Enhancement of Phenol Degradation by Electron Acceptors in Anodic Contact Glow Discharge Electrolysis

Effects of several electron acceptors (Fe³⁺, Cu²⁺, Cr(VI), and H₂O₂) on phenol degradation in anodic contact glow discharge electrolysis have been investigated. Results show that the electron acceptors have positive effects on phenol removal, with the sequence of Fe³⁺?>?Cr(VI)?>?H₂O₂?>?Cu²⁺. Under conditions of voltage 500?V and current 100?mA, 100?mg/L phenol can be removed with 10?min of discharge treatment in the presence of 1.0?mmol/L Fe³⁺, while without any additive only 35?% of phenol is removed in 30?min. The mechanism of the degradation enhancement was discussed based on the reactions taking place in the presence of the different additives.     

Application of Double-Dielectric Barrier Discharge Plasma for Removal of Pentachlorophenol from Wastewater Coupling with Activated Carbon Adsorption and Simultaneous Regeneration

Wastewater containing pentachlorophenol (PCP) was treated by granular activated carbon (GAC) adsorption and double-dielectric barrier discharge (D-DBD) plasma. A packed bed D-DBD reactor was applied for removal of PCP on GAC and GAC regeneration, where the discharge gap was filled with GAC. PCP degradation efficiency of 65% and GAC regeneration efficiency (RE) of 87% were achieved. Effects of discharge power, treatment time and O₂ flow rate on PCP degradation and GAC regeneration were investigated. Increasing discharge power, treatment time and O₂ flow rate were favorable for PCP degradation, and also contributed to GAC RE. C–Cl bonds in PCP were cleaved by D-DBD plasma. Effect of D-DBD plasma on physical and chemical properties of GAC during GAC regeneration process was characterized by N₂ adsorption and Boehm titration. This study is expected to demonstrate the feasibility of applying D-DBD plasma for efficient organic wastewater treatment by coupling with GAC adsorption.     

Synthesis of Fe3O4/CuO/ZnO/RGO and its catalytic degradation of dye wastewater using dielectric barrier discharge plasma

The design of an efficient and green dye degradation technology is of great significance to mitigate water pollution as well as ecological damage. Fe₃O₄/CuO/ZnO/RGO was prepared by solvothermal synthesis and homogeneous precipitation. X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), transmission electron microscopy (TEM), and vibrating-sample magnetometry (VSM) were used to characterize the samples, to explore the morphology and structural composition of the composites. To enhance the degradation efficiency, a dielectric barrier discharge (DBD)–Fe₃O₄/CuO/ZnO/RGO co-catalytic system was created based on the DBD plasma technology. Response surface methodology analysis results demonstrate that the degradation effect of DBD–Fe₃O₄/CuO/ZnO/RGO is optimal and the decolorization rate is 95.06 % when the solution pH is 3, conductivity is 0.5 mS/cm, the input voltage is 90 V, and Fe₃O₄/CuO/ZnO/RGO concentration is 0.18 g/L. Therefore, this study offers a novel method for dye degradation and confirms the viability of a DBD–Fe₃O₄/CuO/ZnO/RGO synergistic catalytic system.     

Degradation of organic dyes in water by electrical discharges

The degradation of several organic compounds in aqueous solution: methyl yellow (C₁₄H₁₅N₃), methyl red (C₁₅H₁₅N₃O₂), methyl orange (C₁₄H₁₄N₃NaO₃S), phenol red (C₁₉H₁₄O₅S) and methylene blue (C₁₆H₁₈ClN₃S), was investigated in a pulsed corona discharge. High voltage pulses of 17 kV amplitude, 24 ns rise time and approximately 200 ns duration (full width at half maximum––FWHM) were applied to an array of six hollow needles. When oxygen was bubbled into the solution through the needle, current pulses of 90 A amplitude were obtained and the average power dissipated in the discharge was 19 W at 100 Hz pulse repetition rate. UV-visible spectra of the aqueous solutions show a significant reduction of the absorbance in the visible range, up to 90%, after plasma treatment, suggesting fragmentation of the compounds. Several aliphatic compounds were identified as oxidation products of methyl yellow, methyl red and methyl orange. For methylene blue and phenol red aromatic reaction products were detected as well.     

Product analysis and mechanism of toluene degradation by low temperature plasma with single dielectric barrier discharge

A single dielectric barrier discharge (DBD) low-temperature plasma reactor was set up, and toluene was selected as the representative substance for volatile organic compounds (VOCs), to study the reaction products and degradation mechanism of VOCs degradation by low-temperature plasma. Different parameters effect on the concentration of O₃ and NO_x during the degradation of toluene were studied. The exhaust in the process of toluene degradation was continuously detected and analyzed, and the degradation mechanism of toluene was explored. The results showed that the concentration of O₃ increased with the increase of the power density and discharge voltage of the plasma device. However, as the initial concentration of toluene increased, the concentration of O₃ basically keep steady. The concentration of NO_x in the by-products increased with the discharge voltage, power density, and initial concentration of toluene in the plasma device, and the concentration of NO₂ was much higher than the concentration of NO. The degradation process of toluene was detected and analyzed. The results showed that the degradation mechanism of toluene by plasma includes high energy electron bombardment reaction, active radical reaction and ion molecule reaction. Among them, the effect of high-energy electrons on toluene degradation is the largest, followed by the effect of free radicals, in which oxygen radicals participated in the reaction mainly through the formation of C–O bond, CO bond, (CO)–O– bond and –OH radical, while nitrogen radicals participate in the reaction mainly through the formation of C–NH₂, (CNH)- bond, CN bond and C–NO₂ bond. The results can provide some data supports for the study of low-temperature plasma degradation of VOCs.     

Removal of Model Pollutants in Aqueous Solution by Gliding Arc Discharge: Determination of Removal Mechanisms. Part I: Experimental Study

The respective roles of short and long-life oxidant species in the degradation of model organic pollutants in water have been investigated in a gas–liquid gliding arc plasma reactor. Three different model pollutants were treated in two configurations: direct discharge mode and spatial post discharge mode. In each case the pollutants were classified according to their ease of removal, from easier to more difficult to remove. The results were as follows: phenol >> 1-heptanol >> pCBA. The removal mechanisms also are different depending on the characteristics of the pollutant treated. Phenol (100 % of phenol was removed for energy density = 1.20 × 10⁵ J/L) was supposed to react strongly with NO₂° radicals produced by the dissociation of N₂O₄ in liquid phase. The degradation of 1-heptanol would proceed by desorption of the liquid phase to the gas phase, where oxidation occurs due to the plasma active short-lived species. In the case of pCBA, oxidation occurs in the liquid solution, but the degradation is low because of its low reactivity with species such as ozone and °NO₂ and insufficient production of OH° radicals in the solution.     

Sustainable activation of peroxymonosulfate by the Mo(IV) in MoS2 for the remediation of aromatic organic pollutants

Although MoS₂ has been proved to be a very ideal cocatalyst in advanced oxidation process (AOPs), the activation process of peroxymonosulfate (PMS) is still inseparable from metal ions which inevitably brings the risk of secondary pollution and it is not conducive to large-scale industrial application. In this study, the commercial MoS₂, as a durable and efficient catalyst, was used for directly activating PMS to degrade aromatic organic pollutant. The commercial MoS₂/PMS catalytic system demonstrated excellent removal efficiency of phenol and the total organic carbon (TOC) residual rate reach to 25%. The degradation rate was significantly reduced if the used MoS₂ was directly carried out the next cycle experiment without any post-treatment. Interestingly, the commercial MoS₂ after post-treated with H₂O₂ can exhibit good stability and recyclability for cyclic degradation of phenol. Furthermore, the mechanism for the activation of PMS had been investigated by density functional theory (DFT) calculation. The renewable Mo⁴⁺ exposed on the surface of MoS₂ was deduced as the primary active site, which realized the direct activation of PMS and avoided secondary pollution. Taking into account the reaction cost and efficient activity, the development of commercial MoS₂ catalytic system is expected to be applied in industrial wastewater.     

辉光放电低温等离子体分解乙醇水溶液制氢

在辉光放电分解乙醇制氢过程中, 高能电子在反应中起到了最为关键的作用, 非法拉第效应使得电流效率获得大幅度提升, 产物产量远远高于理论产量. 本文研究了乙醇水溶液辉光放电等离子体电解制氢的过程. 实验研究发现, 辉光放电分解乙醇水溶液的产物主要以H₂和CO为主, 还有少量的C₂H₄、CH₄、O₂和C₂H₆. H₂体积分数能达到59%以上, CO为20%左右. 通过对影响辉光放电的因素进行实验后发现: 乙醇体积分数的大小不会影响辉光放电的伏安特性参数; 电导率的提高会使‘Kellogg 区’收窄, 同时使放电尽快进入辉光放电. 此外, 乙醇体积分数越高H₂体积分数越低, 产气速率在乙醇体积分数为30%和80%附近时达到极大值; 提高放电电压和电导率对辉光放电的影响规律是相类似的, 其实质都是增大了辉光放电加载在等离子鞘层两端的电压,H₂体积分数基本不随二者的变化而变化, 但提高溶液的电导率更有利于减少辉光放电引起的焦耳热.     

4-Chlorophenol Degradation and Hydrogen Peroxide Formation Induced by DC Diaphragm Glow Discharge in an Aqueous Solution

In the present study, formation of hydrogen peroxide (H₂O₂) and degradation of 4-chlorophenol (4-CP) induced by DC diaphragm glow discharge (DGD) in a sodium sulfate solution were investigated. The discharge was generated in a small hole on a quarts plate interposed between two submersed graphite electrodes. Experimental results showed that 750 V was the optimum voltage for H₂O₂ formation and 4-CP degradation. Both the H₂O₂ formation and the 4-CP degradation proceeded faster in cathodic compartment than in anodic compartment. Lowering the solution pH was favorable for 4-CP degradation but showed no appreciable effect on H₂O₂ formation. Addition of hydroxyl radical scavenger (methanol) to the solution decreased the H₂O₂ formation and the 4-CP degradation. Iron species especially ferric ions enhanced the 4-CP degradation markedly. Based on the analyzes of Current–Voltage characteristics and chemical effects, it was deduced that the mechanism of DGD was similar to that of contact glow discharge electrolysis.     

A mass spectrometric analysis of CF4/O2 plasmas: Effect of oxygen concentration and plasma power

A quadrupole mass analyzer was used to detect the neutral products extracted downstream from a CF₄/O₂ RF discharge at 80 mtorr. mtorr. Stable discharge products were investigated as a junction of plasma power, residence time, oxygen concentration, and plasma voltage standing-wave ratio. In general, as plasma power increased from 10 to 200 W, production of CO increased while the measured mole fractions of CO₂ and COF₂ stabilized. The ratio of' CO to CO, decreases! at low plasma powers as the oxygen concentration increased. ,4n increase in the relative conversion of CF₄ to oxygenated products occurred at both low plasma powers and low oxygen concentrations. Chemical mechanisms are suggested to account for these results.     

Pd–Fe/TiO2 catalysts for phenol degradation with in situ generated H2O2

This study deals with the degradation of phenol over Pd–Fe/TiO₂ catalysts at mild conditions in the presence of in situ generated H₂O₂ from oxygen and formic acid. This catalytic system demonstrated interesting ability to oxidize phenol by Fenton process in a one-pot reaction without the addition of ferrous ion. Lower Pd content catalysts, despite producing a higher hydrogen peroxide amount for bulk purposes, did not reach the same efficiency as the 5Pd–5Fe catalyst in phenol degradation. A close interaction between Pd and iron oxide species is necessary to obtain high active catalysts. These results highlight the advantage of in situ generation of H₂O₂, for oxidation reactions with respect to conventional Fenton process.     

Space- and direction-resolved Langmuir probe diagnostic in rf planar discharges

To consider the anisotropy of the plasma in the sheath regions, Langmuir probe characteristics are measured using a direction-resolving technique. The operation frequency, related to the neutral gas density (/p₀), is chosen in such a way that the electron velocity distribution function (VDF) may be regarded as frozen and the probe diagnostic may be performed without time resolution. In order to prevent convolution of the VDF from the effect of the time-dependent plasma potential, the rf component of the probe bias voltage is compensated to a minimum. The plasma potential, the mean energy of the electrons, and the electron density, averaged over the discharge bulk, are presented as functions of the discharge current and the neutral gas pressure for the O₂ gas. By means of a reactor model based on the theory of plane probes and using the plasma parameter measured, the sheath and bulk portions of the maintaining voltage are separated. In this procedure the thickness of the sheaths in front of the electrodes and the phase difference between discharge current and maintaining voltage are also obtained.     

A Novel Dielectric-Barrier-Discharge Loop Reactor for Cyanide Water Treatment

A novel dielectric-barrier-discharge (DBD) loop reactor was designed for the efficient degradation of cyanide anion (CN^?) in water. The circulation of cyanide water as a falling film through plasma gas discharge zone enhanced gas–liquid mass and energy transfer and induced formation of H₂O₂ which was associated with the efficient destruction of CN^?. It was observed that among different discharge gases, the CN^? degradation rate decreased in the order of Ar > air > H₂/air mixture. Depending on discharge voltage, the treatment time for complete removal of 100 ppm CN^? in this DBD loop reactor is in the range 120–300 min. The dose of Cu²⁺ catalyst in combination with in situ production of H₂O₂ enhanced the destruction of CN^? apparently in this DBD loop reactor. The treatment time for complete degradation of 100 ppm CN^? decreased from 180 min with Ar DBD discharge alone to 40 min with 40 mg/L dose of Cu²⁺ ion in water, making it an efficient means to degrade cyanide water.     

Catalytic Properties of TiO<Subscript>2</Subscript>/Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> Nanoparticles in Plasma Chemical Treatment

We proposed here a new process coupling dielectric barrier discharge (DBD) plasma with magnetic photocatalytic material nanoparticles for improving yield in DBD degradation of methyl orange (MO). TiO₂ doped Fe₃O₄ (TiO₂/Fe₃O₄) was prepared by the sol-gel method and used as a new type of magnetic photocatalyst in DBD system. It was found that the introduction of TiO₂/Fe₃O₄ in DBD system could effectively make use of the energy generated in DBD process and improve hydroxyl radical contributed by the main surface Fenton reaction, photocatalytic reaction and catalytic decomposition of dissolved ozone. Most part of MO (88%) was degraded during 30 min at peak voltage of 13 kV and TiO₂/Fe₃O₄ load of 100 mg/L, with a rate constant of 0.0731 min^?1 and a degradation yield of 7.23 g/(kW h). The coupled system showed higher degradation efficiency for MO removal.     

Enhanced degradation of organic pollutant 4-chlorophenol in water by non-thermal plasma process with TiO2

The combined application of TiO₂ photocatalyst and pulsed high-voltage electrical discharge process for the degradation of organic pollutant parachlorophenol (4-CP) in aqueous solution was tentatively investigated. The optimum conditions for 4-CP removal were applied voltage at 14 kV, electrode distance at 2 cm, pH at 6.5 (close to neutral solution), TiO₂ concentration at 50 mg/L, gas source O₂ at 100 L/h, and hybrid corona-streamer discharge mode. Introduced TiO₂ into pulsed discharge plasma process under such optimum condition, the rate constant of 4-CP degradation (k _cp) was greatly promoted, from 1.56×10⁻³ to 2.81×10⁻³ s⁻¹, and energy efficiency for 4-CP removal was greatly enhanced by approximately one time, and it was attributed to more amounts of chemically active species (e.g., ozone and hydrogen peroxide, especially hydroxyl radicals) produced in pulsed discharge plasma process in combination with TiO₂ photocatalyst.     

Hydrogen Peroxide Formation by Electric Discharge with Fine Bubbles

Pulsed discharge plasma is typical oxidation technology for disposing organic compounds in aqueous solutions. When this electrical discharge plasma was applied in water, it may produce hydrogen peroxide (H₂O₂) without any catalyst or chemical agent. In order to increase H₂O₂ production by electrical discharge plasma in water, fine bubbles were introduced into the electrical discharge plasma in this experiment. Bipolar pulsed voltages were applied to cylindrical electrodes in the water while Ar or O₂ bubbles were introduced, generating a pulsed discharge plasma. The introduction of the bubbles seemed to enhance the dissociation of water molecules and increased H₂O₂ formation, especially with O₂ bubbling. Dissolved oxygen in the water contributed to H₂O₂ formation by pulsed discharge plasma with the bubbles, while dissociation of water molecules was the cause of H₂O₂ formation by pulsed discharge plasma without bubbles. More H₂O₂ was formed by pulsed discharge plasma with O₂ bubbles, because the amount of dissolved oxygen in the water increased upon bubbling with O₂.     

Effect of Spark Discharge Plasma on Water,Physiological Saline,and Hanks’ Solution

The effect of the duration of a current pulse of spark discharge in air on the composition of products formed in liquid both by the action of plasma radiation and with the participation of species formed in the discharge itself has been studied. The products formed in water, 0.9% NaCl, and in Hanks’ solution have been determined. It has been that in all the cases, nitrous acid is one of the primary products. The yield of nitrous acid is the same in all the three solutions. With a decrease in the current pulse duration, the influence of the species formed in the discharge itself on the yield of nitrous acid increases. The products formed in water decompose within up to 13 days. Peroxynitrite and N₂O₃ were identified as degradation products.     

Influence of Solution Conductivity on Contact Glow Discharge Electrolysis

Using a thin platinum anode in contact with an electrolytic solution, normal electrolysis develops spontaneously to contact glow discharge electrolysis (CGDE) at sufficiently high voltage. During this transition, midpoint voltage (V_D) is an important critical value. From V_D on, plasma is sustained by direct current glow discharge between the electrode and the electrolyte surface. And H₂O₂ is the main non-faradaic yield. In this study, effects of conductivity on V_D and the concentration of H₂O₂ have been investigated in Na₂SO₄ and NaCl solution. The results indicate that V_D decreases with the increasing conductivity. And the value of V_D and the concentration of H₂O₂ in NaCl solution are less than those in Na₂SO₄ solution. The concentration of H₂O₂ increases steadily and then decreases to maintain a stationary value.     

Destruction of Styrene in an Air Stream by Packed Dielectric Barrier Discharge Reactors

This study presents the decomposition rates of styrene vapors with non-packed and packed bed dielectric barrier discharge reactors. The concentrations of intermediate byproducts at various plasma operation conditions were evaluated. The results showed that although styrene vapors could be almost completely removed at low styrene inlet concentration of 132 ppm, the selectivity of CO₂ as the major product was rather low in a non-packed bed reactor. It was found that solid carbon containing compound was the major byproduct. An increase in the styrene inlet concentration tended to reduce the styrene removal efficiency, it also led to increase in the solid byproduct. The reactors that packed with glass, Al₂O₃ or Pt–Pd /Al₂O₃ pellets could improve the styrene decomposition efficiency and reduce the formation of intermediate products, of which the best oxidation of styrene to CO₂ could be achieved with a Pt–Pd /Al₂O₃ packed bed reactor. The carbon byproducts could also be reduced if the rector length was increased. The concentrations of ozone formed during the plasma process were also evaluated for the non-packed and packed bed reactors. The plasma reactor that packed with Pt–Pd /Al₂O₃ pellets was proved to have the lowest O₃ concentration.     

Influence of Ferroelectric Materials and Catalysts on the Performance of Non-Thermal Plasma (NTP) for the Removal of Air Pollutants

The introduction of ferroelectric and catalytically active materials into the discharge zone of NTP reactors is a promising way to improve their performance for the removal of hazardous substances, especially those appearing in low concentrations. In this study, several coaxial barrier-discharge plasma reactors varying in size and barrier material (glass, Al₂ O₃, and TiO₂) were used. The oxidation of methyl tert-butyl ether (MTBE), toluene and acetone was studied in a gas-phase plasma and in various packed-bed reactors (filled with ferroelectric and catalytically active materials). In the ferroelectric packed-bed reactors, better energy efficiency and CO₂ selectivity were found for the oxidation of the model substances. Studies on the oxidation of a toluene/acetone mixture in air showed an enhanced oxidation of the less reactive acetone related to toluene in the ferroelectric packed-bed reactors. It can be concluded that the change of the electrical discharge behaviour was caused by a larger number of non-selective and highly reactive plasma species formed within the ferroelectric bed. When combining ferroelectric (BaTiO₃) and catalytically active materials (LaCoO₃), only a layered implementation led to synergistic effects utilising both highly energetic species formed in the ferroelectric packed-bed and the potential for total oxidation provided by the catalytically active material in the second part of the packed bed.