Process for separation and preconcentration of radium from water

A process for preconcentrating and separating radium from a contaminated solution containing at least water and radium includes the steps of adding a quantity of a water-soluble macrocyclic polyether to the contaminated solution to form a combined solution. An acid is added to the combined solution to form an acidic combined solution having an H.sup.+ ! concentration of about 0.5M. The acidic combined solution is contacted with a sulfonic acid-based strong acid cation exchange medium or a organophilic sulfonic acid medium having a plurality of binding sites thereon to bind the radium thereto and to form a radium-depleted solution. The radium-depleted solution is separated from the strong acid cation exchange medium or organophilic sulfonic acid medium. The radium remaining bound to the exchange medium or organophilic reagent is then stripped from the exchange medium or organophilic medium and the activity of the radium is measured.

FIELD OF THE INVENTION 
This invention relates to a process for the separation and preconcentration 
of radium cations from water. More particularly, this invention relates to 
a process for the separation and preconcentration of radium-226 and 
radium-228 cations from water using a strong acid cation exchange medium 
and a water-soluble, macrocyclic polyether. 
BACKGROUND OF THE INVENTION 
The radium content in water supplies has come under intense scrutiny since 
the early 1980s, following the announcement by the U.S. Environmental 
Protection Agency of an acceptable upper limit for the radium content in 
drinking water of 5 picocuries per liter (5 pCi/L). It has been shown 
through recent surveys that this limit is exceeded in many communities, 
particularly those communities that obtain their drinking water from 
wells. 
Radium-226 (Ra-226) and radium-228 (Ra-228), which are products of the 
uranium-238 and thorium-232 decay chains, respectively, are of particular 
environmental concern. Ra-226 has a half-life of 1600 years and is among 
the most toxic long-lived alpha emitters present in environmental samples. 
Thus, considerable interest has developed in improving methods for its 
determination. Likewise, interest has also developed in improving methods 
for determining levels of Ra-228 in water and other samples. 
Because of the low levels of radium generally encountered in such 
environmental samples, radium determination generally requires a 
preliminary preconcentration and separation step to isolate the radium 
from comparatively large quantities of inactive substances and to free the 
radium from other radioisotopes that may interfere with subsequent 
counting. In a typical process, this separation and preconcentration step 
involves co-precipitating the radium with barium and/or lead sulfate. 
Following washing, the precipitate can be subjected to other treatments to 
further reduce the number of other radio-nuclides present or to reduce the 
quantity of the carrier present in the sample. Although such methods are 
generally effective, they can be tedious, particularly the precipitation 
step or steps. 
In addition, radium levels in many barium reagents are not negligible, 
relative to the levels to which the radium is tested in the samples, which 
can require a preliminary purification of the reagents to reduce what is 
commonly referred to as "blank" levels. Moreover, when the precipitate 
containing radium is counted by alpha spectrometry without further 
treatment, the precipitation conditions must be carefully controlled to 
avoid degradation of the alpha spectrum. 
Extraction chromatography has been used as one method for separating and 
preconcentrating radium from samples. In one method, a complex scheme has 
been developed for separating radium and various actinides from copper 
foils using a column of bis(2-ethylhexyl)phosphoric acid (HDEHP) sorbed on 
Celite.RTM. 535. A. Turler et al., Radiochim. Acta 1988, 43, 149-52. The 
uptake of radium by several podands and macrocyclic polyethers supported 
on either Amberlite.RTM. XAD resin or Kieselgel has also been recently 
examined. Radium uptake was observed to be both low and irreproducible 
using the supported macrocyclic polyethers. Similar unsatisfactory results 
were achieved using open-chain extractants. P. Benzi et al., Nucl. Chem. 
Lett. 1992, 164, 211-20. 
Ion exchange techniques have also been used to separate and preconcentrate 
radium cations from various samples. In one procedure, three separate, 
successive cation exchange columns were used to isolate radium from 
geologic samples for subsequent mass spectrometry. A. M. Volpe et al., 
Anal. Chem. 1991, 63, 913-16. In another procedure, a combined 
anion/cation exchange method was used to isolate Ra-226 from human bone 
ash for subsequent electrodeposition and alpha spectrometry. M. Yamamoto 
et al., Radiochim Acta 1991, 55, 163-66. A single column procedure has 
been used to isolate radium from drinking water samples, using a strong 
cation exchange resin and gamma spectrometry. D. A. Clifford et al., 
Health Phys. 1992, 62, 413-22. 
Regardless of the ion exchange procedure used, such methods, like other 
techniques, suffer from various limitations, and particularly, suffer from 
inadequate selectivity and the need for additional cumbersome sample 
treatment steps if the analysis involves the determination of Ra-228. 
Accordingly, there continues to be a need for a simple process for 
selectively separating and preconcentrating Ra-226 and Ra-228 from aqueous 
samples. Such a process permits radium separation and preconcentration 
without sacrificing radium selectivity, and without the need for 
cumbersome sample pretreatment steps. In particular, such a method permits 
radium separation without requiring co-precipitating steps with various 
reagents that can later contaminate the sample. The disclosure that 
follows illustrates one such simple process for selectively separating and 
preconcentrating radium cations. 
SUMMARY OF THE INVENTION 
A process for preconcentrating and separating radium cations from a 
contaminated sample that contains water along with radium and calcium 
cations and can also contain one or both of barium and strontium cations 
is contemplated. The process comprises the steps of admixing a quantity of 
a water-soluble macrocyclic polyether to the contaminated solution to form 
a combined solution, admixing a sufficient amount of an acid to the 
combined solution to form an acidic combined solution having a hydrogen 
ion H.sup.+ ! concentration of about 0.5M, contacting the acidic combined 
solution with a sulfonic acid-based strong acid cation exchange medium to 
bind the radium cations thereto and to form a radium-depleted solution, 
and separating the radium-depleted solution from the strong acid cation 
exchange medium. 
In a preferred process, the macrocyclic polyether is 15-Crown-5, 
18-Crown-6, 21-Crown-7 or 30-Crown-10 (15C5, 18C6, 21C7 or 30C10), and is 
added to the contaminated solution to form a combined solution having a 
macrocyclic polyether concentration of about 10.sup.-3 M to about 
10.sup.-1 M. Most preferably, the macrocyclic polyether is 18-crown-6. 
The process can be carried out in a solid-liquid phase separation system or 
a liquid-liquid phase separation system. In the solid-liquid phase system, 
the cation exchange medium can be a macroporous sulfonic acid-based resin. 
The solid-liquid system can include a chelating agent added thereto to 
prevent sorption by the resin of actinide ions, such as plutonium, uranium 
and the like. 
In a preferred solid-liquid system, after contact between the acidic 
combined solution (during which the radium cations are sorbed on the 
resin), the exchange medium is rinsed with water. The radium cations are 
stripped from the resin by contact with a strong acid strip solution, such 
as 4.0M nitric acid. 
The liquid-liquid separation system contains a water phase and a 
water-immiscible organic phase wherein the sulfonic acid-based strong acid 
cation exchange medium is an organophilic sulfonic acid dissolved in an 
organic diluent. One such organophilic sulfonic acid is 
dinonylnaphthalenesulfonic acid (HDNNS). Exemplary organic diluents are 
o-xylene, diethylbenzene and diisopropylbenzene. The initial water phase 
constitutes the contaminated sample, is adjusted to an acid concentration 
of about 0.5M H.sup.+ ! and has the crown ether added thereto. 
Other features and advantages of the present invention will be apparent 
from the following detailed description, the accompanying drawings, and 
the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Although the present invention is susceptible of embodiment in various 
forms, there is shown in the drawings and will hereinafter be described a 
presently preferred embodiment with the understanding that the present 
disclosure is to be considered an exemplification of the invention and is 
not intended to limit the invention to the specific embodiment 
illustrated. 
A process for preconcentrating and separating radium cations from a sample 
of a contaminated acidic aqueous solution also containing calcium cations 
that can also contain one or both of barium and strontium cations is 
contemplated. That process comprises admixing a relatively small amount of 
a water- soluble macrocyclic polyether (referred to herein as "crown 
ether" or "CE") to the aqueous sample and contacting the sample with a 
strong acid cation exchange medium. Preferably, the crown ether is added 
to a concentration of about 10.sup.-1 M to about 10.sup.-3 M and most 
preferably to a concentration of about 3.times.10.sup.-3 M. A contemplated 
water-soluble crown ether has a minimal solubility in distilled or 
deionized water at 20.degree. C. of about 10.sup.-3 M or more. 
Many common water samples, and in particular water samples taken from wells 
and potable water supplies, contain significant amounts of sodium, 
potassium, magnesium and calcium cations. Unexpectedly, a contemplated 
crown ether significantly increases the uptake by the exchange medium of 
the selected "larger" cations of radium and barium relative to the 
"smaller" cations of magnesium, strontium and calcium. 
Because these "smaller" elemental (lower atomic weight) cations are 
prevalent in common water samples and because they often behave similarly 
to the larger ions of radium and barium, it is beneficial that these 
smaller cations be selectively precluded from sorption by the exchange 
medium. In the present process, the larger alkaline earth cations of 
radium and barium are sorbed by the exchange medium in preference to the 
smaller alkaline earth cations by the addition of the crown ether. 
The process can be carried out in a solid-liquid system, wherein the liquid 
aqueous phase is contacted with a solid exchange medium such as an 
ion-exchange resin. When separation is carried out in the solid-liquid 
system, in order to avoid sorption of cations of actinide metals, such as 
uranium, plutonium and americium, on the exchange resin, a chelating agent 
can be added to the sample. Preferably, a non-hazardous chelating agent 
such as ammonium citrate is used to reduce the uptake of these actinide 
metal cations. 
After addition of the chelating agent, the sample is adjusted to be 
slightly acidic. In a preferred process, the sample is adjusted to an acid 
concentration (H.sup.+ !) of about 0.5M using concentrated aqueous 
hydrochloric acid as is described in greater detail hereinafter. The 
acidified sample is then contacted with the strong acid cation exchange 
medium. 
One preferred solid phase supported exchange resin is Bio-Rad.RTM. 50W-X8 
resin in the H.sup.+ form, which is commercially available from BioRad 
Laboratories, Inc., of Richman, Calif. Other useful strong acid cation 
exchange media include the Dowex.RTM. 50W series of ion exchange resins 
and the Amberlite.RTM. IR series of ion exchange resins that are available 
from Sigma Chemical Co., St. Louis, Mo. 
Another resin that can be used in the present process is a styrene-divinyl 
benzene polymer matrix and includes sulfonic and gem diphosphonic acid 
functional groups chemically bonded thereto. Such a resin is commercially 
available from Eichrom Industries, Inc., of Darien, Ill., under the name 
Diphonix.RTM. brand resin. In the present process, the Diphonix.RTM. resin 
is used in the H.sup.+ form. 
Alternately, the process can be carried out in a liquid-liquid system in 
which the exchange medium is carried in a non-aqueous, water-immiscible, 
organic-based liquid phase that is contacted with the aqueous liquid phase 
sample having the crown ether added thereto. Preferably, the extractant is 
a sulfonic acid-based extractant such as dinonylnaphthalenesulfonic acid 
(referred to herein as HDNNS) dissolved in or carried by xylene or 
di-isopropylbenzene as solvent. 
The crown ethers that have been found to be particularly useful in 
increasing the uptake of radium and barium relative to calcium, in both a 
liquid-liquid phase system and in a liquid-solid phase system include 
15-crown-5 (15C5), 18-crown-6 (18C6), and 21-crown-7 (21C7) The crown 
ether is added to the aqueous solution to a final crown ether 
concentration of about 10.sup.-1 M to about 10.sup.-3 M. In a preferred 
process, the crown ether concentration is about 3.times.10.sup.-3 M. The 
crown ethers are those crown ethers that have only a macrocyclic ring 
system. 
Advantageously, such crown ethers, and in particular, the smaller crown 
ethers (e.g., 15C5 and 18C6) are relatively low cost reagents that provide 
a cost effective and procedurally efficient method for separating radium 
cations from water samples that contain calcium cations and that can also 
contain one or both of strontium and barium cations. 
In carrying out the present process using the solid phase extractant; i.e., 
an ion exchange resin, the crown ether is added to the sample to a 
concentration of about 10.sup.-3 M and preferably about 3.times.10.sup.-3 
M. A chelating agent, such as ammonium citrate can be added to the sample 
to limit sorption of actinide metal cations, such as plutonium, uranium 
and americium, by the resin. Those skilled in the art will recognize the 
tendency of the chelating agent to form a stable complex or complexes with 
such actinide metals. This complex formation inhibits the binding or 
sorption of the actinides by the cation exchange medium. 
The acidity of the sample is then adjusted to a hydrogen ion (H.sup.+ !) 
concentration between about 0.5M and about 1.0M. Where no chelating agent 
is used, the hydrogen ion concentration can be adjusted prior to admixture 
of the crown ether. As will be discussed and illustrated herein, the above 
acidity of the sample favors sorption of the larger alkaline earth 
cations; i.e. radium, barium and strontium, by the ion exchange resin over 
elution of the cations from the resin. 
After the sample is contacted with the resin to bind the cations, the resin 
is rinsed with either water or an aqueous solution of about 0.5M 
hydrochloric acid to improve the separation of radium and barium cations 
from strontium and or calcium cations on the resin. After the rinse 
solution is removed from the resin, essentially only the radium (Ra-226 
and -228) and barium cations remain sorbed on the resin. 
The radium cations, both Ra-226 and -228, as well as the barium cations, 
are stripped from the column using a strong acid strip solution (referred 
to herein as the radium strip solution), preferably an aqueous nitric acid 
solution having a concentration of at least about 3.0M and most preferably 
about 4.0M. The activity of the Ra-226 cations in the radium strip 
solution is then determined by methods that are well known to those 
skilled in the art. 
If an analysis of the Ra-228 initially in the sample (now in the radium 
strip solution) is also desired, its daughter decay product actinium-228 
(Ac-228) is readily isolated for counting. Thus, for example, the strip 
solution is maintained for a suitable period of time for a portion of the 
Ra-228 to decay to Ac-228. Generally, maintaining the solution overnight 
(about 18 hours) suffices to "grow" the Ac-228. 
The radium strip solution that has been so "grown" contains radium and 
Ac-228 and is passed through a subsequent exchange medium, such as an 
extraction chromatographic resin to bind the Ac-228 thereto, and to pass 
the radium. One such extraction chromatographic resin comprises CMPO 
octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide! at 0.75M in 
tri-n-butyl phosphate supported on a non-reactive (inert) polymeric 
support such as Amberlite.RTM. XAD-7 or Amberchrom.TM. CG-71, as discussed 
in Horwitz et al., Analytica Chimica Acta 1993, 283, 361-372. A preferred 
extraction chromatographic resin is TRU.TM. resin available from Eichrom 
Industries, Inc., of Darien, Ill. 
The Ac-228 is recovered from the extraction chromatographic resin by using 
an aqueous acidic actinium strip solution of, for example, 1.0M 
hydrochloric acid, and the activity of the Ac-228 is measured accordingly. 
Based upon the time that the Ac-228 is permitted to "grow" and the 
activity of the Ac-228 recovered from the resin, the initial activity of 
the Ra-228 can be determined. 
It may also be desired to separate strontium-90 (Sr-90) from the 
contaminated water sample. If Sr-90 is to be separated from the sample, 
the radium strip solution is further processed by contacting it with a 
strontium selective extraction chromatographic resin. 
One such resin is "Sr Resin," which is a non-ionic acrylic ester polymer 
bead resin (60 percent to 70 percent) having a coating layer thereon of a 
substituted crown ether such as 4,4'(5')di-t-butylcyclohexano-18-crown-6 
bis-t-butyl-cis-dicyclohexano-18-crown-6! (20 percent to 25 percent) 
dissolved in n-octanol (5 percent to 20 percent). A water-insoluble crown 
ether is typically substituted with two 6-membered ring structures and has 
a solubility in distilled deionized water of about 10.sup.-4 M or less. 
"Sr Resin" is commercially available from Eichrom Industries, Inc. Contact 
of the radium strip solution with the Sr Resin binds the strontium to the 
resin, thus leaving the strip solution with essentially only radium and 
barium cations therein. 
FIGS. 1A-1G, illustrate the uptake or sorption of radium, barium, strontium 
and calcium cations in a solid-liquid phase separation from solutions of 
seven different crown ethers (12C4, 15C5, 18C6, 21C7, 24C8, 27C9 and 
30C10, respectively) in 0.5M hydrochloric acid as liquid phase with 
Bio-Rad.RTM. AG 50W-X8 ion exchange resin as solid phase. Data are 
provided as distribution values (D) relative to the concentration of the 
crown ether. As can be seen from the figures, addition to the aqueous 
phase of even relatively small amounts of crown ether (about 10.sup.-3 M 
to about 10.sup.-1 M) brings about a substantial and unexpected increase 
in D.sub.Ba and D.sub.Ra values relative to D.sub.Sr and D.sub.Ca. As 
shown in FIG. 1C, a pronounced effect is observed on D.sub.Ba and D.sub.Ra 
in as little as 10.sup.-3 M 18C6 crown ether. As seen in FIG. 1G when 
30C10 is used, there is a greater increase in selectivity of the resin for 
radium and barium over strontium and calcium cations that occurs at 
concentrations too high to be practical. 
FIGS. 2A-C and 3A-C show the influence on the sorption of radium, barium, 
strontium and calcium cations at varying concentrations of aqueous 
hydrochloric acid using 15C5 and 18C6, respectively, and the Bio-Rad.RTM. 
AG 50W-X8 ion exchange resin as solid phase. As is apparent from the 
figures, at the lower acid concentrations, the ion-exchange resin exhibits 
increased affinity for; i.e., sorption of the alkaline earth cations, 
whereas the higher acid concentrations favor elution of the same cations. 
FIGS. 4A-4C illustrate the efficacy of the present process even in the 
presence of high concentrations of calcium cations. The uptake of radium 
cations (diamonds), barium cations (squares) and strontium cations 
(circles) is illustrated in solid-liquid phase extraction systems, having 
an aqueous phase that includes various concentrations of 18C6 (shown on 
the abscissa) at hydrochloric acid concentrations of 0.5M, 1.0M and 2.0M 
in FIGS. 4A, 4B, and 4C, respectively. The resin used in this study was 
the aforementioned BioRad.RTM. AG 50W-X8 resin. However, the resin was in 
the Ca.sup.+ form; i.e., essentially saturated with calcium cations. 
Nevertheless, it is apparent from FIGS. 4A-4C and from a comparison of 
those data with those shown in FIGS. 3A-3C, that the increased calcium 
concentrations in these systems have only a slight adverse impact on the 
sorption of radium, strontium and barium cations. In fact, the data of 
FIG. 4A shows that even with complete saturation of the resin with an 
interfering ion (Ca.sup.2+), the uptake of radium and barium cations by 
the resin is still significant, and is sufficiently high to be of utility 
in an analytical scheme. 
As discussed previously, the present process can also be carried out using 
a liquid-liquid exchange medium. Specifically, the process includes the 
use of an exchange medium comprising a strong acid-based cation exchange 
medium, such as an organophilic sulfonic acid dissolved in an organic 
diluent, for removing the large alkaline earth cations from the aqueous 
sample. One such organophilic sulfonic acid is the aforementioned 
dinonylnaphthalenesulfonic acid (HDNNS). Exemplary of the organic diluents 
that can be used for carrying the organophilic sulfonic acid for the 
present liquid-liquid extraction process are xylenes, such as o-xylene, 
other alkylated benzenes such as diisopropylbenzene and diethylbenzene and 
mixtures of any of various well-known tri-C.sub.3 -C.sub.6 alkyl neutral 
organophosphorus ester extractants, such as tributylphosphate (TBP), and a 
paraffinic hydrocarbon. It has been observed that the water-soluble crown 
ethers in contact with the water-immiscible solvent distribute between the 
two phases; i.e., the aqueous phase and the organic phase. 
In conducting the liquid-liquid phase studies, the distribution and 
concentration level of the radioactive component between o-xylene 
solutions of HDNNS alone and in the presence of TBP, and aqueous solutions 
of appropriate composition were measured first by twice pre-equilibrating 
an aliquot of the organic solution with an equal volume of the aqueous 
solutions without the radioactive component. A fresh aliquot of the 
aqueous phase was then spiked with a small quantity of the radioactive 
component (added as a radiotracer for experimental purposes) and the two 
phases were vortexed for about five minutes to achieve equilibrium. 
Radium-223 (Ra-223), barium-133 (Ba-133), strontium-85 (Sr-85) and 
calcium-45 (Ca-45) were used as radiotracers for carrying out the studies 
described herein. 
The solution was then centrifuged to promote phase separation and aliquots 
of both phases were taken for counting. Counting of the samples was 
performed using a Packard Cobra Autogamma counter for the Sr-85 and 
Ba-133, and using liquid scintillation on a Packard Model 2000 CA counter 
for the Ca-45 and Ra-223. Samples containing Ra-223 were counted after 
establishment of secular equilibrium, which required several hours based 
on the half-life of lead-211 (36.1 minutes). The reproducibility of the 
distribution ratio (D) measurements was within about 5 percent. 
FIGS. 5A and 5B illustrate the distribution ratio data for radium in 
two-phase systems including crown ether (CE) in an aqueous acidic phase, 
and 0.01M HDNNS in an o-xylene phase. The distribution ratio, D, is 
defined as the ratio of the concentration of the metal species in the 
organic phase to the concentration of the metal species in the aqueous 
phase. Also shown in these figures is the distribution data for the metal 
species with the crown ethers alone (without the HDNNS) as the lowest set 
of data points (diamonds), and the distribution data for the metal species 
for HDNNS alone (without the crown ethers) as the next higher set of data 
points (squares) in each of FIGS. 5A and 5B. 
As is readily apparent from the figures, the distribution data for the 
metal species in the two-phase system with HDNNS in the organic phase and 
crown ether in the aqueous acidic phase are at least two orders of 
magnitude higher than the data for the HDNNS system alone, and at least 
five orders of magnitude higher than the data for the crown ether system 
alone. The uptake of the metal species is shown as a function of the 
hydrochloric acid concentration of the aqueous phase. As is apparent from 
the data, lower acid concentrations favor partitioning of the metal 
species from the aqueous phase to the organic (extractant) phase. 
Moreover, the magnitude of the increase in the distribution data of the 
metal species to the organic phase in the systems that include both HDNNS 
in the organic phase and the crown ether in the aqueous phase indicates 
that the addition of the crown ether to the aqueous phase produces a 
synergistic effect in the uptake of the metal species by the extraction 
medium. 
More importantly, FIGS. 6A and 6B illustrate the effect of the acid 
concentration in the aqueous phase on the metal species uptake. FIG. 6A 
shows the distribution data for the various metal species in an aqueous 
phase having a hydrochloric acid concentration of 0.1M and having varying 
concentrations of 15C5, between about 10.sup.-5 M and about 10.sup.-2 M. 
Likewise, FIG. 6B shows the distribution data for the various metal cation 
species in an aqueous phase having a hydrochloric acid concentration of 
1.0M and having varying concentrations of 15C5, between about 10.sup.-5 M 
and about 10.sup.-2 M. 
A comparison of the data shown in FIG. 6A with those shown in FIG. 6B 
illustrates that lower acid concentrations favor uptake of the metal 
species by the extraction medium whereas higher acid concentrations favor 
elution. These data also illustrate that the enhancement of radium cation 
sorption induced by crown ether addition is observed at a variety of acid 
concentrations. 
FIGS. 7A and 7B show the distribution data for two-phase systems similar to 
those of FIGS. 6A and 6B, except that an 18C6 crown ether was added to the 
aqueous phase rather than a 15C5 crown ether. Like the data of FIGS. 6A 
and 6B, the data of FIGS. 7A and 7B show that the uptake of the metal 
cations by the extraction medium is adversely effected by an increase in 
acid concentration and that radium cation sorption is enhanced by crown 
ether addition at different acid concentrations. 
A comparison of the data of FIGS. 6A and 6B with that of FIGS. 7A and 7B 
shows increased selectivity by the exchange medium for radium and barium 
over strontium when the crown ether is 15C5. In fact, at crown ether 
concentrations between about 5.times.10.sup.-4 and about 10.sup.-2, there 
is about a five-fold increase in selectivity for radium and barium over 
strontium. In addition, that enhancement does not require a particular 
acidity. 
FIGS. 8A and 8B show the distribution data for two-phase systems similar to 
those of FIGS. 6A and 6B, and FIGS. 7A and 7B, except that a 21C7 crown 
ether was added to the aqueous phase rather than a 15C5 or 18C6 crown 
ether. Like the data of the previous figures, the data of FIGS. 8A (0.1M 
H.sup.+ ! concentration) and 8B (1.0M H.sup.+ ! concentration) show that 
the uptake of the metal cations by the extraction medium is adversely 
effected by an increase in acid concentration, and that radium cation 
sorption is enhanced by addition of the crown ether over a ten-fold change 
in acid concentration. 
A comparison of the data of FIGS. 8A and 8B with that of FIGS. 6A and 6B 
and FIGS. 7A and 7B shows an extreme increase in selectivity by the 
exchange medium for radium over calcium when the crown ether is 21C7. In 
fact, at crown ether concentrations between about 5.times.10.sup.-4 and 
about 10.sup.-2 M, there is about a one thousand-fold increase in 
selectivity for radium over calcium. This is extremely beneficial in that 
calcium cations are the predominant alkaline earth cations in most water 
samples. 
From the foregoing it will be observed that numerous modifications and 
variations can be effectuated without departing from the true spirit and 
scope of the novel concepts of the present invention. It is to be 
understood that no limitation with respect to the specific embodiments 
illustrated is intended or should be inferred. The disclosure is intended 
to cover by the appended claims all such modifications as fall within the 
scope of the claims.