Takayuki Ohshima

Aqueous Phenol Decomposition by Pulsed Discharges on the Water Surface

Decomposition of environmental contaminants such as phenol contained in water was investigated using a pulsed high-voltage gas-phase discharge on the water surface (water surface plasma). The discharge consists of streamer channels that spread out over the water surface. Discharge characteristics were dependent upon the distance between the needle-tip electrode and the water surface, the shape of the submerged ground electrode, and the composition of the gas enveloping the electrode. When the electrode-water distance was decreased, the discharge mode changed from corona to streamer, and then, finally, to a water surface discharge when the distance was small. Argon gas was the most effective enveloping gas for decomposing phenol in water (compared to oxygen or air). When the gas flow rate was increased to carry away the active species formed in the gas phase; the decomposition rate did not change in argon, but decreased in oxygen. The shape of the submerged ground electrode influenced the discharge state and the phenol decomposition rate. A ring-shaped ground electrode was more effective for decomposition of phenol than straight or semicircular shapes. Experiments were performed to identify the mechanism(s) responsible for the decomposition of organic materials in water.

Cloning and Sequencing of a Gene Encoding Nitrite Reductase from Paracoccus denitrificans and Expression of the Gene in Escherichia coli

Abstract A structural gene for nitrite reductase (nirS) was cloned from a denitrifying bacterium Paracoccus denitrificans into Escherichia coli MV1184. The coding sequence of nirS consisted of 1788 nucleotides and the value of the G+C content was 68%. This gene seemed to be in an operon structure. The size of nitrite reductase (NIR) was predicted to be 65.5 kDal from the amino acid sequence, which was similar to the value determined with purified NIR by SDS-polyacrylamide gel electrophoresis analysis. From hydrophathy analysis, the NIR from P. denitrificans seemed to be a periplasmic enzyme. Crude extract from the recombinant E. coli harboring nirS and about 10 kbp of its downstream flanking region had significant activity of NO and N2O formation from nitrite (NO2−), whereas crude extract from E. coli harboring only nirS had only weak activity. This result suggested that the downstream region includes the gene responsible for the protein which involved in NIR activation. In addition, the downstream region seemed to have a NO reductase gene, because NO to N2O conversion activity was also detected in the crude extract from the recombinant E. coli harboring nirS and about 10 kbp of its downstream flanking region.

Decoloration of Organic Dye in Water by Pulsed Discharge Plasma Generated Simultaneously in Gas and Liquid Media

Abstract A new process for decomposing organic contaminants in water was proposed. Pulsed discharge plasma was generated in the gas phase, and the produced plasma was permeated through a pinhole into the water phase. Water (upper) and gas (lower) were separated by an insulating plate, where a pinhole was perforated at the center of the plate. Gas was bubbled into the water phase through the pinhole. In the gas phase, the high voltage pulse was applied between the needle electrode and the ground electrode (immersed in the water phase). The high voltage pulsed discharge plasma was generated in the gas phase, simultaneously the plasma channel was permeated into the water phase accompanying by the gas bubbles. The water phase plasma produced a lot of active species, UV light, and high-energy electrons. Porous ceramic tube was tried to use for producing water phase plasma, instead of the insulating plate in a pinhole reactor. It was observed that the gas phase plasma also permeated through many small pores into the water and generated streamer discharge in water. Chicago sky blue aqueous solution was effectively decolored with oxygen gas bubbling than the cases of argon gas and air. With applying pulsed voltage of 20 kV and pulse frequency of 25 Hz with 500 mL/min oxygen bubbling, the dye aqueous solution with 10 ppm initial concentration was decolored about 95% in 10 min treatment. The decoloration rate increased with increasing electrical conductivity of the solution. This type of simultaneous discharge plasma reactor is expected to have high-energy efficiency degradation rate.

Development of Textile Electrode for Microbial Inactivation with Pulsed Electric Field

We have been studying pulsed electric field (PEF) inactivation of various bacteria, which is based on the destruction of biological membrane due to applying pulsed electric field. In this study, we report a novel treatment system based on the textile electrode, combination of polyester fiber and 0.2 mmφ metal wire. The efficiency of PEF inactivation of Escherichia coli cells depended on the peak voltage and the conductivity of the target solutions. Higher efficiency of PEF sterilization could be achieved with 7 kV of peak voltage and lower solution conductivity. When the solution conductivity was lower, some materials, which seemed to be bacterial residue, were sedimented on high voltage electrode.