Engineered Nanomaterials as Emerging Contaminants in Water - Nanotechnologies for Water Environment Applications - page 580

16.7 Conclusions
Engineered NMs will become common in many commercial products. A
concern exists that NMs, because of their size and high surface area to mass aspect, pose
a risk to bacteria, aquatic organisms, higher trophic organisms and humans. Because of
these known risks and potential unknown dangers, there is an emerging concern about
the release, occurrence and treatment of NMs in aquatic systems. This chapter presented
a framework for understanding NM interactions within aquatic systems based upon
classical colloid theory and the decades of its application to environmental colloids.
Several overarching questions were raised early in this chapter. First, are
engineered NMs critically different from natural NMs which have been studied for
decades? It was shown that engineered NM sorbs dissolved organic matter and interacts
with salts in ways comparable with natural NMs. The DLVO theory for particle-particle
interaction and perikinetic flocculation models is valid for understanding the stability
and aggregation tendency of both engineered and natural NMs. What remains unknown
is if capping ligands from NMs like quantum dots release in the environment, how
rapidly NMs become functionalized via biogeochemical processes, how NMs interact
with heterogeneous mixtures of naturally occurring colloids and particles (i.e.,
suspended solids) or how unique engineered properties (e.g., magnetic NMs) affect their
behavior in the environment. It is also important recognizing that the type, functionality
and properties of engineered NMs used in commercial products may differ significantly
from “reagent” grade NMs for which most research to date has been conducted. For
example, some cosmetic products are labeled as containing fullerenes without
differentiating between functionalized versus non-functionalized, C 60 versus nanotubes,
or how they were emulsified into the commercial product.
Second, what are the likely sources of engineered NMs into aquatic systems? As
with nearly every other active agent in modern times (e.g., pesticides, petroleum
products, fertilizers, pharmaceuticals, personal care products, etc), NMs will be released
into the environment (Figure 16.4). Engineered NMs will enter aquatic ecosystems
through point and non-point source discharges, direct deposition from the atmosphere,
and stormwater runoff from urban and agricultural areas. Of particular attention should
be the discharge of engineered NMs from wastewater treatment plants because this
flows directly into aquatic ecosystems, and represents an opportunity to establish a
barrier against their release.
Third, what types of analytical methods are needed for detecting and tracking
engineered NMs in aquatic systems? NMs already occur in aquatic systems. For
example, the number concentration for just 10 nm sized NMs is roughly 10 9 /mL in
rivers, which equates roughly to a mass concentration of 750 ng/L assuming a density of
1.5 g/cc. Existing and recently developed analytical techniques developed to study