Hydroxyl Radicals In Aqueous Solution Research Articles

DFT insights into the degradation mechanism of carbendazim by hydroxyl radicals in aqueous solution

Advanced oxidation of carbendazim by OH radicals is a central step in its wastewater remediation. However, the understanding of the degradation mechanism of carbendazim has always been a challenge. In this paper, the degradation mechanism of carbendazim by •OH in aqueous solution has been explored using density functional theory (DFT) calculations. On account of the structural and electronic characteristics analysis, the nucleophilic aromatic substitution, dehydrogenation oxidation, and decarboxylation degradation pathways were mainly investigated. These degradation reactions may produce hydroxyl substitution products, oxidized aldehyde and carboxyl products, and decarboxylated carbamic acid products. Computational studies demonstrated that these possible degradation reactions are facile to take place kinetically and have large thermodynamic driving forces, indicating the feasibility of the relevant degradation pathways. Additionally, the ecological risk of carbendazim and its possible degradation products was evaluated, showing that the acute toxicity of degradation products decreases in varying degrees compared with that of carbendazim. The comprehensive mechanistic studies open an avenue for the understanding on the degradation of organic pollutants such as benzimidazole pollutants on molecular level.

Bayesian estimation of the hydroxyl radical diffusion coefficient at low temperature and high pressure from atomistic molecular dynamics.

The hydroxyl radical is the primary reactive oxygen species produced by the radiolysis of water and is a significant source of radiation damage to living organisms. Mobility of the hydroxyl radical at low temperatures and/or high pressures is hence a potentially important factor in determining the challenges facing psychrophilic and/or barophilic organisms in high-radiation environments (e.g., ice-interface or undersea environments in which radiative heating is a potential heat and energy source). Here, we estimate the diffusion coefficient for the hydroxyl radical in aqueous solution using a hierarchical Bayesian model based on atomistic molecular dynamics trajectories in TIP4P/2005 water over a range of temperatures and pressures.

Delafossite-alumina nanocomposite for enhanced catalytic wet peroxide oxidation of anionic pollutants

Mass transfer efficiency and catalytic reactivity are the two major hurdles for heterogeneous catalytic wet peroxide oxidation (CWPO) technologies. To address these issues, nanocomposite CuFeO2/Al2O3 was synthesized and assessed as a novel catalyst for enhanced adsorption and oxidation of anionic pollutants (catechol and reactive red 195 (RR195)) in waters. With a positive charge on the nanocomposite by introducing Al2O3, the adsorption of anionic pollutants was promoted. The surface complexation reaction on CuFeO2/Al2O3, which fits well to the Langmuir isotherm, has engined the mass transfer of pollutants to the nanocatalyst that demonstrated 96.46% and 99.75% removal of catechol and RR195 at pH 3, respectively. CuFeO2/Al2O3 also showed good performance in various reaction media including binary pollutants system and real wastewaters. The hydroxyl radical in aqueous solution played a major role in the pollutants degradation. The CWPO, which followed the Haber-Weiss mechanism, has been accelerated by the Cu and Fe redox cycles. The robustness of the catalyst was verified by negligible amount of metal leaching from the catalysts along with stable catalytic performance after five cycles. Upon the observed results, CuFeO2/Al2O3 with the synergistic effect has shown to be a promising catalyst for removal and degradation of anionic pollutants in CWPO.

Spin Dynamics of the Radicals Produced from Methane Sulfonic Acid and Acetic Acid by Hydroxyl Radicals in Aqueous Solution Studied by Means of Time-Resolved Electron Spin Resonance Spectroscopy.

Fundamental information on the reactions of ·OH radical with alkyl sulfonic acid and carboxylic acid is important for understanding the degradation of the polymer electrolytes for fuel cells. In the present research, the spin dynamics of the organic radicals generated by the ·OH radical was investigated by means of a time-resolved electron spin resonance (TR-ESR) method with the pulsed laser irradiation on H2O2 in aqueous solution. The time profiles of the ESR signals of ·SO3-, ·CH2SO3-, ·CH2COOH, and ·CH2COO- have been analyzed along with the evaluation of characteristics of electron spins such as the g factor, hyperfine coupling constant A, lifetime of radicals, and electron spin relaxation time T1. The effects of the laser repetition frequency on the TR-ESR spectra, the pH dependence on the kinetic parameters, and the mechanism of chemically induced dynamic electron polarization (CIDEP) were discussed. The radical pair mechanism was suggested for the CIDEP, where the electron spins of these radicals are polarized by forming a free radical pair with another radical or the·OH radical, which have remained in the vicinity of photon absorption.

Biological Responses to Hydrogen Molecule and its Preventive Effects on Inflammatory Diseases.

Because multicellular organisms do not have hydrogenase, H2 has been considered to be biologically inactive in these species, and enterobacteria to be largely responsible for the oxidation of H2 taken into the body. However, we showed previously that inhalation of H2 markedly suppresses brain injury induced by focal ischemia-reperfusion by buffering oxidative stress. Although the reaction constant of H2 with hydroxyl radical in aqueous solution is two to three orders of magnitude lower than that of conventional antioxidants, we showed that hydroxyl radical generated by the Fenton reaction reacts with H2 at room temperature without a catalyst. Suppression of hydroxyl radical by H2 has been applied in ophthalmic surgery. However, many of the anti- inflammatory and other therapeutic effects of H2 cannot be completely explained by its ability to scavenge reactive oxygen species. H2 administration is protective in several disease models, and preculture in the presence of H2 suppresses oxidative stress-induced cell death. Specifically, H2 administration induces mitochondrial oxidative stress and activates Nrf2; this phenomenon, in which mild mitochondrial stress leaves the cell less susceptible to subsequent perturbations, is called mitohormesis. Based on these findings, we conclude that crosstalk between antioxidative stress pathways and the anti-inflammatory response is the most important molecular mechanism involved in the protective function of H2, and that regulation of the immune system underlies H2 efficacy. For further medical applications of H2, it will be necessary to identify the biomolecule on which H2 first acts.

Theoretical study on the reaction of anthracene with sulfate radical and hydroxyl radical in aqueous solution

Sulfate radical (SO4−) and hydroxyl radical (OH) generated from persulfate or peroxymonosulfate in AOPs have been widely used in contaminant degradation. Anthracene (ANT) can be decomposed by SO4− and OH. The processes of ANT decomposition were investigated using theoretical calculations in this paper. The initiation reactions of ANT, anthrone, anthraquinone (ATQ) and 1-hydroxylanthraquinone (1-hATQ) by two radicals are studied. The highest free energy barriers of initiation reactions are 22.30 kcal mol−1 in ATQ + SO4− reaction and 6.84 kcal mol−1 in ATQ + OH reaction. Comparing the rate constants of initiation reaction through the two radicals at 273–373 K, it can be concluded that SO4− and OH both play important roles on the initiation of ANT and anthrone at lower pH. For ATQ and 1-hATQ, OH is more important than SO4− in the initiation process, which indicates that the indirect influence of SO4− are more significant in the degradation processes of ATQ and 1-hATQ. This study provides theoretical confirmations for the mechanisms of reactions of ANT with SO4− and OH, and evaluates the importance of SO4− and OH according to the reaction rates. The work can give more insight into the degradation of PAHs by radicals.

Catalytic Effect of Aqueous Solution in Water-Assisted Proton-Transfer Mechanism of 8-Hydroxy Guanine Radical.

Water-assisted proton-transfer process is a key step in guanine damage reaction by hydroxyl radical in aqueous solution. In this article, we quantitatively determine the solvent effect in water-assisted proton-transfer mechanism of 8-hydroxy guanine radical using combined quantum mechanics and molecular mechanism with an explicit solvation model. Atomic-level reaction pathway was mapped, which shows a synchronized two-proton-transfer mechanism between the assistant water molecule and 8-hydroxy guanine radical. The transition-state dipole moment is the largest along the reaction pathway, which electrostatically stabilizes the proton-transfer transition-state complex. The free-energy reaction barrier for this water-assisted proton-transfer reaction was calculated at 19.2 kcal/mol with the density functional theory/M08-SO/cc-pVTZ+/molecular mechanics level of theory. The solvent effect not only has a big impact on geometries, but also dramatically changes the energetics along the reaction pathway. Among the solvent effect contributions to the transition state, the solvent energy contribution is -28.5 kcal/mol and the polarization effect contribution is 19.9 kcal/mol. In total, the solvent effect contributes -8.6 kcal/mol to the free-energy barrier height, which means that the presence of aqueous solution has a catalytic effect on the reaction mechanism and enhances the proton-transfer reactivity in aqueous solution.

What Controls Selectivity of Hydroxyl Radicals in Aqueous Solution? Indications for a Cage Effect.

The oxidation of three isotopologues of methylcyclohexane (MCH: C7H14, C7D14, c-C6D11-CH3) by OH-radicals (•OH) in aqueous solution was investigated. Intermolecular and intramolecular H/D kinetic isotope effects (KIE = kH:kD) for the abstraction of H and D atoms by •OH were measured. These KIEs reflect inter- and intramolecular selectivities of hydrogen abstraction, i.e., the selection of •OH attack on carbon-hydrogen bonds in different molecules and in different positions of one molecule, respectively. The intermolecular selectivity of •OH attack in aqueous solution is largely discriminated against in comparison with the intramolecular selectivity. The observed extent of discrimination cannot be explained by partial diffusion control of the overall reaction rates. A cage model, where •OH and hydrocarbon molecules are entrapped in a solvent cage, is more appropriate. The much higher intramolecular KIEs compared to the intermolecular KIEs of the same chemical reaction, R-H + •OH → R• + H2O, indicate a high degree of mobility of the two reaction partners inside of the solvent cage. This mobility is sufficient to develop an intramolecular selectivity comparable to that of gas-phase reactions of •OH. Furthermore, literature data on KIEs of H-abstraction by •OH in aqueous and gas phases are discussed. There is a general tendency toward lower selectivities in the aqueous phase.

Modified graphitic carbon nitride prepared via a copolymerization route for superior photocatalytic activity

Molecular doping strategy has been applied for regulating the electronic band structure of graphitic carbon nitride (g-C3N4). By incorporating the electron-withdrawing impurity group of N,N′,N″-s-triazine-2,4,6-triyl-tri-p-aminobenzoic acid (H3TATAB) into the network of g-C3N4, a series of H3TATAB-doped g-C3N4 (TABCN) photocatalysts were synthesized via thermal copolymerization at 550 °C for 4 h. H3TATAB doping shifts the VB of TABCN from 1.78 eV (pristine g-C3N4) to a low position (2.07 eV for TCBCN0.8), which enhances the oxidation ability of the VB hole for producing hydroxyl radicals in aqueous solution, and, therefore, the UV photocatalytic activity of TABCN0.8 is estimated to be about twice as high as that of g-C3N4 for the degradation of AO7. Further, doping the H3TATAB impurity group into the g-C3N4 network induces plenty of defect levels below the CB of g-C3N4, which gives a significant absorbance enhancement in the visible region. As a result, the visible light photocatalytic activity of TABCN0.8 is 7.5 times higher than that of g-C3N4 for the degradation of AO7. This work may provide guidance on designing an efficient g-C3N4 polymer photocatalyst with desirable electronic structure.

Multi-level Quantum Mechanics and Molecular Mechanics Study of Ring Opening Process of Guanine Damage by Hydroxyl Radical in Aqueous Solution

Combining multi-level quantum mechanics theories and molecular mechanics with an explicit water model, we investigated the ring opening process of guanine damage by hydroxyl radical in aqueous solution. The detailed, atomic-level ring-opening mechanism along the reaction pathway was revealed in aqueous solution at the CCSD(T)/MM levels of theory. The potentials of mean force in aqueous solution were calculated at both the DFT/MM and CCSD(T)/MM levels of the theory. Our study found that the aqueous solution has a significant effect on this reaction in solution. In particular, by comparing the geometries of the stationary points between in gas phase and in aqueous solution, we found that the aqueous solution has a tremendous impact on the torsion angles much more than on the bond lengths and bending angles. Our calculated free-energy barrier height 31.6 kcal/mol at the CCSD(T)/MM level of theory agrees well with the one obtained based on gas-phase reaction profile and free energies of solvation. In addition, the reaction path in gas phase was also mapped using multi-level quantum mechanics theories, which shows a reaction barrier at 19.2 kcal/mol at the CCSD(T) level of theory, agreeing very well with a recent ab initio calculation result at 20.8 kcal/mol.

Interaction of hydrogen peroxide and thiourea or its oxides with terephthalic acid

The suitability of thiourea and its oxides for the hydroxylation of terephthalic acid in aqueous solutions was explored. Allsulfur-containing compounds tested were found to favor the formation of hydroxyterephthalic acid. It was shown that thiourea and its di- and trioxides can act as both traps and promoters of hydroxyl radicals in aqueous solutions.

Non-catalytic and catalytic degradation of effluent dissolved organic matter under UVA-and UVC-irradiation tracked by advanced spectroscopic tools

Non-catalytic and catalytic photodegradation of effluent dissolved organic matter (EfDOM) was examined under two different light sources (UVA and UVC). The degradation behavior was tracked by dissolved organic carbon (DOC), UV absorbance, and different fluorescent components. Catalytic UV irradiation resulted in much higher degradation rates than those without photocatalysts (TiO2 and ZnO) regardless of the tracking variables. For non-catalytic degradation, the highest removal rates of UV absorbance were found at wavelengths close to the irradiation of either UVC or UVA, while the photocatalytic degradation rates were consistently higher at longer wavelengths. The pseudo first-order rates of UV absorbance individually calculated at several representative wavelengths were very consistent with the sequential orders interpreted from two-dimensional correlation spectroscopy (2D-COS). Excitation emission matrix – parallel factor analysis (EEM-PARAFAC) identified one tryptophan-like (C1) and two humic-like (C2 and C3) components from EfDOM samples. Among those, C1 exhibited the lowest adsorption extent and the highest degradation rates for both photocatalysts, suggesting that the photocatalysis is mainly governed by hydroxyl radicals in aqueous solution. All observed degradation behaviors were well explained by the irradiation wavelengths, the extent of adsorption onto catalysts, and the presumed structure of the tracked component. Our study demonstrated that EEM-PARAFAC and 2D-COS could provide further insights into both non-catalytic and catalytic degradation of EfDOM upon UV-irradiation.

The Effect of pH on OH Radical Generation in Aqueous Solutions by Atmospheric Pressure Glow Discharge

Formation of hydroxyl radicals in aqueous solutions by atmospheric pressure DC glow discharge was investigated. The ferrocyanide was used as an OH scavenger at different pH. Experimental results indicated that pH effect on hydroxyl radical formation rate could be explained by particularities of [Fe(CN)6]4− and reactive species interaction in different media.

Effects of radical scavengers on aqueous solutions exposed to heavy-ion irradiation using the liquid microjet technique

The effects of the radical scavenger ascorbic acid on water radiolysis are studied by fast heavy-ion irradiation of aqueous solutions of ascorbic acid, using the liquid microjet technique under vacuum. To understand the reaction mechanisms of hydroxyl radicals in aqueous solutions, we directly measure secondary ions emitted from solutions with different ascorbic acid concentrations. The yield of hydronium secondary ions is strongly influenced by the reaction between ascorbic acid and hydroxyl radicals. From analysis using a simple model considering chemical equilibria, we determine that the upper concentration limit of ascorbic acid with a radical scavenger effect is approximately 70μM.

Reaction Kinetics of Trans-Sobrerol and 8-p-Menthen-1,2-diol with Hydroxyl Radical in Aqueous Solution: A Combined Experimental and Theoretical Study

Trans-sobrerol (Sob) and 8-p-menthen-1,2-diol (Limo-diol) are the primary products in the atmospheric oxidation of β-pinene and limonene, respectively. Because of their low volatility, they associate more likely to the liquid particles in the atmosphere, where they are subject to the aqueous phase oxidation by the atmospheric oxidants. In this work, through experimental and theoretical study, we first provide the rate constants of Sob and Limo-diol reacting with hydroxyl radical (·OH) in aqueous solution at room temperature of 304±3 K and 1 atm pressure, which are (3.05±0.5)×109 and (4.57±0.2)×109 L/(mol·s), respectively. Quantum chemistry calculations have also been employed to demonstrate the solvent effect on the rate constants in aqueous phase and the calculated results agree well with the measurements. Some reaction products have been identified based on liquid chromatography combined with mass spectroscopy and theoretical calculations.

Ce/SBA-15 as a heterogeneous ozonation catalyst for efficient mineralization of dimethyl phthalate

The catalytic activity of cerium-loaded SBA-15 (Ce/SBA-15) for the ozonation of dimethyl phthalate (DMP) in aqueous solution was studied. Ce/SBA-15 was prepared and characterized by a low angle X-ray powder diffraction (XRD), nitrogen adsorption–desorption and ultraviolet–visible diffuse reflection spectrum (UV–vis DRS). The results showed that the material retained a highly ordered mesoporous structure and processed large surface area. The presence of Ce/SBA-15 slightly accelerated DMP decomposition, but significantly improved its mineralization efficiency. Under the chosen condition, a high mineralization efficiency (88.7%) was achieved by Ce/SBA-15/O3 process at 60min, 2.6 times higher than SBA-15/O3, 3.5 times higher than O3 alone. The comparison among different process indicated that the combination of Ce/SBA-15 and O3 exhibited a significant synergetic effect. Ce/SBA-15 significantly improved ozone decomposition into hydroxyl radicals in aqueous solution and the oxidation of DMP occurred via the free radical mechanism. The reused experiment verified that Ce/SBA-15 kept a good catalytic activity and stability, and it was a promising heterogeneous catalyst.

Rate laws and kinetic modeling of N-ethyl perfluorooctane sulfonamidoethanol (N-EtFOSE) transformation by hydroxyl radical in aqueous solution

The degradation of perfluorochemicals (PFCs) by hydroxyl radical (OH) follows complex pathways resulting in stable products. Kinetic models are needed to predict the product distribution of OH-initiated PFC degradation under environmental and treatment conditions. The bimolecular rate constants were measured in water for the reaction of OH and N-ethyl perfluorooctane sulfonamidoethanol (N-EtFOSE), and intermediates, N-ethyl perfluorooctane sulfonamidoacetate (N-EtFOSAA), N-ethyl perfluorooctane sulfonamide (N-EtFOSA) and perfluorooctane sulfonamidoacetate (FOSAA). Under standard conditions (pH = 6, 25 ± 2 °C, Co PFC = 5–10 μg L−1, Co H2O2 = 10 mM, irradiation intensity = 765 W m−2), the measured constants for N-EtFOSE, N-EtFOSAA, N-EtFOSA and FOSAA were (1.05 ± 0.12) × 109 M−1 s−1, (0.68 ± 0.05) × 109 M−1 s−1, (0.68 ± 0.05) × 109 M−1 s−1 and (0.53 ± 0.05) × 109 M−1 s−1, respectively. Constants in the pH range from 1 to 10 varied within a factor of 2–4 for most compounds. Over a period of 2-days, N-EtFOSE reacted directly (without forming long-lived intermediates) to perfluorooctane sulfonamide (FOSA) (18.8%) and perfluorooctanoic acid (PFOA) (39.1%). N-EtFOSE reacted via oxidation of the ethanolic hydroxyl group to N-EtFOSAA (12.4%) and N-dealkylation to N-EtFOSA (13.3%) and FOSAA (0.2%) and unknown intermediates. In sunlit surface waters, the OH-induced transformation of N-substituted sulfonamide compounds to photostable products occurs on a time scale of days to weeks by model prediction.

Electrochemical oxidation and protein adduct formation of aniline: a liquid chromatography/mass spectrometry study

Historically, skin sensitization tests are typically based on in vivo animal tests. However, for substances used in cosmetic products, these tests have to be replaced according to the European Commission regulation no. 1223/2009. Modification of skin proteins by electrophilic chemicals is a key process associated with the induction of skin sensitization. The present study investigates the capabilities of a purely instrumental setup to determine the potential of commonly used non-electrophilic chemicals to cause skin sensitization by the generation of electrophilic species from the parent compound. In this work, the electrophiles were generated by the electrochemical oxidation of aniline, a basic industrial chemical which may also be released from azo dyes in cosmetics. The compound is a known sensitizer and was oxidized in an electrochemical thin-layer cell which was coupled online to electrospray ionization-mass spectrometry. The electrochemical oxidation was performed on a boron-doped diamond working electrode, which is able to generate hydroxyl radicals in aqueous solutions at high potentials. Without any pretreatment, the oxidation products were identified by electrospray ionization/time-of-flight mass spectrometry (ESI-ToF-MS) using their exact masses. A mass voltammogram was generated by plotting the obtained mass spectra against the applied potential. Oligomerization states with up to six monomeric units in different redox states of aniline were observed using this setup. This approach was extended to generate adducts between the oxidation products of aniline and the tripeptide glutathione. Two adducts were identified with this trapping experiment. Protein modification was carried out subsequently: Aniline was oxidized at a constant potential and was allowed to react with β-lactoglobulin A (β-LGA) or human serum albumin (HSA), respectively. The generated adducts were analyzed by liquid chromatography coupled to ESI-ToF-MS. For both β-LGA and HSA, aniline adducts were successfully generated and identified.

Scavenging of Hydroxyl Radicals in Aqueous Solution by Astaxanthin Encapsulated in Liposomes

Astaxanthin (Asx) is known to be a potent quencher of singlet oxygen and an efficient scavenger of superoxide anion. Therefore, Asx would be expected to be a useful antioxidant for the prevention of oxidative stress, a causative factor in severe diseases such as ischemic reperfusion injury. However, it is still unclear whether Asx has scavenging capability against the most potent reactive oxygen species (ROS), hydroxyl radical, because the hydrophobicity of Asx prevents analysis of hydroxyl radical scavenging ability in aqueous solution. In this study, to solve this problem, liposomes containing Asx (Asx-lipo), which could be dispersed in water, were prepared, and the scavenging ability of Asx-lipo for the hydroxyl radical was examined. The liposomal formulation enabled encapsulation of a high concentration of Asx. Asx-lipo gave a dose-dependent reduction of chemiluminescence intensity induced by hydroxyl radical in aqueous solution. Hydroxyl radical scavenging of Asx was more potent than α-tocopherol. The absorbance of Asx in the liposome decreased after reduction of hydroxyl radicals, indicating the direct hydroxyl radical scavenging by Asx. Moreover, Asx-lipo prevented hydroxyl radical-induced cytotoxicity in cultured NIH-3T3 cells. In conclusion, Asx has potent scavenging capability against hydroxyl radicals in aqueous solution, and this paper is the first report regarding hydroxyl radial scavenging by Asx.

Novel Antiviral Characteristics of Nanosized Copper(I) Iodide Particles Showing Inactivation Activity against 2009 Pandemic H1N1 Influenza Virus

We investigated the antiviral activity of nanosized copper(I) iodide (CuI) particles having an average size of 160 nm. CuI particles showed aqueous stability and generated hydroxyl radicals, which were probably derived from monovalent copper (Cu(+)). We confirmed that CuI particles showed antiviral activity against an influenza A virus of swine origin (pandemic [H1N1] 2009) by plaque titration assay. The virus titer decreased in a dose-dependent manner upon incubation with CuI particles, with the 50% effective concentration being approximately 17 μg/ml after exposure for 60 min. SDS-PAGE analysis confirmed the inactivation of the virus due to the degradation of viral proteins such as hemagglutinin and neuraminidase by CuI. Electron spin resonance (ESR) spectroscopy revealed that CuI generates hydroxyl radicals in aqueous solution, and radical production was found to be blocked by the radical scavenger N-acetylcysteine. Taken together, these findings indicate that CuI particles exert antiviral activity by generating hydroxyl radicals. Thus, CuI may be a useful material for protecting against viral attacks and may be suitable for applications such as filters, face masks, protective clothing, and kitchen cloths.