Halogenated organic compounds are widely spread in the environment (Asplund et al., 1989;26 Gribble, 2015; Winterton, 2000), and many of them have unknown molecular structures27 (Asplund et al., 1989; Hua, 2006). To quantify the total amount of organically bound28 halogens, the sum parameter called AOX representing an approximation of the sum of29 organically bound chlorine, bromine and iodine are used (Asplund et al., 1989).
The30 organically bound halogens in water are usually hydrophilic, but will sorb to activated carbon31 and can be quantified as AOX. At the time of the AOX method development, an32 anthropogenic origin of AOX was generally assumed. Later, the discovery of naturally33 produced AOX in the environment changed the perception about the occurrence and levels of34 AOX (Asplund et al., 1989; Grimvall et al., 1991a; Grimvall et al., 1991b). AOX includes35 halogenated chemicals of different structures and toxicological profiles, of which some36 accumulate in the food chain and pose serious adverse health and environmental effects37 (Schowanek et al.
, 2004). Some halogenated compounds have been found to be carcinogenic38 (IARC, 1995), mutagenic (Cemeli et al., 2006), and geno toxic (IARC, 1999). Due to the39 toxic nature of such halogenated compounds (Cemeli et al., 2006; Robbiano et al.
, 2004), the40 presence of AOX in water is of concern.41 AOX has been reported in natural waters (surface and ground waters) (Flodin et al., 1997;42 Hütteroth et al., 2007; Kaczmarczyk and Niemirycz, 2005) as well as in drinking water43 (Peters et al., 1991). AOX is formed by both natural and anthropogenic processes (Asplund et44 al., 1989; Grimvall et al.
, 1991b). For example, certain industries (predominantly paper,45 chemical, and textile industries) are major sources of anthropogenic AOX (Müller, 1992;46 Savant et al., 2006). However, biogeochemical processes also contribute naturally produced47 AOX (Asplund and Grimvall, 1991) and considered as a natural source for AOX.48 The formation of AOX have been proposed via two fundamental pathways. The first one49 comprises enzymatic biosynthesis (Van Pee and Unversucht, 2003). The other pathway is50 related to chemical oxidation of organic matter in the presence of halogen ions (Hua and51 Reckhow, 2006; Hua and Reckhow, 2007a; Van Pee and Unversucht, 2003).
Therefore,52 during drinking water production, the AOX may be formed as disinfection by-products53 (DBPs) following second pathway, when various forms of chlorine is used for disinfection or54 to prevent growth of pathogens in the distribution system (Ko et al., 2000; Sörensen et al.,55 2001).