Reduction of Contaminants in Soil and Water By Direct Electric Current - Electrokinetics for Petroleum and Environmental Engineers - page 75

may be extremely slow, requiring years. In subsurface soils where there is
less organic matter, the Fe(II) containing minerals reduce Cr(VI) at pH
less than 5 (Eary and Rai, 1991). Electrochemically injected Fe(II) into a
matrix of soil containing hexavalent chromium should facilitate the reduc-
tion of Cr(VI) since electrochemical process produces low pH conditions.
The delivery of Fe(II) has also been shown to enhance the formation of a
chromium-iron hydroxide solid solution [(Cr x Fe 1-x )(OH) 3 (ss)] which has
a low equilibrium solution activity then pure solid phases (Powell, et al.,
1995). Figure 2.33 shows the formation of chromium-iron oxide hydrox-
ide solutions when Fe(II) was delivered electrokinetically into Cr contami-
nated soil where the original form of chromium was Cr(IV). As observed,
most of the post-EK form of Cr coincided with the solid solution distribu-
tion where the Cr is transformed into non-aqueous Cr(III). Figure 2.34
confirms the post EK abundance of Cr(III) per redox-pH measurements
shown on Pourbaix diagram. These results confirm that the electrokineti-
cally delivered Fe(II) had successfully transformed and arrested the Cr in
soil where it is no longer available for leaching out into the groundwater
environment, hence stabilized.
As previously discussed, Faradaic reactions may take place on clay parti-
cle surfaces when current pass in the pathways of the diffuse double layers.
These reactions can aid in electrically enhanced stabilization and or immo-
bilization of certain contaminants. Hence, external supply of electrical
energy can help drive favorable oxidation-reduction reactions on contami-
nated clay surfaces where most of the contaminants tend to reside because
of adsorption or exchange. Controlled laboratory experiments of kaolinite

0.0

-2.0

Cr(OH) 2+

-4.0 x=0.15

x=0.25

-6.0

0.6[Fe]/[Cr]

3.0[Fe]/[Cr]
5.0[Fe]/[Cr]
11.0[Fe]/[Cr]
(Cr,Fe) solid solutions

(Cr x Fe 1-x )(OH) 3

-8.0

-10.0

1 2 3 4

pH

5 6 7

Figure 2.33 Post-EK formation of Cr,Fe solid solutions in soil at various Fe(II)
concentration applications (Weeks and Pamukcu, 2001)