Ecotoxicology of Manufactured Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 269

classifi ed as insoluble in water (CRC Handbook, 2007), whereas its aqueous solubil-
ity ranges from several thousand milligrams per litre at pH 6 to around one milli-
gram per litre at pH 8 (Stumm and Morgan, 1995). For example, one gram of zinc
oxide nanoparticles added to one litre of water at pH 7.8 will generate a dissolved
zinc concentration of about 10 mg/l. Clearly less than 2% of the zinc oxide has dis-
solved but the resulting solution zinc concentration is suffi ciently toxic to kill most
aquatic organisms.
The rate of nanoparticle dissolution is also important, particularly if it is fast
compared to the duration of the toxicity test. Franklin et al. (2007) demonstrated
the rapid dissolution of zinc oxide nanoparticles in algal test media at pH 7.5
(Figure 7.5). Within six hours, suffi cient zinc had dissolved to become toxic.
From a theoretical perspective, the solubility of a particle increases as size
decreases. This is known as the Gibbs-Thompson, or Kelvin, effect and is a ther-
modynamic effect that results from the increased chemical potential found at
curved surfaces (Borm et al. , 2006). The rate of dissolution is dependent on particle
surface area and, consequently, nanoparticulate materials should dissolve faster
than larger sized bulk materials, for the same mass, on surface area considerations
alone (Borm et al. , 2006). Ostwald ripening is a potentially important kinetic effect
which is a consequence of the preferential dissolution of small particles followed
by reprecipitation of larger particles. Particle size therefore changes with time
tending toward larger particles (Borm et al. , 2006 ).
It is interesting to consider the interaction of slowly dissolving nanoparticles
(dissolving over the time course of days) with living cells. A schematic diagram
showing the concentration gradient of a dissolved species with distance from the
surface of the nanoparticle is shown in Figure 7.6. The highest concentrations are
found close to the particle surface. Clearly, organisms in intimate contact with such
particles will receive a higher dose of the dissolving material than is predicted by
measurement of the bulk solution concentration.
The measurement of nanoparticle solubility is not a trivial exercise, as it neces-
sitates the sampling of dissolved species after an appropriate equilibration time.
This can be done by use of equilibrium dialysis (Figure 7.5) or potentially ultrafi l-
tration. Ion selective electrodes, where available, are also suited for such applica-
tions as they can measure the concentration of the dissolving species without the
need for separation.

7.2.3 Oxidation

The major oxidants in environmental systems are oxygen (dissolved or atmo-
spheric), iron(III) and manganese dioxide (MnO 2 ). Iron(III) becomes an impor-
tant oxidant in aqueous solutions at low pH as its solubility increases markedly.
Manganese dioxide is found in the solid phase in soils and sediments and takes part
in various heterogeneous oxidation processes. A number of nanoparticles may be
composed of, or contain, constituents that may be subject to oxidation both in
aquatic and terrestrial environments. These include elemental metal nanoparticles
such as silver (Henglein, 1998; Lok et al. , 2007) and iron (Nurmi et al. , 2005 ). Metal
sulfi des and selenides, which are major components of quantum dots, are also