promoted alkoxy radical generation method with various carbon radical trapping processes. This work opens up new possibilities for the application of alkoxy radicals in organic synthesis.      

Tuneable Redox Chemistry and Electrochromism of Persistent Symmetric and Asymmetric Azine Radical Cations

Molecular organic radicals have been intensively studied in the last decades, due to their interesting optical, magnetic and redox properties. Here we report the synthesis and characterisation of persistent organic radicals from one-electron oxidation of redox-active azines (RAAs), composed of two guanidinyl or related groups. By connecting two different groups together, asymmetric compounds result. In this way a series of compounds with varying redox potential is obtained that could be oxidised reversibly to the mono- and the dicationic charge states. The accessible redox states were fully determined by chemical redox reactions. The standard Gibbs free energy change for disproportionation of the radical monocation into the dication and the neutral molecule in solution, estimated from cyclovoltammetric measurements, varies between 43 and 71 kJ mol⁻¹. While the neutral RAAs absorb predominately UV light, the radical monocations display strong absorptions covering almost the entire visible region and extending for some compounds into the NIR region. A detailed analysis of this highly reversible electrochromism is presented, and the fast switching characteristics are demonstrated in an electrochromic test device.     

Photocatalytic degradation of sulfadiazine in suspensions of TiO2 nanosheets with exposed (001) facets

Antibiotics such as sulfonamides are widely used in agriculture as growth promoters and medicine in treatment of infectious diseases. However, the release of these antibiotics has caused serious environmental problems. In this paper, photocatalytic oxidation technology was used to degrade sulfadiazine (SDZ), one of the typical sulfonamides antibiotics, in UV illuminated TiO₂ suspensions. It was found that TiO₂ nanosheets (TiO₂-NSs) with exposed (001) facets exhibit much higher photoreactivity towards SDZ degradation compared to TiO₂ nanoparticles (TiO₂-NPs) with a rate constant increases from 0.017 min⁻¹ to 0.035 min⁻¹, improving by a factor of 2.1. Under the attacking of reactive oxygen species (ROSs) such as superoxide radicals (O₂^–) and hydroxyl radicals (OH), SDZ was steady degraded on the surface of TiO₂-NSs. Based on the identification of the produced intermediates by LC–MS/MS, possible degradation pathways of SDZ, which include desulfonation, oxidation and cleavage, were put forwards. After UV irradiation for 4 h, nearly 90% of the total organic carbon (TOC) can be removed in suspensions of TiO₂-NSs, indicating the mineralization of SDZ. TiO₂-NSs also exhibits excellent stability in photocatalytic degradation of SDZ in wide range of pH. Even after recycling used for 7 times, more than 91.3% of the SDZ can be efficiently removed, indicating that they are promising to be practically used in treatment of wastewater containing antibiotics.     

Organic Functional Free Radicals

This special column presents a snapshot of recent progress of various organic free radicals including the design, synthesis, characterization, properties and applications. We hope that it can help readers stimulate interest to explore novel persistent/stable organic free radicals and promote the research of organic radical based multi-functional materials.