Stability and aggregation of microplastics suspended in aqueous media

The accumulation of microplastics (MPs) has become a significant problem due to their non-biodegradable nature and ingestion by marine life. This study focused on three types of microplastics of approximate size 1 μm: polystyrene microplastics (PSMPs), carboxylate-modified polystyrene (PS-COOH), and amine-modified polystyrene (PS-NH2). Their behavior was investigated in aqueous media at pH 5 and 9, with the inclusion of humic acid, different ions (Mg2+ or Na+) with ionic strength (IS) range (3mM and 15mM), and kaolinite. Results showed that a divalent cation, Mg2+, promoted aggregation by causing compression of the electric double layer. In lower IS scenarios, repulsion forces generated by electrical charges led to the stability of the PSMPs, inhibiting aggregation. Also, generally, suspensions were more unstable at higher pH values as the surface charge on microplastics may reverse, leading to reduced electrostatic repulsion. Humic acid improved the stability of suspensions through electrostatic or steric repulsion forces. Conversely, kaolinite reduced electrostatic repulsion, making suspensions less stable. Moreover, PS-NH2 suspensions generally exhibited stability with high energy barriers, affected by kaolinite and IS. PS-COOH suspensions remained stable in higher IS NaCl, suggesting that the carboxylic acid groups contributed to their stability. These findings contribute to developing strategies to mitigate the environmental impact of microplastics.