Quantitative hazard assessment of engineered zinc oxide nanoparticles : insights into their regulation

Abstract

As engineered nanomaterials (ENMs) have advanced functional properties over their bulk counterparts, their applications in commercial products have been growing exponentially in recent decades, triggering concerns over their environmental impacts after release. This study first reviewed the current regulations on the application and release of ENMs around the globe, and revealed that there was no specific regulation for ENMs due to their debatable definition, their sophisticated interaction with the environment, and insufficient techniques to quantify them in environmental compartments and predict their risks. Zinc oxide nanoparticles (ZnO-NPs), a popular ENM in commercial products, were adopted as a model in this study, which specifically tested three interrelated overarching hypotheses: (1) different coatings can affect the physicochemical properties of ZnO-NPs and further affect their toxicity to marine organisms, (2) the toxicity of ZnO-NPs can be modulated by the presence of other pollutants such as microplastics, and (3) environmental factors including temperature and salinity can jointly influence the physicochemical properties of ZnO-NPs and the physiology of marine organisms, and hence affect their toxicity. The results of this study provided useful information and insights for regulating the use and release of ZnO-NPs and other metal-based ENMs. The toxic effects of three different types of coated ZnO-NPs on the marine copepod Tigriopus japonicus were compared with the uncoated ZnO-NPs. At acutely toxic concentrations, ZnO-NPs functionalized with hydrophobic coating (i.e., dodecyltrichlorosilane, SA-ZnO-NPs) generated less reactive oxygen species (ROS) compared with uncoated ZnO-NPs and ZnO-NPs coated the with hydrophilic coatings (i.e., (3-aminopropyl)triethoxysilane and 3-(methacryloyloxy)propyltrimethoxysilane). At chronically toxic concentrations (i.e., environmentally relevant concentrations), SA-ZnO-NPs remained intact but all other ZnO-NPs largely dissolved into ions. Evidently, SA-ZnO-NPs were the least toxic to the copepod. To untangle the interacting effect between ZnO-NPs and microplastics, the binary toxicity of uncoated ZnO-NPs and polystyrene plastics at nano-size (PNPs) on the marine rotifer Brachionus koreanus was investigated. Based on the experimental results and three mixture toxicity models, ZnO-NPs were found to be less toxic in the presence of PNPs. This was because the two chemicals could form hetero-agglomerates that lowered their bioavailability to the rotifers. Changes in temperature and salinity could alter the physicochemical properties of ZnO-NPs, and thus their toxicity towards the copepod T. japonicus and the mussel Xenostrobus securis. An increase in temperature facilitated the ROS generation and agglomeration of ZnO-NPs, while a decrease in salinity enhanced their ion dissolution. Both species experienced significantly elevated metabolic cost and thermal stresses at high temperature, which increased their vulnerability to ZnO-NPs. Salinity was a less influential factor than temperature. The results of this study suggested that it is essential to (1) consider the influences of different types of coating on the toxicity of metal-based ENMs, (2) understand the interaction of metal-based ENMs with other environmental pollutants during their environmental risk assessment, and (3) derive toxic effect thresholds of metal-based ENMs with the consideration of natural variations of temperature and salinity, especially temperature. These recommendations would advance the future regulation on the application and release of ENMs for better environmental protection.