## 2 Radiation Technologies
### Water Radiolysis  
The main idea of using ionizing radiation for water and wastewater treatment is based on the simultaneous generation of oxidizing and reducing species during water radiolysis. The same radicals are formed in the reaction environment by the action of gamma radiation and electron beams. Electron accelerators are primarily used in applications due to their safe operation and their high power. The electron beam, in a very short time period (of the order of s), causes the simultaneous ionization of a huge number of water molecules, as according to Equation (1) [7]\[H_{2}\text{O}\ \rightarrow\ {}^{\bullet}OH\ (2.7)\ +e_{aq}^{-}(2.6)+H^{\bullet}\ (0.55)+H_{2}\ (0.45)+H_{2}\text{O}_{2}\ (0.71)+H_{3}\text{O}^{+}(2.6) \tag{1}\]  
The efficiency of the irradiation process is evaluated by calculating the chemical yield of the radiation (G-value, \(\mu\text{M}\) J\({}^{-1}\)), which determines the number of individuals of a given type that is formed or decomposed as a result of 100 eV absorption and is represented by Equation (2). \[G=\frac{6.023\times 10^{23}C}{D\times 6.24\times 10^{16}} \tag{2}\]  
where C is the concentration of formed individuals (M) and D is the absorbed dose (Gy). The radiation efficiencies have the same values for high-energy electron and photon X-rays as well as gamma rays. It is assumed that the temperature does not practically affect the efficiency of water radiolysis products, while the pH value has a significant influence. In acidic solutions, the G (e\({}_{aq}\)\({}^{-}\)) values are lower while the G (\({}^{\bullet}\)H) values are higher, and this is related to Reaction (3):  
\[e_{aq}^{-}+H_{2}\text{O}\leftrightarrow H_{aq}+OH_{aq}^{-}\qquad\qquad pK_{a }=6.3 \tag{3}\]  
Among the reactive species formed during the radiolysis of water, the most reactive is the hydroxyl \({}^{\bullet}\)OH radical, for which the oxidizing potential is in the range of 2.8 V (pH 0) and 1.95 (pH 14) relative to the saturated calomel electrode [8]. The hydroxyl radical is a very nonselective species, it reacts very quickly with most of the organic impurities and the reaction rates are diffusion-controlled 10\({}^{8}\)-10\({}^{10}\) M\({}^{-1}\)s\({}^{-1}\). The decomposition of an organic substance by reaction with a hydroxyl radical can be as follows:  
* Radical addition;
* Hydrogen abstraction;
* Electron transfer;
* Combination/recombination of radicals. The primary products of water radiolysis, mainly hydroxyl radicals, undergo numerous reactions with organic impurities present in the solution. These transformations successively lead to the formation of carbonyl radicals (R\({}^{\bullet}\)_ or R\({}^{\bullet}\)-OH) and then, in reactions with O\({}_{2}\), to the formation of peroxyl radicals (ROO\({}^{\bullet}\)). The end products of these transformations are H\({}_{2}\)O\({}_{2}\) and peroxide, which, as highly reactive individuals, are responsible for the degradation and even mineralization of these pollutants [9]. Hydroxyl radicals have a very short lifetime; they are only generated in situ, in this case, by the action of gamma rays or electron beams on water molecules. The advantage of ionizing radiation-based processes over other AOPs is that, simultaneously alongside the hydroxyl radical, strongly reducing species e\({}_{aq}\)\({}^{-}\) and the hydrogen atom \({}^{\bullet}\)H are generated with the highest efficiency compared to other AOPs. From a basic research point of view, it is important to carry out the process in such conditions where only one selected product of water radiolysis predominates and not their mixture. It is then possible to determine the contribution of individual radicals to the radiation decomposition of the selected impurity, as well as to determine the mechanism of decomposition.