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Elektrodialytisk rensning af jord fra træimprægneringsgrunde
Dette afsnit skal konsekvensrettes i forhold til det danske resumé.In the early 1990s
the first in-situ soil remediation plants based on the electrokinetic phenomenon was
reported by Geokinetics, The Netherlands. The remediation success was quite good in these
first plants, but the results underlined that it was necessary to do more detailed
investigations to predict the progress of the action better. After these first in-situ
plants most research has been carried out in laboratory scale. An increasing number of
researchers are working in the field. From only a few teams in the beginning of the 1990s
there are now more than 55 teams working with different aspects of electrochemical soil
remediation.
Electrochemical soil remediation methods are based on the transport processes that
occur when an electric current is passed through a soil. The electric current tends to
pass the soil in the micropores due to the lower resistance here and this means that
the current is acting exactly where the heavy metals are mainly adsorbed in the soil.
Thus the electrochemical methods are especially suitable for fine-grained soils. Different
names are used for these electrochemical techniques but the most common is electrokinetic
soil remediation.
During the electrochemical soil remediation processes the heavy metals are concentrated
in electrolyte solutions in which the electrodes are placed. It is possible to reuse the
heavy metals from these solutions by different techniques. In the case of e.g. Cu, Zn, Pb
and Cd electroprecipitation in metallic form can be done.
In Denmark a special concept for electrochemical soil remediation has been developed,
in which the electrokinetic method is combined to the process of electrodialysis. Ion
exchange membranes are used to separate the soil and solutions where the heavy metals are
concentrated. This means that current will not be wasted in carrying harmless ions from
one electrode compartment to the other, but the current will carry ions out from the soil
only. The electrodialytic soil remediation method has shown its feasibility in laboratory
scale to remediate different soil and pollution types, and electrode units for a
full-scale plant has been developed and tested with success in a pilot plant.
It has been found that it is beneficial to characterize the soil and the pollution type
in each case and on basis of this characterization it is chosen whether the soil should be
pretreated with an enhancement solution prior to the electrodialytic treatment.
Many sites worldwide are polluted from wood preservation plants and the combination of
inorganic salts most wide spread for wood preservation is CCA (Cu, Cr and As), which are
also the pollutants that are found in the soil polluted from the process. Miljøstyrelsen
amounts the number of wood preservation sites to 150 in Denmark alone.
The electrochemical soil remediation methods cannot be used directly to treat soils
polluted with Cu, Cr and As at the same time, because the three elements are not mobile in
the electric field in the same range of pH. Thus it is necessary to add an enhancement
solution to the soil in order to remediate it.
Miljøstyrelsen supported an investigation where an enhancement solution to a soil from
the former Collstrup wood preservation plant in Stenholtvang, Hillerød, should be found.
Besides this, a 4 m3 pilot plant, in which soil from the former impregnation
plant Galgebakken in Sorø should be remediated, was supported by KAVO. The soil from
Galgebakken was deposited at the landfill of KAVO in Vemmelev in the beginning of the
1990s.
From theoretical considerations it was decided to test ammonia as enhancement solution.
The ammonia ensures a high pH in the soil which mobilizes As. Cr(VI) is mobile at high pH,
too, and in the case of Cr(III) ammonia can form amin-complexes that carries a charge and
is thus mobile. Amine-complexes will also be formed with Cu preventing Cu precipitation,
even at high pH values.
Ammonia was tested as enhancement solution for Collstrup/Stenholtvang in laboratory
scale. The Cr concentration in the soil was 160 mg/kg that corresponds to a klass 2 soil
and thus the work was concentrated on Cu and As. It was found that a 2,5% ammonia solution
was a useful enhancement solution. The As concentration in the soil was reduced from 900
mg/kg to 90 mg/kg and the Cu concentration was reduced from 830 mg/kg to about 300 mg/kg
where the Cu concentration stabilized. This concentration of 300 mg Cu/kg is less than the
target value of 500 mg Cu/kg. The concentration profiles of As did not show any tendency
that the final obtainable level of As had been reached and it was decided to use ammonia
as enhancement solution in the next phase of the project, which was remediation of about
200 kg soil from Collstrup/Stenholtvang.
While the laboratory investigation was carried out with the soil from Collstrup/
Stenholtvang it had been planned that a similar investigation with soil from Galgebakken
should have been made. Meanwhile a problem finding Galgebakke soil polluted with Cu, Cr
and As in the landfill arised. Several investigations in different depths were made, but
it was found that the soil only contained Cu in slightly increased concentrations compared
to the target level. Finally it was decided to remove the soil from the landfill and use
it as cover soil for an other part of the landfill. A total of 3900 m3 soil was
removed. During this work continuously monitoring of Cu, Cr and As concentrations were
made. Neither of the soil samples contained Cr and As in concentrations that exceeded the
class 3 level and thus no Galgebakke soil was found for the pilot plant.
KAVO kindly allowed that the pilot plant was used for remediation of another wood
preservation soil. To make the test valuable for comparison, the soil type should differ
from Collstrup/Stenholtvang and it should be polluted with As and Cu as well as Cr. Soil
from Dansk A-træ in Allerød was chosen. A sample from this site was extremely polluted
with Cu 8780 mg/kg, Cr 8420 mg/kg and As 14,000 mg/kg. The soil varied from
Collstrup/Stenholtvang in a high organic content and a fine fraction of 49% compared to
33% for Collstrup/Stenholtvang.
It was decided to do laboratory experiments on a third soil, too. This soil should be
rich in carbonates, which neither of the first two soils were. Collstrup/Køge was chosen
because the carbonate content in this soil was 24% and it was polluted with all three
elements. The investigation did now cover three very different Danish soil types.
The remediation experiments with the soil from Dansk A-træ and Collstrup/Køge did
show similar results even though they were so different in soil type. Cu was removed well
from both soils. The removal of As was slower, but a total reduction of 46% from the
extremely polluted Dansk A-træ and 60% from Collstrup/Køge, was obtained. In the
experiment with Dansk A-træ the removal of As was measured as a function of time and it
was seen, that at the time the experiment was ended As still was removed at an acceptable
rate. Unfortunately Cr had not been removed to more than 3-4% in both soils. The small
amount of Cr that had been removed was found in the anode compartment and Cr-amine
complexes should have been found at the cathode. The ammonia was thus not an enhancement
solution that could be used for remediation of soils polluted with Cr(III).
Addition of a combination of ammonia and H2O2 to the soil from Dansk
A-træ was tested as enhancement solution to oxidize Cr(III) in order to mobilize it. More
Cr was removed than in the case with ammonia alone and in the soil slice next to the anode
30% Cr was removed but in the rest of the soil, the concentration level was in the same
range as initially. This result must be linked to the fact that this soil was very rich in
organics and instead of oxidizing Cr(III) the enhancement solution could have oxidized the
organic matter. The enhancement solution might be useful for soils with less organic
matter.
Ammonium citrate with a pH of 8 was tested as enhancement solution for soil from Dansk
A-træ, too, and the result was very promising. In an experiment of only one month 65% Cu,
33% Cr and 66% As was removed from this extremely polluted soil sample. Still optimisation
of the ammonia citrate concentration and pH should be carried out.
In the small plant for remediation of 200 kg soil (Megalab) the remediation was carried
out in two steps. In the first step the plant was situated outdoors and the soil was
pre-treated with 2.5% ammonia solution. The duration of this step was 9 ½ month. For the
second step the soil was digged up from the plant and mixed well with a concentrated
ammonia solution. The plant was moved indoor and the distance between the electrodes was
reduced from 90 cm to 60 cm. The duration of second step was 9 month. The progress of the
remediation was much better in the second step than in the first. The main reason for this
is expected to be the higher ammonia concentration in the soil in the second step. In the
first step a total of 24 g Cu and 30 g As was removed from the soil whereas in the second
step 37 g Cu was removed and xx g As was removed. The soil was a class 2 soil according to
Cr but still the Cr concentrations was measured. Not surprisingly, after knowing the
laboratory results, only little Cr was removed, and it was removed as anions.
The final concentrations in the soil after the two steps of the remediation were 450 mg
Cu/kg and 630 mg As/kg.
The large pilot plant for about 8 tonne of soil has lasted 6 month at the time of this
report. The pilot plant was 4 meter long and 1.3 meter wide. Four pairs of electrode units
were placed in the soil and thus each pair of electrode covered about 1 m3 of
soil. The energy consumption in the pilot plant varied between 25 kWh/t and 48 kWh/t and a
total of 430 g Cu was removed.
Soil was sampled for this investigation from three wood preservation sites. The soil
types varied considerably but this had no influence on the remediation results. The main
parameter of importance was found to be the pollution level – high pollution, longer
remediation time. This means that the optimization from now can be done with one soil,
because it should be possible to generalize the results to other soils.
The trial of pilot plants were not optimal but several inputs to improve the plant for
the next trial was found: