Adsorption retention of spiramycin in agricultural calcareous loess soils: Assessing the impact of influential factors and mechanisms

The adsorption retention of different antibiotics and the physical and chemical properties of soil can vary significantly, leading to differences in their retention ability in soil environments. To investigate the adsorption retention characteristics and transfer risk, as well as influential factors of spiramycin (SPI) in three different agricultural calcareous loess soils, which were studied by means of batch equilibrium method. The results show that the adsorption kinetics data can be well described by the pseudo-second-order model, and the sorption isotherms can be described by the Freundlich model. These model parameters indicate that soil organic matter (SOM) and cation exchange capacity (CEC) play a key role in the adsorption retention of SPI, while soil clay content and pH also exert some influence. Furthermore, the desorption amounts of SPI were consistently less than 35%, showing a high degree of irreversibility and a clear desorption hysteresis, making it prone to remaining retention in the soil. The adsorption amount changed little of pH value from 3 to 7. At a pH value of 7, adsorption is promoted by cation bridging between soil and SPI, and the existence of electrostatic attraction. The adsorption amount decreased when the pH value exceeded 7 dues to electrostatic repulsion. Compared with K⁺ and NH⁴⁺, the presence of high valence ions such as Ca²⁺, Mg²⁺ and ions with higher concentration has a significant inhibitory effect on adsorption. The FTIR spectral analysis of the adsorbents revealed that the adsorption retention was mainly governed by cation exchange, hydrogen bonding, and surface complexation. This study provides a scientific basis for assessing the retention ability and transfer risk of typical veterinary antibiotics in the soil environment.