Fundamentals of electrokinetics - In-Situ Remediation of Arsenic-Contaminated Sites - page 90

As shown in Equation (5.8) , there are a number of factors affecting the electroosmotic flow
rate. Even though the permittivity ( ε ) and the viscosity ( µ ) of pore water are changed by the
concentration of dissolved constituents and temperature, they are generally considered as constant
values (Eykholt and Daniel, 1994; Kim, 2001; Shapiro, 1990). As discussed briefly above,
however, the magnitude and sign of the ζ significantly depends on several factors such as pH and
the ionic strength of the pore water, speciation of chemical constituents, temperature, and type
of soil (Page and Page, 2002; Virkutyte et al ., 2002). In particular, the pH and ionic strength of
the pore water are crucial factors impacting ζ (Alshawabkeh et al ., 1999). Since soil surface is
negatively charged in common conditions, the ζ of soil surface appears to be negative. Accordingly,
the electroosmosis takes place from anode towards cathode. However, the magnitude and sign of
ζ continuously changes during the duration of the electrokinetic process because the pH and ionic
strength of the pore water are not constant (Shapiro, 1990). Therefore, the direction and flow rate
of electroomosis do not remain constant. The negative value of zeta potential gradually increases
with the decrease in soil pH and finally becomes positive ( Fig. 5.1 ) . If the sign of ζ changes, then
the direction of electroosmosis is reversed (Kim et al ., 2002a). When electroosmosis occurs from
anode towards cathode, generally speaking, it is called normal electroosmosis. On the contrary,
the direction of reversal electroosmosis is from cathode to anode (Kim, 2001; Probstein, 2003;
Reddy and Camesselle, 2009a).

5.2.1.3 Electrophoresis
When a direct current electric field is imposed on charged clay particles or colloids, those charged
particles and bound contaminants are electrostatically attracted to one of the electrodes and are
repelled from the other (Mitchell and Soga, 2005; Probstein, 2003). For example, negatively
charged clay particles move towards the anode. This movement of charged solid particles or
colloids is called electrophoresis or cataphoresis (Reddy and Cameselle, 2009a). Compared with
electromigration and electroosmosis, electrophoretic transport is not significant and is negligible
in a compact system such as low-permeability soil, because colloids and solid particles are large
in size and exhibit higher frictional drag force in motion. Meanwhile, electrophoretic transport
becomes crucial and dominant for biocolloids (e.g., bacteria) and micelles in soil suspension
systems (Reddy and Cameselle, 2009a).

5.2.1.4 Diffusion
If a concentration gradient exists in a soil system, ionic and molecular contaminants transport
from areas of higher concentration to areas of lower concentration. The transport phenomenon
is called diffusion (Mitchell and Soga, 2005; Probstein, 2003). As shown in Equation (5.1) and
Table 5.1 , the ionic mobility ( u i ) of a charged species i is much greater than its diffusion
coefficient ( D i ), and contaminant transport by diffusion is neglected in the electrokinetic process.

5.2.2 Electrolysis of water
When inert electrodes are placed in water and a direct current is passed through them, water
molecules are electrolyzed at the surface of electrodes. As a result, hydrogen ions are produced at
the anode and hydroxide ions at the cathode. At the electrodes, electrolysis of water takes place
as follows (Kim and Kim, 2002; Reddy and Cameselle, 2009a):

4e − → 4H + + At anode: 2H 2 O − O 2 (g) (5.11)

2e − → 2OH − +

At cathode: 2H 2 O + H 2 (g) (5.12)

The production of H + ions at the anode decreases the pH by Equation (5.11) , but the reaction of
Equation (5.12) increases the pH at the cathode by increasing OH − ions. This electrolysis results
in an acid front at the anode and an alkaline front at the cathode. These fronts travel through soil
media and finally move towards the cathode and the anode, respectively. The propagation of the