Page 280 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Nanoparticle Transport, Aggregation, and Deposition 265

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α = 0.002
α = 0.1
α = 0.4
10

–1 1
λ

0.1

0.01
0.01 0.1 1 10
dp(µm)
Figure 7.20 Travel distance (meters) in a homogeneous porous medium as a func-
–1
tion of particle size (v 0.02 cm sec ). Three different attachment efficiencies
are assumed accounting for unfavorable and favorable deposition conditions.
initial particle concentration n o to a concentration of n.) Plotting recip-
rocal as a function of particle size (Figure 7.20), it is evident that even
for nanoparticles with low attachment efficiencies, the characteristic
distance to which they will travel in a homogeneous media is low. Real
systems present numerous complexities that may increase or decrease
the true mobility of nanoparticles. For example, physical inhomo-
geneities such as fissures may increase mobility by providing prefer-
ential flow paths. Heterogeneities in media size may decrease porosity,
thereby increasing deposition. Heterogeneities in surface chemistry
may decrease the effective surface area available for deposition.
However, particle size also interacts with heterogeneities to deter-
mine the effective surface of the collectors that may be “visible” to a par-
ticle. While larger particles may see an average surface that is
unfavorable to deposition, nanoparticles may be able to sample the sur-
face at a finer scale, seeking out more favorable attachment sites. The
ability to access more of the surface may combine with a higher rate of
transport to the surface due to Brownian diffusion to reduce nanopar-
ticle mobility. In one study by Schrick et al. [2], it was observed that the
mobility of iron nanoparticles in soil columns was less than that of
larger iron particles (Figure 7.21).
This observation has direct implications for the use of nanoparticles
in groundwater remediation applications and removal by granular media
filters. Highly reactive nanoparticles, such as nano-iron, have been pro-
posed as a possible remediation tool for contaminants susceptible to
reduction by Feº. However, both theory and experiments suggest that