## 3 Antibiotic Degradation and Fate Kinetics in Surface Water
### Sorption  
Sorption is the attachment of dissolved substances from the aqueous to the solid phase. Sorption involves accumulating dissolved substances on solid surfaces by adsorption and the penetration or intermingling of substances with solids by absorption. Numerous studies report the high concentration of antibiotics in stream beds and suspended matters, considering that sediments act as sinks of antibiotics [64; 65; 66; 67; 68]. Sorption is vital in determining the reactivity, mobility, persistence, volatilization, and bioavailability of pollutants in natural waters, as it controls the antibiotic concentration between the water, suspended solids, or sediments [69]. The sorption of antibiotics in the water column follows a pseudo-first-order reaction. The water-sediment partitioning coefficient is used to determine the sorption capacity of the solid [25]. The sorption of antibiotics onto the suspended solid and the complexation of dissolved organic matter are determined by the octanol-water portioning constant (\(K_{ow}\)) and the fraction of organic carbon (\(f_{\text{oc}}\)) as presented in Equations (1)-(5) [63]:  
\[\frac{dC_{b}}{dt}=\frac{Ck_{dom}DOM}{1+k_{dom}DOM+k_{dss}TSS} \tag{1}\]  
\[\frac{dC_{p}}{dt}=\frac{Ck_{dss}TSS}{1+k_{dom}DOM+k_{dss}TSS} \tag{2}\]  
\[k_{dss}=f_{\text{oc}}K_{oc} \tag{3}\]  
\[k_{dom}=f_{\text{oc}}K_{oc} \tag{4}\]  
\[LogKoc=1.18 logKow-1.56 \tag{5}\]  
where \(C\) is the concentration of the antibiotics (ng/L), \(C_{b}\) is dissolved organic matter bounded antibiotic concentration (ng/L), \(C_{p}\) is the particulate solid bound antibiotic concentration (ng/L), t is time (day), \(K_{dom}\) is the complexation coefficient between antibiotics and dissolved organic matter (L/kg), and \(K_{dss}\) is the partition coefficient between antibiotics and solid (L/kg). The exchange in the hyporheic zone of river channels is an essential medium in removing antibiotics from natural water bodies. Depletion of compounds in the water and sediment creates a concentration gradient (equilibrium imbalance) between the concentration of the aqueous and bounded form. Consequently, the adsorption and desorption processes rule out the migration of the antibiotics from one phase to the other. Residual antibiotics may sorb onto the solids by adsorption or settle in the sediments. Further, it may diffuse from the benthos' solid-bound phase to the water column's dissolved phase or be absorbed into the solids of the underlying benthos. Antibiotics in the sediment and sediment-water interface can be depleted by biotic activity (biotic depletion), facilitating resuspension and desorption, which cause the water column to be continuously polluted by migrating the antibiotics from the solid phase to the water column. For instance, biotic depletion of sulfamethoxazole in the benthos in the sediment-water interface made a concentration gradient and is absorbed into the sediment, leaving the water column [41]. Conversely, the depletion of sulfamethoxazole in the water column suggests the desorption of sulfamethoxazole from the sediment into the water [70]. As a result, the underlying sediment in the aquatic environment causes the prolonged presence of antibiotics [33]. Hence, sediments are regarded as secondary sources of diffuse pollution in water columns, releasing adsorbed residue to the overlying waters. Sediments are porous matter containing loose particle spacing, enabling them to have many sorption sites on their surfaces. As a result, antibiotics bind to the surface of the sediment particles via electron attraction induced by their functional group. The adsorption of antibiotics is influenced mainly by their physiochemical properties, thecomposition of the adsorbent matrices, and the state of the surrounding environmental element. Antibiotics with amino, hydroxyl, and carboxyl functional groups adsorbed to solid/sediment by cation exchange, hydrogen bonding, cation bridging, and other mechanisms [69; 71; 72; 73].