The aggregation of nanoplastics (NPs) is a key issue in understanding the dynamic nature of NPs in the environment. The aggregation of NPs under various environmental conditions has not yet been studied. We investigated the influences of inorganic ions and natural organic matter (NOM) on polystyrene (PS) NPs aggregation in solutions. Results showed that PS NPs remained stable in wide ionic strength solutions of NaCl (1–100 mM) and CaCl₂ (0.1–15 mM), and only in low ionic strength FeCl₃ solutions (0.01 mM). However, obvious PS NPs aggregation was observed in FeCl₃ solutions with an increase in ionic strength (0.1 and 1 mM). Moreover, NOM had a negligible effect on PS NPs aggregation in all ionic strengths of NaCl and CaCl₂ solutions and in low ionic strength FeCl₃ solutions (0.01 mM). However, NOM reduced PS NPs aggregation in an intermediate ionic strength FeCl₃ (0.1 mM) solution and increased aggregation in a high ionic strength FeCl₃ (1 mM) solution. Based on the theoretical analysis of interaction forces among PS NPs, the Derjaguin–Landau–Verwey–Overbeek force was a contributor governing PS NPs aggregation either in the absence or presence of NOM. In addition, other factors, including electrostatic heterogeneity of PS NPs surfaces, steric repulsion induced by NOM, and clusters formed via bridging effect in the presence of NOM also contributed to altered PS NPs aggregation under selected conditions. The PS NPs-NOM clusters were directly observed using a cryogenic scanning electron microscope.

纳米塑料（NPs）的聚集是理解环境中NPs动态特性的关键问题。尚未研究在各种环境条件下NP的聚集。我们研究了无机离子和天然有机物（NOM）对溶液中聚苯乙烯（PS）NPs聚集的影响。结果表明，PS NP在NaCl（1-100 mM）和CaCl ₂（0.1-15 mM）的宽离子强度溶液中保持稳定，仅在低离子强度的FeCl ₃溶液（0.01 mM）中保持稳定。但是，在FeCl ₃溶液中观察到明显的PS NPs聚集，离子强度增加（0.1和1 mM）。此外，在NaCl和CaCl _2的所有离子强度下，NOM对PS NPs聚集的影响都可以忽略不计。溶液和低离子强度的FeCl ₃溶液（0.01 mM）。但是，NOM降低了中等离子强度FeCl ₃（0.1 mM）溶液中PS NPs的聚集，而增加了高离子强度FeCl _3中的PS NPs的聚集（1 mM）溶液。根据PS NP之间相互作用力的理论分析，在没有NOM或存在NOM的情况下，Derjaguin–Landau–Verwey–Overbeek力是控制PS NP聚集的因素。此外，其他因素，包括PS NPs表面的静电异质性，由NOM引起的空间排斥以及在NOM存在下通过桥接效应形成的簇也有助于在选定条件下改变PS NPs的聚集。使用低温扫描电子显微镜直接观察到PS NPs-NOM团簇。