## 3 Antibiotic Degradation and Fate Kinetics in Surface Water  
Given their high concentration in wastewater, antibiotics have been frequently detected in surface waters [35]. However, the concentrations of antimicrobials in surface water are lower than in wastewater due to degradation (e.g., uptake and metabolism by aquatic microorganisms, photolysis due to sunlight exposure, and hydrolysis). The concentration of antibiotics in the water and sediment occurs as freely dissolved organic matter bound (antibiotics bind to dissolved organic matter) and particulate solid bound (antibiotics bound to solid) [36]. Antibiotics in the dissolved form are transported in the water flow, whereas antibiotics in the bound form are suspended and transported by sediment (particulate organic carbon). The fate and persistence of antibiotics in natural waters are subjectedto various instream reactions as they travel downstream. It attenuates, degrades, decomposes, and transforms into another compound during transportation. The typical instream mechanisms that decrease the aqueous concentration of antibiotics in the water column may be categorized as physical (sorption, resuspension, sedimentation, and volatilization), biological (bioconcentration, uptake/co-metabolism, and biodegradation), and chemical processes (photodegradation, oxidation, and hydrolysis). The effect of reaction processes on a particular antibiotic differs and depends on various factors. For instance, the sorption process is more likely to rely on the geological composition of the medium through which the river flows and the organic carbon content of the suspended solid and underlying sediment. Hence, higher quantities of organic carbon content in underlying sediments could result in a higher sorption rate of antibiotics depending on their physiochemical properties [37]. Further, the hydrolysis of antibiotics is sensitive to the system's ionic strength and pH value. The cationic forms of antibiotics are more susceptible to hydrolysis than neutral and anionic forms [37]. The density of microbes and the presence of high carbon content in the surface water play an increased rate of antibiotic biodegradation. The presence of organic carbon in the river water is limited to sediment, sludge, and wastewater, resulting in a lower biodegradation rate of antibiotics in the water column [37]. Sunlight exposure and the presence of sensitive substances favor a higher rate of photolysis. Therefore, fate modeling of antibiotics in the environment requires a particular investigation of specific rate kinetics. Identifying possible classes of antibiotics detected from specific sources can be helpful for a better understanding of their persistence and evolution in the water environment. The most frequently detected antibiotic classes in natural water channels are sulfonamides (sulfadiazine, sulfadimidine, sulfamethoxazole), Quinolones (fluoroquinolones, norfloxacin, ofloxacin, ciprofloxacin, enrofloxacin), Beta Lactams (penicillin, ampicillin, Carbapenems, carbamazepine), tetracyclines (tetracycline, oxytetracycline, chlortetracycline, and doxycycline) and macrolides (erythromycin, Roxithromycin) [27; 38]. The fate of specific antibiotics may not be equally affected by all the instream degradation reaction mechanisms, as these reactions are influenced by their reactive functional ring. For instance, in a natural river, transport of the tetracycline group of antibiotics has a higher rate of sorbing to sediments than other antibiotic groups [27; 35]. Sulfonamides are highly susceptible to biological degradation, while biotic degradation has a minor role in tetracyclines [39]. Fluoroquinolones are sensitive to photolysis and are reported as the primary removal mechanism, resulting in a high photodegradation rate [40]. Microbial communities readily biodegradable macrolides, while photodegradation has a minor role in macrolide degradation due to its insufficient chromophore groups to absorb sunlight energy [33].