Fluorescent X-Ray Spectrographic Determination of Uranium in Waters and Brines W. L. KEHL and R. G. RUSSELL Gulf Research & Development Co., Pittsburgh 30,
Pa.
In a search for a method sensitive to small concentrations of uranium in water and brine, a modification of a fluorometric method was adopted. The uranium is precipitated from the water or brine by the method of Smith and Grimaldi, and the ashed precipitate is analyzed by fluorescent x-ray spectrography. When an internal standard is required, yttrium nitrate is added. The sample holder of the Norelco x-ray spectrograph was modified to place the small precipitate samples in the most intense part of the bean1 from the molybdenum target x-ray tube. As little as 0.01 mg. of uranium, equivalent to 0.01 p.p.m. in a 1-liter water sample, can readily be measured, when the matrix contains no appreciable concentration of highly absorbing elements. The time for analysis is about 10 minutes per sample, when no internal standard is used, and about 15 minutes when the internal standard is used, exclusive of the precipitation process.
HE recent public interest in the potentialities of oil-field T w a t e r s as a source of uranium led to an investigation of methods for the analysis of waters and brines containing very None Of the methods concentrations Of this described in the literature was entirely satisfactory, particularly for use in a study of a possible economical process for recovering uranium from w-ater and brine solutions.
experiments on recovery methods were analyzed without the usr of an internal standard. Actual field samples, however, required an internal standard, because of the widely varying naturr of the dissolved materials which were carricxl donn with the uranirrni i n the precipitation process. SAMPLE PREP.iR.ATION
The process of precipitating the iiraiiiiriri from solutioti ioll o w exactly the first few steps outlintd in the procedure of Smith and Grinialdi. Briefly summarized, this procedure is a s follows for samples in which no int,crnal .;t:indard is used.
-4 500-ml. aliquot of sample is filtered through glass wool anti acidified with nitric acid. -4 solution containing 20 mg. of alumina is t,hen added, followed by the addition of 60 nig. of diammonium phosphate in solution. The aluminum ion is added to act as a carrier for the precipitation of uranium, while the diammonium phosphate ensures the precipitation of the alriminum and uranyl ions as the phosphates, which are easier to filter than the hydroxides. The solution is boiled for about 1 minute to remove anv carbon dioxide. and tlic i)H is adiusted to the color transition point of methyl red by adding ammonium hydroxide After a 10-minute waiting period, the mixture is filtered through ashless filter paper, and the precipitate is washed T%-ithdilute ammonium nitrate, and ashed. This ach is then analyzed by the fluorescent x-ray method. In the case of field samples, a soltitioil containing 0.28 nig. of \,ttrium is added to the jOO-ml, sample aliquot to provide thc inteIrial standard, The preclpitatioll ocess is then carried Ollt exactly as described, and the J ttrium. \\ hlch behaves like uraiiirini in thib process, is completely recovei c d 1'1 the ash. INSTRUhlENT.iTIO\
Table I.
Results Obtained from Repeated Analyses of Three Synthetic Samples (No internal standard used) Sample No. 1 2
Uranium added, rng. Uranium found, mg.
0.70 0.72 0.70 0.69 0.70 0.70 0.68 0.68
0.50 0.50 0.51 0.51 0.49
0.52 0.50
3
0.10 0.10 0.09 0.09 0.09 0.11 0.10
0.49
0.51 0.48 0.48
Mean
0.70
0.50
0.10
Sorelco spectrographic equipment was used, Kith i)otli tungsten and molybdenum target t,ubes for sample excitation. A lithium fluoride analyzing crystal was used exclusivcxly, anti line intensities were determined by the automatic wanningecording method, with a scanning speed of 0.25" (20) per minute. The volume of the ash samples obtained for analysis was frequently as low as 0.1 to 0.2 cc., and a special sample cup was used which ensured that the sample \vas reproducibly positioned in the area of most intense radiation from the x-ray tube. To accomplish this, the distribution of x-ray illtensity from the tulle was roughly determined by irradiating 3. piece of soft glass in the sample position and noting the intensity of discoloration induced. A cup with the dimensions 15 X 20 X 0.5 mm. was then formed in the top of an aluminum block in a position which corresponded to the area of most intense discoloration of the glass, when tlic block was placed in the sample holder of the spectrograph. PROCEDURE AND RESULTS
Birks and Brooks ( 1 )describe a fluorescent x-ray spectrographic method in which a 1-ml. sample of a water solution of uranium was evaporated in a shallow cup and the residue was analyzed. This method was not directly applicable to brine samples and its sensitivity r a s inadequate for the purposes of the investigation. The optical fluorometric method described by Smith and Grimaldi ( S ) , while accurate for the quantities of uranium found in field waters and brines, was not convenient in this laboratory. However, this method suggested the possibility of following their procedure for precipitating the uranium from solution, and then using fluorescent x-ray spectrography to analyze the ashed precipitate. I n practice, this was found to be a convenient and very satisfactory method. Synthetic solutions prepared in the laboratory for use in the
The precipitates obtained from the synthetic solutions used in the laboratory studies mentioned previously were well suited for analysis without the use of an internal standard. No interfering elements of significance were present, and the absorption of t,he UL,, line in the aluminum phosphate matrix was relatively low and essentially constant from sample to sample. Standard samples %ere prepared by following the regular chemical precipitation procedure with 500-ml. aliquots of water to which 0.10, 0.25, 0.50, 0.70, and 1.0 mg. of uranium had been added as uranyl acetate. Duplicate preparations of each of these wei'r checked and found to be identical, within the limits of reproducibility of the x-ray measurements. The intensity of the UL,, line from all of the standard saniplca was measured at least three times and the peak heights obtaiiietl for each uranium concentration u-ere a\rraged. A straight-linc.
1350
V O L U M E 28, NO. 8, A U G U S T 1 9 5 6 calibration curve was ohtained u l i r i i these average values were plotted as it function of uranium content. The sensitivity, as deduced from this curve, is such that 0.01 mg. of uranium can be measured. This corresponds to 0.02 p.p.m. in the original 500-ml. water sample, but could be altci.et1 to almost ally desired vahic by a proper choice of the volunie of the s:tnipl~~ aliquot used f o i , analysis. Two synthetic samples to nhich 0.70 and 0.50 mg. of uranium had been added were prepared and analyzed repeatedly to test the method. The results are shown in Table I, which also iiicludes measurements made on the 0.10-mg. standard sample, to illustrate the reproducibility obtained in this concentration range. It was found, generally. that, all the results obtained for synthetic samples fell within & : 3 y c of the mean in the 1.0-mg. range and within =k15ycof the niean in t'he 0.10-mg. range. The method, as adapted for the analysis of field saiiipl(Js. is based on the ratio of the intensity of the UL,I line to that of the K , line from the known amount of added yttrium. The use of yttrium as an internal standard for uranium analyses has been reported by Cope and Lingard ( 2 ) , who were interested in the determination of uranium i r i mineral-bearing rocks. The YK, line, with a wave length X = 0.881 .\., is ahsorbed by most elements to about the same extent as the UL,, line, u i t h X = 0.911 A. Hon.ever, gold, platinum, mercury, and tellurium have absorption edges which occur between t,hese two wave lengths, so that an error in the uraniuni analysis will be introduced by t,he presence of any of these elements in the precipitate. An error of smaller niagnitude is also introduced by lead and bromine, which have absorption edges on the near long wave-length side of the T,-Lo,line. Of all the interferirig elemelite mentioned, lead : i i i d tclhirium are thought to be the most likely to occur in solation in field waters. The efTect of lead on the ratio of the GL,, to Y K , line intensities \vas investigated and found to be negligible for concentrations that niight norm:rlly he expected, although a large effect was observed when the lead content of the prcripitate W:IS increased to several huridred milligrams. It is rather improbable that any of the othcr interfering elements will \)e found in brine samples i n sufficieiit concentration to be ti.ouhlcsome. However, it is advisable to check this, at leas: for the first few samples from a new area or field. Obviously, if yttrium is present in the sample as received, this niet,hod cannot be used unless the concentration is determined and a correction applied to the measiired t'otal Y K , intensity. Samples for calibration Rere prepared hy adding 0.28 mg. of yttrium and various amounts of uranium, from 0.08 to 1.25 nig., to 500-nil. water samples, which were then processed as described. , I Y K , from the precipitates were The intensities I W L ~and measured by the scanning-recording method, and a smooth calihration curve was obtained by plotting the ratio I u L , , / I Y x , :ts a function of the uranium content. A synthetic sample containing 0.31 mg. of added uranium was analyzed five times to check the method. Another synthetic sample which contained 0.31 ing. of added uranium as well as approximately 100 mg. each 6f added iron and vanadium was analyzed twice to determine how well the yttrium was functioning as an internal standard. The results are shoim in Table 11. All the analytical values fall within &7% of the mean. Two field samples were analyzed both by a chemical method employing spectrophotometric absorbance and by fluorescent x-ray spectrography. Both samples showed high activity when placed near a Geiger or scintillation counter, but the observed intensity of the UL,, line was near zero for both samples. The sensitivity of the method for these particular samples was then determined by adding 0.1- and 0.3-mg. quantities of uranium t o two other 500-ml. aliquots of each of the brines, which were then processed and the precipitates analyzed. On the basis of these observed UL,, line intensities, the minimum measurable uranium content was determined to be 0.05 mg., so that the Concentration
1351 in the original samples was determined to be less than 0.1 p.p.oi. These results were confirmed by the chemical analyses, which showed a concentration of approximately 0.03 p.p.m. in both samples.
Table 11. Analyses of Synthetic Samples (Showing effect of approximately 100 mg. each of added iron and vanadiuni. Yttrium added to b o t h samples a8 a n internal standard) Sample .4 B Uraniuiii added, m g . 0 31 0 . 3 1 IJIUR Fe and T' Uraniiirn found, m g . 0.34 0.33 0.31 0.29 0,33
Alean
0.31 0.30 0.32
0.31
These findings illustrate a11 observation that has frequently been made-namely, that the activity often found in field waters by Geiger or scintillation coiinter surveys is usually due not to the presenre of uranium hut t.o radium salts or other wvrttersoluble dcc:ty products. I)ISCUSSION
The sensitivity of the method for saiiiples which do not, coiltain any significant quantity of absorbing material other t,haii the added aluminum phosphate is such that 0.01 mg. of uranium can he measured. This can be made to correspoiid to almost, any conccsntration in thc original solution by properly choosing the volume of sample to be analyzed. On the othcr hand, in t,he caw of field samples the sensitivity is reduced by absorption in thc cvtra rnatcriiils that are found iu the precipitates from thcw solutions. Because the quantity of these niaterials is varial)l(l ftom samplc to sample, the sensitivit,y will also vary. In the examples cited, the miriimnm measurable rirariinm cwitent wts 0.05 mg. The procedure for obtaining the precipitates from the water, or brine solutions requires about 1 hour per sample, hut four samples can conveniently be handled simultaneously by oiio person. These precipitates can be analyzed hy one person :it the rate of four per hour when the internal standard method in used, and six samples per hour when no ixrtcmial standard is used. This includes time for loading the spcvimen holder and reduction of data, both of which can usually bc done during the automatic scanning-recording cycle for another sample. These rates would generally be substantially reduced if manual counting techniques were used, but some gain in the reproducibility of the results tvould he realized. If this great,er precision is not required, the method as described can be recommended aa being very convenient, rapid, and in general, satisfactory. ACKNOWLEDGMENT
The authors wish to acknowledge their gratitude t o Blaine B. Rescott, executive vice president, Gulf Research & Development, Co., for permission t o publish this manuscript, and to B. B Osthaus for the analyses of the two field samples by a chemical method employing spectrophotometric absorhance. LITERATURE CITED
(1) Birks, L.S.,Brooks, E. J., AXAL.CHEM.23, 707 (1951). (2) Cope, J. H., Lingard, A. L., Fourth Annual X-Ray Symposium, Industrial Applications of X-Ray Analyses, Denver, Colo.,
August 1955. (3) Smith, A. P., Grimaldi, F. S., Geol. Survey Bull. 1006, 125 (1954). RECEIVXD for review March 28, 1956. Accepted May 15,1956. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., February 29, 1956.
