Significant Enrichment of Engineered Nanoparticles in Water Surface Microlayer

Abstract

Water surface microlayer (SML), as the interface between water and the atmosphere, shows distinct physicochemical properties that differ from those of underlying water. Herein, for the first time, we demonstrate the SML enrichment of nanoparticles (NPs) by using silver nanoparticles (AgNPs) as a model via indoor experiments. The occurrence of SML enrichment of AgNPs was confirmed by the increased concentration of NPs in the SML relative to that in the bulk phase, and the in situ recording of the enhanced Raman spectroscopy intensity of a probe adsorbed on AgNPs in the SML. The significant enrichment of NPs is strongly influenced by environmentally relevant factors such as the solution pH, ionic strength, and natural organic matter. Additionally, the SML enrichment factor was estimated to be 14.6-26.5 for AgNPs in natural waters. Our findings indicate that NPs are inclined to accumulate in the SML, which could cause environmental effects that are differential to the bulk phase.