Molecular sieve abstract
A new microporous crystalline molecular sieve material having the
formula Cs.sub.3 TiSi.sub.3 O.sub.95.cndot.3H.sub.2 O and its hydrothermally
condensed phase, Cs.sub.2 TiSi.sub.6 O.sub.15 are disclosed. The
microporous material can adsorb divalent ions of radionuclides or
other industrial metals such as chromium, nickel, lead, copper,
cobalt, zinc, cadmium, barium, and mercury, from aqueous or hydrocarbon
solutions. The adsorbed metal ions can be leached out for recovery
purposes or the microporous material can be hydrothermally condensed
to a radiation resistant, structurally and chemically stable phase
which can serve as a storage waste form for radionuclides.
Molecular sieve claims
What is claimed is:
1. A crystalline microporous structure having the formula Cs.sub.3
TiSi.sub.3 O.sub.9.5.cndot..3H.sub.2 O.
2. A process for the sorption of divalent radionuclide ions from
an aqueous solution comprising contacting said aqueous solution
with the microporous compound of claim 1.
3. A process for the sorption of divalent radionuclide ions from
a hydrocarbon solution comprising contacting said aqueous solution
with the microporous compound of claim 1.
4. The process of claim 2 wherein the radionuclide is strontium.
5. The process of claim 2 wherein the radionuclide is thorium.
6. A process for the sorption of divalent metal ions from an aqueous
solution comprising contacting said aqueous solution with the microporous
compound of claim 1.
7. The process of claim 6 wherein the metals belong to the class
consisting of chromium, nickel, lead, copper, cobalt, zinc, cadmium,
barium, mercury, and mixtures thereof.
8. A condensed crystalline compound having the formula Cs.sub.2
TiSi.sub.6 O.sub.15.
9. The compound of claim 8 containing sorbed strontium ions.
10. The compound of claim 8 containing sorbed thorium ions.
11. The compound of claim 8 containing sorbed divalent metal ions.
Molecular sieve description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a new silicotitanate molecular sieve
ion exchange material for the capture and immobilization of divalent
cations from aqueous and/or hydrocarbon solutions, including elements
such as radioactive strontium or industrial RCRA metal cations.
The invention further relates to the ability to either recycle the
captured metal for future use or to encapsulate the cation through
thermal treatment of the molecular sieve to a condensed phase.
2. Description of the Prior Art
It is known that crystalline silicotitanates (CST; commercially
available through UOP LLP, as IE-911) can selectively remove 100
ppm Cs and Sr cations from 5M Na.sup.+ solutions over a broad pH
range (1 to 14) and, because of this, they have been found effective
to clean up radioactive .sup.137 Cs and .sup.90 Sr from waste tanks
at the US Department of Energy Hanford waste tanks. To immobilize
the Cs-loaded CSTs, there is used a combination of high activity
waste and melting with borosilicate glass to create a glass log
waste form which can be stored indefinitely. However, since titania
induces crystallization and separation in borosilicate glass, the
Cs-CST must be diluted to a few weight percent, thus increasing
the volume and the cost of waste form production. It has already
been shown by Y. Su, [Y. Su, et. al, MRS Conference Proceedings
(Boston) "Evaluation of Cesium Silicotitanates as an Alternative
Waste Form" p. 457 (1997)]. that a waste form that is more
chemically durable than borosilicate glass logs can be generated
by direct thermal conversion of Cs-CST. However, an improvement
in the isolation of divalent radioactive and industrial waste remains
quite desirable.
Therefore, an object of this invention is to provide a new type
of inorganic molecular sieve materials. Another object is to provide
inorganic materials that are mechanically and chemically stable
and are free from the traditional problems associated with organic-based
molecular sieves. A further object is to provide microporous compounds
that can be used for radionuclides and industrial metals sorption.
A still further object is to provide a molecular sieve that can
be back-exchanged by acid wash to recover/recycle the sequestered
metal cation. Still another object is to provide molecular sieves
that can be thermally condensed to form leach resistant phases for
radionuclide storage.
Additional objects, advantages, and novel features of the invention
will become apparent to those skilled in the art upon examination
of the following description, or will be learned by practice of
the invention.
BRIEF SUMMARY OF THE INVENTION
There is now disclosed that the radionuclides .sup.137 cesium,
and to a lesser extent .sup.90 strontium, as well as divalent cations
of several metals can be selectively removed from solution using
a crystalline silicotitanate (CST) ion exchanger, namely a Cs--Si--Ti--O
phase. However, an improved divalent ion exchanger and its condensed
counterpart phase have now been hydrothermally synthesized, characterized,
evaluated, and described in this specification. The viability of
the new materials for divalent sequestration and subsequent encapsulation
is based on chemical, mechanical, and thermal stability, leachability,
and ion exchange capabilities. The two novel Cs--Si--Ti--O phases
are Cs.sub.3 TiSi.sub.3 O.sub.9.5.3H.sub.2 O (SNL-B), a porous phase
which adsorbs the divalent cation metal, and Cs.sub.2 TiSi.sub.6
O.sub.15 (SNL-A), a condensed stable form in which the metal is
immobilized for storage purposes, if that is desired. The two phases
are also identified by their crystallographic parameters: SNL-B,
orthorhombic, unit cell parameters a=10.83 .ANG., b=7.43 .ANG. and
c=7.11 .ANG.; SNL-A, monoclinic, Cc space group, unit cell parameters:
a=12.998 (2) .ANG., b=7.5014 (3) .ANG., c=15.156 (3) .ANG., .beta.=105.80
(3).degree..
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a phase diagram of the Cs.sub.0.5 --TiO.sub.2 --SiO.sub.2
system which shows the proportions of cesium oxide, titania, and
silica that can yield SNL-A and SNL-B under hydrothermal synthesis
conditions.
FIG. 2 illustrates the power diffraction patterns of both the microporous
phase (SNL-B) and the condensed phase (SNL-A) of the silicotitanates
of the invention.
DETAILED DESCRIPTION OF THE INVENTION
There will now be described the synthesis and characterization
of a completely new type of microporous inorganic molecular sieve
material. The general chemical formula of this new material is Cs.sub.3
TiSi.sub.3 O.sub.9.5.cndot.3H.sub.2 O (SNL-B). The material has
a net negatively charged framework comprised of silicon, titanium,
and oxygen elements. The framework is charge balanced by the occlusion
of cesium cations during its synthesis. The material has a strong
affinity and selectivity for divalent cations, thus allowing the
exchange of the cesium ion for other cations.
The CsO.sub.0.5 --TiO.sub.2 --SiO.sub.2 phase diagram shown in
FIG. 1 delineates, inter alia, the various phases formed by the
hydrothermal treatment of various concentrations of the components
of the system and, more relevantly, the ranges of component concentrations
that can yield the products of the invention, SNL-A and SNL-B. As
can be seen in the diagram, the phases which are obtained by hydrothermal
treatment of the proper concentrations of components at 120.degree.
C. for 5 days, include a compound analogous to pharmacosiderite,
TiO.sub.2 (anastase), and four previously unidentified phases, two
of which are SNL-A and SNL-B. The rest of the diagram area is taken
up by amorphous phases.
EXAMPLE 1
Microporous structure SNL-B was prepared by stirring together titanium
isopropoxide (TIPT) and tetraethyl orthosilicate (TEOS). This mixture
was then added dropwise to a 23 mL Teflon liner for a Parr pressure
reactor containing 50% cesium hydroxide solution and amorphous titania.
After stirring 30 minutes, water was added and the mixture was stirred
for 30 minutes more. The final pH of the mixture was 12.5 and the
final reactants stoichiometry was Cs:Ti:Si:H.sub.2 O=4:1:4:383.
The loaded pressure reactor was placed in a 120.degree. C. oven
for two weeks. The product was collected by filtration and washed
with hot water.
The amorphous titania was prepared by base-catalyzed hydrolysis
of TIPT. Prior to use, it was characterized by TGA for water content,
X-ray diffraction, and ICP for Ti content.
Ion exchange, for instance with Sr.sup.++ ions, takes place without
any framework distortion, as indicated by the X-ray diffraction
spectrum of the ion-exchanged microporous phase. In the case of
Sr, the microporous ion exchange material exhibited high selectivity
with a distribution coefficient K.sub.d >100000 ml/g, i.e.,
no Sr detected in solution after contact. In contrast, a K.sub.d
=19 ml/g was measured for adsorption of Na.sup.+ ions onto this
phase. As intimated earlier, the microporous material of the invention,
SNL-B, can be used to adsorb divalent cations of various industrial
and natural metals, including copper, nickel, thorium, lead, chromium,
cobalt, zinc, cadmiumn, barium, mercury, and the like.
EXAMPLE 2
Three portions of microporous ion exchange material, SNL-B, were
heated at 170.degree. C. for periods of 2.5 days, 5 days, and 10
days, respectively. Temperatures other than that used in this example
will also effect the conversion with time, provided that they are
sufficiently high, e.g., 200.degree. C. As shown by the X-ray diffraction
patterns in FIG. 2 the microporous phase, SNL-B, is converted to
the condensed phase, SNL-A, with time and temperature. To be noted,
only the intensity of the peaks in the patterns differ for each
phase.
EXAMPLE 3
The standard PCT (product consistency test) leach test, a common
technique developed to evaluate chemical durability of nuclear waste
form in aqueous environments, was performed on SNL-A, the condensed
phase. A sample of the material, 0.2 g, was placed in a hydrothermal
bomb with water, 10 g, at 90.degree. C. for 1 2 3 7 and 10 days.
After the designated time of heating, each sample was filtered and
the leachate solution was analyzed for cesium concentration by AAS.
The solid product was analyzed by XRD to determine if any phase
changes had occurred. Such leach tests, as well as irradiation experiments,
showed SNL-A to be extremely resistant to both structural damage
and cesium loss by either method.
Applications
It is envisioned that the microporous phase will be used to (1)
selectively sorb a cation of interest, ether be (2) back exchanged
with acid to recover the metal, or (3) thermally treated into a
stable ceramic form, and then sent to a waste storage facility,
if the cation is a radionuclide, with minimized exposure concerns.
The desirable properties of the novel microporous and the condensed
phases of the materials of the invention can be used advantageously
in a number of different sectors, for instance in environmental
clean-up. Because the condensed phase is robust, i.e., chemically,
mechanically, and thermally stable, it can be used in the government
defense waste clean-up at various sites in the country. Many of
these are sites that have separation needs in extreme pH conditions
with a wide variety of competing cations. To illustrate, the microporous
phase of the new materials has shown selectivity for divalent strontium
ions with competing sodium ions at a pH of 2. The materials are
also applicable to private industrial needs in environmental clean-up
by separations for Resource Conservation Recovery Act (RCRA) heavy
metals, such as chromium, nickel, and lead, and for other naturally
occurring metals, including thorium.
Also, since thin film membranes have been successfully synthesized
from novel inorganic molecular sieves, it is contemplated that the
knowledge and experience in this field can easily be applied to
the molecular sieve of this invention. This would, of course, be
of great interest to the chemical and the petroleum refining industries.
Although the invention has been described with preferred and illustrative
embodiments, it will be appreciated by those skilled in the art
that additions, modifications, substitutions, and deletions may
be made without departing from the spirit and the scope of the invention
defined in the appended claims.
REFERENCE
More data on the nature and properties of the novel compounds of
the invention can be found in the following literature article,
which is hereby incorporated by reference into this specification:
M. Nyman et al., "New crystalline silicotitanate (CST) waste
forms: hydrothermal synthesis and characterization of Cs--Si--Ti--O
phases", Mat. Res. Symp. Proc., 556 71-78 (1999). (Note: This
article disclosed much of the invention. However, Volume 556 was
released to the public around Jan. 5 2000.) |