Sorption of five organic compounds by polar and nonpolar microplastics

Microplastics (MPs) could act as a vector for various kinds of pollutants due to their small size. Compared to nonpolar and nondegradabale MPs, the sorption of organic pollutants on polar and degradable MPs has been seldom studied. In this study, the sorption behavior of two nonpolar polycyclic aromatic hydrocarbons (PAHs, phenanthrene and pyrene), two polar derivates of PAHs (1-nitronapthalene and 1-napthylamine) and a heterocyclic chemical (atrazine) by three polar MPs including polybutylene succinate (PBS), polycaprolactone (PCL) and polyurethane (PU) and a typical nonpolar MP, polystyrene (PS) were investigated. The sorption followed the pseudo-second-order kinetics and sorption equilibrium was achieved within 5 days. Sorption isotherms could be well fitted by both Linear (R²>0.946) and Freundich models (R²>0.945) and the values of nonlinear index (n) from Freundlich model in most cases were close to 1, suggesting that hydrophobic partition was a primary process controlling the sorption. The sorption coefficients (K_d) of the five organic compounds ranged from 29. 6 to 1.42 × 10⁵ (L/kg). The log K_OC/log K_OW of PAHs and derivates of PAHs on polar MPs were greater than 1, especially for 1-naphthylamine (1.30-1.40), confirming the great contribution of hydrogen bonding. PU contains a benzene ring and showed greater sorption compared the other two polar MPs, indicating the existence of π (n) -π electron donor-acceptor interactions. Morevoer, the sorption of phenanthrene and pyrene on PU were better fitted by Langmuir model and the maximum adsorption capacities were 1.06 × 10⁴ and 5.87 × 10³ mg/kg, respectively.