VO
=
VD
=
VP
=
a
=
P
=
~ (+ n 1, u ) = AD
=
XP
=
AD
=
AP
=
V
=
YP
=
volume at which the peak in the general elution curve emerges; see Figure 1 volume at which the peak in the elution curve for daughter species emerges volume at which the peak in the elution curve for parent species emerges parent-daughter separation factor, i.e., Vp/Vu width of elution curve at times the maximum; see Figure 1 incomplete gamma function; see Equation 16 decay constant of daughter species decay constant of parent species fraction of daughter nuclei per unit volume which decay in the time the unit volume flows, i.e., b d t l d V fraction of parent nuclei per unit volume which decay in the time the unit volume flows, i.e., XpdtjdV fraction number fraction number at which parent species elution curve peaks
-
To
4J
=
effective time of separation, i.e., the time at which a hypothetical instantaneous separation of parent and daughter species would yield the same sample as i$ obtained from the column procedure; see E q u a t m 27 column flow rate, d Vldt ACKNOWLEDGMENT
The author expresses his considerable gratitude to Dr, William Rubinson of Brookhaven National Laboratory for his thorough examination of the mathematicai manipulations employed in this work and in particular for his valuable comments and suggestions on simplifying the finel f o r p in which these derivations are Dublished. RECEIVED for review February 2 5 , 1971. Accepted May 19, 1971. Presented in condensed form at the 159th ACS Meeting, Houston, Texas, February, 1970. The author gratefully acknowledges the support of the ACS Petroleum Research Fund, Grant Number 2085-C3, and also partial support by a University Scaife Gtant
~
Separation of Uranium from Seawater by Adsorbing Colloid Flotation Young S. Kim and Harry Zeitlin Department of Chernistrjs and Hawaii Institute of Geophysics, Unirersity of Hawaii, Honolulu, Huw’aii 96822 A procedure is described for the separation from seawater of uranium present as the stable tricarbonatouranyl anion by an adsorbing colloid flotation technique which utilizes a collector-surfactant-air system. At p H 6.7 i 0.1 the uranium is adsorbed effectively on the positively charged ferric hydroxide collector. Upon addition of the anionic surfactant, sodium dodecyl sulfate, and the bubbling of air through the seawater, the colloidal particulates of ferric hydroxide enriched with uranium by absorption are floated within 2-3 minutes to the surface as a stable froth which is easily removed. Uranium was analyzed spectrophotometrically using Rhodamine B. Average recovery of uranium from seawater by this method is 82%.
A m c h r PAPER ( I ) has described the first application of a bubble technique to seawater for the separation of a trace metallic constituent. Under optimal conditions, molybdenum as molybdate is floated to the surface quantitatively and reproducibly in less than five minutes as an easily removable froth by a positively charged iron(II1) hydroxide collector, an anionic surfactant (dodecyl sodium sulfate), and air. The behavior of the collector-surfactant-air system toward a metallic anionic species such as molybdate prompted an investigation to determine whether the flotation method could be applied successfully to other trace metals which exist in seawater as anions. This communication is concerned primarily with the extension of the separation process to uranium believed to be present in seawater as the very stable tricar( I ) Y. S. Kim and H . Zeitlin, Sepur. Sci.. in press. 1390
bonatouranylate ion, UO,(CO.,),~--( 2 ) = 1.7 ?< (3)]. Other methods for the separation of uranium in seawater which is present in the 2.9-3.3 c(g/l. range include coprecipitation with aluminum phosphate and ferric hydroxide and solvent extraction ( 4 ) . In order to determine the separated uranium spectrophotometrically, a modified procedure was worked out involving Rhodamine B (5, 6) which proves to be comparable to other spectrophotometric and fluorometric methods ( 7 , 8), eliminating the need for a fluorescence attachment. Rhodamine B has not been employed previously for the determination of uranium in natural waters. EXPERIMENTAL Apparatus and Equipment. A Beckman DU spectro-
photometer was used for absorbance measurements. The absorbances were read in low volume matched quartz cells of 1.0-cm path length. The pH of the solutions was de(2) E. D. Goldberg, “The Sea,” M . N. Hill, Ed., Vol. 2, lnterscience, New York, N. Y.. 1966, p 5. (3) A. G. Klygin and I . D. Smirnova, R i m . J . Iuorg. Chem., 4, 42 (1959). (4) J. P. Riley and G. Skirrow, “Chemical Oceanography,” Vol. 2, Academic Press. London, New York, 196.5. pp 391-392. (5) Frausto da Silva and Legrand de Moura. 1/7r. Co/?f.P w c r f i t l Uses Ar. Euergy, 28, 537 (1958). ( 6 ) H. H. Ph. Moeken and W. A. H. Van Neste. A i d . Chirn. Acrci, 37, 480 (1967). (7) Academy of Sciences of the USSR, “Analytical Chemistry of Uranium,” Israel Program for Scientific Translations, 1963. (8) E. B. Sandell, “Colorimetric Determination of Traces of Metals,” 3rd ed., Interscience, New York, N . Y., 1965, p 903.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 11, SEPTEMBER 1971
termined by a Beckman Expandomatic p H meter. Millipore filters, HA 47-mm diameter, were used to filter seawater samples. The flotation unit was similar to that described previously (1). Reagents. All chemicals were of analytical grade. Aqueous reagents were prepared in doubly distilled deionized water. A solution of calcium nitrate tetrahydrate containing EDTA was used as a salting reagent in the extraction of uranium (9). Rhodamine B reagent was prepared by the saturation of sodium-dried benzene containing 1 benzoic acid with Rhodamine B. A buffer, p H -7.5 was prepared from ammonium hydroxide-ammonium nitrate (0.05M0.9M). The surfactant was dodecyl sodium sulfate (0.05x in ethanol). A 0.05M ferric chloride solution served as the collector. A standard uranium solution was prepared by dissolving 22.8 mg of sodium tricarbonatouranylate, Nac[U02(CQ3)&(IO) in 1 liter of water and adding 1 gram of sodium bicarbonate to stabilize the solution. Ten milliliters of this solution were diluted to 100.0 ml which provided a solution containing 1 gg of U/ml. Separation of Uranium from Seawater. The flotation setup and procedure were similar to those described previously for molybdenum (I). The principal parameters studied were pH of the seawater sample and volume of ferric chloride solution. For this purpose clear uncontaminated nearshore seawater was filtered through a 0.45-p millipore filter. To the 500 ml samples used in the p H studies were added 3 ml of 0.05M ferric chloride and 6.0 p g of uranium. The samples were adjusted to p H 4.0, 4.5, 5.0, 5 . 5 , 6.0, 6.5, and 7.0 (kO.1) with 1M hydrochloric acid and 4 M ammonia. The flow rate of air was adjusted to 10 f 2 ml/min and allowed to pass for five minutes. Following the flotation, the froth was removed, dissolved in 3-4 ml of 12M HCI-16M H N 0 3 (4:l) and the solution evaporated to a few ml. Uranium was determined by the method described below. The results ar6 given in Figure 1. The optimum volume of ferric chloride solution used as the collector was determined by varying the volume of the 0.05M solution and examining the effect o n recovery at the previously determined optimum pH of 5.7 & 0.1 (Figure 2). Optimum Conditions for Extraction and Determination. When aluminum nitrate was found to be unsuitable as a salting agent for the extraction of uranium, calcium nitrate was substituted satisfactorily. The optimum volume of this reagent was determined by adding, in a series of experiments, 5, 10, 15, 20, and 25 ml of calcium nitrate salting reagent to the near-dry residues contained in vials (see below). Each residue was warmed until it had dissolved. The solution was cooled and 6.0 ml of ethyl acetate were added. Separation of the two layers was carried out with a centrifuge. Five milliliters of the upper organic layer were transferred to a small vial and the solvent evaporated to dryness at 80 to 90 “C. Two drops of dilute nitric acid were added and the evaporation repeated and the residue dissolved in 1 ml of bufler solution (below). In order to obtain the p H for maximum color development of the uranyl-Rhodamine B complex, each residue obtained by extraction with ethyl acetate was dissolved in 1 ml of buffer solution of varying pH (2.2-9.2) and 2.0 ml of Rhodamine B reagent added. The mixture was centrifuged for separation of the two layers. The pink-colored upper layer was transferred to the reduced volume absorption cell by an eye dropper and the absorbance measured against the reagent blank at 5 5 5 nm. Recovery of Uranium. Assessment of recovery was made by comparison of the absorbance obtained from the analysis utilizing the flotation technique under optimal conditions (9) T. M. Florence, D. A. Johnson, and Yvonne J. Farrar, ANAL. CHEM.,41, 1652 (1969). (10) Academy of Sciences of the USSR, “Complex Compounds
of Uranium,” I. I. Chernaev, Ed., Israel Program for Scientific Translation, 1966, p 34.
O’r-----l
Figure 1. Optimum pH of sample solution for separation by flotation
i
Figure 2. Optimum volume of ferric chloride solution for separation by flotation with those obtained from the direct analysis of replicate distilled water standards containing 0.0, 2.0, 4.0, and 6.0 p g of uranium in which the coprecipitation and flotation steps were omitted. The latter absorbances were considered to represent 100% recovery of uranium. The results are given in Table I. Interference by the presence of W04*and Moo4*-which were believed collected by ferric hydroxide was checked by adding these species in amounts four times greater than those in seawater to the Rhodamine B reagent and measuring the effect on the absorbance of the uraniumRhodamine B complex. Precision and Reproducibility. An analytical procedure for the determination of uranium in seawater using Rhodamine B was worked out and its precision and reproducibility of recovery measured at one concentration. Procedure. SEPARATION OF URANIUM FROM SEAWATER. A well-mixed pool of clear uncontaminated seawater filtered through a 0.45-p millipore filter was used as a source of samples. In order to prepare a working curve, 3 ml of 0.05M ferric chloride solution were added to 500-ml samples con-
u 3
Table I. Calibration and Working-Curve Data Recovery, A” A’b AIIC I.rg
0.0 2.0 4.0 6.0
z
0.ooO 0.124 0.255
0.374
0.077 0.181 0.283 0.382
0.ooO 0.104 0.206 0,305
83 81 82 Av 82
Absorbances from standard solutions without coprecipitation and flotation. * Absorbances from 5 W m l seawater samples plus added uranium. Reagents solution served as a blank. A” = A‘ - 0.077. C
ANALYTICAL CHEMISTRY, VOL. 43, NO. 11, SEPTEMBER 1971
1391
-
as
$42 m
: 8
O3 0.2
-
ai
-
0.0
10
I
20
30
4.0
50
bo
20
80
90
100
Figure 3. Effect of pH on formation of uranyl-Rhodamine B complex Table 11. Reproducibility of Recovery Sample No. A bsorbancea Deviation 1 2 3 4 5
0.377 0.396 0.376 0.391 0.394 0.389
-0.012 +0.007 -0.013 +o. 002 $0,005
Mean 0.008 From 500-1111seawater samples plus added uranium (6.0 p g ) . Reagent solution used as a blank. Standard deviation = 0.010. taining 0.0, 2.0, 4.0, and 6.0 pg of uranium. The pH was adjusted to 5.7 f 0.1 with 1 M hydrochloric acid. The flotation equipment and gas flowrate were similar to that described previously. Two milliliters of 0.05 % dodecyl sodium sulfate solution were injected into the cell. After 2 to 3 minutes, the froth was removed and collected into a beaker. ANALYSIS.The froth was dissolved in 3 to 4 ml of 12M HCI-16M H N 0 3 (4:l). The solution was evaporated carefully to near-dryness and a few milliliters of water together with several drops of dilute nitric acid added. The solution was transferred to a 40-ml capacity borosilicate glass vial and evaporated again gently to dryness with baking avoided. Ten milliliters of the salting agent were added to the vial which was warmed to dissolve all salts. After being cooled to room temperature, the uranium was extracted into 6.0 ml of ethyl acetate by shaking the vial manually for one minute. Separation of the layers was effected by centrifugation for two minutes and exactly 5.0 ml of the top solvent layer was removed and transferred to another vial of 10-ml capacity. The solution was evaporated to dryness, a procedure which was most easily accomplished by insertion of the vial into holes suitably positioned in an aluminum metal block heated to about 90 “C by a hot plate. A few drops of dilute nitric acid were added to the residue and the solution again evaporated to dryness. Upon cooling, 1.O ml of the buffer was added and residual salts dissolved. Two ml of the Rhodamine B reagent were added and the mixture was shaken for one minute. The mixture was centrifuged for one minute and the reduced volume absorption cell filled with the solution of pink complex with an eye dropper. The working curve was constructed by plotting absorbances read against the reagent blank at 555 nm obtained from the spiked seawater samples os. concentration. RESULTS AND DISCUSSION
The marked effect of pH on the recovery of uranium from seawater via flotation shown on Figure 1 demonstrates the 1392
maximum recovery based on the spectrophotometric determination as obtained between pH 5.5 and 6.0. A pH of 5.7 f 0.1 was adopted in subsequent work. The results of tests on the effect of the ferric chloride collector on recovery are given in Figure 2. There appears to be no significant difference in the 2- to 6-ml range, and 3 ml of 0.05 Mferric chloride solution were chosen for use in the final procedure. The role of ferric hydroxide as a trace metal collector formed from ferric chloride in seawater was recently clarified (ZZ). At a low pH, ferric hydroxide, whose charge is pH-dependent, has positive charge density and should adsorb on its active surface sites the stable uranyl carbonate complex through electrostatic attraction. The positively charged ferric hydroxide particulates thus enriched with uranium are attached to the anionic surfactant and floated to the surface through bubble formation. Recovery assessment was carried out by measuring the absorbances, following analysis of distilled water standards containing uranium in which the coprecipitation and flotation were omitted, and comparing them with the absorbances obtained from seawater standards to which uranium was added and determined following coprecipitation and flotation. The results show an average recovery of 82 % (Table I). Following separation after according the seawater a similar flotation treatment, approximately 80 of the remaining uranium can be recovered resulting in a combined recovery of about 96 %. The recovery data suggest the presence of the following equilibrium system: 4 Fe8+w
+ 3UOdC03)34-(,q) e Fer [UO~(C03)3h)
It is postulated in accordance with the Paneth-Fajans-Hahn rule (12) that the effectiveness of the collector depends upon the ability of Fe3+ reacting with the counter ion adsorbed to form a compound of low solubility; in this case, ferric uranyl carbonate with the resulting shift of the equilibrium to the right. However, the forward reaction is not quantitative as evidenced by the 82% recovery. In support of this view, previous work ( I S ) has shown that both ferric hydroxide and thorium hydroxide are able to collect molybdenum as molybdate quantitatively from seawater whereas aluminum hydroxide is a very poor collector of molybdate. Solubility tests have shown that ferric and thorium molybdates are insoluble whereas aluminum molybdate is relatively soluble. No information apparently is available on the existence and solubility of ferric uranyl carbonate. An additional factor, as yet unclarified, that may be involved in the recovery of uranium is the effect of pH on the stability of U0~(C03)3~-, particularly at pH 5.7 since the complex may not be stable in acid. Although both mechanisms may be involved to varying degrees it is believed that the former is the more dominant. The statistical studies on a test series of five replicates for recovery of uranium from seawater spiked with 6.0 pg of uranium show a relative standard deviation of 2.6% (Table 11). In the extraction of uranium from an aqueous system, many workers employ aluminum nitrate as a salting agent (7, 8). With its use, the extraction of uranium as shown by the absorbance of the Rhodamine B complex was sharply diminished, This was traced to the small amount of aluminum extracted by the ethyl acetate solvent which precipitated as AI(OH)$ at pH -7-8. The solid aluminum hydroxide ap(11) Y . S. Kim and H. Zeitlin, Anal. Chim. Acfa, 46, 1 (1969). (12) 0. Hahn, “Applied Radiochemistry,” Cornell University Press, Ithaca, N. Y., 1936. (13) Y . S. Kim and H. Zeitlin, Anal. Chim. Acta, 51, 516 (1970).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 11, SEPTEMBER 1971
parently adsorbed the uranyl-Rhodamine B complex on its surface, resulting in decreased absorbance. Calcium nitrate which does not cause precipitation was substituted and worked satisfactorily. In the 6- to 8-pH range the degree of extraction and the absorbance of the pink complex are reproducible (Figure 3). The presence of W04*- and Moo4*- revealed no interference in the formation of the Rhodamine B complex. The calibration data for seawater are given in Table I. The absorbance of 0.077 represented the uranium originally present in the seawater which amounted to 3.2 pg/l., in general agreement with other workers (4,14). Of the trace metals in seawater, uranium is considered to be the only one which might command a high enough market price in the foreseeable future to warrant extraction as a commercially attractive possibility (IS). In excess of 80%
of the dissolved uranium can be separated in less than five minutes o n a laboratory scale by the flotation technique. A process for the large-scale separation of uranium by means of a filtering bed composed of titanium hydroxide has been tested. Preliminary work, however, has shown that 44x of the dissolved uranium was extracted per cycle which requires four days (16). Although the economical extraction of trace metals from seawater appears unlikely at this time, with the possible exception of uranium, the work reported herein suggests that any effort in this direction should consider the use of a flotation technique. RECEIVED for review February 1, 1971. Accepted May 19, 1971. (16) N. J. Keen, J. H. Miler, and K. Spence, “Conference on the Technology of Sea and Sea-Bed,’’Vol. 2, S. B. 21, Her Majesty’s Stationery Office, London, 1967.
(14) Y. Sugirnura, Nature, 204, 464 (1964). (15) R. Spence, Talanta, 15, 1307 (1968).
Analysis of Binary Mixtures by Thermometric Titration Calorimetry Lee D. Hansen and Edwin A. Lewis Chemistry Department, University of New Mexico, Albuquerque, N. M . 87106 The objective of this study was to determine the feasibility of analyzing, by a single calorimetric titration, a binary mixture of reactants having equal or nearly equal equilibrium constants. The relative error i s largely dependent upon the magnitude of the difference in the enthalpy changes for reaction of the two components. Mixtures of sodium acetate and pyridine and of phenol and glycine titrated with perchloric acid and sodium hydroxide, respectively, were used to test the method. The relative error in the method was about 5% for millimolar amounts of each component in the test systems.
BINARY MIXTURES of two reactants can be classified into four general types with respect to the differences in the free energy and enthalpy changes of their reactions with a common titrant. Two of these types, which have nearly equal enthalpy changes and either nearly equal or significantly different free energy changes for reaction with the titrant are of little interest here since the reactants are calorimetrically indistinguishable. The remaining two types of mixtures have enthalpy changes for the reactions which are significantly different. The case where the free energy changes are significantly different has previously been thoroughly studied (1-3). In this case, assuming quantitative reactions, the enthalpogram will exhibit a distinct end point for each reactant. The enthalpogram for the case in which the free energy changes are equal (or nearly equal) and the enthalpy changes are significantly different exhibits only one end point. However, a single calorimetric titration provides sufficient
information to analyze a binary mixture of this type if the reactions are quantitative. (Calorimetric titration as used here refers to the measurement of heat changes as distinguished from thermometric titration which refers to the measurement of temperature changes.) The present study reports the results of analysis by calorimetric titration of test mixtures containing sodium acetate 0) and pyridine (pK, = 5.3, AHI 5), (pK, = 4.8, AHi or glycine (pK, = 9.8, AHr 11) and phenol (pK, = 10.0, AH 6 ) (4). These particular mixtures were chosen because the two reactants in each case have p K values that differ by less than 0.5, AH values that are significantly different, and the mixtures react quantitatively with strong acid and strong base, respectively. The phenol-glycine mixture was also of interest because of the similarity of this system to a polypeptide containing tyrosyl residues and free Q. or c amino groups.
-
-
THEORY
Since the total number of moles, I ~ T , and the total heat, can be determined in a single calorimetric titration (see Figure 2), simultaneous solution of Equations 1 and 2 with known AH values readily yields the number of moles of the reactants A and B, nA and n B , respectively, as shown by Equation 3. QT,
nT QT
(1) L. S. Bark and S. M. Bark, “Thermometric Titrimetry,” Vol. 33, International Series of Monographs in Analytical Chemistry, Pergamon Press, Elmsford, N. Y., 1969. (2) H . J. V. Tyrell and A. E. Beezer, “Thermometric Titrimetry,” Chapman and Hall, Ltd., London, 1968. (3) J. Jordan, in “Treatise on Analytical Chemistry,” I. M. Kolthoff and P. J. Elving, Ed., Part 1, Vol. 4, Wiley-Interscience, New York, N. Y., 1968, pp 5175-5242.
-
-
n.4
=
(QT
= ?I&. &
=
nA
+ n~
(1)
-tnsdHe
(2)
- nTL\HB)/(AH, - AHB)
(3)
(4) J. J. Christensen and R. M. Izatt, Table, Heats of Proton Ionization and Related Thermodynamics Quantities, in “Hand-
book of Biochemistry with Selected Data for Molecular Biology;’ H. A. Sober, Ed., The Chemical Rubber Publishing Company, Cleveland, Ohio, 1968, pp J49-Jl39.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 11, SEPTEMBER 1971
6
1393
