Nanoscale insight into the interaction mechanism underlying the transport of microplastics by bubbles in aqueous environment

The ecological risk of microplastics (MPs) is raising concern about their transport and fate in aquatic ecosystems. The capture of MPs by bubbles is a ubiquitous natural phenomenon in water-based environment, which plays a critical role in the global cycling of MPs, thereby increasing their environmental threats. However, the nanoscale interaction mechanisms between bubbles and MPs underlying MPs transport by bubbles in complex environmental systems remain elusive. This work for the first time directly measured and evaluated the interactions between bubble and polystyrene microplastic (PSMP) under various environmental factors in aqueous media using atomic force microscope (AFM) combined with a Stokes-Reynold-Young-Laplace (SRYL) model. Since hydrophobic interaction was strong enough to act across the repulsive barrier, bubble-PSMP attachment always occurred at different NaCl concentrations, pH and hydrodynamic conditions, and a decay length D0 of hydrophobic interaction was determined as 0.65 +/- 0.05 nm. No bubble attachment was observed during approach for aged PSMP (APSMP) with the weakened hydrophobic interaction (D0 = 0.33 +/- 0.02 nm), while in 100 mM NaCl, APSMP-bubble attachment occurred during retraction due to the hydrodynamic suction effect. The decreased D0 arose from the increased oxygen-containing groups on APSMP surfaces that significantly reduced the hydrophobicity of MPs surface as evidenced by X-ray photoelectron spectroscopy (XPS) and water contact angle measurement. It was further evident from transport tests that aging plays a crucial role in MPs transport driven by bubbles. This work provides nanoscale information on the interaction mechanism underlying the MPs transport by bubbles, with implications to evaluate the fate of MPs in aqueous environments.