Deposition of synthetic polystyrene and low-density polyethylene to quartz sand in different background solutions

Microplastics (MPs) and nanoplastics (NPs) have gained considerable attention as emerging contaminants that can pose potential risks to subsurface environments due to their widespread presence and persistence in the environment. They can act as carriers for other contaminants, such as heavy metals, by adsorbing onto their surfaces, potentially increasing their mobility and consequently causing toxicity to organisms and human health. MPs and NPs can enter groundwater through landfill leachate, agricultural mulches, and wastewater effluent. However, MPs’ and NPs’ behavior in porous media with complicated components has not been thoroughly examined. Therefore, further research is essential to identify the key factors such as aggregation (particles attaching to each other) and deposition (particles attaching to a transport medium), that may influence MPs' and NPs' behavior, fate, and transport mechanisms in soils and groundwater.

The purpose of our research is to investigate how plastic particle properties, pore water chemistry, as well as characteristics of the medium would influence the aggregation and deposition of MPs and NPs.

This study focuses on the attachment of low-density polyethylene micro- and nano-plastics (LDPE) released from macro-plastic pellets and synthesized polystyrene micro-spheres to quartz sand under controlled laboratory conditions. Batch experiments were performed to study the aggregation and deposition of LDPE and synthesized polystyrene micro-spheres onto quartz sand that allow for precise control over environmental variables, facilitating the observation of microplastic-sand interactions in varying background solutions. The influence of two common salts, sodium chloride (NaCl) and calcium chloride (CaCl₂), on the attachment process is systematically investigated. The results from our experiments indicated that similar to polystyrene micro-spheres, the LDPE particles did not adsorb to quartz sand at pH 5 in 3 mM NaCl solution, while a substantial amount of LDPE adsorbed to quartz sand in 1 mM CaCl₂ at pH 5. This could be attributed to the less negative zeta potential of LDPE (~-25 mV) and polystyrene micro-spheres (~-17 mV) in 1mM CaCl₂ background solution as a result of lower electrostatic repulsion between particles.

Results from these experiments provide insights into the complex mechanisms governing MPs' and NPs' behavior in aquatic environments, aiding in the development of strategies to mitigate their impact on ecosystems.