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Description  |
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BACKGROUND OF THE INVENTION
The invention relates to a method for immobilizing radioactive wastes for
long term storage. More specifically, the invention relates to a method
for recovering cesium from solutions containing cesium together with other
metal values and for immobilizing radioactive cesium in a highly stable,
nonvolatile and insoluble product suitable for long-term storage.
The principal long-term problem caused by nuclear reactor accidents is
contamination of the environment with radioactive material as was
evidenced by the Chernobyl nuclear reactor accident in 1986. Cesium is a
particular problem in this respect because it is very volatile and can be
carried in the upper atmosphere for long distances. Therefore, suitable
methods of decontaminating the environment, i.e. water and soil, of cesium
before its ingestion by animals or humans are highly desirable. Also
desirable are methods for recovering radioactive cesium from the
gastrointestinal tracts of contaminated animals.
Certain cation exchange resins and various cation exchangers are available
which are selective for the recovery of cesium from solution. These
include clay minerals and zeolites, both naturally occurring and
synthetic. Furthermore, naturally occuring mordenite zeolite has been
mixed with animal feed to remove ingested cesium-137 from the
gastrointestinal tracts of contaminated animals. Other natural zeolites
include clinoptilolite, erionite and chabazite.
In addition to recovering the ions, the radioactive ions must be
immobilized in a form suitable for longterm storage so that they cannot be
released back into the environment or leached from the storage medium into
the surrounding soil or groundwater.
Many methods and materials have been developed over the years for
immobilizing various nuclear wastes, and especially cesium, for long-term
storage. For example, U.S. Pat. No. 3,161,601 dated Dec. 19, 1964, and
assigned to the common assignee, incorporated the radioactive cesium into
a glass containing cesium oxide, alumina, phosphate and an additive such
as lanthanum or zirconium. Another method was to incorporate the
cesium-137 as cesium carbonate with spodumene or with a mixture of silica
and kaolin at a temperature of at least 1000.degree. C. to form a
synthetic pollucite. Another process mixed an inorganic zeolite containing
radioactive cesium with about 20% additives, mainly iron and calcium
oxides, which was melted at 1500.degree. C. and control cooled to form an
iron-enriched basalt. U.S. Pat. No. 4,537,710, also assigned to the common
assignee, describes a cation exchanger which is a modified tobermorite
containing aluminum, that is selective for a small number of ions
including cesium. The advantage of the modified tobermorite is that it is
compatable with matrix materials such as concrete and, therefore, more
resistant to leaching.
Other, less satisfactory, methods involve incorporating the radioactive
waste directly into a matrix material such as asphalt or concrete before
emplacement for storage.
Almost all of the materials or processes suitable or selective for the
separation and recovery of cesium from contaminated water or radioactive
waste streams require additional processing to immobilize the cesium, in
order to prevent the radioactive ions from being leached or otherwise
separated from the storage medium. For example, the cesium ions may be
eluted from the ion exchanger and incorporated directly into the matrix
material for storage. Preferably, the recovery material (ion exchanger)
containing the radioactive ions, is itself incorporated into the storage
medium, such as a glass or cement, for storage. Alternatively, the
radioactive ions may be mixed with other inorganic materials and by
applying heat and/or pressure, formed into a synthetic mineral which is
satisfactory for storage.
Thus, most processes require several steps to recover and prepare the
cesium for storage. This increases the cost of preparing the radioactive
ions for storage. Furthermore, some processes require high pressures
and/or temperatures which in addition to increasing costs, increases the
opportunities for the loss of radioactive material. Finally, many of the
storage materials are not wholly suitable for long-term storage because
leaching of the radioactive ions can occur.
SUMMARY OF THE INVENTION
It has been found that a modified phlogopite mica is very ion selective for
cesium ions, even from solutions which also contain sodium and calcium
ions. Furthermore, it has been found that the modified phlogopite mica
will trap the cesium ions in such a manner that the phlogopite containing
the cesium is suitable for emplacement for long-term storage, with little
or no additional processing. The modified phlogopite mica of the invention
is a phlogopite mica which has been hydrated and in which the potassium
ions have been replaced by sodium ions. The invention is a process for the
separation and recovery of cesium ions from a feed solution containing
cesium ions, and which may contain other metal ions, by contacting the
solution with the modified phlogopite which is a hydrated sodium
phlogopite mica whereby the cesium ions are selectively taken up by the
modified phlogopite while the other ions remain in the solution, and
separating the modified phologopite containing the cesium ions from the
feed solution thereby recovering the cesium ions. The invention is also a
process for fixing radioactive cesium for long-term storage by contacting
a solution containing radioactive cesium with the modified phlogopite
which is a hydrated, sodium mica maintaining the contact until sufficient
cesium is taken up by the modified phlogopite to reduce the c-axis spacing
an amount sufficient to immobilize the cesium, thereby fixing the
radioactive cesium ions for long-term storage. Alternatively, the cesium
may be fixed by heating the modified phlogopite containing the cesium to a
temperature sufficient and for a period of time sufficient to reduce the
c-axis spacing thereby fixing the cesium ions in the modified phlogopite.
Since the modified phlogopite of the invention is very selective for cesium
ions, it is especially useful for the recovery of radioactive cesium ions
which are present in radioactive waste solutions along with other metallic
ions including sodium and calcium. The solutions may be either low level,
intermediate or high level nuclear wastes. It is also useful for the
recovery of cesium-137 from large volumes of water containing low levels
of cesium such as nuclear reactor coolant systems which have become
contaminated because of fuel element ruptures or from stream or water
supplies which have become contaminated due to Cs.sup.137 fallout.
Furthermore, because of its inertness and stability in an acidic
environment, the modified phlogopite is suitable for ingestion by animals
for the recovery and removal of ingested radioactive cesium fron the
gastrointestinal tract, result from nuclear mishaps like the Chernobyl
incident.
It is therefore one object of the invention to provide a process for
recovering cesium.
It is another object of the invention to provide a process for recovery of
cesium from solutions containing cesium together with other ions.
It is a further object of the invention to provide a process for fixing
cesium for the long-term storage.
It is still another object of the invention to provide a one-step process
for recovering and fixing cesium ions for long-term storage.
Finally, it is the object of the invention to provide a process for
recovering and immobilizing radioactive cesium ions for long-term storage
which does not require conditions of high temperature or high pressure.
DETAILED DESCRIPTION OF THE FIGURES
FIG. 1 is a x-ray diffractogram showing the c-axis spacing of untreated
phlogopite mica.
FIG. 2 is a x-ray diffractogram showing the c-axis spacing of the modified
phlogopite mica of the invention.
FIG. 3 is a x-ray diffractogram of the modified phlogopite mica of the
invention saturated with cesium ions.
FIG. 4 is a graph of the cesium exchange isotherm of modified phlogopite in
the presence of pure CsCl solutions.
FIG. 5 is a diffractogram showing the c-axis spacing of modified phlogopite
containing 50 meq/100 gm cesium ions before and after heating.
FIG. 6 is a diffractogram showing the c-axis spacing of a modified
phlogopite containing 19 meq/100 gm cesium ions before and after heating.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
These and other objects of the invention for recovering and fixing
radioactive cesium ions for long-term storage may be met by contacting a
feed solution containing the cesium together with other metal ions with a
hydrated sodium phlogopite mica which has a c-axis spacing of about
12.23.ANG., whereby the cesium is selectively absorbed by the modified
phlogopite, maintaining said contact until sufficient cesium ions are
absorbed to reduce the c-axis spacing to at least about 11.58.ANG., and
separating the modified phlogopite containing cesium from the feed
solution, thereby recovering the cesium ions from the solution and fixing
the ions for long-term storage. Alternatively, the modified phlogopite
containing the absorbed cesium ions can be heated to at least 150.degree.
C. for a period of time sufficient to dehydrate the modified phlogopite
thus reducing the c-axis spacing and fixing the cesium ions for long-term
storage.
The modified phlogopite is prepared by the method described in Clays Clay
Miner 14, 69 (1966) incorporated herein by reference. As described
therein, naturally occuring phlogopite mica, having the formula: KMg.sub.3
Si.sub.3 AlO.sub.10 (OH).sub.2 is finely ground to about 0.2 to 20 um
particle size and contacted with a solution of about 1.0 N NaCl, 0.3 N
sodium tetraphenylboron (NaTPB) and 0.01 M ethylenediamine-tetracetic acid
(EDTA) for a period of several hours. This results in a complete depletion
of K.sup.+ ions from the interlayers of the phlogopite mica and the
simultaneous saturation of the interlayers with Na.sup.+ ions along with a
monolayer of water molecules. This treatment results in a phase with
12.23.ANG.c-axis (001) spacing as opposed to the original phlogopite mica
which has a c-axis spacig of 10.03A. This 12.23.ANG. phase is ideally
NaMg.sub.3 Si.sub.3 AlO.sub.10 (OH).sub.2.H.sub.2 O or hydrated sodium
phlogopite.
Contact between the solution containing cesium ions may take place by
passing the solution through a packed bed or column of the modified
phlogopite. Alternatively, the modified phlogopite may be mixed with the
solution containing the cesium ions and recovered by filtering.
The modified phlogopite is very selective for cesium ions and should be
able to selectively recover cesium ions from the presence of any other
metal ions.
The theoretical capacity of the modified phlogopite for cesium ions is
about 210 meq/100g. However, the maximum cesium loading which can be
attained is about 93.7 meq/100g at which loading a cesium sodium
phlogopite mica is formed. This incomplete cesium exchange can be
explained by the fact that the interlayer spacing significantly collapses
to about 11.58.ANG. when about half of the exchange sites are occupied by
cesium. This collapse of the c-axis or interlayer spacing by about
0.65.ANG. is effective in preventing any further exchange of cesium ions
from solution. Therefore, just as the cesium ions cannot enter the
structure after the initial exchange, the cesium ions that entered the
structure cannot escape from the collapsed interlayers, effectively
leading to the fixation of the cesium ions. The collapse of the
interlayers is explained by dehydration of the ions in the interlayer
because of the high charge density of the layers and the low hydration
energy of the cesium ions.
Once the cesium ions have entered the phlogopite structure, they are fixed
and not subject to displacement. However, collapse of the c-axis or
interlayer structure and total fixation of the cesium is not believed to
occur until the modified phlogopite contains about 80 meq of cesium. The
spacing can be reduced and the mica formed when the cesium loading is less
than about 80 meq by heating the cesium containing phlogopite to at least
150.degree. C. for a period of time sufficient to partially dehydrate the
phlogopite and reduce the c-axis spacing. Generally, a heating time of
about an hour, depending on the size of the sample has been found
sufficient. Since interlayer collapse is believed caused by dehydration,
there is no minimum loading of cesium on the modified phlogopite before
fixation can take place by heating. However, a modified phlogopite
containing no cesium ions and only sodium ions will require higher
temperatures before any dehydration can take place.
The following examples are given to illustrate the invention and are not to
be taken as limiting the scope of the invention which is defined by the
appended claims.
EXAMPLE I
5 grams of phlogopite mica was ground to a fine powder having a particle
size ranging from about 0.2 to 20.0 um. 5 grams of this powder was
contacted with 100 ml of an aoueous solution of about 1.0 N NaCl, 0.3 N
sodium tetraphenylboron and 0.01 H ethylene-diamine tetraacetic acid for a
period of 24-48 hours. The powder was removed from the solution, washed
with water and acetone, dried and characterized by powder x-ray
diffraction. The diffraction showed that the c-axis (001) spacing was
12.23.ANG., as shown in FIG. 2. This is compared to the original phogopite
mica spacing of 10.03.ANG. shown by FIG. 1.
EXAMPLE II
A 0.015 gm sample of modified phlogopite of Example I was placed in 15 ml
of a CsCl solution containing 26.5 mg Cs per ml for 4 days. The solid and
solution phases were separated by centrifugation after equilibration. The
solution was analyzed for Cs.sup.+ by atomic absorption spectroscopy
(AAS), and the solid phase was characterized by powder x-ray diffraction
(XRD). In a similar manner, a number of tests were made with a constant
solid solution ratio, but increasing amount of cesium. The cesium exchange
solution of modified phlogopite in the presence of pure CsCl solutions
showed that a steady state was attained at a cesium loading of 124.5 mq/gm
which is the equivalent of 93.7 meq/100 g in the presence of Na.sup.+
released from the interlayers during equilibration. The results are shown
in FIG. 4. The K-depleted phlogopite mica has a theoretical exchange
capacity of about 210 meq/100 g so that the cesium exchange that occured
was incomplete.
EXAMPLE III
The cesium loaded philogopite of Example II was characterized by powder
x-ray diffraction. FIG. 3 is a diffractogram which shows that the c-axis
spacing decreased to 11.58.ANG. from 12.23.ANG.. Further examination shows
that cesium mica has formed as revealed by the 10.65.ANG.c-axis spacing
that can be derived from the d(002), d(003) and d(004) spacings of
5.326.ANG., 3.557.ANG. and 2.661.ANG. respectively.
EXAMPLE IV
0.020 gms of the modified phlogoplte as prepared in Example I was contacted
with a solution containing 25 ml containing 0.0002 M CsCl, 0.01 M
CaCl.sub.2 and 0.04 M NaNO.sub.3 to determining the selectivity of the
phlogopite for cesium ions in the presence of excess Na.sup.+ and
Ca.sup.++ ions. In a similar manner, like quantities of other cation
exchangers known to have an affinity for cesium ions were also tried. The
results are shown in Table I.
TABLE I
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Cesium exchange, K.sub.d (ml/g)
Sample 0.01 M CaCl.sub.2
0.04 M NaNO.sub.3
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K-depleted phlogopite mica
664,000 949,000
gamma-zirconium phosphate
27,700 16,000
Mordenite, Nevada
165,000 4,300
Phillipsite, Nevada
34,500 9,800
Clinoptilolite, California
16,600 4,400
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As shown in the Table, the selectivity of the modified phlogopite is much
higher than that of the other cation exchangers used for cesium recovery.
EXAMPLE V
0.010 gm of the modified phlogopite containg varying amounts of cesium was
placed in a solution composed of 10 ml of solution consisting of 0.005 M
CaCl.sub.2, 0.001 M MgCl.sub.2, 0.00025 M KCl and 0.001 M NaCl, which are
the ions most abundant in natural waters. The phlogopite was allowed to
soak for 24 hours before being removed by centrifugation, and dried. The
solution was then analyzed by atomic absorption spectroscopy. The results
are shown in Table II below.
TABLE II
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Initial amount of Cs
Amount of Cs released,
Percentage of
exchanged, meq/100 g
meq/100 g Cs released
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84.4 0.053 0.06
80.7 0.068 0.08
50.0 0.045 0.09
19.9 0.038 0.19
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As shown by the Table, very little cesium was released. Even the small
amounts released appear to have been displaced from the external surfaces
because the modified phlogopite has an exchange capacity of about 3 to 4
meq/100 g on the surface. Thus, all the cesium that entered the
interlayers of modified phlogopite and most of the surface exchanged
cesium was not released by the above treatment. This data clearly shows
that the cesium has been immobilized by the modified phlogopite at room
temperature without any additional treatment.
EXAMPLE VI
0.15 grams of modified phlogopite containing about 50 meq cesium as heated
to about 200.degree. C. for one hour in order to fix the cesium within the
intelayer. The results of a powder x-ray diffraction are shown in FIG. 5
superimposed on an unheated sample. In a like manner, 0.015 gm sample o
modified phlogopite containing 19.9 meq cesium/100 grams was also heated.
The results are shown in FIG. 6, also superimposed on an unheated sample.
The diffractograms show that the heat treatment at only 200.degree. C. for
one hour decreased the c-axis spacing to 10.13-10.14.ANG. from about
12.0.ANG.. This decrease of the c-axis spacing is a result of the
dehydration and collapse of the interlayers and thus trapping cesium.
As can he seen from the above described specification and examples, the
invention for the recovery and storage of cesium in a hydrated, sodium
phlogopite mica provide a suitable new material for the decontamination,
fixation and long-term storage of cesium.
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