Abstract
Where surface-functionalized engineered nanoparticles (NP) occur in drinking water catchments, understanding their transport within and between environmental compartments such as surface water and groundwater is crucial for risk assessment of drinking water resources. The transport of NP is mainly controlled by (i) their surface properties, (ii) water chemistry, and (iii) surface properties of the stationary phase. Therefore, functionalization of NP surfaces by organic coatings may change their fate in the environment. In laboratory columns, we compared the mobility of CeO2 NP coated by the synthetic polymer polyacrylic acid (PAA) with CeO2 NP coated by natural organic matter (NOM) and humic acid (HA), respectively. The effect of ionic strength on transport in sand columns was investigated using deionized (DI) water and natural surface water with 2.2 mM Ca2+ (soft) and 4.5 mM Ca2+ (hard), respectively. Furthermore, the relevance of these findings was validated in a near-natural bank filtration experiment using HA-CeO2 NP. PAA-CeO2 NP were mobile under all tested water conditions, showing a breakthrough of 60% irrespective of the Ca2+ concentration. In contrast, NOM-CeO2 NP showed a lower mobility with a breakthrough of 27% in DI and < 10% in soft surface water. In hard surface water, NOM-CeO2 NP were completely retained in the first 2 cm of the column. The transport of HA-CeO2 NP in laboratory columns in soft surface water was lower compared to NOM-CeO2 NP with a strong accumulation of CeO2 NP in the first few centimeters of the column. Natural coatings were generally less stabilizing and more susceptible to increasing Ca2+ concentrations than the synthetic coating. The outdoor column experiment confirmed the low mobility of HA-CeO2 NP under more complex environmental conditions. From our experiments, we conclude that the synthetic polymer is more efficient in facilitating NP transport than natural coatings and hence, CeO2 NP mobility may vary significantly depending on the surface coating.
Highlights
Cerium dioxide nanoparticles (CeO2 NP) are used in various industrial applications, e.g., in diesel fuels, catalytic converters, and polishing agents (Garcia et al 2005; Reed et al 2014)
The decreasing ζpotential with increasing ionic strength suggests that electrostatic and steric stabilization mechanisms might be responsible for the high stability of polyacrylic acid (PAA)-CeO2 NP
In contrast to PAA-CeO2 NP, natural organic matter (NOM)-CeO2 NP size increased in hard surface water
Summary
Cerium dioxide nanoparticles (CeO2 NP) are used in various industrial applications, e.g., in diesel fuels, catalytic converters, and polishing agents (Garcia et al 2005; Reed et al 2014). CeO2 NP are predicted to reach concentrations in the range of 10−2 μg L−1 (Gottschalk et al 2013). The presence of CeO2 NP in the environment was repeatedly shown to cause negative effects on organisms such as ammoniaoxidizing bacteria (García et al 2012) or earthworms (Collin et al 2014). Damage of human lung cells as well as oxidative stress can be caused by CeO2 NP (Lin et al 2006). For example via drinking water, should be avoided. As riverbank filtrates are an important source for drinking water (Sprenger et al 2017), it must be ensured that CeO2 NP entering the environment will be retained in these systems by natural filtration mechanisms, e.g., straining, adsorption, and attachment to biofilms. To understand the likelihood for transport of NP into environmental compartments such as groundwater, studies on the stability and the transport of NP in natural systems are important
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