Adsorption mechanism of flumequine onto kaolinite: Evidence from density functional theory calculations and the effects of Cu(II)

Flumequine (FLU) is widely applied in animal husbandry and can enter sediment and groundwater systems. However, the adsorption mechanisms of FLU onto clay minerals remain unclear. Density Functional Theory (DFT) calculations were employed to elucidate the adsorption mechanism of FLU onto kaolinite, and batch adsorption experiments were conducted to investigate the adsorption behavior of FLU. Results of DFT calculations demonstrated that the carboxyl group of FLU interacted with the hydroxyl groups on the kaolinite (001) surface via hydrogen bonding and electrostatic interactions, which were the main adsorption mechanisms. The adsorption of FLU onto kaolinite exhibited pH dependence, due to pH-induced variations in both the speciation of FLU and the surface charge of kaolinite. Ionic strength negatively affected FLU adsorption because NaCl could decrease the activity of FLU, and Cl⁻ could also compete for the adsorption sites with FLU⁻. With the addition of Cu(II), the species Cu(FLU⁻)⁺ was formed, and it had stronger adsorption affinity for kaolinite than FLU. However, Cu²⁺ could compete for adsorption sites with Cu(FLU⁻)⁺. Therefore, the molar concentration difference between Cu(FLU⁻)⁺ and Cu²⁺ played a crucial role in FLU adsorption. When the ratio of this difference in the binary systems to the adsorbed FLU in the single system exceeded 1.0, Cu(II) addition significantly enhanced FLU adsorption onto kaolinite; otherwise, it inhibited adsorption. Our findings indicate that hydrogeochemical conditions and coexisting heavy metal ions should be considered when studying the environmental fate of quinolone antibiotics.