Aquatic toxicity of manufactured nanomaterials: challenges and recommendations for future toxicity testing

Abstract

Environmental context. The increased use of nanomaterials in industrial and consumer products requires robust strategies to identify risks when they are released into the environment. Aquatic toxicologists are beginning to possess a clearer understanding of the chemical and physical properties of nanomaterials in solution, and which of the properties potentially affect the health of aquatic organisms. This review highlights the main challenges encountered in aquatic nanotoxicity testing, provides recommendations for overcoming these challenges, and discusses recent studies that have advanced our understanding of the toxicity of three important OECD nanomaterials, titanium dioxide, zinc oxide and silver nanomaterials.Abstract. Aquatic nanotoxicologists and ecotoxicologists have begun to identify the unique properties of the nanomaterials (NMs) that potentially affect the health of wildlife. In this review the scientific aims are to discuss the main challenges nanotoxicologists currently face in aquatic toxicity testing, including the transformations of NMs in aquatic test media (dissolution, aggregation and small molecule interactions), and modes of NM interference (optical interference, adsorption to assay components and generation of reactive oxygen species) on common toxicity assays. Three of the major OECD (Organisation for Economic Co-operation and Development) priority materials, titanium dioxide (TiO 2 ), zinc oxide (ZnO) and silver (Ag) NMs, studied recently by the Natural Sciences and Engineering Research Council of Canada (NSERC), National Research Council of Canada (NRC) and the Business Development Bank of Canada (BDC) Nanotechnology Initiative (NNBNI), a Canadian consortium, have been identified to cause both bulk effect, dissolution-based (i.e. free metal), or NM-specific toxicity in aquatic organisms. TiO 2 NMs are most toxic to algae, with toxicity being NM size-dependent and principally associated with binding of the materials to the organism. Conversely, dissolution of Zn and Ag NMs and the subsequent release of their ionic metal counterparts appear to represent the primary mode of toxicity to aquatic organisms for these NMs. In recent years, our understanding of the toxicological properties of these specific OECD relevant materials has increased significantly. Specifically, researchers have begun to alter their experimental design to identify the different behaviour of these materials as colloids and, by introducing appropriate controls and NM characterisation, aquatic nanotoxicologists are now beginning to possess a clearer understanding of the chemical and physical properties of these materials in solution, and how these materials may interact with organisms. Arming nanotoxicologists with this understanding, combined with knowledge of the physics, chemistry and biology of these materials is essential for maintaining the accuracy of all future toxicological assessments.

Quantifying the dissolution of nanomaterials at the nano-bio interface

“…A growing number of studies suggest that organic (e.g., proteins, metabolites, lipids, DNA, and other natural substances) and/or inorganic (e.g., artificial waters, electrolytes, buffers) components in the exposure media can disturb the dissolution of NMs [49]. It is no longer acceptable to measure dissolution of NMs solely in media or solutions other than those being used in the exposure test [10]. Still, the complexity of the nano-bio interactions will create difficulties in the assessment of NMs dissolution in exposure media, especially in the presence of test organisms.…”

“…), and so on [9]. Meanwhile, the kinetics of dissolution and the saturation concentration are affected by conditions of the surrounding media (e.g., pH, ionic strength, water hardness) and the presence of bio-components (e.g., natural organic matter, polysaccharides, proteins, and the test organism) [9,10].…”

Physicochemical transformation and algal toxicity of engineered nanoparticles in surface water samples

“…It is necessary to strengthen the research on the physicochemical transformation of NPs in a real water environment for the scientific assessment of environmental behaviors and biological effects of the NPs. An increasing number of studies have reported the toxicity of NPs to aquatic organisms with a focus on the effect of water chemistry (Schultz et al, 2014). The higher toxicity of Ag NPs to algal cells was observed in acidic media (Oukarroum et al, 2014) and media with high ionic strength (Chambers et al, 2014), owing to the increased release of silver ions.…”

“…Moreover, environmental concentrations of some NPs in their application or deposition area could be extremely high, e.g., 0.75e50 g/L zero valent iron NPs were used in in situ field scale remediation of contaminated soil and groundwater (Grieger et al, 2010). More and more case studies indicated that the discharged NPs could pose serious risks to aquatic organisms (Ma et al, 2013;Schultz et al, 2014).…”

Bioavailability and toxicity of manufactured nanomaterials

“…Schultz et al [4] outline some of the challenges presented by transformations of MNMs and make recommendations for future toxicity testing. They review the toxicity data for the three most commonly studied metal-based MNMs (TiO 2 , Ag and ZnO) and highlight the emerging themes in the aquatic toxicology of these materials.…”