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.

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.

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).

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...

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.

The splitting of low-lying states for hydroxyl molecule under spin orbit coupling

The splitting of potential energy curves for the states X^2Ⅱ3/2, ^2Ⅱ1/2 and A^2∑＋ of hydroxyl OH under spin-orbit coupling （SOC） has been calculated by using the SO multi-configuration quasi-degenerate perturbation theory （SO- MCQDPT）. Their Murrell Sorbic （M-S） potential functions have been derived, then, the spectroscopic constants for X^2Ⅱ3/2, ^2Ⅱ1/2 and A^2∑＋ have been derived from the M-S function. The calculated dissociation energies for the three states are Do[OH（X^2Ⅱ3/2）]=34966,632cm^-1, Do[OH（^2Ⅱ1/2）]=34922.802cm^-1, and Do[OH（A^2∑ ）]=17469.794cm^-1, respectively. The vertical excitation energy u[^2Ⅱ1/2（v = 0） → X^2Ⅱ3/2（v = 0）] = 139.6cm^-1. All the spectroscopic data for the X^2Ⅱ3/2 and ^2Ⅱ1/2 are given for the first time except the dissociation energy of X^2Ⅱ3/2.

Photooxidation of Methacrolein in Fe(III)-Oxalate Aqueous System and Its Atmospheric Implication

Iron and oxalic acids are widely distributed in the atmosphere and easily form ferric oxalate complex(Fe(III)-Ox).The tropospheric aqueous-phase could provide a medium to enable the photo-Fenton reaction with Fe(III)-Ox under solar irradiation.Although the photolysis mechanisms of Fe(III)-Ox have been investigated extensively,information about the oxidation of volatile organic compounds(VOC),specifically the potential for Secondary Organic Aerosol(SOA)formation in the Fe(III)-Ox system,is lacking.In this study,a ubiquitous VOC methacrolein(MACR)is chosen as a model VOC,and the oxidation of MACR with Fe(III)-Ox is investigated under typical atmospheric water conditions.The effects of oxalate concentration,Fe(III)concentration,MACR concentration,and pH on the oxidation of MACR are studied in detail.Results show that the oxidation rate of MACR greatly accelerates in the presence of oxalate when compared with only Fe(III).The oxidation rate of MACR also accelerates with increasing concentration of oxalate.The effect of Fe(III)is found to be more complicated.The oxidation rate of MACR first increases and then decreases with increasing Fe(III)concentration.The oxidation rate of MACR increases monotonically with decreasing pH in the common atmospheric water pH range or with decreasing MACR concentration.The production of ferrous and hydrogen peroxide,pH,and aqueous absorbance are monitored throughout the reaction process.The quenching experiments verify that·OH and O_(2)^(+)are both responsible for the oxidation of MACR.MACR is found to rapidly oxidize into small organic acids with higher boiling points and oligomers with higher molecular weight,which contributes to the yield of SOA.These results suggest that Fe(III)-Ox plays an important role in atmospheric oxidation.

Colorimetric quantification of aqueous hydrogen peroxide in the DC plasma-liquid system

The quantification of hydrogen peroxide(H_(2)O_(2))generated in the plasma-liquid interactions is of great importance,since the H_(2)O_(2)species is vital for the applications of the plasma-liquid system.Herein,we report on in situ quantification of the aqueous H_(2)O_(2)(H_(2)O_(2)aq)using a colorimetric method for the DC plasma-liquid systems with liquid as either a cathode or an anode.The results show that the H_(2)O_(2)aqyield is 8-12 times larger when the liquid acts as a cathode than when the liquid acts as an anode.The conversion rate of the gaseous OH radicals to H_(2)O_(2)aqis 4-6 times greater in the former case.However,the concentrations of dissolved OH radicals for both liquid as cathode and anode are of the same order of tens of n M.

Heterogeneous kinetics of the OH-initiated degradation of fenthion and parathion

Fenthion and parathion are two representative kinds of organophosphorus pesticides and widely used in agriculture.They are directly or indirectly released into the atmosphere by spraying or volatilization processes.However,their heterogeneous reactivity toward OH radicals has not yet been well understood.Therefore,this work investigated the heterogeneous kinetics of the OH-initiated degradation of surface-bound fenthion and parathion using a flow reactor.The results showed that OH radicals played an important role in the atmospheric degradation of fenthion and parathion.Their average rate constants were(7.20±0.77)×10^(-12) and(10.40±0.60)×10^(−12) cm^(3)/(mol·sec)at a relative humidity(RH)and temperature of 35% and 20℃,respectively,suggesting that they have relatively short lifetimes in the atmosphere.In addition,a negative RH dependence and a positive temperature dependence of the rate constants were observed.The Arrhenius expressions of fenthion and parathion were k_(2)=(1.34±0.48)×10^(-9) exp[-(1432.59±105.29)/T]and k_(2)=(1.96±1.38)×10^(-9) exp[-(1619.98±222.02)/T],respectively,and their overall activation energy was estimated to be(11.88±0.87)and(13.48±1.83)kJ/mol.The experimental results will update the kinetic data of fenthion and parathion in the atmosphere and be helpful to further understand their atmospheric transportation processes.

Time-Effect Relationship of Toxicity Induced by Roundup and Its Main Constituents in Liver of Carassius Auratus

In order to evaluate the eco-toxicological effects of Roundup&reg on Carassius auratus (C. auratus), fish were exposed to 32 μg/L Roundup&reg, isopropylamine salt of glyphosate (G.I.S) and polyoxyethylene amine (POEA) over different periods (0.5, 1, 3, 7 and 14 d). Hydroxyl radical (·OH), malondialdehyde (MDA) and acetylcholinesterase (AChE) in liver were detected in this study. Results showed that the generation of ·OH increased before 7 d, but without significantly difference. ·OH was induced at 1 d for POEA group, 3 d for Roundup&reg group and 7 d for G.I.S group. At 14 d, ·OH generation returned to normal levels. MDA contents all increased significantly (p < 0.01) during 7 days and then reached a normal level at 14 d. AChE activity in all group tests revealed a significant inhibition (p < 0.01) after 7 days exposure and then rebounded a little, but remained below the control after 14 days exposure. The rate of AChE inhibition range from 13% - 42% in Roundup&reg, 6% - 40% in G.I.S, and 21% - 54% in POEA, suggesting that POEA was more toxic compared to Roundup&reg and G.I.S. 32 μg/L Roundup&reg exposure led to the change of physiological and biochemical indexes in C. auratus, which was a reversible process in the long run.

A hydroxyl radical detection system using gas expansion and fast gating laser-induced fluorescence techniques

An OH radical measurement instrument based on Fluorescence Assay by Gas Expansion（FAGE）has been developed in our laboratory.Ambient air is introduced into a low-pressure fluorescence cell through a pinhole aperture and irradiated by a dye laser at a high repetition rate of 8.5 k Hz.The OH radical is both excited and detected at 308 nm using A-X（0,0）band.To satisfy the high efficiency needs of fluorescence collection and detection,a 4-lens optical system and a self-designed gated photomultiplier（PMT）is used,and gating is actualized by switching the voltage applied on the PMT dynodes.A micro channel photomultiplier（MCP）is also prepared for fluorescence detection.Then the weak signal is accumulated by a photon counter in a specific timing.The OH radical excitation spectrum range in the wavelength of 307.82–308.2 nm is detected and the excited line for OH detection is determined to be Q1（2）line.The calibration of the FAGE system is researched by using simultaneous photolysis of H2O and O2.The minimum detection limit of the instrument using gated PMT is determined to be 9.4×10~5molecules/cm^3,and the sensitivity is 9.5×10^（-7）cps/（OH·cm^（-3））,with a signal-to-noise ratio of 2 and an integration time of 60 sec,while OH detection limit and the detection sensitivity using MCP is calculated to be 1.6×10~5molecules/cm^3and 2.3×10^（-6）cps/（OH·cm^（-3））.The laboratory OH radical measurement is carried out and results show that the proposed system can be used for atmospheric OH radical measurement.

Unexpected fast radical production emerges in cool seasons:implications for ozone pollution control

Ozone is a crucial air pollutant that damages human health and vegetation.As it is related to the photo-oxidation of the nitrogen oxides and volatile organic compounds,the summertime reduction of these precursors is the primary focus of current ozone mitigation strategies.During ozone pollution episodes in eastern China,an observed accumulation of daily total oxidants(O_(x)=NO_(2)+O_(3))in cool seasons(spring and autumn:60 ppb and winter 40 ppb)is comparable to that in summer(60 ppb),indicating fast photochemical production of secondary pollutants including ozone over the year.Unrecognized fast radical primary productions are found to counteract the increased termination of hydroxyl radical and unfavorable meteorological conditions to maintain the rapid total oxidant formations in cool seasons.Elucidating and regulating the primary radical sources may be critical for the secondary air pollution control in cool seasons.

Hydroxyl Radical Induced Apoptosis Is Closely Related to Changes in Cellular Energy/Redox Metabolism

Reactive oxygen species (ROS), including the hydroxyl radical ( · OH), are known to be potential modulators of apoptosis. However, the biochemical mechanisms underlying apoptosis induced by ·OH, namely how the radical induces a cell to die, are poorly understood. The present work highlights the changes of the energy/redox status during apoptosis by exogenous ·OH treatment. HeLa cells were induced to undergo typical apoptosis by ·OH generated directly via the Fe 2+ mediated Fenton reaction. The thermodynamics study indicated that the ·OH treatment increased the cellular heat output in the first hours, and then the cellular thermodynamics shifted to endothermic. The data demonstrates that the mitochondria are actively involved in ·OH treatment induced apoptosis, with the cellular oxygen consumption rapidly decreasing after the ·OH treatment for only 0.5 h. But DNA fragmentation, which is the major characteristic of apoptosis, took place 16 h after ·OH treatment. The results suggest that alteration of the energy/redox metabolism and the energy/redox status may be the primary causes among the early events of ·OH induced apoptosis.