## 2 The Degradation of Antibiotics in the Natural Environment
### Photolysis and Hydrolysis
As the pH rises, the electrostatic repulsion between SDZ and soil materials diminishes the adsorption capacity [24]. In addition, the presence of dissolved organic matter (DOM), such as humic acid (HA), significantly influences the photodegradation process of antibiotics. DOM is the primary form and active component of natural organic matter in aquatic environments. Research has demonstrated that DOM plays a dual role in the photolysis of antibiotics. It can either undergo direct photolysis or function as a photosensitizer to facilitate indirect photolysis. The latter involves the generation of reactive oxygen species (ROS), with photochemically reactive DOM being the main source of ROS in surface waters. Antibiotic molecules can experience rapid oxidative degradation in the presence of ROS [25]. DOM can undergo direct photolysis or function as a photosensitizer to facilitate indirect photolysis. This indirect photolysis involves the generation of reactive oxygen species (ROS), with photochemically reactive DOM serving as the primary source of ROS in surface water. Antibiotic molecules can experience rapid oxidative degradation when exposed to ROS [25]. However, there may also be situations that inhibit light degradation [26; 27; 28]. This phenomenon can be attributed to the light-protective capabilities of dissolved organic matter (DOM), which create a shielding effect, quench photoproducts of reactive intermediates (PPRIs), and regulate the inherent excited state characteristics of antibiotic molecules. As a result, this process inhibits the photodegradation of antibiotics [29]. Humic acid (HA) has a dual role in the photodegradation of tetracycline (TC). At low concentrations of HA (<6 mg/L), it acts as a photosensitizer; under simulated sunlight/xenon lamp irradiation, HA absorbs light energy to generate excited triplet state \({}^{3}\)HA\({}^{+}\) and produces reactive oxygen species (ROS) such as \({}^{1}\)O\({}_{2}\), O\({}_{2}\)\({}^{-}\), and \(\bullet\)OH, which indirectly oxidize TC and promote its photodegradation. At high concentrations of HA (>6 mg/L), it competes for photon absorption, reducing the direct photodegradation efficiency of TC and quenching the ROS generated by TC itself (e.g., O\({}_{2}\)\({}^{-}\)), thereby inhibiting TC photodegradation [30]. However, sometimes the promoting effect of HA on TC photodegradation is not obvious, possibly because the ROS generated by HA are quickly quenched by TC or other components, leading to a significant inhibition of TC photodegradation by HA (5 mg/L) at a pH of 8.0-9.0 [31]. The hydrolysis pathway of antibiotics is effective for certain types of pollutants, particularly in the degradation of esters and amides. The hydrolysis of \(\beta\)-lactam compounds typically occurs through the opening of the \(\beta\)-lactam ring and the cleavage of the amide bond. For instance, the hydrolysis of cephalexin primarily results from the rupture of the \(\beta\)-lactam bond and the positional alterations of the branched amide bond, which render the hydrolyzed form of cephalexin more amenable to biodegradation by microalgae [32]. In addition, pH levels and temperature can significantly influence the hydrolysis of antibiotics. For instance, research conducted by [PERSON] assessed the hydrolysis conditions of \(\beta\)-lactams and discovered that the hydrolysis rates of ampicillin, cefazolin, and cefoxitin were positively correlated with temperature. Furthermore, the hydrolysis rate in alkaline environments was markedly higher than in acidic and neutral pH conditions [33].