Study on the Adsorption Process and Mechanism of Oxytetracycline in Different Loess: Roles of Soil Characteristics and Influencing Factors

The presence of veterinary antibiotics (VAs) in the soil environment has raised significant concerns about ecological environmental protection. In this study, the adsorption/desorption behavior and mechanism of oxytetracycline (OTC), a typical VAs, was investigated using a batch equilibrium experiment in four types of agricultural loess soils. The results revealed that OTC exhibited the highest affinity for Heilu soil (HLS), followed by sierozem (SIE), cultivated loessial soil (CLS), and the weakest affinity for gray cinnamonic soil (GCS). Desorption hysteresis was observed for OTC in all four types of loess, indicating a positive hysteresis effect. As for OTC, electrostatic attraction played a significant role when pH < pK_a1; When pK_a1 < pH < pK_a2, hydrophobic distribution, surface complexation, and electron donor–acceptor interactions were the main influencing mechanisms. When pK_a2 < pH, the electrostatic repulsion between the negatively charged target pollutants and soil particles increased, but the cation bridging effect or anion exchange effect, was significantly obvious, making them the dominant mechanisms. The adsorption affinity of OTC on all four types of loess generally decreased or first increased and then decreased with increasing K⁺, Ca²⁺, and Mg²⁺ ion concentrations, the adsorption affinity on the four loess showed a law of continuous decrease or first increase and then decrease, except for Na⁺ ion concentration. It was observed that characterizing the affinity of OTC to different loess types and OTC influencing factors using solid-water adsorption coefficient K_d aided in predicting the OTC affinity effect in the natural environment. Thus, providing a theoretical basis for treating and remediating soil pollution caused by VAs.