Impact of dissolved organic matter characteristics and inorganic species on the stability and removal by coagulation of nanoplastics in aqueous media

The aggregation of rough, raspberry-type polystyrene nanoparticles (PS-NPs) was investigated in the presence of six hydrophobic and hydrophilic dissolved organic matter (DOM) isolates and biopolymers (effluent OM) in NaCl and CaCl₂ solutions using time-resolved dynamic light scattering. Results showed that the stability of PS-NPs mainly depends on OM characteristics and ionic composition. Due to cation bridging, the aggregation rate of PS-NPs in Ca²⁺-containing solutions was significantly higher than at similar Na⁺-ionic strength. Biopolymers rich in protein and carbohydrate moieties showed higher affinity to the surface of PS-NPs than the other DOM isolates in the absence of both Ca²⁺ and Na⁺. Overall, the stability of PS-NPs followed the order of biopolymers > hydrophobic isolates > hydrophilic isolates in the presence of Na⁺ and biopolymers > hydrophilic isolates > hydrophobic isolates in Ca²⁺-containing solutions. In the presence of high MW structures (biopolymers), PS-NPs aggregation in both NaCl and CaCl₂ solutions was attributed to steric repulsive forces. The impact of hydrophilic and hydrophobic isolates on PS-NPs aggregation highly relied on the ionic composition. Coagulation was an effective pretreatment for PS-NPs removal. Using inductively coupled plasma-mass spectrometry, higher removals were recorded with Al₂(SO₄)₃ in the absence of DOM, while PACl more efficiently coagulated PS-NPs in the presence of DOM isolates.