| Nanoplastics?NPs?derived from degradation of macroplastics and microplastics are emerging contaminants in aquatic environments.Previous studies have detected NPs in natural water samples obtained from North Atlantic subtropical gyre and found their potential threat to aquatic biota and human health.Their fate and transport in aquatic systems are mainly governed by aging processes and aggregation behavior.In this study,we simulated plastic aging process using UV irradiation and compared the physiochemical properties of fresh versus aged polystyrene NPs?PSNPs?through various characterizations including Fourier Transform Infrared Spectroscopy?FTIR?,contact angle measurements,potentiometric titration,total organic carbon?TOC?analysis,and X-ray photoelectron spectroscopy?XPS?;Meanwhile,time-resolved dynamic light scattering?TR-DLS?was employed to investigate the influence of water chemistry?i.e.,p H and ionic strength?,UV irradiation and natural organic matter on the aggregation kinetics of NPs in aqueous systems.The study found that:?1?After UV irradiation,the particle size of PSNPs decreased and TOC concentration in solutions of the irradiated PSNP samples increased,which could be related to the change in PSNPs surface structure likely due to chain scission by attacks from free radicals such as peroxyl?ROO·?,alkoxyl?RO·?and hydroxyl?·OH?radicals;On the other hand,the hydrophilicity of nanoplastics was enhanced as reflected by the decreased contact angle from108.2°to 73.7°,and the PSNP surface charge became more negative,which might be attributed to the induced O-containing functional groups such as carboxyl group after irradiation.?2?Fresh PSNPs had strong colloidal stability under typical freshwater,and electrolytes could induce PSNP aggregation and exhibited both reaction-and diffusion-limited aggregation regimes,in agreement with classic Derjaguin-Landau-Verwey-Overbeek?DLVO?theory.The effects of different salts on PSNP aggregation followed the Schulze-Hardy Rule;The aging process inhibited PSNP aggregation in Na Cl solutions by increasing the negative charge on PSNP surface and the organic matter content in solution,while promoted PSNP aggregation in Ca Cl2 solutions due to interactions between Ca2+and carboxyl groups formed on aged PSNP surface;The data of DLVO predication was calculated by MATLAB software and derived Hamaker constants for PSNPs decreased from 3.5 to 1.5×10-21 J as the irradiation time increased.Such reduced Hamaker constant for UV-irradiated PSNPs may lower their van der Waals attraction and thus enhance their colloid stability compared to fresh PSNPs.?3?Organic macromolecules exerted different influence on PSNP stability that enhanced the stability of PSNPs in Na Cl solutions but distablized PSNPs in Ca Cl2 solutions.In Na Cl solutions,PSNPs became more stable due to steric hindrance originated from adsorbed macromolecular layer on PSNP surface.BSA was the most effective in retarding PSNP aggregation,followed by HA,EPS,and SA.In Ca Cl2 solutions,SA significantly destabilized PSNPs via alginate bridging with Ca2+with such effect enhanced with concentrations of both SA and Ca Cl2;whereas the destabilization effects of other three macromolecules were governed by the interplay among molecular bridging,electrical double layer?EDL?compression.Through this study,we revealed the homoaggregation mechanisms of NPs in aquatic system,deepened people's understanding of the migration and transformation for NPs in natural water,and provided theoretical fundament for environmental risk assessment and control of NPs. |
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