Sinking of microbial-associated microplastics in natural waters

Area of Science:

• Environmental Science
• Marine Biology
• Polymer Science

Background:

• Microplastics (MPs) are pervasive pollutants in aquatic environments, from urban waters to the open ocean.
• The natural removal of buoyant MPs via deposition is hindered by their low density, but biological attachment can alter this.
• The specific influence of biological ballast on microplastic aggregate dynamics remains poorly understood.

Purpose of the Study:

• To experimentally investigate how the biological fraction of microplastic aggregates affects their physical properties and settling velocity.
• To develop a predictive model for microplastic aggregate settling velocity that incorporates biological content and aggregate structure.

Main Methods:

• Utilized a novel Optical Measurement of CEll colonisation (OMCEC) system for experimental analysis.
• Created polyurethane microplastic aggregates with varying percentages of biological ballast (5% to 80%).
• Measured aggregate size, shape, and settling velocity under controlled conditions.

Main Results:

• Microplastic aggregates with 80% biological ballast were nearly twice the size of those with 5% biological ballast.
• Aggregates with higher biological content exhibited significantly slower settling velocities (approximately two times slower).
• A new settling velocity equation was derived, accurately predicting the behavior of virgin and biologically-associated MPs with 7% error.

Conclusions:

• The biological fraction of microplastic aggregates plays a critical role in determining their size, shape, and vertical transport dynamics.
• The developed settling velocity equation provides a valuable tool for estimating the fate of diverse microplastic aggregates in aquatic systems.
• Understanding these bio-geophysical interactions is essential for assessing the environmental impact and transport of microplastic pollution.