The structure of agricultural microplastics (PT, PU and UF) and their sorption capacities for PAHs and PHE derivates under various salinity and oxidation treatments

Microplastic (MP) pollution and its potential to concentrate and transport organic contaminants in environments have recently gained widespread attention. Compared to traditional nonpolar plastics such as polypropylene (PP) and polyethylene (PE), study about the environmental behavior of polyurethane (PT), polyuria (PU) and urea-formaldehyde resin (UF), which are typically used as shell materials for pesticide microcapsules and have polar structure is scarce. In the present study, we investigated the sorption capacities and binding mechanisms of PT, PU and UF for three polycyclic aromatic hydrocarbons (PAHs, naphthalene, phenanthrene (PHE), and pyrene) and two PHE derivates (ethylphenanthrene-2-carboxylate (2-CPHE) and 2-methylphenathrene (2-MPHE)) selected as the model compounds, and the effects of salinity and UV and/or H₂O₂ aging treatments on PHE sorption to MPs. The results showed that PT, PU and UF had negative surface charges, micron-scaled sizes and abundant polar functional groups containing O and N elements. PT, PU and UF could sorb PAHs efficiently with sorption coefficients (K_d) being in the range of 8.11 × 10³-9.53 × 10⁵ (L/Kg) and partitioning was the main sorption mechanism with polar interactions (H-boning and p/π-π EDA interactions) also contributing. The sorption capacity of the three MPs changed mainly depending on their glass transition temperatures (T_g). Furthermore, high salinity decreased the surface zeta-potential of the MPs and enhanced PHE sorption to MPs. In addition, aging treatments with UV and/or H₂O₂ markedly decreased the T_g of PT and enhanced its sorption capacity for PHE, while opposite results were obtained for PU. The findings on the sorption mechanisms of PAHs to agricultural MPs are useful for predicting the transport, fate and persistence of the co-existing HOCs in agricultural ecosystems and provide a scientific basis for the comprehensive risk assessment of agricultural MPs.