Effects of exposure of polyethylene microplastics to air, water and soil on their adsorption behaviors for copper and tetracycline

The adsorption behaviors of Cu(II) and tetracycline (TC) on polyethylene microplastics that were exposed to air (A-PE-MPs), water (W-PE-MPs) and soil (S-PE-MPs) environments for a long time were investigated with batch experiments in this study. FT-IR and GC-MS analysis confirmed that phthalates (PAEs) were released from PE microplastics after exposure to UV light. When exposed to water and soil environments, PE microplastics were colonized by microorganisms and biofilms were formed on their surface. Compared with virgin microplastics (V-PE-MPs), the environment-exposed microplastics possessed higher adsorption and stabilization capacity for Cu (II) and TC. The adsorption isotherm data of V-PE-MPs and A-PE-MPs were well fitted by Freundlich model, whilst the adsorption by W-PE-MPs and S-PE-MPs were better described by Langmuir model. The Boyd equation used in kinetic experiments revealed that V-PE-MPs and A-PE-MPs were additionally affected by intra-particle diffusion compared with W-PE-MPs and S-PE-MPs. The glycosyl composition analysis suggested that the higher adsorption capacities of S-PE-MPs than W-PE-MPs could relate to the different components of extracellular polysaccharides in biofilms. Moreover, density functional theory further verified that Cu(II) and PAE adsorption on microplastics could be enhanced by the generation of Cu-PAE and Cu-PAE-TC complexes. In addition dissolved organic matter exerted opposite effects on the adsorption onto A-PE-MPs and S-PE-MPs due to their different affinity for Cu(II), TC and DOM. This study demonstrated that the contribution of microplastics to heavy metal and antibiotic migration could be affected by the way they exposed to environments.

Automated summary

The adsorption behaviors of Cu(II) and tetracycline (TC) on polyethylene microplastics exposed to different environments varied, with marine- and soil-exposed microplastics showing higher adsorption and stabilization capacities. The adsorption isotherms were better described by Langmuir model for water- and soil-exposed microplastics, while kinetic experiments revealed differing intra-particle diffusion effects. Additionally, the glycosyl composition and density functional theory analysis suggested that biofilm components and metal-antibiotic complexes played a role in the adsorption process.