Environmental Engineering Reference
In-Depth Information
between truly dissolved solutes and colloids (or NPs). CFUF is also known to be
prone to artefacts if the procedure and membrane selection are not optimized
carefully. There are several studies that have examined both experimental setup
and operating conditions to obtain good recovery and as precise a cut-off as pos-
sible in both seawater and freshwater (Guo
et al.
, 2000 ; Larsson
et al.
, 2002 ; Morrison
and Benoit, 2004; Doucet
et al.
, 2005 ). Ultrafi ltration can not only be used to frac-
tionate the samples, but also to obtain a concentration factor (1-100) but this
increased concentration of natural colloids does change their physicochemical
properties (e.g. agglomeration)(Liu and Lead, 2006 ).
6.2.5.3
Centrifugation
Centrifugation can be used as a pre-fractionation method or as the main nanopar-
ticle fractionation method (ultracentrifugation). Centrifugation can induce agglom-
eration between particles due to differential settling velocities, but it has been
shown to cause less perturbations than fi ltration for soil colloids (Gimbert
et al.
,
2005). From the settling in the centrifuge gravitational fi eld the fractionation of
particles with a certain equivalent volumetric diameter (d
V
) can be calculated,
providing that the particle density (
ρ
part
) is known, using the following equation:
(
)
(
x
x
2
ρρ
η
−
=
part
)
2
(
)
ln
d
2
π
v
t
−
t
(6.5)
V
0
9
0
where x
0
and x are the starting and ending position distances from the centrifuge
centre at time t
0
and t,
is the viscosity of
the medium. Centrifugation can also be used to study dynamics of agglomeration
and phase separation in analytical centrifugation with an integrated photometer by
acquiring the particle mass profi le in the centrifuge tube as a function of time
(Lerche, 2002; Colfen
et al.
, 2003 ) (Figure 6.6 ).
ν
is the centrifuge rotational speed and
η
6.2.5.4
Field - Flow Fractionation
Field-Flow Fractionation (FFF) is a family of separation techniques for colloids,
polymers and NPs that uses the combination of distribution of particles in some
kind of fi eld and a laminar fl ow in thin channels. The different FFF subtechniques
are distinguished by the type of fi eld, where the two most common are a second
hydrodynamic fi eld (Flow FFF) and a centrifugal fi eld (Sedimentation FFF). The
fi eld forces the particles against one of the walls in the channel and the particle
mean distribution is governed by a balance between the Brownian motion and on
how the particles are affected by the fi eld (Figure 6.7). The particle mean height is
inversely proportional to the particle size and proportional to the transport velocity
through the channel since the transport is faster close to the middle of the laminar
fl ow (Giddings, 1993). Not only can FFF separate NPs but the elution time can be
directly related to physical properties (diffusion coeffi cient or hydrodynamic diam-
eter for Flow FFF and buoyant mass or equivalent spherical volume diameter for
Sedimentation FFF) (Giddings, 1993 )
Search WWH ::
Custom Search