Case Study Challenges in Human Health Hazard and Risk Assessment of Nanoscale Silver - Safety of Nanomaterials along Their Lifecycle - page 424

review in Chapter 15). However, under conditions of a wastewater treatment plant
also AgCl nanoparticles are transformed into Ag 2 S (Lombi et al. 2013). As has
been postulated by the Smoluchowski coagulation theory, heteroaggregation with
natural colloids in surface waters leads to rapid sedimentation of nanoparticles.
This has also been demonstrated for silver nanoparticles (Quik et al. 2013; Zhou
et al. 2012), whereas humic acids appear to have little influence on aggregation and
sedimentation (Piccapietra et al. 2012).
During textile washing the surfactants can coat the particles and the different
molecules have an influence on the aggregation, both positive and negative depend-
ing on various factors including charge (Hedberg et al. 2012; Skoglund et al. 2013).
Upon exposure of organisms, nanosilver particles may come into contact with a
variety of bodily fluids, prominently the lung surfactant and blood. Especially, the
contained proteins, lipids, and polysaccharides add to the formation of a so-called
corona, a layer of such molecules covering the surface of the particle (see Chapter 4).
It has been shown that the composition of the protein corona varies with the material,
size, and surface modification of the nanoparticles (Monopoli et al. 2012). Initial for-
mation of the protein corona is rapid and is followed by maturation over time (Tenzer
et al. 2013). Interestingly, in the investigated set of nanoparticles, the majority of pro-
teins in the corona exhibited an overall negative charge independent of the charge of
the surface modification (Tenzer et al. 2013). Moreover, changes in the environment
will dynamically alter the corona (Lundqvist et al. 2011). Importantly, the composi-
tion of the corona influences the cellular uptake of nanoparticles (Treuel et al. 2013).
Apparently, more proteins are shared amongst silver nanoparticles of the same size
with different surface modifications compared to the corona on different sized par-
ticles with the same surface modification (Shannahan et al. 2013). Lipids that are
found in the mucus of the lung can form a bilayer around particles, which does not
significantly affect ion release. Silver ion release from these particles was dependent
on the pH value and varied from 10% at pH 3 to 2% at pH 5, with negligible dissolu-
tion at pH 7 over 14 days (Leo et al. 2013). Similarly, the calculated half-life of silver
nanoparticles of different sizes after inhalation exposure of rats for 28 days was in
the range of 24-260 days for the different tissues with an average of 80 days (Lee
et al. 2013b). In fact, deposition of silver in human skin is considered permanent in
argyria after oral or intravenous application of silver (Drake and Hazelwood 2005).
Thus, it is apparent that silver nanoparticles exhibit a high tendency to accumulate
in organisms and the environment with a slow release of silver ions. Moreover, silver
ions themselves show a high propensity to bioaccumulate and persist in the environ-
ment (Fabrega et al. 2011).
In conclusion, there are a multitude of potential transformation reactions that any
given nanosilver particle can undergo during its lifecycle, depending on both the initial
product properties and the environments encountered. These reactions determine the
form of silver to which exposure occurs, ultimately affecting the resulting risk.
19.6 HAZARD IDENTIFICATION AND CHARACTERIZATION
For the purpose of risk assessment, the characterization of human health hazards of
nanosilver should address the routes of exposure and time frames that are of relevance