Chemical Modeling in Aqueous Systems - ACS Publications

6 Conditional Stability Constants for Copper Ions w i t h Ligands i n Natural Waters CONSTANT M. G. VAN DEN

BERG and JAMES R. KRAMER

Downloaded by UNIV LAVAL on July 14, 2016 | http://pubs.acs.org Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch006

Department of Geology, McMaster University, Hamilton, Ontario, Canada L8S 4M1

Heavy m e t a l s g e n e r a l l y o c c u r i n n a t u r a l w a t e r s i n t h e f o r m s o f i n o r g a n i c (1^, p. 2 3 8 - 2 9 9 ) a n d o r g a n i c com p l e x e s ( 2 , p. 2 9 7 - 3 1 3 ) a n d a d s o r b e d , o r s u r f a c e complexed, on c h a r g e d c o l l o i d s . This r e s u l t s i n a lowering of the free metal ion concentration. F r e q u e n t l y copper i s used f o r experiments w i t h a q u a t i c organisms s i n c e i t produces e f f e c t s at very low c o n c e n t r a t i o n s , p o s s i b l y a t l e v e l s as f o u n d i n n a t u r a l w a t e r s . Kamp N i e l s e n ( 3 ) and Steeman N i e l s e n and Wium-Anderson (4) d e t e r m i n e d a d e p r e s s i o n and d e l a y o f a l g a l g r o w t h a t c o p p e r c o n c e n t r a t i o n s a s l o w a s 2 χ 1 0 ° M. M i l l i m o l a r concen t r a t i o n s o f sodium and p o t a s s i u m r e d u c e t h e e f f e c t o f copper ( 3 ) . A l s o , the presence of c o l l o i d a l Fe(0H)3 and e x c r e t i o n o f o r g a n i c m a t e r i a l a p p e a r s t o a f f e c t t h e t o x i c i t y ( 4 ) . The c o n c e n t r a t i o n o f f r e e c o p p e r i n s e a w a t e r u p w e l T i n g f r o m s u b s u r f a c e w a t e r s , may be h i g h enough t o s u p p r e s s p l a n k t o n g r o w t h ( 4 ) . H u t c h i n s o n ( 5 , p. 817) m e n t i o n e d t h i s p o s s i b i l i t y s o m e t i m e e a r l i e r . D a v e y et: a ^ . ( 6 ) u s e d t h e s e n s i t i v i t y o f a d i a t o m t o c u p r i c i o n as a t o o l t o d e t e r m i n e t h e c o p p e r c o m p l e x i n g c a p a c i t y of seawater. _

A p p a r e n t l y the t o x i c i t y of c o p p e r to p l a n k t o n depends upon the f r e e m e t a l c o n c e n t r a t i o n , as i s shown i n e x p e r i m e n t s w i t h v a r y i n g c h e l a t o r c o n c e n t r a t i o n s (7, 8J. C a l c u l a t i o n s u s i n g the R E D E Q L c o m p u t e r m o d e l f o r m e t a l s p e c i a t i o n (8, 9.) r e l a t e d data f r o m t o x i c i t y e x p e r i m e n t s to f r e e m e t a l c o n c e n t r a t i o n s . P a r t i a l g r o w t h i n h i b i t i o n i s found i n the a c t i v i t y -11 -9 range 4 x 1 0 t o 2 χ 10 M c o p p e r (7, 8), and e f f e c t s on d i a t o m s a r e c a l c u l a t e d to be l i n e a r l y dependent upon the f r e e c o p p e r c o n c e n t r a t i o n , when p £ C u J i s b e t w e e n 8 and 12 (9_). +

0-8412-0479-9/79/47-093-115$05.00/0 © 1979 American Chemical Society Jenne; Chemical Modeling in Aqueous Systems ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

116

CHEMICAL MODELING IN AQUEOUS SYSTEMS

M e t h o d s to M e a s u r e the F r e e M e t a l Ion C o n c e n t r a t i o n


E v i d e n t l y i t i s m o r e i m p o r t a n t to d e t e r m i n e the c u p r i c i o n c o n c e n t r a t i o n than the total c o p p e r c o n c e n t r a t i o n i n n a t u r a l waters. P o l a r o g r a p h i c m e t h o d s have b e e n u s e d to m e a s u r e the l i g a n d c o n c e n t r a t i o n ( c o m p l e x i n g c a p a c i t y ) (10) and to d e t e r m i n e s t a b i l i t y constants f o r s o m e s t r o n g c h e l a t o r s (11 ). T h e p l a t i n g s t e p , h o w e v e r , s t r i p s the m e t a l out of its c o m p l e x and i n t r o d u c e s a s y s t e m a t i c e r r o r w h i c h b e c o m e s l a r g e r when the c o m p l e x is w e a k e r . A d d i t i o n a l l y , a d s o r p t i o n effects o n the e l e c t r o d e m a y o b s c u r e the m e a s u r e m e n t (12). T h e c o n c e p t of the m e t h o d u s e d h e r e goes b a c k a t l e a s t to 1922, to the w o r k of G u n t h e r - S c h u l z e (1_3» 14). She u s e d a n a t u r a l z e o l i t e a s a n i o n e x c h a n g e r to d e t e r m i n e c o m p l e x a t i o n Ι οί c o p p e r by i n o r g a n i c a n i o n s , s u c h as CI , S O . a n d B r , a n d «2 even i n her l a r g e s t dilution ( 5 x 1 0 M ) she s t i l l found C u C l ^ complexes. T h e b a s i c change thatwas m a d e after 56 y e a r s was to m e a s u r e l i g a n d s at s i x o r d e r s o f m a g n i t u d e l o w e r c o n c e n t r a tion. W i t h the d e v e l o p m e n t of s y n t h e t i c , o r g a n i c i o n exchange r e s i n s , the i o n exchange m e t h o d has been a p p l i e d f r e q u e n t l y to the d e t e r m i n a t i o n of s t a b i l i t y constants f o r c o m p l e x e s of o r g a n i c anions and m e t a l c a t i o n s , e . g . S c h u b e r t (15, 16), S c h n i t z e r and H a n s e n (17), G a m b l e et^aT. (18), A r d a k a n i and S t e v e n s o n (19). T h e s e ion exchange r e s i n s have a s t r o n g a f f i n i t y f o r the m e t a l i o n s , h o w e v e r , so one n e e d s e i t h e r a v e r y s t r o n g c o m p l e x i n g l i g a n d o r a high c o n c e n t r a t i o n of a weak c o m p l e x i n g l i g a n d to be able to r e a d i l y m e a s u r e the affinity of the l i g a n d for the m e t a l i o n . In a d d i t i o n , p o l a r o g r a p h i c (a. s. v . ) m e a s u r e m e n t s of the f i l t r a t e of a C h e l e x - 1 0 0 s u s p e n s i o n showed that f r a g m e n t s o r m o l e c u l e s a r e r e l e a s e d w h i c h p a s s the 0.45 μτη f i l t e r and a r e able to c o m p l e x c o p p e r o r i n s o m e o t h e r way o b s c u r e the p o l a r o g r a p h i c a l m e a s u r e m e n t of c o p p e r at p H 6. A s an a l t e r n a t i v e to the s t r o n g , o r g a n i c , i o n exchange r e s i n s , i n o r g a n i c o x i d e s w i t h i n t e r m e d i a t e i o n exchange p r o perties were c o n s i d e r e d . Manganese dioxide ( £ - Μ η θ £ ) p r e p a r e d as d e s c r i b e d (20) was c h o s e n b e c a u s e of its s t a b i l i t y o v e r a l a r g e p H r a n g e and b e c a u s e of its r a t h e r s t r a i g h t f o r w a r d i o n exchange c a p a b i l i t y . It is n e g a t i v e l y c h a r g e d at the p H - r a n g e of i m p o r t a n c e to n a t u r a l w a t e r , pH> 3, i n w h i c h v i c i n i t y the p H of z e r o p o i n t of c h a r g e is (21, 22, 23).

Jenne; Chemical Modeling in Aqueous Systems ACS Symposium Series; American Chemical Society: Washington, DC, 1979.


6.

VAN

DEN BERG AND

Copper

KRAMER

I oris with Ligands

117

The a u t h o r s ' p r o c e d u r e has been d e s c r i b e d i n d e t a i l (20), h o w e v e r a s h o r t d e s c r i p t i o n i s g i v e n h e r e a l s o . To 45 0 m l of a f i l t e r e d (0.45 μτο) s a m p l e a s m a l l quantity (3 m l ) of aged MnC>2 d i s p e r s i o n i s added, to a c o n c e n t r a t i o n of 42 μπι. C o n s t a n t i o n i c s t r e n g t h (0. 01 M K N O 3 ) , c o n s t a n t t e m p e r a t u r e (25°C) and a f i x e d p H a r e m a i n t a i n e d . N i t r o g e n i s b u b b l e d c o n t i n u o u s l y to r e m o v e a l l c a r b o n a t e s f r o m s o l u t i o n . C o p p e r i s added i n nine steps f r o m 0. 5 to 16 μΜ ; one h o u r e q u i l i b r i u m i s a l l o w e d a f t e r e a c h a d d i t i o n b e f o r e a s u b s a m p l e (30 m l ) i s f i l t e r e d and a c i d i f i e d . T o t a l d i s s o l v e d c o p p e r i s m e a s u r e d i n the f i l t r a t e b y d.p.a.s.v. C a l i b r a t i o n of Μ η θ £ w i t h C u i s c a r r i e d out at the s a m e c o n c e n t r a t i o n , t e m p e r a t u r e , i o n i c s t r e n g t h and p H c o n d i t i o n s . T h e c u p r i c i o n c o n c e n t r a t i o n can then be c a l c u l a t e d u s i n g the L a n g m u i r equation. M a s s b a l a n c e f r o m the m e a s u r e d t o t a l C u and the c u p r i c i o n g i v e s the c o m p l e x e d c o p p e r c o n c e n t r a t i o n , C u L ; the t o t a l l i g a n d c o n c e n t r a t i o n and the c o n d i t i o n a l s t a b i l i t y constant, Κ , f o r the f o r m a t i o n of C u L , C u + L = CuL, +2 (Cu ) a r e then c a l c u l a t e d b y p l o t t i n g ( C u ) v s . , \ and u s i n g +2) 2 (CuLj (Cu 1 (Cu ) __, 1 _ . _ , , , = 7+ — . . The ligand concentration (CuL) K_ ( L . , ) ( L ) * L total total i s o b t a i n e d f r o m the s l o p e and the c o n d i t i o n a l s t a b i l i t y c o n s t a n t is o b t a i n e d f r o m the slope d i v i d e d b y the Y - a x i s i n t e r c e p t . The c o r r e l a t i o n c o e f f i c i e n t i s c a l c u l a t e d f r o m a l e a s t s q u a r e s a n a l y s i s of a l l the data. The m e t h o d has b e e n t e s t e d o n s o m e r e a s o n a b l y w e l l c h a r a c t e r i z e d l i g a n d s (20) and a p p e a r s to w o r k w e l l at l i g a n d c o n c e n t r a t i o n s found i n n a t u r a l w a t e r s (0. 2 μ Μ ). The c o n d i t i o n a l s t a b i l i t y c o n s t a n t s o b t a i n e d f o r n a t u r a l l y o c c u r r i n g l i g a n d s f a l l w i t h i n the r a n g e of s t a b i l i t y constants f o r known and t e s t e d l i g a n d s . The r e s u l t s of t i t r a t i o n s w i t h c o p p e r of a n u m b e r o f l a k e s a n d r i v e r s u s i n g this m e t h o d , a r e g i v e n h e r e . A n a t t e m p t has b e e n m a d e to c o r r e l a t e the data to o t h e r f a c t o r s s u c h a s U V - s p e c t r o p h o t o m e t r i c m e a s u r e m e n t s f o r o r g a n i c content and p H . + 2

+ 2

ir

T

+

v

L

Sample Description S a m p l e s , u s u a l l y two l i t e r s , w e r e t a k e n , f i l t e r e d as soon a s p o s s i b l e (0.45 μπι M i l l i p o r e ) and s t o r e d i n the d a r k u n d e r r e f r i g e r a t i o n . S a m p l e s w e r e c o l l e c t e d f r o m the following fresh water environments:




118

- d y s t r o p h i c w a t e r s : low p H ( 4 . 6 ) b r o w n c o l o u r e d w a t e r s , c o n t a i n i n g m u c h o r g a n i c m a t t e r , f l u s h e d out of s o i l s . T h e R i v e r s D i c k i e N o . 5,6 a n d 10, R e d C h a l k N o . 3 and 4 and L a k e D i c k i e b e l o n g to this g r o u p . - m e d i u m a l k a l i n i t y (ΛΙΟ. 5 m e q / l j , low p r o d u c t i v i t y , n o n - p o l luted: Lake Windy - h i g h a l k a l i n i t y (rJ 2 m e q / L ) , n o n - p o l l u t e d : L a k e H u r o n - h i g h a l k a l i n i t y (rj2 m e q / L ) , m e d i u m h i g h p r o d u c t i o n (no bloom): G l o u c e s t e r P o o l , L a k e O n t a r i o , Onaping R i v e r (flows out of L a k e O n a p i n g ) . - r a t h e r h e a v i l y p o l l u t e d w i t h m e t a l s , low a l k a l i n i t y (*g0. 1 m e q / L ) and h a v i n g a m e d i u m h i g h p r o d u c t i o n : W h i t e w a t e r l a k e . - F A : s u p p l i e d to us by S c h n i t z e r i n d r i e d f o r m (15). It has b e e n e x t r a c t e d f r o m the s o i l . R e s u l t s and D i s c u s s i o n C o n d i t i o n a l S t a b i l i t y C o n s t a n t s and C o m p l e x i n g C a p a c i t y . T h e r e s u l t s of c o p p e r t i t r a t i o n s i n p r e s e n c e of M n O ^ of a n u m b e r of n a t u r a l w a t e r s a r e g i v e n i n T a b l e I . Initial analyses done i n contact w i t h a i r a t m o s p h e r e a r e l e s s a c c u r a t e (low c o r r e l a t i o n c o e f f i c i e n t s ) due to the l a r g e a m o u n t of c o p p e r carbonate complexation. C o r r e c t i o n i n v o l v e s the s u b t r a c t i o n of the c a l c u l a t e d C u C O ° c o n c e n t r a t i o n f r o m the m e a s u r e d d i s -

+2 s o l v e d c o p p e r c o n c e n t r a t i o n to o b t a i n C u and C u L . In a l l c a s e s the c o r r e l a t i o n c o e f f i c i e n t s a r e c a l c u l a t e d f o r nine data p o i n t s and a r e thus c o m p a r a b l e a m o n g t h e m s e l v e s . The l o g c o n d i t i o n a l s t a b i l i t y constants of s m a l l , m o n o p r o t i c a c i d s c a n be a s s u m e d to be l i n e a r l y and on a o n e - t o one b a s i s dependent upon the p H , due to c o m p e t i t i o n between

+

+2

Η and C u , u n t i l the p H r e a c h e s the value of the a c i d i t y constant. S i n c e the c o m p l e x a t i o n r e a l l y i s c o m p o s e d of two competitive reactions: 1. Cu + L " = C u L , K_ L + 2

and

2.

HL = H

+

+ L", Κ a

W h e n p H > Κ , equation 1 i s s u f f i c i e n t and K _ i s a L

constant

Ο


6.

VAN DEN BERG AND KRAMER

Table I.

I OTIS With

Ligands

119

C o m p l e x i n g c a p a c i t i e s and c o n d i t o n a l s t a b i l i t y con stants of n a t u r a l w a t e r s . In m o s t c a s e s only one type of c o m p l e x i n g site p e r l i g a n d m o l e c u l e was found of r e l e v a n c e to the n a t u r a l w a t e r s y s t e m . O r i g i n a l s a m p l e p H shown i n b r a c k e t s , r = c o r r e l . coeff. , o r i g i n a l , undiluted, ligand concentration given i n brackets.

pH Downloaded by UNIV LAVAL on July 14, 2016 | http://pubs.acs.org Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch006

Copper

log

K

L

Complexing Capacity

r

* Gloucester Pool

8. 4

9. 3

0. 51 μ Μ

0. 90

*Lake Huron

8. 3

9. 2

0. 20

0. 70

* Whitewater Lake

8. 0

8. 6

0. 68

0. 98

* Onaping River

7. 8

8. 6

0. 38

0. 95

* Windy Lake

6. 6

7. 2

0. 20

0. 57

Lake

Ontario

8. 4

9. 5

0. 33

0. 97

Lake

Ontario

7. 4(8. 4)

8. 6

0. 34

0. 986

D i c k i e No.

5

8. 4(4. 6)

8. 5

5. 35(20)

0. 98

D i c k i e No.

5

7. 6(4. 6)

7. 8

2. 47(20)

0. 987

D i c k i e No.

6

7. 6(4. 6)

7. 8

5. 75

0. 98

D i c k i e No.

10

7. 6(4.9)

7. 8

4.95(10.9)

0.991

Lake Dickie

7. 6(4. 6)

7. 8

2. 19

0. 996

Red Chalk #3

7. 6(6.3)

7. 7

3. 35

0.991

Red

7. 6(4. 7)

7.9(K )

2. 43

0. 992

7-2(K )

5.93

0. 998

7. 8

2. 24

0.986

Chalk#4

L i

L z

Fulvic Acid

7. 6(4. 6)+

*has b e e n d e t e r m i n e d i n p r e s e n c e of c a r b o n a t e , i n a i r a t m o s phere. +pH of F A d i s s o l v e d i n d i s t i l l e d w a t e r .



120 When p H ^ K

, Κ

a

L

, as u s e d h e r e , b e c o m e s :

[c„-][„ ]'

[,„•] •

L

Now l o g Κ constant,

L Κ

is l i n e a r l y p H dependent.

B u t s i n c e the a c i d i t y

, i s u n k n o w n , the a u t h o r s w e r e unable to c a l c u l a t e


Ο

L a r g e poly electrolytic m o l e c u l e s , however, approach colloidal particles in their behaviour. Then Κ determines L the e q u i l i b r i u m s i t u a t i o n for the r e a c t i o n between a m e t a l i o n and c h a r g e s on the c o l l o i d a l p a r t i c l e . So Κ should v a r y with L the d e g r e e of n e u t r a l i z a t i o n and the i o n i c s t r e n g t h to the s a m e extent as the i o n i z a t i o n c o n s t a n t , but i n o p p o s i t e d i r e c t i o n (24). A c c o r d i n g l y it has b e e n found i n s e v e r a l c a s e s that l o g Κ

L v a r i e d w i t h the p H but not w i t h a s l o p e e q u a l to o n e . Takamatsu and Y o s h i d a (25) found that the o v e r a l l constants f o r the f o r m a t i o n of the M L ^ c o m p l e x ( L = H A ) ,

P2

L

M L

2

]

fed y Cu Pb Cd

2+ 2+ 2+

, vvaarryy as f o l l o w s for d i f f e r e n t m e t a l i o n s : 2

:

l o g β..

8. 65 + 0.65

(pH-5)

:

log β

2

8. 35 + 0.30

(pH-5)

:

l o g β..

6. 25 + 0.63

(pH-5)

F o r b e t t e r u n d e r s t a n d i n g of s u c h c o r r e l a t i o n s it i s n e c e s s a r y to know the a c i d i t y constants i n v o l v e d . G e n e r a l l y , these have b e e n d e t e r m i n e d a t h i g h l i g a n d c o n c e n t r a t i o n s (around 10"^ M ) and r a t h e r low p H . The a c i d i t y constants a l s o have b e e n found to be v e r y p H dependent and i o n i c s t r e n g t h dependent, a g a i n fitting the c o m p a r i s o n w i t h c o l l o i d a l p a r t i c l e s . Thus G a m b e (26) found l o g constants a r o u n d 3.6 f o r and 4 . 3 for K.£ a t p H 3 and p H 4 r e s p e c t i v e l y , (27) found l o g

= 5.5

for F A ' s .

C o l e m a n et a l .

for p e a t .



6.


Copper

lOYlS With

Ligdtlds

121

A l l these c o n s i d e r a t i o n s l i m i t the data w i t h w h i c h the r e s u l t s of this study can be c o m p a r e d , s i n c e they w e r e d e t e r m i n e d a t a h i g h e r pH. R e s u l t s of s i m i l a r s t u d i e s a r e s u m m a r i z e d i n T a b l e I I , The d e t e r m i n a t i o n s by B r a n i c a (28) and S h u m a n and W o o d w a r d (11 ) w e r e a c t u a l l y p e r f o r m e d f o r c o n d i t i o n s of s e a w a t e r and l a k e w a t e r , r e s p e c t i v e l y . The o t h e r a u t h o r s p r e c o n c e n t r a t e d t h e i r l i g a n d s . P o l a r o g r a p h i c a l methods have the d i s a d v a n t a g e that the c o m p l e x m a y s p l i t up d u r i n g m e a s u r e m e n t , u n l e s s the m e a s u r e m e n t i s p e r f o r m e d m u c h f a s t e r than the k i n e t i c s of the c o m p l e x d i s s o c i a t i o n . The a p p a r e n t s t a b i l i t y constants thus d e t e r m i n e d m a y w e l l be too s m a l l . M a n t o u r a and R i l e y (30), who w o r k e d at a p H s i m i l a r to the p r e s e n t e x p e r i m e n t s and who a l s o u s e d a s y s t e m w h e r e l i g a n d and m e t a l a r e i n e q u i l i b r i u m w i t h e a c h o t h e r , found constants q u i t e s i m i l a r to those of the p r e s e n t study. The v a l u e d e t e r m i n e d by van D i j k (31 ) i s m e n t i o n e d to show how a c o m p a r a t i v e l y s i m p l e m e t h o d can p r o d u c e the o r d e r of m a g n i t u d e of the s t a b i l i t y constant. He c o m p a r e d the s t r e n g t h of s o m e known c o m p l e x i n g agents w i t h that of HA i n the p r e s e n c e of an i o n exchange r e s i n . E f f e c t of p H on the C o n d i t i o n a l S t a b i l i t y Constant. A n u m b e r of w a t e r s have b e e n a n a l y s e d a t t h e i r o r i g i n a l pH, s o m e have b e e n a d j u s t e d to p H 7.6 f o r i n t e r c o m p a r i s o n , and s o m e have b e e n a n a l y s e d t w i c e at d i f f e r e n t pH's. The r e s u l t s a r e p l o t t e d i n F i g u r e 1. The d y s t r o p h i c w a t e r s a l l g i v e v e r y s i m i l a r v a l u e s at p H 7. 6, and the l o g c o n s t a n t i n c r e a s e s w i t h a s l o p e c l o s e to one w i t h the pH: f o r D i c k i e No. 5, l o g Κ i s 8.5 a t p H 8.4, and 7. 8 at p H 7. 6 The o t h e r s a m p l e s p r o v i d e d g e n e r a l l y h i g h e r c o n s t a n t s . The d e t e r m i n e d f o r L a k e O n t a r i o , i s an o r d e r of m a g n i t u d e h i g h e r than F A . A g a i n , a s h i f t w i t h the p H w i t h a s l o p e c l o s e to one was o b s e r v e d : L a k e O n t a r i o , l o g Κ i s 9. 5 at p H 8.4, and 8.6 at p H 7.4. The a u t h o r s o b s e r v e d that the c o n d i t i o n a l s t a b i l i t y constants s t i l l i n c r e a s e at the h i g h e s t pH's m e a s u r e d (pH 8.6). A p p a r e n t l y the l i g a n d i s at l e a s t s t i l l p a r t i a l l y i o n i z e d so t h e r e has to be a l o g d i s s o c i a t i o n c o n s t a n t l a r g e r than 8.6. By s p e c i f i c a l l y b l o c k i n g a c t i v e g r o u p s on o r g a n i c m a t t e r , S c h n i t z e r and S k i n n e r (32 ) showed that m e t a l s a r e bound by s i m u l t a n e o u s a c t i o n of a c i d i c c a r b o x y l g r o u p s and p h e n o l i c h y d r o x y l g r o u p s . One c o u l d s p e c u l a t e that the h y d r o x y l g r o u p s have a r a t h e r h i g h d i s s o c i a t i o n constant s i n c e they a r e l e s s a c i d i c .


Jenne; Chemical Modeling in Aqueous Systems ACS Symposium Series; American Chemical Society: Washington, DC, 1979. χ

(Κ )

2

(K )

7. 8

FA

(2) F A e x t r a c t e d f r o m peat.

(1) F A e x t r a c t e d f r o m l a k e w a t e r

7

4.5-5.7

2

7. 16 ( K )

8.51

8.05

8.80 (K )

HA

Lakewater

FA

(2)

F A (1)

5. 1-6.2

7.46

Seawater

HA

Log Constant

Ligand

7.6

6

6.5

8. 0

6.8(?)

8.2

pH

ion

exchange

competition

a. s. v.

gelpermeation

dialysis

d.p.p.

Method

0. 01

0. 1

0. 01

0. 01

0. 7

Ionic Strength

this paper

(31.)

(3 0)

(29)

(28)

Reference

T a b l e I I . C o m p a r i s o n of c o n d i t i o n a l s t a b i l i t y c o n s t a n t s f o r c o m p l e x e s of c o p p e r with naturally occurring organics


6.

VAN

DEN

BERG AND

KRAMER

Copper Ions With

123

Ligands

The l i g a n d s that w e r e m e a s u r e d i n the f i r s t g r o u p of ( d y s t r o p h i c ) w a t e r s m a y w e l l be d e r i v e d f r o m s o i l s . F u l v i c a c i d s , b e i n g the s m a l l e s t and m o s t s o l u b l e of the h u m i c c o m pounds, a r e f l u s h e d out m o s t e a s i l y and f o r m a m a j o r p a r t of d i s s o l v e d o r g a n i c s i n n a t u r a l f r e s h w a t e r s (33). The s i m i l a r i t y of the m e a s u r e d s t a b i l i t y constants of F A and dy s t r o p h i c w a t e r s s u g g e s t s that s i m i l a r ( c a r b o x y l ) g r o u p s m a y be r e s p o n s i b l e f o r b i n d i n g and that the r e m a i n i n g p a r t of the m o l e c u l e s i s not v e r y m u c h d i f f e r e n t f r o m e a c h o t h e r .


A d s o r p t i o n of the M e t a l C o m p l e x on MnCL,.

Adsorption

of the m e t a l - o r g a n i c c o m p l e x , o r p a r t of i t , on the i o n exchange m e d i u m w o u l d a f f e c t the d e t e r m i n a t i o n of the s t a b i l i t y constant i n that the d i s s o l v e d c o p p e r c o n c e n t r a t i o n w i l l be d e c r e a s e d . P r e v i o u s s t u d i e s u s i n g i o n exchange r e s i n s o b s e r v e d no ad s o r p t i o n e f f e c t s (15 ) o r do not m e n t i o n i t . The p r e s e n t r e s u l t s w i t h known l i g a n d s (20) show that a d s o r p t i o n , i f any, of the c o m p l e x does not a f f e c t the m e a s u r e m e n t and the r e s u l t s . It was o b s e r v e d , h o w e v e r , that the o r g a n i c m a t t e r has s o m e s o r t of s u r f a c e a c t i v e e f f e c t on the MnO^ d i s p e r s i o n : s u b s a m p l e s , taken d u r i n g the t i t r a t i o n w i t h c o p p e r , of w a t e r s c o n t a i n i n g a h i g h l i g a n d c o n c e n t r a t i o n (10 μΜ ) take m o r e t i m e , up to about t w i c e as long to be f i l t e r e d than s a m p l e s c o n t a i n i n g a v e r y low l i g a n d c o n c e n t r a t i o n (0.3 μΜ ). A l s o , the type o f s a m p l e s e e m s to a f f e c t the f i l t e r speed. A s a m p l e of L a k e O n t a r i o , c o n t a i n i n g 0.5 μΜ l i g a n d s , has r o u g h l y the same e f f e c t as a 5 μιη F A s o l u t i o n . A p p a r e n t l y the s i z e and c o m p l e x i t y of the m o l e c u l e s d e t e r m i n e s i n w h a t way the MnO^ d i s p e r s i o n w i l l be stabilized. F r o m C o u l o m b i c - f o r c e s p o i n t of v i e w one can s p e c u l a t e that t h e r e i s s o m e a d s o r p t i o n at v e r y l o w copper c o n c e n t r a t i o n [^Cu^J < Q ^ ^ J ' o s t of the l i g a n d s a r e c o m p l e x e d and w

n

e

n

m

have a p o s i t i v e c h a r g e , w h i l e the MnO^ s t i l l has i t s n e g a t i v e c h a r g e . A s soon as £ C u r J > {^Lig^^J , b o t h the c o m p l e x and the s u r f a c e of the MnO w i l l have a p o s i t i v e c h a r g e and w i l l r e p e l e a c h o t h e r e l e c t r o s t a t i c a l l y . T h i s w i l l be d i s c u s s e d f u r t h e r in a later publication. M o l e c u l a r W e i g h t o f FA. FA was S c h n i t z e r i n d r i e d form (32). I f one c o m p l e x a t i o n o f one c o p p e r i o n b y one can c a l c u l a t e the m o l e c u l a r w e i g h t o f

s u p p l i e d t o us by assumes the F A - m o l e c u l e , one FA t o be 8 4 1 ,



124

which compares w e l l to the v a l u e o f 950 as found b y Schnitzer

(34).


Determination o f Conditional Stability Constants for M i x e d L i g a n d s and f o r C o m p l e x e s o t h e r than 1:1 . Schubert's (15) study o f c o m p l e x ions by a n i o n exchange technique was c o m m e n t e d upon by I. F e l d m a n , who m e n t i o n e d that the m e t h o d w o r k s o n l y f o r 1:1 c o m p l e x e s . T h i s is i n h e r e n t to any m e t h o d , h o w e v e r , w h i c h does not v a r y the l i g a n d c o n c e n t r a t i o n . In c a s e of a m i x t u r e of l i g a n d s , a n a v e r a g e s t a b i l i t y constant is d e t e r mined (35). T h e d e t e r m i n a t i o n of two s t a b i l i t y constants f o r 8H y d r o x y l - Q u i n o l i n e (20) i n the p r e s e n t study shows that the i o n exchange m e t h o d is c a p a b l e o f a c c u r a t e l y d e t e r m i n i n g two s i t e s o r two l i g a n d s i f t h e s e two s i t e s a r e p r e s e n t i n e q u a l c o n c e n t r a t i o n , and i f the sites r e s u l t i n d i f f e r e n t enough s t a b i l i t y constants r e l a t i v e to d i s c e r n i n g s l o p e c h a n g e s . In o t h e r c a s e s the g r a p h i c a l r e p r e s e n t a t i o n of the t i t r a t i o n w i l l be s l i g h t l y c u r v e d c o n c a v e l y . E x c e p t i n one c a s e , the r e s u l t s produced almost straight lines with high c o r r e l a t i o n coeff i c i e n t s s h o w i n g that one site o r l i g a n d is at l e a s t i n a v e r y d o m i n a n t p o s i t i o n . C o n t r a d i c t o r y r e s u l t s have b e e n p u b l i s h e d e a r l i e r . A r d a k a n i and S t e v e n s o n (19) found o n l y 1:1 c o m p l e x e s of m e t a l s w i t h H A and o b s e r v e d o n l y a s i n g l e c o m p l e x i n g s i t e . S c h n i t z e r and H a n s e n (17) o b s e r v e d the s a m e f o r F A ( f r o m soil). M a n t o u r a a n d R i l e y (30) found two s i t e s o n F A e x t r a c t e d f r o m water. L i g h t A b s o r p t i o n at 260 n m and C o m p l e x i n g C a p a c i t y . T h e c o n c e n t r a t i o n of h u m i c m a t t e r in w a t e r has b e e n r e l a t e d to m e a s u r e m e n t s of l i g h t a b s o r p t i o n at d i f f e r e n t w a v e l e n g t h s at 365 n m and at 250 n m . S c a n n i n g s p e c t r o p h o tome t r i e m e a s u r e m e n t s w e r e m a d e f r o m 220 to 380 n m . T h e p e a k height g e n e r a l l y d e c r e a s e s going to h i g h e r w a v e l e n g t h s and the p e a k was r a t h e r s m a l l to m a k e a c c u r a t e m e a s u r e m e n t s at 365 n m . In m o s t s a m p l e s , h o w e v e r , a p l a t e a u that l a s t e d f r o m 255 to 265 n m , and s o m e t i m e s f r o m 250 to 270 n m , was f o u n d . It was then d e c i d e d to m e a s u r e the a b s o r p t i o n at the height o f this p l a t e a u , at 260 n m . T h e low p H s a m p l e s w e r e b r o u g h t to p H 7 by the a d d i t i o n of s o d i u m b i c a r b o n a t e buffer u n t i l -3 10 M b i c a r b o n a t e was p r e s e n t . The results a r e given in F i g u r e 2, as a b s o r p t i o n v s . c o m p l e x i n g c a p a c i t y . T h e data s c a t t e r w i d e l y w h i c h s u g g e s t s that i t i s not p o s s i b l e to a c c u r a t e l y p r e d i c t the c o m p l e x i n g c a p a c i t y f r o m a n a b s o r p t i o n



Copper

IOUS With

Ligands


log K,

Figure

1.

Conditional mined. (%)=

stability constants vs. pH at which they have been deter Dystrophic waters and FA; (X) = other waters.

0

10

20

COMPLEXING CAPACITY (μΜ) Figure 2.

Light absorption

at 260 nm vs. complexing

capacity



126


m e a s u r e m e n t . One cannot s a y m o r e than that h i g h a b s o r p t i o n might indicate a high complexing capacity. P o s s i b l y only part of the a b s o r p t i o n i s c a u s e d b y c o m p l e x i n g m a t e r i a l . The r e s t i s c a u s e d by o t h e r o r g a n i c m a t t e r . Complexation of C u by some Known Ligands and N a t u r a l l y O c c u r r i n g L i g a n d s at p H 8.3. The e f f e c t i v e n e s s i n l o w e r i n g the f r e e m e t a l i o n c o n c e n t r a t i o n by s o m e known c o m p l e x i n g l i g a n d s and b y F A a n d l i g a n d s as i n L a k e O n t a r i o w a t e r i s s h o w n i n F i g u r e 3. The r e l a t i o n s h i p s have b e e n c a l c u l a t e d at p H 8.3, a s i n s e a w a t e r , a n d f o r 25°C a n d 0. 01 M i o n i c s t r e n g t h . The s t a b i l i t y constants of the k n o w n l i g a n d s have b e e n c o r r e c t e d f o r the p H u s i n g t h e i r a c i d i t y constants a s found i n S i l l en a n d M a r t e l l (36). F o r F A and L a k e O n t a r i o the constants have b e e n t a k e n f r o m T a b l e l a n d a d j u s t e d f o r p H 8. 3. In the c a l c u l a t i o n s , 1 0"^ M c o m p l e x i n g l i g a n d s a n d 10~8 m t o t a l d i s s o l v e d c o p p e r w e r e a s s u m e d . T h e s e c o n c e n t r a t i o n s a r e s i m i l a r to v a l u e s found i n o p e n o c e a n w a t e r w h i c h has about 1 mg. L~*. C (37); i f the o r g a n i c c a r b o n o c c u r r e d as a c o m p o u n d w i t h the Mw. of F A , i t w o u l d be 10"^ M l i g a n d s , but b e c a u s e o f the v e r y l o n g r e s i d e n c e t i m e the m o l e c u l e s p r o b a b l y a r e m o r e c o m p l e x a n d h e a v i e r than F A . H i s t i d i n e , w h i c h has b e e n r e p o r t e d to o c c u r a t l e v e l s o f -8 -7 10 M (38) to 10 M (39), w o u l d b r i n g the f r e e c o p p e r i o n -1 0 8 d o w n to a v e r y l o w l e v e l o f 10 " M; N T A w h i c h m i g h t e n t e r the s e a a s a r e s u l t of p o l l u t i o n , w o u l d b r i n g i t down to -12 10 M , b u t i t i s b i o d e g r a d a b l e a n d i s not p r o d u c e d i n s i t u . Organic matter o c c u r r i n g in Lake Ontario would produce a p [ c u J o f 10. 5. A n d e r s o n and M o r e l (8) c a l c u l a t e d w i t h the R E D E Q L c o m p u t e r m o d e l a ρ £ c u ^ J o f 9. 6 b y i n o r g a n i c c o m plexation. The p r e s e n c e of o n l y 10"^ M o r g a n i c c o m p l e x i n g l i g a n d s m a y w e l l be s u f f i c i e n t to d i m i n i s h the f r e e i o n c o n c e n tration by an o r d e r of magnitude. A n inorganic particulate such as M n O ^ w o u l d not be a s u c c e s s f u l c o m p e t i t o r f o r c u p r i c i o n s + 2

+

under these

circumstances.

C o m p l e x a t i o n o f C u a t V a r y i n g p H . In o r d e r to c o m p a r e the e f f e c t s o f c o m p l e x a t i o n by l a k e o r g a n i c s a t d i f f e r e n t pH's, the f r a c t i o n (a) of t o t a l d i s s o l v e d c o p p e r (Cu^,) w a s c a l c u l a t e d w h i c h i s i n the f o r m o f the f r e e m e t a l i o n



6.

Copper Ions with Ligands

V A N DEN BERG AND KRAMER

127

Figure 3. The effect of complexation at pH 8.3 by naturally occurring ligands and some artificial ligands (on Cu ) shown against the conditional stability con stant (K ) at this pH. Conditions for the plot are total ligand concentration L = J0" M, total copper concentration Cu = 10~ M, 25° C, μ = 0.01. +2

L

T

7

T

8


128



Figure

4. Model for complexation of copper by lake organics. Ligand tai = log K = 7.8, and log K0n,ert0 = 8.8 at pH 7.6, log K oh = 6.0, log Kcucos = 6.8, pCO, = 10' , = 0.01, 25°C. (Cu ) = . (Cu ), = 1/ (K · [L] + 1), Cu = 2 χ 10 U. 1:L = OH", 2:L = COf, 3:L = FA, 4:L = ligand in Lake Ontario. to

W M, 6

F

Cu

A

0

L

35

T

μ

+2

a

+

T

a

7


6.

VAN D E N BERG AND KRAMER

+ 2

(Cu ):

[cu

+

2

Copper

]= . [Cujand 0

IoUS With

α

=K

L

Llgands

129

. £ L ]+ 1'


F r o m the r e s u l t s , i n F i g u r e 4, one c a n see that u n t i l about p H 9, o r g a n i c s a r e m u c h m o r e i m p o r t a n t i n c o m p l e x i n g c o p p e r than a r e c a r b o n a t e and h y d r o x y l ion. A t a l o w e r p H of 6, a n d w i t h o r g a n i c s w i t h a s t a b i l i t y c o n s t a n t a s found i n L a k e O n t a r i o , 9 0% o f the copper p r e s e n t i s c o m p l e x e d b y 10" M o r g a n i c s , w h e r e a s a t a p H o f 8, 99. 9% o f the c o p p e r i s complexed. Conclusions C o n d i t i o n a l s t a b i l i t y constants f o r c o m p l e x a t i o n of n a t u r a l o r g a n i c s w i t h c o p p e r have v a l u e s , a t p H 7.6, between 7 8 88 10 , f o r F A , and 10 , f o r ligands i n Lake Ontario. G e n e r a l l y , o n l y one c o m p l e x i n g s i t e p e r m o l e c u l e has been found w i t h a s t a b i l i t y constant of i m p o r t a n c e a t the con c e n t r a t i o n s found i n n a t u r a l w a t e r s , f o r F A and o t h e r n a t u r a l ly occurring ligands. Organic matter occurring i n natural waters is a sig nificant complexing ligand f o r free metal ions. A cknowledgements T h i s w o r k was s u p p o r t e d by g r a n t s f r o m the N a t i o n a l R e s e a r c h C o u n c i l o f C a n a d a and Inland W a t e r s Subvention P r o gram, Environment Canada. Abstract Toxicity to copper in natural waters may be related to the cupric ion concentration. Therefore, a method is developed using MnO as weak ion exchanger, to assess the complexing 2

capacities and conditional stability constants for compounds in natural waters. Constants found are in the range of 10 for fulvic acid to 10 for a ligand in Lake Ontario, at pH 7. 6, 2 5 ° C , and 0. 01 M ionic strength. Calculation of the complexa tion of copper by 10 M naturally occurring ligands, at different pH's, shows that the free metal ion concentration is lowered considerably between pH 5 and 9. 7·

8.8

-6


8

130


L i t e r a t u r e Cited 1. 2.

3.


4. 5. 6.

7. 8. 9.

10.

11. 12.

13.

14.

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