Chemical Properties of Radon - ACS Symposium Series (ACS

Chapter 18

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Chemical Properties of R a d o n Lawrence Stein Chemistry Division, Argonne National Laboratory, Argonne, IL 60439 Radon is frequently regarded as a totally inert ele ment. It is, however, a "metalloid" -- an element which lies on the diagonal of the Periodic Table be tween the true metals and nonmetals and which exhibits some of the characteristics of both. It reacts with fluorine, halogen fluorides, dioxygenyl salts, fluoronitrogen salts, and halogen fluoride-metal fluoride complexes to form ionic compounds. Several of the solid reagents can be used to collect radon from air but must be protected from moisture, since they hydrolyze readily. Recently, solutions of nonvolatile, cationic radon have been produced in nonaqueous sol vents. Ion-exchange studies have shown that the radon can be quantitatively collected on columns packed with either Nafion resins or complex salts. In its ionic state, radon is able to displace Η , Na , Κ , Cs , Ca , and Ba ions from a number of solid materials. +

2+

+

+

+

2+

+

Since the discovery of the first noble gas compound, Xe PtF^ (Bartlett, 1962), a number of compounds of krypton, xenon, and radon have been prepared. Xenon has been shown to have a very rich chemistry, encompassing simple fluorides, XeF , XeF^, and XeF^; oxides, Xe0 and XeO^; oxyfluorides, XeOF, XeOF^, and Xe0 F ; perxenates; perchlorates; fluorosulfates; and many adducts with Lewis acids and bases (Bartlett and Sladky, 1973). Krypton com pounds are less stable than xenon compounds, hence only about a dozen have been _prepared: KrF^ and derivatives of KrF , such as KrF SbF^, KrF VF , and KrF Ta F . The chemistry of radon has been studied by radioactive tracer methods, since there are no stable isotopes of this element, and it has been deduced that radon also forms a difluoride and several complex salts. In this paper, some of the methods of preparation and properties of radon compounds are described. For further information concerning the chemistry, the reader is referred to a recent review (Stein, 1983). 2

3

2

+

2

+

6

2

2

n

0097-6156/87/0331-0240$06.00/0 © 1987 American Chemical Society

In Radon and Its Decay Products; Hopke, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

2

18.

STEIN

241

Chemical Properties of Radon

Clathrate Compounds Radon forms a series of clathrate compounds (inclusion compounds) similar to those of argon, krypton, and xenon. These can be pre pared by mixing trace amounts of radon with macro amounts of host substances and allowing the mixtures to c r y s t a l l i z e . No chemical bonds are formed; the radon i s merely trapped i n the l a t t i c e of surrounding atoms; i t therefore escapes when the host c r y s t a l melts or dissolves. Compounds prepared i n this manner include radon hydrate, Rn 6H 0 (Nikitin, 1936); radon-phenol clathrate, Rn 3C H 0H ( N i k i t i n and Kovalskaya, 1952); radon-p-chlorophenol clathrate, Rn 3p-ClC H OH ( N i k i t i n and Ioffe, 1952); and radon-pcresol clathrate, Rn op-CR^CÔH (Trofimov and Kazankin, 1966). Radon has also been reported to c o - c r y s t a l l i z e with sulfur dioxide, carbon dioxide, hydrogen chloride, and hydrogen s u l f i d e ( N i k i t i n , 1939). 2


6

5

6

4

Radon Difluoride Most chemical experiments with radon have been carried out with isotope R n ( h a l f - l i f e 3.82 days), which decays by α-emission as shown i n Figure 1. The B" and γ-emitting daughters *Pb and * B i , as well as the α-emitting daughters P o and *Po, grow into radioactive equilibrium with the parent within four hours. We have used the 1.8 MeV γ-ray of * B i , which can be measured conven i e n t l y through the walls of glass or metal apparatus, to determine the position of radon i n tracer experiments, since the bismuth f o l lows the radon as i t moves from one location to another. Figure 2 shows a Monel vacuum l i n e , constructed with Autoclave Engineers Type 30 VM6071 valves and a 0-1000 torr Helicoid pressure gauge, which i s currently used for the preparation of radon com pounds. The radon i s obtained from a Pyrex bulb containing an aqueous solution of radium chloride (approximately 30 mCi of R a C l i n d i l u t e HCI). The bulb i s suspended i n a p l a s t i c cup surrounded by lead bricks and i s attached to the l i n e by a KovarPyrex seal. In a t y p i c a l experiment, approximately 0.1 to 1.0 mCi of R n i s pumped from the bulb through a bed of D r i e r i t e , con densed i n a cold trap at -195°C, d i s t i l l e d i n vacuum from the trap at 23°C to the reaction vessel at -195°C, mixed with a f l u o r i n a t i n g agent, and allowed to react at either room temperature or an e l e vated temperature. The reaction vessel may be a closed-end Monel tube, as shown, or a Kel-F p l a s t i c test tube. When radon i s heated to 400°C with f l u o r i n e , a nonvolatile f l u o r i d e i s formed (Fields et_ a l . , 1962, 1963). It has been de duced from the chemical behavior that the product i s radon d i f l u oride, RnF . (Products of the tracer experiments have not been analyzed because of their small mass and intense r a d i o a c t i v i t y . ) 2 2 2

211

2 1 l

2 1 8

21l

2 1 l

2 2 6

2

2 2 2

2

Rn + F

2

• 400°C

RnF

2


(1)

242

RADON AND ITS DECAY PRODUCTS

2 2 2

Rn (3.823d)

Po

2

0.02%


(RaA,3.05m)

At

2 1 8

(1.3s)

99.98% 2

Pb (RaB,26.8m)

Bi

2

2

99.96%

(RaC,19.7m)

I

Po

4

(RaC',1.6 χ 10" s)

0.04%

JJ210

β_

n

m

r

o

* ~ioo°/ ~ioo%* (RaD,22y) (RaE,5.01d) (RaF,138.4d) 0

(RaC, 1.32m)

6

1.8x 10- % α 2 0 6

Hg · (8.5m) Figure 1.

Figure 2. pounds.

5

5xl0- % 2

Tl (4.3m)

2 0 6

• Pb (Stable)

The decay scheme of radon-222.

Monel vacuum

l i n e f o r the preparation of radon com


18. STEIN

243


The compound can be reduced w i t h hydrogen t i v e l y r e c o v e r e l e m e n t a l radon.


RnF

• Rn + 2HF (2) 500°C I f a l a r g e amount of radon, such as 10 mCi, i s mixed w i t h f l u o r i n e i n a s m a l l v e s s e l ( e . g . , a 30 ml K e l - F t e s t t u b e ) , t h e f l u o r i n a t i o n o c c u r s s p o n t a n e o u s l y i n t h e gas phase a t room temperature o r i n l i q u i d f l u o r i n e a t -195°C. The a c t i v a t i o n i s p r o v i d e d by t h e i n t e n s e α r a d i a t i o n , which produces l a r g e numbers of i o n s and e x c i t e d atoms. 2

+ H

a t 500°C t o q u a n t i t a

2

Complex F l u o r i d e s Radon r e a c t s s p o n t a n e o u s l y a t room temperature w i t h many s o l i d com pounds t h a t c o n t a i n o x i d i z i n g c a t i o n s , such as B r F , I F ^ , 0 , and N F ( S t e i n , 1972, 1973, 1974; S t e i n and H o h o r s t , 1982). Xenon a l s o r e a c t s w i t h a few compounds of t h i s type which have v e r y h i g h o x i d a t i o n p o t e n t i a l s ( S t e i n , 1973, 1974). The xenon p r o d u c t s have been a n a l y z e d by Raman and mass s p e c t r o m e t r i c methods and shown t o c o n s i s t of xenon ( I I ) complex f l u o r i d e s . 2

2

+

2

Xe + 2 0 S b F ^ — • 2

+

XeF Sb F7 2

1

+ 20

Xe + N F SbF£ — • XeF+SbF^ + N

(3)

2

(4)

+

2

2

(5)

Xe + BrFÂsF^ — • XeF+AsF^ + Br F 5 radon i s b e l i e v e d t o form t h e f o l l o w i n g p r o d u c t s . Rn + 2 0 S b F ^ — • 2

+

RnF Sb Fj 2

1

+ 20

Rn + N F S b F ^ — • R n F S b F ^ + N

(6)

2

(7)

+

+

2

Rn + N F 3 S b F ^ — • 2

2

x

+

RnF Sb F7 2

1

2

+ N F 2

2

(8)

+

(9)

Rn + B r F S b F ^ — • R n F S b F ^ + BrF

+

(10)

Rn + B r F T a F ^ — • R n F + T a F j T + BrF

(11)

Rn + BrF BiF£ — • R n F B i F ^ + BrF

(12)

Rn + ClF SbF£ — • R n F S b F ^ + C1F 2

2

2

+

2

Rn + B r F ^ S b ^ j L — • RnF SbF£ + BrF SbF£ +

2

+

Rn + IF^SbF^ — • R n F S b F ^ + I F

5

(13) (14)

H y d r o l y t i c R e a c t i o n s of Radon Compounds Radon d i f l u o r i d e i s q u a n t i t a t i v e l y reduced t o e l e m e n t a l radon by water i n a r e a c t i o n which i s analogous t o t h e r e a c t i o n s of k r y p t o n d i f l u o r i d e and xenon d i f l u o r i d e w i t h w a t e r . Complex s a l t s of radon also hydrolyze i n this fashion.


244

RADON AND ITS DECAY PRODUCTS KrF

2

+ H0 —•Kr 2

+V2O2 + 2HF l

XeF + H 0 — • Xe + /2°2 2

2 H F

+

2 H F

(

1

6

)

2

l

RnF + H 0 — • Rn + / 2

2

2

°2

l

Rn + / °2

+

RnF SbF^ + H 0

+

H

F

+

(17

^

H S b F

2

2


+

(15)

6

1 8

^ ^

This behavior provides evidence that i n each of the compounds, radon i s i n the +2 oxidation state. When higher-valent xenon compounds, such as XeF^ and XeF^» are hydrolyzed, water-soluble xenon species (XeO^ and XeO^~) are produced (Malm and Appelman, 1969). We have observed no radon species corresponding to these xenon species i n hydrolysis experiments. Russian s c i e n t i s t s (Avrorln et a l . , 1981, 1985) have reported that reactions of complex mixtures of radon, xenon, metal f l u o rides, bromine pentafluoride, and fluorine y i e l d a higher f l u o r i d e of radon which hydrolyzes to form RnO-j. However, efforts to confirm these findings have been unsuccessful. In s i m i l a r experiments which have been carried out at Argonne National Laboratory (Stein, 1984), i t has been found that radon i n the hydrolysate i s merely trapped i n undissolved s o l i d s ; centrifugation removes the radon from the l i q u i d phase completely. This i s i n marked contrast to the behavior of a solution of XeO^, which can be f i l t e r e d or centrifuged without loss of the xenon compound. Hence there i s no r e l i a b l e evidence at present for the existence of a higher oxidat i o n state of radon or f o r radon compounds or ions i n aqueous s o l u tions. E a r l i e r reports of the preparation of oxidized radon species i n aqueous solutions (Haseltine and Moser, 1967; Haseltine, 1967) have also been shown to be erroneous (Flohr and Appelman, 1968; Gusev and K i r i n , 1971). Solutions of Ionic Radon Stable solutions of radon d i f l u o r i d e can be prepared i n nonaqueous solvents, such as halogen fluorides and hydrogen fluoride (Stein, 1969, 1970). Radon reacts spontaneously at 25°C or at lower temperatures with each of the halogen fluorides except I F . It also reacts with mixed solvent-oxidant pairs, such as HF-BrF^, HFB r F , and I F - B r F , and solutions of K N i F i n HF. 5

5

5

3

2

6

Rn + 2C1F — • RnF + C l 2

(19)

2

Rn + B r F — • RnF + BrF

(20)

Rn + BrF 5 — • RnF + BrF^

(21)

Rn + I F — • RnF + I F

(22)

3

2

2

?

2

5

Rn + NiF^~ — • RnF + N i F 2

2

+ 2F~

(23)

Electromigration studies, which have been carried out with a Kel-F p l a s t i c c e l l (Figure 3), have shown that radon i s present i n many of these solutions as a cation (Stein, 1970, 1974). When the c e l l


18.

245


STEIN

i s f i l l e d with inactive solvent and a solution of radon d i f l u o r i d e i s added to the center leg, the application of a D.C. voltage to the n i c k e l electrodes causes the radon to move to the cathode. In a pure conducting solvent, such as bromine t r i f l u o r i d e , the follow ing i o n i z a t i o n occurs: RnF ^ ± 2

+

R n F ^ ± Rn Downloaded by UCSF LIB CKM RSCS MGMT on November 21, 2014 | http://pubs.acs.org Publication Date: February 5, 1987 | doi: 10.1021/bk-1987-0331.ch018

(24)

+

RnF + F" 2 +

(25)

+ F'

In a solution containing added e l e c t r o l y t e , however, d i s s o c i a t i o n may be suppressed, and the radon may form anionic complexes. Recently, a search has been conducted f o r solvents which are less hazardous to handle than halogen fluorides and hydrogen f l u o ride and which can be used to prepare solutions of the cationic species. Two have been found that are highly oxidation-resistant and suitable for this purpose: 1,1,2-trichlorotrifluoroethane and s u l f u r y l chloride. Solutions of cationic radon prepared by o x i d i z ing R n with halogen fluorides i n these solvents have been shown to be stable when stored i n capped FEP Teflon bottles at room tem perature for several weeks (Stein, 1985). Because of their ease of preparation and r e l a t i v e safety, solutions of this type can be used most readily to study reactions of radon. 2 2 2

Ion-Exchange Reactions of Cationic Radon Figure 4 shows the behavior that i s observed when a solution of c a t i o n i c radon i n 1,1,2-trichlorotrifluoroethane i s passed through a column packed with KPF^* The radon displaces potassium ion and adheres i n a narrow band at the top of the column. It can be washed repeatedly with d i l u t e BrF^ i n the halocarbon solvent, then eluted rapidly with 1.0 M BrF^ i n s u l f u r y l chloride. The radon daughters remain on the column during elution and decay jLn_ s i t u ; new daughters are generated i n the radon-containing eluant f r a c tions. We have found that s i m i l a r behavior occurs when the column i s packed with other salts of Group I elements, such as _NaSbF^ NaÂlF^, or K^NiF^, or with Nafion ion-exchange resins (H or Κ forms). In batch e q u i l i b r a t i o n experiments, using 1 g amounts of s o l i d s s t i r r e d with 5-15 ml volumes of solutions, we have found that the radon ions can also be collected on the compounds CsBrF^, C a ( B r F ) , and Ba(BrF ) « Thus i t i s apparent that, i n i t s o x i dized state, radon can displace H , Na , K , C s , Ca , and Ba ions from a number of s o l i d materials. By measuring the d i s t r i b u t i o n c o e f f i c i e n t , K^, of c a t i o n i c radon on Nafion resin (H form) i n BrF^-trichlorotrifluoroethane solutions as a function of the concentration of BrF^, we have been able to show that the charge on the radon cation i s +2 and that the parent molecule i s RnF . This physico-chemical method makes use of the fact that BrF^ produces the univalent cation B r F , which com petes with Rn for sites on the r e s i n . The following e q u i l i b r i a occur i n this system (R~ represents the anion of the r e s i n ) : 4

2

4

2

+

+

+

+

+

2

2


246


NICKEL ANODE


NICKEL CATHODE

INACTIVE S O L V E N T OR SOLUTION RADON

ION

MIGRATION

Figure 3. Kel-F p l a s t i c e l e c t r o l y s i s c e l l . Reproduced with per mission from Stein, L., Ionic Radon Solutions. Copyright 1970 the American Association f o r the Advancement of Science.

Solution of Radon A Fluoride Bed

of

- Radon Band

IOPF;

Π

π

Effluent Fractions

Figure 4. The ion-exchange column packed with K PF^.

behavior of cationic radon on a


18.

Rn

2 +

+

2+

+ 2H R^=ÊTRn (R~) +

BrF2 + H R2BrF


247


STEIN

2+

2

+ Rn (R")

2

2

+ 2H

BrF^T + H

+

(26)

+

2BrF R~ + R n

(27) 2 +

(28)

2

Many f i n e l y divided solids w i l l partly remove trace amounts of radon from solution by physical adsorption, but we have found that removal by ion-exchange i s much more e f f i c i e n t . The d i s t r i b u t i o n c o e f f i c i e n t , K j , of cationic radon ranges from about 90 to 4000 ml/g on ion-exchange materials that have been tested thus f a r i n d i l u t e BrF^-trichlorotrifluoroethane solutions. In contrast, the c o e f f i c i e n t i s less than 10 ml/g on materials which do not undergo ion-exchange with the radon species, such as L i F , MgF , PbF A1 0^, and Ce0 « These studies show that radon can be c l a s s i f i e d as a metalloid element, together with boron, s i l i c o n , germanium, arsenic, a n t i mony, tellurium, polonium, and astatine. Like these elements, radon l i e s on the diagonal of the Periodic Table between the true metals and nonmetals (Figure 5) and exhibits some of the characteri s t i c s of both (Stein, 1985). 2

2

2

2

Possible Applications of Radon Chemistry Tests i n the laboratory with radon-air mixtures have shown that two reagents, dioxygenyl hexafluoroantimonate (O^SbF^) and hexafluoroiodine hexafluoroantimonate (IF^SbF^) are p a r t i c u l a r l y well suited for trapping radon (Stein et_ at., 1977; Stein and Hohorst, 1982). In the form of powders or p e l l e t s , both compounds remove more than 99% of the radon from a i r at low or moderate flow rates. The a i r must f i r s t be dried by passage through a bed of desiccant, however, since the compounds are decomposed by moisture. Figure 6 shows one type of metal and p l a s t i c cartridge containing 1.0-1.5 g of 0 SbF^ powder that has been tested as a device f o r analyzing R n . To perform an analysis with this system, a measured volume of a i r i s drawn by a battery-operated pump through a drying tube and the cartridge, which captures the radon as a nonvolatile compound. After radioactive equilibrium has been established between radon and i t s short-lived daughters (approximately 4 hours), the γ-emission of the cartridge i s measured with a well-type s c i n t i l l a t i o n counter. The amount of radon i s then calculated from the γ-emission rate. In tests conducted with samples of a i r containing 3.5 to 11,700 pCi/1 of R n (simulating the wide range of radon concentrations that can occur i n a uranium mine), this type has been found to have a count rate of 2.74 counts/min/pCi of R n ( a l l γ-ray energies). An advantage of this method over the c o l l e c t i o n of radon with charcoal i s that no refrigerant i s required; both 0 SbF^ and IF^SbF^ have been shown to operate e f f i c i e n t l y at temperatures ranging from 10° to 40°C and probably can also be used at higher or lower temperatures. An advantage over the grab sampling technique i s that only the radon i s collected; a large volume of a i r (10-50 l i t e r s , for example) can be sampled, a l l of the radon retained, and the a i r discarded. This reduces the volume of sample that must be transported to a laboratory for analysis. A disadvantage of the 2

2 2 2

2 2 2

2 2 2

2

American Chemical Society, Library 15thProducts; St, N.W. In Radon and1155 Its Decay Hopke, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987. Washington. D.C. 20036

248



1 H 3 Li

4 Be

W//

11 Na

12 Mg

13 AI

19 Κ

20 Ca

21 Sc

22 Ti

23 V

24 Cr

25 Mn

26 Fe

27 Co

28 Ni

29 Cu

30 Zn

31 Ga

37 Rb

38 Sr

39 Y

40 Zr

41 Nb

42 Mo

43 Tc

44 Ru

45 Rh

46 Pd

47 Ag

48 Cd

49 In

50 Sn

55 Cs

56 Ba

57 La

72 Hf

73 Ta

74 W

75 Re

76 Os

77 Ir

78 Pt

79 Au

80 Hg

81 TI

82 Pb

83 Bi

87 Fr

88 Ra

89 Ac

104 Rf

105 Ha

58 Ce

59 Pr

60 Nd

61 Pm

62 Sm

63 Eu

64 Gd

65 Tb

66 Dy

67 Ho

68 Er

90 Th

91 Pa

92 U

93 Np

94 Pu

95 Am

96 Cm

97 Bk

98 Cf

99 Es

100 Fm

Wt

Figure 5 . The arrangement of the metalloid shading) i n the Periodic Table.

6 C

2

1 H

Ηθ

7 Ν

8 0

9 F

10 Ne

15 Ρ

16 S

17 Cl

18 Ar

35 Br

36 Kr

'/AM/, 5 3 1

54 Xe

70 Yb

71 Lu

101 102 Md No

103 Lr

77Π7, 3 4

UP

Se

vrTr/ W

69 Tm

elements

(dark

KEL-F INLET TUBE

0

I 2 CENTIMETERS

3

Figure 6. Kel-F p l a s t i c cartridge containing 0 S b F reagent for the analysis of radon gas (from Stein et a l . , 1977). 2

6



18. STEIN

249


method is that the background count rate of a γ-scintillation counter is usually higher than that of a Lucas flask-photomultiplier combination, a very sensitive α counter. However, the back ground count rate can be reduced by using lead shielding, several scintillators, and anti-coincidence circuitry. Chemical methods have been proposed for purifying radon-laden air in uranium mines (Stein, 1975, 1983). The cost of operating a full-scale air purification system using C^SbF^ reagent in a mine has been estimated at 0.245 U.S. dollars per 1000 standard cubic ft of treated air (1975 dollars) (Lindsay et_ al., 1975). This in cludes the cost of drying the air beforehand with a refrigerantdesiccant system. Comparable costs for removing the radon by phys ical methods have been estimated as follows: cryogenic condensa tion, $0.117; adsorption on charcoal at -80°C, $0.170; adsorption on Molecular Sieve at -80°C, $0.376; and membrane permeation, $0.402 per 1000 standard cubic ft of treated air. All of these methods are considered to be too expensive, in comparison to venti lation, to be used at present. Some mixtures of noble gases can be separated by chemical methods. A mixture of radon and xenon, for example, can be separ ated by selectively oxidizing the radon with ClF^SbF^, BrF^SbF^, or IF7SbF . Xenon can be trapped with stronger oxidants, such as °2 6» °2 6» 2 6> P ted from krypton and lighter noble gases. Ternary mixtures of krypton, xenon, and radon can be separated by successive oxidation of the radon and xenon. bF

6

PtF

o r

N

F+SbF

a n d

t h u s

se

ara

Acknowledgments Work performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, U. S. Department of Energy, under Contract W-31-109-Eng-38. Literature Cited Avrorin, V. V., Krasikova, R. N., Nefedov, V. D., and Toropova, Μ. A., Production of Higher Fluorides and Oxides of Radon, Radiokhim. 23:879-883 (1981). Avrorin, V. V., Krasikova, R. Ν., Nefedov, V. D., and Toropova, Μ. Α., Coprecipitation of Radon Oxide with Cesium Fluoroxenate, Radiokhim. 27:511-514 (1985). +

-

Bartlett, Ν., Xenon Hexafluoroplatinate (V), Xe PtF , Proc. Chem. Soc. 218 (1962). 6

Bartlett, N. and Sladky, F. O., The Chemistry of Krypton, Xenon, and Radon, in Comprehensive Inorganic Chemistry (A. F. TrotmanDickenson, ed), Vol. 1, pp. 213-330, Pergamon Press, Oxford, England (1973). Fields, P. R., Stein, L., and Zirin, Μ. Η., Radon Fluoride, J. Am. Chem. Soc. 84:4164 (1962). Fields, P. R., Stein, L., and Zirin, Μ. Η., Radon Fluoride: Further Tracer Experiments with Radon, in Noble-Gas Compounds (Η. H. Hyman, ed), pp. 113-119, University of Chicago Press, Chicago, IL (1963). In Radon and Its Decay Products; Hopke, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

250


Flohr, K. and Appelman, Ε. Η., The Resistance of Radon to Oxidation in Aqueous Solution, J. Am. Chem. Soc. 90:3584 (1968). Gusev, Yu. K. and Kirin, I. S., The Chemical State of Rn in the α-Decay of Ra in Certain Solutions and Crystals, Radiokhim. 13:916-918 (1971). 222

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August 5, 1986


Chemical Properties of Radon - ACS Symposium Series (ACS

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