Reduction of Contaminants in Soil and Water By Direct Electric Current - Electrokinetics for Petroleum and Environmental Engineers - page 74

center cell compartment. The concentration of the formic
acid decreased in the anode chamber pursuant to migration
of some of the compound into the soil as well. Lab study
confirmed that infield recycling of potassium from drilling
fluid is possible by filtering it through soil using EK.

2.3.2 Electrokinetially-Aided Stabilization and Immobilization

In environmental restoration, stabilization is defined as fixing the toxic
substance in place thereby rendering it less likely to move elsewhere under
the ambient hydrogeological conditions. In the subsurface, stabilization of
a toxic substance can be accomplished by delivering an appropriate oxidiz-
ing or reducing agent that subsequently will: (i) degrade the contaminant;
or (ii) change it to a non-toxic or immobile species; or (iii) enhance sta-
ble sorption and incorporation of the contaminant into the clay minerals.
Zero-valent iron enhanced degradation of TCE (Ho et al., 1995), and Fe(II)
degradation of toxic Cr(VI) to less toxic and less mobile Cr(III) are exam-
ples of such processes (Haran et al., 1995; Pamukcu et al., 1997; Pamukcu
et al., 2008; Gomes et al., 2012; 2013a; 2013b).
The metals that could be altered in such a manner are those that are least
likely to be extracted by electrokinetic treatment. Owing to their complex
electrochemistry, metals such as Cr, As, Hg are possible candidates for
electrokinetic containment. A good example of electrochemical stabili-
zation may be the relatively well studied reduction of Cr(VI) to Cr(III),
by delivering iron (Fe(0), Fe(II), or Fe(III)) with co-reagents in aqueous
environments (Powell, et al., 1995; Anderson et al., 1994; Eary and Rai,
1991). Chromium exists in two possible oxidation states in soils: the triva-
lent Cr(III) and the hexavalent Cr(VI) chromium. At low pH conditions
(2 to 6.5) the predominant form of the hexavalent chromium is chromate
or dichromate ion. Due to their negative charge they are not readily
adsorbed or exchanged at clay surfaces, therefore remain in soil pore water
and can be readily transported. At sufficiently low pH, the soil surface
may become positively charged and tend to retain and accumulate these
anions. Therefore complete removal may not be achieved unless precise
control of pH is maintained at the anode during an electrokinetic process.
As discussed previously, Figure 2.20 shows a good example where only a
small mass of Cr is removed with increasing duration of treatment and
applied current.
Hexavalent chromium can be reduced to Cr(III) under normal soil and
pH conditions, for which soil organic matter acts as the electron donor (Rai
et al., 1987). Bartlett (1991) reported that in natural soils, this reduction