Generation Of OH Radicals Research Articles

Computational Fluid Dynamics Modeling of Rotating Annular VUV/UV Photoreactor for Water Treatment

The ultraviolet photochemical degradation process is widely recognized as a low-cost, environmentally friendly, and sustainable technology for water treatment. This study integrated computational fluid dynamics (CFD) and a photoreactive kinetic model to investigate the effects of flow characteristics on the contaminant degradation performance of a rotating annular photoreactor with a vacuum-UV (VUV)/UV process performed in continuous flow mode. The results demonstrated that the introduced fluid remained in intensive rotational movement inside the reactor for a wide range of inflow rates, and the rotational movement was enhanced with increasing influent speed within the studied velocity range. The CFD modeling results were consistent with the experimental abatement of methylene blue (MB), although the model slightly overestimated MB degradation because it did not fully account for the consumption of OH radicals from byproducts generated in the MB decomposition processes. The OH radical generation and contaminant degradation efficiency of the VUV/UV process showed strong correlation with the mixing level in a photoreactor, which confirmed the promising potential of the developed rotating annular VUV reactor in water treatment.

Preparation of chitosan modified montmorillonite biocomposite for sonocatalysis of dyes: Parameters and degradation mechanism

In this study, the natural chitosan biopolymer was immobilized onto montmorillonite (MMT) surface for efficient ultrasound-assisted removal of dyes in aqueous solution. The MMT, chitosan, and prepared chitosan/MMT samples were characterized using XRD, HR-SEM/EDX, FT-IR, and BET analyses. The sonocatalytic activity of the chitosan/MMT biocomposite was compared in degradation Acid Orange 7 (AO7), Basic Red 46 (BR4), Basic Yellow 2 (BY2) and Basic Yellow 28 (BY28). To define the optimized reaction conditions, the removal efficiency of BY2 was examined by various operational variables, such as biocomposite dosage, initial BY2 concentration, degree of high power, and pH. Besides, the effects of various treatment processes and scavengers were tested. The experimental results revealed that the highest removal efficiency, 82.74%, was obtained at natural pH of 6.95 for the processing conditions of 1.0 g/L catalyst dosage, 200 mg/L dye concentration, 350 W high power, and 60 min reaction time. Moreover, the obtained results pointed out that the sonocatalytic process was the most effective process due to the increased mass transfer and the degradation of BY2 by •OH radicals generation. The kinetic of sonocatalytic process can be represented by the combination of both pseudo-second order and intra-particle diffusion models. Among the Langmuir, Freundlich, BET, Temkin, and Dubinin-Radushkevich isotherm models, the Langmuir model was found to be the best conformity. The trapping experiments suggested that the hydroxyl radicals (•OH) for BY2 oxidation have a predominant role, whereas hydroperoxyl (HO2•) radicals have less effect. GC-MS analysis was also used for the determination of by-products that occurred throughout the removal process.

Improved catalytic wet peroxide oxidation of phenol over Pt-Fe2O3/SBA-15: Influence of platinum species and DFT calculations

In this study, Pt-Fe2O3, Pt, and Fe2O3 catalysts supported on SBA-15 were prepared for catalytic wet peroxide oxidation (CWPO) of phenol at low temperatures. Pt-Fe2O3/SBA-15 and Pt/SBA-15 had higher H2O2 decomposition rates at 20 °C compared with that of Fe2O3/SBA-15, which is representative of a conventional CWPO catalyst. Although H2O2 was decomposed at a higher rate by Pt/SBA-15 than Pt-Fe2O3/SBA-15, Pt/SBA-15 barely exhibited any phenol removal activity. Meanwhile, Pt-Fe2O3/SBA-15 showed a superior phenol removal activity, indicating that Pt-Fe2O3/SBA-15 efficiently utilizes H2O2 for the CWPO reaction. To establish a correlation between the H2O2 decomposition activity and the phenol removal activity, characterization of the catalysts and density functional theory (DFT) calculations were conducted. TEM, XPS, and H2-TPR verified that Fe2O3 affected the state of the impregnated Pt via metal-support interactions, which enabled the formation of Pt4+ species in the Pt-Fe2O3/SBA-15 catalyst. DFT calculations revealed that PtO2 can selectively generate OH radicals, while Pt converts OH radicals into H2O via further reactions. As predicted by DFT calculations, EPR spectra demonstrated that OH radicals were formed by Pt-Fe2O3/SBA-15 but not by Pt/SBA-15. In conclusion, Pt4+ species in the Pt-Fe2O3/SBA-15 catalyst via interactions between Pt and Fe2O3 led to highly efficient phenol removal with H2O2.

Fenton reaction mechanism generating no OH radicals in Nafion membrane decomposition

Mechanism of Fenton reaction, which is a most widely-used degradation test for organic materials using hydrogen peroxide (H_2O_2) and iron (Fe) cations, is revealed for the decomposition of hydrated Nafion membrane. This reaction mechanism has been assumed to generate OH radicals. For a doubly-hydrated Nafion membrane model, Fenton reaction with divalent and monovalent Fe (Fe^{2+} and Fe^+) cation hydration complexes is explored for experimentally-supported hydration numbers using long-range correction for density functional theory. As a result, it is found that H_2O_2 coordinating to the Fe^{2+} hydration complexes first approaches Nafion side chains in high humidity, then leads to the C–S bond dissociation of the side chain to produce carbonic acid group and sulfonic acid ion. On the other hand, once electron transfer proceeds between iron ions, the O–O bond of the coordinating H_2O_2 is extended, then the C–S bond is dissociated to produce trihydroxymethyl group and sulfur trioxide, which are rapidly transformed to carboxyl group and sulfonic acid ion in aquo. This mechanism is confirmed by the vibrational spectrum analysis of the decomposed product. Collective Nafion decomposition mechanisms also suggest that the decomposition reaction uses the recycle of generated Fe cation hydration complexes under acidic condition near membrane surface.

Evaluation of the sono-assisted photolysis method for the mineralization of toxic pollutants

Persistent organic pollutants (POPs), which are highly toxic and potential health hazards, are discharged in wastewater and cannot be mineralized by conventional biological and physicochemical methods of wastewater treatment. Advanced oxidation processes (AOPs) such as electrochemical, sonochemical, photolysis, Fenton, and Fenton like processes seem to be good and promising wastewater treatment technology alternatives. However, flaws limit their use in industrial applications. Recently, combined AOPs have gained attention among researchers to overcome these flaws of single AOPs. In this review, we studied the combined sonolysis and photolysis techniques for the degradation of the POPs. The efficiency of sono-photolysis has been examined by considering the single strategies, i.e., sonolysis and photolysis process, and the effect of various parameters of sono-photoreactor on the kinetics of degradation. Combined irradiation of ultrasound and ultraviolet light has been reported to give the synergistic effect under different mechanisms. The physical feature of ultrasound provides the intense mixing of the pollutant, which supports UV light to deeply penetrate the medium and accelerate the hydroxylation/oxidation reaction where toxic pollutants generated due to photolysis were degraded. H2O2 generated during the transient collapse of microbubble experienced a photocleavage to produce additional •OH radicals, which increases the degradation of the pollutants. A significant synergistic effect has been reported during combined treatment due to the increase in •OH radicals generation and photolysis of H2O2 either added externally or formed by the US, which also avoids the secondary pollution. Critical analysis of the geometry of various types (batch and continuous) of the sono-photo reactor used during treatment have also been evaluated. The kinetics of the degradation process has been reviewed in detail. This review article demonstrates a thorough and critical analysis (1998–2020) of sono-photolysis techniques, which improve the understanding of combined treatment and provides an outline direction for future research.

On the propagation of the OH radical produced by Cu-amyloid beta peptide model complexes. Insight from molecular modelling.

Oxidative stress and metal dyshomeostasis are considered as crucial factors in the pathogenesis of Alzheimer's disease (AD). Indeed, transition metal ions such as Cu(ii) can generate Reactive Oxygen Species (ROS) via O2 Fenton-like reduction, catalyzed by Cu(ii) coordinated to the Amyloid beta (Aβ) peptide. Despite intensive effort, the mechanisms of ROS-induced molecular damage remain poorly understood. In the present paper, we investigate on the basis of molecular modelling computations the mechanism of OH radical propagation toward the Aβ peptide, starting from the end-product of OH radical generation by Cu(ii)·Aβ. We evaluate (i) the OH oxidative capacity, as well as the energetics of the possible Aβ oxidation target residues, by quantum chemistry Density Functional Theory (DFT) on coordination models of Cu(ii)/OH/Aβ and (ii) the motion of the OH˙ approaching the Aβ target residues by classical Molecular Dynamics (MD) on the full peptide Cu(ii)/OH/Aβ(1-16). The results show that the oxidative capacity of OH coordinated Cu(ii)Aβ is significantly lower than that of the free OH radical and that propagation toward Aβ Asp and His residues is favoured over Tyr residues. These results are discussed on the basis of the recent literature on in vitro Aβ metal-catalyzed oxidation and on the possible implications for the AD oxidative stress mechanism.

Tungsten passivation layer (WO3) formation mechanisms during chemical mechanical planarization in the presence of oxidizers

Effects of single and mixed oxidants of Fe(NO3)3 and H2O2 containing acidic silica slurries were studied to investigate the mechanism of tungsten (W) chemical mechanical planarization (CMP). The W polishing rate obtained from the CMP test depicted high W polishing rate in the presence of mixed oxidants of Fe(NO3)3 and H2O2 as compared to a single oxidant of either H2O2 or Fe(NO3)3. The formation of a passive layer of tungsten oxide (WO3) and W dissolution could be the reason for these results as confirmed by XPS. Further investigation revealed that the generation of much stronger oxidants of hydroxyl radicals (OH) was solely responsible for WO3 layer formation. Quantitative evaluation of OH generation was estimated using a UV–visible spectrophotometer and confirmed that in-situ generation of hydroxyl radicals (OH) could be a main driving force for the high W polishing rate by converting a hard W film into a soft passive film of WO3. WO3 film formation was further confirmed using potentiodynamic polarization studies, which showed a smaller value of corrosion current density (Icorr) in mixed oxidants of Fe(NO3)3 and H2O2 as compared to the large values of Icorr observed for H2O2 alone. This study revealed that a single oxidizer of either Fe(NO3)3 or H2O2 was not capable of achieving a high W removal rate. Rather, only mixed oxidants of Fe(NO3)3 and H2O2 could cause a high W polishing rate due to excessive in-situ generation of OH radicals during the W CMP process.

Photocatalytic degradation of aniline blue, brilliant green and direct red 80 using NiO/CuO, CuO/ZnO and ZnO/NiO nanocomposites

Photocatalytic degradation of three dyes, aniline blue (AB), brilliant green (BG) and direct red 80 (DR 80) by transition metal oxide nanocomposites having p-p isotype heterojunction, NiO/CuO and p-n type heterojunction, CuO/ZnO and ZnO/NiO has been explored under sun light and UV light. The effect of reaction conditions such as initial dye concentration, catalyst dosage, pH, contact time and presence of electron acceptor on photocatalytic degradation of these dyes was investigated. A suitable mechanism has been proposed for the photocatalytic degradation of dyes by the nanocomposites with illustration. The results show that NiO/CuO has higher degradation ability on AB dye (91 %) than BG (82 %) and DR 80 dyes (67 %). Among the three nanocomposites, the enhanced degradation ability shown by NiO/CuO are due to its lowest particle size, reduced electron/hole pair recombination, high BET surface area and increased OH radical generation during the photocatalytic reaction. The percentage degradation of all the dyes was carried out under sun light and UV light. It has been found that the rate of dye degradation under sun light is slightly higher than in UV light due to the existence of all type of radiations in sun light.

Hydrophilic properties of soot particles exposed to OH radicals: A possible new mechanism involved in the contrail formation

This work investigates the role of soot particles in the early steps of formation of condensation trails (contrails). Contrails are thin linear clouds that form behind cruising aircrafts and can evolve into persistent clouds, and therefore can contribute to the radiative forcing of the atmosphere. Mitigating their contribution is considered an efficient way for decreasing the radiative forcing. Condensation freezing is proposed as a major pathway leading to the heterogeneous nucleation of ice particles in the aircraft exhausts. Soot particles, freshly produced in the flames and not significantly oxidized in the post-flame, are hydrophobic. Therefore, many theoretical and experimental approaches that investigate contrails formation introduce prior activation by surface adsorption of sulfur compounds issued from kerosene combustion. Even though the role of sulfur content of the fuel is unclear, and may only weakly impacts contrails formation, many simulations still rely on this assumption. In this work, to elucidate the role of sulfur compounds on the activation of soot chemically aged with OH radicals, a comparative study is performed with soot sampled from a turbulent jet flame burning a sulfur-containing fuel (kerosene) and a sulfur-free fuel (diesel). Soot aging experiments are performed in the atmospheric simulation chamber CESAM, with controlled generation of OH radicals. Soot activation (ratio between the number of nucleated droplets and seeding particles) is measured in water supersaturation conditions. OH exposure significantly enhances soot activation regardless of the sulfur presence in the fuel. The possibility for soot oxidation by OH radicals formed in aeronautical engines to be sufficient to promote soot activation is explored. Large eddy simulation in the high pressure turbine is used to model the fluid particle and OH trajectories and their chemical evolution. Residence time in the turbine is found sufficient to activate soot particles, opening a new possible route to explain ice particles formation.

Characterization and Kinetic Studies on Photocatalytic Degradation of Phenol in Aqueous Solution

Recently, many researchers tend to study the performance of hybrid polyoxometalates (HPOM) as photocatalyst due to its greater performance in photocatalytic degradation. The process that based on the light enhanced generation of reactive OH radical that plays an important role in order to convert organic compound completely into water, CO2 and inorganic compounds. In this research, the photocatalytic degradation of phenol was studied using HPOM as photocatalyst. The HPOM was synthesized and used to study its effect on photocatalytic degradation of phenol. The synthesized HPOM were characterized using SEM and FTIR. It was found that the surface morphology of the synthesized HPOM le displayed an irregular spherical-like shape, granular structure with different size and least distribution of non-agglomerated rod-like nanoparticles. The present of WO6 octahedral and PO4 octahedral indicated that Keggin type structure of synthesized hybrid polyoxometalates. The kinetics of photocatalytic degradation of phenol via hybrid polyoxometalates follows pseudo first order kinetic reaction.

Nitrogen Doped Carbon Nanomaterial as Electrocatalyst for Oxygen Reduction Reaction in Acidic Media: To use in Electro‐Fenton

AbstractThe doped heteroatom in carbon matrices plays a crucial role in the electro‐Fenton process. Here, we investigated the electrocatalytic activities of the acetonitrile –derived (by flame synthesis) nitrogen‐doped carbon nanomaterials for the two‐electron oxygen reduction reaction (ORR) in the electro‐Fenton process, focusing on the peroxide generation and degradation of chlorophenoxy acid herbicides from contaminated water. Besides, we studied the anodic oxidation of chlorophenoxy acid herbicides from the water, and comparatively electrochemically generated OH radicals yield the higher degradation efficiency. To study the fundamental property, the linear sweep voltammetry studies of nitrogen‐doped carbon cathode material exhibits the excellent electrocatalytic activity towards 2 electron transfer oxygen reduction reaction in acidic conditions and successfully applied for the degradation of herbicides This work provides a feasible synthetic approach for scalable production of acetonitrile derived high‐performance carbon‐based ORR catalyst. The degradation results point out that, nitrogen‐doped carbon nanomaterial is one of the promising cathode materials for the degradation of organic pollutants by electro‐Fenton process

Antioxidative potential of metformin: Possible protective mechanism against generating OH radicals

Exposure to particulate matter (PM) canlead to the excessive accumulation of reactive oxygen species (ROS), which causes oxidative stress and endangers human health. In this study, the effects of metformin on PM-induced radicals were investigated, and the antioxidation reaction mechanism of metformin was analyzed by the density functional theory (DFT) method. The corresponding results revealed that the consumption rate of dithiothreitol (DTT) increased as the metformin concentration (0–40 mmol/L) increased under exposure to PM active components. Moreover, the OH radical content decreased as the metformin concentration increased. This result may be related to the consumption of PM-induced OH radicals by metformin, which promotes the DTT consumption rate. Additionally, because the initiation reaction has a high barrier, the oxidation reaction rate between metformin and ·O2 − is not very fast, although various catalysts may be present in the human environment. Importantly, we found that the barrier of metformin induced by OH radicals is only 9.6 kcal/mol while the barrier of metformin induced by oxygen is 57.9 kcal/mol, which shows that the rate of the ·OH-initiated oxidative reaction of metformin is much faster and that this reaction path occurs more easily. By sample analysis, the mean OH radical generation was 55 nmol/min/g (ranging from 5 to 105 nmol/min/g) on haze days and 30 nmol/min/g (ranging from 10 to 50 nmol/min/g) on non-haze days. Moreover, OH radical generation was higher on haze days than on neighboring non-haze days. Taken together, all data suggest that metformin could consume the PM-induced radicals, such as OH radicals and ·O2 −, thereby providing health protection.

Electrocatalytic oxidation of PCP-Na by a novel nano-PbO2 anode: degradation mechanism and toxicity assessment

This study aims at investigating the electrocatalytic oxidation of sodium pentachlorophenate (PCP-Na) using a novel nano-PbO2 powder anode. The nano-PbO2 powder (marked as HL-PbO2) was prepared by a simple hydrolysis process, and hydrothermal treatment was followed to improve the activity of HL-PbO2. The HL-PbO2 treated for 24h by hydrothermal process (HL/HT-PbO2-24) was confirmed to possess higher crystallinity, higher oxygen evolution potential, and more active sites, resulting in stronger OH radical generation capacity and higher electrochemical activity. Compared with conventional electrodeposited PbO2 (ED-PbO2) anode, the HL/HT-PbO2-24 anode showed higher PCP-Na degradation rate. Under the same operating conditions, the mineralization current efficiency at HL/HT-PbO2-24 was 2.7 times than that at ED-PbO2. Five intermediates were detected in PCP-Na degradation solution and possible degradation mechanism of PCP-Na was discussed. In addition, the acute toxicity of PCP-Na degradation solution to zebrafish embryos and the oxidative stress induced in zebrafish embryos/larvae were studied to evaluate the ecological security of electrocatalytic oxidation of PCP-Na.

Binary α‐Fe2O3–Co3O4 nanostructures for advanced oxidation process: Role of synergy for enhanced catalysis

Iron and its binary oxides are meticulously exploited for environmental remediations. However, only limited studies have been carried out on the degradation of industrial organics by advanced oxidation process. In this study, iron oxide, cobalt oxide, and iron–cobalt binary oxides were synthesized by a modified hydrothermal method as heterogeneous Fenton‐like catalysts for the removal of methylene blue (MB) from wastewaters. The oxide nanostructures were characterized by different analytical techniques. Studying the effects of various parameters such as catalyst dose, MB concentration, and H2O2 concentration, the reaction conditions were optimized to enhance the removal of MB dye. The results revealed that α‐Fe2O3–Co3O4 shows much higher activity than both Co3O4 and α‐Fe2O3 for the degradation of MB at room temperature and beyond. The binary α‐Fe2O3–Co3O4 shows degradation efficiency of 96.4% at 65 °C within 60 min. Furthermore, the binary α‐Fe2O3–Co3O4 catalyst retains its activity for up to four successive cycles. A probable mechanism is also proposed, involving the generation of ‧OH radical as well as Fe2+/Fe3+ or Co2+/Co3+ redox couple of the binary α‐Fe2O3–Co3O4 catalyst.

Electrocatalytic degradation of perfluoroocatane sulfonate (PFOS) on a 3D graphene-lead dioxide (3DG-PbO2) composite anode: Electrode characterization, degradation mechanism and toxicity

In this work, a three-dimension grapnene-PbO2 (3DG-PbO2) composite anode was prepared using coelectrodeposition technology for electrocatalytic oxidation of perfluorooctane sulfonate (PFOS). The effect of 3DG on the surface morphology, structure and electrocatalytic activity of PbO2 electrode was investigated. The results indicated that the 3DG-PbO2-0.08 anode (3DG concentration in electrodeposition solution was 0.08 g L−1) possessed the best electrocatalytic activity due to its stronger ·OH radicals generation capacity, more active sites and smaller charge-transfer resistance. The degradation rate constant of PFOS on 3DG-PbO2-0.08 anode was 2.33 times than that of pure PbO2 anode. Additionally, the by-products formed in electrocatalytic degradation of PFOS were identified and a PFOS degradation pathway was proposed accordingly, which was dominated by the dissociation of –CF2- groups via the attack of ·OH radicals. Finally, the toxicity evolution of degradation solution was examined to evaluate the ecological risk of electrocatalytic oxidation of PFOS by acute toxicity assays to zebrafish embryos.

A dual surface inorganic molecularly imprinted Bi2WO6-CuO/Ag2O heterostructure with enhanced activity-selectivity towards the photocatalytic degradation of target contaminantst.

In this work, a high-surface-area dual inorganic molecularly imprinted (DIMI) Bi2WO6/CuO/Ag2O photo-catalyst was developed for the selective photocatalytic degradation of methyl green (MG) and auramine O (AO) dyes as target pollutants. The DIMI-Bi2WO6/CuO/Ag2O heterojunction was synthesized by a sono-chemically assisted sol-gel method by coating a layer of molecularly imprinted Ag2O/CuO on the surface of Bi2WO6 nanocubes with MG and AO as the templates. This was followed by calcination for the removal of target molecules and annealing for Ag/Cu oxide preparation. This novel photocatalyst was prepared to overcome the challenge of the co-existing non-target molecules, which has limited the photocatalytic degradation performance. The surface DIMI sites could act as surface defects for accelerating the separation of photogenerated holes and electrons, which led to the increased generation of OH radicals. Moreover, the DIMI sites had increased binding affinity toward MG and AO via the formation of multiple H bonds and electrostatic bonds, which were confirmed by FTIR spectroscopy, PL and EIS studies. The surface DIMI sites led to the increased adsorption and improved local concentration of MG and AO on Bi2WO6/CuO/Ag2O. Consequently, the heterojunction properties of the final DIMI product accelerated the transfer and separation of photogenerated carriers. The high binding affinity of the DIMI sites to MG and AO confirmed the selective recognition, which was tested in the presence of coexisting pollutant dyes. The other characterizations confirmed the successful fabrication and high photocatalytic activity of the high-surface-area DIMI-Bi2WO6/CuO/Ag2O heterostructured composite. In general, the superior interfacial electronic interactions, high migration efficiency of photoinduced charge carriers, and strong visible light absorption of the prepared photocatalyst resulted in good photocatalytic performance.

Synthesis of spherical titanium dioxide microspheres and its application to degrade rifampicin

The presence of antimicrobials in the aquatic system is a form of contamination that has been linked with increasing cases of antimicrobial resistance hence the need to find ways in which they can be remediated. In this study, spherical titanium dioxide microspheres were synthesized, characterized and used as a catalyst to degrade rifampicin in different pH media. Presence of OH functional groups in the IR spectrum and sharp intense peaks associated with TiO2 in the XRD diffractograms confirmed the presence of crystalline TiO2 microspheres. The synthesized microspheres were spherical in nature with diameters ranging between 200−1000 nm, a high thermal stability and low ash content characteristic of metallic particles. Degradation of rifampicin in basic, neutral and acidic media using the microspheres further revealed that degradation of the antibiotic was pH dependent as the rate of degradation was higher in acidic media than in neutral and basic media. The percent degradation was higher at pH 12 as compared to pH 3 and pH 6.5 and it was calculated to be 57.6, 62.9 and 63.8 % for pH 3, 6.5 and 12, respectively. In the presence of H2O2, the degradation effeciency was enhanced due to generation of OH radicals that aided in the degradation of the antibiotic and they followed first order reaction model. As a result, TiO2/H2O2 were effective in degrading the antibiotic hence could be used to remediate the concentration of the drug when found in water.

Iron precursor salt effect on the generation of [rad]OH radicals and sulfamethoxazole degradation through a heterogeneous Fenton process using Carbon-Fe catalysts

The influence of the iron precursor salt on the preparation of Carbon-Fe catalysts was evaluated based on the generation of OH radicals in a heterogeneous Fenton process applied to the degradation of sulfamethoxazole (SMX). Carbon catalysts were obtained with 9% Fe by weight using three iron salts: iron acetate (C-AC-AFe), iron sulfate (C-AC-SFe) and iron nitrate (C-AC-NFe). Characterization of catalysts was evaluated by N2 physisorption, X-ray diffraction, scanning electron microscopy and spectroscopic techniques (FTIR, EDX, XPS); these properties were related to the OH generation kinetics and to SMX degradation rate. The iron precursor salt favors the anchoring of the metal in different oxidation state on the catalyst in a proportion of: Fe2+ / Fe3+ = 4.1 for C-AC-AFe, 1.5 for C-AC-SFe and 1.7 for C-AC-NFe, which is related to the OH generated: 53.8 μM g−1, 37.9 μM g−1 and 42.4 μM g−1, respectively. The OH generation kinetics were described by a pseudo-first-order model with rate constants of 0.0252 and 0.0299 min−1 for C-AC-AFe and C-AC-SFe respectively, which coincide with the kinetic constants for the degradation of SMX (0.0262 and 0.0297 min−1), therefore, the oxidation process is carried out by OH. In the case of C-AC-NFe, Fe was fixed within the texture of the carbon and the OH generation was the lowest (0.0005 min−1), explaining the reaction is limited by diffusion. For SMX, the degradation percentage achieved for 20 mg L−1 was: 98.2 % in 120 min for C-AC-AFe, 98.1 % in 180 min for C-AC-SFe and 92.8 % in 210 min for C-AC-NFe.

Disproportionation Channel of the Self-reaction of Hydroxyl Radical, OH + OH → H2O + O, Revisited.

The rate constant of the disproportionation channel 1a of the self-reaction of hydroxyl radicals OH + OH → H2O + O (1a) was measured at ambient temperature as well as over an extended temperature range to resolve the discrepancy between the IUPAC recommended value (k1a = 1.48 × 10-12 cm3 molecule-1 s-1, discharge flow system, Bedjanian et al. J. Phys. Chem. A 1999, 103, 7017) and a factor of ca. 1.8 higher value by pulsed laser photolysis (2.7 × 10-12 cm3 molecule-1 s-1, Bahng et al. J. Phys. Chem. A 2007, 111, 3850, and 2.52 × 10-12 cm3 molecule-1 s-1, Altinay et al. J. Phys. Chem. A 2014, 118, 38). To resolve this discrepancy, the rate constant of the title reaction was remeasured in three laboratories using two different experimental techniques, namely, laser-pulsed photolysis-transient UV absorption and fast discharge flow system coupled with mass spectrometry. Two different precursors were used to generate OH radicals in the laser-pulsed photolysis experiments. The experiments confirmed the low value of the rate constant at ambient temperature (k1a = (1.4 ± 0.2) × 10-12 cm3 molecule-1 s-1 at 295 K) as well as the V-shaped temperature dependence, negative at low temperatures and positive at high temperatures, with a turning point at 427 K: k1a = 8.38 × 10-14 × (T/300)1.99 × exp(855/T) cm3 molecule-1 s-1 (220-950 K). Recommended expression over the 220-2384 K temperature range: k1a = 2.68 × 10-14 × (T/300)2.75 × exp(1165/T) cm3 molecule-1 s-1 (220-2384 K).

High performance of Ag/BiVO4 photocatalyst for 2,4-Dichlorophenoxyacetic acid degradation under visible light

With the aim of exploring and comparing the photocatalytic degradation of 2,4-Dichlorophenoxyacetic acid (2,4-D) under visible light, BiVO4 was synthesized by using coprecipitation and hydrothermal methods and modified with silver nanoparticles (AgNP) employing a photo-deposition method. X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy were some of the characterization techniques used. A particular synergy was observed when decorating the hydrothermally prepared BiVO4 with AgNP, showing the higher degradation/mineralization of 2,4-D. According to the results, AgNP served as efficient co-catalysts of BiVO4, allowing the electron separation/transfer to generate the OH radicals required for the reaction. A parallel pathway of 2,4-D photodegradation was proposed, including 2,4-Dichlorophenol and oxalic acid as the main intermediary compounds. Active species trapping experiments demonstrated that the photogenerated holes play a fundamental role in promoting both the direct oxidation of 2,4-D and generation of OH radicals.