Salinomycin and lasalocid adsorption/desorption by different soils and bioadsorbents from Galicia (NW Spain).

Salinomycin and lasalocid are polyether ionophore antibiotics commonly used in animal production as anticoccodia. These antibiotics are partially metabolised and excreted at least partially in the active form. Therefore, these drugs enter the environment mainly by direct deposition of urine and faeces on soil, or when manure is applied to soils as fertilizer, facilitating their subsequent entry into the food chain, as well as the development of bacterial resistance and risks to human and animal health. Soils can potentially reduce environmental risks related to these antibiotics through the adsorption on their components, which implies its immobilization. Adsorption depends on the antibiotic characteristics and physicochemical soil´s properties. Some soils have a low capacity to retain antibiotics, for this reason is necessary to investigate low-cost strategies to increase the adsorption capacity of the soils, minimizing environmental pollution. Pine bark, oak ash, mussel shell and olive residue could be used as bioadsorbents of these contaminants, since they have a high adsorption capacity for other antibiotics and are produced in large quantities in several countries. The objective of this work was to investigate the adsorption/desorption capacity of salinomycin and lasalocid of three soils from Galicia (NW Spain) with different properties, and also that of the four residues previously indicated, which could be added to soils to improve the adsorption capacity of these antibiotics. To carried out this work, batch experiments were performed, adding different concentrations (5; 10; 20; 50; 100; 200; 400; 800 and 1000 µmol L^-1) of these antibiotics at 2 grams of soil or 0.5 grams of bioadsorbent samples; the antibiotic concentration in equilibrium solution was measured by HPLC-UV

The results obtained showed that lasalocid was adsorbed completely by the soils in all cases, whereas salinomycin was adsorbed totally by soils when the concentration added was between 5-200 µmol L^-1, however when the concentration added increased (1000 µmol L^-1), adsorption decreased up to 68%.  The results showed that soil with a high pH value (pH=7.97), presented slightly lower adsorption values. In the case of the bioadsorbents, pine bark (pH=3.99), olive residue (pH=5.95) and ash (pH= 11.31) adsorbed 100% of salinomycin in all added concentrations, therefore, these residues could increase the capacity of the soil to adsorb this antibiotic. However, in the case of lasalocid the adsorption by ash was 75%, by olive residue was 94% and 95% by pine bark, when the maximum concentration of antibiotic was added. Mussel shell (pH= 9.39) was the bioadsorbent that presented the lowest adsorption for both antibiotics, 87% for salinomycin and 22% for lasalocid, when 1000 µmol L^-1 was added. Desorption results of salinomycin showed that this is always less than 10% for soils and less than 5% for bioadsorbents. In the case of lasalocid, desorption was less than 3% for soils and bioadsobents.

In conclusion, soils adsorbed 100% of lasalocid, however, salinomycin adsorption was lower. To retain this antibiotic, the use of bioadsorbent would be interesting, specially, pine bark, olive residue and ash, which adsorbed all the added salinomycin.