Environmental Engineering Reference
In-Depth Information
As a result, the coagulation rate rises owing to an
increase in collisions, resulting in aggregation of
particles.
Raindrop impact on the soil surface can promote
detachment of colloidal particles from soil aggre-
gates. Most of this fi ne particulate matter is dispers-
ible in water, giving rise to hydrophobic colloidal
solutions that can remain stable for some time (from
minutes to hundreds of years) (Seta & Karathanasis
1996). At high fl ow rates, particle release can be due
to hydraulic shear stress on larger particles.
At low fl ow rates (in soil or natural subsurface
water), the causes of particle release are more likely
due to changes in soil solution or groundwater chem-
istry. For instance, particle detachment can be the
result of changes in pH or ionic strength, which
modify the balance of forces at the particle-grain
interface (Seta & Karathanasis 1996).
In conclusion, water dispersible colloids in soil
play an important role in soil erosion. Moreover,
dispersed soil colloids that remain stable in subsur-
face moisture are potential carriers of contaminants
to groundwater because they are abundant in the
subsurface. The surface sites of these particles can
bind contaminants with low solubility, such as radio-
nuclides and hydrophobic organic compounds by
aqueous-phase transport models (Czigány
et al.
2005).
Factors such as pH and salinity are important in
the dispersibility of natural particles. A critical salt
concentration (CSC) can be experimentally deter-
mined, meaning the salt concentration below which
fi ne particles are released from the matrix surface
(Nowicki & Nowicka 1991; Blume
et al.
2005).
According to the DLVO theory, this situation can be
achieved at low indifferent electrolyte concentration,
when repulsive forces between colloidal particles and
the matrix surface surpass binding forces, stabilizing
the colloidal dispersion.
Blume
et al.
(2005) show the importance of deter-
mining the CSC in understanding the behavior of a
highly radioactive waste deposit, which had been
leaking into the vadose zone of the Handford
Formation (USA) for several years. The hypersaline
waste solution (
in salt concentration is important because it gave
rise to detachment of the fi ne particles from the sedi-
ments, enhancing radionuclide migration or reduc-
ing the permeability of the formation (owing to the
settling of these colloidal particles within fi ne layers).
In addition, pH plays a role in the dispersibility of
natural particles, because part of the variable charge
on fi ne particles is due to surface reactive sites that
are dependent on pH, such as carboxylic or phenolic
groups in humic acids, hydroxilic groups in clays or
Fe and Al oxyhydroxides. The zero point of charge
of the particle (pH
zpc
) represents the pH where it has
no net charge; at pHs lower than the pH
zpc
the par-
ticle would be positively charged, owing to the addi-
tion of protons to some of the reactive surface
groups. Above the pH
zpc
, the loss of protons at these
sites makes the particle negatively charged. Therefore,
when the soil pH is different from the pH
zpc
, the net
surface charge of the particle will be either positive
or negative. Similar particles have the same charge,
giving rise to double-layer repulsive forces between
them, enhancing their dispersibility. At pHs near the
zero point of charge, van der Waals attractive forces
prevail and the colloidal solution is no longer stable;
that is, water-dispersible particles are at a minimum.
3.2.6 Organic and inorganic carbon
The carbon content in soils, sediments, and natural
particulates can be present as inorganic or organic
forms; the former is largely found in carbonate min-
erals, whereas the latter is present in organic matter.
In most cases, inorganic carbon in river sediments is
found as calcite (CaCO
3
) and dolomite (CaCO
3
.
MgCO
3
), derived from sedimentary rocks. Sometimes
other forms of carbonate, such as siderite (FeCO
3
),
are also present (Galy
et al.
2007). In soils derived
from calcareous parent material under arid condi-
tions, the inorganic C concentration can be higher
than organic carbon (Nelson & Sommers 1996). In
marine sediments, signifi cant amounts of carbonates
can be present, occurring mostly as calcite and arag-
onite (anhydrous CaCO
3
) from organisms such as
molluscs, and dolomite, incorporated into the sedi-
ments from weathered soil parent materials and
transported to the sea by river fl ow (Schubert &
Nielsen 2000).
The organic carbon content of sediments and soils
is from animal and plant residues at different stages
5M Na
+
) included radionuclides
and other toxic metals. According to the authors,
migration of this solution from the leaking tanks
through the soil and into the vadose zone had been
accompanied by substantial dilution. The decrease
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