Page 272 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Nanoparticle Transport, Aggregation, and Deposition 257

particles aggregated more slowly at lower pH values, while the 30 nm par-
ticle dispersions remained stable under these conditions. These differ-
ences were attributed to additional interfacial interactions (hydration and
the presence of a gel layer) that became more pronounced with decreas-
ing particle size.

Deposition
The deposition of nanoparticles on stationary surfaces will be a key
factor in determining their mobility, persistence, and fate in environ-
mental and engineered systems. Similarly, the effective exposure and
dose of nanoparticles experienced by organisms will be influenced to a
large degree by the potential for deposition on biological surfaces such
as gills and lungs. Analogous to particle aggregation, we will consider
particle deposition as a process of transport and attachment, focusing
on the case of particle deposition from aqueous suspensions in porous
media such as aquifers or filters.

Particle deposition in porous media
The mobility of submicron-sized particles in aqueous systems has been
explored extensively, particularly for the case of latex suspensions [6].
Analysis of the transport and deposition of particles in porous media typ-
ically begins with the convective-diffusion equation, which under steady-
state conditions is generally written as [12]:

'n j # # # D F 'n j
#
1= sun d 5= sD =n d 2=a n b 1 (17)
j
j
j
't kT 't RXN
is the number concentration of particles of size or type j in the
where n j
suspension; D is the particle diffusion tensor; u is the particle velocity
'n j
vector induced by the fluid flow; F is the external force vector, and
't
RXN
is the reaction of these particles with a surface (deposition) or with
other particles (aggregation). The external force vector may be used to
account for forces such as those arising from gravity and interfacial
chemical interactions typically accounted for in the extended DLVO
model (see the section entitled “Physicochemical Interactions”).
As in the case of aggregation, particle deposition can be considered
as a sequence of particle transport (in this case to an immobile collec-
tor) followed by attachment to the surface [58]. In a porous medium
such as a filter or groundwater aquifer, fluid flow can be described by
the Happel sphere in cell model [59], in which grains within the porous