Effects of organic matter on interaction forces between polystyrene microplastics: An experimental study

Area of Science:

• Environmental Science
• Marine Biology
• Physical Chemistry

Background:

• Marine microplastics undergo aggregation and deposition, transferring them to the deep sea.
• Oceanic organic matter significantly influences the kinetics of these microplastic processes.

Purpose of the Study:

• To investigate the morphology and interaction forces of microplastics influenced by organic matter.
• To understand how biofilms affect microplastic aggregation and sinking.

Main Methods:

• Environmental scanning electron microscopy (ESEM) for morphology analysis.
• Atomic force microscopy (AFM) for interaction force measurements.
• Experiments conducted with varying concentrations of organic matter, specifically humic acid.

Main Results:

• Humic acid formed a film around polystyrene microplastics, increasing their size up to 11% at 50 mg/L total organic content (TOC).
• The formed biofilm reduced inter-particle repulsion, significantly promoting microplastic aggregation.
• A modified eXtended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory was proposed to account for biofilm influences.

Conclusions:

• Organic matter, particularly humic acid, plays a crucial role in marine microplastic aggregation and transport.
• Biofilm formation alters microplastic surface properties and interaction forces, facilitating their sinking.
• The developed theoretical model aids in predicting microplastic behavior in marine environments.