Page 300 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Nanoparticle Transport, Aggregation, and Deposition 285

influence of gel charge, i.e. the Donnan potential (Figure 7.32a), is
thus maximized in the pH range of 3.5 < pH < 9, where
is maximized
(Figure 7.32b) and the average diffusion coefficient D is minimized
g
(Figure 7.32c) due to electrostatic attractions with oppositely charged par-
ticles. The electrostatic contribution can thus be easily cancelled by
neutralizing the sum of the electrostatic interactions by changes in pH
3
or ionic strength. A minimum salt concentration of approximately 10
M NaCl has been found to be required for cancellation. This salinity
is largely below that of natural sea or river waters, implying the bioavail-
ability of nanoparticles via diffusion through charged or uncharged
organic gels.

Specific chemical bonds. When the particle surface presents a specific
chemical affinity for sites S on the gel fibers, an adsorption reaction
occurs to form a complex SA according to the following intrinsic equi-
int
librium constant K A :

[SA]
int
K A 5 (35)
[A] [S]
P
where [SA] and [S] are the respective concentrations of complexes SA
and free sites S. Then, when 0, by combining Eqs. 30, 34, and 35,
one obtains the following expression for :

Z F
[SA] 1 [A] P A int
5 5 expa2 b[1 1 K A sSd] 5 pa (36)
[A] W kT
The existence of specific chemical interactions has been put in evidence
) and amine-
for example in the case of silica nanoparticles (ludox HS 30
or carboxylate-terminated dendrimers, diffusing in an agarose gel
[99, 100], where the diffusion coefficients measured by FCS showed
more reduced diffusivity, compared to the pure steric effect expected.

Airborne Nanoparticles
A detailed consideration of the origins, transport, and characteristics of
airborne nanoparticles is beyond the scope of this chapter. However,
these materials are of major concern for human health and occupa-
tional safety [113]. In fact, nanoscale particles are perhaps more
ubiquitous in the atmosphere than in any other environment. Clinical
studies have suggested a strong link between particulate air pollution
and respiratory disease. Many of the principles governing nanoparticle
transport in aqueous systems apply to atmospheric systems as well.
In particular, the framework for analyzing the kinetics of particle