Reduction of Contaminants in Soil and Water By Direct Electric Current - Electrokinetics for Petroleum and Environmental Engineers - page 43

0.01 M ), the concentration and
the type of the ion appear to control the K ef factor, as all soil types show
very close to constant normalized values of K ef for the three ions involved
(except MS with Sr). Subsequently, as the concentrations of the major con-
tributing ions increase beyond 0.01 M, fewer soil types comply with the
categorization, because the mineralogy of the soil, and its unique chemi-
cal interaction with the dominant ion comes into effect. Because of the
uniqueness of this interaction the normalized values show greater varia-
tion for each ion involved at high concentration (e.g., Cr, Pb, Zn) then the
low concentration (e.g., Cs, Cd, Sr) group.
K ef is computed easily by measuring the total volume of flow through the
clay matrix and the corresponding total current in the laboratory. In equa-
tion 2.5 the total current, i is given as the summation of the bulk current,
i b and the surface current, i s . Unless the contribution of the surface current
is uncoupled, the measured K ef based on total current, i will always dis-
play high dependency on soil composition and mineralogy, and the type
and concentration of the major contributing ion. During electrokinetic
transport, as the bulk fluid chemistry changes so does the electric double
layer properties of the clay medium. Equation 2.7 derived earlier, shows a
ratio of the H-S coefficient of electroosmotic conductivity k eo and the sur-
face conductivity, s s , which should remain constant as they are uniquely
related. Only when this constancy (= k eo /σ s =constant) is true, so is the
expression given in equation 2.8. It is important to note here that the use
of equation 2.8 necessitates the presence of distributed surface conductiv-
ity that enables the surface currents to travel within the electrochemical
system for electroosmosis to occur. Yin and co-workers (1995) backed up
Khan's theory, which relate electroosmotic mobility and surface conduc-
tivity. They found that there is no apparent relationship between electro-
osmotic mobility and the applied electric field. The term, electroosmotic
mobility, defined as the average velocity achieved by the pore water relative
to the solid skeleton due to an externally applied electrical field of unit
strength (cm 2 /s-V), appeared to be directly proportional to the specific
conductance (mho/cm) of the specimens at different water saturations in
independent tests (Yin et al., 1995).

concentrations (e.g., Cs, Cd and Sr, all ≤

2.2.3 Theoretical Considerations: Mathematical Modeling
of Transport
In the past two decades, many researchers studied and proposed theoreti-
cal models of ion transport under electric field as it applied to contami-
nated clays. In most cases, dilute solutions, rapid dissociation-association