Phosphate affects the adsorption of tetracycline on two soils with different characteristics

Tetracyclines (TCs) have been widely used as feed additives in intensive livestock production, and are often detected in animal manure and agricultural soils. Their fate in the environment, e.g., adsorption, leaching and degradation, is influenced by soil characteristics. This study evaluated the adsorption of tetracycline (TC; H3L) on two soils as affected by phosphate (P). Freundlich equations fit the adsorption data of TC on the soils well with high correlation coefficients (0.994–0.999). Compared with the control (without P addition), the Kf values of TC after addition of 31 mg P l− 1 decreased from 9210 to 5700 on the Red soil (RS, Udic Ferrosols, with lower pH and organic matter), and from 3730 to 2470 on the Wushan soil (WS, Anthrosol, with higher pH and organic matter). Solution pH strongly affected TC adsorption on the soils with and without P. Addition of 31 mg P l−1 as orthophosphate increased TC adsorption on the RS at pH < 4 and then decreased with further increasing pH. However, no significant influence of P on TC adsorption was observed on the WS in a wide pH range. Phosphate is known to convey negative charges to soil particle surface, which will decrease the zeta potential of the soils, and consequently affect TC adsorption on the soils. The adsorption model, in which species-specific sorption coefficients (Kdi) were weighted by pH-dependent fractions of each species, fits the experimental data very well. The addition of P increased the sorption coefficients of H3L+ 00 about 123% and 14.1% on the RS and WS, respectively, while decreased the sorption coefficients of H2L+−0 and HL+−−. Our results suggest that application of high rate of P fertilizer may enhance TC leaching, especially from the soils with variable charges.