Environmental Engineering Reference
In-Depth Information
factor,
=
R
g
/
R
h
, was used to give an indication of
how far the particles deviated from a spherical shape
and
R
g
was used as a control to ensure undisturbed
fractionation. Baalousha
et al.
(2005b) demonstrated
that FIFFF-SLS coupling was a valuable method to
fractionate and characterize particle size in river col-
loids. Ledin
et al.
(1995) found that seasonality
affected size distribution of colloidal matter in a
Swedish lake using DLS, with the smallest between
120 and 340 nm in spring, and between 280 and
700 nm during summer and fall. Li
et al.
(2007) veri-
fi ed that sediments were disturbed by different wind
velocities, especially in shallow lakes. The three-
dimensional fractal expression (
D
f
) of resuspended
sediment particles was measured by light-scattering
techniques because
I
(
ρ
ions are spherical, the ionic radii of cations are lower
than O and OH, so the arrangement of ions within
a crystalline structure is ruled by O and OH in
compact or hexagonal planes with alternating
cations. According to the arrangement of compact or
hexagonal planes face to face, two polyhedral forms
are found: the tetrahedron and octahedron.
The cation in the middle of the polyhedron can
coordinate four oxygen ions in the tetrahedron, six
or eight in an octahedron, and 12 when outside the
polyhedron, in the interlayer. The ratio between the
cationic radius,
x
, and the oxygen radius,
o
, (
R
x
/
R
o
)
determines the cation inside the polyhedron, as
reported in Dixon & Weed (1989). Therefore, ratios
less than 0.41, such as Si
4+
and Al
3+
, determine that
these cations can coordinate four molecules of
oxygen inside the tetrahedron; ratios between 0.41
and 0.73, such as Al
3+
, Mg
2+
, Fe
3+
, and Fe
2+
, deter-
mine the possible cations inside tetrahedrons. Cations
with values higher than 0.73, for example K, can
only be located in the interlayer. When inside poly-
hedrons, cations with different valencies create a
defi cit of positive charge.
The formation of a mineral crystal structure
depends on the organization of successive ionic
layers or sheets, namely tetrahedrons or octahedrons.
The combination of these layers results in 1 : 1 and
2 : 1 layers or even 2 : 1 layers with one more octahe-
dron layer, resulting in chlorite, for example. The
successive arrangement of these layers forms inter-
layer spaces of H
+
bonds with a large amount of
energy. This bond occurs in the union between tet-
rahedron basal oxygen and the OH
−
of the octahe-
dron in phyllosilicates 1 : 1, resulting in a
non-expanded interlayered rigid structure. On the
other hand, when octahedron basal oxygen atoms
are face to face bonded by van der Waals forces in
a 2 : 1 structure, it can expand in response, for
example, to water content. However, when non-
hydrating K ions occupy the spaces between layers,
or siloxane cavities, they are bonded to permanent
charges in the tetrahedron layers, for example micas,
and expansion does not occur.
The identifi cation of the clay mineral species is
based on the sequence of ionic planes that are part
of the structure, and are represented by the Miller
index (
hkl
) (Brindley & Brown 1980; Bouchet
et al.
2000). Plane 00l refers to the basal distance of clay
minerals in orientation (c).
q
−
D
f
.
D
f
was between 2.26
and 2.44 at different depths and under various wind
velocities. Fractal geometry is a well-established
means of describing the complicated structure of
aggregates in colloidal suspensions.
θ
)
α
3.3.2 Identifi cation of minerals by X-ray
diffraction
X-ray diffraction (XRD) is one of the main methods
to identify minerals and has been in use since the
1960s. Owing to the discovery of X-ray emissions in
1895 and the discovery of their diffraction patterns
through various materials in 1912, techniques and
equipment to study minerals have been developed. It
is fundamental to know the crystalline structure of
minerals and their behavior when exposed to X-rays.
Thus, mineral species can be identifi ed in a hetero-
geneous sample from X-ray diffractograms after
several tests, for instance the expansivity and col-
lapse of interlayers in minerals (Dixon & Weed
1989; Bouchet
et al.
2000).
This section briefl y presents the background to
identifi cation of fi ne material such as clays, which at
less than 2
m are the main fraction studied as they
are the most reactive.
Silicate minerals are predominantly found at the
surface of the Earth, and contain silica (Si
4+
) in the
crystal lattice. However, other cations, as Al
3+
, Mg
2+
,
Fe
2+
, and Fe
3+
are part of the lattice in minerals and
coordinate oxygen atoms, O
2−
or OH
−
(Schulze
1989). The coordination number depends on the
valency of the cation and its ionic radius, following
Pauling's law (Pauling 1929, 1947). Assuming that
μ
