Beyond surface functionalization: Reassessing aging claims in micro- and nanoplastics for hazard interpretation

Area of Science:

• Environmental Science
• Materials Science
• Toxicology

Background:

• Microplastics are often labeled "aged" based on surface charge, implying environmental relevance.
• This assumption equates engineered functionalization with actual environmental weathering.
• Limited empirical validation exists for this descriptor-based aging equivalence.

Purpose of the Study:

• To challenge the assumption that surface charge similarity indicates functional equivalence in microplastics.
• To investigate the mechanistic basis of microplastic aging.
• To propose a framework for accurately assessing microplastic aging states.

Main Methods:

• Synthesized evidence comparing engineered, laboratory-weathered, and environmentally aged microplastics.
• Analyzed aggregation behavior, contaminant interactions, and biological responses.
• Evaluated particle states based on ζ-potential and transformation history.

Main Results:

• Particles with similar ζ-potentials, regardless of origin (engineered vs. aged), showed divergent behaviors.
• Surface charge similarity does not confer functional equivalence for microplastics.
• Current assumptions introduce bias in microplastic fate, exposure, and toxicity studies.

Conclusions:

• Microplastic "aging" must be defined by transformation history, not just surface descriptors.
• A new transformation-based framework distinguishes engineered, weathered, and aged particle regimes.
• This framework improves the interpretability of microplastic research for environmental assessment.