Aquatic behavior and toxicity of polystyrene nanoplastic particles with different functional groups: Complex roles of pH, dissolved organic carbon and divalent cations.

Herein we systematically examined the roles of water chemistry (pH, dissolved organic carbon (DOC), and divalent cations) and particle surface functionality that control the aqueous stability, aggregation, and toxicity of engineered nanoplastic particles in simulated natural environmental conditions. Model polystyrene latex nanoparticles (PLNPs) with three different functional groups, namely unmodified (uPLNPs), amine-modified (aPLNPs), and carboxyl-modified (cPLNPs), were investigated. Results indicate that the presence of only DOC increased the surface charge and exhibited negligible effects on the size distribution of the PLNPs in aqueous suspensions. The presence of the divalent cations (Ca2+ and Mg2+) was observed to decrease the surface charge and increase the size of the PLNPs. The coexistence of DOC and the divalent cations enhanced the extent of aggregation of the PLNPs in the water columns. The surface modification and pH were sensitive factors influencing the stability of PLNPs during long-term suspension when DOC and the divalent cations coexisted. Direct visual further testified the conclusions on the combined effects of solution and surface chemistry parameters. Furthermore, in situ transmission electron microscope observations revealed that the enhancement of PLNP aggregation in the presence of DOC and the divalent cation was caused by bridge formation. Toxicity test indicated the PLNPs exhibited acute toxicity and physical damage to Daphnia magna. The more complex the solution conditions, the more toxicity the aPLNPs and cPLNPs. Analysis of mode of toxic action implied that the PLNPs mainly caused the accumulation of oxidative damage to the gut of D. magna.