Strong but reversible sorption on polar microplastics enhanced earthworm bioaccumulation of associated organic compounds

Sorption/desorption of two organic compounds (OCs), phenanthrene (PHE), and 1-nitronaphthalene (1-Nnap) on three polar and one nonpolar polypropylene (PP) microplastics (MPs) and earthworm bioaccumulation of MP associated PHE were systematically studied. Poly-butylene succinate (PBS) with the lowest glass transition temperature (Tg) showed the highest sorption toward PHE and 1-Nnap (Kd: 25,639 +/- 276 and 1673 +/- 28.8 L kg-1, respectively), while polylactic acid (PLA) with the highest Tg showed the least sorption (182 +/- 5 and near 0), confirming that hydrophobic partition was the main driving force of sorption. However, polar interactions also contributed to the preferential sorption of 1-Nnap on polar poly-hydroxyalkanoates (PHA). Moreover, small particle size favored the sorption of MPs and simulated weathering enhanced sorption on MPs with medium/high Tg. As for desorption, slight hysteresis was observed in most cases with near-zero hysteresis index (HI), and PHE generally had higher HI than 1-Nnap. The simulated digestive solution could further promote the desorption of PHE. The PHE concentrations in earthworms with the presence of 5% PBS or PP MPs in soil were 1.50-2.35 or 1.59-1.75 times that of the control without MPs; and PBS MPs with the smallest particle size showed the greatest enhancement. The results of this study confirmed that polar MPs could strongly but reversibly sorb both polar and nonpolar OCs and hence promote the bioaccumulation of OCs to soil organisms.

Automated summary

The study found that polar interactions of microplastics contributed to the preferential sorption of 1-Nnap, while small particle size favored the sorption of microplastics and simulated weathering enhanced sorption on microplastics with medium/high Tg.