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Engineered nanoparticle (NP) size and natural organic matter (NOM) composition play important roles in determining NP environmental behaviors. The aim of this work was to investigate how NP size and NOM composition influence the colloidal stability of polyvinylpyrrolidone coated platinum engineered nanoparticles (PVP-PtNPs). We evaluated PVP-PtNP aggregation as a function of the NP size (20, 30, 50, 75, and 95 nm, denoted as PVP-PtNP 20–95 ) in moderately hard water (MHW). Further, we quantified the effect of the hydrophobic organic acid (HPOA) fraction of NOM on the aggregation of PVP-PtNP 20 and PVP-PtNP 95 using 6 NOM samples from various surface waters, representing a range of NOM compositions and properties. NOM samples were characterized for bulk elemental composition ( e.g. , C, H, O, N, and S), specific ultraviolet absorbance at 254 nm (SUVA 254 ), and molecular level composition ( e.g. , compound classes) using ultrahigh resolution mass spectrometry. Single particle-inductively coupled plasma-mass spectrometry (sp-ICP-MS) was employed to monitor the aggregation of PVP-PtNPs at 1 μg PVP-PtNP per L and 1 mg NOM per L concentrations. PVP-PtNP aggregate size increased with decreasing primary PVP-PtNP size, likely due to the lower zeta potential, the higher number concentration, and the higher specific surface area of smaller NPs compared to larger NPs at the same mass concentration. No aggregation was observed for PVP-PtNP 95 in MHW in the presence and absence of the different NOM samples. PVP-PtNP 20 formed aggregates in MHW in the presence and absence of the six NOM samples, and aggregate size increased in the presence of NOM likely due to interparticle bridging of NOM-coated PVP-PtNPs by divalent counterions. PVP-PtNP 20 aggregate size increased with the increase in NOM elemental ratio of H to C and the relative abundance of lignin-like/carboxyl rich-alicyclic molecules (CRAM)-like compounds. However, the aggregate size of PVP-PtNP 20 decreased with the increase in NOM molecular weight, NOM SUVA 254 , elemental ratio of O to C, and the relative abundance of condensed hydrocarbons and tannin-like compounds. Overall, the results of this study suggest that the composition and sources of NOM are key factors that contribute to the stability of PVP-PtNPs in the aquatic environment. 

Adsorption of natural organic matter (NOM) on nanomaterials (NMs) results in the formation of interfacial area between NMs and the surrounding environment (referred to as NOM-corona), giving rise to NMs' unique surface identity. This unique surface identity is determined by the ligands and their interactions with NM surfaces. Since the chemical structure and functionality is heterogeneous and polydisperse, the molecular composition of NOM-corona is the result of competitive adsorption of NOM molecules on the NM surface. Here, we investigate the molecular composition of NOM-corona formed from two different NOM samples (isolated from the Yukon River and Milwaukee River) on the surface of AgNMs using electrospray ionization-Fourier-transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS). The composition of AgNM-NOM corona varied with the composition of the original NOM. In general, AgNM-NOM corona is rich with N- and S-containing compounds. Furthermore, AgNM-NOM corona is rich with compounds with high molecular weight, high unsaturation, and high number of oxygenated groups. However, CHOS (carbon, hydrogen, oxygen and sulfur) compounds adsorbed on AgNMs from the Yukon River NOM have low molecular weight (LMW) and low saturation index, which might be due to selective adsorption via chemical complexation (Ag–S). On the other hand, NOM compounds with LMW and low unsaturation or compounds containing few oxygenated groups (mainly alcohols and ethers) are preferentially maintained in solution phase. The results here provide evidence of molecular interactions between NOM and NMs, which are critical to understanding NM behavior and toxicity in natural environments.