Fate and transport of nanoplastics in complex natural aquifer media: Effect of particle size and surface functionalization.

Environmental processes of nanoplastics in heterogeneous natural groundwater systems remain unclear. In this study, the control of particle size and surface functional groups on the fate and transport of nanoplastics in an organic matter (OM) rich aquifer was explored using batch and column tests. The carboxyl-modified 200 nm (200CNP), carboxyl-modified 50 nm (50CNP), and amino-modified 50 nm (50ANP) polystyrene latex beads were used as surrogates for nanoplastics of contrasting sizes and surface functional groups. Aquifer sand and natural groundwater sampled from an agriculture-impacted shallow sandy aquifer were processed to obtain granule beds with/out surface minerals and groundwater containing different-sized fractions of OM. Results show that particle size controlled the hetero-aggregation rate of nanoplastics with OM and Ca: a larger size resulting in a lower reaction rate led to a higher stability of 200CNP than 50CNP and 50ANP. Meanwhile, surface functional groups appeared to affect the affinity of OM and Ca to nanoplastics, i.e. the amino group allowed the adsorption of dissolved OM on the particle but inhibited the adsorption of Ca and suspended OM, while the carboxyl group allowed adsorption of the all. The resulting variable OM coatings formed on the different nanoplastics played a critical role in determining the particle stability and mobility, i.e. the suspended OM increased both the particle stability and mobility while the dissolved OM reduced both. These findings suggest that: 1. Depending on the OM properties, the influence of particle size and surface group on the nanoplastic processes might be secondary to the OM impact; 2. In evaluating the OM impact, not only the OM concentration but also the size and surface physiochemistry of the OM should be characterized. The insight gained is important to predict the concentration evolution pattern of weathered nanoplastics in OM-impacted sandy aquifers.