## 1 Introduction  
Alkanolamines encompassing monoethanolamine (MEA), diethanolamine (DEA), diisopropanolamine, triethanolamine (TEA), _N_-methyldiethanolamine (MDEA), etc. are effective absorbents to capture acid gas components, such as CO\({}_{2}\) and H\({}_{2}\)S from natural or mixed gases [1; 2; 3; 4; 5; 6; 7]. Particularly, MDEA, a tertiary alkanolamine, has been most widely used in the field because it selectively reacts with CO\({}_{2}\) and H\({}_{2}\)S [1; 3; 8]. However, all alkanolamines including MDEA should be properly treated prior to being discharged into the environment since they are classified as hazardous substances [8]. Petrochemical and coal pyrolysis plants are major point sources generating a large volume of acid gases, and many of them adopt an acid gas capture process using alkanolamines [2; 3; 5]. In the acid gas capture process, varied organic and inorganic anions, such as glycolate, acetate, formate, oxalate, chloride, nitrite, nitrate, sulfate, and so on, are produced through the reactions between alkanolamines and acid gases. These anions lead to the accumulation of heat stable salts (HSS) within the process, which causes amine loss as well as decrease of process efficiency during the operation. To remove HSS, reclaimer unit processes, such as distillation, ion exchange, and electrodialysis, can be incorporated into the acid gas capture process [6; 9]. When using a reclaimer unit process, however, so-called reclaimer wastewater containing high concentrations of alkanolamines and other organic matters is generated, so its adequate treatment should be considered together. In general, wastewater containing alkanolamines is very difficult to treat by biological method because of low biodegradability of alkanolamines [1; 2; 3; 4; 7; 8]. As alternatives,advanced oxidation processes (AOPs), such as Fenton, ozone (O\({}_{3}\)), and photocatalytic oxidation, have been explored and reported to be effective not only in removing alkanolamines, but also in improving the biodegradability of the wastewater [2; 3; 4; 7; 10; 11; 12; 13; 14]. Fenton oxidation is a method that can decompose refractory pollutants present in wastewater through a chain reaction of hydroxyl radical (-OH), which is a powerful oxidizer produced by the reaction of ferrous ion (Fe\({}^{2+}\)) and hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)). This method has been known to be able to decompose various alkanolamines including MEA, DEA, and MDEA [2; 3; 7]. Ozone oxidation, meanwhile, use a chain reaction of hydroxyl radicals generated by ozone decomposition. This method is known to have an advantage of selective degradation for some target pollutants compared to the Fenton method [15; 16]. Although ozone has been widely used in the field for a long time to treat various kinds of recalcitrant pollutants, its application cases for alkanolamine degradation are not widely studied in the literature [3; 11; 12; 13]. The photocatalytic oxidation method has also been successfully used for the treatment of alkanolamines [4; 10; 14]. This method, however, is not suitable for treating a large amount of wastewater due to the maintenance problem of UV lamps [17]. Some combinatorial innovative AOP schemes, such as photo-Fenton, electro-Fenton, and peroxone using H\({}_{2}\)O\({}_{2}\) together with ozone, have also been developed for the treatment of refractory pollutants [18; 19; 20; 21]. However, more case studies are needed for their practical application in the field. AOPs have usually been used to treat the wastewater with low concentrations of refractory pollutants or to resolve turbidity issue at the final stage of wastewater treatment [20; 22]. In the case of high concentration wastewater, however, it is more efficient to use AOPs as pretreatment method for subsequent biological treatment, considering their high facility investment and reagent costs [2; 3; 4; 7; 15; 23]. [PERSON] et al.