Degradation of toluene by tube-tube coaxial dielectric barrier discharge: power characteristics and power factor optimization.

In this paper, the power characteristics and power factor optimization were investigated in a coaxial tube-tube dielectric barrier discharge (DBD) reactor. The effects of several parameters, including discharge voltage, discharge length, discharge frequency and gas flow rate on discharge power and power factor have been evaluated. The experiment results showed that higher discharge power can be obtained by increasing the discharge voltage, discharge frequency and electrode length. But for the power factor, with the increase of discharge frequency, the power factor increased firstly and then decreased. Moreover, with the discharge length increased, the discharge frequency when the power factor reached the maximum value reduced. The response surface method (RSM) and artificial neural network (ANN) were used to optimize the power factor, and their results were relatively consistent. The result of the ANN showed that when discharge voltage was 9.58 kV, discharge frequency was 8.69 kHz, discharge length was 15.8 cm, and gas flow rate was 1.5 L/min, the power factor reached the maximum value of 0.362. The degradation experiment of toluene was carried out in the reactor and its degradation effect was analyzed. The toluene degradation rate is positively correlated with the power factor, and the discharge voltage, gas flow rate and initial concentration are also the key parameters to determine the degradation of toluene. When the discharge voltage, gas flow rate, and initial concentration are 10 kV, 70 mL/min, and 50 ppm, respectively, the power factor and toluene degradation rate reach 0.34 and 74.3%.

p-Nitrophenol contaminated soil remediation in a spray-type coaxial cylindrical dielectric barrier discharge plasma system.

In the present work, plasma remediation of p-nitrophenol (PNP) contaminated soil was performed in a novel spray-type coaxial cylindrical dielectric barrier discharge (DBD) system at ambient temperature. This system is capable of generating large-size nonthermal plasma (NTP) and improving the diffusion and transfer of chemical active species around the dispersed soil particles. Several key parameters including plasma treatment time, discharge voltage, soil granular size, the entry speed of soil, PNP initial concentration, gas variety, and gas flow rate were investigated in terms of PNP degradation and energy efficiencies. Under the optimized experimental conditions, 54.2% of PNP was degraded after only 50 s discharge treatment, indicating that the spray-type coaxial cylindrical DBD system can degrade organic pollutants in soil more quickly compared to other plasma systems due to its efficient transfer of reactive oxygen and nitrogen species (RONS) into the contaminated soil. The possible PNP degradation pathways were proposed based on intermediates identification results and the role of reactive species analysis. The toxicological assessment of the PNP decomposition products was conducted by quantitative structure-activity relationship (QASR) analysis. This work is expected to provide a potential plasma technology for rapid and efficient processing of industrial organic pollutants contamination soil.

The post plasma-catalytic decomposition of toluene over K-modified OMS-2 catalysts at ambient temperature: Effect of K+ loading amount and reaction mechanism.

The present work is devoted to study the post plasma-catalytic (PPC) degradation of toluene using packed-bed discharge (PBD) plasma over K-modified manganese oxide octahedral molecular sieve (OMS-2) catalysts at ambient temperature. Compared to plasma alone, PPC can significantly improve the toluene degradation and mineralization performance simultaneously, and the generation of discharge byproducts and organic intermediates is suppressed. The catalytic capacity of OMS-2 for toluene degradation is greatly promoted by tuning potassium ions (K+) content in OMS-2 tunnel, which might be owing to the formation of more surface active oxygen species derived from weak Mn-O bonds, plenty of oxygen vacancies, as well as more superior low-temperature reducibility. Highest toluene degradation efficiency (89.4%) and COx selectivity (88.9%) can be achieved in plasma-catalysis system over K-modified OMS-2 sample with K/Mn molar ratio of 2 at the SIE of 658 J/L. A long-term stability test has also been successfully carried out to evaluate the stability of K-modified OMS-2 with the assistance of plasma. Possible reaction mechanism for plasma-catalytic degradation of toluene on K-modified OMS-2 catalyst has been proposed based on the plasma diagnosis, catalysts characterization, and organic intermediates identification. This work aims to gaina deeperunderstandingof plasma-catalytic degradation mechanism and provides an environmentally friendly and energy-efficient method for practical volatile organic compounds (VOCs) abatement in PPC process.

Degradation of toluene by pulse-modulated multistage DBD plasma: Key parameters optimization through response surface methodology (RSM) and degradation pathway analysis.

In the present work, a pulse-modulated high-frequency (HF) dielectric barrier discharge (DBD) plasma has been employed and utilized to evaluate the feasibility of toluene degradation in a multistage rod-type reactor at room temperature. Experimental result indicates that the energy consumption is significantly reduced and heating effect can be effectively suppressed when the DBD plasma is ignited in pulse-modulated mode instead of continuous mode. The response surface methodology (RSM) based on central composite design (CCD) model has been proposed to evaluate the contribution of key operating parameters including duty cycle and modulation frequency. The proposed model offers a good fit for actal data. The contribution of the modulation frequency is observed to be more dominant compared to the duty cycle for both the degradation efficiency and the energy yield. According to the results provided by the proposed model, the toluene degradation efficiency of 62.9 % and the energy yield of 0.90 g/kWh are obtained under the optimal conditions of 400 Hz modulation frequency and 56 % duty cycle. The effect of initial toluene concentration and gas flow rate have also been investigated. Increasing toluene initial concentration and gas flow rate are found to be unfavorable for the degradation of toluene, however, which are of benefit to the energy yield. A long-time experiment to assess the stability of pulse-modulated DBD has been successful performed. The possible pathways in plasma degradation of toluene is proposed based on the intermediates identification using GC-MS and FTIR.

Characterization of highly effective plasma-treated g-C3N4 and application to the photocatalytic H2O2 production.

Plasma treated g-C3N4 (PT-g-C3N4) was obtained by a simple and rapid DBD plasma modification process on the pristine g-C3N4. Compared with the pristine g-C3N4, the grain size of the PT-g-C3N4 decreased from 99.2 nm to 57.2 nm, the specific surface area and the pore volume increased by 15% and 33.8%, respectively. Oxygen-containing groups such as -NO2 and -COOH were observed to form on the surface of PT-g-C3N4 so the hydrophilic property of PT-g-C3N4 was much higher than that of pristine g-C3N4. More importantly, the photocatalytic H2O2 production activity of PT-g-C3N4 was significantly improved on account of the treatment in plasma atmosphere for only 5 min, the H2O2 yield of which was about 13 times that of the pristine g-C3N4. Our finding is not only of great significance for effectively promoting the production of H2O2 under mild conditions, but also proposes an innovative DBD plasma method to modify the g-C3N4 photocatalyst, which effectively promotes the improvement of photocatalytic activity and provides valuable insights for catalyst modification studies.

Pulsed discharge plasma induced WO3 catalysis for synergetic degradation of ciprofloxacin in water: Synergetic mechanism and degradation pathway.

Pulsed discharge plasma (PDP) was adopted to induce WO3 for synergetic degradation of ciprofloxacin (CIP) in water. WO3 was firstly characterized by scanning electron microscope (SEM), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET), X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), UV-Vis diffuse reflectance spectroscopy (DRS), Photocurrents and Photoluminescence. The degradation results showed that PDP could induce WO3 photocatalysis successfully, and a synergetic effect was established in PDP/WO3 system. After 60 min treatment time, 0.16 g/L WO3 increased the CIP removal from 71.3% to 99.6%, with the enhancement of the first-order kinetic constant from 0.020 min-1 to 0.081 min-1. Then, the effect of peak voltage, air flow rate and pH on CIP removal was evaluated. Active species trapping test verified that ·OH and ·O2- played the major role for plasma-degradation of CIP degradation, whereas OH and h+ were conductive to catalytic degrade CIP. WO3 addition lead to the decline of O3 and enhancement of OH no matter in deionized water or CIP solution. The degradation process was explored using fluorescence spectrograph, liquid chromatography-mass spectrometry (LC-MS) and ion chromatography (IC). Finally, the possible degradation pathways of CIP degradation were proposed. The reuse test suggested WO3 possessed excellent catalytic performance as well as good stability.

Degradation of antibiotic chloramphenicol in water by pulsed discharge plasma combined with TiO2/WO3 composites: mechanism and degradation pathway.

Pulsed discharge plasma (PDP) combined with TiO2/WO3 composites for chloramphenicol (CAP) degradation was investigated. The prepared TiO2/WO3 composites were characterized by scanning electron microscope, transmission electron microscope, nitrogen adsorption apparatus, zeta sizer, X-ray diffraction, Raman spectra, UV-Vis absorption spectroscopy, X-ray photoelectron spectroscopy, photocurrent and electrochemical impedance spectroscopy. The degradation performance showed that the addition of TiO2/WO3 composites significantly enhanced the removal efficiency of CAP in PDP system. At a peak voltage of 18 kV, the highest removal efficiency of CAP could reach 88.1% in PDP system with 4 wt% TiO2/WO3, which was 36.8% and 26.0% higher than that in sole PDP system and PDP/TiO2 system, respectively. The TiO2/WO3 composites significantly accelerated interfacial charge transfer process compared to the pure TiO2. Besides, the effect of catalyst dosage and peak voltage on CAP removal was evaluated. OH, O3O2-, h+ and high-energy electrons contributed to CAP degradation in PDP-TiO2/WO3 system. Addition of TiO2/WO3 composites can decompose O3 and produce more OH and H2O2. The degradation intermediates were measured by liquid chromatography-mass spectrometry (LC-MS) and ion chromatography (IC). The cycling degradation experiment showed that the TiO2/WO3 composites have good reusability as well as stability.

Plasma-catalytic destruction of xylene over Ag-Mn mixed oxides in a pulsed sliding discharge reactor.

Plasma-assisted catalytic degradation of xylene was performed in a pulsed sliding dielectric barrier discharge (SLDBD) reactor based on three-electrode geometry over Ag-Mn bimetallic oxides catalysts at room temperature. Experimental results showed that more active species were distributed uniformly in the SLDBD plasma than traditional surface dielectric barrier discharge (SDBD), contributing to higher degradation and energy performance. The xylene degradation efficiency and energy yield in the SLDBD reactor driven by both +pulse (+18 kV) and -DC (-10 kV) were 40% and 2.3 g/kWh higher, respectively, than in the SDBD reactor energized by +pulse alone. The combination of SLDBD plasma with catalysts significantly improved the xylene degradation efficiency and CO2 selectivity than the plasma-only system. The incorporation of Ag into Mn oxide further enhanced its catalytic activity for xylene degradation, and the catalytic activity of Ag-Mn oxides was closely correlated with the Ag/Mn molar ratio. Ag-Mn/γ-Al2O3 (1:2) presented the best performance in plasma-catalysis process, with 91.5% of degradation efficiency and 80.1% of CO2 selectivity at 4.6 W. The higher proportion of surface Oads and better reducibility through the interaction between Ag and Mn species can explain the excellent reactivity of Ag-Mn/γ-Al2O3 (1:2).

Synergistic degradation of trans-ferulic acid in aqueous solution by dielectric barrier discharge plasma combined with ozone.

Trans-ferulic acid (FA), extensively used in pharmaceutical and olive oil industries, causes huge risks to ecological environment due to its biotoxicity and phytotoxicity, leading to the difficulty of biochemical processes in treating FA wastewater. In this study, synergistic degradation of FA via dielectric barrier discharge (DBD) plasma and O3 (plasma-ozone) was studied. The results showed that FA degradation efficiency reached 96.9% after a 40-min treatment by plasma-ozone process, and the energy efficiency of FA degradation was increased by 62.5 and 24.5% compared to single DBD plasma and ozonation treatment. Moreover, FA degradation rate constant in plasma-ozone process was 41% higher compared with the sum of single DBD plasma and ozonation, indicating a significant synergistic effect. Radical diagnosis experiments reveal that a profound increase of ·OH yield through peroxone (H2O2/O3) and UV/O3 pathways is the important mechanism of synergistic degradation of FA in plasma-ozone process, while eaq- played little role in FA degradation. A degradation pathway of FA by plasma-ozone was also proposed according to the detected intermediates from EEM and LC-MS. This work revealed that plasma-ozone process is an alternative process for FA treatment, and the findings are helpful for understanding FA degradation characteristics and synergistic mechanisms in plasma-ozone process.

Optical characteristics of the filamentary and diffuse modes in surface dielectric barrier discharge.

Surface dielectric barrier discharge (DBD) plasmas generally exhibits filamentary and diffuse discharges at atmospheric air. The focus of this investigation is on the different optical characteristics and quantitative research about morphological features of two discharge modes. The temporally and spatially resolved characteristics of discharge phenomenon together with the gas temperature are presented with microsecond time scale. Discharge area is estimated by the sum of pixels that equal to "1" in MATLAB software. The formation of diffuse plasma mainly depends on an increase of the ionization coefficient and a creation of sufficient seed electrons by the Penning effect at low electric fields. Accordingly, experimental measurements show that diffuse discharge during the negative half cycle has good uniformity and stability compared with filamentary discharge during the positive half cycle. The rotational temperatures of plasma are determined by comparing the experimental spectra with the simulated spectra that have been investigated. The plasma gas temperature keeps almost constant in the filamentary discharge phase and subsequently increased by about 115K during the diffuse discharge. In addition, it is shown to be nearly identical in the axial direction. Non-uniform temperature distribution can be observed in the radial direction with large fluctuations. The plasma length is demonstrated almost the same between two discharge modes.

Performance evaluation of non-thermal plasma injection for elemental mercury oxidation in a simulated flue gas.

The use of non-thermal plasma (NTP) injection approach to oxidize elemental mercury (Hg(0)) in simulated flue gas at 110°C was studied, where a surface discharge plasma reactor (SDPR) inserted in the simulated flue duct was used to generate and inject active species into the flue gas. Approximately 81% of the Hg(0) was oxidized and 20.5µgkJ(-1) of energy yield was obtained at a rate of 3.9JL(-1). A maximal Hg(0) oxidation efficiency was found with a change in the NTP injection air flow rate. A high Hg(0) oxidation efficiency was observed in the mixed flue gas that included O2, H2O, SO2, NO and HCl. Chemical and physical processes (e.g., ozone, N2 metastable states and UV-light) were found to contribute to Hg(0) oxidation, with ozone playing a dominant role. The deposited mercury species on the internal surface of the flue duct was analyzed using X-ray photoelectron spectroscopy (XPS) and electronic probe microanalysis (EPMA), and the deposit was identified as HgO. The mercury species is thought to primarily exist in the form of HgO(s) by adhering to the suspended aerosols in the gas-phase.

Effects of electrode geometry on the performance of dielectric barrier/packed-bed discharge plasmas in benzene degradation.

In this study, the effects of electrode geometry on benzene degradation in a dielectric barrier/packed-bed discharge plasma reactor with different electrodes were systematically investigated. Three electrodes were employed in the experiments, these were coil, bolt, and rod geometries. The reactor using the coil electrode showed better performance in reducing the dielectric loss in the barrier compared to that using the bolt or rod electrodes. In the case of the coil electrode, both the benzene degradation efficiency and energy yield were higher than those for the other electrodes, which can be attributed to the increased role of surface mediated reactions. Irrespective of the electrode geometry, the packed-bed discharge plasma was superior to the dielectric barrier discharge plasma in benzene degradation at any specific applied voltage. The main gaseous products of benzene degradation were CO, CO2, H2O, and formic acid. Discharge products such as O3, N2O, N2O5, and HNO3 were also detected in the outlet gas. Moreover, the presence of benzene inhibited the formation of ozone because of the competing reaction of oxygen atoms with benzene. This study is expected to offer an optimized approach combining dielectric barrier discharge and packed-bed discharge to improve the degradation of gaseous pollutants.

Innovative approach for benzene degradation using hybrid surface/packed-bed discharge plasmas.

An innovative plasma reactor, which generates hybrid surface/packed-bed discharge (HSPBD) plasmas, was employed for the degradation of benzene. The HSPBD reactor was found to display remarkably better benzene degradation, mineralization, and energy performance than surface or packed-bed discharge reactors alone. The degradation efficiency, CO2 selectivity, and energy yield in the HSPBD reactor were 21%, 11%, and 3.9 g kWh-1 higher, respectively, than in a surface discharge reactor and 30%, 21%, and 5.5 g kWh-1 higher, respectively, than in a packed-bed discharge reactor operated at 280 J L-1. Particularly, the benzene degradation in the HSPBD reactor exhibited an unambiguous synergistic enhancement rather than a simple additive effect using the surface discharge and packed-bed discharge reactors. Moreover, in the HSPBD reactor, the formation of byproducts, such as NO2, was suppressed, while O3 was promoted. The use of N2 as the carrier gas was found to be effective for benzene degradation because of the fast reaction rate of N2(A3∑u+) with benzene, and oxygen species derived from the dissociation of O2 were found to be significant in the mineralization process. Thus, the addition of O2 to N2 allows for efficient degradation of benzene, and the optimized amount of O2 was determined to be 3%.

[Emission spectra of hydroxyl radical generated in air corona discharge].

In this paper, the relative emission intensity of the 309 nm transition band of hydroxyl radical (OH) was measured by a CCD imaging spectrometer in a pin-plane corona discharge scheme of one atmosphere pressure air injected with unsaturated water vapor from the central hole of the used pins. The influences of several factors on the OH radical production were investigated by relative emission intensity measurement. The production of OH radical increased with a limited increment of water vapor concentration in the mixed gas. Compared with positive DC corona discharges, more OH radicals were generated in positive pulsed corona discharges and less in negative DC corona discharges. The spatial distribution of OH radical production was also observed. Most OH radicals were produced within the range of 5 mm off the discharge pin electrode. In conclusion, this means of optical emission spectroscopy, compared with more sophisticated laser fluorescence measurements used for plasma OH production diagnostics investigation, is simpler and more effective for characterizing the OH radical potential for pollutant oxidation.