Effects Of Coexisting Ions On Sorption Of Trimethoprim Onto Typical Clay Minerals

Montmorillonite and kaolinite are the most important clay minerals in soils or sediments.Their sorption of polar organic pollutants affects the transport of pollutants in groundwater environment.In this study,trimethoprim?TMP?,a sulfonamide antibiotic with high detection rate in the environment,was selected as the representative of polar organic pollutants.The sorption/desorption behaviors of TMP onto kaolinite and montmorillonite under the coexistence of inorganic cations and amino acids were investigated by batch experiments.The main conclusions are as follows:?1?Solution pH can change the species of TMP and the surface properties of clay minerals,resulting in different mechanisms of TMP sorption by clay minerals.When the solution pH is lower than the pHpzc of clay minerals,the surface of clay minerals is positively charged and cation exchange is the sorption mechanism of cationic TMP on clay minerals;when the solution pH is higher than the pH_pzczc of clay minerals,the surface of clay minerals is negatively charged,cation exchange and electrostatic attraction dominate the sorption of cationic TMP;while the sorption mechanism of molecular TMP is dominated by hydrogen bonding.?2?There was a hysteresis in the desorption of bivalent cationic TMP on montmorillonite,and its desorption rate was less than 10%.This may be due to its easier entry into the interlayer of montmorillonite and exchange with interlayer ions,while TMP in interlayer structure is more difficult to release.When TMP was dominant in solution,there was desorption hysteresis,and the desorption rate was higher?more than56%?.?3?Because of the different pHs of the equilibrium systems,TMP mainly existed in molecular state in montmorillonite system and cationic state in kaolinite system.The effects of Na⁺on TMP sorption on montmorillonite and kaolinite were different.With the increase of ionic strength?0 M⁰.05 M?,the sorption capacity of molecular TMP on montmorillonite increased slightly under the influence of salting-out effect,while the sorption capacity of cationic TMP on kaolinite decreased significantly due to the competition of Na⁺.?4?The effects of alanine and lysine on the sorption and desorption of TMP on kaolinite were different.Positive lysine inhibited the sorption by occupying the sorption sites,while dipole ionic alanine promoted the sorption of TMP on kaolinite by increasing the surface electronegativity of kaolinite.The desorption hysteresis of TMP on kaolinite was obvious when lysine coexisted,and the desorption hysteresis was weakened when lysine was added to the desorption solution.Lysine has one more amino group than alanine,which makes it more effective to bind to kaolinite and compete with TMP for sorption sites.