Sorption of Antibiotics in Agricultural Soils as a Function of pH

This study aims to understand the adsorption/desorption process in six agricultural soils of two antibiotics, Ciprofloxacin (CIP) and Trimethoprim (TRI), widely used today and the influence of pH on this process. Antibiotics can reach the soil through the application of sludge and effluents from wastewater treatment plants and are directly influenced by changes in pH, once in the soil. Therefore, this study with batch experiments allows us to know the adsorption process in a pH range between 2 and 12, in six soils with different organic carbon content, between 1% and 7.7%. The results obtained show that the adsorption of CIP has its maximum at pH between 5 and 7; above and below this range, the adsorption decreases. The soils with the highest organic carbon content (between 4.4% and 7.7%) are those with the highest adsorption. The values for each forms in which the CIP molecule is found are: for KdCIP+, between 0.887 and 8.289 L kg-1; for KdCIP-, between 0.670 and 5.440 L kg-1, while for KdCIP0, the values do not differ from 0, except soils 1 and 3, whose values are 0.206 and 0.615 L kg-1, respectively. Regarding TRI, the maximum adsorption takes place at acidic pHs, below 6 for all soils. Above these values, desorption decreases. The Kd values for each of the species vary between 0.085 and 0.218 L kg-1 for KdTRI+, between 0.011 and 0.056 L kg-1 for KdTRI0, and between 0.092 and 0.189 L kg-1 for KdTRI-. For both antibiotics, the highest adsorption was achieved in the soil with the highest organic carbon content (7.7%). Comparing both antibiotics, we see that CIP presents the highest adsorption, and in the case of desorption, for CIP, it varies between 3.7% and 75.8%, with the maximum desorption at basic pHs. In the case of TRI, desorption is higher, varying between 9.4% and 99.1%, with the maximum around neutrality, except for two soils, whose maximums are at pH of 4.3 and 9.5. These results should be taken into account, as once they reach the soil, pH will be a determining factor in their behaviour and fate.