Biochar impacts on sorption-desorption of oxytetracycline and florfenicol in an alkaline farmland soil as affected by field ageing

Abstract Few studies have been conducted to examine the effects of biochar amendment on sorption and desorption of antibiotics in alkaline soils under field ageing conditions. In this study, sorption and desorption of two antibiotics (oxytetracycline (OTC) and florfenicol (FFC)) were investigated for a calcareous farmland soil with different biochar treatments (1 and 5 wt%, including fresh and aged soil-biochar mixtures). The fresh and recovered field-aged biochar particles were characterized by various methods. The results indicated that, after one-year-field-ageing, the abundance of amorphous carbon on biochar surface and the carbon content of biochar particles decreased markedly whereas the surface area and the oxygen-containing functional groups increased. It was found in batch experiments that the effects of biochar application and ageing on sorption and desorption characteristics were apparently different between the two antibiotics. OTC (a more hydrophobic and less polar antibiotic) showed much higher sorption capacity than FFC. Biochar-derived dissolved organic matter (DOM) was found to compete for sorption sites on soil surface, particularly for the 5 wt%-freshly-amended soil. The adherent excessive DOM had lowered OTC sorption and enhanced its partitioning to the solution phase. On the other hand, enhanced partitioning of FFC to the amorphous carbon of biochar accounted for the observed increase of its sorption to the soils with fresh biochar amendment. However, the above effects on sorption affinity for both antibiotics weakened after ageing in the studied calcareous purple soil. Thus, due attention should be paid to the potential risk of leaching increase of OTC in alkaline soils upon biochar amendment, particularly within the very beginning period after application.