Water, radiolysis mechanism

Flan P, Bartels DM (1992) Fl/D isotope effects in water radiolysis. 4. The mechanism of (Fl)aq <=> (e )aq interconversion. J Phys Chem 96 4899-4906 Hart EJ (1964) The hydrated electron. Science 146 19-25 Hart EJ, Anbar M (1970) The hydrated electron. Wiley, New York... 

Many undesirable organochlorine compounds are produced by the chlorination of drinking water. Among these are chloroacetic acids and trihalo-methanes. Studies have reported the radiolytic decomposition of CHC13 and related compounds [11-14], An example is shown in Fig. 1, where the concentration decrease for various bromochloromethanes is plotted vs. absorbed dose. A proposed mechanism for the decomposition of CHC13 and formation of by-products, involving the three important radical products of water radiolysis is shown ... 

DNA suffers radiation damage by both a direct mechanism and an indirect mechanism. The direct damage results from radiation-induced ionization of DNA itself. The indirect damage results from attack on DNA by other free radicals. Since water is the predominant molecule in cells, the major source of indirect radiation DNA damage is from water radiolysis. 

Even if irradiation with heavy ions concerns evidendy many materials such as solids or liquids and gases, this chapter will deal with the effect of high-energy heavy particles in liquid water. Actually, chemical mechanisms in pure water have been depicted a few decades ago and this medium is undoubtedly the one in the nature, more generally in the living systems, and also in the industry to be extensively used. Therefore this chapter tries to give the new trends of the research on water radiolysis with heavy ion beams, the methods developed with their associated problems and the facilities used for these studies. 

From a fundamental point of view the origins of H2O2 and H2 take place at the earliest step after ionization basically the recombination of OH forms H202, and the recombination of hydrated electron forms H2 and a non-scavengeable part of H2 directly originates from the ionization of water molecule and then enters the reaction mechanism of water radiolysis very early. 

In order to demonstrate the differences in the rates and mechanisms of reactions of organic compounds with the primary radicals of water radiolysis several representative examples are collected in Table 1. The original sources of these data can be found in the rate constant compilations mentioned above). 

At the radio-sterilization doses, simulation predicts a greater loss than in the experimental results, possibly because some radiolytic products react with the water radiolysis radicals thus protecting the drug solute [16]. The simulations also predict similar solute concentrations without dose rate effects for E-beam or gamma irradiations whereas the opposite isfound in experimental results [17].The complexity ofthe radiolysis mechanisms at sterilization doses appears with the increase ofthe analytical efficiency. 

Figure 3 Mechanism ofperoxidation oflinoleica cid (18 2) initiated by OH free radicals from water radiolysis.

Effects of ionizing radiation on lipid molecules have been understood by studying model systems which are simpler than the real biological membranes, such as PUFA micelles and liposomes. The formation of lipid oxidative modifications of PUFAs appears as a dynamic process initiated by hydroxyl free radicals generated by water radiolysis, amplified by a propagating-chain mechanism involving alkyl and peroxyl free radicals, and leading not only to hydroperoxides but also to a lot of other lipidic oxidized end-products. Kinetic data, such... 

Although the radiation yield of hydrogen from irradiated polymers became no concern from the point of view of safety, it remained the object of interest from the point of view of mechanisms of radiation induced chemical reactions. In analogy to water radiolysis, the yield of hydrogen become of interest if radiations of higher LET values were used for irradiation of polymers [19], These are proton beams and alpha radiation, which can suggest different yields of hydrogen, and they really do. 

Radiolysis Mechanisms. In the earlier work (34) with water vapor a number of parameters relevant to understanding the experimental system were discussed in detail. These are summarized in Table I for H20 and D20. Briefly, it can be calculated that given a pressure in the irradiated vapor stream of — 0.05 torr, second- or third-order processes of radical decay cannot occur in the vapor phase on a time scale < 10 r> sec. On the other hand, ion-molecule reactions with a rate constant k — 5X 10"10 cc./molecule-sec. (16), could have reaction half-lives of this order and may go to completion before condensation at 77 °K. However, it is possible for radical-scavenger reactions to occur in this system. For example, it was shown that 0.14 mole % CH3I reacts readily with electrons produced from irradiated water vapor to give CHS radicals (34). We suggested that this reaction may occur in the transient liquid phase immediately before solidification at 77 °K. Irrespective of the phase where such a reaction occurs, it can be used for detecting the presence of electrons. 

The decontamination factors mentioned above may be affected over time by mechanisms which lead to a revolatilization of fission products from the pool water. When the bubbles break up at the pool surface, they produce new aerosol droplets by which non-volatile fission products can be carried to the containment atmosphere. Calculations using the Sparc code (Owczarski and Burk, 1991), however, showed that radionuclide entrainment caused by this mechanism is very small and, thus, can be ignored in source term analyses. Iodine, on the other hand, can be revolatilized if the pH of the aqueous phase is decreased as a consequence of water radiolysis. In the presence of alkalizing substances such as CsOH, such a decrease in pH leading to a lower I2 partition coefficient is not to be expected. The existing codes (such as Sparc, Supra, Busca) do not take chemical reactions into consideration and, thus, do not provide any information on chemical iodine revolatilization from the pool. 

The first study of the radiolysis of water at supercritieal temperatures was apparently that of Bums and Marsh published in 1981 164 jjjggg measurements were made at 400 C and the techniques employed were generally adopted by later workers in repeating and expanding upon this initial, pioneering work. (Fig. 91) The Bums and Marsh sturfy produced a radiolysis mechanism and associated rate constants and activation energies that need to be verified or modified, as the case may be. 

The attack on the aromatic nucleus by hydroxyl radicals is probably analogous to that by phenyl and methyl radicals, Eq. (34a,b). Evidence that the first step is the addition of hydroxyl radical to benzene, rather than abstraction of a hydrogen atom, has recently been adduced from a study of the radiolysis of water-benzene mixtures. The familiar addition complex may undergo two reactions to form the phenolic and dimeric products respectively, Eq. (34a,b). Alternative mechanisms for the formation of the dimer have been formulated, but in view of the lack of experimental evidence for any of the mechanisms further discussion of this problem is not justified. 

It has been found that the activity which is conventionally referred to as the "unattached" fraction is actually an ultrafine particle aerosol with a size range of 0.5 to 3 nm. The hydroxyl radical from water molecule radiolysis is a key element to the particle formation mechanism. By injecting different concentrations of S02 into the test chamber, a possible particle formation mechanism has been suggested as follows Oxidizable species such as S02 reacts promptly with hydroxyl radicals and form a condensed phase. These molecules coagulate and become ultrafine particles. 

From the preceding discussion it may be inferred that in frozen aqueous solutions there are basically two different mechanisms by which H atoms can be formed (1) by reaction of the electron with acid anions according to Equation 4, and (2) by direct radiolysis of water molecules according to Equation 1. Earlier investigations of aqueous solutions at room temperature have also lead to the same conclusion (70). Although there are no experimental data from either the room temperature studies or from those on the frozen solutions, as to the actual nature of the second process, it is believed (70) that it is the dissociation of excited water molecules formed in the tracks of the fast electrons. 

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