Agriculture Reference
In-Depth Information
loid. Stability of NPs in aqueous environments is a key factor controlling
their transport and ultimate fate in aqueous environments. Large aggregate
of NPs will quickly precipitate out and their transport and bioavailabil-
ity will be greatly restricted. However, well-dispersed NPs will be widely
transported and have higher chances to interact with and cause potential
harm to organisms. System pH is a major factor determining the zeta po-
tential of colloids. When pH is at point of zero charge (pzc) or isoelec-
tric point, the colloidal system exhibits minimum stability (i.e., exhibits
maximum coagulation/flocculation). When the pH is lower than the pzc
value, the colloid surface is positively charged and the zeta potential will
increase with decreasing pH below the pzc. Conversely, at pH above pzc,
the surface is negatively charged and the zeta potential will be more nega-
tive with increasing pH. High zeta potential (negative and positive) will
impart stability to the NP suspension, whereas NPs with low zeta potential
tend to coagulate or flocculate.
9.7
BEHAVIOR OF NPS IN TERRESTRIAL ENVIRONMENT
After entering into soil environment, NPs may be retained by soil matrix
or break through soil matrix and reach ground water, which is also deter-
mined by the properties of NPs and soil. There are significant physical and
chemical similarities between the most widely manufactured ENPs and
naturally occurring nanoparticles, although in a number of cases the exact
size, shape, and coatings/surface functional groups may be quite differ-
ent from ENPs. Also while the term nanoparticle may not yet be widely
used in ecology, earth scientists have been studying at least some major
classes of natural nanoparticles for many decades. For example, it was
realized years ago that the charged surfaces of clays in soil-an example of
natural nanoparticles-were known to form electrostatic bonds with ions
(e.g., NH
4
+
, Ca
2+
, K
+
, and Mg
2+
) that contribute substantially to soil fertility
by preventing the loss of these vital nutrients to groundwater (Eriksson,
1952; Gieseking, 1939). Now modern nanoscience has become an integral
part of soil science that goes far beyond the study of clay minerals (Ho-
chella, 2008; Yuan, 2008). This and other aspects of nanogeoscience, as it
is now called, have extensively developed relatively recently, particularly
in the last decade
(Banfield, 2001; Hochella, 2002; Hochella, 2008). This
is because an exceptionally wide variety of nanoparticle exist on earth,
