Radionuclide-Humic Acid Interactions Studied by Dialysis - ACS

10 Radionuclide-Humic Acid Interactions Studied by Dialysis LARS CARLSEN, PETER BO, and GITTE LARSEN

Downloaded by FUDAN UNIV on December 16, 2016 | http://pubs.acs.org Publication Date: March 8, 1984 | doi: 10.1021/bk-1984-0246.ch010

Riso National Laboratory, Chemistry Department, DK-4000 Roskilde, Denmark

Dialysis has been used to study the interaction between radionuclides and humic acid. The technique gives information on 1) the complexing capacity of the humic acid samples, i.e. the concentration of complexing s i t e s , and 2) the radionuclide - humic acid complexes by a) i t s stoichiometry ( i . e . metal ion/ligand r a t i o ) , and b) interaction constants. The applicability of the technique i s i l l u s t r a t e d by studies on the interaction between a humic acid and Cs+, Sr , Co , and Eu . 134

85

2+

60

2+

154

3+

In the course o f time i t has been unambiguously demonstrated that humic- and f u l v i c a c i d s i n t e r a c t with metal cations by forming rather s t a b l e , and o f t e n s o l u b l e complexes(_l,2^) . The i n c r e a s i n g awareness o f a p o s s i b l e p o l l u t i o n of the environment, e.g. i n connection w i t h the d i s p o s a l o f nuclear waste, emphasizes the need f o r a d d i t i o n a l knowledge about the i n t e r a c t i o n between r e l e v a n t metal i o n s , e.g. r a d i o n u c l i d e s commonly present i n n u c l e a r waste, and humic substances. The p o s s i b l e presence o f s o l u b l e and rather s t a b l e complexes may p l a y an important r o l e i n determining the migration behavior o f the metal ions under shallow land b u r i a l c o n d i t i o n s . The i n f l u e n c e o f humic- and f u l v i c a c i d s on the migration behavior o f metal ions has been discussed p r e v i o u s l y (2^6), A v a r i e t y o f techniques, i n c l u d i n g ion-exchange e q u i l i b r i u m (7), potentiometric t i t r a t i o n ( 8 , 9 ) , a p p l i c a t i o n o f i o n - s e l e c t i v e electrodes(9,10), s p e c t r o s c o p i c measurements(11), l i q u i d - l i q u i d p a r t i t i o n ( 1 2 ) , and g e l f i l t r a t i o n ( J L 3 ) , have a p p l i e d t o s t u d i e s on metal i o n - s o i l organic matter i n t e r a c t i o n s . The former of these was o r i g i n a l l y developed by Schubert (7_) and l a t e r was s u c c e s s f u l l y modified by Ardakani and Stevenson (14). Together w i t h the a p p l i c a t i o n o f i o n - s e l e c t i v e e l e c t r o d e s , the ion-exchange e q u i l i b r i u m technique i s , f o r the time being, the more a p p l i e d method. However, a general disadvantage

0097-6156/84/0246-0167S06.00/0 © 1984 American Chemical Society

Barney et al.; Geochemical Behavior of Disposed Radioactive Waste ACS Symposium Series; American Chemical Society: Washington, DC, 1984.


168

GEOCHEMICAL BEHAVIOR OF RADIOACTIVE WASTE

u s i n g these techniques i s the l a c k of immediate i n f o r m a t i o n about the complexing c a p a c i t y of the l i g a n d . In the past, the e f f e c t i v e l i g a n d c o n c e n t r a t i o n [AJ has been expressed i n terms of molar u n i t s (11, 15), amount of l i g a n d m a t e r i a l (16), and c o n c e n t r a t i o n of complexing s i t e s (17, 18); the l a t t e r of these terms seems to be the more appropriate as i t c h a r a c t e r i z e s the humic a c i d samples more s a t i s f a c t o r i l y than an average molecular weight. Recently, Weber and co-workers reported on the a p p l i c a t i o n of d i a l y s i s to the determination of the complexing c a p a c i t y of f u l v i c a c i d f o r a s e r i e s of metal c a t i o n s (19, 20). In the present paper we r e p o r t a f u r t h e r e l a b o r a t i o n on the d i a l y s i s technique to allow simultaneous determination of complexing c a p a c i t y f o r the humic a c i d sample, and i n t e r a c t i o n constants f o r the metal i o n - humic a c i d complexes, as w e l l as the s t o i c h i o m e t r y of the l a t t e r . The technique furthermore opens up a p o s s i b i l i t y f o r an i n d i r e c t determination of apparent i o n i z a t i o n constants f o r the humic a c i d s . Experimental Samples (10 mL) of humic a c i d i n the a p p r o p r i a t e b u f f e r (μ = 0.050.1), t y p i c a l l y c o n t a i n i n g between 0.1 and 2.0 grams of humic a c i d per l i t r e (determined by e l e c t r o n i c a b s o r p t i o n spectroscopy), were placed i n regenerated c e l l u l o s e d i a l y s i s bags. The c l o s e d bags were placed i n 100 mL b o t t l e s , p r e v i o u s l y f i l l e d with 40 mL of the b u f f e r c o n t a i n i n g v a r y i n g concentrations (10"" - 10 M) of the metal ions C s , S r , Co* , or E u , and t r a c e amounts of Cs+, S r , C o , or W + , respectively. The present s t u d i e s were c a r r i e d out u s i n g commercially avail a b l e humic a c i d , obtained as the corresponding sodium s a l t (EGA HI,675-2) and d i s s o l v e d i n a phosphate b u f f e r (pH = 6.99), and an acetate b u f f e r (pH - 4.47), r e s p e c t i v e l y . The humic a c i d s o l u t i o n s were d i a l y z e d against pure b u f f e r s o l u t i o n p r i o r to t h e i r use i n the complex formation experiments i n order to remove any low molecular f r a c t i o n s that could pass through the d i a l y s i s membrane. The c l o s e d b o t t l e s were a g i t a t e d gently f o r approximately 48 hours, the temperature being kept at 25.0 ± 0.1°C by a Heto 02 PT 923 thermostat. A f t e r e q u i l i b r a t i o n , samples mL of the s o l u t i o n s outside the d i a l y s i s bag were withdrawn, and the contents of metal i o n s , [ M ] , were determined by γ counting (Kontron MR 252 Automatic Gamma Counting System) and comparison with s t a n dard s o l u t i o n s . The r e l a t i v e l y high i o n i c s t r e n g t h of the b u f f e r s o l u t i o n s (μ = 0.05-0.1) ensures an equal d i s t r i b u t i o n of low molecular species that pass the d i a l y s i s membrane, i . e . [M] measured i n the s o l u t i o n outside the d i a l y s i s bag equals that i n s i d e the bag. B l i n d - t e s t s , i . e . using d i a l y s i s bags c o n t a i n i n g pure b u f f e r s o l u t i o n , revealed that e q u i l i b r i u m was i n general obtained a f t e r 25 - 30 hours. Furthermore the b l i n d - t e s t s a f f o r d e d i n f o r m a t i o n on the p o s s i b l e s o r p t i o n of metal ions on the d i a l y s i s bags as f u n c t i o n of...metal i o n c o n c e n t r a t i o n : Z ( [ M ] ) . 7

+

13,+

8 5

2 +

6 0

2 +

2 +

+

-1+

3 +

1 5

0

Q

Q


10.

Radionuclide-Humic Acid Interactions

C A R L S E N ET A L .

169

D i a l y s e s are normally c a r r i e d out with f o u r d i f f e r e n t humic a c i d c o n c e n t r a t i o n s , each of which i s combined with seven d i f f e r e n t metal i o n concentrations (up to 2 χ 10" mol/L). For the determi n a t i o n of the complexing c a p a c i t y a f u r t h e r s e r i e s of three metal i o n concentrations (up to c a . Ι Ο " mol/L) was added i n the case of the lowest humic a c i d c o n c e n t r a t i o n , i n order to occupy a l l s i t e s available. 5

4

Theory


Metal i o n s , M, r e a c t with humate anions, A, to form complexes of the general type MAj, t a k i n g only mononuclear species i n t o account. M+jAsMAj

(1)

[MAj]

M

MM''

" NflAlJ

(2)

c

The l a t t e r r e l a t i o n being v a l i d s i n c e [MAj] » [M] . Since the metal ions i n t e r a c t with the humate anions t o form the complexes the d i s s o c i a t i o n of the humic a c i d should be taken i n t o account: C

HA=H*A [HA] [Al

(3)

M s

K

(4)

A

As [A] i n the present context i s the c o n c e n t r a t i o n of humate anions, which may p o s s i b l y p a r t i c i p a t e i n the complex formation has to be regarded as an apparent i o n i z a t i o n constant, not of nesc e s s i t y equal t o that obtained by acid-base t i t r a t i o n . The t o t a l c o n c e n t r a t i o n of complexing s i t e s a v a i l a b l e i s given by the expression (5), the number of l i g a n d s being occupied i n the complex formation being equal to ^[MA-J] = j [ w ] . c

[A] -[HAl+lAl+jlMlc t

l ) [A1 + j[M]

(jçU

c

(5)

I t i s important to note that [Α]^ cannot be regarded as a u n i v e r s a l constant f o r a given humic a c i d sample, as i t may be depen dent of the nature of the p a r t i c i p a t i n g metal i o n s , M (12). The t o t a l amount of metal i o n s , M , can be expressed as a sum of the amounts of metal ions i n the d i a l y s i s bag, i n the s o l u t i o n outside the l a t t e r , and the amount p o s s i b l y sorbed on the bag ( c f . F i g . 1). T

M

t

-

Vj ( M

c

+ Mo) + v M +z([Mlo) 0

0


(6)

170


A rearrangement of the r e l a t i o n (5) gives the f o l l o w i n g p r e s s i o n of the a c t u a l f r e e l i g a n d c o n c e n t r a t i o n [ A ] .

[A] -

([Al

- j[M] )

t

ex

(7)

c

Combining the equations (2) and (7) a f f o r d s the expression (8) f o r the s o - c a l l e d pH dependent Metal i o n - Humic a c i d I n t e r a c t i o n constant 3. I

(Ml

-iM Y


M,(W, where [ M ]

c

=

( 1 ^ )

c

,

S

P

( 8 >

i s given by

(Ml,

I t s h a l l be remembered that part of the metal i o n s , which are not engaged by the humic a c i d complexation, may i n t e r a c t w i t h the b u f f e r s o l u t i o n , c o n t a i n i n g a c e r t a i n c o n c e n t r a t i o n of ' b u f f e r l i g a n d s , [ L ] , i . e . [M]f < [ M ] . The metal i o n - b u f f e r - l i g a n d i n t e r a c t i o n i s given by the f o l l o w i n g set of equations. 1

q

M * L = M L

M

L

-

L

M

T

MLi-i • L = ^

L

(10a)

l

(10b)

MLi

(10c) n

[ML il /

' / M , lu'

=

PMLJ » π κ

Since [M] • [M]* + H ^ i ] i o n , (12), between [Mjf and [M] . w

Q

e

(ID

Μ ί |

o b t a i n the

rather simple

relat

0

In cases where [ M ] « [ A ] eqn's (8) and f i e d as f o l l o w s , i n c o r p a r a t i n g (12) c

| . ιαψί

pMfIA)

t

.

. (

which e a s i l y i s rearranged i n t o the expression

(9) can be s i m p l i

1 +

^)

I M l o

(14):

κ - - ^ ( - H * (:·*)} I t i s seen that f o r f i x e d values of 3 and

da)

Q (i.e. fixed


(14) pH),

10.

C A R L S E N ET AL.

171


V , and V p l o t s of [ M ] V S . ( M - Z ( [ M L ) ) / V w i l l r e s u l t i n s t r a i g h t l i n e s , the slope being dependent of [ A j only, i . e . the amount of humic a c i d . I t i s noted that an i n c r e a s i n g amount of humic a c i d causes a decrease i n the slope α ( c f . F i g . 2 ) . 0

0

±

±

T

t

(

- ' - Ι κ * ^ ) Introducing the term y =

οΓ

1

- (1 + V /V±)

1

5

)

we have

0

y = plAlj^

(16)


which gives log y = log ρ +jlog[Aj -logQ

(17)

t

By means of the complexing c a p a c i t y , w (eq/g), [ A ] i s expres sed i n terms of the amount ( i n g/L) of humic a c i d , or sodium humate, (HA) , present i n the s o l u t i o n . t

W

18

[Al «w(HA)w

< >

t

Accordingly

(17) can be rearranged

into

logy = (logp • jlogw-logQ) + jlog(HA)

^

w

1 9

^

A p l o t of l o g y as f u n c t i o n of l o g ( H A ) represents a s t r a i g h t l i n e , the slope being equal to j , the number of l i g a n d s p r . metal i o n ( c f . F i g . 3 ) . The i n t e r c e p t , l o g 3 + j l o g w - l o g Q, may i n p r i n c i p l e be used f o r the c a l c u l a t i o n of 3» i f w i s known, s i n c e l o g 3 • ( l o g 3 + j l o g w - l o g Q) - j l o g w + l o g Q. However, since determinations of i n t e r c e p t s i n general may be r a t h e r d e f e c t i v e , i t seems more reasonable t o use the d i a l y s i s r e s u l t s d i r e c t l y , c a l c u l a t i n g 3 according t o eqn. (8). I n general up t o 20-30 i n d i v i dual s e t s of [A]t and L ] c / [ ] f w i l l be a v a i l a b l e f o r t h i s purpose v i d e supra). Determination of the complexing c a p a c i t y i s c a r r i e d out by i n c r e a s i n g the metal i o n c o n c e n t r a t i o n t o a l e v e l , where a l l s i t e s a v a i l a b l e i n the humic a c i d , under the a c t u a l pH-cond i t i o n , w i l l be occupied. For convenience a solutions, with low humic a c i d c o n c e n t r a t i o n , ( H A ) (0.1 - 0.2 g/L) are used f o r t h i s purpose. A p l o t of [M]f (= [ M ] / Q ) as a f u n c t i o n of [ M ] w i l l f e a t u r e a v e r t i c a l asymptote at [M] = [ M ] , (cf. F i g . 4), the complexing c a p a c i t y , w, being determined by W

M

M

W

0

C

C

W, - i ' M W

=

w

C

M A X

HA

« 'w

(

2

0

)

which gives w

"

iK:,max/

(HA,

w

(21)

The t o t a l c o n c e n t r a t i o n of complexing s i t e s ( i n eq/L) i s ex t r a c t e d from eqn. (20), s i n c e w and ( H A ) a r e both known. Finally 3 can, as mentioned, be c a l c u l a t e d by eqn. ( 8 ) . W


GEOCHEMICAL BEHAVIOR O F RADIOACTIVE WASTE

6


3x10"

0.1 xlO"

4

0.5x10*

Mo FIGURE 1.

Sorption of europium ions on d i a l y s i s bags as f u n c t i o n of europium concentration.

5x10"

FIGURE 2.

V a r i a t i o n i n europium c o n c e n t r a t i o n o u t s i d e the d i a l y s i s bag as f u n c t i o n of the t o t a l amount of europium.





FIGURE 4,

Free europium i o n c o n c e n t r a t i o n as f u n c t i o n of the c o n c e n t r a t i o n of complexée! europium.


174


Results and D i s c u s s i o n 3 +

Based on p r e v i o u s l y reported r e s u l t s , i t i s expected that E u ions w i l l i n t e r a c t s t r o n g l y with the humate anions, l e a d i n g t o complexes e x h i b i t i n g r a t h e r high s t a b i l i t y constants. Bertha and Choppin(i8) reported simultaneous formation of 1:1 and 1:2 complexes, the corresponding s t a b i l i t y constants being l o g 3 = 7.8 and l o g 3 = 10.7, r e s p e c t i v e l y (pH = 4.50). A somewhat lower s t a b i l i t y constant f o r a 1:1 E u - f u l v i c a c i d complex was found ( l o g 3 = 6.90)(18) i n c l o s e agreement with r e s u l t s reported by Marinsky et a l . ( l o g 3 • 6.36)(21). On the other hand i t i s expected that C o ions and humate anions w i l l lead to conside r a b l y l e s s s t a b l e complexes. Chimielewska (17) reported on Co~HA complexes i n the pH-range 4.7 -5,8, the corresponding v a r i a t i o n i n l o g 3 ranged from 4.65 to 5.49 and 5.65 to 6.29, r e s p e c t i v e l y , f o r two d i f f e r e n t humic a c i d samples. The corresponding v a r i a t i o n s i n the number of l i g a n d s per metal i o n , j , were 1.08-1.3 and 1.4-1.59, r e s p e c t i v e l y . A d h i k a r i et a l . (22) reported on a 1:1 Co-HA complex, the s t a b i l i t y constant being l o g 3 = 3.01 (pH = 4, Τ = 30°C). A d d i t i o n a l l y some r e s u l t s on Co-FA complexes are a v a i l able (11, 15), In the cases of strontium and cesium no r e p o r t s on the a c t u a l i n t e r a c t i o n w i t h humate anions i n s o l u t i o n have appeared. However, i t has been reported that both types of ions sorb onto s o l i d humic a c i d (24, 25). An i n t r o d u c t o r y s e r i e s of experiments on the p o s s i b l e s o r p t i o n of the metal ions on the d i a l y s i s bags r e v e a l e d that only euro pium, to any s i g n i f i c a n t extent, sorbed to the c e l l u l o s e bags. F i g u r e 1 d e p i c t s the s o r p t i o n of E u by d i s p l a y i n g the c o r r e c t i o n term Z ( [ E u ] ) as f u n c t i o n of [ E u ] . In F i g u r e 2 the r e l a t i o n between [ M ] and (Μς - Z ( [ M ] ) ) / V i i s v i s u a l i z e d f o r the humic a c i d i n t e r a c t i o n w i t h E u ions i n an ace t a t e b u f f e r (pH = 4.47, μ = 0.05, Τ = 25.0°C). I t should be remem bered that the r e l a t i o n given i n eqn. (14) i s v a l i d f o r [M] « [ A ] only. In cases where the l a t t e r c o n d i t i o n i s not s a t i s f i e d , strong d e v i a t i o n s from the s t r a i g h t l i n e s are observed, as i n d i c a ted i n F i g u r e 2 by the points i n parentheses. The slopes of the l i n e s were determined by a l e a s t square procedure to be 8.04xl0~ , 5.00xl0" , 2.03X10"" , and 1.40xl0~ , f o r ( H A ) equal to 0.23, 0.43, 1.13, and 2.21 g/L, r e s p e c t i v e l y . The corresponding l o g y-values ( c f . eqn. (17)) are 0.871 1.18, 1.65, and 1.82, r e s p e c t i v e l y . In F i g u r e 3 the p l o t of l o g y as a f u n c t i o n of l o g (HA) i s shown, the slope being determined to be j • 0.99 ± 0.08. In order to c a l c u l a t e [ E u ] (eqn. (12)) three d i f f e r e n t euro pium - acetate complexes have to be considered: EuAc , EuAco > and EuAc3, the corresponding s t a b i l i t y constants being Ι Ο · , 1 0 · and 10 *· *, r e s p e c t i v e l y , (26)corresponding to a c o r r e c t i o n f a c t o r Q = 8.48. In F i g u r e 4 the dependence of [ E u ] as f u n c t i o n of [ E u ] i s depicted, the p l o t f e a t u r i n g the v e r t i c a l asymptote at [ E U ] = 5 . 1 x l 0 ~ mol/L. Since the corresponding amount of sodium humate 1

2

3 +

1

1


2 +

3 +

3

3 +

Q

Q

0

0

3 +

C

t

2

2

2

2

W

W

3 +

f

2

1

1

7 9

3

1 1

21

3

f

c

C

5


10.



175

i s 0.23 g/L, the complexing c a p a c i t y can be c a l c u l a t e d , according to eqn. (21), to be (2.20 ± 0.18)xl0" eq/g. Based on f o u r d i f f e r e n t l i g a n d concentrations [A]. 5.06X10~ 9.46xl0~ , 2.49x10"**, and 4.86xl0~ eq/L and seven d i f f e r e n t con c e n t r a t i o n s of europium i o n s , ranging from 2.66xl0~ to 2.01xl0" mol/L, corresponding to 28 mutually connected sets of data, the o v e r a l l Eu-HA i n t e r a c t i o n constant at pH • 4.47, l o g 3, i s c a l c u l a ted (eqn. (8)) to be 5.86 ± 0.15. The r e s u l t s obtained f o r the d i f f e r e n t i a l r a d i o n u c l i d e s are summarized i n Table I. l+

5

5

l+

7


+

2 +

5

2 +

3 +

TABLE I. I n t e r a c t i o n Between C s , S r , C o , and E u and Humic a c i d i n D i f f e r e n t Buffer Systems (Ph: phosphate b u f f e r , Ac: acetate b u f f e r ) M

Cs Cs Sr Sr Co Co Eu

pH

μ

7.00(Ph) 4.49(Ac) 6.99(Ph) 4.49(Ac) 7.01(Ph) 4.49(Ac) 4.47(Ac)

Q

0.1 0.05 0.1 0.05 0.1 0.05 0.05

1.0*> 1.0*> 1.56 1.13 6.49 1.97 8.48

1

w xlO * ( e q / g )

2.16 0.34 1.36 0.51 2.20

+ ± ± + +

a

j

0.82 0.71 1.06 1.00 0.99

0.40 0.02 0.23 0.01 0.18

± + + + ±

log 3

0.15 0.04 0.18 0.01 0.08

Radionuclide-Humic Acid Interactions Studied by Dialysis - ACS

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