[Sorption Behaviors of Copper Ions and Tetracycline on Microplastics in Aqueous Solution]

The interaction between microplastics, heavy metals, and antibiotics can lead to combined pollution, which could result in greater environmental damage. The pathway and mechanism of the interaction between microplastics, heavy metals, and antibiotics are the preconditions for evaluating the associated environmental risk; however, these are not well understood. As probe sorbates, the sorption behaviors of copper ions (Cu²⁺) and tetracycline (TC) on two microplastics [high density polyethylene (HPDE) and general-purpose polystyrene (GPPS)] in aqueous solution were investigated and the welding theory with relevant experimental results were discussed. The adsorption capacity of HDPE was greater than that of GPPS in a single Cu solution, whereas the reverse situation occurred in a single TC solution. Moreover, the adsorption capacity of the microplastics in a Cu²⁺-TC binary solution was larger than that in the single solutions. The pseudo-second-order kinetic models to describe the adsorption process were reasonable and the entire process could be divided into two phases:surface adsorption and internal diffusion. The Langmuir model provided a better fit of the data than did the Freundlich model. In the single solutions, the saturated adsorption amounts of Cu²⁺ and TC were 0.178 μmol·g^-1 and 0.257 μmol·g^-1, respectively, for GPPS, and 0.334 μmol·g^-1 and 0.194 μmol·g^-1, respectively, for HDPE. In the binary solution, the corresponding numerical values were 0.529 μmol·g^-1 and 0.411 μmol·g^-1, respectively, for GPPS and 0.471 μmol·g^-1 and 0.341 μmol·g^-1, respectively, for HDPE. The variations in the surface morphological characteristics and chemical functional groups were the main reasons for the difference in the adsorption behavior of microplastics. The variation of the pH of the adsorption system could change the existing forms and surface electrical properties of microplastics and adsorbed objects, and subsequently affected the equilibrium adsorption capacity. When the ambient temperature was in the range of 15 to 35℃, increasing the temperature was unfavorable for the adsorption process. Cu²⁺ and TC could produce a synergistic effect under the conditions of coexistence. The formation of complexes and bridging make Cu²⁺ and TC more easily adsorbed by microplastics.