Stressors and Soil Contamination - Sustainable Practices in Geoenvironmental Engineering - page 59

TABLE 2.1
Charge Characteristics, SSA, and CEC for Some Clay Minerals

Range of
Charge
(meq/100 g)

Reciprocal of
Charge Density
(nm 2 /charge)

CEC
(meq/100 g)

Surface Area
(m 2 /g) Soil Fraction Isomorphous Substitution Source of Charges

Kaolinite 5-15 10-15 5-15 0.25 Dioctahedral; two thirds of

positions illed with Al

Surface silanol and edge
silanol and aluminol groups
(ionization of hydroxyls
and broken bonds)

Clay micas and chlorite 10-40 70-90 20-40 0.5 Dioctahedral; Al for Si

Trioctahedral or mixed Al
for Mg

Silanol groups, plus
isomorphous substitution
and some broken bonds at
edges

Illite 20-30 80-120 20-40 0.5 Usually octahedral

substitution Al for Si

Isomorphous substitution,
silanol groups and some
edge contribution

Montmorillonite a 80-100 800 80-100 1.0 Dioctahedral; Mg for Al Primarily from isomorphous

substitution, with very little
edge contribution

Vermiculite b Primarily from isomorphous

substitution, with very little
edge contribution
Source: Adapted from Yong, R.N., Geoenvironmental Engineering: Contaminated Soils, Pollutant Fate and Mitigation , CRC Press, Boca Raton, FL, 307 pp., 2001.
Note: That ratios of the external to the internal surface areas are highly approximate since surface area measurements are operationally deined, i.e., they depend on
the technique used to determine the measurement.
a Surface area includes both external and intralayer surfaces. Ratio of external particle surface area to internal (intralayer) surface area is approximately 5:80.
b Surface area includes both external and internal surfaces. Ratio of external to internal surface area is approximately 1:120.

100-150 700 100-150 1.0 Usually trioctahedral

substitution Al for Si