Environmental Engineering Reference
In-Depth Information
(Cs, Sr, UO
2
) the fraction of pore volume of flow ranged
between 0.25 to nearly 1.00. The flow for the anionic form of
Cr (Cr
2
O
7
=
), was consistently lower than that of the cationic
species. This is potentially due to the water dragging action
of the hydrated anions as they migrate in the opposite direc-
tion of the net flow toward cathode.
Although significant removal of metal into the liquid
phase was achieved in most cases, a clear trend of metal accu-
mulation was observed at the discharge locations (cathode
end for cations; anode end for anions) in all soil specimens.
The accumulation of the metal is due to several conditions.
One condition deduced from the experiments is that the
time rate of migration of the metal reduces significantly as it
approaches to the discharge end. This reduction is triggered
by: (i) concentration increase, (ii) precipitation of the met-
als, and (iii) increased retention capacity of the soil at the
discharge ends due to the local pH levels. As the concentra-
tion of a particular ion increase near the discharge chamber
(anode or cathode), the mobilities of the ions or the micelles
decrease (Kohlrausch law). A slight reduction of the con-
taminant mobility may trigger a progressive concentration
build up at that location if the discharge is inhibited by other
mechanisms simultaneously. These mechanisms come about
by the variation of soil pH from anode to the cathode end.
All the metals, both anionic and cationic, accumulated at
their discharge locations creating zones of high concentra-
tions of metal (sometimes over that of the original). These
zones are often narrow residing at the interface between the
soil and the electrode chamber, which is analogous of depo-
sition of the metal on an electrode as if it was in contact with
soil. In the case of cations, the production of hydroxide in
the cathode chamber is one of the major causes of the phe-
nomena. As the hydroxide ions (OH
-
) migrate into the soil
from the catholyte, they create a high pH zone in the soil
before they can encounter the oncoming hydronium ions
(H
+
) and be neutralized. This zone may lessen in thickness
as the acid front penetrates deeper toward the cathode side
of the soil (Hamed et al., 1991). Subsequently, the high pH
generated at the electrode site solution and the lower pH
at the soil-water interface can create a large pH gradient
across this interface, inhibiting the discharge of the metals.
Another effect of high pH is on the adsorption capacity of
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