The nano cocktail: Ecotoxicological effects of engineered nanoparticles in chemical mixtures

Around 2003, the first concerns related to potential environmental exposure and related effects of engineered nanoparticles (ENPs) were raised in the scientific literature. Today it is evident that the widespread and diverse use of products containing ENPs, as well as the expected increase in the number of such products, will lead to emissions to the environment. Parallel to escalating industrial production, increasing focus has been placed upon their ecotoxicological effects. As is the case for traditional chemicals, ENPs will be part of a complex mixture, rather than existing as single contaminants, when present in the environment. However, only a limited number of studies, exist on the effects of ENPs in mixtures with other contaminants. Mixture interactions between ENPs and other chemical compounds may influence bioavailability and, hence, the effects, of the individual compounds. Seen from the point of view of the organism, these interactions can be beneficial as well as negative, but are in both cases difficult to predict at our present state of knowledge. Interactions between ENPs and other chemicals will take place already at the stage of particle production, for example, in the processes of synthesis and functionalization. When the particles are used in industrial applications, medical, and consumer products, they again come into contact with chemical compounds such as surfactants and preservatives. Finally, the different routes of disposal also lead to interactions with environmental contaminants present in, for instance, wastewater streams, which may include both metals and organics. Thus, unintentional mixing of ENPs with other compounds is inevitable before, during, and after their intended use. Furthermore, some ENPs are also produced with coatings or doped with metallic or nonmetallic compounds. This is done, for example, to shift the activation wavelengths of the photocatalyst TiO2 from ultraviolet to visible wavelengths (Zaleska 2008). A relatively inert and low-toxicity particle thereby becomes a carrier of potentially toxic compounds. Combined effects of traditional chemicals in a mixture do not necessarily require a direct physical interaction between the individual compounds. This is the case when 2 compounds are competing for the same binding site, thereby leading to a less than additive effect, or when 1 compound is promoting the uptake of another, leading to synergistic effects. In the case of ENPs interacting with other chemical compounds, other interaction scenarios need to be included due to the presence of a solid particle phase. This issue is particularly relevant because ENPs have certain properties that may be favorable for sorption and interactions with other contaminants: a large surface area relative to mass, and a size possibly small enough for particles to enter into organisms, organs, and cells. It is evident that there are several possible scenarios for the interactions between ENPs, environmental contaminants, and aquatic organisms. Although some of these scenarios are so far purely theoretical, others have already been documented in toxicity studies involving algae, crustaceans and fish. Below we briefly comment on the scenarios outlined in Figure 1. Different scenarios of nanoparticles (NP) acting as modifying factors on the effect of environmental contaminants (EC) toward an organism (in this case algae). Scenario 1: No interaction between NP and EC. The effect of the EC is unchanged. The NP has no effect on the organism. Scenario 2: No interaction between NP and EC. Effect—and possibly uptake—of both independently. Scenario 3: Interaction between NP, EC, and organism. Increased bioavailability of the EC as a result of increased local EC concentration (or at least the sorbed EC is bioavailable). Scenario 4: Interaction between NP, EC, and organism. Increased bioavailability as a result of membrane rupture (caused directly or indirectly by NP) leading to increased uptake and/or effect of EC. Scenario 5: Interaction between NP and EC. EC uptake facilitated by NP (trojan horse) and increased EC body burden. Scenario 6: Release of free ions from NP, leading to competition with EC (in this case a metal) for binding sites. This will result in reduced EC uptake and effect. However, the overall effect on the organism may be unchanged, reduced, or decreased depending on the specific EC and NP. Scenario 7: Interaction (sorption) of the EC onto NP. The bioavailability (and effect) of the EC is reduced. Possible inherent effect of the NP on the organism. In some cases the physicochemical properties of the ENPs and coexisting contaminants may not allow for interactions (corresponding to Scenarios 1 and 2). We have previously investigated changes in toxicity of 4 chemical compounds (atrazine, methyl parathion, phenanthrene, and pentachlorophenol) in the presence of C60 nanoparticles (Baun et al. 2008). For atrazine and methyl parathion, we observed a limited sorption to the C60 aggregates, and the addition of C60 did not affect the toxicity of these 2 compounds (Scenario 1). In the same study, toxicity of phenanthrene to both green algae and daphnids was found to increase in the presence of C60, despite 85% sorption of phenanthrene to C60-aggregates. This indicates that phenanthrene sorbed to C60 aggregates is available to the organisms (Scenario 4) (Baun et al. 2008). High sorption of PAHs to C60 has also been observed by others, indicating that the presence of C60 can influence the environmental fate and biological exposure of PAHs (e.g., Hu et al. 2008). In a recent study, we found that cadmium sorption to TiO2 nanoparticles resulted in reduced concentrations of dissolved cadmium species. However, the negative effects on algal growth rates were higher than could be explained by dissolved cadmium concentrations. The reason for this is not yet known, but it may be a result of either bioavailability of sorbed cadmium, local high concentrations (Scenario 3), the TiO2 nanoparticles affecting cell permeability leading increased cadmium bioavailability (Scenario 4), or a carrier effect of TiO2, transporting cadmium into the algal cells (Scenario 5) (Hartmann et al. 2009). The ability of ENPs to disrupt cell membranes (prerequisite for Scenario 4) has been studied (Leroueil et al. 2007) and found to be related to the size and charge of the nanoparticles. That ENPs can act as carriers for other contaminants into aquatic organisms has been demonstrated by Zhang et al. (2007). The presence of TiO2 nanoparticles (Degussa P25) was found to increase the accumulation of cadmium in carp (Cyprinus carpio) compared with the accumulation in the presence of natural sediment particles. It was found that TiO2 nanoparticles have a stronger sorption capacity for cadmium than the natural soil particles and that cadmium accumulated in the carp together with TiO2 (Scenario 5). This increase may partly reflect the presence of cadmium sorbed onto TiO2 and the increase in cadmium concentration could be partly reversible through excretion and desorption of TiO2 from the intestine, skin, and scales, respectively. However, an increase in muscle bioconcentration of cadmium indicates actual uptake (Zhang et al. 2007). The presence of black carbon (an anthropogenic carbon-rich nanoparticle originating from incomplete combustion) has been shown to reduce the toxicity of diuron (a widely used herbicide) to green algae (Knauer et al. 2007). This reduction in toxicity is explained by sorption, leading to reduced bioavailability of the contaminant (Scenario 6). Also, the addition of 300 nm TiO2 particles was found to reduce toxicity of cadmium to algae through sorption (Hartmann et al. 2009). Finally, the release of metal ions from ENPs (e.g., from Ag and ZnO) can lead to a competition with other metal ions. The presence of dissolvable metal or metal oxide nanoparticles is, therefore, likely to reduce the bioavailability and toxicity of other metals (Scenario 7). Changes in overall toxicity will, however, depend on the toxicities of the 2 individual metal ions. ENP aggregation behavior in the test medium; Physical interactions between 1) particles and contaminants (sorption, desorption) and 2) particles and organisms (attachment, uptake, excretion); Potential release of free metal ions; and Potential for cell membrane damage and/or increased cell permeability. We strongly encourage more researchers to move into this so far almost overlooked area of nanoecotoxicology.