Engineering properties of peat and organic soils - Geotechnics of Organic Soils and Peat - page 64

The ζ can be measured by electrophoresis, the measurement is very direct. The
sample is placed in a chamber which is called an electrophoresis cell. Electric field is
then activated. Then the colloids move with a velocity that is proportional to their zeta
potential. The direction of the movement can show whether their charge is positive or
negative.
The Smoluchowski equation, the most elementary expression for ζ , gives a direct
relation between the zeta potential and electrophoretic mobility:

EM
D t

ζ = 4 πV t (3.8)

where
ζ = Zeta potential

=

EM Electrophoretic mobility at actual temperature

=

V t Viscosity of the suspending liquid

D t dielectric constant

It is preferable to calculate ζ in millivolts instead of in electrostatic units. The
formula then becomes:

=

11300 V t EM
D t

=

ζ (3.9)

ζ for a colloid is usually negative, but the magnitude and sign depend on the
interfacial chemistry of both the liquid and the solid phase (Eykholt and Daniel, 1994;
Vane and Zang, 1997; Yukselen and Erzin, 2008). ζ is a good indicator of the thickness
of the double layer. As ζ increases, the thickness of the double layer increases (Hunter,
1981).
The ζ potential of organic soils varies from 43mV at

pH 11.5 (Asadi et al ., 2009d). It is almost zero at pH 2.5 to 3.5 (Figure 3.15). The
variations in ζ with pH are probably related to the nature of the electrical energy field
in organic soils. The sign of the natural ζ in organic soils is negative. The charge is
affected by pH. As the pH goes up, the net negative charge is produced. As the pH
drops, there is less and less negative charge. The ζ behaviour of organic soils in the
presence of the NaOH is possibly related to dissociation of H + . The negative charge
of humus is generally believed to be due to the dissociation of H + from carboxylic
and phenolic functional groups (Stevenson, 1994; Yu, 1997). All charge on humus
is strongly pH-dependent, with humic substances behaving like polyprotonated weak
acids. Humic substances are a series of relatively high molecular weight, brown- to
black-coloured substances formed by secondary synthesis reactions (Madaeni et al .,
2006).
The natural ζ of peats increase with increase in the natural organic content. In the
peats, organic contents has more influence over increase of the natural ζ in comparison
with organic soils having less organic content, which are under influence of mineral
portions. Thus, the relationship between organic contents and zeta potential are not
only under influence of organic contents, but also under influence of minerals portions
and degree of humification (Asadi et al ., 2009d).

+ 41mV at pH 1.91 to −