Sorption and Desorption Characteristics of Tylosin in Three Louisiana Soils

The macrolide antibiotic tylosin is widely used in animal production, but its environmental fate is not fully understood. Objectives of this study were to determine the effect of pH on tylosin A sorption and desorption in three sandy loam soils from Louisiana, USA, that had long histories of poultry waste application, to model sorption and desorption, and to estimate the effect of high soil organic matter on sorption. Twenty-four-hour sorption isotherms (5 to 200 mg L-1 in 0.01 M CaCl2) at pH 4.5, 6.0, and pH 7.5 were described by the Freundlich model. Desorption from the 200-mg L-1 set at constant pH by 10 24-h extractions with 0.01 M CaCl2 recovered 43 to 98% of the added tylosin A, and further desorption with methanol increased recovery from 66 to 100%. Single-point distribution coefficient, KD, as a function of pH from 4 to 9 in 0.01 M CaCl2 exhibited maxima from pH 6 to 7, reflecting increasing sorption of the positively charged form with increasing pH up to about the pKa. The data were well described, modeling pH-dependent negative charge according to Henderson-Hasselbalch along with tylosin speciation. Using soil from which organic matter had been removed by H2O2 gave three to six times greater KD at pH 6 to 7, but without or with reduced maxima. The data could be approximately described without invoking pH-dependent surface charge. Further study is needed to confirm whether an increase in soil organic matter with poultry waste application decreases tylosin sorption, thus increasing environmental risk.