Adsorption–Desorption Behavior of the Antibiotic Ofloxacin in Calcium-Rich Sediments at Riverbank Sewage Discharge Outlets: Implications for Sustainable Water Quality Management in an Arid Inland River Basin

Fluoroquinolone antibiotics are widely detected emerging pollutants, yet their interfacial behavior in the alkaline, calcium-rich sediments of arid inland river basins remains poorly quantified, which limits regional risk assessment. This study selected riverbank calcareous sediments from the Ili River and ofloxacin (OFL), a representative fluoroquinolone, and used batch adsorption–desorption experiments to quantify the retention of OFL and the regulatory roles of pH and coexisting cations. Adsorption was biphasic, with a fast stage (<1 min) followed by slow equilibration, and equilibrium was approached within 8 h. When the pseudo-first-order (PFO), pseudo-second-order (PSO), and Elovich models were all fitted by nonlinear least squares on the original q-t scale, the Elovich model gave the best description at both 1 and 10 mg L−1 (R2 = 0.829 and 0.589, lowest AIC), whereas PFO and PSO performed poorly over the full time range (R2 = 0.20–0.53). Boyd analysis indicates that particle diffusion dominates for the two XFD datasets, whereas a significant positive intercept in a second sediment series shows that film diffusion also contributes. Desorption kinetics were also best described by the Elovich model (R2 = 0.929). Isotherm analysis with 4–11 data points per temperature (rather than the three points used previously) showed that the Langmuir and Freundlich models fit the data comparably, so the results do not justify discriminating between monolayer and multilayer uptake (difference in AIC ≤ 1 at 15 and 35 °C, and 6.5 in favor of Langmuir at 25 °C); the corresponding Dubinin–Radushkevich mean free energy of 0.08–0.25 kJ mol−1 indicates physisorption-dominated uptake. Thermodynamic parameters calculated from the 15/25/35 °C series by the van’t Hoff relation gave Delta-G = −20 kJ mol−1 (spontaneous), Delta-H = −14.5 ± 13.2 kJ mol−1, and Delta-S = 20.5 ± 44.3 J mol−1 K−1, i.e., a weakly exothermic, entropy-assisted process whose enthalpy term is not statistically significant; the 5/15/25 °C series was non-monotonic and is therefore not used to derive the enthalpy. Desorption exceeded adsorption (maximum desorbed/adsorbed ratio 106–136%) and the Freundlich-based hysteresis index rose from −0.09 at 5 °C to 0.37 at 25 °C, so part of the sorbed OFL is retained against desorption. Coexisting cations inhibited OFL adsorption through competition for surface sites, following the order Mg2+ > Ca2+ > Na+ at matched molarity (0.01 mol L−1), and the inhibition intensified with increasing cation concentration. Because Na+ was applied at the same molarity as Mg2+ as an ionic-strength control while Ca2+ was also applied at a higher concentration, the ranking reflects specific competition by the divalent cations combined with a weaker double-layer (ionic-strength) effect for the monovalent ion. Acidic pH enhanced adsorption and alkaline pH suppressed it, consistent with the change in OFL speciation from cationic through zwitterionic to anionic. This work quantifies the adsorption–desorption behavior of OFL on arid-region calcareous sediments and provides parameter values with uncertainty estimates that support risk assessment and pollution control in the Ili River Basin. From a sustainability perspective, the work converts these site-specific measurements into transferable, uncertainty-bounded parameters (Langmuir and Freundlich capacities, the linear partition coefficient Kd and the hysteresis index) for a sediment type that is widespread in arid inland river basins. It therefore supplies directly usable input for the definition and quantification of water-quality sustainability indicators, the monitoring of the sediment buffering function over time, and the setting of discharge-outlet effluent limits and the seasonal screening of secondary-pollution risk during the warm, low-flow irrigation period, thereby connecting a mechanistic sorption study to the wider objective of sustaining safe water use in a water-scarce region.