Size/Shape – Property Relationships of Non - Carbonaceous Inorganic Nanoparticles and their Environmental Implications - Environmental and Human Health Impacts of Nanotechnology - page 97

There are many indications that nanoparticle aggregates behave differently from
their well dispersed counterparts. For example, 3 nm zinc sulfi de nanoparticles have
more highly ordered crystal structures when aggregated (Huang et al. , 2004 ). The
activation energies for phase transitions in titanium dioxide nanoparticles were
found to be lower for less aggregated nanoparticles. This was attributed to a lower
surface energy of the more aggregated nanoparticles due to interparticle interac-
tions (Zhang and Banfi eld, 2007). Aggregation has also been shown to affect the
thermal conductivity of nanoparticle solutions (Hong et al. , 2006 ). Recently, aggre-
gates of 9 nm ferrihydrite (Fe 3 O 4 ) nanoparticles were found to reduce carbon tet-
rachloride, a common organic contaminant, more slowly than non-aggregated
ferrihydrite (Vikesland et al. , 2007). As per Table 3.1, in studies of metal sorption
onto nanoparticles, in some cases it was suggested that the size dependent differ-
ences observed were due to aggregation (Gao et al. , 2004 ).
One of the well known effects of aggregation is its contribution to nanoparticle
growth (Banfi eld et al. , 2000 ; Guyodo et al. , 2003; Penn and Banfi eld, 1998, 1999).
Simply infl uencing growth is very important, as changing nanoparticle size may
mean changing physical and chemical properties. Aggregative growth processes
also may have an impact upon metal sequestration. Waychunas and co-workers
(2005) found that when Zn 2+ was added to mixtures of - FeOOH (goethite)

nanoparticles during aggregation, zinc was either incorporated directly into the
crystal or that a secondary zinc-containing precipitate was formed. X-ray absorp-
tion spectroscopy (XAS) results from Zn 2+ sorbed to the nanoparticles after aggre-
gation were very different (Waychunas et al. , 2005 ).
The connection between degree of aggregation and chemical/physical properties
is quite intriguing and invites further exploration.

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3.7 Environmental Implications: General Discussion, Recommendations
and Outlook

Nanoparticles have size dependent properties that may affect many environmental
processes. Nevertheless, it still remains diffi cult to predict what these actual envi-
ronmental effects might be. It is clear that more research will be conducted regard-
ing the fate and impact of nanoparticles in the environment with size dependent
properties in mind. Here, commentary and recommendations on future directions
in this research are discussed.
First, it is critical to establish how and where nanoparticles are being released
into the environment. This topic is addressed for atmospheric nanoparticles in
Chapter 5, but this area continues to merit much more study. Knowing the sources
and means of entry for nanoparticles will reveal which environmental exposure
scenarios are the most likely. The knowledge will in turn guide researchers attempt-
ing to study the most environmentally relevant nanoparticle systems. As many
nanoparticle-based technologies are still emerging as they will for some time,
various release scenarios may have to be postulated. Some examples of nanopar-
ticles in current and emerging technologies likely to be released (or they are
already being released) into the environment are as follows: