[Coadsorption of Heavy Metal and Antibiotic onto Humic Acid from Polder River Sediment]

To clarify the interactions between heavy metals, antibiotics, and humic acid, copper (Cu²⁺), oxytetracycline (OTC), norfloxacin (NOR), and humic acid samples from river sediment in the Polder area were selected to build single and coexisting systems. Groups of experiments were designed to investigate the kinetics, thermodynamics, and isotherms of Cu²⁺, OTC, and NOR adsorption onto humic acid in single and Cu²⁺+OTC and Cu²⁺+NOR coexisting systems (concentration ratio=1:1). The physicochemical properties of humic acid were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), BET tests, and IR spectroscopy, and the possible adsorption mechanisms are discussed. The results showed that the humic acid was a typical amorphous material with a negative charge and non-uniform porous structure, and the pore size was at the mesoporous scale. In the single systems, the saturated adsorption capacity (q_m) of Cu²⁺, OTC, and NOR onto humic acid was 33.043, 19.512, and 26.676 mg·g^-1, respectively. In the Cu²⁺+OTC system, the q_m of Cu²⁺ and OTC was 38.053 and 25.965 mg·g^-1, respectively. In the Cu²⁺+NOR system, the q_m of Cu²⁺ and NOR was 39.187 and 32.728 mg·g^-1, respectively. The adsorption behaviors in the single and coexisting systems were similar and the adsorption processes were well fitted by the pseudo-second-order kinetic equation; the Sips model provided good descriptions for the isothermal adsorption equilibrium. Moreover, adsorption thermodynamics were characterized by spontaneous endothermic reactions with the reduction of free energy and the increase of enthalpy and entropy. It can be concluded that Cu²⁺ combines with OTC and NOR to form complexes, which increases the number of species available for adsorption by humic acid. Also, adsorbed Cu²⁺ can combine with free OTC and NOR in a bridging manner. Thus, a more favorable adsorption situation occurred in the coexisting systems. The IR characteristics of the carboxyl, phenolic hydroxyl, ketone, and aldehyde groups of humic acid changed by different degrees after adsorption, indicating that oxygen-containing functional groups generally participated in the adsorption reactions.