Reduction of Contaminants in Soil and Water By Direct Electric Current - Electrokinetics for Petroleum and Environmental Engineers - page 39

layer properties, the fluid viscosity, the dielectric constant, solute distribu-
tion and electrical field can vary significantly from one point to another,
both along the flow path and in the orthogonal direction. Hence, aver-
aging microscopic parameters (e.g., zeta potential) to use alongside with
macroscopic parameters (e.g., E ) in an H-S equation can lead to the non-
linear effects.
he zeta, potential is the electrical potential at the junction between the

fixed and mobile parts of the electrical double layer. The

ζ

potential is influ-
enced by the type and concentration of the electrolytes added to the par-
ticle suspension (Kruyt, 1952; Smith and Narimatsu, 1993). For clay soils,
ζ

ζ

potential is usually negative because of the net negative charge on clay
particle surfaces. The magnitude and sign of this potential highly depends
on pore fluid chemistry. As the hydrogen and hydroxyl ions are the poten-
tial determining ions, the lower pH will reduce

ζ potential in magnitude

for most clay. At low enough pH, potential may become positive. Hunter

and James (1992) observed that adsorption of partially hydrolyzed metal
cations such as Co 2+ , Cd 2+ , and Cu 2+ cause

ζ

potential reversals for kaolin-
ite. As the concentration of hydrolyzable metal ions increases,

ζ

potential
becomes more positive at low pH levels due to the accumulation of the
cations in a compressed electrical double layer bearing a larger charge than
is present on the solid surface (Kruyt, 1952). The effect is largest at an inter-
mediate pH, slightly above the value at which precipitation of the metal
hydroxide would be expected in the bulk solution. Due to the sorption of
hydrolyzable metal ions, the sign reversal of the

ζ

potential can make the
net electroosmotic flow insignificant in clay soils with high pore fluid elec-
trolyte concentrations. In this case, and also for saline soils, electromigra-
tion becomes the dominant mechanism of electrokinetic transport.
Recognizing the inability to uncouple v eo from E in the classical H-S
equation for macroscopic systems, Khan (1991) proposed a modified
theory of electroosmotic flow in clay soil. In Khan's work, the ionic con-
ductance through the bulk fluid and the electronic conduction through
the electric double layer at the solid-liquid interface were lumped in series,
representing resistances in parallel or conductance in series. When model-
ing mass flow in the direction of the electric field, it is intuitive to line up
the conducting elements in the direction of the electric field. The electroki-
netic transport theory clearly shows that it is the electronic conductance at
the electric double layer that induce electroosmotic transport and the ionic
conductance in the bulk fluid that result in the ionic transport. Hence, the
following relations are expected to hold:

ζ

(2.5) ii i

s

=+

b