Civil Engineering Reference
In-Depth Information
The computed value for the zeta potential may vary significantly from the true
value because of the uncertainty of values for the various constants in the equation.
As a result, many researchers report experimental results in terms of electrophoretic
mobility, instead of zeta potential. The electrophoretic mobility may be computed from
the equation:
dx
(10-2)
tIR
s
where:
electrophoretic mobility,
/
s
/
V
/cm
d
distance traveled
/s
x
cross-sectional area of cell, cm
2
t
time, seconds
I
current density, amperes
R
s
specific resistance of suspension, ohm-cm
Mobilities are positive (
) for those particles that migrate to the negative pole of
the cell, and are negative (
) for those that migrate toward the positive pole. The
point at which there is no particular migration (zero mobility) is considered to be the
isoelectric point (IEP).
Coagulation Mechanisms
Coagulation can be accomplished through any of four different mechanisms:
20,21
Double-layer compression.
•
Adsorption and charge neutralization.
•
Enmeshment by a precipitate (sweep-floc coagulation).
•
Adsorption and interparticle bridging.
•
Double-Layer Compression
This mechanism relies on compressing the diffuse
layer surrounding a colloid. This is accomplished by increasing the ionic strength of
the solution through the addition of an indifferent electrolyte (neutral salt). The ex-
planation for this phenomenon lies in the Schulze-Hardy rule for anions, which was
based on Schulze's work on the coagulating power of cations. He noted that coagu-
lating power increases in the ration of 1:10:1000 as the valency increases from 1 to 2
to 3. The Schulze-Hardy rule for anions is:
The coagulating power of a salt is determined by the valency of one of its ions. The
prepotent ion is either the negative or positive ion, according to whether the colloidal
particles move down or up the potential gradient. The coagulating ion is always of the
opposite electrical sign to the particle.
This rule is valid for indifferent electrolytes, which are those that do not react with
the solution. If such an electrolyte is added to a colloidal dispersion, the particle's
