The Mobility of Plastic Nanoparticles in Aqueous and Soil Environments: A Critical Review

Plastic nanoparticles (PNPs) are now widely recognized as a significant, and ever increasing, hazard in aquatic and soil environments. These particles, defined here as plastics <1000 nm across their largest dimension, are released into the environment both as primary products and through the degradation of larger plastic waste. They have the potential to be highly mobile, given their small size, which raises concerns about their global spread. Furthermore, PNPs are capable of adsorbing a variety of other pollutants, including heavy metals, pharmaceuticals, and pesticides, and in doing so may increase the mobility of these harmful materials. Dozens of studies focused on the toxicity of PNPs with and without other adsorbed contaminants for organisms from algae to humans have been published in the past few years. It is also critical to understand the transport of these particles in different environments to fully assess the threat that they pose. In this critical review, we discuss the behavior of suspended PNPs in aqueous systems and water-saturated soil/sediment with a particular emphasis on the processes that may alter their mobility. We describe how PNPs of different sizes and surface functionalities interact with the typical constituents of natural solutions, including metal cations, dissolved and particulate organic matter, and inorganic colloids. These variables all affect PNP aggregation in a range of complex, interrelated ways and can ultimately lead to particle sedimentation and removal from the mobile fluid phase. We also discuss the reactive transport of these PNPs, and PNP aggregates, through soils and sediments, during which PNP mobility can be limited by physical straining and/or attachment to the substrate. Finally, we conclude with some suggested future directions for research within this field, particularly the need for direct measurement of PNP abundances and characteristics in real environmental samples, which to date has been severely limited due to analytical challenges. Plastic nanoparticles do present a serious potential threat to the environment and even to human health, and a thorough understanding of the transport and mobility of these pollutants is critical in assessing these risks.