Sorption Behavior Of Typical Fluoroquinolone Antibiotics On Kaolinite:Batch Experiments

Fluoroquinolone antibiotics(FQs)have been frequently detected in watersoil/sediments systems.Sorption of antibiotics on soils and sediments is a key process that controls their transport and transformation in the environment.The coexistence of dissolved organic matter(DOM)and heavy metals may alter the sorptive behavior of antibiotics.Dissolved humic acid(DHA)is one of the main components of DOM,and Cu(II)is a common heavy metal pollutant,which is widely used in industry and agriculture.Therefore,the sorption and desorption of ofloxacin(OFL)and flumequine(FLU)on kaolinite under the effects of DHA and Cu(II)were investigated by using the methods of batch experiments,microscopic characterization and simulation.The main conclusions are as follows:(1)Cation exchange and hydrogen bonding played an important role in OFL sorption to the negatively charged kaolinite.The effect of ionic strength on the sorption of OFL+ was not obvious.However,the OFL± sorption decreased with increased ionic strength.This indicates that the sorption affinity of OFL species to kaolinite decreased in the order OFL+ > OFL± > OFL-.The sorption coefficient Kd(L/kg)of OFL to kaolinite reached its maximum at pH around 7.0 in the in the pH range of 4.0-10.0.Hydrogen bonding and electrostatic attraction were the sorption mechanisms of FLU on kaolinite.The maximum Kd value of FLU appeared at pH around 6.0 in the pH range of 4.0-10.0.As the FLU activity decreased with the increase of ionic strength,ionic strengths showed a negative effect on the FLU sorption to kaolinite.In the cosorbate system,OFL had a stronger affinity to kaolinite than FLU.Both OFL and FLU were easily desorbed from kaolinite,especially for FLU.(2)When solution pH was lower than 7.0,Cu(II)promoted sorption of OFL and FLU to kaolinite because of the higher affinity of the complexes.However,Free Cu(II)in solution caused a decrease of OFL and FLU sorption due to its competition for the sorption sites.The promoting or inhibiting effect depended on the initial concentrations of OFL and Cu(II).For pH higher than 7.0,extra Cu(II)ions that do not take part in complexation reaction mainly form Cu(OH)2,so there was no competitive sorption.Cu(II)obviously facilitated the removal of OFL and FLU from solution due to the formation of complexes,and the sorption amount of OFL and FLU increased with the increase of Cu(II)concentration.In addition,Cu(OFL-)2(s)would precipitate on the surface of kaolinite.In the desorption process,Cu(II)would promote the desorption of OFL and FLU from kaolinite due to the formation of complex with the sorbed OFL or FLU.In addition,the cation exchange interaction of Cu(II)and OFL also contributed to the desorption.(3)The interactions between DHA and OFL or FLU were pH-dependent in different systems.Their binding abilities were weakened with increasing pH(4.0-10.0).The affinities of DHA-OFL and DHA-FLU to kaolinite decreased with the increasing solution pH,which affected sorption process.Under the acidic condition,coprecipitation was an important removal mechanism.In desorption process,DHA would form complex with the sorbed OFL or FLU,thus pulling them from kaolinite and promoting desorption.