Adsorption and desorption of the ionophore antibiotics narasin and monensin in soils and bioadsorbents from Galicia (NW Spain)

Ionophore antibiotics, such as narasin and monensin, are widely used in the poultry industry and are the only type of antibiotics that the EU allows to incorporate as feed additives, and use them without veterinary prescription, with the consequent risk of favouring antibiotic bacterial resistance. In the case of the narasin it has been proved that, in poultry, the use of this antibiotic may increase the number of enterococci bacteria that are resistant to the human antibiotic vancomycin; meanwhile ruminal Prevotella strains might become resistant to monensin. Furthermore, these drugs are very toxic to humans. For these reasons, the entrance of antibiotics in the environment due to the use of manure and slurry as fertilizers in agricultural soils is an important environmental problem and a risk for human and animal health. The soil can adsorb these antibiotics and prevent their entry into the food chain, but sometimes its retention capacity is low and could be improved by incorporating residues that can act as contaminant adsorbents. The objective of this study was to analyse the adsorption and desorption processes in three soils and four different biadsorbents for narasin and monensin. The study was perform using one forest soil under Eucalyptus and two crop soils, and four different by-products as bioadsorbents (wood ash, pine bark, mussel shell and olive residue). Different concentration (5, 10, 20, 50, 100, 200, 400, 800, 1000 µmol L^-1) of both antibiotics were added to both soils and bioadsorbents and adsorption / desorption test were performed by means of HPLC.

The results showed that the soils adsorbed 100% of the added monensin at low concentrations, and this percentage decrease to 80-86% when 1000 µmol L^-1 of antibiotic were added. The adsorption was irreversible for the low concentrations and the desorption increase up to a maximum of 1-12% for the higher ones. Regarding the bioadsorbents, for the higher concentration of monensin added, the olive residue and wood ash adsorbed 99 and 98% of the antibiotic, respectively. On the other hand, the pine bark and mussel shell adsorbed a maximum of 64 and 48%, respectively, for the lower concentrations and these percentages decreased to 25 and 34% when 1000 µmol L^-1 were added, with generally small desorption values. For the antibiotic narasin, the soils adsorbed 100% of the added antibiotic, and only decreases to 99% when 1000 µmol L^-1 are added; and they desorbed less than 1% of the absorbed at that concentration. Regarding the bioadsorbents, the olive residue adsorbs irreversibly 100% of the narasin added for all concentrations, meanwhile the mussel shell, wood ash and pine bark adsorbed 86, 89 and 96%, respectively, for the highest narasin concentration, with no desorption for any antibiotic concentration.

The soils, olive residue and wood ash were good bioadsorbents for both antibiotics, due to their high adsorption capacity, irreversible in most cases. The pine bark and mussel shell were as well good bioadsorbents for naransin, but that´s not the case for monensin.