Effects of Soil/Solution Ratios and Cation Types on Adsorption and Desorption of Tetracycline in Soils

The adsorption and desorption behavior of tetracycline on an Alfisol and an Ultisol was investigated at varying soil/solution ratios (1:10, 1:50, and 1:100) and different cation types (CaCl2, KCl, and NaCl) and H2O using a batch equilibration experiment. Kinetic studies revealed that the adsorption of tetracycline on the two soils was rate limited, and the adsorption kinetics could be well described by the Elovich equation and exponent equation. Changes in the soil/solution ratio affected the relative equilibration time in the Ultisol, but there was no effect of the initial tetracycline concentration. The adsorption rate in the soils decreased as the soil/solution ratio increased from 1:50 to 1:10. The adsorption and desorption data on tetracycline in the two soils could be described by the Freundlich isotherm equation, with r values >0.952 under all experimental treatments. The Ultisol always had stronger adsorption (higher distribution coefficient [K-d] value) than the Alfisol under all experimental conditions. The K-d values decreased with increasing soil/solution ratios in the two tested soils at 1 or 1.25 mg L-1 of tetracycline; however, this trend did not occur at 25 mg L-1 of tetracycline. The adsorption isotherm had an S-type curve that changed into an L-type curve with decreasing soil/solution ratios from 1:10 to 1:100. The presence of cations in the soil solution, especially CaCl2, can significantly reduce the adsorption of tetracycline in soils. The presence of cations also significantly reduced the nonlinearity of the adsorption isotherms.