Remediation Techniques for Hg-Contaminated Sites
125
mercury by cathodic reduction. Results of a test which was done with a synthetic
solution seem to be promising, but this synthetic solution can behave also in a
different way than soil leachates.
The use of potassium iodide has also been investigated by Wasay et al. (1995)
but contrary to Foust they added acid (HC!), just for desorption, instead of the
oxidant iodine. The best extraction rate was found at pH <2. Mercury was
removed with higher efficiency than by aqua regia digestion. It should be
considered that this process seems to work only in special cases: all mercury
present in the soil should be oxidized already before starting the leaching.
Beyer et al. (1990) describes the extraction ofHg from contaminated soil using
a mixture of HNOj with NaCI in the patent (DE 38 12986 C2). The solution is
heated to a temperature of 50-60°C before use. After separation, Hg is removed
from solution by precipitation with S-containing agents. The lixivant is
recirculated. Finally, the soil is washed and dried.
Several techniques are available for separating the clean soil particles from the
pregnant extracting solution. Classifiers, hydrocyclones or flotation columns are
state of the art and are based on the density, size or shape of the particles.
Electrokinetic Techniques. In the literature several different terms are used to
describe techniques based on the same principles: electrokinetic remediation,
electrokinetic extraction, electro reclamation, electrorestoration or electrodialysis.
Actually, these techniques are identical or differ from each other only in details;
but these slight differences can have a great impact on the success: for instance, the
use of ion-exchange membranes or resin to separate the electrode compartments
from the soil can avoid an increase in pH in the soil, and consequently avoid an
undesirable precipitation of mercury. Commonly, their use is reported concerning
the cleaning of soils polluted with other heavy metals apart from mercury. There
are only a few reports about mercury, which behaves in a different way.
Three transportation phenomena are responsible for electrokinetic mercury
movement in soils. The transport mechanism for any particulate mercury with
charged surfaces, elemental mercury (HgO) or colloidal precipitates, for example,
is called electrophoresis. By electro migration, all ionic species can be transported
to one of the two electrodes. Since dissolved mercury occurs at high pH or high
halide concentration as an anion complex it usually moves towards the anode.
For high concentrations of mercury species in the liquid phase, the process
becomes pH-sensitive because of precipitation. In addition, charged as well as
uncharged species present in the pore liquid can be transported towards the
cathode by electroosmosis (see Fig. 4). Experiments with artificially polluted clay
soil (produced by contacting a mercury chloride solution) were performed by
Hemmings et al. (1992). They used a cation selective membrane to separate the
anode chamber. The analyte contained different "driver ions" (H+ or Na+
respectively) and it was found that Na + was disadvantageous because of mercury
precipitation. Cox et al. (1996) compared the cleaning of an artificially polluted
loam soil with a field-contaminated sandy-loam soil in a bench-scale cell using an
iodine/iodide lixivant for controlling the redox potential. They proved that the
height of the oxidation potential is essential for success. When iodine was added,
up to 99% of the mercury present in artificially polluted soil could be removed.
125
mercury by cathodic reduction. Results of a test which was done with a synthetic
solution seem to be promising, but this synthetic solution can behave also in a
different way than soil leachates.
The use of potassium iodide has also been investigated by Wasay et al. (1995)
but contrary to Foust they added acid (HC!), just for desorption, instead of the
oxidant iodine. The best extraction rate was found at pH <2. Mercury was
removed with higher efficiency than by aqua regia digestion. It should be
considered that this process seems to work only in special cases: all mercury
present in the soil should be oxidized already before starting the leaching.
Beyer et al. (1990) describes the extraction ofHg from contaminated soil using
a mixture of HNOj with NaCI in the patent (DE 38 12986 C2). The solution is
heated to a temperature of 50-60°C before use. After separation, Hg is removed
from solution by precipitation with S-containing agents. The lixivant is
recirculated. Finally, the soil is washed and dried.
Several techniques are available for separating the clean soil particles from the
pregnant extracting solution. Classifiers, hydrocyclones or flotation columns are
state of the art and are based on the density, size or shape of the particles.
Electrokinetic Techniques. In the literature several different terms are used to
describe techniques based on the same principles: electrokinetic remediation,
electrokinetic extraction, electro reclamation, electrorestoration or electrodialysis.
Actually, these techniques are identical or differ from each other only in details;
but these slight differences can have a great impact on the success: for instance, the
use of ion-exchange membranes or resin to separate the electrode compartments
from the soil can avoid an increase in pH in the soil, and consequently avoid an
undesirable precipitation of mercury. Commonly, their use is reported concerning
the cleaning of soils polluted with other heavy metals apart from mercury. There
are only a few reports about mercury, which behaves in a different way.
Three transportation phenomena are responsible for electrokinetic mercury
movement in soils. The transport mechanism for any particulate mercury with
charged surfaces, elemental mercury (HgO) or colloidal precipitates, for example,
is called electrophoresis. By electro migration, all ionic species can be transported
to one of the two electrodes. Since dissolved mercury occurs at high pH or high
halide concentration as an anion complex it usually moves towards the anode.
For high concentrations of mercury species in the liquid phase, the process
becomes pH-sensitive because of precipitation. In addition, charged as well as
uncharged species present in the pore liquid can be transported towards the
cathode by electroosmosis (see Fig. 4). Experiments with artificially polluted clay
soil (produced by contacting a mercury chloride solution) were performed by
Hemmings et al. (1992). They used a cation selective membrane to separate the
anode chamber. The analyte contained different "driver ions" (H+ or Na+
respectively) and it was found that Na + was disadvantageous because of mercury
precipitation. Cox et al. (1996) compared the cleaning of an artificially polluted
loam soil with a field-contaminated sandy-loam soil in a bench-scale cell using an
iodine/iodide lixivant for controlling the redox potential. They proved that the
height of the oxidation potential is essential for success. When iodine was added,
up to 99% of the mercury present in artificially polluted soil could be removed.