Biomedical Engineering Reference
In-Depth Information
10.2.3 Parameters Affecting Environmental Fate and Translocation
An important question is how NM interactions with the environmental
matrices can exert a toxic effect on the environmental receptors. The behavior
of NPs in the environment may follow the same laws as for natural colloids;
their stability should depend on (a) physical properties (including particle
size, solubility, state of aggregation, concentration, shape, crystal structure,
surface area, zeta-potential or surface charge, and pHpzc—the pH at which
the net particle surface charge is zero); (b) the nature of the surface chemistry
of the NMs (including elemental composition, presence of impurities, pres-
ence of coating); and (c) environmental conditions such as solution chemistry
(pH, composition, and ionic strength of solution), redox potential, and pos-
sibly biochemical reactions over time (aging properties) [23,25,39,42].
Na no -TiO
2
is one of the best-studied NMs from the standpoint of environ-
mental fate, and is considered to be a possible sentinel NM [44]. It is found that
the pH of the milieu can affect surface charge properties, and subsequently
the aggregation, agglomeration, potential “bioavailability,” and reactivity of
n a no -TiO
2
[45], and high concentrations of Na
+
and K
+
(high ionic strength)
could promote its aggregation [46]. The presence of surfactants (bronchoal-
veolar lavage fluid or combinations of albumin and complex lipids) can
also stabilize the TiO
2
NPs [47]. The interaction between TiO
2
and organic
matter (as humic acid or equivalents) was found to increase the stability of
the NP [48]. In studies conducted with soil columns, both surface potential
and aggregate size influenced the mobility of TiO
2
NPs [49-51]. As expected
for colloids, the smaller particles could be transported for longer distances.
Similarly, results from several studies indicate that nano-TiO
2
particles and
their aggregates/agglomerates might be very mobile in subsurface and in
groundwater [39,49]. In fact, results from a recent study suggest that trans-
port of TiO
2
NP would be favored by low ionic strength and by the presence
of large soil particles; conversely, high clay content, natural organic matter,
and salinity would favor soil retention of the same particles [52]. Studies with
other NMs, such as nanoaluminum, yield similar conclusions [53]. Transport
of aluminum particles also depends on the size, surface charge, and agglom-
eration rate, which is inversely related to the size of agglomerates and can
be affected over time by the ionic strength of the medium. Additionally, the
surface charge of the particles and that of the receiving matrices (e.g., soil)
are important for small agglomerates, in which they tend to bind to large soil
particles of opposite charge [53].
In another study, the mobility of zero-valent iron was found to be enhanced
by natural organic matter [54]. This effect was completely evident at high
particle loads, but was still measurable at low concentrations. Likewise, the
stability of CNTs can be greatly improved by the presence of natural organic
matter or humic acids [41,42]. It seems that untreated CNTs as received from
producers are undispersable in water [42]. Owing to their hydrophobicity, it
was thought earlier that CNTs could not be sufficiently bioavailable to cause
