Development of a comprehensive understanding of aggregation-settling movement of CeO2 nanoparticles in natural waters☆
Graphical abstract
Introduction
Nanotechnology has been one of the fastest developing technologies in recent decades, and the subsequent use of nanomaterials (ENMs) in a wide variety of applications is due to their overwhelming advantages, including a small size and high surface-to-volume ratio (Forster et al., 2011; Praetorius et al., 2014). However, the rapid growth of the market indicates that the release of nanoparticles (NPs) from manufactured ENMs are increasingly being discharged into aquatic environments and that their environmental levels are growing (Keller and Lazareva, 2014). Consequently, an ecological risk assessment of released NPs has also raised concerns, and increasing evidence has shown that the release of NPs into an aquatic environment has potential adverse environmental risks, causing damage to different organisms (Adam et al., 2014; Adam et al., 2016; Yang et al., 2014). Once NPs are released into an aquatic system, their environmental transport and transformation are dependent on their sizes and surface properties, and the physicochemical characteristics of the aquatic medium (Li et al., 2017; Lin et al., 2016; Miao et al., 2016; Oriekhova and Stoll, 2016). Thus, to assess the subsequent risk to an ecosystem, a clear understanding of the NP behavior (e.g., aggregation, sedimentation, and dissolution) in water is crucial for fully elucidating the life cycle and transport mechanisms of NPs in the environment.
Owing to the presence of different solid particles in natural waters, the aggregation-settling process of NPs cannot be directly detected and measured using existing instruments such as dynamic light scattering (DLS). Therefore, Luo et al. investigated the application of the Turbiscan Stability Index (TSI) for measurements of the CeO2/TiO2 NP heteroaggregation in natural waters, the results of which showed that TSI can effectively measure the heteroaggregation process of NPs (Luo et al., 2017). In addition, Kloster et al. employed UV–VIS spectroscopy to quantify the progress of the aggregation reaction (Kloster et al., 2013). Chen et al. reported that a Fourier transformation infrared (FTIR) process using a fluorescence excitation−emission matrix is effective when investigating the heteroaggregation of TiO2 NPs (Wei et al., 2014). Nevertheless, the detection of NPs in natural water remains extremely challenging, and suitable analytical methods are still under development (Klaine et al., 2011). Under this situation, a modeling approach has gradually become a new method for estimating the exposure concentration of NPs in an environment (Arvidsson et al., 2011; Garner et al., 2017; Praetorius et al., 2012; Quik et al., 2010). However, most modeling studies have considered the aggregation-settling of NPs in static water systems, focusing on the effects of the water characteristics, such as the pH, ionic strength (IS), and dissolved organic matter (DOM) (Fang et al., 2017; Quik et al., 2012). One crucial component in controlling the NP aggregation-settling process that has been overlooked is the hydrodynamic force (Chekli et al., 2015; Lv et al., 2016b). In addition, research results on the effects of the shear force on the environmental behavior of NPs in existing studies are contradictory, and a comprehensive understanding is therefore lacking. For example, the generally higher aggregation rate (khet) in turbulent waters is explained by either a higher attachment efficiency or a higher collision frequency, leading to a faster and more extensive aggregation (Fang et al., 2017; Ilona et al., 2014). In contrast, some studies have reported that a disaggregation occurs in stirred samples because the shear force can induce a breaking of the aggregates (Chekli et al., 2015; Frédéric et al., 2013; Philippe and Schaumann, 2014). Our previous study also showed that the aggregates formed by a weak interaction, such as a van der Waals force, can be easily broken up by the shear forces (Lv et al., 2017). Such conflicting results suggest that the effect of the shear force in a natural water environment is complex and should not be simply defined. Moreover, water flows can yield a dramatic increase in the suspended sediment (SS) in a water column, which may dominate the NP sedimentation (Cross et al., 2015). Velzeboer et al. investigated the heteroaggregation of NPs in dynamic sediment–water systems, but failed to consider the influence of natural colloids (Ncs) (Velzeboer et al., 2014). In short, the realistic conditions include IS, pH, DOM, SS, and water flows. However, to date, there have been no comprehensive modeling studies on the fate of NPs in an aquatic environment under different hydrodynamic conditions. Furthermore, such available studies on the behavior of the NPs in an aquatic environment have applied a theoretical analysis, lacking validation data based on laboratory research. This indicates that certain parameters are chosen arbitrarily and are not based on data. In addition, the low NP concentrations applied in these studies are within the range of 1–200 mg/L, some of which were much higher than the predicted environmental concentrations in natural waters (Chekli et al., 2015; Hoecke et al., 2009; Wang et al., 2015b; Zhou et al., 2012). Although such studies have helped in understanding the behavioral mechanisms, their explanations remain problematic because an increase in the NP concentration has a decisive effect on the aggregation rate.
In the present study, CeO2 NPs were chosen as the model particles owing to their extensive commercial applications and insolubility, which means their table physicochemical properties and data availability make them appropriate for revealing the underlying mechanism of NPs in natural waters (Dahle et al., 2015; Keller and Lazareva, 2014; Keller et al., 2010; Quik et al., 2010). The aim of this study is to investigate how the hydrodynamic force changes the fate of NPs in natural waters and their associated mechanisms. Both the settling and aggregation rates were calculated based on the aggregation-settling experiment data of CeO2 NPs in different natural waters. The hydrodynamic effect was explained through a calculation of the collision frequency. Analytical experiments were also conducted to provide reliable evidence for the modeling data. Thus, the fate of NPs after entering an aquatic system can be clearly understood and the uncertainty in the models can be reduced, which will ultimately serve to accurate predict the transport distance of NPs in natural waters.
Section snippets
Nanoparticles
In this study, commercially available CeO2 NPs (25 nm) were purchased from Sigma-Aldrich (St. Louis, MO, USA). The CeO2 NP stock dispersions were characterized using several techniques. Details are provided in the Supplementary Material section. In particular, the nanoparticle tracking analysis (NTA) measurement shows that the concentration of particles under mass concentrations of 1 and 10 mg/L were 2.02 × 108 ± 1.66 × 107/mL and 3.20 × 109 ± 2.31 × 108/mL, respectively. In particular, the
Settling of CeO2 nanoparticles under different environmental conditions
The different environmental conditions show significant differences in the settling of CeO2 NPs (Fig. 1). As expected, in seawaters with a much higher IS and relative low DOC, the concentration of NPs decreased rapidly during day 1 and then decreased slightly until day 5 (Fig. 1A). Settling rarely occurred, and the NP concentration remained the same. This phenomenon is consistent with other studies and can be explained as a high IS compressing the electrostatic double layer of the NPs and thus
Acknowledgment
We are grateful for the grants for Project supported by the National Natural Science Funds for Excellent Young Scholar (No. 51722902), the National Science Funds for Creative Research Groups of China (No.51421006), the Outstanding Youth Fund of Natural Science Foundation of Jiangsu, China (BK20160038), the China Scholarship Council (201806710076) and PAPD.
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This paper has been recommended for acceptance by Bernd Nowack.