Rapid Determination of Lead-210 and Polonium-210 in Environmental Samples by Deposition on Nickel RICHARD
1. BLANCHARD
U. S. Department of Health, Education, and Welfare, Robert A. Toft Sanitary Engineering Center, Cincinnati, Ohio
A procedure for determining 210Po and ll0Pb simultaneously in environmental samples was developed by finding the optimum conditions for the spontaneous deposition of 210Po and 2lOBi on nickel from an HCI solution. The effects of HCI concentration, hydrazine concentration, temperature, time of deposition, surface area of the planchet, and the presence of the HCIO4 used to ensure oxidation of environmental samples were studied. Recoveries of 210Po, *l0Bi, and 210Pb were determined for various environmental samples spiked with a 210Pb210Bi-210Po standard. The average efficiencies for the deposition of 210Po and 210Bi were 96 f 4% and 94 f 3%. The average efficiency for the deposition of *loPb,determined by decay measurement, was 13 f 2%. The 210Poconcentrations were determined by alpha counting. The 21OPb concentrations were determined by beta counting the 210Bidaughter. An aluminum absorber was used to eliminate the 21OPo alpha and 210Pb beta particles.
T
o DETERMINE the relative importance of 210Poas a contributor to the background radiation and the behavior of 210Poin environmental and biological systems, it is often helpful to know the relationship between 210Po and 210Pb(5, 7 , 9 -11, 13, 18). The method of determining 210Po by spontaneous deposition on silver has been thoroughly investigated (2, 4, 5) and used in determining polonium in biological and environmental samples (10-12, 18-21). Lead-210 has been determined by either measuring the growth of 210Poin a sample three to six months after the initial 210Pohad been removed (S), or by measuring the 210Bi ingrowth after separating the lead by electrodeposition (8, 17), ion exchange (15), or solvent extraction (1, 6, 14). Because these methods require either a number of months to allow for 210Po ingrowth or a number of chemical manipulations, a sensitive procedure was developed for rapidly determining both 210Pband 210Po. When nickel is used for the deposition of polonium from an HC1 solution, the quantitative deposition of polonium is
reported to be accompanied by the deposition of some bismuth and lead (4, 20). The oxidation potentials (EO) for the reactions of polonium(IV), bismuth (111), and lead(I1) with nickel are (26) :
+ 2 Nio = Po0 + 2 Ni(1I) Eo = + 0.98volt 2 Bi(II1) + 3 Xio = 2 BiO + 3 Xi(I1) Eo = + 0.57 volt Pb(I1) + Nio = PbO + Ki(I1) Eo = + 0.12 volt Po(1V)
Although other factors will influence these oxidation potentials and the spontaneous deposition of these radionuclides on nickel, all three may be expected to deposit spontaneously on nickel to some degree. Lead-210 can be determined if either lead or its bismuth daughter is deposited quantitatively. When radioactive equilibrium is known to exist between 210Bi and zloPb the deposition of 210Biwould be preferable because the 1.17-m.e.v. beta particles of 210Biare much easier to count than the relatively weak 0.018-n1.e.v. beta particles of 210Pb. Since the half-life of 210Biis only 5.0 days, radioactive equilibrium with 210Pbis achieved in a short time. EXPERIMENTAL
Apparatus. Depositions were made with a multiple unit consisting of four stirrers and a constant temperature bath. Polished 0.025-inch by 1.0inch and 1.5-inch diameter nickel disks were used. X thin coating of polyethylene was melted on the unpolished side of the disk to prevent deposition. Adhesion of the polyethylene to the disk was enhanced by first sanding the nickel surface with garnet paper. The polished side was cleaned consecutively with trichloroethylene, HCl, and distilled water. The alpha activity was determined by exposing the deposited sample to a ZnS(Ag) screen on a 2-inch E M 9536B photomultiplier tube-scaler system (23). The beta activity of *lOBi was determined with an anticoincidence low-background beta counter. A 5.22 mg./sq. cm. aluminum absorber was used to absorb the 5.30-m.e.v. alpha particles of 210Poand the 0.018-m.e.v. beta particle group of 210Pb. An 8% absorption correction was applied to the measured 210Bibeta activity.
Standards. A 210Pb-210Bi-2qrPo standard in radioactive equilibrium was obtained from the United Kingdom Atomic Energv -" Authoritv, Amersham, U.I(. Procedure. The sample is weighed, dissolved in HNOs. and then completely oxidized in-'hot' concentrated HN03 and 72% HC10, (3, 5 ) . All traces of HKOI are removed by heating t o fumes of HCIOa. The solution is cooled, its volume adjusted to 60 ml. with distilled water, and neutralized with 1 8 5 SaOH. The solution is transferred to a 150-ml. beaker and diluted to 100 ml. with distilled water and made 0.1S in HCl. Two-hundred milligrams of ascorbic acid are added and the solution is heated to 85' C. beneath a watch glass (.lscorbic acid is required to eliminate the interference of iron by reducing it to the ferrous state.) The sample is placed in the deposition apparatus at 85" C. A 1.5-inch nickel disk is suspended in the solution with a platinum wire and the deposition is allowed to proceed for 3 hours a t a stirring rate of 400 r.1i.m. The hides of the beaker are washed down with 0.l.Y HC1 every hour. The nickel di.sk is removed, rinsed with distilled water, and dried a t room temperature. The 210Pocontent of the sample is determined by alpha counting and the 210Pbcontent by beta counting the 2lOBi daughter with a 5.22-mg./sq. cm. aluminum absorber. The alpha counting time was 24 to 48 hours and the beta counting time was 1000 to 2000 minutes, depending upon the count rate. Since a fraction of the 210Pbdeposits Tvith the 21'JPoand *l0I3i,the determination of the radioactivity associated with 2IOBi is complicated. If the sample is counted immediately following deposition, little or no correction is necessary because the aluminum absorbs the beta activity associated with the 210Pb. If a t all possible, this practice should be adopt'ed. If, however, it is not possible to determine the activity of 210Bi soon after deposition, fresh 210Bi will grow int'o the sample from the deposited *I0Pb. The initial 210Bi activity may then be determined, however, by counting the sample a second time after 4 or 5 days have elapsed, and solving the following equations simultaneously for d o B i . "
VOL. 38, NO. 2, FEBRUARY 1966
I
189
In the above equations, A1 and Az are the beta activities with absorbers a t times T1and Tz, A0 refers to the initial activities, T1 and T z are the elapsed time from deposition to counting, and X is the decay constant of 2lOBi. It is assumed that the only beta-emitting nuclides deposited are 21OBi and 210Pb, and that only the beta particles of 2lOBi are counted. The number of radioactive cations in the environment which will spontaneously deposit on nickel from a 0.1N HC1 solution is very limited. Contamination of the nickel with other radioactive isotopes of Bi, Pb, or 'OSRu, the latter a fission product, can, due to their different half-lives, be easily detected by decay measurements and the count rate corrected. Also, the ruthenium cation will volatilize when fumed with HC1O4. Preliminary Study. A preliminary study determined the optimum conditions for the deposition of zloPo and
210Bi on nickel from distilled water solutions. The variables considered were the HCl concentration, hydrazine concentration, temperature, time of deposition, surface area of the nickel disc, and the presence of HC104 used to insure oxidation of environmental samples. To determine the effect of HClO4, 210Po-210Bi-210Pbtracers were added to various amounts of 72% HC104. After the solution was neutralized with NaOH, the solution was diluted to 100 ml. with distilled water and made 0.1N HCl. The solution was then placed in a water bath a t 85" C., and deposition allowed to proceed with stirring for 11/2hours on a 1.5-inch nickel disk. To complete the preliminary study, samples of various environmental material were weighed, spiked with 500 d.p.m. of a 21'JPo-210Bi-z10Pb standard, and wet-ashed with concentrated "03 and 72% HC104. The method used was that given under "Procedure." Following the initial 3-hour deposition, a second nickel disk was placed in the solution for an additional 11/* hours.
Table I. Recovery of *loPo and 210Bi vs. HCI Concentration at 60" C.
ZlOBi, 70 55.5 56.1
ZlOPo, yo 60.1 58.7 52.4 49.0
HC1, N 0.1 0.5 1.0
2.0
50.1
43.5
Table II. Recovery of zloPo and zlOBi vs. Temperature in 0.1N HCI
c.
IlOBi, yo 55.5 61.7 69.6
ZlOPo, % 60.1 67.3 73.5 72.3
60 70 80 90
70.1
Table 111. Recovery of zlOPoand 210Bi vs. Time in 0.1N HCI at 85' C.
ZlOPo, 70 79.2 80.0 92.0
Hours 1 2
3
210Bi,Yo 78.8 82.0
92.4
Table IV. Recovery of 210Po, 210Bi,and 2lOPb vs. Time in 0.1 N HCI, on 1 I/zInch Planchet
210P0, yG 210Bi,70210Pb,% 100 83 10 98 97 13 103 101 16
Hours '/Z
1 1'/2
Table V.
HClO4 1.2N 2.3N 4.6N 7.ON
190
RESULTS AND DISCUSSION
The results of the preliminary study are shown in Tables I t o IV. One determination was made a t each change of conditions and the recoveries listed were measured on 1-inch nickel disks unless specified differently. At a deposition time of 1 hour, the ~ recovery of both zlOBi and 2 1 0 Pdecreased when the HC1 concentration exceeded 0.5N. The results in Table I1 illustrate the increased recoveries obtained by increasing the temperature from 60" to 80" C. for a deposition time of 1 hour. Higher temperatures did not appear to improve the deposition. The recoveries of 210Bi and zloPb were affected little by the addition of hydrazine. Hydrazine, therefore, is omitted in the procedure unless the sample is thought to contain such interfering ions as gold, mercury, platinum, and tellurium (4). Although it has also been reported that hydrazine retards the discoloration of silver ( 4 ) , the discoloration on nickel was not noticeably affected by hydrazine. The recoveries of both polonium and bismuth were improved by increasing the time of deposition, as shown in Table 111. Quantitative yields were not obtained, however, even when the deposition was continued for 3 hours. The 1-inch nickel disks were replaced by similar 1.5-inch diameter disks and
The Recovery of 2 1 0 Pand ~ 21OBi from HClO4-HCl Solutions Z10P0, % 2l0Bi, % ' Condition of planchet 102 100 Very little discoloration 92 Very little discoloration 98
102 88
ANALYTICAL CHEMISTRY
92 75
Noticeably discolored Considerable discoloration
quantitati4e recoveries of both polonium and bismuth were obtained when the deposition was continued for 1 hour or longer. The results are given in Table IV. Considerably less discloration was observed on the larger disks, which may have retarded the deposition on the smaller planchets. Because of the volatility of polonium, it is not possible to dry-ash environmental samples a t high temperatures. Consequently, the preparation of many types of environmental samples requires wet ashing with strong oxidizing agents to assure complete digestion. Because HC104 is commonly used for this purpose, the recoveries of polonium, bismuth, and lead from HC104-HC1 solutions were investigated. The results are shown in Table v. Polonium was recovered quantitatively from a 4.6N perchlorate solution while the recovery from a 7 N perchlorate solution was reduced to 88%. The recovery of bismuth was quantitative from a 1.2N perchlorate solution, was a constant 92% for 2.3N and 4.6N solutions, and was reduced to 75% when deposited from a 7 N perchlorate solution. Table V I gives the recoveries obtained from a 4.6N perchlorate solution for various times of deposition. The deposition of polonium and bismuth appears to be complete after 2 hours Immediately following the initial depositions, a second nickel disk was placed in the solution and the deposition continued for an additional ll/z hours, When the initial deposition was equal to or greater than 2 hours, less than 1% of the initial zlOPoand zlOBiactivities was found on the second disk. When the perchlorate concentration was increased to 7 N and the deposition time extended to 3 hours, 97 f 3% of the polonium and 95 f 3% of the bismuth were deposited. The activity associated with the second disk that was placed in the solution immediately following the initial deposition for 11/2 hours was equivalent to 1% of the polonium and 4% of the bismuth initially present. The uncertainty is quite large for the ZloPb results shown in Table VI since these values were calculated from the decay of the deposited 210Bi,and since such small quantities of lloPb were deposited. It is evident, however, that
Table VI. The Recovery of ZlOPo, 210Bi, and 210Pbfrom 4.6N HC104-0.1 N HCI Solution vs. Time of Deposition
Hours
210Po,Yo
zlOBi,70
zlOPb,7 0
l'/z
102 f 3 99f3 103 f 3
92 f 3
6 i5 7 f 5 12 i 5
2
3
lOOf3
98
f3
Table VII.
Sample White
potatoes
Radishes Carrots Lettuce Cabbage '&'hole hea t Well water Bone ash Beef tissues
w
Quantity of sample, grams 110.7
106.0 97.5 102.3 104.1 20.0 1000 10.0
25.4
Recovery of Added zlOPo,210Bi, and *l0Pb from Environmental Samples
210Po,% (f3)
Digestive agent HC104-HN03 "03
" 0 3
HClOd-HNOa HClOL-HNOs HC~O~-HNO~ HClO4 HC1 HClOrHNOa
a small per cent of the lead in a sample will be deposited along with the polonium and bismuth. The results of the preliminary study indicate that under certain conditions i t is possible to deposit spontaneously both polonium and bismuth quantitatively on nickel. These studies, however, were conducted on tracer solutions and not on actual solutions of environmental materials. Therefore, the recoveries of 210Po-210Bi-210Pb were investigated from solutions of various environmental material. Listed in Table VI1 are the quantity of sample and digestive agent used, and the recoveries obtained for zlOPo,210Bi, and ZloPb from the environmental samples investigated. I n the last two columns are the per cents of the initial activities of ZlOPo and 210Bithat were deposited on the second nickel disk. The average recoveries measured for ZlOPo and 210Bi were 96 f 4% and 94 f 3%, respectively. The errors are based on counting statistics for one standard deviation. The low polonium yield from the sample of radish may have been due to incomplete digestion since the complete recovery of polonium was not attained on the combination of both nickel disks, and " 0 1 alone was used as the digestive agent. The total recoveries of zlOPoand 210Bi obtained on both disks were 99% and 97%, respectively, which indicate that both nuclides are available for deposition on nickel when complete digestion of the sample is achieved. The recoveries measured for 210Pb varied from 8 to 20% with an average of 13%. These values were obtained by observing the decay of the deposited ZlOBi. The activity which adsorbed on the polyethylene covering the back of the nickel disk varied from 1 to 5%. The higher adsorption was noted when poor adhesion was obtained between the metal and the polyethylene. The sensitivity of the method is dependent upon the quantity of sample available, the counting time, and the alpha and beta backgrounds. Since alpha backgrounds of less than 0.5
Table VIII.
98 83 97
96 89 91 93 90 96 95 91 102
101
102 91
101
98
90
14 12 13 15 13 20 15 9 8
Second planchet zlOBi, 70
ZlOPO, 7 0
