Characteristics of Underwater Capillary Discharge and its Application for Antibacterial Activity

Abstract

Formation of underwater plasma discharge has been an attractive avenue for the researchers since last few decades due to some rare and critical applications. Underwater plasma discharges have wide range of applications in environmental, biological, industrial and semi-conductor industry. The basic understanding of the physical and chemical characteristics of these underwater discharges is of extensive importance for their future applications and research. Due to the high dielectric constant of water the control on power consumption is first challenge for researchers working for liquid plasma. Various methods were adopted in past to reduce power consumption and to generate plasma in large volume of water effectively. Among them use of different electrode assemblies with pulse power source, high frequency power sources, and wave-heated sources were adopted to generate plasma at low power consumption. Using some electrical, spectral and imaging diagnostic techniques some work has been done on the characterization of liquid plasmas. The aim of this research work was to generate flowing water discharge at low power consumption by injecting stream of gas. Moreover the determination of best experimental conditions to yield high concentration of reactive species and their advance antibacterial application was another objective of this research work. To achieve these objectives a flowing water capillary discharge method has been adopted in this research for the diagnostics and applications of underwater plasma discharge. The discharge was created in a quartz tube with two different power sources, negative DC and high frequency AC. The tungsten electrodes were installed in pin-pin electrode assembly and discharge was created in flowing water. In order to reduce the power consumption gas bubbles were introduced in capillary tube. Three different gasses argon, oxygen and air were used in all xperiments to compare the electrical, spectral and chemical effects of each of them. Two types of water was used, a tab water and high conductivity water to compare the physical and chemical haracteristics of two types of water. Hydrogen peroxide was also mixed in water for different amounts to observe its effect on the yield rate of oxidant species. The observed oxidant species were OH' radicals, ozone (O3), hydrogen peroxide, reactive hydrogen and reactive oxygen. Various spectral and chemical mechanisms were adopted for the determination of these oxidant species generated by underwater plasma discharge. In negative DC discharge air and oxygen injected discharge was compared. The effect of each gas on electrical parameters like breakdown voltage, frequency, time between the occurrence of discharge pulses, energy per discharge pulse and power of discharge pulses were determined and compared. The effect of each gas on the yield rate of OH' radicals has been explored. Oxygen was proved to be more efficient compared to air from above mentioned prospects. Electron temperature and number density was determined in argon air and oxygen injected negative DC discharge and their effect on the concentration of oxidant species including OH' radicals, ozone and hydrogen peroxide has been explored. A direct relation of electron temperature and oxidant species was determined. In high frequency capillary discharge the effect of hydrogen peroxide addition along with argon injection for tab water and water with various conductivities was performed. In tab water the effect of hydrogen peroxide addition and argon injection on the electrical and spectra characteristics was determined. While in case of higher conductivities then tab water the effect of water conductivity on the electrical and spectral characteristics were explored. Hydrogen peroxide was observed ineffective for electrical parameters of discharge like break down voltage, power and frequency of discharge pulses while it effected highly the concentration of OH' radicals. High conductivity of water reduced required breakdown voltage and enhances the concentration of OH' radicals. On the basis of DC and high frequency AC underwater capillary discharge conclusions large volume flowing water sterilization was obtained by plasma discharge at low input power. Twenty capillaries were connected in parallel and high flow rate (0.2L/min) of water was treated by high frequency power source. Total 4 L/min water sterilization was obtained by this mechanism. Based on spectral and electrical diagnostics of underwater capillary discharge, its application on the antibacterial activity (Gram-negative E. Coli) was observed. The oxygen injected and hydrogen peroxide added capillary discharge was adopted for the disinfection of bacterial effect in drinking water. The addition of oxygen and hydrogen peroxide mixing resulted in high yield rate of ozone, OH' radicals, hydrogen peroxide, reactive hydrogen and reactive oxygen, which played a vital role in bacterial disinfection.