Engineered Nanomaterials as Emerging Contaminants in Water - Nanotechnologies for Water Environment Applications - page 562

Aquatic systems (rivers, groundwaters, oceans) contain trillions of “natural”
NMs per milliliter of ~ 10 nm size range and exhibit logarithmic relationship between
decreasing NM size and increasing particle numbers (Buffle and van Leeuwen, 1992;
Buffle and Leppard, 1995a, b). Nanoparticle number concentrations can be roughly
converted to mass concentrations by making some simplifying assumptions: (a)
nanoparticle is spherical and discrete, (b) has a fixed density. Assuming a diameter of
10 nm and density of 1.5 g/cm 3 a number concentration of 10 9 particles/cm 3 equates to
roughly 750 ng/L on a mass concentration basis. Considering that many trace metals
exist at concentrations above 1 μg/L in surface and ground waters, this type of mass
concentration appears reasonable (Hem, 1992).
A wide variety of natural inorganic and organic NMs exist, although these are
commonly referred to as colloids with a size of less of 0.45 micron (450 nm) (Stumm,
1992; Hiemenz and Rajagopalan, 1997). For example, inorganic salts form as aerosols
above the sea surface or via gas-phase atmospheric chemical reactions (Bates et al.,
1998). Various mineral phases of iron oxide/hydroxide colloids form in water as iron
undergoes biogeochemical transformations (Allard et al., 2004). Colloidal silica and
silicates also exist ( Ryan et al., 1999; Schemel et al., 2002; Ballousha and Lead, 2007).
Viruses range in size from 20 to a 200 nm and are classic examples of organic natural
NMs (Stumm, 1992). Other biogenic organic NMs (i.e., organic colloids) exist,
although they remain poorly characterized (Leppard, 1997; Santschi et al., 1998). For
example, these may consist of cell wall fragments which are refractory to biodegradation
(e.g., acetylamide structures) (Leenheer, 1992; Rostad et al., 1997). Our research group
is finding that biogenic NMs are comprised of long fibrils that exist either as individual
structures or bundled together, possibly similar to cellulose fibrils bundled together by
lignin (Figure 16.2). These fibrils have similar dimensions as engineered carbon
nanotubes. Other cellular debris (e.g., lyposomes, etc) occur on the < 100 nm scale too.
While there is little scientific research available on the precise role of these “natural”
NMs on aquatic organisms (Wigginton et al., 2007), clearly they tolerate and thrive in
the presence of a large number of natural NMs already. Engineered NMs are in the
same size range as natural NMs (e.g., Figure 16.3), yet some research suggests they pose
risks to aquatic organisms (Nel et al., 2006; Balbus et al., 2007).
16.4 Sources, Detection and Fate of Engineered Nanomaterials in
Aquatic Systems
Engineered NMs will likely be mass produced and widely incorporated into
products used by society over the next few decades. At multiple points of their use and
production engineered NMs may enter the environment (Figure 16.4). During
manufacturing accidental or incidental release of NMs may occur into the air or through
disposal to the sewer or landfills. As consumer products containing NMs are used by