## 2 Advanced Treatment Processes for Emerging Contaminants
AOPs have been and are still widely researched: A non-comprehensive bibliography search counted over 20,000 published works on their application for the removal of pharmaceutical residues, endocrine disruptors, natural organic matter, bio-recalcitrant organics, generic and specific pollutants, and pathogens from water and wastewater [11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32]. AOPs promote the degradation of organic compounds into simpler molecules: Partial decomposition of non-biodegradable organics may lead to more biodegradable intermediates or, more rarely, to their mineralization and to the destruction of pathogenic organisms, by the effect of strong oxidants. The oxidation potential is hence the strength and, at the same time, the limit of AOPs: Some contaminants, such as, for example, chlorinated and brominated compounds, are poorly or not degraded at all by oxidation. While AOPs and their applications in the water sector are well known, advanced reduction processes (ARPs) are a recently emerged class of processes combining activation methods similar to those used by AOPs (UV, ultrasound, microwave) and reductive agents (sulfite, ferrous iron, sulfide, dithionite) to generate reactive radicals capable of strong reducing reactions (hydrated electrons, e\({}^{-}\)aq, hydrogen \(\cdot\)H, and sulfite radicals \(\cdot\)SO\({}_{3}\)\({}^{-}\)). These, unlike those generated by AOPs, can easily degrade oxidized contaminants in solution [33; 34]. Reductants are selected for ARPs based on the applied activation method's capacity to produce reducing radicals, or other effective reducing agents. Dithionite (i.e., sodium hydrosulfite, Na\({}_{2}\)S\({}_{2}\)O\({}_{4}\)), for example, is a water-soluble salt used as reducing agent in aqueous solutions. By cleaving the S-S bond upon absorption of 315-nm UV radiation, dithionite can be broken into two sulfur dioxide radical anions (SO\({}_{2}\)\({}^{-}\)), a strong reductant. Additionally, sulfite (SO\({}_{3}\)\({}^{2-}\)), upon irradiation, will react to create a sulfite radical anion (\(\cdot\)SO\({}_{3}\)\({}^{-}\)) and e\(-\)aq, also a strong reductant, while the sulfite radical could act indifferently as an oxidant or reductant, accepting an electron to form sulfite or donating one to form sulfate. Sulfide, S\({}^{2-}\), adsorbs UV light at 230 nm, promoting the formation of hydrogen in solution, similarly to ferrous iron. While the chemical mechanisms of reducing radical formation are fairly well known, little research has been conducted so far on the application of these processes to water treatment and contaminant degradation. Preliminary results [33] have demonstrated that ARPs can degrade a wide variety of contaminants, including: Perchlorate (a highly oxidized chlorine form normally difficult to reduce) [35], 1,2 dichloroethane [36], perfluorooctanoic acid (PFOA, a synthetic, difficult to treat, fluorinated organic acid) [37], 2,4-dichlorophenol (2,4-DCP), a chlorinated highly toxic phenol derivative used as herbicide preparation intermediate [38], several halogenated compounds [39], nitrate (a ubiquitous groundwater contaminant that can be removed by chemical or electrochemical methods) [40; 41], and metals (e.g., arsenic, selenium) [42]. Despite these encouraging results, however, this technology has not  
Figure 1: Various process technologies in the ATP realm. reported real-scale applications so far. Considering the fact that some CECs can only be removed by strong reduction reactions, these processes will likely gain greater consideration in the near future. Advanced oxidation-reduction processes (AORPs), a third class of ATPs, combine both types of radical reactions to directly degrade contaminants. They are perhaps the most interesting development in advanced treatment technology. Concurrently generated radicals carry out simultaneous oxidation and reduction reactions, achieving accelerated destruction of a wide range of contaminants [43].