Dissipation kinetics of oxytetracycline, tetracycline, and chlortetracycline residues in soil.

The dissipation of different residual states of tetracycline antibiotics (TCs) including oxytetracycline (OTC), tetracycline (TC), and chlortetracycline (CTC) laboratory microcosm systems was investigated in this study. The residues were fractionated by stepwise extractions into aqueous state (KCl solution extracts), organic state (MeOH extracts), residual state I (citric acid-sodium citrate buffer and ethyl acetate extracts) and residual state II (acetonitrile-EDTA-McIlvaine buffer extracts) for accurate evaluation of TCs pollution. The antibiotics in the aqueous state were hardly detected, whereas the antibiotics in the organic state dissipated relatively fast (not detectable within 15 days after application) and followed simple first-order kinetics (SFOK) (R (2) from 0.929 to 0.990). While first-order double-exponential decay model (FODED) (R (2) from 0.840 to 0.999) and availability-adjusted first-order model (AAFO) (R (2) from 0.939 to 0.999) had a better fit on the dissipation of both residue state I and II than SFOK. TCs in these states were likely sequestered into a dormant undegradable phase since no degradation product was detected during the entire experiment. In addition, the overall 50 % dissipation values (i.e., stability) of the three TCs were OTC > TC > CTC. The TCs tend to dissipate faster in the high water content and organic matter soil.