Collaborative sorption behavior of tylosin and enrofloxacin in loess soil: implications for veterinary antibiotic fate†

Multiple veterinary antibiotics (VAs) coexist in soils, but their interaction and their effects on sorption in soils have not been extensively studied. This study focused on investigating the sorption properties of two widely used VAs, tylosin (TYL) and enrofloxacin (ENR), in loess soil. Through a comprehensive batch sorption method, we systematically evaluated single and binary sorption behaviors using varying ions (Ca²⁺, Mg²⁺, NH₄⁺, and K⁺), ionic strengths (from 0.01 to 0.1 M) and pH (4–10). The results showed that batch experiments fitted well with pseudo-second-order kinetics (equilibrium times: 4 h for TYL and 2 h for ENR) and Freundlich isotherms (1/n = 0.364–0.831), revealing distinct sorption pathways. The results showed that TYL enhanced ENR sorption by up to 88% in binary systems, acting as a “surrogate organic phase” via hydrophobic interactions, with Freundlich coefficients (K_f) increasing from 0.112 (TYL) and 0.331 (ENR) in single systems to 0.215 and 0.411, respectively. High Ca²⁺ (0.10 M) inhibited adsorption due to site competition (sorption capacity order: NH₄⁺ > K⁺ > control > Ca²⁺ > Mg²⁺). While single-system sorption affinity K_d declined with pH from 4 to 10 (ENR from 0.888 to 0.126 mg g⁻¹; TYL from 79.1 to 2.13 L kg⁻¹), binary systems reversed TYL's pH dependency, peaking at pH 8 (K_d = 649 L kg⁻¹) due to ENR-induced surface charge modification and Ca²⁺-bridging. These results underscore the critical role of co-sorption in modulating antibiotic mobility and advocate for integrating multi-pollutant interactions into risk assessments for calcareous soils, directly informing contamination management in vulnerable semi-arid ecosystems.