RADIATION-INDUCED DEGRADATION OF WATER POLLUTANTS--STATE OF THE ARTt

-The radiation-induced decomposition of biological resistant pollutants in drinking as well as in wastewater is briefly reviewed. First, some important units, definitions etc., radiation sources, as well as dose-depth curves in water as functions of the electron energy and 6°Co-y-rays are mentioned. Following is a schematical presentation of water radiolysis and of characteristics of primary free radicals. Then the degradation of some aliphatic and aromatic chlorinated compounds in the presence of air is presented. Some spectroscopic and kinetic data of transients resulting from chlorinated phenols are also quoted in order to illustrate and to explain the rather complicated degradation mechanisms. In this respect the synergistic effect of radiation and oxygen as well as that of ozone is also discussed. Finally, a scheme for technical application of high energy elelctron beam is presented. l. INTRODUCTION 2. UNITS AND CONVERSION FACTORS As a result of the strong development of various industries and the rapid growth of world population a rather heavy overloading of water resources is observed. In addition to this, the usage of various chlorine-containing presticides and other chemicals, as well as fertilizer in modern agriculture, contributes to contamination of ground water. The killing of microorganisms in drinking water containing humic compounds by chlorination leads to the formation of various halogenated hydrocarbons (Rook, 1974). Such substances are carcinogenic and are usually removed by filtration through activated charcoal which is subsequently burned up. The resulting chlorine oxides contribute to the occurence of acid rain. Due to the above mentioned pollution sources, environmental and toxicological effects have increased considerably (Hutzinger et al, 1974, 1982, Fishbein, 1979; Strobel and Dieter, 1990). This is also reflected in the health of the populat ion in industrial areas. Fundamental studies have shown that a complete degradation of biologically resistant compounds, disinfection of sewage sludge as well as killing of microorganisms can be achieved by ionizing radiation treatment. Abundant literature is available on this topic. This is now the basis for technical electron beam remediation in water. In order to present a more complete picture of the subject matter and as an introduction for newcomers to this field, some basic knowledge in radiation chemistry is briefly mentioned. tPresented at the IMRP-9 in Istanbul, Turkey, 11-16 September, 1994. For convenience some radiation units, definitions and conversion factors used in the radiation technology are given in Table 1. 2.1. Radiation sources For radiation processing of polluted water high energy electrons, such as 't-rays (e.g. from 6°Co, zl /==5.26 years, Ey = 1.17 and 1.33MeV; ~37Cs, ~/2 = 29 years, E~, = 0.66 MeV etc.) or X-rays can be principally used. Based on the present state of technological achievements, preference is given to the electron-accelerator machines (EA-machines). Some typical radiation sources are given in Table 2 for comparison in respect to their output power. Obviously the EA-machines are far the best for this purpose, because of their very high dose rate. 3. PENETRATION DEPTH OF RADIATION IN WATER Only the absorved radiation energy can initiate physical, chemical or biological effects. The energy absorption in a medium, e.g. water, takes place in 10 -~5 s, in the course of which the dose distribution is not uniform, because of electron scattering and the "bui ld-up" effects occuring during the interaction between radiation and matter. As a result of this dose distribution gradients are observed (Wiesner, 1979; Seizer and Berger, 1987; Miller, 1990; Getoff, 1992a, 1993a). This effect is illustrated by curves, representing depth-dose (in %) for a set of electron energies from 1 to 14 MeV, shown in Fig. 1. Taking the curve e.g. for 10 MeV electrons the extrapolated range in water, R¢~ = 5 cm (Fig. 1). This