The investigation of OH radicals produced in a DC glow discharge by laser-induced fluorescence spectrometry

In this paper the OH radicals produced by a needle-plate negative DC discharge in water vapor,N_(2)+H_(2)O mixture gas and He+H_(2)O mixture gas are investigated by a laser-induced fluorescence(LIF)system.With a ballast resistor in the circuit,the discharge current is limited and the discharges remain in glow.The OH rotation temperature is obtained from fluorescence rotational branch fitting,and is about 350 K in pure water vapor.The effects of the discharge current and gas pressure on the production and quenching processes of OH radicals are investigated.The results show that in water vapor and He+H_(2)O mixture gas the fluorescence intensity of OH stays nearly constant with increasing discharge current,and in N_(2)+H_(2)O mixture gas the fluorescence intensity of OH increases with increasing discharge current.In water vapor and N_(2)+H_(2)O mixture gas the fluorescence intensity of OH decreases with increasing gas pressure in the studied pressure range,and in He+H_(2)O mixture gas the fluorescence intensity of OH shows a maximum value within the studied gas pressure range.The physicochemical reactions between electrons,radicals,ground and metastable molecules are discussed.The results in this work contribute to the optimization of plasma reactivity and the establishment of a molecule reaction dynamics model.

Effect of N2 and O2 on OH radical production in an atmospheric helium microwave plasma jet

UV-pulsed laser cavity ringdown spectroscopy of the hydroxyl radical OH(A–X)(0–0)band in the wavelength range of 306–310 nm was employed to determine absolute number densities of OH in the atmospheric helium plasma jets generated by a 2.45 GHz microwave plasma source.The effect of the addition of molecular gases N2 and O2 to He plasma jets on OH generation was studied.Optical emission spectroscopy was simultaneously employed to monitor reactive plasma species.Stark broadening of the hydrogen Balmer emission line(Hβ)was used to estimate the electron density ne in the jets.For both He/N2 and He/O2 jets,ne was estimated to be on the order of 10^15 cm^?3.The effects of plasma power and gas flow rate were also studied.With increase in N2 and O2 flow rates,ne tended to decrease.Gas temperature in the He/O2 plasma jets was elevated compared to the temperatures in the pure He and He/N2 plasma jets.The highest OH densities in the He/N2 and He/O2 plasma jets were determined to be 1.0×10^16 molecules/cm^3 at x=4 mm(from the jet orifice)and 1.8×10^16 molecules/cm^3 at x=3 mm,respectively.Electron impact dissociation of water and water ion dissociative recombination were the dominant reaction pathways,respectively,for OH formation within the jet column and in the downstream and far downstream regions.The presence of strong emissions of the N2^+ bands in both He/N2 and He/O2 plasma jets,as against the absence of the N2^+ emissions in the Ar plasma jets,suggests that the Penning ionization process is a key reaction channel leading to the formation of N2^+ in these He plasma jets.

Increasing the·OH radical concentration synergistically with plasma electrolysis and ultrasound in aqueous DMSO solution

In recent years,significant increases in waste processing and material engineering have been seen by using advanced oxidation processes.The treatment results and energy yields of these processes are largely determined by the generation and retention of reactive oxygen species(ROS).However,increasing the amount of ROS remains a key challenge because of the unavailability of performance-and energy-efficient techniques.In this study,plasma electrolysis,ultrasound,and plasma electrolysis combined with ultrasound were used to treat dimethyl sulfoxide(DMSO)solutions,and the results showed that the two methods can synergistically convert filament discharge into spark discharge,and the conversion of the discharge mode can significantly increase the concentration of OH radicals and effectively improve the efficiency of DMSO degradation.We verified the rationality of the results by analyzing the mass transfer path of ROS based on the reaction coefficients and found that the OH radicals in aqueous solution were mainly derived from the decomposition of hydrogen peroxide.These findings indicated that the synergistic action of plasma electrolysis and ultrasound can enhance the production of chemically reactive species,and provide new insights and guiding principles for the future translation of this combined strategy into real-life applications.Our results demonstrated that the synergistic strategy of ultrasound and plasma electrolysis is feasible in the switching mode and increasing the ROS,and may open new routes for materials engineering and pollutant degradation.

Detection of OH Radical in the Photodissociation of p-Aminobenzoic Acid at 266 nm

Photodissociation of p-aminobenzoic acid at 266 nm was investigated by probing the nascent OH photoproduct employing the laser-induced fluorescence technique. It was found that the nascent OH radical was vibrationally cold and its rotational state distribution conformed to be a Boltzmann behavior, characterized by a rotational temperature of 1040±110 K. The rotational energy of OH was determined to be 8.78±0.84 kJ/mol. Between the two spinorbit states of OH, ^2Ⅱ3/2 and ^2Ⅱ1/2, the former was found to be preferentially populated. The distribution of the II（A＇） state for the A-doublet was dominant. Finally, a probable mechanism for the formation of OH produced from the photodissociation of p-aminobenzoic acid is discussed.

On the Production of OH Radical through Plasma Electrolysis Mechanism for the Processing of Ammonia Waste Water

Plasma produced many active species such as OH radical and H radical. As well known, OH radical plays an important role in degrading complex pollutants. This study aims to measure the production of OH radicals and evaluate important parameters that have influent in degradation process of waste water contains ammonia in circulated system and analyze the level of energy consumptions are resulted by this research. The production of OH radical was detected by formation of hydrogen peroxide which was resulted by recombination reaction between OH radicals during plasma electrolysis process. From the measured concentration of hydrogen peroxide, obtained concentration of OH radical is 2,020 ppm. The depth of anode, applied voltage and ammonia initial concentration have affected ammonia degradation percentage and energy consumption level. The highest result for ammonia degradation percentage is 63.2% which gets from applied voltage 700 V, with depth of anode 1 cm, initial concentration of ammonia 100 ppm, and lowest energy consumption of 110 KJ/mmol.

Photolysis and Rate Constant of 2,3-butanedione with OH Radicals in the Aqueous Phase under Tropospheric Conditions

Photolysis rate （J1） and reaction rate constants （kl） for the biacetyl （butane-2,3-dione） were evaluated in aqueous phase using a continuous photolysis system with a conventional Xe-Hg arc lamp as a light source. The OH radicals was generated by H2OE/UV process and biacetyl （CH3C（O）C（O）CH3） concentrations were monitored using 2,4-DNPH derivatization method. 2,3-butanedione molecule is widely present in the atmosphere, it have been detected in hydrometeors （fogs, rain, and clouds） and at a significant yield （up to 10μmolar）. The measurements were performed at 294 K and with free pH values. Our results lead to the following obtained values： J1= 3×10^-4 S^-1 and k1 = （6.17±0.95）×10^8 M^-1·s^-1.The uncertainty listed above is ±15%.

Theoretical insights into the degradation mechanisms, kinetics and eco-toxicity of oxcarbazepine initiated by OH radicals in aqueous environments

As an anticonvulsant,oxcarbazepine(OXC)has attracted considerable attention for its potential threat to aquatic organisms.Density functional theory has been used to study the mechanisms and kinetics of OXC degradation initiated by OH radicals in aqueous environment.A total of fourteen OH-addition pathways were investigated,and the addition to the C8 position of the right benzene ringwas themost vulnerable pathway,resulting in the intermediate IM8.The H-abstraction reactions initiated by OH radicalswere also explored,where the extraction site of the methylene group(C14)on the seven-member carbon heterocyclic ring was found to be the optimal path.The calculations show that the total rate constant of OXC with OH radicals is 9.47×10^(9)(mol/L)^(−1)sec^(−1),and the half-life time is 7.32 s at 298 K with the[·OH]of 10^(−11) mol/L.Moreover,the branch ratio values revealed that OH-addition(89.58%)shows more advantageous than H-abstraction(10.42%).To further understand the potential eco-toxicity of OXC and its transformation products to aquatic organisms,acute toxicity and chronic toxicity were evaluated using ECOSAR software.The toxicity assessment revealed that most degradation products such as OXC-2OH,OXC-4OH,OXC-1O-1OOH,and OXC-1OH’are innoxious to fish and daphnia.Conversely,green algae are more sensitive to these compounds.This study can provide an extensive investigation into the degradation of OXC by OH radicals and enrich the understanding of the aquatic oxidation processes of pharmaceuticals and personal care products(PPCPs).

Effect of gas components on the postdischarge temporal behavior of OH and O of a non-equilibrium atmospheric pressure plasma driven by nanosecond voltage pulses

OH radicals and O atoms are two of the most important reactive species of non-equilibrium atmospheric pressure plasma(NAPP),which plays an important role in applications such as plasma medicine.However,experimental studies on how the gas content affects the postdischarge temporal evolutions of OH and O in the noble gas ns-NAPP are very limited.In this work,the effect of the percentages of O_(2),N_(2),and H_(2)O on the amounts of OH and O productions and their post-discharge temporal behaviors in ns-NAPP is investigated by laser-induced fluorescence(LIF)method.The results show that the productions of OH and O increase and then decrease with the increase of O_(2)percentage.Both OH and O densities reach their maximum when about 0.8%O_(2)is added.Further increase of the O_(2)concentration results in a decrease of the initial densities of both OH and O,and leads to their faster decay.The increase of N_(2)percentage also results in the increase and then decrease of the OH and O densities,but the change is smaller.Furthermore,when the H_(2)O concentration is increased from 100 to 3000 ppm,the initial OH density increases slightly,but the OH density decays much faster,while the initial density of O decreases with the increase of the H_(2)O concentration.After analysis,it is found that OH and O are mainly produced through electron collisional dissociation.O(^(1)D)is critical for OH generation.O_(3)accelerates the consumption processes of OH and O at high O_(2)percentage.The addition of H_(2)O in the NAPP considerably enhances the electronegativity,while it decreases the overall plasma reactivity,accelerates the decay of OH,and reduces the O atom density.

Photooxidation of hydrochlorofluocarbons and hydrofluorocarbons initiated by OH radicals

Under simulated atmospheric condition, photoomdation for HCFC-22 + H2O2, HCFC-22 + H2O2+O2, HFC-134A + H2O2 and HFC-134A + H2O2+ O2 systems were studied.H2O2 was irradiated by low pressure mercury lamp and produced OH radicals. The OH radicals can initiate photooxidation of HCFC-22 and 134A. The products of photooxidation were determined by a Fourier Transform infrared Spectroscopy with a 20ml long path cell. The products were COF2,CO2, HCI, H2O and HF for HCFC-22 + H2O2 system, HO, CO2, HCI and HF for HCFC-22 +H2O2 +O2 system, HCOF, CF3OOCF3,CO2, H2O and HF for HFC-134A +H2O2 system, HCOF, CO2, H2O, and HF for HFC-134A + H2O2+ O2 system. Based on those results, the mechanisms of photooxidation were suggested.

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

Exposure to particulate matter(PM)can lead 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·O_(2)–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·O_(2)–,thereby providing health protection.

Photocatalytic OH radical formation and quantification over TiO_2 P25:Producing a robust and optimised screening method

The development of photocatalytic technology has grown significantly since its initial report and as such,a number of screening methods have been developed to assess activity. In the field of environmental remediation, a crucial factor is the formation of highly oxidising species such as OH radicals. These radicals are often the primary driving force for the removal and breakdown of organic and inorganic contaminants. The quantification of such compounds is challenging due to the nature of the radical,however indirect methods which deploy a chemical probe to essentially capture the radical have been shown to be effective. As discussed in the work presented here, optimisation of such a method is fundamental to the efficiency of the method. A starting concentration range of coumarin from 50 mmol/L to 1000 mmol/L was used along with a catalyst loading of 0.01 g/L to 1 g/L Ti TiO2 to identify that 250 mmol/L and 0.5 g/L Ti TiO2 were the optimum conditions for production. Under these parameters a maximum production rate of 35.91 mmol/L（Rmax= 0.4 mmol/L OH＊ min-1） was achieved which yielded at photonic efficiency of 4.88 OH＊moles photon-1 under UV irradiation. The data set presented also highlighted the limitations which are associated with the method which included; rapid exhaustion of the probe molecule and process inhibition through UV light saturation. Identifying both the optimum conditions and the potential limitations of the process were concluded to be key for the efficient deployment of the photocatalytic screening method.

Quantitative measurement of hydroxyl radical(OH) concentration in premixed flat flame by combining laser-induced fluorescence and direct absorption spectroscopy

An accurate and reasonable technique combining direct absorption spectroscopy and laser-induced fluorescence（LIF）methods is developed to quantitatively measure the concentrations of hydroxyl in CH;/air flat laminar flame. In our approach, particular attention is paid to the linear laser-induced fluorescence and absorption processes, and experimental details as well. Through measuring the temperature, LIF signal distribution and integrated absorption, spatially absolute OH concentrations profiles are successfully resolved. These experimental results are then compared with the numerical simulation. It is proved that the good quality of the results implies that this method is suitable for calibrating the OH-PLIF measurement in a practical combustor.

Detection of hydroxyl radical in plasma reaction on toluene removal

A new method was introduced to detect the concentration of OH radical in dielectric barrier discharge(DBD)reaction.A film, which was impregnated with salicylic acid,was used to detect OH radical in plasma reaction at room temperature and atmospheric pressure.Salicylic acid reacts with OH radical and produces 2,5-dihydroxybenzoic acid(2,5-DHBA).Then,a high performance liquid chromatography(HPLC)was carried out to detect the concentration of 2,5-DHBA.Therefore,OH radical in nonthermal plasma reaction could be...

Population Change of OH and H2O in Water Vapor Glow Discharge Measured Using Concentration Modulation Spectroscopy

A distributed feedback laser with a wavelength of 2.8μm was used to measure the species produced by water vapor glow discharge.Only the absorption spectra of OH radicals and transient H2O molecules were observed using concentration modulation(CM)spectroscopy.The intensities and orientations of the absorption peaks change with the demodulation phase,but the direction of one absorption peak of H2O is always opposite to the other peaks.The different spectral orientations of OH and H2O reflect the increase or the decrease of the number of particles in the energy levels.If more transient species can be detected in the discharge process,the dynamics of excitation,ionization,and decomposition of H2O can be better studied.This study shows that the demodulation phase relationship of CM spectrum can be used to study the population change of molecular energy levels.

Numerical study of the effect of water content on OH production in a pulsed-dc atmospheric pressure helium–air plasma jet

A numerical study of the effect of water content on OH production in a pulsed-dc atmospheric pressure helium-air plasma jet is presented. The generation and loss mechanisms of the OH radicals in a positive half-cycle of the applied voltage are studied and discussed. It is found that the peak OH density increases with water content in air （varying from 0% to 1%） and reaches 6.3 x 10^18 m-3 when the water content is 1%. Besides, as the water content increases from 0.01% to 1%, the space-averaged reaction rate of three-body recombination increases dramatically and is comparable to those of main OH generation reactions.

Reaction mechanism of 3-chlorophenol with OH,H in aqueous solution

The reaction mechanism of 3 chlorophenol with OH, H in aqueous solution was studied by transient technology. The 3 chlorophenol aqueous solutions have been saturated with air or N 2 previously. Under alkaline condition, the reaction of OH radical with 3 chlorophenol produces 3 chlorinated phenoxyl radical, with the absorption peaks at 400 nm and 417 nm. Under neutral condition, the reaction of OH radical with 3 chlorophenol produces OH adduct with the maximal absorption at about 340 nm. And in acid solution, the reaction of H with 3 chlorophenol produces H adduct with the maximal absorption at about 320 nm. 3 chlorophenol is compared with 4 and 2 chlorophenols from the free radical pathways. The results show that the positions of chlorine on the aromatic ring strongly influence the dehalogenation and degradation process.

LIF diagnostics of hydroxyl radical in a methanol containing atmospheric-pressure plasma jet

In this paper, a pulsed-dc CH;OH/Ar plasma jet generated at atmospheric pressure is studied by laser-induced fluorescence（LIF） and optical emission spectroscopy（OES）. A gas–liquid bubbler system is proposed to introduce the methanol vapor into the argon gas, and the CH3OH/Ar volume ratio is kept constant at about 0.1%. Discharge occurs in a 6-mm needle-to-ring gap in an atmospheric-pressure CH;OH/Ar mixture. The space-resolved distributions of OH LIF inside and outside the nozzle exhibit distinctly different behaviors. And, different production mechanisms of OH radicals in the needle-to-ring discharge gap and afterglow of plasma jet are discussed. Besides, the optical emission lines of carbonaceous species, such as CH, CN, and C;radicals, are identified in the CH;OH/Ar plasma jet. Finally, the influences of operating parameters（applied voltage magnitude, pulse frequency, pulsewidth） on the OH radical density are also presented and analyzed.

Atmospheric chemistry of 2-nitrobenzaldehyde:Initiated by photo-excitation,OH-oxidation,and small TiO_(2) clusters adsorption catalysis

The fate of 2-nitrobenzaldehyde(2-NBA)is of interest in atmospheric chemistry as it is a semi-volatile organic compound with high photosensitivity.This study presents a quantum chemical study of the gas-phase reactions of 2-NBA photo-excitation and OH-oxidation in the absence and presence of small TiO_(2) clusters.To further understand the unknown photolysis mechanism,the photo-reaction pathways of ground singlet state and the lying excited triplet state of 2-NBA were investigated including the initial and subsequent reactions of proton transfer,direct CO,NO_(2),and HCO elimination routes in the presence of O_(2) and NO.Meanwhile,the OH-mediated degradation of 2-NBA proceeded via five H-extraction and six OH-addition channels by indirect mechanism,which follows a succession of reaction steps initiated by the formation of weakly stable intermediate complexes.The H-extraction from the-CHO group was the dominant pathway with a negative activation energy of-1.22 kcal/mol.The calculated rate coefficients at 200–600 K were close to the experimental data in literature within 308-352 K,and the kinetic negative temperature independence was found in both experimental literature and computational results.Interestingly,2-NBA was favored to be captured onto small TiO_(2) clusters via six adsorption configurations formed via various combination of three types of bonds of Ti…O,Ti…C,and O…H between the molecularly adsorbed 2-NBA and TiO_(2) clusters.Comparison indicted that the chemisorptions of aldehyde oxygen have largest energies.The results suggested adsorption conformations have a respectable impact on the catalysis barrier.This study is significant for understanding the atmospheric chemistry of 2-nitrobenzaldehyde.

Effect of ethanol addition on flame characteristics of waste oil biodiesel

Biodiesel is a kind of clean and renewable energy. The effect of ethanol addition on the flame characteristics of waste oil biodiesel is studied by using OH-PLIF technique from the perspective of OH radical evolution. Ethanol addition leads to the appearance of diffusion flame reaction interface ahead of schedule and shortens the diffusion flame height. The experimental results show a linear correlation between the flame height and the fuel flow rate for a given fuel and oxidant. The same conclusion is drawn from the theoretical analysis of the approximate model. In addition. ethanol addition makes the average OH signal intensity of flame at different fuel flow rate tend to be consistent and the fuel flow rate enlarge where the flame field shows the strongest oxidation performance. Average OH signal intensity begins to weaken at larger fuel flow rate, which indicates that fuel flow rate of fuels blended with ethanol can change in larger range and does not significantly affect the uniformity of combustion.

Electrochemical degradation of acidified reed pulp black liquor with three-dimensional electrode reactor

The electrochemical degradation of reed pulp black liquor containing lignin pretreated by acidification method was investigated using a three-dimensional electrode reactor. Using activated carbon as particle electrode, the effects of p H value, reaction temperature, electrolysis time and current on residual concentration of total organic carbon(TOC) were discussed in detail. The optimal conditions were obtained: pH 2.5, influent flow rate of 200 mL/min, 25 °C, 300 mA and 2h of electrolysis time, and the removal efficiency of TOC maintains at 35.57 %. The results of the electrochemical method indicate that ·OH radicals are produced in activated carbon anode in the electrolysis process and then adsorbed on the activated carbon surface. Microcell consists of ·OH radicals and the absorbed lignin. With the microcell reaction, the lignin is degraded, while the anodic polarized curve illustrates that the lignin is obviously oxidized in the anode. The contributions of direct and indirect electrolyses to the TOC removal ratio are about 50%, respectively.