Environmental Engineering Reference
In-Depth Information
ecules or macro-ions. Therefore, hydrophilic colloids
became known as macromolecular colloids or poly-
electrolyte solutions. Their colloidal properties are a
consequence of the large size of dispersed molecules
compared with the size of molecules in the liquid
medium (van Olphen 1977).
In contrast with the spontaneous creation of mac-
romolecular colloids (hydrophilic), colloidal disper-
sions of a material such as gold in water are diffi cult
to obtain. Hence, they are called hydrophobic col-
loids, meaning that the colloidal particles repel
water. However, this term is misleading to some
extent, because actually at least one or two mono-
molecular layers of water are held tightly by the
particle surface, owing to adsorption forces.
Clay solutions are hydrophobic colloids that are
widespread in natural waters; they are actually
homogeneous dispersions of very small particles.
Depending on the dimensions of the particles, they
may not settle within a reasonable time; in this case
they are called a colloidal solution or sol. If the par-
ticles are large enough, settling is faster and the dis-
persion is called a suspension. The distinction
between a sol and a suspension is based on the dif-
ferent settling rates of the particles. Usually, particles
with an equivalent spherical radius (Stokes' radius)
smaller than 1 micrometer are placed in the colloidal
size range. The equivalent spherical radius of a par-
ticle of any shape is obtained using Stokes' law for
spherical particles to compute its settling velocity.
A remarkable difference between hydrophobic and
macromolecular (hydrophilic) colloidal solutions is
the way in which they are affected by the addition
of salt. In the presence of small amounts of salt,
hydrophobic sols fl occulate, whereas macromolecu-
lar sols are rather insensitive. Several macromolecu-
lar compounds remain dissolved even in highly
concentrated salt solutions.
Colloidal particles in hydrophobic sols are small
enough to undergo Brownian motion, which results
in collisions between particles. These collisions can
cause particle aggregation. An explanation was
therefore required to address the fact that some
hydrophobic sols are stable for relatively long
periods. The explanation came in the theory of the
stability of hydrophobic sols, developed independ-
ently by four individual researchers: Derjaguin,
Landau, Verwey, and Overbeek, in the middle of the
20th century. In honor of these authors, it is called
and bioavailability (Entwistle
et al.
2003; Citeau
et al.
2006; Buffl e 2006).
In this context, this chapter aims to (i) comment
on the most important properties of fi ne particles in
order to contextualize the methodologies used in
studies of sediment mineralogy, and (ii) present some
methodologies used the characterization of mineral
particles making up sediments.
3.2 Behavior of particles in water
3.2.1 Colloids
Aquatic suspended particles comprise a continuous
particle-size distribution, including several at submi-
crometer levels, such as clays, iron (Fe) and alumi-
num (Al) (hydro)oxides and humic substances.
Particles measuring between 1 and 1000 nm in at
least one direction are called colloids (Stumm 1993).
Although colloidal particles are made up of many
atoms or molecules, they are still too small to see
using optical microscopy. Colloids are widespread in
fresh surface waters, groundwaters, and interstitial
soil and sediment waters. They pass through most
paper fi lters, but can be observed by light scattering
and sedimentation.
The word colloid (meaning glue-like) was coined
by Thomas Graham (1805-69), a Scottish chemist,
who studied diffusion through membranes separat-
ing pure water from aqueous solutions of several
substances. Graham observed that most salts in solu-
tion diffused freely, but some substances, such as
gelatins and Arabic gum, had low mobility in water,
as well as a tendency to stick to the membrane.
Graham referred to these species as colloids and he
became convinced that these particles were aggre-
gates of small molecules (Graham 1861; van Olphen
1977).
Colloidal solutions prepared from organic macro-
molecular substances such as natural and synthetic
gums were initially classifi ed as hydrophilic colloids,
owing to the great affi nity observed between the
particles and water. This affi nity is due to the chemi-
cal similarity between the colloid particle and the
solvent, for example a hydroxyl group that can bind
with water using hydrogen bonds.
With the growing knowledge of colloidal systems,
it has been recognized that this kind of colloid should
be better considered as a true solution of macromol-
