Capacity of Agricultural Soils for the Simultaneous Adsorption of Amoxicillin, Cefuroxime, and Azithromycin

In the last years, the consumption of antibiotics increased significantly and due to their poor absorption in the digestive tract, these antimicrobials are excreted by feces and urine, causing their entry into the environment. β-lactams are the most common antibiotics found in waste and surface water and quinolones are also emerging groups of antibiotics. One of the environmental compartments where these antimicrobials can reach is the soil, due to the use of sewage sludge as fertilizers for agricultural soils. This study aims to determine the capacity of six soils with different physicochemical properties to adsorb three antibiotics, two B-lactams (amoxicillin and cefuroxime) and a quinolone (azithromycin) when added simultaneously and compare it to when they appear individually. For this purpose, batch experiments were carried out, adding increasing concentrations (0; 2.5; 5; 10; 20; 30; 40; 50 µmol L^-1) of the three antibiotics into 2 grams of soil. Finally, the antibiotic concentration in equilibrium solution was measured by HPLC-UV.

The results indicated that azithromycin exhibited the highest adsorption values in soils, followed by cefuroxime and lastly amoxicillin. Soil properties can affect the adsorption, it is noted that the ones with higher pH adsorbed more azithromycin and cefuroxime. However, in the case of amoxicillin, the opposite occurs, with low-pH soils showing the highest adsorption values. Comparing the results obtained when the three antibiotics were added together with those observed when they were added individually, amoxicillin adsorption by soils increased in presence of cefuroxime and azithromycin, ranged the percentages from 49% to 76% in simple system and from 44% to 88% in ternary system. However, the adsorption of the other two decreased when added simultaneously, indicating the existence of competition between them. In the case of cefuroxime, adsorption percentages oscillated between 79% and 99% in simple system and between 43% and 96% in ternary system and for azithromycin, in simple system the adsorption percentage was 100% in all cases, while in ternary system these values oscillated between 42% and 100%. Adsorption results were adjusted to Freundlich and Linear models and five out of six soils studied achieved a good adjustment, since they presented R²>0.9. Regarding desorption, amoxicillin was the antibiotic which presented the higher desorption values, reaching values of up to 7.68% (simple system) and 29.21% (ternary system) for the highest concentration of antibiotic added. In the case of cefuroxime, the maximum desorption reached 8% (simple system) and 3% (ternary system) and azithromycin presented null desorption in the most cases. In conclusion, amoxicillin was the only antibiotic favoured by the presence of the other two antibiotics, as an increase in adsorption was observed. However, desorption also increased, indicating that the formed bonds are weaker.