Nanoscale interaction mechanism between bubbles and microplastics under the influence of natural organic matter in simulated marine environment

Bubble-microplastic (MP) interaction is a significant process that changes the routes of MP circulation in marine environment and thereby determines the risk of MPs, which could be strongly influenced by natural organic matter (NOM) in oceans. However, the quantitative interaction mechanisms between bubbles and MPs under the effect of NOM remain elusive. Herein, bubble-MP interactions in simulated seawater were quantified at nanoscale based on atomic force microscope coupled with the Stokes-Reynold-Young-Laplace model. Bubble-polystyrene (PS)/polyvinyl chloride (PVC) MP interactions exhibited stronger hydrophobic interactions (decay length D of 0.60 ± 0.03 nm/0.43 ± 0.02 nm for PS/PVC MP) than polymethyl methacrylate (PMMA) MPs. Humic acid (HA) considerably reduced the D of hydrophobic interaction from 0.43-0.60 nm to 0.30-0.32 nm for PS and PVC MPs by introducing oxygen-containing components as evidenced by spectroscopic analysis. In contrast, alginate (Alg) accumulated less on PS/PVC MP surfaces, thereby negligibly affecting the D value. While for PMMA MPs, virtually identical D values were observed despite the presence of HA/Alg. Therefore, the bubble-driven transport of PS/PVC MPs were modulated by different types of NOM, whereas PMMA MPs could only be slightly affected. This work provides nanoscale insights into quantitative bubble-MP interactions, shedding light on understanding MPs global cycling.