Uranium adsorbent

A uranium adsorbent is prepared by reacting a polymer containing nitrile groups with hydroxylamine, wherein the amount of imidedioxime groups formed is relatively larger than that of amidoxime groups formed in terms of the integral values of areas of peaks respectively assigned to imidedioxime groups and amidoxime groups in the .sup.13 C-NMR spectrum chart of the adsorbent. The reaction may be effected: 1) in an organic solvent containing water under substantially neutral or weakly alkaline conditions, or 2) under acidic conditions. The uranium adsorptivity of the adsorbent is improved by hydrolyzing its residual nitrile groups under alkaline conditions.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a uranium adsorbent for use in trapping 
uranyl ions, and more particularly to a uranium adsorbent for use in 
trapping uranyl ions contained in aqueous solutions such as seawater. 
2. Discussion of the Related Art 
Attempts to recover uranyl ions from aqueous solutions containing uranyl 
ions, such as seawater and water drained from dams, have recently been 
made in various quarters. Uranyl ions have been attracting attention as a 
fuel material of the atomic energy which has accounted for an increasing 
proportion among the various sources of energy production these days. 
Thus, developed nations have been competing in research with a view to 
developing effective methods of recovering uranyl ions from inexhaustible 
seawater. 
Conventional methods of trapping uranyl ions contained in an aqueous 
solution such as seawater include a process comprising the step of 
contacting an aqueous solution such as seawater with an adsorbent such as 
a polymer adsorbent bearing an inorganic metallic oxide (e.g. titanic 
acid) or a macrocyclic hexacarboxylic acid, or an amidoxime-containing 
adsorbent prepared by reacting a nitrile-containing polymer with 
hydroxylamine, and the step of eluting uranyl ions adsorbed on the 
adsorbent. 
There are a number of known serviceable forms of amidoxime-containing 
adsorbents, examples of which include not only grainy and fibrous 
adsorbents prepared using as a starting material a nitrile-containing 
polymer available at a low price, but also composite adsorbents comprising 
an amidoxime-containing substance in powdery form incorporated into a 
polymer matrix. 
As described above, an adsorbent containing amidoxime groups is prepared by 
a reaction of a polymer containing nitrile groups with hydroxylamine. It 
has been said that, when hydroxylamine is used in the form of an aqueous 
solution thereof in the above-mentioned reaction, a large number of 
functional groups such as hydroxamic acid groups, carboxylic amido groups 
and carboxylic acid groups, which take little part in adsorption of 
uranium, are formed as by-products through hydrolysis of amidoxime groups 
in addition to formation of amidoxime groups, which have heretofore been 
believed to take direct part in adsorption of uranium. In view of this, it 
has been recommended that hydroxylamine should be used in the form of a 
methanol solution thereof rather than an aqueous solution thereof in order 
to secure amidoxime groups (see Egawa, Nippon Kagaku Kaishi, 1980, p. 
1767). 
However, an amidoxime-containing adsorbent prepared using a methanol 
solution of hydroxylamine is generally poor in the capacity of adsorbing 
uranyl ions contained in seawater (hereinafter referred to as the "uranium 
adsorptivity"). A treatment of the above-mentioned amidoxime-containing 
adsorbent with an alkali to improve the uranium adsorptivity thereof was 
proposed (see Kato et al., Bulletin of the Society of Sea Water Science, 
Japan, Vol. 35, p. 156 (1881), and Japanese Patent Publication No. 
16,812/1884). The reason for such an improvement in the uranium 
adsorptivity of an amidoxime-containing adsorbent through a treatment 
thereof with an alkali has not thoroughly been elucidated yet, but an 
increase in the hydrophilicity of the adsorbent through hydrolysis of 
nitrile groups remaining in the above-mentioned amidoxime-containing 
adsorbent is claimed to be responsible for the improvement (Kato et al, 
Nippon Kagaku Kaishi, 1982, p. 1455). 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a uranium adsorbent which 
is much higher in uranium adsorptivity than the aforementioned 
conventional uranium adsorbents. 
In accordance with the present invention, there is provided a uranium 
adsorbent prepared by reacting a polymer having nitrile groups with 
hydroxylamine, wherein the amount of imidedioxime groups formed is 
relatively larger than the amount of amidoxime groups formed as determined 
by the integral values (areas) of peaks respectively assigned to 
imidedioxime groups and amidoxime groups in the .sup.13 C-NMR spectrum 
chart of the uranium adsorbent. 
Such a uranium adsorbent can be prepared, for example, by reacting a 
nitrile-containing polymer with hydroxylamine in an organic solvent 
containing water under substantially neutral or weakly alkaline 
conditions, or reacting a nitrile-containing polymer with hydroxylamine 
under acidic conditions, as will be described later in detail. 
In accordance with the present invention, there also is provided a uranium 
adsorbent improved in uranium adsorptivity which adsorbent is prepared by 
hydrolyzing nitrile groups remaining in the uranium adsorbent of the kind 
as described above and containing the residual nitrile groups under 
alkaline conditions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention will now be described in detail. 
The present inventors have not only studied the mechanism of uranium 
adsorption but have also made extensive investigations with a view to 
developing a uranium adsorbent superior in uranium adsorptivity to the 
conventional adsorbents. As a result, they have made the following new 
findings. 
It has been discovered that imidedioxime groups are higher in uranium 
adsorptivity than amidoxime groups which have heretofore been believed to 
be functional groups taking direct part in the adsorption of uranium. 
It has also been discovered that imidedioxime groups are formed in a 
relatively larger amount (with respect to the aforementioned integral 
values in the aforementioned NMR spectrum chart; the same basis of 
comparison will apply hereinafter) than amidoxime groups in a reaction of 
a polymer containing nitrile groups with hydroxylamine when the reaction 
is effected in a mixed solvent system of water and an organic solvent 
under substantially neutral or weakly alkaline conditions. 
It has further been discovered that imidedioxime groups are formed in a 
relatively larger amount than amidoxime groups in a reaction of a polymer 
containing nitrile groups with hydroxylamine when the reaction is effected 
under acidic conditions. 
It has still further been discovered that, when the uranium adsorbent of 
the present invention, prepared in the manner as described above to form a 
relatively larger amount of imidedioxime groups than that of amidoxime 
groups, is superior in uranium adsorptivity to the conventional uranium 
adsorbents, when used to adsorb thereon uranyl ions. 
It has still further been discovered that, when the uranium adsorbent of 
the present invention, prepared in the manner as described above to form a 
relatively larger amount of imidedioxime groups than that of amidoxime 
groups while leaving some nitrile groups intact, is subjected to 
hydrolysis of the residual nitrile groups under alkaline conditions, the 
uranium adsorptivity of the resultant adsorbent is remarkably increased. 
Although it has already been known that a treatment of a uranium adsorbent 
containing amidoxime groups with an alkali improves the uranium 
adsorptivity of the adsorbent as described hereinbefore, the 
above-mentioned uranium adsorbent according to the present invention which 
has been subjected to hydrolysis of the residual nitrile groups under 
alkaline conditions is improved in uranium adsorptivity to an extremely 
large extent, as compared with the above-mentioned conventional alkali 
treatment. 
The reason for such a remarkable increase in the uranium adsorptivity of 
the polymer adsorbent according to the present invention through 
hydrolysis of the residual nitrile groups thereof under alkaline 
conditions has not been elucidated yet, but is believed to be that an 
increase in the hydrophilicity of the polymer adsorbent through hydrolysis 
of the residual nitrile groups thereof is extremely effectively in the 
case of the present invention. 
A description will now be made of processes for producing a uranium 
adsorbent according to the present invention. 
Examples of the polymer containing nitrile groups that can be used for the 
production of the uranium adsorbent of the present invention include 
homopolymers of monomers containing a nitrile group(s), copolymers of 
different monomers containing a nitrile group(s) with each other, and 
copolymers of monomers containing a nitrile group(s) with other types of 
comonomer(s). 
Examples of the monomers containing a nitrile group(s) that can be used in 
the present invention include acrylonitrile .alpha.-substituted with a 
monomer containing a nitrile group(s). Examples of such compounds include 
olefins such as ethylene, propylene, and butenes; unsaturated 
monocarboxylic acids such as acrylic acid and methacrylic acid; various 
acrylic and methacrylic esters; aliphatic vinyl compounds such as vinyl 
chloride, vinyl fluoride, vinyl alcohol, vinyl acetate, and vinyl ethyl 
ether; vinylidenes such as vinylidene chloride and vinylidene fluoride; 
aromatic vinyl compounds and their derivatives such as styrene, 
.alpha.-methylstyrene, and vinyl toluene; heterocyclic vinyl compounds 
such as vinylcarbazole and vinylpyrrolidone; dienes such as butadiene; 
divinyl compounds such as divinylbenzene and divinyltoluene; unsaturated 
dicarboxylic acids and their anhydrides such as maleic acid and maleic 
anhydride; allyl compounds such as diallyl phthalate; polyhydric alcohol 
esters of (meth)acrylic acid such as ethylene glycol diacrylate, ethylene 
glycol dimethacrylate, and glycerol dimethacrylate; and mixtures thereof 
In addition to the aforementioned polymers containing nitrile groups, use 
can be made of other nitrile-containing polymers, examples of which 
include cyanoethylated polymers such as cyanoethylated cellulose and 
cyanoethylated polyvinyl alcohol; polymers prepared by treating an active 
halogen-containing polymer such as polyvinyl haloacetal or a 
halomethylated copolymer of an aromatic monovinyl compound and a polyvinyl 
compound with, for example, iminodiacetonitrile to introduce nitrile 
groups thereinto; and condensation resins prepared using as one component 
an aromatic or aliphatic compound having a nitrile group(s), such as 
dicyandiamide, hydroxybenzonitrile or dihydroxybenzonitrile. 
The polymer containing nitrile groups can be prepared by a known method or 
is commercially available. 
The polymer containing nitrile groups may be used in any one of various 
forms including not only grainy, spherical, rod-like, plate-like, 
membranous, tubular, annular and fibrous forms, but also threadlike, 
netty, stringy, cloth-like, felty, mat-like and other forms shaped by 
further fabrication. 
According to the first method of reacting a polymer containing nitrile 
groups with hydroxylamine to form imidedioxime groups, the reaction is 
effected in an organic solvent containing water under substantially 
neutral or weakly alkaline conditions In this first method, a hydrous 
organic solvent such as a lower alcohol containing water is used as a 
solvent, while using hydroxylamine either as such or in the form of a 
solution thereof prepared by adding a substantially equivalent or slightly 
excessive amount of an alkali to a solution of a salt thereof, such as 
sulfate, hydrochloride, phosphate or acetate, to neutralize the salt. The 
reaction is usually effected at a temperature of room temperature to 
110.degree. C., and preferably 80.degree. to 90.degree. C. The reaction 
may be effected under pressure in an autoclave. Because the reactivity of 
hydroxylamine with nitrile groups largely differs from polymer to polymer, 
preferable reaction conditions should be set depending on the kind of 
nitrile-containing polymer chosen. It has heretofore been reported that 
the use of water more or less causes a side reaction to lower the uranium 
adsorption performance of the resulting hydroxylamine-treated polymer. In 
the present invention, however, the use of an organic solvent containing 
water provides a polymer type uranium adsorbent with a high uranium 
adsorptivity wherein the amount of imidedioxime groups formed is 
relatively larger than that of amidoxime groups formed. 
Too large an amount of water unfavorably causes a side reaction to form 
functional groups taking little part in the adsorption of uranium, while 
too small an amount of water unfavorably decreases the yield of 
imidedioxime groups. From the foregoing point of view, the water content 
of the mixed solvent system composed of an organic solvent(s) and water 
is, usually 10 to 90 vol. %, preferably 40 to 60 vol. %, and more 
preferably about 50 vol. %. 
According to the second method of reacting a polymer containing nitrile 
groups with hydroxylamine to form imidedioxime groups, the reaction is 
effected under acidic conditions. Solvent systems usable in the second 
method include organic solvent systems consisting of an organic solvent(s) 
alone such as a lower alcohol(s), and hydrous organic solvent systems such 
as lower alcohols containing water. Use is made of an acidic solution of 
hydroxylamine prepared by adding an alkali to a solution of a salt thereof 
such as sulfate, hydrochloride, phosphate or acetate to partially 
neutralize the salt; or an acidic solution of hydroxylamine prepared by 
adding an acid such as sulfuric acid or hydrochloric acid to a solution of 
hydroxylamine or a solution of a salt thereof such as sulfate, 
hydrochloride, phosphate or acetate which solution has been neutralized to 
neutrality. The reaction is usually effected at a temperature of room 
temperature to 110.degree. C., preferably 60.degree. to 100.degree. C. The 
reaction may be effected under pressure in an autoclave. Because the 
reactivity of hydroxylamine with nitrile groups largely differs from 
polymer to polymer, preferable reaction conditions should be set depending 
on the kind of nitrile-containing polymer chosen. 
Besides the aforementioned lower alcohols such as methanol and ethanol, 
other water-soluble polar solvents such as dimethylformamid, dimethyl 
sulfoxide, tetrahydrofurane and dioxane can be used as organic solvents 
that may be used in either of the foregoing two methods. 
Since the polymer type uranium adsorbent thus prepared according to the 
present invention has a relatively larger amount of imidedioxime groups 
formed than that of amidoxime groups formed, it is superior in uranium 
adsorptivity to the conventional uranium adsorbents. When the 
above-mentioned polymer type uranium adsorbent, if containing some 
residual nitrile groups, is further subjected to hydrolysis of the nitrile 
groups under alkaline conditions, the resulting adsorbent is further 
remarkably improved in uranium adsorptivity. 
Such hydrolysis under alkaline conditions of the uranium adsorbent 
according to the present invention wherein a relatively larger amount of 
imidedioxime groups than that of amidoxime groups have been formed while 
leaving some nitrile groups intact is effected by contacting the 
above-mentioned adsorbent with a solution of an alkali such as potassium 
hydroxide, sodium hydroxide, lithium hydroxide, calcium hydroxide, barium 
hydroxide, potassium carbonate, sodium carbonate, or lithium carbonate. 
The alkali concentration of the above-mentioned solution may be at least 
0.05 N (normal), preferably at least 0.5 N, and can be suitably chosen 
within the range of up to about 10 N, though the upper limit thereof is 
not particularly restricted. 
The treatment temperature and time are widely variable depending on the 
kind of alkali to be used and the alkali concentration of the solution. As 
the alkali concentration is lowered, the treatment must be effected at a 
higher temperature for a longer period of time. The treatment temperature 
and time are usually chosen arbitrarily from within the range of 
ice-cooled bath temperature to 100.degree. C. and from within the range of 
10 minutes to 200 hours, respectively. 
The uranium adsorbent of the present invention can be used to trap and 
recover uranium from all kinds of solutions containing uranium, 
particularly low-concentration aqueous solutions of uranium such as 
seawater. When the uranium adsorbent of the present invention is brought 
into contact with an aqueous solution of uranium, imidedioxime groups form 
a stable chelate with uranium. The chelate formation constant is so high 
that uranium can be highly efficiently adsorbed on the uranium adsorbent 
of the present invention at a high speed of adsorption. 
The adsorbent of the present invention can be used in various manners 
according to the form thereof. For example, the adsorbent of the present 
invention may be packed into a column or tower, through which a solution 
containing uranium is then passed. Alternatively, the adsorbent of the 
present invention may be immersed into a solution containing uranium. 
The adsorbent of the present invention on which uranium is adsorbed is 
subsequently brought into contact with an eluant to elute and recover 
uranium therefrom. Examples of the eluant that can be used include 
inorganic acids such as nitric acid, sulfuric acid, hydrochloric acid, and 
phosphoric acid; carbonates such as sodium carbonate, sodium 
hydrogencarbonate, and ammonium carbonate; various organic acids; and 
various amino acids. 
The present invention will now be specifically illustrated by the following 
Examples, which, however, should not be construed as limiting the scope of 
the present invention. 
EXAMPLE 1 AND COMATIVE EXAMPLE 1 
Two kinds of 3 w/v % (weight/volume %) solutions of hydroxylamine were 
prepared by dissolving hydroxylamine respectively in two kinds of 
solvents, methanol alone (Comparative Example 1) and ethanol containing 50 
vol. % of water (Example 1). 21 ml of each solution was added to 1 g of 
hollow polyacrylonitrile fibers, which were then heat-treated in the 
solution for 24 hours. A small amount of each of the two kinds of thus 
treated fibers were dissolved in d.sub.6 -DMSO and examined by .sup.13 
C-NMR spectroscopy to take an NMR spectrum chart thereof. The integral 
ratio (areal ratio) of a peak at .delta.=148 ppm, assigned to imidedioxime 
groups, to a peak at .delta.=155 ppm, assigned to amidoxime groups, was 
found from the spectrum chart. The results are shown in Table 1. 
TABLE 1 
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Reaction Integral Ratio 
Solvent Temperature (imidedioxime:amidoxime) 
______________________________________ 
methanol 65.degree. C. 
1:3 
ethanol/water 
85.degree. C. 
3:2 
(50%) 
______________________________________ 
FIG. 1 shows the NMR spectrum chart of the uranium adsorbent of Example 1. 
EXAMPLE 2 
1.07 g of hydroxylamine hydrochloride was dissolved in ethanol containing 
50 vol. % of water. 0.788 g of potassium hydroxide was added to the 
resulting solution to neutralize part of the hydroxylamine hydrochloride 
to thereby prepare an acidic solution of hydroxylamine, to which 1.49 g of 
polyacrylonitrile fibers (copolymer of 80 mol % acrylonitrile with 10 mol 
% methyl acrylate) were then added and heat-treated at 85.degree. C. for 
24 hours. A small amount of the thus treated fibers were examined by CPMAS 
(cross polarization magic angle spin) 13C-NMR spectroscopy to take an NMR 
spectrum chart thereof. The integral ratio (areal ratio) of a peak at 
.delta.=150 ppm, assigned to imidedioxime groups, to a peak at .delta.=158 
ppm, assigned to amidoxime groups, was found from the spectrum chart. The 
result is shown in Table 2. 
TABLE 2 
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Reaction Integral Ratio 
Solvent Temperature 
(imidedioxime:amidoxime) 
______________________________________ 
ethanol/water (50%) 
85.degree. C. 
9:1 
______________________________________ 
FIG. 2 shows the NMR spectrum chart of the uranium adsorbent of Example 2. 
EXAMPLE 3 
The uranium adsorbent of Example 2 according to the present invention, 
wherein the amount of imidedioxime groups formed was relatively larger 
than that of amidoxime groups formed, was immersed in a 1 N solution of 
sodium hydroxide at room temperature. The uranium adsorbent thus immersed 
was taken out in a small amount each time at given intervals to take NMR 
spectrum charts thereof by CPMAS .sup.13 C-NMR spectroscopy. It was 
discovered from these spectrum charts that the peak assigned to amidoxime 
groups gradually dwindled as the immersion time was prolonged. In the 
CPMAS .sup.13 C-NMR spectrum chart of the uranium adsorbent, after 24 
hours of the immersion, the peak assigned to imidedioxime groups remained, 
while no substantial peak assigned to amidoxime groups was recognizable 
and a peak at .delta.=184 ppm, assigned to carbonyl groups which newly 
emerged, suggesting that the residual nitrile groups had been hydrolyzed. 
EXAMPLE 4 AND COMATIVE EXAMPLE 2 
100 ml of a 3 w/v % solution of hydroxylamine in a mixed solvent composed 
of ethanol and 50 vol. % water was added to 1 g of acrylonitrile fibers 
(.phi.=12 .mu.m), which were then heat-treated in the solution at 
85.degree. C. for hours. The thus treated fibers were separated from the 
solution by filtration, sufficiently washed with warm water, and 
sufficiently dried in a vacuum to obtain a uranium adsorbent according to 
the present invention (sample A). 
50 mg of the thus obtained uranium adsorbent (sample A) according to the 
present invention was added to 5 liters of natural seawater, followed by 
stirring at 25.degree. C. for 24 hours. The fibers of the adsorbent were 
separated from the seawater by filtration, and washed with distilled 
water. Thereafter, an elution operation with 10 ml of 1 N hydrochloric 
acid was repeated three times. The amount of uranium in the whole 
collected eluate was determined by the Arsenazo III method to find the 
uranium adsorption (i.e., the amount, per day and per unit weight of 
adsorbent, of uranium which had been adsorbed on the uranium adsorbent) 
(Test No. 1). 
20 mg of the above-mentioned fibers of the adsorbent (sample A) was 
immersed in 10 ml of a 1 N solution of sodium hydroxide at room 
temperature for 24 hours. Separately, the same immersion was continued for 
72 hours. The two kinds of fibers after the immersion were each 
neutralized and sufficiently washed with water to prepare two kinds of 
uranium adsorbents according to the present invention which had the 
residual nitrile groups hydrolyzed under alkaline conditions. Each of the 
thus obtained uranium adsorbents was added to 5 liters of natural 
seawater. The uranium adsorption by each of these uranium adsorbents was 
determined in the same manner as described above (Tests Nos. 2 and 3). 
Part of the conventional uranium adsorbent of Comparative Example 1 was 
immersed in an alkali solution in the same manner as described 
hereinbefore for 24 hours. 50 mg of each of the uranium adsorbent of 
Comparative Example 1 and the conventional alkali-treated uranium 
adsorbent thus prepared was added to 5 liters of natural seawater. The 
uranium adsorption by each of these uranium adsorbents was determined in 
the same manner as described hereinbefore (Test Nos. 4 and 5). 
The results are shown in Table 3. 
TABLE 3 
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Integral Ratio 
Uranium 
Test Alkali Immersion 
(imidedioxime/ 
Adsorption 
No. Time (hours) amidoxime) (.mu.gU/g .multidot. Ad .multidot. 
______________________________________ 
day) 
1 0 3/2 100 
2 24 -- 550 
3 72 -- 850 
4 0 1/3 50 
5 24 -- 150 
______________________________________ 
As can be seen in Tables 1 and 2, the uranium adsorbents according to the 
present invention, not subjected to the alkali immersion treatment, had a 
relatively larger amount of imidedioxime groups formed than that of 
amidoxime groups formed. As can be seen in Table 3, the uranium adsorbent 
according to the present invention, not subjected to the alkali immersion 
treatment (Test No. 1), exhibited twice as much uranium adsorption as the 
conventional uranium adsorbent (Test No. 4) having a relatively larger 
amount of amidoxime formed than that of imidedioxime groups formed. 
As can be seen in Table 3, the conventional adsorbent, when subjected to 
the alkali treatment (Test No. 5), exhibited merely about three times as 
much uranium adsorption as when not subjected to the alkali treatment 
(Test No. 4), whereas the above-mentioned uranium adsorbent according to 
the present invention, when subjected to the alkali immersion treatment, 
exhibited 5.5 times (Test No. 2) to 8.5 times (Test No. 3) as much uranium 
adsorption as when not subjected to the alkali immersion treatment (Test 
No. 1), thus proving a remarkably increased effect of hydrolysis of the 
residual nitrile groups under alkaline conditions. 
As will be understandable from the foregoing description, the uranium 
adsorbent of the present invention has various advantages, including a far 
larger uranium adsorption than conventional uranium adsorbents such that 
the amount of uranium adsorbed thereon per unit amount of the adsorbent 
can be greatly increased to provide an eluate having a higher uranium 
concentration than those obtained in the case of the conventional uranium 
adsorbents, particularly when the adsorbent is used to adsorb and separate 
uranium from seawater.