Determination of plutonium in sediments by solvent extraction and α-spectrometry

Analytica Chimica -4cta.111 (1979) 265-274 o EIsevier Scientific Publishing Company, Amsterdam

DETERlMINATION

EXTRACTION NARAYANI E. WRENNT

OF PLUTONIUM

Printed in The Netherlands

IN SEDIMENTS

BY SOLVENT

AND a-SPECTROMETRY

P. SINGH*t

PAUL LINSALATA,

Institute of Environmental Medicine. Tuxedo. A’\‘. Y. 109s 7 (U.S.A.) [Received

-

RICK

GENTRY

and MCDONALD

New York University dlcdical Center. BOX SI 7.

10th April 1979)

SUMMARY A simple technique for the determination of environmental levels of plutonium in a highly complex matris (sediments containing very high amounts of iron and other metals) is reported_ The sediments, collected from the Hudson River Estuary with an Emory dredge, were hand-homogenized before a sampIe aliquot was taken. Samples were airdried, weighed, spiked with z’2Pu tracer, and heated at 400°C for 24 h. Plutonium was leached from the sediment with an acid mixture. The leachate was filtered, and plutonium coprecipitated with iron by adding ammonia solution_ After dissolution, plutonium was estracted with 20% trilaurylamine in sylene, the extracts were thoroughly acid-washed to remove uranium and thorium traces, and plutonium was then back-extracted with 2 hl sulfuric acid prior to electrodeposition onto a platinum planchet. The isotopic composition of plutonium was determined by a-spectrometry. Tracer yield and plutonium concentrations determined on aliquots of the same samples by this method and by an ionexchange technique were not significantly different.

Many different biological and environmental samples, e.g. air, food, water, escreta, fused rocks and soil, have been analyzed for plutonium for moni-

toring purposes [l-8] . Plutonium has often been separated from other elements by an ion-eschange technique [9--111. To estend a study of the isotopic distribution of plutonium in the Hudson River Estuary along with other r-emitting radionuclides, a method reported by Chu [12] was tried. Although the method gave satisfactory tracer recovery, it was time-consuming on a routine basis. Therefore, a method involving solvent estraction with trilaurylamine was undertaken. Veselsky [ 131 has reported a technique for the determination of plutonium in environmental samples by solvent extraction with trilaurylamine including back-washing with hydrochloric and hydrofluoric acids. This paper deals with certain modifications, particularly the complete removal of thorium-228, which has an a-energy spectplm overlapping that of plutonium-238. *Present address: Radiobiology U.S.4.

Division, University of Utah, Salt Lake City, Utah 84112,

Trilaurylamine (TLA) was chosen because of its high distribution coefficient for plutonium within a wide range of nitric acid concentrations (Z-8 M) and also because of its low extraction coefficient for uranium [14] ; it has been used successfully to determine plutonium [ 151 and thorium [ 161 in soft tissues. EXF’ERIMENTAL

Reagents

and apparatus

All reagents used were of analytical grade. Electrodeposition apparatus_ The plating cell consisted of an elongated 22-mm cap which held a 1-0~. polyethylene bottle from which the bottom had been removed. The cap and space for a platinum plating disc (17.6 mm diameter, 0.005 in. thick, mirror-finished on one side) supported by a nickel backing disc (17.6 mm diameter) and could be firmly screwed into the polyethylene bottle forming a leak-tight plating cell. A threaded brass bushing was molded into the cap, thus making the electrical contact with the platinum disc cathode by clip leads The cell was supported on a lucite pedestal which rested in an ice-water bath. The anode was a platinumiridium rod (4 in. long, l/16 in. diameter) with a circular beading on the bottom end. It was connected through a constant-speed stirrer to the positive outlet of the power supply [l] _ The power supply furnished a constant current in the range O-10 A and a constant voltage in the range O-36 V. Sample prepara fion

Sediment samples were collected during the summer and fall of 1976 at a variety of locations occurring between 19.3 miles north (Newburgh Bay) and 7 miles south ‘(Buoy 14 East) of Indian Point nuclear power station_ Sediment samples, collected on board a 22-foot Aqua-Sport with an Emory dredge, were immediately hand-mixed to obtain homogeneity and placed inside 500-ml polyethylene containers. The Emory dredge is believed to capture the upper 2-4 inches of bottom sediment, the area thought to contain the highest concentration of plutonium and other radionuclides [17]. After collection, sediment samples were air-dried, crushed to obtain uniform particle size, and stored. An aliquot (20 g) was weighed and transferred to fused quartz baking dishes, and an aliquot (2--3 dpm) of plutonium242 tracer (by weight) was dropped on the surface of the sediment sample. The advantages of “‘Pu instead of ‘36Pu as tracer have been adequately discussed [lS, 191. After the tracer had dried, the sample was heated at 400°C for 24 h in a muffle furnace to remove organic matter. The sample was cooled to room temperature, and plutonium was leached with 400 ml of a mixture of concentrated nitric and hydrochloric acids (3 + 1). After the sample had been filtered through Whatman no. 42 paper, the sediment portion was again leached, the leachates were combined, and the remaining sediment residue and filter paper were discarded_

26’7

Procedures

After plutonium had been leached from the sediment sample with two 400-ml portions of aqua regia, filtered twice, and the two leachates combined, the solution was boiled down to 100 ml. The solution was diluted to 300 ml with distilled-deionized water, and iron was precipitated by the slow, careful addition of ammonia liquor until precipitation was complete. After centrifugation, the aqueous portion was discarded and the precipitate washed with a dilute ammonia solution (1 + 20). Washing and centrifugation were repeated at least twice to insure the removal of the sulfate moiety (confirmed by adding BaCl, to a few ml of solution)_ After the aqueous portion had been discarded, the precipitate was dissolved in a minimum volume of concentrated nitric acid and divided into two equal portions. Solvent extracfion. The acidity of both portions of the solution was adjusted to 8 M following the addition of ca, 100 mg of sodium nitrite and gentle heating of the solution. The sample was cooled to room temperature (ice bath) and transferred to a 250-ml separator-y funnel. An equal volume of 20% TLA in xylene, pre-equilibrated with ea. 10 ml of 8 M HN03, was added to the separatory funnel, which was shaken gently for 10 min. After the two phases had separated, the aqueous phase was removed and discarded. (To enhance recovery, a second and third extraction proved necessary.) An equal volume of 10 M HCl was added and the separator-y funnel was shaken gently for 5 min to remove traces of thorium. After the aqueous phase had been discarded, the organic phase was shaken twice (5 min each time) with an equal volume of nitric acid (1 + 1) to remove uranium. The aqueous phase was discarded and plutonium was back-extracted with 2 M H&30, by shaking gently for 10 min. Back-extraction with 2 M HzS04 repeated twice, further enhances recovery. The other portion of the solution was Anion-exchange separation_ processed by the ion-exchange technique reported by Chu [ 121_ The plutonium solution, previously adjusted to 8 M in nitric acid and heated in the presence of sodium nitrite, was loaded onto a 20-ml column containing BioRad AGl-X4 (100-200 mesh) anion-exchange resin pre-equilibrated with nitric acid (1 + 1). Plutonium was eluted from this column with 100 ml of 0.4 M HN03-0.01 M HF solution. This solution, after evaporation and subsequent additions of 8 M nitric acid, was Ioaded onto a smaller (10 ml) column containing the pre-equilibrated AGl-X4 resin. This column was then rinsed consecutively with 10 M HCI and 8 M HN03 to remove traces of thorium and uranium, respectively_ Plutonium was again eluted with 100 ml of 0.4 M HNO,-0,Ol M HF solution_ The solution was evaporated to dryness and the residue was converted to the chloride form prior to electrodeposition by adding several l-ml portions of concentrated hydrochloric acid and evaporating to dryness after each addition E~ec~rode~osi~~o~_ The solution obtained after back-extraction of plutonium with 2 M H&lo4 in the extraction procedure was evaporated to dryness. If black organic particles floated during the evaporation, several drops

of concentrated nitric acid and 30% hydrogen peroxide were added to decompose them (during the back-washing of plutonium from the TLA phase, some organic matter was usually entrained with the aqueous phase). The solution thus obtained was evaporated to dryness. The residue containing plutonium was dissolved in 1 ml of 2 M H&GO4 and transferred to the plating cell. The beaker was washed twice with l-ml portions of 2 M H,SO, and the washings were transferred to the cell. Following the addition of one drop of methyl red indicator, concentrated ammonia followed by (1 + 1) ammonia was added dropwise until a yellow color appeared- A few drops of 2 &I H,SO, were added to restore the red color and finally 3-4 drops of ‘2 M HzS04 were added to- give the required pH of the plating solution. The platinum anode was positioned l-2 mm above the plating disc and the plutonium was electroplated at a current of l-2 A for 1 h. The electrolyte was quenched with 3-4 drops of ammonia solution (1 + 1) at the completion of the plating process. The cell was dismantled and the platinum disc was rinsed with distilled-deionized water followed by ethanol and heated to redness with a Bunsen burner. Plutonium tracer recovery and the isotopic composition were determined by ar-spectrometry_ In the case of the ion-eschange technique, plutonium was electroplated from hydrochloric acid solution_ The residue containing plutonium was dissolved in 1 ml of 1 iv1HCl and transferred to the plating cell. The beaker was washed twice with l-ml portions of 1 M HCl and transferred to the plating cell- The desired pH was achieved as described earlier except that 1 iv1 HCl was used in place of 2 M H,SO,. Plutonium was electroplated as described above. A superior final electroplating procedure 1301 is currently being used. Basically, the procedure involves the complete removal of organic compounds entrained in the back-extract by treating with nitric acid and hydrogen peroxide in l--2 ml of 18 AI H2S0,; the volume is then reduced to 0.5 ml and the beaker and watch glass are rinsed with 3 ml of distilled--deionized water. After addition of 2 drops of 0.1% thymol blue solution, the solution is adjusted to a yellowish color by addition of concentrated ammonia vapor, which has the advantage of avoiding both an increased plating volume and the possible formation of any local polymeric plutonium at this critical stage. The soiution is then placed in the electroplating celI along with three washings of the watch glass and beaker with 0.1s M H$O_,. Plutonium is electroplated for up to 2 h at 1.2 A, and then 10 ml of 1.5 M ammonia solution are added to quench the solution_ The electrode is removed after a quenching period of 1 min with the power supply still on. The electroplating cell is rinsed with an additional 10 ml of 1.5 hl ammonia sclution. The planchet is then rinsed, flamed, and counted as usual. Procedure modifications Attempts to improve chemical yield were made by extracting plutonium from 3 W HNO, solution; by back-extracting plutonium with different

agents including 0.03 M hydroxylammonium HN03--0.01 M HF, and with 2 M H2S04.

chloride

in 0.01 IM HCl, 0.04 M

Finally, changes were made in the percentage of TLA used, the number of HN03 and HCl scrubbings, and the number of back-extractions. In an accompanying experiment, the leachability of fallout plutonium with aqua regia was tested by splitting one sample into two aliquots. One aliquot was analyzed after being treated only with aqua regia (i.e., by the usual procedure) and the other aliquot was analyzed after being treated with aqua regia followed by dissolution in hydrofluoric acid. The results from the aqua-regia leach and the hydrofluoric acid dissolution were compared for plutonium-239, 240 content_ Standardization

Counting efficiency for the lOOO-channel Nuclear Data analyzer supplied with a 300-mm Ortec surface barrier detector was determined by counting a platinum planchet standardized with known amounts of ‘47_Pu (4.90 MeV), ‘“9Pu (5.16 MeV) and *“Pu (5.50 McV). The average counting efficiency was 17%; the analyzer energy calibration averaged 5.6 (keV/channel), and background activity in the energy regions of interest. was usually less than 0.001 cpm. A further reduction in counter background and simultaneous increase in counting efficiency (26%) was achieved by counting samples in a 16,384-channel Ortec 6240B multichannel analyzer equipped with 8 dual-port a-spectrometric modules. RESULTS

AND

DISCUSSION

Comparison of two techniques The results of repeating the split sample experiment 11 times are shown in Table 1; mean recoveries were not significantly different (two sided “Ttest” with Q’ = O-05), averaging 35% for solvent extraction and 37% for the ion-exchange procedure_ Although the mean recoveries for the two techniques were very similar, the superiority of solvent extraction, in terms of time, cannot be overlooked_ The low variable recoveries from both methods may be due to the very complex matris of these sediments, including a very high iron concentration as well as other metals [231. The plutonium-239,240 concentrations reported for the two techniques, treated as paired data, showed no significant difference in the sign test (p < 0.01). The ratios of the plutonium 239,240 contents given by each procedure are sufficiently close to 1.0 (mean ratio = 1.004) to indicate an absence of systematic bias between the two procedures. The precision of the solvent extraction method was checked by analyzing two or three aliquots of the same sediment specimen separately_ Four specimens were analyzed; the individual results (Table 2) fall within one standard deviation (representing the counting error) of each other, indicating that the samples were sufficiently homogenized prior to analysis and

2'70 TABLE

1

Comparison of plutonium-242 ion-eschange techniques Samult

Darcof collection

TLX

extraction

Recovery I 2

3 1 5 6 7 8 9 10

11

-18~9 29 ~6 132-z 14 23 33 r3 26 t-1 195 511 71-6 236 116 214 %18 11rl

7+1 921 50 -3 71 I3 31x2 6912 6852 16e2 12 +2 42 x2 Mlmn

by TLA and

Ionexchange

Pu found" (5%) (pCi kz-'. dry)

921

11/19/76 11/19/56 10/15/i6 11/19!76 11119176 11119/76 81-l/76 11/19/76 8/11i6 8/-l/76 4129/ii

aPlutonium-239,

tracer recoveries and sediment concentrations

Pu founda Rrccxer?; (Cc) (pCikg_'-dry,-) 11 +1 6123 ii z 4 10 +1 34 z!? 2s I-' 25 I2 38 k3 52 I 5 -16 14 38 z-2

0.98

49 t8 29t3 16 = 2 23 +i 22 c-4 2-i + 4 178 ~12 6-I +8 20859 221 112 12z.2

1.0 0.81 0.61 1.15 0.96 1.10 1.11 1.13 0.97 0.92 Mean

Xlcan = 37Fc

= 35%

Ratio ofPu found TLXlion cschange

ratio = 1.004

210.

T_4BLB 2 Plutonium concentrations sample by TLA extraction Sample

Sampling station

in Hudson River sediments from aliquots of the same sediment

Date of collection

23*pu

t39_2-OPU (pCi kg-‘, -_---_---

1s 11 16 t In = 21.5 t 21

(pCi kg-‘,

dry) --

15 r 6 3?1 9x3

1X 1B 1c

5

2A 2B

2

11/19/‘76

3s = 9 32 t 5 Mean f 10 = -IO t 11

252 0.0

3A 3B 3c

6

10/15/i6

2s t 7 16 ! 5 1s 2 4 Mean I 10 = 21 I 6

-it3 0.0 2t2

4A -lB

3

10/15/76

13 ? 4 11 56 Mean : 1 u=12!1

l?l ‘7,s

S/4/i6

214 t 195 ? 236 t Mean

dry)

that the method yields precise results. The overall precision varied between 8 and 28%; clearly most of this could result from counting error. Repeated analyses by the two techniques showed some interesting characteristics. The major observation was that ca. 10 times as much uranium [a (238U) = 4.2 MeV and CY(134U) = 4.77 MeV] found its way through the

271

procedure_ solvent extraction process compared to the ion-exchange Assuming a sediment uranium concentration of 1 pCi g-* [ 211, the discrimination factor for solvent extraction, incorporating three 7 M HN03 washes of the TLA phase, is greater than 90%. As the a-energy difference between 23JU and the tracer “‘Pu (4.902 MeV) is only 130 keV, it is essential to add sufficient tracer (3-10 dpm) and to obtain a sample planchet with good resolution (<70 keV) in order to minimize any recovery error caused by the spectral interference of uranium-234. Although traces of polonium-210 (a-energy z 5.30 MeV) survived the ionexchange procedure, causing some overlap with plutonium 239,240, it was not present in sediments treated by solvent extraction_ Although the presence of thorium-228 (a-energy Z 5.43 MeV) has been observed in ion exchange [22] and solvent extraction [ 131 techniques, it was not found in either of the techniques described here. It appears that washing the TL,4 phase twice with 10 M or 11 M HCI is sufficient to remove ail thorium traces. Figures la and lb provide typical a-spectra from sediment samples analyzed by the so!vent extraction and ion-eschange procedures, respectively _

kl)

-

120 100

80 60 40 20

MeV *

MeV +

Fig. 1. Typical a-spectra for a sedimentsample: (a) aher TLA estraction (count time 3784 min. chemicalyield 42 + 2%); (b) after ion-exchangeseparation(count time 1271 min. chemicalyield 25 + 2%).

272

The leachability u f fallout plu toniunt Conflicting views have been presented regarding the physical characteristics of pIutonium in the environment, Concern has arisen that some of the plutonium may be in a refractory form which may not be completely leaehable from the sediment, thus giving erroneous results. Results of the split sample experiment described, in which one portion of a sample was analyzed by the usual procedure (i-e_, leaching) while the remaining portion was analyzed by leaching with aqua regia followed by hydrofluoric acid dissolution, indicated plutonium-239, 240 concentrations of 13 c 2 and 9 +- 5 pCi kg-’ (dry weight), respectively_ This preliminary result indicates that both methods are equally efficient for leaching plutonium from these scdiments. Similar results were found by Chu [ 121 when the plutonium concentrations after leaching with aqua regia were compared with an aqua-regia leach followed by a fusion method. Factors affecting recovery Unsuccessful attempts were made to improve the recovery of plutonium (single estractions) by extracting from 3 M H-NO, solutions_ The results, summarized in Table 3, indicate t.hat the mean tracer recovery was slightly less (ZS%) when the extraction was carried out from 3 31 HN03 instead of 7 M HN03 (3X%), and more uranium survived the procedure when the extraction was done from 3 M HNO+ A further attempt was made to improve tracer recovery by backextracting plutonium from the TLA phase with a reducing agent such as 0.03 M hydroxylamine hydrochloride in 0.01 XI hydrochloric acid medium; for 3 samples the highest recovery was only 34% with no improvement in the variability of percentage recovery (Table 3). With this technique a lot of material persisted even after heating with sulfuric acid-nitric acid mixtures, eventually yielding a planchet with considerable mass and poor resolution_ The choice of 2 M H,SO, for hack-extracting plutonium was based on the fact that the distribution coefficient for plutonium in 20% TLA from 2 M H,SOJ is O-004 [ 141, insuring quantitative recovery from the TLA phase; an additional advantage is that entrained organic materials are usually easily decomposed by adding only a few drops of concentrated nitric acid before electrodeposition Because chemical yieids were highly variable and relatively low, a simple experiment was done which allowed the yield to be checked after each experimental step. It was found that negligible amounts of tracer were lost. during the leaching and precipitation phases, and the majority of the plutonium-242 tracer was lost during the estraction procedure_ Approximately 3% of the tracer activity that survived the extraction procedure was lost during the combined washings with S M HNOJ and 11 M HCl, and 7% of the tracer activity that survived the entire procedure (i.e., that up to and including back-extraction) was lost during electrodeposition. The final recovery during this experiment was 33%.

273

TABLE

3

Plutonium tracer recovery under different conditions Sample

Sample station

and sediment (see text) Tracer

concentrations

recovery

W) ~_

t?slractiorr

from

found

by extraction

2,Y.

2411pu

(pCi

kg-‘,

with

:3xpu dry)

___ - _--_____--.

3 JI IINO,

1 2

1 1

3

6

15 ! 2

18 ‘. 2

0.0

1

2

1s

32

0.0

61 f 3 19 k 2

of 0.03

2 3

6

dl IlydI-os~lanlnlo,lircm 31 + 4

1 3

721 19 t 4

13

Mean _4ddilion 1

TLA

9r2 it2

0.45 3.3

! 7

t 0.05 ? 2.10

= SS% chloride 1.5

1s + 4 125 11

r 20 r6

L 1.6

1-S k 6.0 6.6 t 5.4

There is reason to believe that the high iron concentration (up to 20%. w/w) in Hudson River sediments [ 231 along with other metals may interfere in TLA extraction and ion-exchange techniques_ The presence of organic material persisting after heating at 400°C for 12 h and that contributed from the various filtrations may also be responsible for large tracer losses during estraction. The current esperimental procedure calls for additional ashing of the sediments with nitric acid and hydrogen peroside after the muffle furnace treatment and prior to leaching. Ashing must be continued before the second leach to remove any organic contribution from the filter paper. Following precipitation and adjustment to a 7 M HNO, solution, three estractions of 20% TLA in sylene are recommended, followed by three washings with ‘7 M HNOJ, to remove uranium and two washings with 10 M HCl to remove thorium. Plutonium is then back-estracted twice with a slightly more selective agent., 0.4 M HNOj--O.O1 M HF, which, compared with the 2 %I H$O., previously used, slightly decreased the amount of uranium and iron stripped from the organic phase. The back-estracting solution was evaporated and ashed complet.ely with H,SOa, HNOj and H,O, before electroplating according to the previously outlined procedure utilizing thymol blue. The resulting planchets were clean and free of mass. This modified technique has reduced the recovery variability and increased the overall mean recovery to about 40%. REFERENCES 1 Health and Safety Laboratory, hlanual of Standard-Procedures, 2 H. Levine and A. Lamanna, Health Phys., 11 (1965) 117.

1976.

274 3 P_ J. Mango, P. E. Kauffman and B. Schleien, Health Phys., 13 (1967) 1325. 4 E. E. Campbell and W. D. Moss, Health Phys., ll(l965) 737. 5 K. Wolfsberg, W_ R. Danieis, G. P. Ford and E. T. Hitchcock, Nucl. Appl. Technol., 3 (1967) 375. 6 M. C. deBortoIi, Anal. Chem., 39 (1967) 375. 7 K. C. PiIlai, R. C_ Smith and T. R. Foison, Nature, 203 (1964) 568. 8 E. L. Geyer, Health Phys., 1 (1959) 405. 9 D. B. James, Los Alamos Scientific Laboratory Ref. TID 4500, January 1967. 10 I. K. Kressin and G. R. Waterbury, Anal. Chem., 34 (1962) 1598. 11 R- F- Buchanan, J_ P. Ferris, K. A_ Orlandini and T. P. Hughes, U.S.A.E.C. Ref. TID 7560 (1958) 179. 12 N_ Y. Chu, Anal. Chem., 43 (1971) 449. 13 J. C. Veselsky, Int. J. Appl. Radiat. Isot., 27 (1976) 499. 14 N_ Srinivasan, M. V. Ramaniah, C. L. Rao, P. K. Khopkar, G. M. Nair and N. P. Singh, Bhabha Atomic Research Centre, Report no. 374 (1968). 15 N. P. Singh, S. A. Ibrahim, N. Cohen and M. E. Wrenn, Anal. Chem., 50 (1978) 357. 16 N. P. Singh, S. A. Ibrahim, N. Cohen and M. E. Wrenn, Anal. Chem., 51 (1979) 207. 17 D. N. Edgington and J. A. Robbins, Proceedings of an International Symposium on Radiological Impacts of Release from Nuclear Facilities in Aquatic Environment, held by IAEA, IAEA/SM-198140 (1975) 245. 18 C. W. Sill, Health Phys., 29 (1975) 619. 19 I. D. Kressin, W. D. Moss, E. E. Campbell and H. F. Schulte, Health Phys., 28 (1975) 41. 20 N. A. Talvitie, Anal. Chem.. 43 (1971) 1827. 21 New York State Department of Environmental Conservation, Annual Report of Environmental Radiation in New York State, 1975. 22 H. J. Simpson and S. C. Williams, Annual Technical Progress Report prepared for ERDA, Dot. COO-2529-l (1975). 23 A. W. McCrone, Geogr. Rev_, LVI(2) (1966) 175.