Radon and Its Decay Products - ACS Publications - American

Chapter 40 The Feasibility of Using Activated Charcoal To Control Indoor Radon 2

ReyBocanegra1and Philip K. Hopke 1Department of Nuclear Engineering and Institute for Environmental Studies, University of Illinois, Urbana, IL 61801

Departments of Civil Engineering and Nuclear Engineering and Institute for Environmental Studies, University of Illinois, Urbana, IL 61801 Indoor contamination by radon-222 and its decay products has recently been the focus of attention for the Environmental Protection Agency, the news media, and the public in general. This new aware ness of the health hazard posed by radon has lead to increased efforts to understand and alleviate the problem. Numerous methods for radon and/or decay product removal from uranium mines and indoor air are found in the literature. A method often cited involves radon adsorption on charcoal. We have studied radon adsorption on activated charcoal as a means of radon mitigation for indoor air. We have identified several important adsorption parameters which need to be investigated. These studies will eventually lead to the design and construction of an apparatus capable of operating within reasonable constraints. The parameters are flow rate, humidity, temperature, and charcoal type. Other interfering organic and inorganic gases will also be examined.

Downloaded by CORNELL UNIV on June 9, 2017 | http://pubs.acs.org Publication Date: February 5, 1987 | doi: 10.1021/bk-1987-0331.ch040

2

There is an increasing concern regarding the exposure of the general population to increased levels of radon decay products in indoor air. The exposure to the public from increased levels of natural radiation has been the subject of a conference held in Maastricht, The Netherlands, in March 1985 where a number of reports were presented showing high levels of radon in the indoor environment. Other reports in this volume also demonstrate that high levels of radon are not as uncommon as had been previously thought. Radon in indoor air arises primarily from radium in the soil. The radon in the soil gas flows under a pressure gradient from the soil into the building. In some cases building practices can lead to high radon levels in the living areas of the house. Radon is chemically quite inert and does not pose a significant radiation health hazard in itself because the retained fraction in the body is so low (Mays et al., 1958). It is, however, an excellent vehicle for the dispersion of its short-lived radioactive decay products. 0097-6156/87/0331-0560$06.00/0 © 1987 American Chemical Society

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

40.

BOCANEGRA AND HOPKE

Using Activated Charcoal To Control Radon

Background Measurements done i n houses i n t h e Reading Prong a r e a o f Penn s y l v a n i a and i n New J e r s e y have found a number o f houses w i t h v e r y h i g h decay product c o n c e n t r a t i o n s . Radon t r a n s p o r t e d through t h e ground tends t o accumulate i n t h e s e houses. I n one case t h e a c t i v i t y was i n e x c e s s o f f i f t y t i m e s t h e maximum p e r m i s s i b l e c o n c e n t r a t i o n f o r workers (0.1 WL) i n uranium mines (Guimond, 1985). The E n v i r o n m e n t a l P r o t e c t i o n Agency (EPA) has recommended a l i m i t o f 150 Bq/m (4 p C i / 1 ) o f radon as an e q u i v a l e n t t o 0.02 WL f o r houses i n a r e a s o f h i g h n a t u r a l r a d i o a c t i v i t y (Guimond, 1985). A l t h o u g h radon c o n c e n t r a t i o n v a l u e s f o r houses i n t h e U.S. have been r e p o r t e d i n t h e g e n e r a l range of about 4 - 1000 Bq/m (0.1 - 27.0 p C i / 1 ) (Nero e t a l . , 1983), t h e r e have been cases r e p o r t e d showing radon c o n c e n t r a t i o n s an o r d e r o f magnitude h i g h e r . V a r i a t i o n s i n h o u s e h o l d c o n c e n t r a t i o n s a r e due m a i n l y t o radium c o n c e n t r a t i o n i n t h e s o i l , a i r p e r m e a b i l i t y , and volume o f h i g h p o r o s i t y s o i l ( S e x t r o e t a l . , 1986). Sources o f radon i n f i l t r a t i o n a r e sometimes found t o be v e r y l o c a l i z e d (Grimsrud e t a l . , 1983). S e a l i n g c r a c k s , d r a i n s , and o t h e r p a t h s f o r radon can be an e f f e c t i v e c o n t r o l measure i n t h e s e oases. S m a l l f a n s can a l s o be used t o p r o v i d e s u b f l o o r v e n t i l a t i o n f o r s o i l gas d i l u t i o n and removal b e f o r e e n t r y i n t o the house. There may be cases where t h e s e measures a r e not e f f e c t i v e , and l a r g e r , more e x p e n s i v e v e n t i l a t i o n systems must be i n s t a l l e d . The EPA has i n s t a l l e d v e n t i l a t i o n systems i n houses i n t h e Reading Prong a r e a i n an e f f o r t t o reduce the radon c o n c e n t r a t i o n . I n some c a s e s , because o f d i f f i c u l t t o c o n t r o l p r e s s u r e d i f f e r e n t i a l s , t h e v e n t i l a t i o n systems a c t u a l l y caused t h e radon c o n t e n t t o i n c r e a s e (Henschel and S c o t t , 1986). Thus, a l t e r n a t i v e methods t o reduce t h e radon l e v e l s c o u l d prove u s e f u l . A c o n c e i v a b l e way o f r e d u c i n g t h e exposure t o radon and i t s decay p r o d u c t s i s t o t r a p t h e decay p r o d u c t s i n a f i l t e r and t h u s remove them. However, i n s p e c t i o n o f F i g u r e 1 r e v e a l s t h a t w i t h i n a m a t t e r o f minutes e q u i l i b r i u m w i t h polonium-218 i s r e e s t a b l i s h e d and t h e r e i s a s u b s t a n t i a l i n c r e a s e i n t h e second and t h i r d decay p r o d u c t s as w e l l . T h e r e f o r e , c o n t r o l o f t h e radon i s t h e key t o exposure c o n t r o l . One method o f radon removal i s by t r a p p i n g i t on some a d s o r b e r . The a b i l i t y o f c h a r c o a l t o a d s o r b n o b l e gases i s w e l l documented. Adams e t a l . (1959) have shown t h e s u p e r i o r i t y o f c h a r c o a l over o t h e r a d s o r b e r s i n c l u d i n g m o l e c u l a r s i e v e , s i l i c a g e l , and a l u m i n a . V a l u e s f o r t h e dynamic a d s o r p t i o n c o e f f i c i e n t f o r radon a t room temperature have been r e p o r t e d i n the g e n e r a l range of 2000-7000 c n r / g . T a b l e I summarizes d a t a a v a i l a b l e i n t h e l i t e r a t u r e f o r v a r i o u s radon a d s o r b i n g c h a r c o a l s . 3


3

Theoretical The b a s i c mechanism f o r radon a d s o r p t i o n on c h a r c o a l and t h e e f f e c t s o f competing p r o c e s s e s have been w e l l s t u d i e d . S e v e r a l t r e a t m e n t s o f t h e a d s o r p t i o n o f radon (and n o b l e gases i n g e n e r a l ) on c h a r c o a l have appeared i n the l i t e r a t u r e (Adams e t a l . , 1959; S t r o n g and L e v i n s , 1978; K a p i t a n o v e t a l . , 1967; S i e g w a r t h e t a l . , 1972). C o n s i d e r a c h a r c o a l bed c o n s i s t i n g o f Ν t h e o r e t i c a l s t a g e s as


561


562

RADON AND ITS DECAY PRODUCTS

Figure 1. Given a constant radon source, equilibrium i s quickly established between radon-222 and i t s s h o r t - l i v e d daughters.


40.


BOCANEGRA AND HOPKE

represented i n Figure 2. A mass balance f o r the adsorbate each of the Ν stages i s given by

dt

1

kM

1

+

563

across

1


3 where k = dynamic adsorption c o e f f i c i e n t i n cm /g F = flow rate through the charcoal bed i n cm /min Y = volume f r a c t i o n of adsorbate i n the gas phase leaving the i t h stage at time t M = t o t a l mass of the adsorber i n grams t = time i n minutes. The adsorbate concentration i n the Nth stage along the charcoal bed can be found by solving the series of Ν d i f f e r e n t i a l equations. These solutions represent the concentration p r o f i l e i n the Nth stage. For a unit pulse of adsorbate at time t = 0, the s o l u t i o n reduces to Y

N

=

N

N

1

1

N

F* "

t "

(N-1)!

1

(kM)

exp(-NFt/kM)

(2)

N

The time at which the concentration of the adsorbate i s maximum, max» y s e t t i n g the derivative of Equation (2) equal to zero. The s o l u t i o n i s given by t

i s

f

o

u

n

d

b

=

(3)

Since the concentration p r o f i l e given by Equation (2) i s nearly symmetrical, i t can be assumed that t occurs at the midpoint of the curve, therefore, t h i s value i s equivalent to the mean residence time, . For columns consisting of a large number of t h e o r e t i c a l stages, the quantity (N-O/N approaches unity and Equation (3) becomes

v K

_ F M

(4)

For the case of constant input, the concentration p r o f i l e f o r the Nth stage can be found by i n t e g r a t i n g Equation ( 2 ) . _t

C

N

=

(5)

T„ dt " 0 1

At time = tmax ( i e . time = ), Y · = 0, and Y » = C ' . Therefore, the mean residence time f o r the constant input case i s the point of i n f l e c t i o n of the concentration p r o f i l e represented by Equation (5). Figure 3 shows a t y p i c a l radon concentration p r o f i l e . N



564

Table 1 Adsorption C o e f f i c i e n t f o r Various Radon Adsorbing Charcoals

Dynamic Adsorption Coefficient (cm STP Air/g)


Type

SKT-2M No. 2 SKT-1 No. 4 MSKT (5) SKT (82) No. 7 SKT (84) Ag-2 No. 12 Witco AC-337 Peat Sutcliffe-Speakman 207c Norit RFL 3 Norit RFL 111 Ultrasorb Pittsburgh PCB Lime Wood

a. b. c. d.

Temperature (°C)

Reference

a a a a a a a a a a a b c c c c c d

18 18 18 18 18 18 18 18 18 18 20 20 25 25 25 25 25 20

96 50 6300 9200 7600 7400 6000 4500 5700 4250 2000 5660 2250 3530 4610 46 6 0 5000 56 90 7000

Kapitanov e£ a_L., 1967 Scheibel £ L Α Ι · , 1980 Strong and Levin, 197 8 Gubeli and S t o r i , 1954

Adsorbate Entrance

,

1

I

Adsorbate Exit

- Ν Theoretical

Stages

Figure 2. A carbon bed consisting of Ν theoretical stages.




40. BOCANEGRA AND HOPKE

Figure 3. Adsorption and desorption curves f o r the sorption of radon on coconut based charcoal. (Jebackumar, 1985).


565


566

The dynamic a d s o r p t i o n c o e f f i c i e n t c a n be used t o gauge t h e performance o f a c h a r c o a l bed under v a r i o u s c o n d i t i o n s . The temperature s t r o n g l y a f f e c t s the a d s o r p t i o n p r o c e s s . At about 100 C, the d e s o r p t i o n o f radon from c h a r c o a l i s r a p i d and e f f i c i e n t enough t o f u l l y r e g e n e r a t e a c h a r c o a l bed i n a r e a s o n a b l e amount o f t i m e . The d e s o r p t i o n c o n c e n t r a t i o n p r o f i l e a l s o f e a t u r e s an i n f l e c t i o n p o i n t (see F i g u r e 3). E q u a t i o n (4) can t h e r e f o r e a l s o be a p p l i e d t o f i n d an o p t i m a l temperature f o r the d e s o r p t i o n process.


Discussion I t i s c l e a r from T a b l e I t h a t a d s o r p t i v e carbon i s a v a i l a b l e f o r use i n an a i r c l e a n i n g system. A radon removal a p p a r a t u s would be r e q u i r e d t o c l e a n a volume o f a i r l a r g e enough t o e q u a l the r a t e o f e n t r a n c e i n t o the house i n o r d e r t o m a i n t a i n the radon c o n c e n t r a t i o n below the 150 Bq/m (4 p C i / 1 ) l i m i t recommended by the EPA. F i g u r e 4 shows a p o s s i b l e c o n c e p t u a l d e s i g n o f an i n d o o r radon removal a p p a r a t u s based on c h a r c o a l a d s o r p t i o n . I t c o n s i s t s o f two p a r a l l e l c h a r c o a l beds. The p r i n c i p l e o f o p e r a t i o n i s a s f o l l o w s : A i r i s p u l l e d i n t o i n l e t A and passed through Bed 1 w h i l e v a l v e s V1 and V2 a r e shut and V3 and V4 a r e open. Once b r e a k t h r o u g h i s d e t e c t e d i n d e t e c t o r D1, v a l v e s V3, V4, V5, and V6 w i l l c l o s e and v a l v e s V1, V2, V7, and V8 w i l l open. Heated a i r w i l l then be f o r c e d i n t o i n l e t 0S1 through Bed 1 and out o u t l e t 0S2. W h i l e the radon i s b e i n g desorbed from Bed 1, Bed 2 w i l l be i n t h e a d s o r p t i o n c y c l e . The d e s o r p t i o n p r o c e s s t a k e s a much s h o r t e r t i m e than the a d s o r p t i o n t h e r e b y i n s u r i n g t h a t the o t h e r bed w i l l be a v a i l a b l e t o b e g i n the next c y c l e . There are s e v e r a l e n g i n e e r i n g c o n s i d e r a t i o n s t h a t need t o be taken i n t o a c c o u n t . The b a c k f l u s h i n g d u r i n g the d e s o r p t i o n stage i s a c h i e v e d u s i n g a i r from o u t s i d e o f the b u i l d i n g t o p r e v e n t the c r e a t i o n o f a p r e s s u r e g r a d i e n t t h a t might enhance the d i f f u s i o n o f radon from the s u r r o u n d i n g w a l l s . The c h a r c o a l beds must be t h e r m a l l y i s o l a t e d from each o t h e r . The a i r i n l e t s must be p o s i t i o n e d f a r enough a p a r t s o as t o m i n i m i z e feedback o f c l e a n a i r back i n t o the system. To p r e v e n t t h e a c c u m u l a t i o n o f radon i n the house i n the event o f a v a l v e f a i l u r e , a l l v a l v e s s h o u l d be p r o v i d e d w i t h backups. The volume o f a i r c l e a n e d per u n i t mass o f carbon i n c r e a s e s e x p o n e n t i a l l y w i t h d e c r e a s i n g temperature ( K a p i t a n o v e t a l . , 1967). Thus g r e a t l y i n c r e a s e d a d s o r p t i o n c a p a c i t y can be o b t a i n e d by c o o l i n g the c a r b o n below ambient temperature. Although t h i s p r o c e s s w i l l r e q u i r e a d d i t i o n a l energy i n p u t , i t may be w o r t h w h i l e t o c o n s i d e r some form o f c o o l i n g . 3

There may be problems from o t h e r a d s o r b i n g s p e c i e s i n t h e house. C a r b o n - d i o x i d e and w a t e r vapor have been found t o have an a d v e r s e e f f e c t on t h e a d s o r p t i o n c o e f f i c i e n t ( S t r o n g and L e v i n s , 1978; S i e g w a r t h e t a l . , 1972). The l i k e l i e s t p l a c e f o r i n d o o r radon t o accumulate i n houses i s i n the basement o r c r a w l space where a l a r g e s u r f a c e a r e a i s i n d i r e c t c o n t a c t w i t h t h e s o i l , and t h u s the most l i k e l y p l a c e t o put an a d s o r p t i o n system i s i n t h e s e l o c a t i o n s . However, these a r e a s a r e a l s o commonly used t o s t o r e v a r i o u s household c h e m i c a l s such a s p a i n t i n g s u p p l i e s , e t c . These household items s t o r e d i n basements can r e l e a s e contaminants t h a t may be c l a s s i f i e d i n t o 4 broad c a t e g o r i e s ; a r o m a t i c s , p a r a f f i n s ,


BOCANEGRA AND HOPKE


OUTLET


0S2

0S1 Backflush Inlet Figure 4 . Conceptual design of an apparatus f o r the removal of indoor radon.


567



568

h a l o g e n a t e d h y d r o c a r b o n s , and combustion b y - p r o d u c t s . P a i n t , p a i n t remover, and p a i n t t h i n n e r a r e examples o f a r o m a t i c compounds. P a r a f f i n s may be r e l e a s e d from m a t e r i a l s used f o r h o b b i e s o r r e c r e a t i o n . Examples o f t h e s e a r e propane l a n t e r n s and w e l d i n g / b r a z i n g equipment. Household c l e a n i n g c h e m i c a l s and garden c a r e p r o d u c t s a r e the b i g g e s t s o u r c e o f h a l o g e n a t e d compounds. The e f f e c t o f basement a i r b o r n e contaminants on t h e a d s o r p t i o n o f radon on c h a r c o a l i s p r o b a b l y t h e most i m p o r t a n t and l e a s t s t u d i e d a r e a o f i n d o o r radon c o n t r o l . I n o r d e r t o e v a l u a t e t h e p r a c t i c a l f e a s i b i l i t y o f t h e system d e p i c t e d i n F i g u r e 4, s e v e r a l e x p e r i m e n t s w i l l be performed t o f i n d t h e c o n d i t i o n s f o r o p t i m a l performance. The f o l l o w i n g parameters w i l l be i n v e s t i g a t e d : Carbon t y p e : G i v e n a f i x e d s e t o f c o n d i t i o n s ( i e . f l o w r a t e , t e m p e r a t u r e , mass o f c h a r c o a l , h u m i d i t y ) t h e type o f carbon demon s t r a t i n g t h e h i g h e s t dynamic a d s o r p t i o n c o e f f i c i e n t w i l l be identified. Flow r a t e : The system must be c a p a b l e o f p r o c e s s i n g a moderate volume o f a i r per u n i t time t o be o f p r a c t i c a l use. However, t h e e m p i r i c a l f o r m u l a t i o n s f o r t h e dynamic a d s o r p t i o n c o e f f i c i e n t d e s c r i b e d i n t h i s paper a r e v a l i d o n l y f o r a c e r t a i n range o f c o n d i t i o n s . Experiments w i l l be performed t o i d e n t i f y t h e f l o w r a t e / f l o w c h a n n e l d i a m e t e r c o m b i n a t i o n beyond which the f o r m u l a t i o n s are no l o n g e r v a l i d . Contaminants: Indoor contaminants a r e expected t o compete f o r a d s o r p t i o n s i t e s on the c h a r c o a l . We w i l l e x p e r i m e n t a l l y f i n d the e f f e c t t h a t t h e s e contaminants have on t h e dynamic a d s o r p t i o n c o e f f i c i e n t and on t h e l i f e - t i m e o f the c h a r c o a l bed. S i n c e t h e number o f radon atoms i n even the most s e r i o u s l y c o n t a m i n a t e d houses i s v e r y s m a l l , decay p r o d u c t b u i l d u p i s not e x p e c t e d t o pose a s i g n i f i c a n t problem. The f e a s i b i l i t y o f b u i l d i n g a radon removal a p p a r a t u s w i l l depend on f i n d i n g t h e parameters which w i l l g i v e the h i g h e s t dynamic a d s o r p t i o n c o e f f i c i e n t under o p e r a t i o n a l c o n d i t i o n s . However, i t i s recognized that r e a l world constraints l i k e i n i t i a l costs, o p e r a t i n g c o s t s , and p h y s i c a l s i z e may f o r c e the i d e a l parameters to be compromised. D e v i c e s o f t h i s t y p e a r e i n t e n d e d f o r use i n p r i v a t e homes where t h e s e c o n s t r a i n t s cannot be i g n o r e d . Acknowledgments A l t h o u g h t h e i n f o r m a t i o n d e s c r i b e d i n t h i s a r t i c l e has been funded w h o l l y by t h e u n i t e d S t a t e s E n v i r o n m e n t a l P r o t e c t i o n Agency under a s s i s t a n c e agreement EPA C o o p e r a t i v e Agreement CR810462 t o t h e Advanced E n v i r o n m e n t a l C o n t r o l Technology R e s e a r c h C e n t e r , i t h a s not been s u b j e c t e d t o t h e Agency's r e q u i r e d peer and a d m i n i s t r a t i v e r e v i e w , and t h e r e f o r e does not n e c e s s a r i l y r e f l e c t t h e views o f the Agency and no o f f i c i a l endorsement s h o u l d be i n f e r r e d . Literature Cited Adams, R.E., W.E. Browning, J r . , and R.D. A c k l e y , Containment o f R a d i o a c t i v e F i s s i o n Gases by Dynamic A d s o r p t i o n , I n d u s t r i a l and E n g i n e e r i n g C h e m i s t r y , 51:1467-1470 (1959).


40.

BOCANEGRA AND

HOPKE


Grimsrud, D.T., W.W. N a z a r o f f , K.L. Revzan, and A.V. Nero, C o n t i n uous Measurements o f Radon E n t r y i n t o a S i n g l e F a m i l y House, P r o c e e d i n g s o f 76th Annual M e e t i n g o f t h e A i r P o l l u t i o n C o n t r o l A s s o c i a t i o n , pp. 83-98 (June, 1983) . G u b e l i , 0. and M. S t o r i , Zur M i s c h a d s o r p t i o n von Radon an A k t i v o h l e m i t V e r s c h i e d e n e n Tragergasen, H e l v e t i c a Chim. A c t a 37:2224-2230 (1954).


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Radon and Its Decay Products - ACS Publications - American

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