Molecular weight fractionated extracellular polymeric substances (EPS) impart different aggregation characteristics on polystyrene nanoplastics

This study explores how EPS with varying molecular weights (MW) adsorb onto polystyrene nanoplastics coated with bare (PS-Bare), carboxylic (PS-COOH), or amine (PS-NH₂) functional groups. Experiments were performed in NaCl and CaCl2 to characterize the formed eco-corona and their impacts on nanoplastics aggregation kinetics. The results show that while eco-corona shares similar chemical compositions, their thickness (ECT) correlates positively with both adsorbed EPS mass and MW. In NaCl, PS-Bare exhibited the narrowest ECT range (0.88–7.17 nm), followed by PS-COOH (1.78–9.43 nm), whereas PS-NH₂ displayed the broadest span (2.33–13.95 nm). The ECT further increased in CaCl2. Enhanced ECT caused higher critical coagulation concentration (CCC). In NaCl, PS-Bare’s CCC rose from 343 mM to 466–1291 mM, and PS-COOH’s from 339 mM to 577–1373 mM. Under 5 mg C L⁻¹ EPS, PS-NH₂ maintained a stable hydrodynamic diameter (Dh ≈ 150 nm) in both NaCl and CaCl2, indicating strengthened steric stabilization. In CaCl2 with 5 mg C L⁻¹ EPS, similar stabilization trends were observed. However, at 10 mg C L⁻¹, high-MW EPS reduced CCC via bridging effects. For PS-NH2, low-MW EPS (5 mg C L⁻¹) increased Dh to 900–1200 nm through charge neutralization and weak steric hindrance. The EPS <3kDa further enlarged Dh to 1400 nm by patch-charge attraction. This study reveals that EPS molecular weight, by regulating eco-corona thickness and surface charge distribution, is a key factor influencing the colloidal stability of nanoplastics.