Civil Engineering Reference
In-Depth Information
surface charge will remain the same, but the added electrolyte will increase the charge
density in the diffuse layer. This results in a smaller diffuse-layer volume being re-
quired to neutralize the surface charge. In other words, the diffuse layer is ''com-
pressed'' toward the particle surface, reducing the thickness of the layer. At high
electrolyte concentrations, particle aggregation can occur rapidly.
Adsorption and Charge Neutralization
The energy involved in an electrostatic
interaction having a 100-millivolt potential difference across the diffuse layer between
a colloidal particle and monovalent coagulant ion is only about 2.3 kcal / mole. This
compares with covalent bond energies in the range of 50 to 100 kcal / mole. Based on
these facts, it is apparent that some coagulants can overwhelm the electrostatic effects
and can be adsorbed on the surface of the colloid. If the coagulant carries a charge
opposite to that of the colloid, a reduction in the zeta potential will occur, resulting
in destabilization of the colloid. This process is quite different from the double-layer
compression mechanism described above.
The hydrolyzed species of Al(III) and Fe(III) can adsorb onto the colloids and
destabilize the particles. However, at higher doses of Al(III) or Fe(III) coagulation is
caused by enmeshment of the colloidal particles in the precipitated metal hydroxide.
This aspect is discussed in the next section. Destabilization by adsorption is stoichi-
ometric. Therefore, the required coagulant dosage increases with increasing concen-
trations of colloids in the solution.
Enmeshment by a Precipitate
(
Sweep-Floc Coagulation
)
The addition of certain
metal salts, oxides, or hydroxides to water in high dosages could result in the rapid
formation of precipitates. These precipitates enmesh the suspended colloidal particles
as they settle.
5
Coagulants such as aluminum sulfate (Al
2
(SO
4
)
3
), ferric chloride
(FeCl
3
), and lime CaO or Ca(OH)
2
) are frequently used as coagulants to form the
precipitates of Al(OH)
3
(s), Fe(OH)
3
(s) and CaCO
3
(s). The removal of colloids by this
method has been termed
sweep-floc coagulation
.
This process can be enhanced when the colloidal particles themselves serve as
nuclei for the formation of the precipitate. Therefore, the rate of precipitation increases
with an increasing concentration of colloidal particles (turbidity) in the solution. Some-
times additional turbidity (e.g., bentonite particles) is artificially added to the raw water
to enhance the sweep-floc coagulation. Packham reported the inverse relationship be-
tween the optimum coagulant dose and the concentration of the colloids to be re-
moved.
5
Benefield explained this phenomenon as follows:
20
At low colloidal concentrations, a large excess of coagulant is required to produce a large
amount of precipitate that will enmesh the relatively few colloidal particles as it settles.
At high colloidal concentrations, coagulation will occur at a lower chemical dosage be-
cause the colloids serve as nuclei to enhance precipitate formation.
This method of coagulation does not depend upon charge neutralization, so an opti-
mum coagulant dose does not necessarily correspond to minimum zeta potential. How-
ever, an optimum pH does exist for each coagulant.
Destabilization by Interparticle Bridging
Synthetic polymeric compounds have
been shown to be effective coagulants for the destabilization of colloids in water. These
coagulants can be characterized as having large molecular sizes, and multiple electrical
