Case Study Challenges in Human Health Hazard and Risk Assessment of Nanoscale Silver - Safety of Nanomaterials along Their Lifecycle - page 423

additional uncertainty factor of 10 in quantitative risk assessment to account for
the quality of the database (US EPA 2011).
19.5 TRANSFORMATION (SPECIATION) OF NANOSILVER
MATERIAL ALONG ITS LIFECYCLE
Nanosilver particles can undergo transformation reactions, most notably oxida-
tive dissolution with release of silver ions. Ion release from coated silver nanopar-
ticles has been shown to be dependent primarily on available surface area, thus
increasing with decreasing particle size (Ma et al. 2012). In principle, the pres-
ence of surface modifiers could be expected to control nanosilver degradation.
Unfortunately, the current literature seems to be contradictive in this respect,
which may be due to an exchange and/or release of the surface modifier depend-
ing on the particular environment and interaction between modifier and particle
core. Modification of 5 nm sized silver particles by addition of 0.4-10 mM citrate,
0.4 mM sodium sulfide, or 4 mM 11-mercaptoundecanoic acid was shown to slow
the time-dependent silver release (Liu et al. 2010). Although the effect of citrate
was comparatively weak (~50% reduction), the sulfur compounds had a major
impact with >90% reduction in silver release. However, the surface modifiers used
during wet chemistry synthesis of nanosilver are usually noncovalently bound such
as those evaluated by Ma et al. (2012). These authors measured silver release from
particles synthesized using different methods, including wet chemistry and gas
phase condensation, and which were either unmodified or coated with PVP or
gum arabic. Notably, the method of synthesis and the different coatings only had a
small effect on ion release in this study, whereas the effect of surface area and size
was confirmed (Ma et al. 2012). This apparent discrepancy might be explained by
the oxygen-mediated formation of a surface layer of Ag 2 S on silver nanoparticles,
termed sulfidation (Liu et al. 2011). This process generates a highly insoluble Ag 2 S
shell, which reduces the rate of silver oxidation (Levard et al. 2011). It has been
proposed that the sulfide-rich, anoxic environment of sewage treatment plants also
facilitates rapid sulfidation of silver nanoparticles (Kim et al. 2010). Importantly,
Ag 2 S is less toxic to microorganisms than elemental silver nanoparticles and silver
ions (Levard et al. 2012). Similarly, a modeling approach predicted that sulfidation
will reduce oxidation and ion release of silver nanoparticles in sediments (Dale
et al. 2013). Depending on redox conditions, sulfidized silver nanoparticles might
persist in the environment with a half-life ranging from a few years to over a
century (Dale et al. 2013). The seasonally variable availability of organic carbon
and dissolved oxygen dictates silver speciation and persistence (Dale et al. 2013).
In contrast, desulfurization of proteins and amino acids by silver nanomaterials
and concomitant formation of Ag 2 S appears to be negligible (Chen et al. 2013).
Nevertheless, in the case of argyria Ag 2 S and Ag 2 Se are formed in the skin upon
sun exposure (Liu et al. 2012). Some oxidation to Ag 2 O occurs as well in solution,
possibly generating a shell around the particle (Li et al. 2012b). AgCl nanoparticles
can be formed from elemental silver nanoparticles in the presence of bleach, for
instance by textile washing (Impellitteri et al. 2009). They are also used in func-
tional textiles as part of nanocomposites (Lorenz et al. 2012) (compare the detailed