## 2 The Degradation of Antibiotics in the Natural Environment  
### Photolysis and Hydrolysis  
The natural degradation of antibiotics in the environment primarily occurs through photolysis and hydrolysis. The photolysis process depends on the absorption of specific wavelengths of light by antibiotic molecules, which initiates photochemical reactions, including both direct and indirect photolysis. The process of direct photolysis refers to the direct absorption of sunlight by antibiotic molecules, particularly the photon energy in the ultraviolet-visible light spectrum, which triggers the breaking of chemical bonds or structural rearrangements, leading to degradation. The direct photolysis pathway is evident in the photochemical degradation of drug molecules that contain photon-absorbing groups, such as quinolones [19]. Indirect photolysis requires an excitation donor, known as a photosensitizer, to transfer excitation energy to an acceptor molecule, such as an antibiotic, in order to facilitate a photochemical transformation. This can include reactions such as reactive oxygen species (ROS, such as \({}^{1}\)O\({}_{2}\), O\({}_{2}\)\({}^{-}\), \(\bullet\)OH, H\({}_{2}\)O\({}_{2}\), etc. )-induced photo-oxidative degradation and hydrated electron-induced photo-oxidative degradation. However, the efficiency of these processes is significantly influenced by a variety of environmental conditions. [PERSON] discovered that quinolone antibiotics, such as ciprofloxacin, exhibit a higher affinity for soils and sediments. Additionally, oxidizers and catalysts, including MnO\({}_{2}\), Fe (III), and Al (III), which are commonly found in the natural environment, may significantly influence their oxidation and indirect photolysis. Among them, Fe\({}^{3+}\) and Al\({}^{3+}\) form complexes with the carboxyl or carbonyl groups of ciprofloxacin, while MnO\({}_{2}\), a metal oxide, fixes ciprofloxacin molecules through surface adsorption, thereby affecting its photostability. Additionally, Fe\({}^{3+}\) and MnO\({}_{2}\), as strong oxidants, can directly oxidize organic matter or generate reactive oxygen species through electron transfer when exposed to promote indirect photolysis. Conversely, Al\({}^{3+}\) alters the molecular conformation of ciprofloxacin through complexation or electrostatic adsorption, thereby inhibiting its photolysis. Other factors, such as humic substances, clay types, and pH, can also impact photolysis in the solid phase. At the same time, TiO\({}_{2}\) can generate strong oxidizing electron-hole pairs (e\({}^{-}\)-h\({}^{+}\)) under ultraviolet (UV) light, producing hydroxyl radicals (\(\bullet\)OH) and superoxide radicals (O\({}_{2}\)\({}^{-}\)). It has high photocatalytic activity, is low-cost, highly stable, and non-toxic, making it the most commonly used catalyst in photolysis [20]. Other factors, such as the type of clay, pH levels, and the presence of humic substances, can also impact photolysis in solid phases [21]. For example, dissolved humic acid (HA) contains various functional groups, including carboxyl groups, phenolic hydroxyl groups, carbonyl groups, amino groups, and methoxy groups. These negatively charged functional groups can influence van der Waals forces and hydrogen bonding during the adsorption of organic pollutants in the soil [22]. Furthermore, the effect of dissolved organic matter (DOM) on the environmental behavior of antibiotics varies significantly. For instance, DOM derived from biochar enhances the adsorption of sulfamethoxazole (SMX) while inhibiting the adsorption of chloramphenicol onto biochar [23]. The pH level directly affects the migration and transformation of sulfonamides (SAs) in the environment by altering their chemical state. At lower pH levels, sulfidiazine (SDZ) predominantly exists in a cationic form, leading many positively charged SAs to electrostatically bind to adsorption sites on the soil surface, which significantly increases the adsorption rate of SDZ.