Use of crystalline intergrowth molecular sieves for removing contaminant metal ions from liquid streams

A process for removing contaminant metal ions from a liquid stream is disclosed. The process involves contacting the liquid stream with a crystalline molecular sieve which has a crystal structure which is an intergrowth of the pharmacosiderite and sitinakite structures. The molecular sieve has an empirical formula of: EQU A.sub.((4-4x)(n) (M.sub.x Ti.sub.1-z Ge.sub.y).sub.4 (Ge.sub.1-p Si.sub.p).sub.q O.sub.r where A is a cation such as sodium or potassium and M is a metal such as niobium or tantalum. These molecular sieves are particularly effective in removing cesium and strontium ions from aqueous streams.

FIELD OF THE INVENTION 
This invention relates to a process for removing contaminant metal ions 
such as cesium from liquid streams, especially aqueous streams using a 
novel molecular sieve which has a crystal structure which is an 
intergrowth of the pharmacosiderite and sitinakite structures. 
BACKGROUND OF THE INVENTION 
Zeolites are crystalline aluminosilicate molecular sieves which have a 
microporous three-dimensional framework structure. In general, the 
crystalline zeolites are formed from corner-sharing AlO.sub.2 and 
SiO.sub.2 tetrahedra and are characterized by having pore openings of 
uniform dimensions, having a significant, ion-exchange capacity and being 
capable of reversibly desorbing an adsorbed phase which is dispersed 
throughout the internal voids of the crystal without significantly 
displacing any atoms which make up the permanent crystal structure. 
Zeolites can be represented on an anhydrous basis, by the empirical formula 
EQU M.sub.2/n O:Al.sub.2 O.sub.3 :XSiO.sub.2 
where M is a cation having the valence n, X is generally equal to or 
greater than 2. In naturally occurring zeolites, M can be Li, Na, Ca, K, 
Mg and Ba. The M cations are loosely bound to the known. 
Other crystalline microporous compositions are known which are not 
zeolitic, i.e., do not contain AlO.sub.2 and SiO.sub.2 tetrahedra as 
essential framework constituents, but which exhibit the ion-exchange 
and/or adsorption characteristics of the zeolites. One such group of 
microporous compositions, i.e., molecular sieves, contain titanium and 
silicon as the framework elements. For example, U.S. Pat. No. 3,329,481 
discloses a crystalline titano-silicate molecular sieve having the x-ray 
diffraction pattern of pharmacosiderite. U.S. Pat. No. 4,853,202 discloses 
a titano-silicate molecular sieve having large pores, while U.S. Patent 
No. 4,938,939 discloses a small pore titano-silicate molecular sieve. 
Sandomirskii and Belov, in Sov. Phys. Crystallogr., 24(6), 
November-December 1979, pp. 686-693 report on the structure of an alkali 
titanosilicate known as zorite. Sokolova et al. in Sov. Phys. Dokl., 
34(7), July 1989, pp. 583-585 disclose the structure of a natural sodium 
titanosilicate. This mineral has a unique structure related in only one 
crystallographic direction to the pharmacosiderite structure, with unique 
structural features in the other two principal crystallographic 
directions. This new mineral was designated SiTNaKite by the geologist who 
originally discovered it in Zap. Vseross Mineral O-va 121(1), 1992, pp. 
94-99. More recently, Poojary, Cahill and Clearfield reported the 
preparation and structural characterization of a porous titanosilicate 
with the sitinakite structure in Chem. Mater., 6, 1994, pp. 2364-2368. 
One property of these molecular sieves is that they can undergo cation 
exchange. For example, the alkali metal cations present in these molecular 
sieves can be exchanged for other metals such as cesium, strontium, 
mercury, and silver cations. Owing to this property, these molecular 
sieves can be used to remove various metals from waste streams or may find 
utility in hydrometallurgical separations of technologically important or 
precious metals. The effectiveness of any one molecular sieve is 
determined primarily by its ring or channel diameter, framework charge 
density and dimensionality of the intracrystalline pores. In particular, 
the pharmacosiderite structure has a three-dimensional pore structure 
which offers facile diffusion of cations such as sodium, potassium, 
strontium, mercury and silver, although the structures selectivity for 
certain cations is inferior to that of sitinakite. The sitinakite 
structure has a one-dimensional pore system which has been shown to 
display high selectivity for cations such as strontium and cesium but 
which potentially displays diffusion kinetic limitations due to the low 
dimensionality of its channel system. 
Applicant has synthesized molecular sieves which have a structure which is 
an intergrowth of the pharmacosiderite and the sitinakite structures. What 
this means is that these novel molecular sieves display the beneficial ion 
exchange characteristics of both the pharmacosiderite and sitinakite 
structures. The molecular sieves of this invention have an empirical 
formula of 
EQU A.sub.((4-4x)(n) (M.sub.x Ti.sub.1-z Ge.sub.y).sub.4 (Ge.sub.1-p 
Si.sub.p).sub.q O.sub.r 
where A is an exchangeable cation selected from the group consisting of 
alkali metals, alkaline earth metals, hydronium ion, ammonium ions, 
alkylammonium ions having C.sub.1 or C.sub.2 alkyl groups and mixtures 
thereof, n is the valence of A and has a value of +1 or +2, M is a metal 
selected from the group consisting of niobium, tantalum, antimony or 
mixtures thereof, x has a value from about 0.01 to about 0.99, z=x+y, y 
has a value from 0 to 0.75, p has a value from 0 to about 1, q has a value 
from about 2.01 to about 2.99 and r has a value from about 14.02 to about 
15.98. 
It has been found that these novel molecular sieves have good ion exchange 
properties, especially for cesium ions. 
SUMMARY OF THE INVENTION 
This invention relates to a process for purifying a waste stream using 
novel molecular sieves. One specific embodiment is a process for removing 
a metal ion contaminant from a liquid stream comprising contacting the 
stream with a molecular sieve for a time sufficient to adsorb the metal 
contaminant onto the molecular sieve, the molecular sieve characterized in 
that it has a crystal structure which is an intergrowth of 
pharmacosiderite and sitinakite structures and has a chemical composition 
represented by an empirical formula: 
EQU A.sub.((4-4x)(n) (M.sub.x Ti.sub.1-z Ge.sub.y).sub.4 (Ge.sub.1-p 
Si.sub.p).sub.q O.sub.r 
where A is an exchangeable cation selected from the group consisting of 
alkali metals, alkaline earth metals, hydronium ion, ammonium ions, 
alkylammonium ions having C.sub.1 or C.sub.2 alkyl groups and mixtures 
thereof, n is the valence of A and has a value of +1 or +2, M is a metal 
selected from the group consisting of niobium, tantalum, antimony or 
mixtures thereof, x has a value from about 0.01 to about 0.99, z=x+y, y 
has a value from 0 to 0.75, p has a value from 0 to about 1, q has a value 
from about 2.01 to about 2.99 and r has a value from about 14.02 to about 
15.98. 
This and other objects and embodiments of the invention will become more 
apparent after a more detailed description of the invention.

DETAILED DESCRIPTION OF THE INVENTION 
One embodiment of the present invention is a novel molecular sieve which is 
an intergrowth of the pharmacosiderite and sitinakite structures. The 
molecular sieve of this invention is represented by the empirical formula 
of: 
EQU A.sub.((4-4x)(n) (M.sub.x Ti.sub.1-z Ge.sub.y).sub.4 (Ge.sub.1-p 
Si.sub.p).sub.q O.sub.r 
where A is a cation having a valence of n where n is +1 or +2 and A is 
selected from the group consisting of alkali metals, alkaline earth 
metals, hydronium ions, ammonium ions, alkylammonium ions having C.sub.1 
or C.sub.2 alkyl groups, and mixtures thereof. The alkali metals include 
sodium, potassium, rubidium, lithium, and cesium, the alkaline earth 
metals include magnesium, calcium, strontium and barium, while the 
alkylammonium cations include tetramethylammonium cations and the 
protonated forms of ethylenediamine and methylamine. A mixture of sodium 
and potassium is preferred. M is a metal selected from the group 
consisting of niobium, tantalum, antimony and mixtures thereof. The other 
variables have the following values: x has a value from about 0.01 to 
about 0.99; z=x+y, y has a value from 0 to 0.75, p has a value from 0 to 
about 1; q has a value from about 2.01 to about 2.99 and r has a value 
from about 14.02 to about 15.98. 
The molecular sieves of the present invention are intergrowths of the 
pharmacosiderite and sitinakite structures. By intergrowth is meant that 
both structures are present in a major portion of the crystals in a given 
sample. This intergrowth of structures is possible when the two structures 
have nearly identical spacial arrangements of atoms along certain 
directions of their crystal structure. FIG. 1 shows a polyhedral drawing 
of the A-B plane (which is equivalent to the B-C and A-C planes because of 
the cubic structure) of pharmacosiderite, as well as the A-B plane of the 
sitinakite structure. FIG. 2 shows a polyhedral drawing of the B-C plane 
(which is equivalent to the A-C plane because of the tetragonal structure) 
of sitinakite. It can be seen from these drawings that the A-B plane of 
sitinakite is identical to that of pharmacosiderite, thereby allowing the 
two structures to intergrow with interfaces at the compatible A-B planes, 
as shown in FIG. 3. 
An intergrowth is not a physical mixture of the two molecular sieves. 
Electron diffraction, transmission electron microscopy and x-ray 
diffraction analysis are employed to show that a material is an 
intergrowth instead of a physical mixture. Usually lattice image data of 
the crystals is most definitive in determining whether one has produced an 
intergrowth because it provides direct visual evidence of the existence of 
both structures within one crystal. 
Intergrowth of the two structures can be seen within individual crystals by 
imaging the crystals in a direction perpendicular to the intergrowth 
vector, which corresponds to the C-direction of the tetragonal unit cell 
of the sitinakite structure and the &lt;100&gt; direction of the isotropic 
pharmacosiderite structure. The intergrowths are indicated by the 
appearance of bands of different thicknesses. Bands of 5.9.+-.0.2 .ANG., 
corresponding to sections of the sitinakite structure, are interspersed 
with bands of 7.8.+-.0.2 .ANG., corresponding to sections of the 
pharmacosiderite structure in the lattice images. FIG. 4 shows the lattice 
image of an intergrowth sample prepared in Example 1. Sections of 
pharmacosiderite that are present in the intergrowth structure are 
indicated with arrows. The thinner sections making up the balance of the 
crystal in the image correspond to blocks of the sitinakite structure. 
The x-ray diffraction patterns of these intergrowths contain at least one 
peak at a d-spacing between 7 .ANG. and 8 .ANG. with a relative intensity 
of 100. More specifically, the x-ray powder diffraction patterns display a 
combination of sharp and broad peaks. The sharp diffraction peaks 
primarily coincide with the hk0 index reflections that are common to the 
two end member structures while most of the diffraction peaks that would 
have indexed as hkl in sitinakite when l is a non-zero integer are broad 
or absent in the intergrowths. 
Since the intergrowth sieves of this invention can have varying amounts of 
the two separate components, it is to be understood that the relative 
intensity and line width of some of the diffraction lines will vary 
depending on the amount of each structure present in the intergrowth. 
Although the degree of variation in the x-ray powder diffraction patterns 
is theoretically predictable for specific intergrown structures, the more 
likely mode of intergrowth is random in nature and therefore difficult to 
predict without the use of large hypothetical models as bases for 
calculation. Qualitatively, most of the variations in the x-ray patterns 
can be predicted to be in those reflections which contain contributions 
from the intergrowth vector direction, or the hkl reflections with 
non-zero l values. 
The intergrowth molecular sieves are prepared by hydrothermal 
crystallization of a reaction mixture containing reactive sources of the 
desired elements and water. For the A cation, the reactive sources include 
potassium hydroxide, sodium hydroxide, cesium hydroxide, rubidium 
hydroxide, sodium carbonate, potassium carbonate, cesium carbonate, 
rubidium carbonate, barium hydroxide, barium carbonate, 
tetramethylammonium hydroxide, ethylenediamine, methylamine and ammonium 
hydroxide. When A is an amine it becomes protonated during the mixing of 
the reaction mixture. Specific examples of the reactive sources of 
germanium are germanium oxide, germanium tetrachloride and germanium 
alkoxides, e.g., germanium ethoxide and germanium isopropoxide. Specific 
examples of M sources are niobium pentaoxide hydrate, tantalum ethoxide, 
antimony trichloride, niobium oxalate, niobium ethoxide, freshly 
precipitated hydrated niobium oxide. Sources of titanium include titanium 
trichloride, titanium tetrachloride, titanium tetraethoxide and amorphous 
titanium oxide. Finally, examples of reactive sources of silicon include 
sodium silicate, fumed silica, precipitated silica and silicon 
tetrachloride. 
Generally, the hydrothermal process used to prepare the intergrowth 
molecular sieves of this invention involves forming a reaction mixture 
which is expressed by the formula in terms of mole ratios of the oxides 
of: 
EQU aA.sub.2/n O:bM.sub.2 O.sub.5 :cGeO.sub.2 :dTiO.sub.2 :eSiO.sub.2 :fH.sub.2 
O 
where a has a value from about 0.5 to about 4, n is the valence of A, b has 
a value from about 0.05 to about 1.0, c has a value from 0 to about 0.8, d 
has a value from about 0.25 to about 1.0, e has a value from about 0.2 to 
about 1.3, and f has a value from about 25 to about 300. 
The pH of the mixture needs to be adjusted to a value of about 8 to about 
14 and preferably from about 10 to about 13. The pH can be adjusted by 
adding an hydroxide such as sodium hydroxide, potassium hydroxide, cesium 
hydroxide, or tetramethylammonium hydroxide. 
Having formed the reaction mixture, it is next reacted at a temperature of 
about 130.degree. C. to about 225.degree. C. for a period of time of about 
4 to about 336 hours. The reaction is carried out under atmospheric 
pressure or the reaction vessel may be sealed and the reaction run at 
autogenous pressure. Preferably the reaction is run at a temperature of 
about 150.degree. C. to about 200.degree. C. and a time of about 24 to 
about 168 hours. 
One function of the A cation is to act as a structure directing agent. 
Since the A cation acts as a structure directing agent, a portion of the A 
cation will be present in the pores of the molecular sieve. These A 
cations can be exchanged for other cations using ion exchange methods well 
known in the art. For example when A is an alkali metal it can be 
exchanged with ammonium ions to give the ammonium form of the molecular 
sieve. 
Due to their approximately 4 .ANG. pore size, the crystalline materials of 
this invention are capable of separating water and other small molecules 
from larger molecular species and can thus be used as desiccants, gas 
drying agents, as well as in separations of ammonia and hydrogen from gas 
streams. 
The crystalline materials of this invention are also capable of selective 
ion exchange of various contaminant metal ions from liquid streams such as 
aqueous streams thereby removing these metals from the liquid streams. 
Illustrative of the contaminant metal ions which can be removed from 
liquid stream are cesium, strontium, mercury, silver, lead, transition 
metal ions, lanthanide metal ions and actinide metal ions. These metal 
ions can be removed from the liquid stream by contacting the stream with 
the molecular sieve for a time sufficient to remove the metal ions and 
trap them on the molecular sieve. The contacting can be carried out either 
in a batch mode or in a continuous mode. In a batch mode, the desired 
molecular sieve is placed in an appropriate container and the stream to be 
treated mixed therewith. Contacting is carried out for a time of about 0.1 
to about 100 hr. In a continuous mode, the molecular sieve is placed in a 
column and the stream to be treated is flowed through it, usually 
downflow, until the contaminant metal is detected in the effluent of the 
column. 
Additionally, as stated above, the molecular sieves can be exchanged with a 
different cation prior to its use in an ion exchange process. The criteria 
for choosing the cation are: 1) compatibility with the solution to be 
treated and 2) the relative ion exchange selectivities of the cation 
versus the metal ion to be removed. Such modifications of molecular sieves 
are well known in the art. For example, if the molecular sieve is 
synthesized in the potassium form and the stream contains sodium ions in 
addition to contaminant ions, the potassium ion should preferably be 
exchanged with sodium ions prior to using the molecular sieve to remove 
contaminants in order to prevent adding potassium ions to the treated 
stream. 
In order to more fully illustrate the instant invention, the following 
examples are set forth. It is to be understood that the examples are only 
by way of illustration and are not intended as an undue limitation on the 
broad scope of the invention as set forth in the appended claims. 
EXAMPLE 1 
In a container equipped with a magnetic stirrer 2.81 g of NaOH and 1.36 g 
of KOH were mixed with 19.2 g of distilled water. To this solution there 
were added 2.06 g of Nb.sub.2 O.sub.5.nH.sub.2 O and the mixture was mixed 
well. In a separate container 11.0 g of Ti(OC.sub.3 H.sub.7).sub.4 and 9.2 
g of tetraethylorthosilicate were mixed well to give a clear solution. 
This clear solution was now added dropwise to the solution containing 
sodium, potassium and niobium and the resultant mixture stirred for 10 
minutes. Finally, 95.6 g of distilled water was slowly added, followed by 
stirring for 15 minutes. The pH of this reaction mixture was 12.89. A 
portion of the reaction mixture (34.2 g) was placed in a reactor and 
heated at 200.degree. C. for 24 hours. After this time, the mixture was 
vacuum filtered, the solid was washed with distilled water and dried in 
air at room temperature. Elemental analysis showed that this material had 
the composition: 
EQU 0.38 Na.sub.2 O:0.21 K.sub.2 O:0.18 Nb.sub.2 O.sub.5 :1.0 SiO.sub.2 :1.24 
TiO.sub.2 :2.41 H.sub.2 O 
A sample of the intergrowth product was analyzed by Transmission Electron 
Microscopy (TEM) to obtain lattice imaging of the crystals. The lattice 
image of this material is presented in FIG. 4. The arrows in FIG. 4 
indicate the sections of pharmacosiderite present in the intergrowth 
sample. The thinner sections making up the balance of the crystal in the 
image correspond to blocks of the sitinakite structure. 
EXAMPLE 2 
A sample of Example 1 was tested to determined its ability to adsorb cesium 
by determining its cesium distribution coefficient (K.sub.d) as follows. 
On hundred milligrams of the sample was placed into a 25 mL polyethylene 
terephthalate (PET) plastic vial. To this vial there were added 10 mL of a 
solution containing 5.7M NaNO.sub.3, 0.6M NaOH and a 100 mg/L 
concentration of cesium chloride solution. The vial was capped and placed 
in an environmental orbital shaker maintained at 25.degree. C. The sample 
was agitated for about 18 hours at 300 revolutions per minute, removed 
from the shaker and the powder material allowed to settle. Next the 
supernate was vacuum filtered with a 0.2 micron membrane filter, diluted 
10:1 and then analyzed for cesium by flame atomic absorption spectroscopy. 
The K.sub.d value was calculated using the following formula: 
##EQU1## 
where: V=volume of waste simulant (mL) 
Ac=concentration of cation absorbed on ion-exchanger (g/mL) 
W=mass of ion-exchanger evaluated (g) 
Sc=concentration of cation in post reaction supernate (g/mL) 
The sample of Example 1 was found to have a cesium K.sub.d of 300. The 
above K.sub.d was obtained using the weight of the sample as synthesized. 
On an anhydrous basis, the K.sub.d was 361. 
EXAMPLE 3 
In a container equipped with a magnetic stirrer 0.7 g of NaOH and 0.3 g of 
KOH were mixed with 27.4 g of distilled water. To this solution there were 
added 0.5 g of Nb.sub.2 O.sub.5.nH.sub.2 O and the mixture was mixed well. 
In a separate container 2.62 g of Ti(OC.sub.3 H.sub.7).sub.4 and 2.20 g of 
tetraethylorthosilicate were mixed well to give a clear solution. This 
clear solution was now added dropwise to the solution containing sodium, 
potassium and niobium and the resultant mixture stirred for 2 hours at 
room temperature. The pH of this reaction mixture was 12.06 and the 
mixture had the following composition: 
EQU 9.24 K.sub.2 O:0.71 Na.sub.2 O:0.15 Nb.sub.2 O.sub.5 :0.77 TiO.sub.2 :0.87 
SiO.sub.2 :125 H.sub.2 O 
The reaction mixture was placed in a reactor and reacted at 200.degree. C. 
for 72 hours under autogenous pressure with mixing. At the end of this 
time period, the solid was isolated, washed with distilled water and dried 
in air at room temperature. 
A sample of this product was analyzed by diffraction and TEM which showed 
that it was an intergrowth of the pharmacosiderite and sitinakite 
structures. A sample of this product was also tested according to the 
procedure of Example 2 and was found to have a K.sub.d of 350 on an 
anhydrous basis. 
EXAMPLE 4 
In a container equipped with a magnetic stirrer 6.9 g of NaOH and 3.2 g of 
KOH were mixed with 50.2 g of distilled water. To this solution there were 
added 3.3 g of Nb.sub.2 O.sub.5.nH.sub.2 O and the mixture was mixed well. 
In a separate container 17.5 g of Ti(OC.sub.3 H.sub.7).sub.4 and 14.5 g of 
tetraethylorthosilicate were mixed well to give a clear solution. This 
clear solution was now added dropwise to the solution containing sodium, 
potassium and niobium and the resultant mixture stirred for 20 minutes. 
Next 132.7 g of distilled water was added followed by the dropwise 
addition of 16.2 g of a 50% KOH solution and finally the dropwise addition 
of 35.64 g of a 50% NaOH solution. The pH of this reaction mixture was 
13.4 and the mixture had the following composition: 
EQU 1.26 K.sub.2 O:3.85 Na.sub.2 O:0.15 Nb.sub.2 O.sub.5 :0.77 TiO.sub.2 :0.87 
SiO.sub.2 :140 H.sub.2 O 
One portion of the above mixture was reacted at 200.degree. C. for 120 
hours (Sample A) while a second portion was reacted at 170.degree. C. for 
72 hours (Sample B) under autogenous pressure. After this time, both 
products were isolated, washed with distilled water and dried in air at 
room temperature. an intergrowth of the pharmacosiderite and sitinakite 
structures. Portions of Samples A and B were tested for cesium adsorption 
per Example 2 and were found to have the following K.sub.d on an anhydrous 
basis: 
EQU Sample A K.sub.d =353 
EQU Sample B K.sub.d =289 
EXAMPLE 5 
In a container equipped with a magnetic stirrer 8.58 g of NaOH and 4.01 g 
of KOH were mixed with 342.4 g of distilled water. To this solution there 
were added 6.22 g of Nb.sub.2 O.sub.5.nH.sub.2 O and the mixture was mixed 
well. In a separate container 33.0 g of Ti(OC.sub.3 H.sub.7).sub.4 and 
27.0 g of tetraethylorthosilicate were mixed well to give a clear 
solution. This clear solution was now added dropwise to the solution 
containing sodium, potassium and niobium and the resultant mixture 
stirred. The pH of this reaction mixture was 11.96 and the mixture had the 
following composition: 
EQU 9.24 K.sub.2 O:0.71 Na.sub.2 O:0.15 Nb.sub.2 O.sub.5 :0.77 TiO.sub.2 :0.87 
SiO.sub.2 :125 H.sub.2 O 
The mixture was placed into an 0.6 liter Parr Stirred Reactor and reacted 
at 200.degree. C. for 72 hours with stirring at 150-200 RPM under 
autogenous pressure. After this time, the solid was isolated, washed with 
distilled water and dried in air at room temperature. 
A portion of this product was analyzed by x-ray diffraction which showed 
that it was an intergrowth of the pharmacosiderite and sitinakite 
structures. Elemental analysis gave the following empirical formula: 
EQU 0.33 K.sub.2 O:0.67 Na.sub.2 O:0.28 Nb.sub.2 O.sub.5 :1.61 TiO.sub.2 :1.0 
SiO.sub.2 :3.83 H.sub.2 O 
Another portion of this product was tested for Cs adsorption per Example 2 
and was found to have a K.sub.d of 454 on an anhydrous basis.