Abstract
In this study, we investigate whether the influence of natural organic matter (NOM) on the aquatic ecotoxicity of engineered nanoparticles (ENPs) can be described quantitatively for the purpose of risk assessments based on existing ecotoxicity studies. A review of the literature studying the aquatic ecotoxicity of ENPs in the presence of NOM identified 66 studies in total, covering the metal and metal oxide ENPs most commonly used in consumer products. It was found that 80% of the studies show a reduction in ENP ecotoxicity in the presence of NOM. Analyses of ecotoxicity data based on 50% effect/inhibition/lethal concentrations (collectively referred to as XC50) were conducted. Correlations of XC50 values with the concentrations of NOM were investigated through Spearman's rank correlation coefficient as well as linear, power law, polynomial, exponential and logarithmic correlations. Furthermore, multiple linear regression (MLR) analyses, including also the pH in the reviewed ecotoxicity test systems (mainly in the range pH 7.0–8.5), were conducted. While none of these statistical approaches provided strong empirical correlations between XC50 values, NOM concentration and pH, an empirical rule of thumb was discovered for the ratio between XC50 values with and without NOM over an environmentally realistic concentration range for NOM (0.1–10 mg/L): XC50 values obtained in experiments with NOM present tended to be a factor of 1–10 higher than those without NOM. Until more accurate correlations are provided, a pragmatic approach for environmental risk assessments of ENPs might therefore be to use observed XC50 values from experiments without NOM present as reasonably conservative proxies for XC50 values with NOM present. Further studies are needed to confirm or falsify this rule of thumb for different ENPs, environmental conditions and metrics.
Highlights
The research on environmental risks of engineered nanoparticles (ENPs) has to date largely been about pristine particles, i.e. as they are when manufactured, which are usually bare or coated with some stabilising substance
We investigate whether the influence of natural organic matter (NOM) on the aquatic ecotoxicity of engineered nanoparticles (ENPs) can be described quantitatively for the purpose of risk assessments based on existing ecotoxicity studies
Multiple linear regression (MLR) analyses, including the pH in the reviewed ecotoxicity test systems, were conducted. While none of these statistical approaches provided strong empirical correlations between XC50 values, NOM concentration and pH, an empirical rule of thumb was discovered for the ratio between XC50 values with and without NOM over an environmentally realistic concentration range for NOM (0.1–10 mg/L): XC50 values obtained in experiments with NOM present tended to be a factor of 1–10 higher than those without NOM
Summary
The research on environmental risks of engineered nanoparticles (ENPs) has to date largely been about pristine particles, i.e. as they are when manufactured, which are usually bare or coated with some stabilising substance. One way of interaction between ENPs and NOM is the adsorption of NOM onto ENPs, which can occur through different mechanisms: hydrophobic interaction, electrostatic interactions, van der Waals forces, ligand exchange, chelation, cation bridging and hydrogen bonding (Philippe and Schaumann, 2014). In this context, the eco-corona ( called environmental corona) concept has emerged as an analogy to the bio-corona ( called protein corona), which is a (possibly partial) layer of proteins formed around ENPs (Cedervall et al, 2007; Fadeel et al, 2013; Ke et al, 2017). NOM can interact with ENPs in other ways depending on the type of NOM, for example by forming larger aggregates (Buffle et al, 1998)
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