Molecular and Environmental Drivers of Microplastic–Humic Acid Association: A Synthesis of Mechanisms, Polymer Selectivity, and Aquatic Fate

Humic acids (HA) associate selectively with microplastics (MPs), not as an inert bystander. This review compares MD, spectroscopy, sorption-competition, and field studies to ask: how consistent is HA's polymer selectivity across independent groups and conditions? Polystyrene (PS) shows the strongest HA affinity via π–π stacking, while PE/PP rely on van der Waals forces. This ranking is almost always tested under one fixed pH/salinity — the one study that varied both reversed adsorption entirely.