Geoscience Reference
In-Depth Information
12.2.3 Modeling ENP Transport in Porous Media
Gravitational sedimentation, interception, and Brownian diffusion are the basic
capture mechanisms of colloidal particles in porous media (Schrick et al.
2004
).
Transport of ENPs in porous media and soils depends largely on the rate of their
capture or filtration by the stationary granular surface. Lin et al. (
2010
) suggest use
of particle filtration theory to understand the transport behavior of ENPs in soil-
subsurface systems. They developed a schematic diagram for ENP transport
(Fig.
12.25
) showing that sedimentation (Fig.
12.25
a) and interception
(Fig.
12.25
b) mechanisms mainly control the transport of larger pristine nano-
particles and nanoparticle aggregates. In the case of nanoparticles, diffusion is the
dominant mechanism (Fig.
12.25
c), because of the high diffusivity of nanoparti-
cles which leads to a higher incidence of collisions with the porous media solid
phase. When ENPs aggregate, they become microscale particles, which are
retained by the porous medium according to a physical screening (Fig.
12.25
d).
Particle filtration processes are determined largely by colloid-porous medium
surface interactions and may be affected by particle surface chemistry, water pH,
ionic strength, and organic matter content. In addition, physical parameters such as
particle size and morphology, fluid velocity, and soil-subsurface water tempera-
ture should be considered in defining ENP breakthrough capability.
Characteristics of the soil-subsurface system influence ENP filtration. Both
physical (e.g., porosity and aggregate size) and surface properties (e.g., charge) of
the porous media solid phase affect ENP transport through soil-subsurface sys-
tems. For example, when soil particles are negatively charged, ENPs with high
negative charge are more mobile and may be transported through the soil-sub-
surface down to the groundwater zone, while positively charged ENPs will be
retained in the soil upper layer.
12.2.4 Fate of ENPs in Soil Column Experiments
Relatively few experimental studies of nanoparticle transport in laboratory col-
umns of natural geological materials exist in the literature. One example is given
by Sagee et al. (
2012
), who studied the transport of silver nanoparticles (AgNPs) in
a set of soil column experiments (Figs.
12.26
,
12.27
,
12.28
). The effect of soil
aggregate size on AgNP transport is shown in Fig.
12.26
a as can be seen from the
similar breakthrough of the AgNPs in all three cases, the nanoparticle transport
rate was not affected by the size of the aggregate. Moreover, the amount of AgNPs
moving through the soil column is affected by the aggregate size and that the larger
the aggregates, the more nanoparticles are eluted. For the coarse (largest aggre-
gate) soil, about 70 % of the AgNPs that enter the soil column elute, while for
the fine (smallest aggregates) soil, only about 30 % of the nanoparticles entering
the soil column eluted. Sagee et al. (
2012
) noted that the AgNPs moved through
