Study on the Mechanism of Polypropylene Microplastics Affecting the Adsorption-Desorption of Ciprofloxacin in Soil

Microplastics (MPs) and emerging pollutants such as antibiotics are commonly detected in soil and exhibit characteristics of combined pollution. Their migration is influenced by soil adsorption-desorption processes. However, existing research has primarily focused on the adsorption-desorption of antibiotics in pure soil or the adsorption of antibiotics on pure microplastics, with limited studies on the adsorption-desorption of antibiotics in soil-microplastic composite systems. Based on the detected concentrations of antibiotics and the environmental abundance of MPs, this paper selects ciprofloxacin (CIP) and polypropylene (PP) as representative antibiotics and MPs, respectively. Through adsorption-desorption experiments, the adsorption-desorption process of CIP in soil added with different proportions of PP was investigated. High-performance liquid chromatography (HPLC) was used to detect CIP concentration, and experimental data were fitted using two kinetic models (pseudo-first-order, pseudo-second-order) and two adsorption isotherm models (Freundlich, Linear). The results showed that the adsorption of CIP in soil with and without PP added conformed to both pseudo-first-order and pseudo-second-order kinetic models (R² ≥ 0.997). Compared to the Linear model (R ≥ 0.952), the isothermal adsorption-desorption characteristics of CIP in soil with PP added were more consistent with the Freundlich model (R² ≥ 0.991), indicating that the adsorption of CIP in soil with PP added was influenced by a combination of cation exchange and hydrophobic partitioning mechanisms. There was no significant difference in the equilibrium adsorption capacity of CIP between soil with and without PP added, suggesting that soil still played a dominant role in CIP adsorption even with up to 20% PP added. However, the isothermal adsorption fitting parameters indicated that as the proportion of PP added increased, the adsorption capacity of soil for CIP decreased, while the desorption capacity increased. Using the desorption hysteresis index (HI) to analyze the desorption experiment results, it was found that the HI value (0.043–0.066) increased monotonically with the increase in PP addition proportion, indicating a significant desorption hysteresis phenomenon of CIP. The higher the proportion of PP added, the weaker the desorption hysteresis effect, making CIP easier to release from soil and increasing the environmental migration risk. This study reveals the impact of PP on the adsorption-desorption of CIP in soil, providing data support for further exploring the interaction mechanism and environmental behavior of MPs and antibiotics in soil.