Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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IRON REMOVAL FROM ~DTA SOLIJTIONS
BACKGROUND OF THE INVENTION
Deposits that contain radioactive elements often
form in the cooling systems of nuclear reactors. In order
- to safely maintain and repair the cooling system, it is
necessary to remove these radioactive deposits. This can be
accomplished, for example, by using an oxidizing solution of
alkali permanganate followed by a decontamination solution
of oxalic acid, citric acid, and ethylenediaminetetraacetic
acid (EDTA). The ~DTA forms a complex with the radioactive
metal ions in the deposits, which solubilizes them. The
decontamination solution is circulated between the cooling
system and a cation exchange resin which exchanges the metal
ions on the resin and frees the EDTA to solubilize
additional metal ions.
s
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~ major difficulty with this process, however,
is that EDTA does not readily yield up metal ions, par-
ticularly the ferric ion, to the cation exchanye resin.
Thus, the concentration of the metal ion-EDTA complex
builds up in the decontamination solution until it is no
longer effective in solubilizing the metal ions in the
deposits. When this happens, it is necessary to add fresh
EDTA to the solution. This means that the solution must
be constantly monitored to determine if the EDTA has been
depleted so that more can be added. Also, great care must
be taken not to add excess EDTA since EDTA is not very
soluble unless it has fcrmc~ a complex with metal ions,
and precipitated EDTA can itself be difficult to remove
from the cooling system. Moreover, if excess EDTA is
added, not only is the reagent wasted, but the additional
EDTA must be removed from the solutisn at a later stage
which adds to the volume of radioactive waste.
SUMMARY OF THE INVENTION
We have discovered a process for removing tran-
-ition metal ions from a decontamination solution contain-
ing EDTA while regenerating the EDTA. Unlike the prior
process which used a cation exchange resin, our process
uses an anion exchange resin. The anion exchange resin is
preloaded with EDTA anion so that the entire metal ion-EDTA
complex deposits on the ion exchange resin, releasing
fresh EDTA from the anion exchange resin into the solution.
Thus, the concentration of uncomplexed EDTA in the solution
remains fairly constant and it is not necessary to monitor
the solution for the EDTA concentration or to add fresh
EDTA.
DESCRIPTION OF THE INVENTION
The process of this invention can be applied to
any solution containing a complex of a transition metal
with a complexing agent having an equilibrium constant for
the ferric ion complex formation reaction of greater than
1022. Examples of such complexing agents include ethylene-
diaminetetraacetic acid, trans, 1, 2-diaminocyclohexane-
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3 51,129
tetraacetic acid (DCTA), and oxybis (ethylenediaminetetra-
acetic acid) (EEDTA). Common transition metals found in
nuclear reactor decontamination solutions include iron,
cobalt, nickel, and chromium. The temperature of the solution
should be at least about 40C in order to keep the EDTA in
solution and prevent it from precipitating. The temperature
of the solution should be below about 100C, however, as anion
exchange resins and the reagents used in the solution may de-
compose above that temperature. The pH of the solution is not
critical but it is typically about 2 to about 2 1/2 for most
decontamination solutions due to the acidity of reagents which
are present.
In the first step of the process of this invention,
an anion exchange resin is loaded with EDTA, DCTA, or EEDTA.
Any anion exchange resin is suitable and may be used in this
invention. The resin should be loaded with only EDTA, DCTA, or
EEDTA and not with any other complexing agents because as the
metal EDTA, DCTA, or EEDTA complex is absorbed by the resin,
another anion (i.e., NTA, citric, or oxalic) would be released,
diluting the concentration of EDTA, DTCA, or EEDTA in the solu-
tion. Other complexing agents, such as NTA, or organic acids,
such as citric acid and oxalic acid form much weaker transition
metal complexes compared to those formed with EDTA, DTCA, or
EEDTA and metals complexed with these other agents can be removed
from solution by cation exchanges. This is not the case for
EDTA, DCTA, or EEDTA metal complexes, and as a result, the metal
remains in solution using conventional removal methods.
The anion exchange resin is most conveniently loaded
with the EDTA, DCTA, or EETA anion by preparing a solution of
the EDTA, DCTA, or EEDTA and passing the solution tnrough the
anion exchange resin. It is preferable to use a solution of an
EDTA, DCTA, or EEDTA salt, preferably an alkali metal salt, such
as sodium EDTA, DCTA, or EEDTA to load the anion exchange resin
with the EDTA, DCTA, or EEDTA anion, or this releases sodium
hydroxide rather than just water into the solution. Since NaOH
is highly alkaline, (pH~12-14) the pH of the solution exiting
the colurnn, after an initial rise, will fall back down to the
pH of
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the sodium EDTA, DCTA, or EEDTA (pH~4-5) as fewer hydroxide
groups of the preferred strong base anion exchange resin are
replaced by the EDTA, DCTA, or EEDTA anion. Thus, by monitoring
the pH of the solution leaving the resin, one can then determine
when the resin has been fully loaded. After the pH falls to
below abou-t 6, the resin should be considered to be fully loaded
with EDTA, DCTA, or EEDTA anion. While the acid form of EDTA,
DCTA, or EEDTA can be used, it is more difficult to determine
when the resin has been loaded because, without the presence of
the sodium ion, the solution leaving the columns will be at
approximately a neutral pH value (~7). Thus, the difference in
pH values of the column feed (about 4.5) and the column effluent
(about 7) is significantly less than when the sodium salt is
used. Also, the acid form of EDTA, D~TA, or EEDTA is not very
soluble in water, which means that the solution must be more
dilute.
In the next step of the precess of this invention,
the decontamination solution containing the metal ion-EDTA, DCTA,
or EEDTA complex is circulated between the EDTA, DCTA, or EEDTA
loaded anion exchange resin and the reactor cooling system, or
the portion thereof that is being decontaminated, such as the
steam generator of a pressurized water reactor or a boiling
water reactor. As the metal ion-EDTA, DCTA, or EEDTA complex
is absorbed onto the EDTA, DCTA, or EEDTA anion exchange resin,
fresh EDTA, DCTA, or EEDTA is released into the contamination
solution. The solution is circulated until the concentration of
metal ions in the solution leaving the cooling system is not
substantially greater than the concentration of metal ions in
the solution entering the cooling system.
After the metal ion-EDTA, DCTA, or EEDTA complex has
been removed, the EDTA and any remaining ions in the solution
can be removed by passing the solution through a fresh anion
exchange resin or a mixed anion-cation exchange resin, which
results in relatively pure water. When the preloaded anion
exchange resin has been saturated with -the metal ion-EDTA, DCTA,
or EEDTA complex, it is disposed of as radioactive waste.
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The following examples further illustrate this
invention.
, !EXAMPLE
e f ~
A 1 inch ~m~~glass column 18 inches long was
partially filled with 100 ml of an anion exchange resin
sold by Rohm and Haas under the trade designation "IRA-
400," a strong-based polystyrene resin having a particle
size between 16 and 50 mesh. A solution was prepared of
100 grams/liter of the di~odium salt of EDTA. The solu-
tion, which had a pH of 4.38, was fed through the top ofthe column at 1-3 bed volumes/hr. and the pH of the solu-
tion leaving the bottom of the column was measured. The
following table gives the pH of the solution leaving the
column after various bed volumes of the solution had
flowed through the column.
BED VOL~ME pH
0.5 11.85
1.0 12.86
1.5 12.94
2.0 12.36
2.5 6.56
3.0 5.90
3.5 5.64
4.0 5.47
4.5 5.29
5.0* 5.15
6.0 5.07
*A new solution was prepared having a pH of 4.49.
The above table shows that, after an initial
start-up time, the pH of the solution leaving the resin
fell to close to the pH of the solution entering the
resin. This indicated that the column was almost satur-
ated with EDTA.
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XAMPLE 2
Simulated spent decontamination solutions were
prepared by dissolving 50, lO0, and 200 ppm of iron (from
magnetite, Fe304) in three 0.5 weight percent solutions of
a commercially available decontamination agent believed to
be 30% citric acid, 30% oxalic acid, 40% EDTA, and contain-
ing an inhibitor believed to be thiourea. The three
solutions were mixed in beakers with the preloaded anion
exchange resin prepared in Example 1 at 54C. After 5
hours the solutions were tested and were found to contain
3, 11, and 46 ppm of iron, respectively. This established
that the EDTA-loaded anio.. exchange resin successfully
removed iron from the solutions.
EXAMPLE 3
A 100-ml sample of the EDTA-loaded anion exchange
resin prepared as in Example 1 was placed in a 1 inch
glass column 18 inches long. A 0.5% solution of the
commercially available decontamination agent (described in
Example 2) which contained 80 ppm of iron was passed
through the column at 12 bed volumes/hr. from top to
bottomJand the iron, oxalate, citrate, and EDTA concentra-
tions in the solution leaving the column were measured.
The following table gives their concentrations.
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IRON OXALATE CITRATE EDTA
~ED VOLUME ~ppm) (mg/ml) ~ mi) (ppm,
Feed 80 1080 1000 1384
10 to 11 39 10 290 4100
5 20 to 21 35 - 10 760 4366
40 to 41 37 10 1370 3352
60 to 61 14 78 1340 2521
83
80 to 81 8.8 430 1130 1783
430
90 to 91 3.8 500 910 984
100 to 101 1.0 830 970 1332
The above table shows that, after an initial
start-up period, the EDTA-loaded column successfully
removed iron in the solution to levels below lO ppm.
