Mechanistic inferences from empirical and LSER modeling approaches concerning sorption of organic compounds by pristine and aged PE microplastics

This study investigates the effect of aging of polyethylene (PE) microplastics (MP) on its interaction with organic compounds (OCs). Initially, pristine PE MPs were subjected to UV-aging, followed by characterization of their chemical structure and thermal properties. UV-aging resulted in formation of new functional groups such as carbonyl (CO), –OH, and unsaturation, along with changes in crystallinity and melting temperature. Complimentary sorption experiments were conducted with a suite of environmentally significant and structurally related OCs i.e., phenol, 2,3,6-trichlorophenol, triclosan, 1,1,2,2-tetrachloroethane, tetrachloroethylene and hexachloroethane, using pristine and UV-aged PE MPs. In addition to the distribution coefficients (i.e., K_PEW) obtained experimentally, relevant data from the literature was also gathered for the purpose of developing a poly-parametric linear free energy relationship (pp-LFER) model. Two models were developed for predicting sorption onto: (i) only UV-aged PE, yielding an R² = 0.96, RMSE = 0.19 (n = 16), (ii) PE that has undergone various types of aging, yielding an R² = 0.83, RMSE = 0.68 (n = 36). Lastly, a direct comparison was performed between two pp-LFERs developed for the interaction of the same OCs with pristine vs. aged PE (n = 7). In addition to the predictive strength, the system coefficients enabled mechanistic inferences to be made; such that while molecular volume or non-specific hydrophobic interactions govern OC-pristine PE interactions, polar interactions and H-bonding also play important roles for OC-aged PE interactions. Overall, findings suggested that changes of MP surfaces under environmentally relevant aging conditions indicated an impact on their interactions with OCs in the environment.