Geoscience Reference
In-Depth Information
The formula indicates that for the same valence the concentration contributes to
the thickness of the electrical double layer and consequently could reduce the zeta
potential.
The effects of divalent and trivalent electrolytes are higher than those of Na
+
and
K
+
because of their valence. The effect of Na
+
in decreasing the zeta potential is lower
than the effect of K
+
and the effect of Mg
2
+
in is lower than that of Ca
2
+
, indicating the
possibility that cations with a higher hydrated radius have a lower effect on decreasing
the zeta potential. The hydrated radius of Na
+
(0.79 nm) is higher than the hydrated
radius of K
+
(0.53 nm), and the hydrated radius of Mg
2
+
(1.08 nm) is also higher than
the hydrated radius of Ca
2
+
(0.96). Thus, the higher hydrated radius ions show a lower
capacity to decrease the zeta potential (Asadi, 2010).
3.12.3 Resistivity of organic soils and peat
The resistivity of a soil depends on the surface conductivity of the colloids (i.e. clay
or/and humus), presence of ions, porosity, moisture content, and temperature; and is
determined according to Ohm's law. Resistance is that property of a conductor which
opposes electrical current when a voltage is applied across the two ends and is given
by the ratio of the applied voltage to the resulting current flow. The resistance of a
conductor depends on the atomic structure of the material or its resistivity, which is
that property of a material that measures its ability to conduct electricity. The resis-
tivity is measured in ohm metres, and can be derived from the resistance, length and
cross-sectional area of the conductor. The mathematical equation that describes this
relationship is:
E
I
A
L
ρ
=
(3.12)
where
ρ
=
resistivity of soil (
m)
E
=
applied voltage across the sample (V)
cross-sectional area (m
2
)
A
=
=
I
current (amp)
=
L
Length of the sample (m)
A resistivity cell can be used to measure the resistivity of organic and peaty soils.
In order to increase the degree of accuracy, different constant electrical potentials of
40, 70 and 90 V can be applied across the specimen.
The resistivity of organic soils is affected by the water content and temperature
as depicted in Tables 3.10 and 3.11. The resistivity decreases as the water content or
the temperature increase. A higher degree of peat humification results in a lower peat
resistivity (Asadi
et al
, 2009c, Asadi, 2010).
The resistivity of both very slightly decomposed and highly decomposed peat
increases as the organic content increases. The porosity of peat is also an important
factor in its resistivity. Peat is a high-porosity material. Since it tends to have a high
water content due to the high level of organic matter and plant remains, most of the
void could be peat water. Therefore, as the porosity of the peat increases, the potential
