Ecotoxicology of Manufactured Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 271

Dissolved
Metal
Concentration

DBL

Bulk solution
concentration

Distance from particle surface

Figure 7.6 Idealised diagram showing the concentration gradient that develops around a
slowly dissolving spherical metal oxide nanoparticle in aqueous solution. DBL refers to the
diffusive boundary layer. Note the high localised dissolved metal concentrations close to the
nanoparticle surface.

susceptible to oxidation, which may result in the release of soluble metal ions that
are potentially toxic, such as Cd 2+ (Derfus et al. , 2004). The high surface area of
nanoparticles may increase oxidation rates beyond those observed for bulk materi-
als. Oxidation may result in the build up of an oxide coating on the surface of the
nanoparticle; this can act as an impermeable barrier reducing further oxidation.
However, the oxidation products themselves may also be toxic.

7.2.4 Adsorption Reactions

The high surface area and chemical composition of some nanoparticles may lead
to high adsorptive affi nity for various dissolved molecules, including essential nutri-
ents, cations and trace metals. Iron nanoparticles rapidly form outer iron hydroxide
layers, which are effective in adsorbing both anions and cations (Waychunas et al. ,
2005). This may be problematic in algal growth bioassays where nutrient phosphate
and other micronutrients are required to ensure adequate growth. Carbon-based
nanomaterials may also adsorb hydrophobic organic molecules including many
contaminants. In this respect, nanoparticles may be considered as nanovectors.
Contaminants may be preconcentrated by adsorption onto nanoparticles only to
be released inside organisms or in environments where the physico- chemical condi-
tions favour desorption.

7.3 Mechanisms of Nanoparticle Toxicity in the Environment

7.3.1 Exposure Routes

Mechanisms by which nanoparticles may have deleterious effects on the environ-
ment are likely to be more diverse than those for human health and will be infl u-