Effect of livestock manure on chlortetracycline sorption behaviour and mechanism in agricultural soil in Northwest China

Abstract Adding livestock manure to soil to improve soil quality is a common agricultural practice. However, information on the effect of manure soil amendments on the sorption of veterinary antibiotics is scarce. Batch equilibrium experiments were utilized to investigate the effects of livestock manures on the sorption of chlortetracycline (CTC) onto sierozem. Kinetic sorption results showed that sorption equilibrium for CTC was reached within 2 h, and the presence of livestock manures lengthened the time to equilibrium attainment and promoted the sorption capacity of CTC onto sierozem. A good fit of the sorption kinetic data was achieved with a pseudo-second-order model with or without the presence of animal manure. Both boundary layer control and intraparticle diffusion were related to the sorption kinetic process. The CTC sorption isotherm of sierozem fit well with both Freundlich isothermal and linear models. The sorption of CTC onto sierozem exhibited strong pH dependence and was strongly governed by the ability of each substance to ionize as a function of pH. A decrease in CTC sorption was observed with increasing ionic strength as electrolyte cations competed with the positively charged CTC species and complexes for sorption sites. CTC sorption capacity declined when the solution contained Ca2+ compared to Na+ and Mg2+ solutions at similar ionic strength. The sorption of CTC by sierozem affected by livestock manures exhibited strong dependence on the amendment amount. The results suggest that manure application may change the sorption behavior and mechanism of CTC on loess soil. These findings contribute to understanding the fate and risk of veterinary antibiotics in loess soil affected by manure application.