Geoscience Reference
In-Depth Information
Within the first few days of electrokinetic processing, electrolysis reactions decrease the
pH at the vicinity of the anode and increase the pH in the vicinity of the cathode. These
changes depend on the total current applied (Acar
et al
., 1990; Acar and Alshawabkeh,
1993).
The acid generated at the anode advances through the soil towards the cathode.
This is due to ionic migration and electroosmosis. The base generated at the cathode
initially advances towards the anode. The base advance is because of diffusion and
ionic migration. However, the counter flow because of electroosmosis makes the back-
diffusion and migration of the base front slower. The advance of the base front is slower
than the advance of the acid front because of (1) the counteracting electoosmotic flow
and (2) the ionic mobility of H
+
is higher than OH
−
(Acar
et al
., 1990; Alshawabkeh
and Acar, 1992; Probstein and Hicks, 1993). Geotechnical reactions in the soil pores
significantly impact electrokinetic phenomena and can enhance or make slower the
electrokinetic process. Geomechnical reactions, including precipitation, dissolution,
sorption and complexation reactions, are highly dependent on the pH conditions (Acar
et al
., 1990; Alshawabkeh and Acar, 1992).
7.2.2 Electroosmosis in organic soils and peat
Peats generally have a very high water content, which can be in excess of 1500%,
compared with mineral soils (sand, silt, and clay) whose values in the field may range
from 3% to 100% (Huat, 2004; Tan, 2008). Peats tend to have high water content
due to their organic content. Fibrous peat also tends to have higher water content than
humified peat. The bulk densities of peat are in the range 0.8-1.2Mgm
−
3
compared
with the bulk densities of mineral soils which are in the range 1.8-2Mgm
−
3
(Huat,
2004). This is due to the lower specific gravity (1.2-1.4) of the solids found and the
higher water-holding capacity in the peat (Huat, 2004). A saturated mass is needed in
electrokinetic phenomena and the peat environment is a good medium.
In electroosmotic dewatering, the frictional drag is produced by the movement
of hydrated ions (Yeung and Datla, 1995). The quantity of these ions depends on
the soil cation exchange capacity (CEC). The CEC range of humus is from 100 to
300 cmol
+
kg
−
1
, which is highest among colloids. The mineral fractions of tropical
regions are dominated by kaolinite, aluminium oxides and iron oxides. The CEC range
of kaolinite and Al, Fe oxides is 5-10 and 2-6 cmol
+
kg
−
1
of soil, respectively. There-
fore the humus is the key component in creating the potential for water momentum in
electroosmotic phenomena (Asadi
et al
., 2011b).
The charge on humus, kaolinite and Fe and Al oxides is affected by pH (Stevenson,
1994). Dissociation of H
+
is under the control of pH. In all Al and Fe oxides, as well
as some silicate clays, exposed OH
−
groups in moderate to acid conditions experience
protonation. This occurs as an H
+
attaches to the OH
−
. Since humus has charge, the
occurrence of a water flow is expected in electroosmotic phenomena. However, the
charge behaviour of peat needs to be investigated (Asadi
et al
., 2010).
Electroosmosis can affect the pore fluid and distribution of exchangeable ions in
soil and consequently can change the intensity of the forces holding the water films
between soil and water (Fang and Daniels, 2006). Since the compressibility of a soil
indicates the intensity of those forces, the electroosmotic environment can affect the
compressibility behaviour of a soil. It is noteworthy that the water is attracted to a
