## 2 Advanced Treatment Processes for Emerging Contaminants
Irradiation of a dilute (total solute concentration lower than 10%) solution with ionizing radiation instantaneously (t \(\leq 10^{-12}\) s) splits water molecules into strongly oxidative hydroxyl radicals (OH), and strongly reductive species (e\({}^{-}\)\({}_{\text{aq}}\cdot\text{H}\)), in addition to other species, such as H\({}^{+}\), H\({}_{2}\), H\({}_{2}\)O\({}_{2}\), H\({}_{3}\)O\({}^{+}\), and O, without chemical or catalyst addition [50]. The yields of reactive species formed in this process are expressed by the G-value, a measure of the number of molecules formed/consumed per unit of absorbed energy. For example, in the simplest water radiolysis reaction:  
Figure 1:0  
the _G-Values_ of the generated species are 0.28, 0.28, and 0.06, respectively, much higher than in other conventional AOPs [51]. However, the possible reaction mechanisms of hydroxyl radicals are the same as those in AOPs, i.e.,:  
Figure 1:1  
Figure 1:2  
Figure 1:3  
Figure 1:4  
Gamma rays are still used in many experimental radiolysis studies, but, due to their high organic matter penetration, they require strong precautions and shielding protection to avoid harm to living cells. Furthermore, \(\gamma\)-ray sources are highly radioactive isotopes requiring periodical replacement and special (and costly) high-risk handling procedures. The efficiency of a \(\gamma\)-irradiation process depends on the  
Figure 1:5 Non-ionizing and ionizing radiation ranges: frequency and energy. \"age\" of the radiation sources, which determines the deliverable dose. EB technology, instead, does not rely on constantly decaying isotopes but on electromagnetic particle generation, and can therefore be turned off when the process is not needed, much like a television set. Old cathodic tube television sets were actually quite similar, in principle, but at lower energy, to modern electron accelerators (Figure 4). Safety and practical considerations have therefore contributed to widespread industrial adoption of EB, commercially used in many manufacturing processes requiring an improvement of the material properties by molecular modification (e.g., submarine data cables, membranes grafting, medical hydrogels), sterilization or disinfection (including food processing), and environmental areas (industrial fine gas treatment, sludge processing, textile effluents processing) [52]. However, despite the advantages of this practical process, few full-scale water and wastewater treatment applications have been reported to date [53]. Studies on radiolytic processes for water treatment date back to the 1950s, when this technology was initially tested on domestic and industrial wastewater [51]. Since at that time CECs had not yet been addressed nor identified, treatment of organic matter solely in terms of COD, though successful, immediately appeared anti-economical, given the availability of cheaper established biological processes, and the high cost of irradiation equipment at the time. Some applications were also attempted with the use of X-rays [54], which have not been followed up further, due to the low specific efficiency of this source. While \(\gamma\)-rays are still often used in laboratory studies, optimal irradiation dose-rates can be achieved more easily, in practice, with EB systems. The dose-rate parameter, one of the main indicators for process applicability, and the process time are in fact much more favorable for EB irradiation than for \(\gamma\) or X-rays, as reported in Figure 1:6 Dose-rate determines the process contact time for an established irradiation dose, as indicated in the table: A dose of 1 kGy represents a relatively low value used, for example, to achieve wastewater disinfection, and is in the low range of those commonly used in the pretreatment of foods in industrial distribution applications. It should also be noted that although 1 Curie (37 GBq in SI units) corresponds to a very high value of radioactivity, it still generates a low dose-rate even compared to small EB units.