## 2 Radiation Technologies
### Water Radiolysis
A commonly used method of ensuring the predominance of \({}^{\bullet}\)OH radicals in irradiated solutions is their saturation with N\({}_{2}\)O, which, as a result of the following Reaction (4):  
\[e_{aq}^{-}+N_{2}\text{O}+H_{2}\text{O}\to N_{2}+OH^{-}+\ ^{\bullet}OH\ \ \ \ \ k=9.1\times 10^{9}M^{-1}\text{s}^{-1} \tag{4}\]  
causes the conversion of e\({}_{aq}\)\({}^{-}\) into an additional amount of \({}^{\bullet}\)OH radicals, under such conditions only \({}^{\bullet}\)OH and \({}^{\bullet}\)H remain in the solution in a percentage ratio of 90/10 [10]. Moreover, when H\({}_{2}\)O\({}_{2}\) is present in the irradiated solution, hydroxyl radicals dominate as a result of Reaction (5) [11]:  
\[e_{aq}^{-}+H_{2}\text{O}_{2}\rightarrow\ ^{\bullet}OH+OH^{-}\ \ \ \ \ \ \ \ \ \ \ \ k=1.2\ \times\ 10^{10}M^{-1}\text{s}^{-1} \tag{5}\]In research practice, N\({}_{2}\)O is much more commonly used due to the fact that N\({}_{2}\)O itself and the nitrogen formed during the process are practically inert towards the compounds present in the solution. In the case of hydrogen peroxide, it is difficult to estimate the exact amount needed to achieve the intended effect. When H\({}_{2}\)O\({}_{2}\) is present in the solution in excess of the substance being decomposed, it can also react with the \({}^{\bullet}\)OH radical as well as other radicals, as according to Reaction (6) [7]:  
\[{}^{\bullet}OH+H_{2}O_{2}\to H_{2}O+O_{2}^{\bullet-}\hskip 28.452756 ptk=2.7 \ \times\ 10^{7}M^{-1}s^{-1} \tag{6}\]  
On the other hand, in order to eliminate hydroxyl radicals from the reaction environment, solutions before irradiation should be deaerated and an appropriate amount of tert-butyl alcohol (0.2-1 M) should be added [12], which acts as a scavenger of hydroxyl radicals, as according to Reaction (7):  
\[{}^{\bullet}OH+(CH_{3})_{3}COH\to H_{2}O+\ ^{\bullet}CH_{2}(CH_{3})_{2} COH\ \ \ k=6\ \times\ 10^{8}M^{-1}s^{-1} \tag{7}\]  
Other classic hydroxyl radical scavengers include thiourea, dimethylurea, or several alcohols such as methanol and ethanol. The pH of the irradiated solution also plays an important role (Figure 1). When the experiment is carried out in an acidic environment, it is practically only the hydrogen atom present, while in neutral and alkaline solutions the equilibrium shifts towards the formation of e\({}_{\text{aq}}\)\({}^{-}\) (Reactions (8) and (9)). \[e_{aq}^{-}+H^{+}\ \rightarrow\ H^{\bullet} \tag{8}\]  
\[H^{\bullet}+OH^{-}\ \rightarrow\ e_{aq}^{-} \tag{9}\]  
Water and wastewater subjected to purification and treatment processes are very complex matrices. There is a whole spectrum of chemical compounds that significantly affect the efficiency of the purification process. As mentioned earlier, the reactive individuals formed by radiolysis of water are non-selective to the substances present in the reaction environment. In real wastewater, there is competition between the pollutants to be degraded and the anions present in the solution, i.e., Cl\({}^{-}\), CO\({}_{3}\)\({}^{2-}\) HCO\({}_{3}\)\({}^{-}\), NO\({}_{3}\)\({}^{-}\), or SO\({}_{4}\)\({}^{2-}\), as well as the humic compounds for reaction with primary radiolysis products of the water. This competition can affect the efficiency of the purification process [14], these substances are called \({}^{\bullet}\)OH radical scavengers as they usually cause a decrease in the degradation efficiency [15; 16]. Wojnarovits and Takacs [17] showed that the pH value of a radiation-treated solution has a significant effect on the progress of competitive reactions with hydroxyl radicals. The reaction with chloride ions is predominant at pH \(<\) 5, while the reaction with carbonate and bicarbonate ions is predominant at a neutral or slightly alkaline pH. Very often, in order to reduce the amount of the dose necessary to achieve a satisfactory degree of decomposition of organic pollutants, chemical support is used by adding, for  
Figure 1: Effect of pH on G-values of primary products of water radiolysis [13]. example, ozone [18] or hydrogen peroxide [19; 20].