Hydroxyl Radical Formation Rate Research Articles

Harnessing ultrasound-derived hydroxyl radicals for the selective oxidation of aldehyde functions.

Ultrasonic irradiation holds potential for the selective oxidation of non-volatile organic substrates in the aqueous phase by harnessing hydroxyl radicals as chemical initiators. Here, a mechanistic description of hydroxyl radical-initiated glyoxal oxidation is constructed by gleaning insights from photolysis and radiation chemistry to explain the yields and kinetic trends for oxidation products. The mechanistic description and kinetic measurements reported herein reveal that increasing the formation rate of hydroxyl radicals by changing the ultrasound frequency increases both the rates of glyoxal consumption and the selectivity towards C2acid products over those from C-C cleavage. Glyoxal consumption also occurs more rapidly and with greater selectivity towards C2acids under acidic conditions, which favor the protonation of carboxylate intermediates into their less reactive acidic forms. Leveraging such pH and frequency effects is crucial to mitigating product degradation by secondary reactions with hydroxyl radicals and oxidation products (specifically hydrogen peroxide and superoxide). These findings demonstrate the potential of ultrasound as a driver forthe selective oxidation of aldehyde functions to carboxylic acids, offering a sustainable route for valorizing biomass-derived platform molecules.

TiO2-Coated Meltblown Nonwoven Fabrics Prepared via Atomic Layer Deposition for the Inactivation of E. coli as a Model Photocatalytic Drinking Water Treatment System

The controlled manufacturing of semiconductor photocatalysts is crucial to their development for drinking water treatment. In this study, TiO2-coated meltblown nonwoven fabrics prepared via Atomic Layer Deposition (ALD) are applied for the inactivation of Escherichia coli (E. coli). It is observed that in the presence of an ultraviolet light-emitting diode (UV-LED) light source (255 nm), 1.35 log E. coli inactivation is achieved. However, exposure to catalyst-coated fabrics in addition to the light source resulted in >4 log E. coli inactivation, suggesting a much higher rate of hydroxyl radical formation on the surface, leading to cell death.

Annealing crystallization control mechanism of catalytic degradation properties of Fe-based amorphous ribbons

Amorphous alloys are meta-stable materials with long-range disordered atomic structure, which have excellent catalytic degradation performance and are also susceptible to crystallization, but the mechanism of the effect of crystallization on their catalytic properties has not been clarified. Therefore, the effect of the annealing crystallization process on the microstructure of Fe-Si-B-Cu-Nb industrial amorphous ribbons and their catalytic degradation properties for acid orange 7 are investigated in this work. It is found that the catalytic degradation performance of the ribbons decreases dramatically after having been annealed at 460–580 ℃ , and its reaction rate constant is less than 0.01 min<sup>–1</sup>. The main reason is the formation of <i>α</i>-Fe precipitation phase in the ribbons after having been annealed at high temperatures and the destruction of the substable amorphous structure. These reduce the rate of hydroxyl radical formation. In contrast, the catalytic degradation performance of the 650–700 ℃ annealed ribbons increases significantly, which increases to 3.77 times the degradation rate of the as-cast ribbons. The decolorization rate of acid orange 7 by the annealed ribbons reaches 99.22% within 15 min, which is 1.12 times that of the as-cast ribbons. The improvement of the catalytic degradation performance is attributed to the primary cell effect between the crystalline phase and the metal compounds and the substitution reaction between the Cu-enriched clusters and zero-valent iron. In this study, the influence mechanism of annealing crystallization on the performance of Fe-Si-B-Cu-Nb industrial amorphous ribbons for degrading azo dyes is revealed, which provides theoretical and experimental support for using aged iron-based amorphous ribbons to purify printing and dyeing waste-water and achieve “purification of waste-water by using alloy waste”.

Chemical Stability Enhancement of Aromatic Proton Exchange Membranes Using a Damage Repair Mechanism

Harmful radical species (HO•, HOO•) are formed during fuel cell operation. Out of these species, the highly electrophilic hydroxyl radical is the most reactive (1.8 – 2.7 V vs. NHE). Consequently, the most electron-rich constituents, aromatic rings, are preferentially attacked and undergo rapid degradation. Hydroxyl radical formation rates depend on operating conditions and cell parameters, but even with low formation rates the damage accumulates over time and can lead to thinning or embrittlement of the membrane, and will cause irreversible damage to the cell. Due to these effects, long-term operation becomes challenging. Therefore, membranes with enhanced radical stability are increasingly desirable. In our previous work, we have shown, using kinetics measurements, that it should be possible for Ce(III) to act as a repair agent for formed damaged intermediates (cf. Figure).[1] In our follow-up work we demonstrate that Ce(III) can indeed reduce degradation rates.[2] These experiments were performed in a fuel cell setup using accelerated stress tests at open circuit voltage (OCV), a cell temperature of 80 °C, 100% RH and high H2 and O2 partial pressures of 2 bar. In-situ measurements and post-test analyses unequivocally show that bound Ce(III) can mitigate degradation of poly(α-methylstyrene sulfonate)-based membranes. We believe that this approach could also be applicable for other aromatic-based polymers if a sufficiently long-lived intermediate is formed upon radical attack.

Advanced oxidative chemical recycling of carbon-fiber reinforced plastic using hydroxyl radicals and accelerated by radical initiators

We herein report a method for the chemical depolymerization of the epoxy resin present in a carbon composite through an advanced oxidation reaction using hydroxyl radicals. It was found that decomposition of the epoxy resin by the hydroxyl radicals generated from NaOCl was complete within 2 h at 100 °C and ambient pressure in an aqueous solution. The effects of different radical initiators (i.e., potassium persulfate (KPS) and 4,4′-azobis(4-cyanovaleric acid) (ACVA)) on the acceleration of hydroxyl radical generation from NaOCl were also evaluated. More specifically, the rate of hydroxyl radical formation in the presence and absence of a radical initiator was determined using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical method, and it was confirmed that the radical generation rate was increased by up to 3 times when the KPS radical initiator was employed (c.f., in the absence of an initiator). Interestingly, it was confirmed that the reaction rate constant (k) increased by up to 5.3 times when the combination of NaOCl and radical initiator KPS was applied to the recycling of carbon fiber-reinforced plastic (CFRP). In addition, X-ray photoelectron spectroscopy, Raman spectroscopy, and Field-emission scanning electron microscopy observations of the reclaimed carbon fiber demonstrated that no significant damage occurred during the reaction. This result is expected to lead to novel methods for enhancing the rate of CFRP chemical recycling processes.

Influence of strong iron-binding ligands on cloud water oxidant capacity

Iron (Fe) plays a dual role in atmospheric chemistry: it is involved in chemical and photochemical reactivity and serves as a micronutrient for microorganisms that have recently been shown to produce strong organic ligands. These ligands control the reactivity, mobility, solubility and speciation of Fe, which have a potential impact on Fe bioavailability and cloud water oxidant capacity.In this work, the concentrations of Fe-binding ligands and the conditional stability constants were experimentally measured for the first time by Competitive Ligand Exchange-Adsorptive Cathodic Stripping Voltammetry (CLE-ACSV) technique in cloud water samples collected at puy de Dôme (France). The conditional stability constants, which indicate the strength of the Fe-ligand complexes, are higher than those considered until now in cloud chemistry (mainly Fe-oxalate). To understand the effect of Fe complexation on cloud water reactivity, we used the CLEPS cloud chemistry model. According to the model results, we found that Fe complexation impacts the hydroxyl radical formation rate: contrary to our expectations, Fe complexation by natural organic ligands led to an increase in hydroxyl radical production. These findings have important impacts on cloud chemistry and the global iron cycle.

Degradation of natural organic matter using Ag-P25 photocatalyst under continuous and periodic irradiation of 405 and 365 nm UV-LEDs

Removal of natural organic matter (NOM) in drinking water is an important objective imposed by water utilities due to plant operational problems, taste and odor issues. Photocatalysis using titanium dioxide (TiO2) has been increasingly explored in water treatment studies as a potential technology to remove NOM, with most studies employing 365 nm UV light. However, photocatalysis under this wavelength limits the application of UV/TiO2 to larger scales as it is not cost-effective and energy-efficient. We approached this limitation by exploring the impact of switching between periodic and continuous irradiation when removing NOM under both non-visible and visible light exposure (365 and 405 nm). In addition, we used silver (Ag) to increase the photocatalytic efficiency of TiO2 in the visible region. The optimum concentration of Ag was first investigated by comparing the surface area, and hydroxyl radical (˙OH) formation rate of Ag-P25 with different Ag:P25 ratios. Of the Ag:P25 ratios explored, 1.29% Ag-P25 was found to be the optimal based on high surface area and ˙OH formation rate. The effect of periodic irradiation on Suwanee River NOM removal using two wavelengths was then investigated. Dissolved organic carbon (DOC) concentrations and UV absorbance at 254 nm (UV254) indicated that the 1.29% Ag-P25 was more efficient than P25 at a higher irradiation cycle of 365 nm. The electrical energy per order (EEO) values calculated for each treatment further indicated that using 1.29% Ag-P25 resulted in a lower energy requirement for removing DOC and UV254 than P25.

Constructing magnetic separable BiOBr/MnFe2O4 as efficient Z-scheme nanocomposite for visible light-driven degradation of palm oil mill effluent and inactivation of bacteria

Z-scheme BiOBr/MnFe2O4 nanocomposite was successfully prepared by a surfactant-free hydrothermal method. A series of characterizations revealed that the MnFe2O4 nanoparticles were intimately decorated on the BiOBr nanosheets. The as-prepared BiOBr/MnFe2O4 displayed lower charge recombination and higher hydroxyl radical formation rates compared to BiOBr and MnFe2O4. Visible light photocatalytic experimental results showed that the BiOBr/MnFe2O4 possessed excellent performances towards the palm oil mill effluent degradation and Escherichia coli inactivation. The high photoactivity of BiOBr/MnFe2O4 was attributed to the successful establishment of Z-scheme heterojunction between BiOBr and MnFe2O4. Moreover, the BiOBr/MnFe2O4 can easily be recovered using an external magnet and exhibited good repeatability of photoactivity.

Revisiting terephthalic acid and coumarin as probes for photoluminescent determination of hydroxyl radical formation rate in heterogeneous photocatalysis

We investigated in detail the applicability of two radical scavengers – coumarin (COUM) and terephthalic acid (TA) – as probe compounds in a widely used photoluminescent (PL) determination of OH∙ radical rate formation of photocatalysts. The study reveals that precautions must be taken when using either of the two compounds as neither allows a direct determination of the absolute OH∙ radical formation rate. Chromatographic analyses of irradiated COUM and TA solutions revealed that both probe compounds reacted via more than one pathway, out of which, only one pathway in each case yielded the measured photoluminescent 7-hydroxycoumarin (7-OHC) and 2-hydroxyterephthalic acid (TAOH), respectively. Applicability of both probes was also tested on three model TiO2-based catalysts (anatase TiO2 nanorods (TNR), anatase TiO2 nanoparticles (TNP), amorphous TiO2 nanorods (a-TNR)). Regardless of the probe compound used, the order of relative OH∙ radical formation rates determined for these catalysts was the same (TNR < TNP < a-TNR) and in good correlation with the order of bisphenol A (BPA) degradation rates. This demonstrates that (i) formation of OH∙ radicals is the predominating criterion when probing a photocatalyst’s activity (but not the only one) and that (ii) TA and COUM potentially enable a relative evaluation of photocatalytic materials, despite the numerous shortcomings of these probes. However, in order to allow for general inter-laboratory comparisons the photoluminescence method should clearly be standardized with precisely defined experimental parameters.

Formation Rate and Energy Yield of Hydroxyl Radicals in Water under the Action of Gas-Discharge Plasma

The kinetics of hydroxyl radical formation in water under the action of a pulsed discharge in vapor-gas bubbles formed near the surface of a graphite electrode immersed into water is studied. The reaction of potassium hexacyanoferrate(II) oxidation is used to determine the concentration and formation rate of OH radicals in the liquid phase. The power dissipated in the discharge is estimated from measurements of the time dependences of the current through the discharge cell, the voltage applied to the electrodes, and the integrated intensity of the discharge radiation. The yields of OH radicals per 100 eV of the energy applied to the plasma are calculated. It is found that, with an increase in power from 13 to 100 W, the rate of radical generation increases within the range of (2.8–34.8) × 10–7 mol/(L s). The radical yield weakly depends on power and is equal to 0.12 ± 0.03 particles/100 eV.

Photochemistry of the Cloud Aqueous Phase: A Review.

This review paper describes briefly the cloud aqueous phase composition and deeply its reactivity in the dark and mainly under solar radiation. The role of the main oxidants (hydrogen peroxide, nitrate radical, and hydroxyl radical) is presented with a focus on the hydroxyl radical, which drives the oxidation capacity during the day. Its sources in the aqueous phase, mainly through photochemical mechanisms with H2O2, iron complexes, or nitrate/nitrite ions, are presented in detail. The formation rate of hydroxyl radical and its steady state concentration evaluated by different authors are listed and compared. Finally, a paragraph is also dedicated to the sinks and the reactivity of the HO• radical with the main compounds found in the cloud aqueous phase. This review presents an assessment of the reactivity in the cloud aqueous phase and shows the significant potential impact that this medium can have on the chemistry of the atmosphere and more generally on the climate.

Reaction kinetics of sonochemical oxidation of potassium hexacyanoferrate(II) in aqueous solutions.

We studied sonochemical reactions resulting from ultrasonic treatment of potassium hexacyanoferrate(II) in aqueous solutions using a custom-built apparatus working at 536kHz. We concluded that primary reactions are completely dominated by oxidation of Fe(II) to Fe(III) and did not find any evidences for degradation of cyanide. At the highest concentration used in the present study (0.1M) we detected formation of pentacyanoaquaferrate(II) complex, which is most probably formed in reactions between hexacyanoferrate(III) anions and hydrogen atoms or hydrated electrons formed in sonochemical processes. We also determined that hydroxyl radicals formation rate in our system, (8.7±1.5)∙10-8 M∙s-1, is relatively high compared to other reported experiments. We attribute this to focusing of the ultrasonic wave in the sample vessel. Finally, we suggest that oxidation rate of hexacyanoferrate(II) anions can be a convenient benchmark of efficiency of sonochemical reactors.

A new source of ammonia and carboxylic acids in cloud water: The first evidence of photochemical process involving an iron-amino acid complex

In the present study we investigate the photochemical properties and impact on cloud water of ferric-aspartate complex (Fe(III)−Asp) under irradiation. Fe(III)−Asp complex was investigated as proxy of ferric-amino acids complexes that could be expected to be present in cloud water. First, the fate of complex under monochromatic (313 and 365 nm) and polychromatic (λ ≥ 290 nm) irradiation is investigated and formation quantum yields of Fe2+ are determined. Moreover, hydroxyl radical quantification in the presence of hydrogen peroxide is investigated at cloud water relevant concentrations and under irradiation. Obtained results suggest that hydroxyl radical formation rates are expected to be lower than those obtained considering all iron as aquacomplex or complexed with oxalate. Finally, a phototransformation mechanism for inorganic and organic products formation such as ammonia, malonic, oxalic and formic acids is proposed.

Influencing investigation of metal ions on heterogeneous catalytic ozonation by ceramic honeycomb for the degradation of nitrobenzene in aqueous solution with neutral pH

Abstract Influence of metal ions on heterogeneous catalytic ozonation by ceramic honeycomb for degradation of nitrobenzene in aqueous solution was investigated through the semi-continuous batch experiments to compare apparent reaction rate constant (kapp) and hydroxyl radical ( OH) formation rate ( k · OH ) in the presence of different metal ions including monovalent (Na+, K+ and Ag+), divalent (Zn2+, Ni2+, Co2+, Mn2+, Ca2+, Fe2+, Cu2+ and Mg2+) and trivalent ions (Fe3+, Al3+ and Cr3+). Each metal ion can increase kapp and k · OH in the process of ozonation alone under the present experimental conditions. Except for the positive influence of Mn2+, the others all show the early enhancement effect and the latter inhibition effect on kapp and k · OH , namely existing a critical maximum concentration of metal ion for catalytic ozonation. The ratio of OH initiation from heterogeneous and homogeneous systems decreases with the increasing concentration of metal ion, indicating that the contribution of OH initiation derived from heterogeneous catalytic system decreases. Results confirm that the increasing concentration of metal ion causes the decrease in density of surface hydroxyl groups on ceramic honeycomb, and a novel correlation is established between the density decrease rates of surface hydroxyl group and the standard reduction potentials of metal ions.

The effects of sunlight on the composition of exopolymeric substances and subsequent aggregate formation during oil spills

Abstract During the Deepwater Horizon oil spill, a large amount of marine oil snow (MOS) was formed in surface waters of the Gulf of Mexico (GOM) that eventually settled to the bottom. MOS consists of a microbially colonized matrix of extracellular polymeric substances (EPS), to which cells, and both ionic (e.g., trace nutrients, Ca 2+ ) and non-ionic (e.g., toxic oil) substances can attach. The secretion of EPS is one of the microbial defense strategies against harmful or stressful environmental situations. In the surface waters, the toxicity of oil can be enhanced by elevated oxidative stress through UV radiation . To test the effects of sunlight on the composition and secretion of EPS and the subsequent aggregation process, we conducted short-term irradiation experiments in three treatments, i.e., control (GOM coastal seawater), water accommodated fraction of oil (WAF), and chemically-enhanced WAF (CEWAF). EPS composition (mainly carbohydrates and proteins) was quantified in the colloidal and aggregate fractions. In addition, bacterial abundance, live/dead cell ratio, particle size distribution , and the ambient hydroxyl radical ( OH) formation rate were measured under these conditions. We found that in the presence of oil, natural sunlight stimulated polysaccharide secretion, coinciding with increased reactive oxygen species (ROS; i.e. OH) production. Moreover, formation of larger sized aggregates (>10 μm) was observed in the irradiated WAF treatments. The results support the hypothesis that sunlight plays an important role in MOS formation during an oil spill.

One-dimensional ZnO nanorods doped with neodymium for enhanced resorcinol degradation under sunlight irradiation

ABSTRACTNeodymium-doped ZnO nanorods (Nd/ZNRs) were prepared for the first time by a simple and surfactant-free solvothermal route. The synthesized samples were characterized using different instrumental techniques. The photoluminescence results showed that the Nd(2.0 at%)/ZNRs exhibited the highest separation rate of charge carriers and the highest formation rate of hydroxyl radicals. The photocatalytic activities of synthesized samples were investigated toward the degradation of endocrine disrupting chemical resorcinol under natural sunlight irradiation. Among the samples, Nd(2.0 at%)/ZNRs showed considerable improvement in the photocatalytic activity for the resorcinol degradation as compared to ZNRs and commercial TiO2. Kinetic studies revealed that the photocatalytic degradation of resorcinol obeyed pseudo-first-order kinetic. The high-performance liquid chromatography and total organic carbon analyses also demonstrated the progressive mineralization of resorcinol into carbon dioxide and water. Furthermore, high stability of Nd/ZNRs in photocatalytic reaction also demonstrated promising potential toward practical applications in purifying environmental pollutants.

Non-linear hydroxyl radical formation rate in dispersions containing mixtures of pyrite and chalcopyrite particles

The formation of hydroxyl radicals was studied in mixed pyrite-chalcopyrite dispersions in water using the conversion rate of adenine as a proxy for hydroxyl radical formation rate. Experiments were conducted as a function of pH, presence of phosphate buffer, surface loading, and pyrite-to-chalcopyrite ratio.The results indicate that hydroxyl radical formation rate in mixed systems is non-linear with respect to the rates in the pure endmember dispersions. The only exception is a set of experiments in which phosphate buffer is used. In the presence of phosphate buffer, the hydroxyl radical formation is suppressed in mixtures and the rate is close to that predicted based on the reaction kinetics of the pure endmembers.The non-linear hydroxyl radical formation in dispersions containing mixtures of pyrite and chalcopyrite is likely the result of two complementary processes. One is the fact that pyrite and chalcopyrite form a galvanic couple. In this arrangement, chalcopyrite oxidation is accelerated, while pyrite passes electrons withdrawn from chalcopyrite to molecular oxygen, the oxidant. The incomplete reduction of molecular oxygen leads to the formation of hydrogen peroxide and hydroxyl radical. The galvanic coupling appears to be augmented by the fact that chalcopyrite generates a significant amount of hydrogen peroxide upon dispersal in water. This hydrogen peroxide is then available for conversion to hydroxyl radical, which appears to be facilitated by pyrite as chalcopyrite itself produces only minor amounts of hydroxyl radical. In essence, pyrite is a “co-factor” that facilitates the conversion of hydrogen peroxide to hydroxyl radical. This conversion reaction is a surface-mediated reaction.Given that hydroxyl radical is one of the most reactive species in nature, the formation of hydroxyl radicals in aqueous systems containing chalcopyrite and pyrite has implications for the stability of organic molecules, biomolecules, the viability of microbes, and exposure to dust containing the two metal sulfides may present a health burden.

Hydroxyl radical generation with a high power ultraviolet light emitting diode (UV-LED) and application for determination of hydroxyl radical reaction rate constants

We propose a simple, efficient and selective hydroxyl radical generation system based on the photolysis of submicromolar concentrations of nitrite using a high-power ultraviolet light emitting diode (UV-LED). Hydroxyl radical formation by the 6.75-W UV-LED was at least 10 times greater than that by a 300-W Xe lamp. In the UV-LED system, the hydroxyl radical formation rate from nitrite was about four orders of magnitude larger than that from nitrate and two orders of magnitude larger than that from hydrogen peroxide. Such efficient and selective hydroxyl radical formation can be attributed to the overlap of the emission spectrum of the UV-LED and absorption of nitrite. The system was used to determine the reaction rate constants between hydroxyl radicals and chemicals based on the competition method with terephthalate as the hydroxyl radical probe. The reaction rate constants between hydroxyl radicals and some low-molecular-weight organic compounds and inorganic halide salts with various reaction rate constants were determined. The values obtained ranged from 105 to 1010 M−1 s−1, and agreed well with previously reported values. The potential of the developed method to determine the reaction rate constants of hydroxyl radicals is discussed.

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.

Noble metal modified TiO2 microspheres: Surface properties and photocatalytic activity under UV–vis and visible light

Composite photocatalysts that consist of TiO2 and noble metal nanostructures have been considered to be the promising and pivotal material for accessible enhancement of the efficiency in the photocatalytic process carried out in the aqueous and gas phases. In this work we fabricated porous TiO2 microspheres through a hydrothermal process followed by photochemical reduction of noble metal nanoparticles at the TiO2 surface. The morphology and structure of M-TiO2 spheres (M=Ag, Au, Pt and Pd) were studied with the use of various techniques, including transmission electron microscopy (TEM), X-ray powder diffraction analysis (XRD), photoluminescence (PL) and UV–vis diffuse-reflectance spectroscopy (DRS). The effect of metal amount (from 0.1 to 1wt.%.) on the photocatalytic activity during toluene degradation in gas phase and phenol degradation in aqueous phase was investigated. Additionally, the photocatalytic activity of the M-TiO2 samples was evaluated by measuring the formation rate of photo-induced hydroxyl radicals (OH) under UV–vis light irradiation using coumarin as a probe. The obtained results indicated that toluene could be mostly removed from the air over TiO2 microspheres modified with Ag, Au, Pt, and Pd nanoparticles. UV-mediated photoreactivity was almost similar for all samples obtained by loading metals from solutions consisting of 0.1 and 1wt.% of metal precursors. Under visible light, except for Au, in gas phase toluene oxidation, the optimized loading of the metals was 0.1wt.-% (photoreactivity changed in order: Ag-TiO2≈Pd-TiO2>Pt-TiO2>>Au-TiO2). In case of phenol degradation in the aqueous phase, in the presence of UV irradiation the highest amount of metal (1.5wt.%) was profitable, while under the Vis light reaction the medium amount of metal (0.5wt.%) was beneficial. Additionally, it was noticed that phenol was degraded not only via oxidation by OH radicals but probably also in direct reaction with the photogenerated carriers (e−/h+), particulary in the presence of TiO2 spheres loaded with Au and Ag nanoparticles.