Effects of dissolved humic acid on fluoroquinolones sorption and retention to kaolinite

Fluoroquinolones (FQs) are widely used in human and veterinary medicaments, and as such are ubiquitous environmental contaminants. Dissolved organic matter (DOM) is widely distributed in natural water and sediment and dissolved humic acid (DHA) is a major component of DOM. The coexistence of DHA might influence the sorption, migration and transformation of FQs, thus determining their environmental fate. In this study, the interaction of DHA and ofloxacin (OFL)/flumequine (FLU) was evaluated using dialysis-bag assays. The sorption of OFL and FLU to kaolinite in the presence of DHA under different pH conditions was investigated. The results revealed that the binding affinities of FQs to DHA were weakened with increasing pH from 4.0 to 10.0 due to the increased negative charge of DHA and subsequent electrostatic repulsion. Sorption experiments indicated that co-precipitation was an important mechanism for OFL/FLU removal from the aqueous phase under acidic conditions. At pH 7.0, the affinity of OFL-DHA/FLU-DHA to kaolinite was weaker than that of OFL/FLU thus suppressed its sorption. At pH 9.5, the affinity of OFL-DHA to kaolinite was stronger than that of OFL and consequently promoted its sorption, but there was no observed effect of DHA on FLU sorption. During desorption, DHA could bind to OFL/FLU and promote its desorption from kaolinite at neutral pH. In binary solute systems of OFL and FLU, OFL was a more effective competitor for the sorption sites of kaolinite than FLU.