Adsorption/Desorption and competition of amoxicillin, ciprofloxacin, and trimethoprim on the sites of three bioadsorbents.

Antibiotics are essential for treating infectious diseases in humans, animals, and plants, but their massive use in recent decades, together with their low absorption by the body, has led to their presence in water and soil, representing a serious threat to human health and ecosystems. The main risks include the proliferation of resistant bacteria, ecotoxicity, and their incorporation into the food chain. The average total consumption of systemic antibacterial agents in Europe in 2022 was estimated at 17.8 daily doses per 1,000 inhabitants per day in hospitals, with a very similar value for consumption outside hospitals. Up to 90% of ingested antibiotics are excreted through feces and urine, accumulating in slurry pits or sewage sludge from wastewater treatment plants. Once in the soil, these compounds can degrade, leach, or be retained, preventing their transfer to water and plants. However, these processes depend on the properties of the soil and the antibiotic. The adsorption method, as an environmental control of these drugs, is the one that presents the most advantages since it is simple, low-cost, highly efficient, non-polluting, and renewable. Consequently, this work is going to study the retention capacity of two forestry residues (pine bark and oak ash) and one food residue (mussel shell) to retain three antibiotics widely used in human medicine (amoxicillin, trimethoprim, and ciprofloxacin). Batch-type experiments were conducted, adding increasing concentrations (0–400 µmol/L) of the three antibiotics in both single (individual) and binary systems, that is, two at a time in all combinations. Additionally, the adsorption results were fitted to the Freundlich isotherms.

The results show that in all systems, simple and binary, ash is the most effective for adsorbing amoxicillin (with rates close to 100% in competitive systems), and there is virtually no desorption, indicating its suitability as an adsorbent for this drug. Pine bark retains almost the entire amount of ciprofloxacin and trimethoprim added in all cases and desorbs very little trimethoprim (desorption rate lower than 5% for all added concentrations of trimethoprim and below 20% for the two highest added concentrations of ciprofloxacin), making it an interesting adsorbent for trimethoprim and ciprofloxacin (the latter at high doses). Although ash and mussel shells also adsorb a high proportion of ciprofloxacin (adsorption rates higher than 50%), desorption is high, especially at low added doses, meaning that ash and mussel shells could be used for higher doses. In general, for all antibiotics and systems, the least suitable bioadsorbent is the shell due to the high desorption rates it exhibits. The results suggest a limited competition between the three antibiotics for the adsorption sites of the bioadsorbents; conversely, it is more common to observe a synergistic effect of ciprofloxacin and trimethoprim on amoxicillin in the case of the pine bark, and of amoxicillin on ciprofloxacin and trimethoprim in ash. The results showed a good fit to the Freundlich isotherms, with an R² greater than 0.92 in all cases.