The application of strong electric fields in water and organic liquids has been studied for several years, because of its importance in electrical transmission processes and its practical applications in biology, chemistry, and electrochemistry. More recently, liquid-phase electrical discharge reactors have been investigated, and are being developed, for many environmental applications, including drinking water and wastewater treatment, as well as, potentially, for environmentally benign chemical processes. This paper reviews the current status of research on the application of high-voltage electrical discharges for promoting chemical reactions in the aqueous phase, with particular emphasis on applications to water cleaning.

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Electrohydraulic Discharge and Nonthermal Plasma for Water Treatment

There is a continuing need for the development of effective, cheap and environmentally friendly processes for the disinfection and degradation of organic pollutants from water. Ozonation processes are now replacing conventional chlorination processes because ozone is a stronger oxidizing agent and a more effective disinfectant without any side effects. However, the fact that the cost of ozonation processes is higher than chlorination processes is their main disadvantage. In this paper recent developments targeted to make ozonation processes cheaper by improving the efficiency of ozone generation, for example, by incorporation of catalytic packing in the ozone generator, better dispersion of ozone in water and faster conversion of dissolved ozone to free radicals are described. The synthesis of ozone in electrical discharges is discussed. Furthermore, the generation and plasma chemical reactions of several chemically active species, such as H2O2, O•, OH•, HO2•, O3*, N2*, e-, O2-, O-, O2+, etc, which are produced in the electrical discharges are described. Most of these species are stronger oxidizers than ozone. Therefore, water treatment by direct electrical discharges may provide a means to utilize these species in addition to ozone. Much research and development activity has been devoted to achieve these targets in the recent past. An overview of these techniques and important developments that have taken place in this area are discussed. In particular, pulsed corona discharge, dielectric barrier discharge and contact glow discharge electrolysis techniques are being studied for the purpose of cleaning water. The units based on electrical discharges in water or close to the water level are being tested at industrial-scale water treatment plants.}

https://iopscience.iop.org/article/10.1088/0963-0252/10/1/311/pdf

Water purification by electrical discharges

Initiation of apoptosis, or programmed cell death, is an important issue in cancer treatment as cancer cells frequently have acquired the ability to block apoptosis and thus are more resistant to chemotherapeutic drugs. Targeted and perhaps selective destruction of cancerous tissue is desirable for many reasons, ranging from the enhancement of or aid to current medical methods to problems currently lacking a solution, i.e., lung cancer. Demonstrated in this publication is the inactivation (killing) of human Melanoma skin cancer cell lines, in vitro, by Floating Electrode Dielectric Barrier Discharge (FE-DBD) plasma. Not only are these cells shown to be killed immediately by high doses of plasma treatment, but low doses are shown to promote apoptotic behavior as detected by TUNEL staining and subsequent flow cytometry. It is shown that plasma acts on the cells directly and not by “poisoning” the solution surrounding the cells, even through a layer of such solution. Potential mechanisms of interaction of plasma with cells are discussed and further steps are proposed to develop an understanding of such systems.

Floating Electrode Dielectric Barrier Discharge Plasma in Air Promoting Apoptotic Behavior in Melanoma Skin Cancer Cell Lines

Decomposition of toxic organic compounds is a major environmental problem. Increasingly, research is being conducted to use plasmas to decompose these wastes in water, as it is simple, effective and does not require the addition of other chemical agents. Different groups have used varied reactor designs. This study calculates and compares relative energy yields of about 27 major types of plasma reactors. The results reveal dramatic differences in the energy yields, up to five orders of magnitude. The most efficient are pulse powered reactors, in which plasma is formed in gas phase and the waste solution sprayed into it. Factors that account for this improved energy yield are discussed. This study will help narrow down efficient reactors for further studies, development and commercial uses.

Water Purification by Plasmas: Which Reactors are Most Energy Efficient?

Partial electrical discharges in a water solution as a function of conductivity have been studied experimentally. Using needle-plate electrodes it has been demonstrated that the discharge evolves in two phases. During the first streamer-like phase, the discharge propagated with a velocity of 106 cm/s, while during the second arc-like phase the length of the discharge remained almost constant although the current still increased. Higher solution conductivity resulted in the generation of shorter channels, in larger discharge current, and in a higher plasma electron density. Degradation of phenol by the discharge has also been demonstrated. A special metallic electrode covered by a thin layer of porous ceramic has been developed and used for generation of a multichannel discharge. At comparable solution conductivity the ceramic-coated electrode produced plasma with very similar parameters as the needle-plate electrode configuration. Generation of strong focused shock waves by the multichannel discharge in a...

Pulse electrical discharges in water and their applications

Results obtained using a pulsed-streamer corona discharge for organic compound removal are reported in this investigation. The removal of phenol and the effects of various parameters on the removal efficiency were studied. The intermediate products and final byproducts were also studied. It was found that the organic contaminants in aqueous solution could be destroyed effectively by the pulsed-streamer corona discharge and that the organic compound removal was greatly influenced by gas injection (bubbling). In addition, a preliminary study of the reaction mechanism was performed. Byproducts were determined for phenol removal. The main intermediate products produced by the pulsed-voltage discharge during the treatment process were hydroquinone, pyrocatechol and 1,4-benzoquinone. These intermediate products disappeared when the residence time was increased. By analysing the total carbon content of the treated phenol solution, it was found that 83% of the total organic carbon was removed without oxygen bubbling and 86% was removed with oxygen bubbling. Carbon dioxide emitted in the gas phase was measured by an absorption method.

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Use of a pulsed high-voltage discharge for removal of organic compounds in aqueous solution

In this investigation, results obtained using a pulsed discharge for organic compound removal are presented. The degradation of phenol by a streamer corona discharge and spark discharge, the effects of hydrogen peroxide additive on removal efficiency, and photochemical oxidation by ultraviolet light from the discharge plasma channel were investigated. The intermediate products and final byproducts formed by the spark discharge were also studied. A preliminary study of the degradation mechanism inside and outside the plasma channel was carried out. It was found that the removal efficiency of organic contaminants in the aqueous solution was higher for the spark discharge than for the streamer corona discharge and was greatly influenced by the discharge type and additive. The energy efficiency was the highest (6.91 × 10-3 μmol/J) for the case with hydrogen peroxide injection and spark discharge. The main intermediate products produced by the spark discharge during the treatment process were hydroquinone, pyr...

Oxidative Processes Occurring When Pulsed High Voltage Discharges Degrade Phenol in Aqueous Solution

Abstruct- The primary mechanism for sterilization of microorganisms by high-voltage pulses has been considered to be an electrical breakdown of the cell membrane. However, it is expected that many kinds of chemically active species would be generated by an electrical discharge in a needle-plate or rodrod electrode system. Therefore it is necessary to identify the chemical species produced by the discharge and to investigate lethal effects of the active species on microorganisms. Using a nozzle-plate electrode configuration, the authors previously reported that magenta colored streamers propagated from the nozzle tip during a pulsed discharge in water with various conductivities. The authors also investigated the generation of ozone from oxygen bubbled through the discharge. In the present study, the formation of active species in water (without 02 flow) and their effects on yeast cells were investigated using needleplate electrodes. In the presence of the streamer discharge, H and OH radicals were detected by means of emission spectroscopic analysis of the discharge light. Hydrogen peroxide (H202) was also detected by absorption spectrophotometry using a reaction of peroxidase and catalase. The effect of the electrical conductivity of the water on the formation of the active species was investigated. Maximum .OH and Hz 02 concentrations were obtained at a water conductivity of about S/cm. The H202 formation mechanism was considered to be a recombination reaction of .OH. The lethal effects on beer yeast of .OH and H2 02 generated by the pulsed electrical discharge in water were also investigated. It was found that .OH had almost no effect in reducing the survivors. However, the H~02 did kill the yeast cells: the logarithm of the survival ratio decreased linearly with increasing HZ 02 concentration.

J. S. Clements

Papers

Use of a pulsed high-voltage discharge for removal of organic compounds in aqueous solution

Oxidative Processes Occurring When Pulsed High Voltage Discharges Degrade Phenol in Aqueous Solution

Chemical Species an ei icroorganisms Usi -Voltage ischarge in