Effects of Low-Molecular-Weight Organic Acids on the Transport of Polystyrene Nanoplastics in Saturated Goethite-Coated Sand Columns

The widespread use of nanoplastics inevitably contributes to pollution in aquatic environments and soils. Therefore, it is crucial to understand how these particles migrate in soils with diverse organic matter. This study investigated the effects of low-molecular-weight organic acids (LMWOAs) on the migration of polystyrene nanoplastics (PS-NPs) in goethite-coated quartz sand. The experiments utilized two organic acids, propanoic acid (PA) and tartaric acid (TA), under varying aqueous conditions, including pH levels (4.0, 7.0), ionic strengths (1 mM, 10 mM), and cations (Na+, Ca2+, Ba2+). The experimental results indicated that with the presence of Na⁺, organic acids promoted the migration of PS-NPs through electrostatic forces and steric hindrance, with TA having a greater effect than PA. When pH < pHpzc, increased concentrations of positively charged goethite coating provided favorable deposition sites for the negatively charged PS-NPs, thereby increasing their deposition. Using the DLVO theory, low pH and high ionic strength (IS) decreased the energy barriers between PS-NPs and porous media, whereas high pH and low IS increased these barriers, thus enhancing PS-NPs transport. Divalent cations Ca2+ and Ba2+ enhanced the migration of PS-NPs through complex-forming and -bridging agents. These findings offered significant insights for predicting and analyzing the migration behavior of plastic nanoparticles.