Pulse Radiolysis of Supercritical Water
1.Introduction
Spercritical water can be used as a benign solvent for reaction, separation, waste destruction, etc. Compared to ambient liquid water, supercritical has some advantages, e.g. markedly decreased dielectric constant and H-bonding. There is an increasing number of papers on the application and fundamental research of supercritical water in the past few years. Probably more than 50% of the publications are concerning with supercritical water oxidation (SCW). SCWO is considered as a promising method for disposal of toxic compounds such as chemical weapon and PCB. However, corrosion of the wall caused by Cl- in SCWO remains a major concern. In addition of SCWO, hydrolysis of biomass and production of H2 from coal in supercritical water are feasible ways to utilize natural sources.
In radiation chemistry, it is known for several decades that radicals such as OH and eaq- and molecular products such as H2 and H2O2 can be formed as water isradiated by high energy ionizing source. Hydrated electron (eaq-) is a strong reducing agent, it can be easily determined because of its large absorption coefficient. Some early workers reported the absoprion spectrum from room tmperature to 300 C. Absorption spectrum was found to shift to the side of long wavelength with incresing temperature.
A project was initiated in our group (Prof. Y. Katsumura) to understnd radiation-induced processes in supercritical water. As the first step, temperature dependence of eaq- was investigated over the temperature range 25-400C including conditions of supercritical state. It is basically important to confirm the eaq- formation of eaq- and record its absorption spectrum in supercritical water, because the dielectric constant of supercritical water could be as low as that of nonpolar organic solvent. In addition, knoweldge of radiation-induced processes in supercritical water is indispensible for design of future generation of nuclear reator. Some fundamental calculatiions show that operating efficiency acan be considerably increased with using supercritical water as coolant.
2. Experimental
Pusle radiolysis experiment was performed at 25-400 C using heavy water (D2O). The air dissolved in water was removed by bublng with Ar gas. Below is shown the high-temperature irradiation cell and flow system.
The cell was made of Hasteloy, withstanding temperatures up to 500 C and pressures up to 500 atm. Optical path of the cell is 15 mm and the internal volume of the cell is about 0.4 mL.
Pressure of water was handled by the pressure regulator. Temperature of water was tetermined by immersing a thermocouple inside the cell.
3. Results and Discussion
Transient absorbance of hydrated electron was observed under any conditions including supercritical water. The decay of hydrated electron increases with temperature at temperatures up to 300 C but then slows down at 350 and 400C.
It is clear that absorption peak shifts to longer wavelength and the band becomes wider as the temperature rises. For instantce, the peak is 700 nm at room temperature but 1200 nm at 400C. Inset: pressure dependence in supercritical water.
The energy of absorption maximum decreases gradually with temperature. Our data are compared with previously reported ones. There is a good agreement among various research groups below 250C. However, above 300C, our values are larger than the literature values, being equivalent to difference of 100-200 nm in absorption peak. Such a difference is considered to be caused by the pressure difference. The previous work is probably carried out in the vapor phase.
Change of water property with temperature is responsible for the change in energy of absorption maximum. Since many proterties, e.g. dielectric constant and H-bonding, change simultaneously with increasing temperature, we cannot resolve them. The inserted figure indicates a linear correlation between dielectric constant and energy.
4. Conclusion
(1) Hydrated electron is observed in supercritical water;
(2) Absorption peak shifts significantly with temperature.
5. Publication>
1. G. Wu, Y. Katsumura, Y. Muroya, X. Li, Y. Terada,
Chemical Physics Letters, 2000 (accepted)
2. G. Wu, Y. Katsumura, Y. Muroya, X. Li, Y. Terada,
"International Symposium on Prospects for Application of Radiation Towards the 21st Century."
March 13-17, 2000. Waseda University, Tokyo, Japan.
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e-mail wu@tokai.t.u-tokyo.ac.jp