Reduction of Contaminants in Soil and Water By Direct Electric Current - Electrokinetics for Petroleum and Environmental Engineers - page 56

The mitigation outcomes of electrokinetic treatment can be catego-
rized in three general processes, not exclusive of each other: extraction
and separation; immobilization ; and transformation, as discussed below:

2.3.1 Electrokinetially-Aided Separation and Extraction

Electrokinetic extraction is analogous to soil washing whereby the con-
taminant is extracted from the soil pore space and subsequently collected
in aqueous phase in a collection well or deposited at the electrode site.
The alkali metals, e.g., Na(I), K(I), and Cs(I), and alkali earth metals, e.g.,
Sr(II) and Ca(II) tend to remain ionic under a wide range of pH and
redox potential values, therefore they are expected to electromigrate and
be extracted from soils readily unless they become preferentially sorbed
onto solid surfaces and clay interstices in soil. Under ideal conditions, the
predominant cation and its accompanying anion may be caused to separate
efficiently by electromigration only, for which little or no electroosmotic
water advection may be necessary (Weinberger, 1993). The electroneutral-
ity can be maintained by the hydrogen and the hydroxide ions produced
by the electrolysis reactions at the electrodes and transported into soil by
migration.
Described below are several selected laboratory experiments conducted
using the Lehigh Elektrokinetic Test Cell (Wittle and Pamukcu, 1993;
Pamukcu, 1994) to exemplify the variety of electrokinetic extraction and
separation processes that can be engineered to address site specific situa-
tions. The soil samples used in all these experiments were the actual con-
taminated field soils.

A. NaCl Extraction/Separation : The first example of ionic spe-
cie separation and extraction is provided for NaCl removal
from drilling mud obtained at an Eastern Pennsylvania site.
Figures 2.13 and 2.14 show the separation of sodium (Na)
and chloride (Cl) from drilling mud soil samples of differ-
ent water saturations under constant electric potential. The
final pH profiles attained at the end of the tests are super-
imposed on the graphs. As observed in figure 2.13, close
to 100% recovery of the Na is accomplished in the 81%
saturated specimen, S1 at the termination of the test, while
about 70% of Na is recovered for the 53% saturated speci-
men, S2. The specimen designated as S1 shows a substantial
recovery of Cl (figure 2.14), although not as high a recovery
as Na. The inability to account for all the Cl transported