Stressors and Soil Contamination - Sustainable Practices in Geoenvironmental Engineering - page 60

that forms a monolayer coating on the surface of the particles. The choice of the adsorbate
and the availability of soil particles in a totally dispersed state are important factors in pro-
duction of the inal sets of data. Because of the dependence on techniques used, we con-
sider a laboratory measurement of the SSA of a soil sample to be an operationally deined
property (i.e., dependent on technique, adsorbate used, and degree to which the soil has
been properly dispersed).
To explain the reciprocal of charge density shown in Table 2.1, we need to explain what
surface charge density means. The surface charge density is the total number of electrostatic
charges on the clay particles' surfaces divided by the total surface area of the particles.
The common procedure is to express this surface charge density in terms of its reciprocal,
as shown in Table 2.1. We have omitted the values for the hydrous oxides such as goethite
[-FeOOH] and gibbsite [-Al(OH) 3 ] from Table 2.1 because the range of values for these
types of soil fractions are dependent upon (a) their structure, (b) the speciically adsorbed
potential-determining ions, and (c) the pH of the porewater.
The CEC is deined as the quantity of exchangeable ions held by a soil, and is generally
equal to the amount of negative charge in the soil. This is usually expressed in terms of
milliequivalents per 100 g of soil (meq/100 g soil). Exchangeable cations are associated
with clay minerals, amorphous materials, and natural soil organics. Many of the surface
functional groups of these soil fractions are direct participants in cation exchange, e.g., the
oxygen-containing functional groups of SOM such as the carboxyl and phenolic functional
groups. Although not reported in Table 2.1, we see measured values for CEC ranging from
15 to 24 meq/100 g soil for Fe-oxides, from 10 to 18 meq/100 g soil for Al-oxides, and from
20 to 30 meq/100 g for allophanes. CEC values of up to 100 meq/100 g soil for goethites and
hematites, and from 150 to 400 meq/100 g soil for organic matter at a pH of 8 have been
reported by Appelo and Postma (1993). Their empirical relationship for the CEC of a soil is
given in terms of the percentage of clay less than 2 μm and the organic carbon as follows:
CEC (meq/100g soil) = 0.7 Clay% + 3.5 OC%
where Clay% refers to the percentage of clay less than 2 μm and OC% refers to the percent-
age of organic carbon in the soil.
By combining the density of charges with the amount of surface areas available and the
CEC of the speciic clay mineral, we will obtain some appreciation of the degree of reactiv-
ity of the clay mineral in question. This should not be construed as a quantitative estimate
since actual ield soils will not have all particles and their surfaces available for exposure
to contaminants. Aggregate groups of particles such as loccs, domains, peds, and clusters
will diminish the total calculated surface area obtained from single particle theory.
2.5.5 Surface Properties
The surface properties of soils are important because it is these properties, together with
those surface properties of contaminants themselves and the geometry and continuity of
the pore spaces that will control the transport processes of the contaminants. We have pre-
viously deined reactive surfaces to mean those surfaces which by virtue of their properties
are capable of reacting physically and chemically with solutes and other dissolved matter
in the porewater. The chemically reactive groups, which are molecular units, are found
on the surfaces of the various soil fractions, are deined as surface functional groups . These
surface functional groups give the surfaces their reactive properties.