Deposition of NMs into the environment and its implications

With the heightened incorporation of NMs into a range of industrial processes, the release of NMs into environmental sectors is unavoidable. The small size of NMs provides them with exclusive physiochemical characteristics which make them attractive candidates for industrial processes, though it is these same properties that can prompt physiological reactions within organisms upon exposure. The projected use of NMs manufactured and used in industrial processes is estimated to be half a million tonnes by 2020 (Stensberg et al., 2011). There are various entry mechanisms for NMs to enter the environment (Mueller and Nowack, 2008) though it is challenging to evaluate concentrations of manufactured NMs

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present in the environment at a specific point for several reasons; 1) NMs entering the environment at low (ng/L) concentrations (Weinberg et al., 2011); 2) characterization of NMs at these levels depends on the detection limit of instruments; 3) the high amount of already naturally existing colloids in the environment (as discussed previously in section 1.2.1) makes it difficult to distinguish manufactured NMs; and 4) the incapability of instrumental methods to separate manufactured NMs from an environmental sample without altering the original sample. Table 1.3 is a summary of commonly used NMs and their predicted environmental concentrations from their various entry pathways.

Table 1.3. Different types of NMs and their predicted environmental concentrations under various environmental entry pathways.

NM Pathway PEC Reference

Au Surface water 468 pg/L (Mahapatra et al., 2015)

Effluent 440 pg/L (Mahapatra et al., 2015)

Sludge 124 µg/kg (Mahapatra et al., 2015)

carbon-based Surface water 0.001–0.8 ng/L (Mueller and Nowack, 2008) Effluent 3.69–32.66 ng/L (Gottschalk et al., 2009)

Sludge 0.0093–0.147 mg/kg (Gottschalk et al., 2009) Ag Surface water 0.088–10 000 ng/L (Mueller and Nowack, 2008)

Effluent 0.0164–17 μg/L (Gottschalk et al., 2009, Blaser et al., 2008)

Sludge 1.29–39 mg/kg (Gottschalk et al., 2009) (Blaser et al., 2008) ZnO Surface water 1–10 000 ng/L (Gottschalk et al., 2009)

Effluent 0.22–1.42 μg/L (Gottschalk et al., 2009) Sludge 13.6–64.7 mg/kg (Gottschalk et al., 2009)

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1.4.1.1. NM transformations

Once NMs are released into environmental compartments (e.g. surface water, wastewater, etc.), they are highly likely to be transformed from their original pristine state as their surroundings influence factors such as stability which are no longer under their original conditions. Some of these transformations include change in surface charge resulting from differences in ionic strength of the surrounding medium, dissolution of NMs, agglomeration and interaction with biological macromolecules present in the environment. These transformations affect NM transport and toxicity, and thus for toxicity assays in particular, it is essential to understand and test the environmentally relevant, or transformed NMs rather than the as-produced or pristine NMs.

1.4.1.1.1 Ionic strength

The addition of charged groups around NMs provides stability to NMs in suspension. For example, sulphate ester groups on NM surfaces are highly acidic and dissociate at a low pH resulting in negatively charged surfaces encouraging electrostatic repulsion between the NMs and therefore improving stability (Romdhane et al., 2015). There is an opposition of forces between the attractive Van der Waals and the repulsive electrostatic forces between NMs. With NMs that have a charge, a layer of opposite charge forms in the fluid around them which is called the ‘electrical double layer’. At low ionic strength (where there are a low amount of ions in the suspension), the electrical double layer is large and domineers over any Van der Waal forces causing repulsion to be the dominant force, inhibiting NM agglomeration (Badawy et al., 2010). At high ionic strengths, the electrical double layer shrinks usually resulting in NM agglomeration. In addition, pH can drastically influence ionic strength (Badawy et al., 2010) as a lower pH increases the presence of positively charged protons. In environmental waters where the pH and ionic strength can vary, NMs that are deposited into the environment have a high chance of either stabilization or agglomeration, either of which can affect toxicological and biological responses to the NMs. This is an important property to

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consider as D. magna medium used throughout this thesis has a variety of salts within it and consequently high ionic strength and therefore the stability of all types of NMs will be assessed.

1.4.1.1.2 Dissolution

Dissolution of NMs is an important transformation that provides some metal oxide NMs with antimicrobial properties which in turn affects their toxicity. Dissolution is when ions from a solid diffuse into the surrounding medium (Misra et al., 2012). The process of dissolution can cause the release of lethal ions, which is a question to be highlighted with regards to nanotoxicology of whether toxicity of dissolving NMs is due to (i) the NM itself or; (ii) due to the ions released by the NMs or; (iii) a combination of both NMs and released ions. As a general rule it has been noted that smaller NMs are more soluble than larger particles of equivalent composition (Misra et al., 2012). The medium surrounding NMs also has an influence on NM transformation, whereby acidity of the medium, presence of salts or other ions in the medium and water hardness all effect dissolution potential and rate (Misra et al., 2012). This is important as NMs released into environmental waters, which have a range of pH and ionic strength, may be subject to transformations that will cause them to change from their pristine state. Assessing NMs under only one condition therefore may not be representative of the various transformations they may undergo in the environment. Shape and morphology also have an impact on solubility as this influences the specific surface area, whereby smaller NMs with a contingent smaller diameter have a convex curving making them more likely to be energetically unstable and more prone to dissolve (Borm et al., 2006) though also there will generally be more dissolution with smaller NMs due to a larger available surface area and proportionally more of the atoms at the surface. It is important in the case of dissolving NMs to test ionic controls so that toxicity can be attributed correctly to the NMs or the corresponding concentration of released ions. Though the contents of this

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thesis investigate PS and gold NMs which poorly dissolve (Carvalho et al., 2007) though dissolution is still an important NM transformation to discuss.

1.4.1.1.3 Presence of biological matter

Natural organic matter (NOM) and biomolecules may replace surfactants or other stabilising agents which originally provided charge or coating to the NM surface in order to ensure proper dispersion (Maurer-Jones et al., 2013). The acquisition of a layer of macromolecules that originally existed in the environment is usually termed the ‘eco-corona’ and can change the identity of the NM, which may cause either stabilization or destabilization leading to NM agglomerates, changing the NM size which in turn affects toxicity. The presence of an eco- corona around a NM also transforms the NM surface wherein biomolecules now present on the surface of the NM may change its shape and through molecular recognition cause it to be able to interact with receptors on the surface of cells potentially instigating other responses such as internalisation, for example, 15 nm gold NMs incubated in protein contained medium had increased binding of NMs to A549 cells leading to increased uptake as well as retention within the cell. This was due to increase in protein concentration influencing NM aggregation and cell receptor compatibility and association (Albanese et al., 2014). The presence of an adsorbed corona around NMs and their subsequent effects on toxicity will be more thoroughly discussed later in this chapter.