The elimination of chlorinated ethylenes from drinking water

Sub-ppm quantities of chlorinated ethylenes dissolved in drinking water can be decomposed by high energy radiation very effectively. The decomposition reaction is initiated mainly by OH·radicals originating from water radiolysis. For perchloroethylene the main decomposition products are CO2 and Cl-, for trichloroethylene beside these formic acid is formed. However, as an undesirable side reaction, a small fraction of the nitrate present in the water is converted to nitrite. The presence of ozone during the irradiation prevents the nitrite formation. Additionally, the radiation catalyzes the conversion of ozone into OH·radicals. Therefore, the effect of the combined treatment is always higher than the sum of the individual effects of ozone and radiation. Compared to ozonation alone the combined treatment results in decomposition rates for per- and trichloroethylene higher by about two powers of ten. Compared to γ-irradiation alone, the doses necessary for a 90 % pollutant reduction could be lowered by factors of about 6 (for perchloroethylene) and 10 (for trichloroethylene) when an initial ozone concentration of 5 ppm is provided. Electron beams are less effective than γ-rays and require higher ozone concentrations if the results of the combined ozone/γ-irradiation treatment should be realized. (author)