ADSORPTION AND DESORPTION OF NARASIN IN GALICIAN SOILS: INFLUENCE OF SOIL PROPERTIES

Introduction
Ionophore antibiotics, such as narasin, are widely used in intensive livestock production to control coccidiosis, and up to 90% of the administered dose is excreted, reaching soils through the application of slurry and manure. This poses an environmental risk by promoting bacterial resistance, ecotoxicity, and entry into the food chain. Given their high toxicity to humans, investigating the adsorption capacity of soils is essential to limit their mobility.

Methods
Batch adsorption experiments were conducted by adding increasing concentrations of narasin (0–600 µmol L⁻¹) to eight Galician soils with different physicochemical properties.

Results
In soils with pH < 5, adsorption ranged from 85 to 100%, reaching 100% at the highest added concentration; the forest soil exhibited 100% adsorption at all concentrations. In soils with pH > 5, adsorption varied between 20 and 90%. In two soils, adsorption increased with increasing narasin concentration, consistent with a cooperative adsorption model, whereas in the remaining soils, adsorption was high at low concentrations, decreased at intermediate concentrations, and increased again at the highest concentrations. At pH < 5, adsorption occurred mainly through electrostatic interactions and hydrogen bonding, whereas at higher pH values, cation bridging predominated. The hydrophobic nature of narasin and its high affinity for soil organic matter also enhanced adsorption. Desorption was below 10%, indicating that adsorption was essentially irreversible.

Conclusions
The studied soils exhibited a high capacity to adsorb narasin, mainly governed by soil pH and organic matter content, thereby reducing its mobility and the risk of incorporation into the food chain.