The Effect of Polymer Type and Particle Concentration on Microplastic Transport Mechanisms in Saturated Porous Media

The widespread presence of microplastics (MP) is posing a potential threat to soils and groundwater. However, the mechanisms governing MP transport and retention within porous media and groundwater remain largely unknown. In this study, we provide new evidence for the complex mechanisms governing the transport of different types of MPs within porous media.Using saturated quartz sand column models, we investigated the transport of five different MP polymer types, including Polyethylene (PE), Polyamide (PA), Polypropylene (PP), Polymethyl methacrylate (PMMA), and Polyethylene terephthalate (PET) at particle concentrations of 5,000; 50,000; and 500,000. MP Breakthrough curves and retention profiles were obtained to determine the polymer type and concentration specific transport and retention rates. Results indicate that MP transport capacity in porous media does not always exhibit linear correlation with MP particle concentrations. Specifically, as the particle injection concentration increased, the transport capacity of PE and PP initially increased and then decreased, reaching a maximum at 50,000. In contrast, the transport capacity of PET and PMMA increased markedly as the injection concentration rose from 5,000 to 50,000, but showed no further significant change when the concentration was increased from 50,000 to 500,000. In addition, biofilm growth on MP particles was found to alter the physicochemical properties of the MP particle surface, thereby modifying particle-matrix interactions and particle retention, and changing the overall MP transport behavior through porous media.These findings indicate that concentration impacts on particle transport behavior must be fully accounted for when applying column experiments to investigate particle transport, highlighting the importance of determining and applying environmentally realistic MP concentration ranges and particle conditions for testing. Furthermore, biofilm attachment to MP surfaces can alter critical surface properties and particle-matrix interactions, thereby modifying transport rates within subsurface environments.