## 3 Antibiotic Degradation and Fate Kinetics in Surface Water
### Sorption
For instance, cation exchange and hydrophobic partition play a vital role in the sorption of fluoroquinolone and sulfonamides. However, this does not necessarily mean other mechanisms do not affect their sorption. The mechanisms depend on and are influenced by the sediment's pH, ionic strength, temperature, organic matter, type, and size. The high-level organic carbon content in the sediment enhances sorption capacity [37]. Finer particles, such as silt, clay, and organic matter/detrius, are the ideal solvents to have a greater sorption capacity and transport sorbate due to their large surface area-to-volume ratio. Antibiotics carry polar and charged ions throughout the environment, which are influenced by pH [10; 65; 69; 71]. The polar and charged ion interacts with the surface charge of the solid. Antibiotics with a positively charged functional group have an affinity to interact with negatively charged sediment via cation bridging. Antibiotics with the polar functional group may interact with the acidic group of sediment by hydrogen bonding. Overall, the opposing charges of the contaminant and the environmental medium facilitate antibiotic adsorption. Tetracycline's ammonium group interacts with negatively charged sites via cation exchange, while the negatively charged tricarbonyl methane keto-enol moiety (tetracycline derivative) interacts with negatively charged sites through cation bridging [69]. Furthermore, a fraction of the sediment sorbed may desorb due to environmental changes (salinity, pH, and temperature), resulting in hysteresis. This reversal process is highly dependent on the energy of the chemical bond between the sorbate and binding sites and thus affects the antibiotic sorption rate [71]. The availability and composition of different ions in the system will give the sorption capacity, and Kd values different degrees of change. The presence of metallic ions Fe\({}^{3+}\), Na\({}^{+}\), K\({}^{+}\), Ca\({}^{2+}\), and Mg\({}^{2+}\) in an acidic solution compete with Tylosin, an antibiotic from the erythromycincin family, which decreases the adsorption capacity of the solid to macrolides [33; 74]. In neutral and alkaline solutions, metallic ions act as proton donors and may promote sorption [33]. In most cases, the positively metallic minerals are attracted to negatively charged surfaces forming a metallic ion complex with functional groups of antibiotics, inhibiting the sorption process and playing the decisive role [71; 72; 75; 76]. Most fluoroquinolones' affinity to sorption is controlled by cation exchange, as most of them are cationic species in natural pH and can be easily attracted to negatively charged solids or sediments [72]. Conversely, sulfonamides poorly adsorb into the sediments and are detected in relatively high concentrations in the pore water than the sediment [74]. This is attributed to the electrostatic repulsions of sulfonamides, primarily acidic chemicals, which adsorb poorly in alkaline conditions [74]. Thus, due to their weak adsorption into solids and sediments, sulfonamides pass the hyporheic zone and swiftly reach and contaminate groundwater [39]. The change in pH affects the dissociation of antibiotics and the surface charge of the solid [71]. The pH is the primary factor affecting the sorption process. In most cases, the sorption rate of antibiotics decreases with the increase in pH. The sorption rate of chlortetracycline in natural water decreased from 0.68 to 0.54 at a pH of 7.9 and 9 [71]. The sorption rate of tetracycline decreases linearly with an increase in pH from 7, 7.5, 8, and 8.5 [69]. Temperature is another factor that influences the sorption of antibiotic contaminants. Increased temperature beyond normal room temperature decreases sorption rates. The variation in water temperature influences the solubility of pollutants (the higher the temperature, the higher the solubility) and influences the dissociation accordingly.