## 2 Advanced Treatment Processes for Emerging Contaminants
AORPs are chemical-less high-rate energy-efficient processes, with a wide applicability range. AORPs may be activated by high-energy inputs, such as photoionization (Vacuum UV -VUV- with irradiation at 185 nm and 254 nm) [44], non-thermal plasma electric discharges [45], or ionizing radiations [46]. In the VUV process, photoionization and hemolysis of water molecules occur with the absorption of high-energy photons in water, according to the following reactions:  
\[\mathrm{H_{2}O+VUV_{185 nm}\to\mathrm{OH+H^{+}+e^{-}_{aq}}}, \tag{1}\]  
\[\mathrm{H_{2}O+VUV_{185 nm}\to\mathrm{OH+\cdot H}}, \tag{2}\]  
\[\mathrm{H_{2}O+H\to H_{3}O^{+}+e^{-}_{aq}}, \tag{3}\]  
\[\mathrm{O_{2}+VUV_{185 nm}\to 2\cdot O}, \tag{4}\]  
\[\mathrm{O_{2}+O\to O_{3}}, \tag{5}\]  
\[\mathrm{O_{3}+H_{2}O+UV_{254 nm}\to 2\cdot OH+O_{2}} \tag{6}\]  
In non-thermal plasma processes (NTPs), high-voltage electrical pulses generate a corona discharge, which in turn excites electrons in the air above a liquid solution with an ionization effect, producing singlet oxygen atoms, which generate ozone and hydroxyl radicals in the underlying water layer [47]. In other alternative NTP configurations, dielectric barrier discharges, corona discharges in aerosols or in the water matrix itself, are applied without substantial differences in the products of the reactions sequence previously indicated (Equations (1)-(6)). NTP technology is confined to thin-layer low-flow applications, due to the limited water penetration of excited electrons. An example of a pilot-scale NTP unit is schematized in Figure 2. This \"electrode-to-plate\" scheme consists of two units: In the first, water flowing in a thin film (~5 mm) over a ground electrode (anode) is exposed to high-voltage pulses from a carbon fiber electrode (cathode). Electric pulses with a frequency in the range 500-1000 Hz, at maximum voltage of 8.0 kV, 100 A current, are released by a generator. Upon leaving this reactor, water enters an ozone contactor, where it is mixed with ozone-rich air (generated in the previous reactor's headspace). Other configurations, such as wetted-wall or falling film reactors, have been used experimentally [48; 49]. Figure 2: NTP “electrode-to-plate” reactor scheme described in [45]. The most common AORP technology, however, operates through the use of ionizing radiations (Figure 3). Ionizing radiation is defined as any electromagnetic wave (at a frequency higher than \(10^{17}\) Hz) carrying sufficient energy to ionize or remove electrons from an atom. Unlike non-ionizing radiation (e.g., radiofrequency, infrared, and UV), with the kinetic energy of particles (photons, electrons, etc.) too small to generate charged ions when passing through matter, the energy of ionizing radiation particles can form ions by losing electrons (i.e., ionize) from target atoms. Two types of electromagnetic waves can do that: X-rays, and \(\gamma\)-rays, emitted by radionuclides, such as \({}^{60}\)Co and \({}^{137}\)Cs. The same effect may be achieved by particulate radiation, consisting of subatomic particles (electrons, protons, etc.) carrying energy in kinetic form, such as those emitted by electron accelerators (electron beam, EB). These types of ionizing sources do not have the capability of making their targets radioactive, although they can break chemical bonds and damage DNA. Neutron radiation, such as that emitted from nuclear fission, on the other hand, is not able to directly ionize atoms (due to its lack of charge) but can make them unstable, turning materials radioactive, and must not be confused with the former. Ionizing irradiation therefore results in the cleavage of one or more molecular bonds, dissociating the exposed molecules (radiolysis) without radioactivity induction. The process differs from \"conventional\" AOPs, for instance, photolysis, where lower energy UV sources are used in conjunction with chemicals or catalyzers.