Competitive adsorption and desorption of three tetracycline antibiotics on bio-sorbent materials in binary systems

Batch-type experiments were used to study competitive adsorption/desorption for the antibiotics tetracycline (TC), oxytetracycline (OTC), and chlortetracycline (CTC), onto by-products from forest and food industries (oak ash, pine bark, and mussel shell). These antibiotics are frequently present in manures and slurries spread on agrosystems. Binary competitive systems were performed by setting the dose of one antibiotic to 200 μmol L^-1, and varying the concentration of a second antibiotic from 50 to 600 μmol L^-1. In the cases where a concentration of 200 μmol L^-1 was used for each antibiotic, the results of the binary experiments were also compared with those obtained in parallel tests corresponding to simple and ternary systems using the same concentration. The results indicated that pine bark can adsorb most of the antibiotics added, with desorption being less than 5% in most cases. Oak ash showed high adsorption for all three antibiotics in the simple systems (100% of CTC, 90% of TC, and 80% of OTC), but clearly decreased in the binary systems (up to values below 40%), especially for higher antibiotics concentrations, although desorption was generally less than 5%. Mussel shell showed adsorption results lesser than 25% for OTC and CT in simple systems, while increased up to 65% in binary systems in which CTC was present at high concentrations, but desorption was generally very high. CTC was the antibiotic with the highest adsorption onto all three by-products, and the one showing less decrease for its adsorption in the binary systems. Overall, the smallest differences among the various competitive systems were obtained when the adsorbent used was pine bark, and especially for the CTC antibiotic. These results could aid to develop management practices, based on the use of low-cost bio-sorbents, which would decrease risks of pollution due to tetracycline antibiotics spread in agroecosystems and affecting the environment.