## 3 Antibiotic Degradation and Fate Kinetics in Surface Water
### Photodegradation  
Photodegradation is the abiotic degradation of compounds caused by sunlight absorption. Photolysis is the degradation pathway for most antibiotics in natural water and wastewater treatment [54; 55]. Photodegradation is the primary elimination pathway for fluoroquinolones, sulfonamides, and tetracyclines [40; 56]. There are two forms of photodegradation: direct and indirect (reaction due to photochemical-produced reactive intermediates). Direct photodegradation is the chemical degradation of a compound due to the direct absorption of solar radiation, leading to the formation of highly reactive intermediates or radicals that can cause the compound to break down into less complex molecules [57]. Indirect photodegradation occurs when photochemically produced reactive oxidant species (singlet oxygen (O\({}_{2}\)), superoxide (O\({}_{2}\)\({}^{-}\)), hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)), peroxyl radicals (OOR), triplet excited state dissolved organic matter (DOM), and hydroxyl radicals (OH)) transform compounds into a molecule [56; 58]. Photosensitizers photochemically produce the reactive oxidant species under light exposure. A series of intermediate (reactive oxidant) species could be generated when light absorption transforms a substance surrounding the antibiotics into an excited state. Consequently, the active species reacts with the antibiotic and initiates a faster decomposition by shortening the half-life of residual antibiotics [59]. The photolysis removal of antibiotic residues in the environment is influenced by various factors, such as water composition (such as organic and inorganic compounds, type and content of dissolved organic matter), water property (conductivity, pH, and temperature), photo intensity, structure and property of organic pollutants, the functional group of the antibiotics and algal biomass. The photolysis mechanism is primarily influenced by latitude in addition to cloud cover, a fraction of daylight, water level (depth), and bacterial and algal biomass. Algal and bacterial biomass secrete photoactive extracellular and intracellular organic matter. The degradation rate of antibiotics increases when the radical photocatalyst species grow due to the high concentration of algal release of organic matter [57]. However, the produced organic matter may either induce the generation of photosensizers (active species, such as chlorophyll, protein, humic, and fulvic dissolved organic matter) upon light absorption [59], where they may improve the photolysis rate or absorb irradiation by masking the light and scavenging of reactive oxygen species, playing a decisive role in the photolysis rate of antibiotics. Many studies report the effectiveness of the algal-mediated photodegradation of antibiotic residues [57; 59; 60]. [PERSON] et al. [61] reported a 90% removal of chlortetracycline by algal-induced extracellular organic matter, while [57] reported a 38% elimination ofloxacin by algal matter-induced photolysis. Increasing the concentration of organic matter (fluvic acid) from 5.0 to 50.0 mg/L under a simulated sunlight irradiation of light wavelength of 300-800 nm and temperature 21 \({}^{\circ}\)C decreased the rate constant of sulfamethoxazole from \(3.0\times 10^{-3}\) to \(1.0\times 10^{-3}\) min\({}^{-1}\)[62], attributed to the scavenging of oxidant species. Thus, photo-reactive intermediates produced by DOMs trigger the removal of most human antibiotics in the aquatic environment by indirect photodegradation [58]. However, the composition of DOMs influences the degradation of a particular antibiotic. For instance, the DOM with an oxygenated environment increases the degradation rate of sulfathiazole, while the steady purge of N\({}_{2}\) gas (deoxygenerated condition) decreases the degradation of sulfarrazine [58]. The pH of a solution changes the photolytic forms of organic matter and antibiotics, as well as the rate at which reactive intermediates are generated.