Behaviour of sulfadimethoxine on farming soils from Buenos Aires, Argentina

Therapeutic drugs are routinely supplied to farm animals to prevent bacterial infections, coccidiosis, and as growth promoters as intensive livestock production systems become more common. It is required to develop local data to be used in environmental risk assessments in order to predict the fate of these compounds. In this work, a sulfadimethoxine (SDM) adsorption study was conducted in soils used for agricultural and livestock activities. To carry out the adsorption/desorption experiments, two soil samples from the Pampa Plain region (Argentina) were obtained. Both soils, denoted as MS and US, presented similar physicochemical properties and textural classification of clay loam. Batch experiments revealed that the adsorption/desorption isotherms of SDM on soils were well adjusted to the Freundlich model, where adsorption constants resulted in 5.35 and 8.94 g (1-1/n) mL (1/n) g(-1) for MS and US, respectively. These values were normalized according to the organic matter content in both soils, obtaining values of 171.52 and 195.62 mu g(1-1/n)mL(1/n) g(-1), respectively. Regarding kinetics results, adsorption processes followed a pseudo-second-order law whose rate constants showed major differences, being 3.8 x 10(-3) and 0.012 g g(-1) min(-1), respectively. Moreover, the equilibrium period ranged from a few minutes (US) to 5 h (MS) suggesting that SDM mobility in soils could be different besides the SDM adsorption constants were similar in both. In conclusion, equilibrium studies could not be sufficient to predict the quantity of SDM adsorbed on soils because the adsorbed quantity predicted only from equilibrium studies could be overestimated in a real-life scenario.