Effect of low-molecular-weight organic acids on the transport of polystyrene nanoplastics in saturated porous media

Low molecular weight organic acids (LMWOAs) are extensively present as soluble organic matter in the environment, potentially influencing the transport of polystyrene nanoplastics (PSNPs) in soil and groundwater environments. In this study, we studied the impact of three LMWOAs (Acetic Acid (AA), Malic Acid (MA), and Citric Acid (CA)) on PSNPs migration under varied pH and Ionic Strength (IS) conditions in the saturated porous medium. The results demonstrated that the low LMWOAs concentrations (0.0001 mol L-1) promoted PSNPs migration rate, while high concentrations (0.001, 0.01 mol L-1) reduced the migration rate and increased the deposition. Due to the different relative molecular weights and number of functional groups of different LMWOAs, the order of promoting (0.0001 mol L-1) /inhibiting (0.001, 0.01 mol L-1) effects of LMWOAs on PSNPs migration rate under various physicochemical conditions in this study was AA < MA < CA. The decrease in IS and increase in pH promoted the migration of PSNPs. Electrostatic repulsion and spatial potential resistance affected PSNPs migration. This study offers theoretical support for the understanding of migration patterns and mechanisms of nanoparticles in soil-water environments.