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Chapter 9
Photochemistry of Copper Complexes in Sea Water James W. Moffett and Rod G. Zika
Downloaded by COLUMBIA UNIV on March 8, 2013 | http://pubs.acs.org Publication Date: December 8, 1987 | doi: 10.1021/bk-1987-0327.ch009
Rosenstiel School of Marine and Atmospheric Sciences, Division of Marine and Atmospheric Chemistry, University of Miami, Miami, FL 33149
The photochemistry of copper in seawater has been studied to assess the role sunlight plays in the speciation and redox chemistry of copper in the upper water column of the ocean. A variety of photo chemically induced redox reactions involving Cu(I)/Cu(II) interconversion have been studied at elevated copper concentrations in the laboratory. To evaluate the net effect of these processes at natural copper levels, Cu(I) and [Cu(II)] /[Cu(II)] were measured as a function ofsunlightirradiation and depth in the water column. The results indicate that sunlight has an important influence on copper speciation in the upper water column. T h e r e i s now c o n s i d e r a b l e e v i d e n c e t h a t a v a r i e t y o f p h o t o c h e m i c a l l y i n d u c e d redox p r o c e s s e s o c c u r i n n a t u r a l water systems. A t t h e p r e s e n t t i m e h o w e v e r , l i t t l e i s known a b o u t how t h e s e p r o c e s s e s i n f l u e n c e redox e q u i l i b r i a o f minor elements such as c o p p e r o r o t h e r t r a n s i t i o n m e t a l s i n t h e water column. A v a r i e t y of elements e x i s t i n t h e upper water column i n v a l e n c e s t a t e s which a r e u n s t a b l e w i t h r e s p e c t t o t h e (^/H^O c o u p l e . However, s i n c e l i t t l e k i n e t i c d a t a i s a v a i l a b l e f o r p r o b a b l e photoredox p r o c e s s e s i n v o l v i n g these e l e m e n t s , i t i s i m p o s s i b l e t o a s s e s s t h e importance of p h o t o c h e m i s t r y i n r e l a t i o n t o b i o l o g i c a l l y mediated p r o c e s s e s . The c h e m i s t r y o f copper has been s t u d i e d e x t e n s i v e l y i n r e c e n t y e a r s because o f i t s t o x i c i t y t o a q u a t i c organisms and p o s s i b l e r o l e i n l i m i t i n g p r i m a r y p r o d u c t i v i t y under c e r t a i n c o n d i t i o n s (1,2). These s t u d i e s i n d i c a t e t h a t t h e s p e c i a t i o n o f copper determines the extent of i t s t o x i c i t y i n aqueous solution. U n f o r t u n a t e l y , t h e parameters which i n f l u e n c e i t s s p e c i a t i o n a r e p o o r l y u n d e r s t o o d , i n p a r t because o f i t s s t r o n g c h e l a t i o n by p o o r l y c h a r a c t e r i s e d o r g a n i c c h e l a t o r s i n seawater. Most s t u d i e s have f o c u s e d on t h e c o m p l e x a t i o n c h e m i s t r y o f C u ( I I ) . However, t h e chemistry of Cu(I)/Cu(II) interconversion has been studied extensively i n c h e m i c a l and b i o l o g i c a l systems and t h e r e i s
0097-6156/87/0327-0116$06.00/0 © 1987 American Chemical Society
In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
9.
MOFFETT AND ZIKA
117
Photochemistry of Copper Complexes
considerable evidence that s i m i l a r reactions occur i n n a t u r a l waters. In t h i s work, the p h o t o c h e m i s t r y of copper complexes i n seawater has been i n v e s t i g a t e d w i t h the f o l l o w i n g g o a l s i n mind; a.
b.
To i d e n t i f y p r o b a b l e p r i m a r y and secondary p h o t o c h e m i c a l redox r e a c t i o n s i n v o l v i n g copper complexes and t o e v a l u a t e t h e i r p o t e n t i a l s i g n i f i c a n c e i n the marine environment, To s t u d y t h e o v e r a l l e f f e c t o f s u c h r e a c t i o n s on c o p p e r s p e c i a t i o n i n seawater by measuring C u ( I ) and I^u(II)]^ / [Cu(II)] i n seawater a t n a t u r a l l e v e l s . Examine trie i m p l i c a t i o n s of t h i s work f o r the b i o a v a i l a b i l i t y of copper and the d e g r a d a t i o n of o r g a n i c c h e l a t o r s . Examine the i m p l i c a t i o n s of t h i s work f o r the c h e m i s t r y and f a t e of H^O^ i n seawater. r e e
t
c.
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d.
Methods P o t e n t i a l l y s i g n i f i c a n t r e a c t i o n s were s t u d i e d i n model systems a t e l e v a t e d copper l e v e l s , f o l l o w i n g C u ( I ) f o r m a t i o n o r decay u s i n g a s e l e c t i v e , s e n s i t i v e procedure i n v o l v i n g c o l o r i m e t r i c d e t e r m i n a tion of a C u ( I ) complex with bathocuproine d i s u l f o n i c acid (2,9,-dimethyl,-4,7,-diphenyl 1,10-phenanthroline d i s u l f o n i c a c i d ) . The method i s d e s c r i b e d i n R e t a i l elsewhere (3) and has a l i m i t of d e t e c t i o n of 1 χ 10 mol L . The a d d i t i o n o f e t h y l e n e d i a m i n e has been shown t o e f f e c t i v e l y mask C u ( I I ) i n t e r f e r e n c e . The procedure was e x t e n d e d t o s t u d i e s a t n a t u r a l l e v e l s by u s i n g a s i m i l a r l i g a n d , n e o c u p r o i n e ( 2 , 9 - d i m e t h y l , 1 , 1 0 - p h e n a n t h r o l i n e ) , which forms a s o l v e n t e x t r a c t a b l e C u ( I ) complex. The complex i s e x t r a c t e d i n t o m e t h y l e n e c h l o r i d e a n d j j a c k e x t r a c t e d w i t h 10% HNO^ The l i m i t of d e t e c t i o n i s 0.02 χ 10 mol L · E t h y l e n e d i a m i n e i s a g a i n used as a masking l i g a n d ( 4 ) . The r a t i o [ C u ( I I ) ] /[Cu(II)] was determined u s i n g a l i g a n d exchange, l i q u i d Lfquid p a r t i t i o n technique. Acetylacetone was added t o s e a w a t e r , f o r m i n g a copper a c e t y l a c e t o n a t e complex. D e t e r m i n a t i o n of the d i s t r i b u t i o n of copper between t h a t complex and natural complexes enabled calculation of [Cu(II)] / [Cu(II)] . The procedure i s described i n d e t a i l elsewhere(5;. S t u d i e s a t the l a b o r a t o r y were c a r r i e d out i n s y n t h e t i c media or i n seawater c o l l e c t e d a t the RSMAS dock, B i s c a y n e Bay, F l o r i d a . Most water was f i l t e r e d through 0.2/Λη Nucleopore f i l t e r s , a l t h o u q h s y s t e m a t i c d i f f e r e n c e s between f i l t e r e d and n o n f i l t e r e d w a t e r s were not o b s e r v e d . S h i p b o a r d samples were c o l l e c t e d u s i n g t e f l o n c o a t e d Go F l o b o t t l e s ( G e n e r a l O c e a n i c s ) and were n o t f i l t e r e d . A l l d e t e r m i n a t i o n s a t sea and i n the l a b a t n a t u r a l copper l e v e l s were c a r r i e d o u t i n a l a m i n a r f l o w hood. Go f l o b o t t l e s were r a c k e d i n a c l e a n a r e a of the s h i p and samples t r a n s f e r r e d d i r e c t l y t o the hood v i a t e f l o n t u b i n g . A l l s a m p l i n g l o c a t i o n s r e f e r r e d t o i n t h i s s t u d y a r e shown i n F i g u r e 1. O r g a n i c m a t e r i a l i s o l a t e d f r o m B i s c a y n e Bay, F l o r i d a , by u l t r a f i l t r a t i o n was u s e d i n some e x p e r i m e n t s a t h i g h e r c o p p e r levels. An A m i c o n u l t r a f i l t r a t i o n a p p a r a t u s w i t h A m i c o n PM10 f i l t e r s was used. e
t
f
In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
1 18
PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS
Photoreduction processes i n v o l v i n g
Cu(II)
P r i m a r y P r o c e s s e s . The most common p r i m a r y photochemical p r o c e s s e s i n v o l v i n g c o p p e r ( I I ) complexes a r e l i g a n d t o metal charge t r a n s f e r (LMCT) r e a c t i o n s ( 6 , 7 ) , a l s o r e f e r r e d t o as charge t r a n s f e r t o metal (CTTM) r e a c t i o n s . LMCT r e a c t i o n s occur when l i g h t absorbed i n an LMCT a b s o r p t i o n band o f the C u ( I I ) complex l e a d s t o r e d u c t i o n of the m e t a l and o x i d a t i o n o f the l i g a n d .
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Cu(II)-L Cu(II)-L*
^ —-
Cu(II)-L* Cu(I) + L
Where L i s an o r g a n i c l i g a n d , o x i d a t i o n g e n e r a l l y r e s u l t s i n the d e s t r u c t i o n o f the l i g a n d . LMCT r e a c t i o n s have been observed f o r many C u ( I I ) complexes, which a r e l i s t e d i n Table 1. Table Complex ( 2
CuCl
1.
P h o t o r e a c t i v e C o p p e r ( I I ) Complexes Products
"
x )
2
x
X
CuOCOCH
+
2
_
7
B r
2
_
7
+
Cu ,
3
References
C 1
Χ
CuBr < - >
Detected*
CH OH, C H 3
2
6
7
2Cu(malonate)
co 2
2Cu(oxalate)
7
2
co , Cu
+
7, t h i s work
co , Cu
+
7
co
Cu
+
7, t h i s work
co , Cu
+
7, t h i s work
2
2
2CuCglutamate)^
2
2
Cu(P-alanine) ~ 2
2 >
2Cu(glycine) Cu(en)
2
2
2 +
NH , C 0 ,
2
3
2
Cu(NTA) " 2
* Other products l i s t e d here.
co detected
2
2
Cu
Cu 0 2
+
f o r many o f t h e s e
7 8, t h i s work processes
but not
The l i s t i n d i c a t e s t h a t most l i g a n d s w i t h c a r b o x y l a t e and amino functional groups form Cu(II) complexes which a r e photoreactive. F o r complexes where the LMCT band extends above 300nm the r e a c t i o n s h o u l d proceed i n s u n l i g h t i r r a d i a t e d seawater. T h i s h a s been d e m o n s t r a t e d i n t h i s l a b o r a t o r y by u s i n g C u ( I I ) c o m p l e x e s w i t h NTA, o x a l a t e , g l y c i n e and a l a n i n e i n s e a w a t e r exposed t o s u n l i g h t and m o n i t o r i n g t h e f o r m a t i o n of C u ( I ) . C o p p e r ( I I ) i s the predominant valence s t a t e of copper i n o x y g e n a t e d marine waters and forms a v a r i e t y of complexes w i t h i n o r g a n i c and o r g a n i c l i g a n d s . C o n s i d e r a b l e a t t e n t i o n i n r e c e n t
In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
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9.
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119
Photochemistry of Copper Complexes
y e a r s has been f o c u s e d on C u ( I I ) o r g a n i c c h e l a t o r s and t h e i r importance i n C u ( I I ) s p e c i a t i o n i s now f i r m l y e s t a b l i s h e d . Since oxygen and n i t r o g e n donor f u n c t i o n a l groups such as c a r b o x y l a t e and amine groups a r e p r o b a b l y i n v o l v e d i n C u ( I I ) c h e l a t i o n by n a t u r a l o r g a n i c c h e l a t o r s (9,10) i t i s r e a s o n a b l e t o expect t h a t they a r e photoreactive. S p e c t r a l d a t a i n d i c a t e t h a t C u ( I I ) complexes w i t h o r g a n i c matter i s o l a t e d by u l t r a f i l t r a t i o n from seawater absorb s i g n i f i c a n t l y above 300nm ( F i g u r e 2 ) . S i n c e t h i s a b s o r p t i o n i s p r o b a b l y an LMCT band i t i n d i c a t e s the p o t e n t i a l f o r these processes i n s u n l i g h t i l l u m i n a t e d seawater. In a d d i t i o n , phenolic or s a l i c y l a t e type s i t e s may be i m p o r t a n t i n the c h e l a t i o n o f copper by d i s s o l v e d o r g a n i c matter (11,12). _ I n t h i s work i r r a d i a t i o n o f C u ( I I ) s a l i c y l a t e i n 0.7 molL CI a t 296.7 nm and 313nm l e d t o the f o r m a t i o n o f C u ( I ) . However i t was not c l e a r t h a t a LMCT p r o c e s s was r e s p o n s i b l e and i n d e e d some r e d u c t i o n o c c u r e d a f t e r c e s s a t i o n o f i r r a d i a t i o n , i n d i c a t i n g a second o r d e r process was i n v o l v e d . The o b s e r v a t i o n t h a t i r r a d i a t i o n l e a d s t o the f o r m a t i o n of a r e l a t i v e l y l o n g l i v e d r e d u c t a n t i s i n t e r e s t i n g , but r e n d e r s s a l i c y l a t e l e s s u s e f u l as a model f o r p r i m a r y r e a c t i o n s . I r r a d i a t i o n of CuCO^ i n 0.7 M NaCl s o l u t i o n a t 313nm l e a d s t o Cu(I) formation, probably v i a CuCO
3
Cu
+
+ CO
3
The r e a c t i o n i s p o t e n t i a l l y i m p o r t a n t because the complex absorbs s i g n i f i c a n t l y above 300nm and CuCO^ i s the major i n o r g a n i c copper complex i n seawater ( 1 2 ) . C u ( I ) p r o d u c t i o n was e x t r e m e l y slow, negligible i n s u n l i g h t and p r o b a b l y not s i g n i f i c a n t i n the e n v i r o n m e n t . The low quantum y i e l d may be^ due to_^back r e a c t i o n of Cu w i t h CO,, a l t h o u g h a d d i t i o n o f 10 mol L 2-propanol t o r e a c t w i t h the CO^ r a d i c a l d i d not l e a d t o an i n c r e a s e i n r a t e . Secondary R e a c t i o n s . The r e d u c t i o n o f a C u ( I I ) c o m p l e x by a p h o t o c h e m i c a l l y generated r e d u c i n g agent c o n s t i t u t e s a secondary photochemical r e a c t i o n . Reductants such as 0 , H 0 and o r g a n i c 9
?
9
T h e r e f o r e we have s t u d i e d the r e d u c t i o n k i n e t i c s o f C u ( I I ) by H^O^ to a s s e s s i t s e n v i r o n m e n t a l importance. A g e n e r a l mechanism f o r t h i s process has been p o s t u l a t e d by numerous workers ( 1 6 ) . (1) (2) (3) (4)
The r e a c t i o n has been s t u d i e d i n a v a r i e t y o f aqueous media but not i n c h l o r i d e media (17-19). T h e r e f o r e we i n v e s t i g a t e d the
In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
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PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS
Figure
1.
Map of F l o r i d a showing sampling l o c a t i o n s .
•03^ LU
200
250
300
350
WAVELENGTH/nm
F i g u r e 2. S p e c t r a o f C u ( I I ) complexes formed by t h e a d d i t i o n of C u ( I I ) t o a c a r b o n a t e f r e e s o l u t i o n o f marine o r g a n i c m a t t e r isolated from Biscayne Bay water by ultrafiltration. P r e c o n c e n t r a t e d x _ i 0 0 . ( a ) [ C u ( I I ) ] = 2 χ 10 mol L ; ( b ) , [ C u ( I I ) ] - 4 χ 10 mol L ; ( c ) [ C u ( l T ) ] = 8 χ 10 mol L , no u l t r a f i l t r a t e ; (d) [ C u ( I I ) ] = 8 χ 10 mol L . f
R
In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
121
Photochemistry of Copper Complexes
9. MOFFETT AND ZIKA
r e a c t i o n i n s i m p l e c h l o r i d e media and i n seawater t o e s t a b l i s h the r e a c t i o n r a t e and determine which C u ( I I ) s p e c i e s a r e k i n e t i c a l l y important. The e f f e c t o f pH, [CI ] and [CO^ ] on t h e r e a c t i o n r a t e i n c h l o r i d e m e d i a was s t u d i e d . The r e s u l t s , w h i c h a r e d e s c r i b e d i n d e t a i l elsewhere (20) showed t h e r e a c t i o n has a f i r s t o r d e r d e p e n d e n c e on c h l o r i d e c o n c e n t r a t i o n and a s e c o n d o r d e r dependence on h y d r o x i d e . These o b s e r v a t i o n s a r e c o n s i s t e n t w i t h the f o l l o w i n g mechanism. fCu , ν (aq) 2 +
CuL 2 +
Cu
( a q )
(5)
+ OH" + C l ~ ^ CuOHCl
CuOHCl + H0 ~ —
CuOHCl" + H 0
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2
(6) (7)
2
I n t h i s mechanism CuL r e p r e s e n t s t h e major Cu complexes i n s o l u t i o n which a r e r e l a t i v e l y u n r e a c t i v e towards p e r o x i d e . Steps ( 5 ) and (6) r e p r e s e n t l i g a n d exchange processes which m a i n t a i n e q u i l i b r i u m steady s t a t e c o n c e n t r a t i o n s o f CuClOH. R e a c t i o n ( 7 ) i s t h e r a t e d e t e r m i n i n g s t e p . The r e a c t i o n pathway o f t h e HO^ formed i n r e a c t i o n (7) i n seawater i s n o t known. However i n t h e model s y s t e m s a t e l e v a t e d Cu l e v e l s i t was assumed t h a t i t r e a c t e d v i a r e a c t i o n s ( 3 ) and ( 4 ) above. T h i s mechanism i s c o n s i s t e n t w i t h e x p e c t a t i o n s from charge t r a n s f e r theory which i n d i c a t e s that the r a t e of the charge t r a n s f e r s t e p i s p r o p o r t i o n a l t o t h e f r e e energy change on g o i n g from t h e C u ( I I ) complex t o the C u ( I ) complex^ R e d u c t i o n o f CuClOH l e a d s £p f o r m a t i o n o f the v e r y s t a b l e CuClOH s p e c i e s . Reduction o f Cu , . o r CuCO^ does n o t l e a d t o d i r e c t f o r m a t i o n o f a s t a b l e C u ( I ) s p e c i e s . T h i s s i m p l e e x p l a n a t i o n does n o t e x p l a i n why o t h e r s p e c i e s such as C u C l are r e l a t i v e l y unreactive i n s p i t e of the h i g h s t a b i l i t y of the corresponding C u C l complex. A more c o m p l e x , i n n e r sphere charge t r a s f e r mechanism i s p r o b a b l y i n v o l v e d , as suggested i n p r e v i o u s s t u d i e s w i t h o t h e r C u ( I I ) complexes (17-19). The o v e r a l l second o r d e r r a t e c o n s t a n t f o r ^ C u i l l ^ r e d u c t i o n by H 0 i n o r g a n i c f r e e seawater i s 6.3 + 0.2 χ 10 mol L s . C u ( I I ) complexes w i t h marine o r g a n i c m a t t e r a r e u n l i k e l y t o be r e a c t i v e t o w a r d s H 0 because C u ( I I ) complexes w i t h c a r b o x y l a t e and amine l i g a n d s a r e g e n e r a l l y more s t a b l e than t h e i r C u ( I ) c o u n t e r p a r t s . T h e r e f o r e , i n seawater, t h e importance o f C u ( I I ) r e d u c t i o n by H 0 i s a f u n c t i o n o f the f r a c t i o n o f i n o r g a n i c a l l y complexed C u ( I I ) , deetrmined by t h e s t a b i l i t y c o n s t a n t s and c o n c e n t r a t i o n s o f o r g a n i c c h e l a t o r s , and t h e l a b i l i t y o f these C u ( I I ) o r g a n i c complexes. The importance o f 0 , generated i n t h i s r e a c t i o n o r by o t h e r p a t h w a y s i s d i f f i c u l t t o a s s e s s because t h e r e i s no d a t a f o r i t s c o n c e n t r a t i o n i n seawater. The c o n c e n t r a t i o n s a r e undoubtedly l o w , b u t i t i s v e r y r e a c t i v e w i t h Cu · The r a t e c o n s t a n t f o r t h e reaction 2
2
2
2
2
2
2
2
2
Cu
2 +
+
+ 0„" — C u + 0„ (8) ^2 — ^ ^ ^2 9 -1 -1 9 has been r e p o r t e d as Ls (,2 1 as 1.9 χ 10 mol mo . ) . From t h i s _ d a t a we can c a l c u l a t e a minimum steady s t a t e c o n c e n t r a t i o n o f 0 required
In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
122
PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS
f o r t h e r e a c t i o n t o be comparable w i t h C ^ ^ I I ) r e d u c t i o n by H J ^ . The c o n c e n t r a t i o n o b t a i n e d i s a b o u t 10 J8 ' - f ^elow e s t i m a t e s o f t h e upper c o n c e n t r a t i o n l i m i t o f 10 mol L ( 2 2 ) . I r r a d i a t i o n o f c o n c e n t r a t e d humic s o l u t i o n s and u l t r a f i l t r a t e i n s e a w a t e r leads to Cu(I) formation. C u ( I I ) was added t o u l t r a f i l t r a t e w h i c h had been i r r a d i a t e d f o r 1 hour a t 313 nm. Initially a b o u t 5% o f t h e t o t a l added copper was reduced but then a s t e a d y s t a t e was r e a c h e d w i t h no f u r t h e r r e d u c t i o n o c c u r i n g e v e n i n deoxygenated solution. C u ( I ) y i e l d s were small i n these experiments as Cu(I) oxidation occured simultaneously, by photochemically generated oxidants i n these concentrated s o l u t i o n s . This experiment indicates t h e problem i n working in a preconcentrated media where s o u r c e s sinks and s t e a d y state c o n c e n t r a t i o n s o f r e a c t i v e t r a n s i e n t s may be much d i f f e r e n t than i n seawater and a r e n o t r e a d i l y c h a r a c t e r i z a b l e . Such rapid Cu(I)/Cu(II) interconversions are less probable a t seawater concentrations where t r a n s i e n t s i n v o l v e d p r o b a b l y r e a c t r a p i d l y w i t h more abundant s p e c i e s than c o p p e r .
Downloaded by COLUMBIA UNIV on March 8, 2013 | http://pubs.acs.org Publication Date: December 8, 1987 | doi: 10.1021/bk-1987-0327.ch009
M O
Cu(I)
Oxidation
L
i n Seawater
P r o b a b l e o x i d a t i v e pathways f o r C u ( I ) i n seawater have been i n v e s t i g a t e d by t h i s group t o a s s e s s i t s p o t e n t i a l t o accumulate a t s i g n i f i c a n t s t e a d y s t a t e l e v e l s . The o x i d a t i o n o f C u ( I ) by 0^ has b e e n s t u d i e d e l s e w h e r e (14 ) . The o v e r a l l s e c o n d o r d e r r a t e c o n s t a n t i n seawater f o r t h e r e a c t i o n Cu
+
+ 0
—*
2
Cu
2 +
+ 0 "
(9)
2
1
H
h
a
s
a
l
s
o
D
e
e
n
i s 6.1 mol Ls The r a t e o f o x i d a t i o n by 2 ° 2 i n v e s t i g a t e d i n seawater and t h e o v e r a l l second o r d e r r a t e c o n s t a n t f o r the r e a c t i o n Cu
+
+ H 0 2
2
2
—
Cu 1
2 +
+ OH + OH"
(10)
1
i s 1.0+0.1 χ 10 mol" L s " . T h i s i n d i c a t e s t h a t o x i d a t i o n by 0 i s t h e most s i g n i f i c a n t p r o c e s s s i n c e p e r o x i d e c o n c e n t r a t i o n s a r e at l e a s t three orders of magnitude lower i n surface w a t e r s . E x t r a p o l a t i o n of t h i s rate data to natural concentrations i s s i m p l e r f o r C u ( I ) than f o r C u ( I I ) because i n t e r a c t i o n s w i t h o r g a n i c l i g a n d s a r e u n l i k e l y t o be s i g n i f i c a n t and c h l o r i d e c o m p l e x a t i o n d o m i n a t e s . T h e r e f o r e s p e c i a t i o n i n t h e s e model systems a t e l e v a t e d l e v e l s i s representative of s p e c i a t i o n a t n a t u r a l l e v e l s . Studies of t h e c h l o r i d e and. pH dependence o f b o t h t h e s e r e a c t i o n s i n d i c a t e t h a t Cu and CuCl a r e t h e most r a p i d l y o x i d i z e d s p e c i e s . The major species, CuCl and CuCl are r e l a t i v e l y inert to o x i d a t i o n , accounting Tor the s t a b i l i t y of Cu(I) i n s e a w a t e r ^ The predominant C u ( I ) s p e c i e s i n seawater i s C u C l ^ (14) which undergoes p h o t o l y t i c o x i d a t i o n v i a a charge t r a n s f e r t o s o l v e n t (CTTS) r e a c t i o n (_7). 2
CuCl
2 3
"
—
A quantum y i e l d
CuCl
+
+ 2 C l " + e"
o f 0.23 was d e t e r m i n e d
(11) a t 296nm and 313nm.
In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
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9. MOFFETT AND ZIKA
Photochemistry of Copper Complexes
123
However, the complex absorbs o n l y weakly above 300nm and so does not r e a c t s i g n i f i c a n t l y i n s u n l i g h t . C a l c u l a t i o n s of p h o t o l y s i s r a t e s u s i n g a m o d i f i e d v e r s i o n of the Zepp and C l i n e p h o t o c h e m i c a l model (23) i n d i c a t e t h a t a t s u r f a c e seawater under optimum s u n l i g h t c o n d i t i o n s the h a l f l i f e o f Cu(I) i s >6 hours assuming t h i s i s the only sink. The r e s u l t s o f these s t u d i e s i n d i c a t e t h a t Cu(I) o x i d a t i o n by 0« i s t h e major o x i d a t i v e pathway but t h a t o x i d a t i o n by H^O^ i s also significant. Other o x i d a n t s may be i n v o l v e d , but no concentration or k i n e t i c data i s a v a i l a b l e . I n p a r t i c u l a r , 0^ may be i m p o r t a n t as an o x i d a n t here i n a d d i t i o n t o i t s r o l e as a reductant o u t l i n e d previously. These r e s u l t s i n d i c a t e t h a t a v a r i e t y o f r e a c t i o n s may l e a d t o Cu(I)/Cu(II) interconversions i n seawater. The r e s u l t s a r e summarised s c h e m a t i c a l l y i n F i g u r e 3 t o demonstrate how copper redox c y c l i n g probably occurs. The b a s i c f e a t u r e s a r e a dynamic redox c y c l e i n v o l v i n g the r e l a t i v e l y minor but k i n e t i c a l l y r e a c t i v e s p e c i e s which r e a c t w i t h oxygen and hydrogen p e r o x i d e as d e s c r i b e d previously. They a r e p r o b a b l y r e a c t i v e w i t h o h t e r p h o t o c h e m i c a l l y generated oxidants and r e d u c t a n t s , such as 0^ and organic radicals. C u C l r e d u c t i o n may a l s o be i m p o r t a n t d e s p i t e i t s n e g l i g i b l e r e a c t i o n w i t h H^O^ as i t i s r a p i d l y reduced by o t h e r r e d u c t a n t s such as f e r r o u s cytochrome C ( 2 4 ) . The major Cu(I) and C u ( I I ) s p e c i e s , i n d i c a t e d i n the boxes, do not p a r t i c i p a t e i n these r e a c t i o n s but do undergo p r i m a r y p h o t o l y s i s . The above s t u d i e s i n d i c a t e that the p h o t o l y s i s o f l-3 CuCO^ a r e not s i g n i f i c a n t but p h o t o l y s i s o f C u ( I I ) o r g a n i c complexes may be, based on the model l i g a n d s . T h i s scheme i s h i g h l y q u a l i t a t i v e a t the p r e s e n t t i m e . The s p e c i a t i o n o f C u ( I ) i s p r o b a b l y d o m i n a t e d by c h l o r i d e e v e n a t n a t u r a l l e v e l s so e x t r a p o l a t i o n o f t h e l a b o r a t o r y d a t a i s v a l i d . However, f o r C u ( I I ) there i s s t i l l widespread disagreement about t h e n a t u r e , c o n c e n t r a t i o n s and s t a b i l i t y c o n s t a n t s o f n a t u r a l o r g a n i c C u Q l ) c h e l a t o r s , r e s u l t i n g i n estimates f o r the f r a c t i o n o f f r e e Cu ( i . e . [Cu(II)] /[Cu(II)] ) which v a r y by two o r d e r s of magnitude (12,25, fg). T h i s matces i t d i f f i c u l t t o e s t i m a t e i n s i t u r a t e s o f secondary r e d u c t i o n r e a c t i o n s i n v o l v i n g minor C u ( I I ) s p e c i e s which a r e p r o p o r t i o n a l t o t h i s f r a c t i o n . And t h e r e i s no d i r e c t evidence t h a t LMCT r e a c t i o n s i n v e s t i g a t e d u s i n g model l i g a n d s a c t u a l l y o c c u r i n seawater. A further complication i s t h a t steady s t a t e c o n c e n t r a t i o n and r a t e d a t a a r e u n a v a i l a b l e f o r many p o t e n t i a l l y i m p o r t a n t r e a c t i v e s p e c i e s i n v o l v e d i n copper redox c y c l i n g , such as 0 . I n o r d e r t o determine the o v e r a l l e f f e c t t h a t these p r o c e s s e s m i g h t have w i t h o u t h a v i n g q u a n t i t a t i v e d a t a f o r e v e r y p o t e n t i a l l y i m p o r t a n t r e a c t i o n i n v o l v e d procedures were developed t o measure two i m p o r t a n t parameters i n the copper system a t n a t u r a l l e v e l s ; Cu(I) and [ C u ( I I ) ] /[Cu(II)] . ?
C u C
a n d
r
f r e e
t o t a l
Measurement o f C u ( I ) i n Seawater The f o r m a t i o n o f Cu(I) i n c o a s t a l waters exposed t o s u n l i g h t was investigated. Seawater c o l l e c t e d from B i s c a y n e Bay was s t o r e d i n t h e d a r k f o r 2 weeks and exposed t o s u n l i g h t i n round bottomed
In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
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124
PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS
quartz f l a s k s . The r e s u l t s , shown i n F i g u r e 4, i n d i c a t e t h a t photochemical f o r m a t i o n o f C u ( I ) o c c u r s and c o r r e l a t e s w i t h t h e photochemical p r o d u c t i o n o f h^C^» I n a second experiment, a l l f l a s k s were exposed t o 5 hours o f s u n l i g h t and t h e decay i n t h e Cu(I) s i g n a l monitored ( F i g u r e 5 ) . C u ( I ) f o r m a t i o n observed here i s p r o b a b l y t h e r e s u l t o f a c o m b i n a t i o n o f p r i m a r y and secondary p r o c e s s e s . Primary p r o c e s s e s may be i m p o r t a n t i n d e s t r o y i n g C u ( I I ) complexes w h i c h are i n e r t t o s e c o n d a r y r e a c t i o n s , thereby making secondary p r o c e s s e s more important. The decay o f C u ( I ) a f t e r i r r a d i a t i o n i s much slower t h a n t h e e x p e c t e d , s i n c e i t s h a l f l i f e i n t h e presence o f 0~ i s under 6 m i n u t e s . T h e r e f o r e , the presence o f r e l a t i v e l y l o n g l i v e d r e d u c t a n t s p e c i e s such as must be i n v o k e d . Artificial seawater c o n t a i n i n g u l t r a f i l t r a t e a l s o produced C u ( I ) upon s o l a r I r r a d i a t i o n , but c h l o r i d e f r e e media d i d not ( 4 ) , i n agreement w i t h k i n e t i c e x p e c t a t i o n s based on t h e e x t r e m e l y r a p i d o x i d a t i o n o f Cu(I) i n c h l o r i d e f r e e media. C u ( I ) was measured i n the water column o f f the F l o r i d a Coast on the SOLARS I I I c r u i s e . R e s u l t s from a c a s t taken a t a s t a t i o n west o f Tampa a r e shown i n F i g u r e 6. The c a s t was made a t 1400 on a sunny, f l a t calm day; o p t i m a l c o n d i t i o n s f o r photochemical activity. Hydrographie data, obtained during a CTD c a s t i m m e d i a t e l y a f t e r w a r d s i n d i c a t e d a w e l l s t r a t i f i e d water column. The p r o f i l e i n d i c a t e s t h a t Cu(I) i s p r e s e n t a t the s u r f a c e where i t c o n s t i t u t e s a b o u t 15% o f t h e t o t a l c o p p e r . The c o n c e n t r a t i o n d e c l i n e s w i t h depth and i s below t h e l i m i t o f d e t e c t i o n a t 90m. The H 0 p r o f i l e shows s i m i l a r c h a r a c t e r i s t i c s , w i t h a s u r f a c e maximum. A s t u d y o f t h e d i u r n a l v a r i a b i l i t y o f Cu(I) was made a t a c o a s t a l s i t e c o n t a i n i n g h i g h DOC, o f f t h e F l o r i d a Everglades (Station 1). The r e s u l t s i n d i c a t e v i r t u a l l y no C u ( I ) above t h e l i m i t o f d e t e c t i o n d e s p i t e h i g h daytime p e r o x i d e l e v e l s shown i n F i g u r e 7. T h i s may be a r e s u l t o f more e x t e n s i v e C u ( I I ) c h e l a t i o n by o r g a n i c matter i n t h i s h i g h l y p r o d u c t i v e c o a s t a l s i t e . Or Cu(I) l i f e t i m e s may be e x t r e m e l y s h o r t because o f a h i g h photochemical production of short l i v e d r a d i c a l oxidants. The r e s u l t s i n d i c a t e t h a t , a p a r t from t h e c o a s t a l s i t e , t h e r e i s a g e n e r a l c o r r e l a t i o n between C u ( I ) l e v e l s and H 0 c o n c e n t r a t i o n . T h i s agreement may be c o i n c i d e n t a l because o t h e r r e d u c t a n t s o f photochemical o r i g i n may have a s i m i l a r d i s t r i b u t i o n t o p e r o x i d e . 2
2
2
2
Measurement o f [ C u ( I I ) K /[Cu(II)l _ tree total Ί
i
n
Measurements o f [CixC XI) ] f /1CuC IX) ] i seawater, were c a r r i e d o u t t o c a l c u l a t e i n s i t u r a t e s o f secondary r e a c t i o n s such as t h e r e d u c t i o n o f C u ( I I ) by H 0 and t o l o o k f o r evidence o f primary photoreactions involving Cu(II) organic complexes. The a c e t y l a c e t o n e l i q u i d - l i q u i d p a r t i t i o n procedure was used t o measure [Cu(II)] /[Cu(II)] as a f u n c t i o n o f depth o f f the c o a s t o f F l o r i d a a P s t a t i o n 2 Surlng the SOLARS I I I c r u i s e . Data are shown i n F i g u r e 6, a l o n g w i t h p e r o x i d e and Cu(I) d a t a . P r o f i l e s o f the t h r e e parameters have s i m i l a r c h a r a c t e r i s t i c s . ^Cu(II)]^ / [Cu(II)] _ p r o f i l e s a r e c h a r a c t e r i s e d by a s u r f a c e maxima and d e c r e a s i n g v a l u e s w i t h depth t o r e l a t i v e l y u n i f o r m low v a l u e s from ree
2
t o t : a
2
f
r e e
t o t a
In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
9. MOFFETT AND ZIKA
125
Photochemistry of Copper Complexes
CuCIo
11
3
CuCI.
.red
CuCI
2
2
C u O H C r M ? - CuOHCl
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η CuCI
cucr
Cut.
Cu
IL 2
CuCOo
II
3
CuOrganlc
F i g u r e 3· Summary of c y c l i n g i n seawater,
processes
involved
in
Cu(I)/Cu(II)
4r
o p
ο Δ Ο
• Ο
·
Cu(l)
Ο Δ
30 •25
60 90 TIME/MINUTES -51(38) -76
150
120 · 99(78)
1-21
2
LIGHT FLUX (WATT HOUR METER' )
F i g u r e 4. P h o t o c h e m i c a l f o r m a t i o n o f C u ( I ) i n seawater exposed t o s u n l i g h t i n February,Ο y and A p r i l , # , 1985. L i g h t f l u x v a l u e s i n b r a c k e t s a r e f o r F e b r u a r y , 1985.
In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS
6
f
5
3
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I
12
24 "
" 36
48
TIME(HOURS)
F i g u r e 5. Dark decay of C u ( I ) s i g n a l i n seawater exposed t o 5 hours s u n l i g h t i r r a d i a t i o n .
1
7
4
[ H ^ m o l L " χ 10 ) ICudlMludl^ixlO ) 0 (fe 10 15 0 2 4 6 8 0
10
[CuCOKmoldxIO ) 1 2 3 4
F i g u r e 6. Depth p r o f i l e o f C u ( I ) , [ C u ( I I ) ] 7[Cu(II)] and . H 0„ w i t h accompanying h y d r o g r a p h i e S i f t , s t a t i o n-^ September 1985. 9
In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
t o t
^
127
Photochemistry of Copper Complexes
9. MOFFETT AND ZIKA
50m t o the deepest depths sampled. There was about a t w e n t y f o l d i n c r e a s e b e t w e e n t h e deep v a l u e s a n c ^ t h e s u r f a c e v a l u e s . To d e t e r m i n e i f t h e i n c r e a s e i n f r e e Cu towards the s u r f a c e i s p h o t o c h e m i c a l l y i n d u c e d , samples o f seawater c o l l e c t e d i n Biscayne Bay were exposed t o s u n l i g h t i n q u a r t z f l a s k s m a i n t a i n e d a t 25 i n a water bath and [Cu(II)]. /[Cu(II)l ^ measured with i r r a d i a t i o n time. The r e s u l t s , i n F i g u r e 8 , d e m o n s t r a t e t h a t s u n l i g h t d o e s l e a d t o an i n c r e a s e , p r o b a b l y as a r e s u l t o f p h o t o - d e s t r u c t i o n of the l i g a n d and/or copper complex, p o s s i b l y i n v o l v i n g LMCT r e a c t i o n s . The minimum was l o c a t e d a t the base o f t h e mixed l a y e r and c o i n c i d e d w i t h what i s g e n e r a l l y the zone of maximum p r o d u c t i v i t y . An e x p l a n a t i o n f o r the observed s p e c i a t i o n t r e n d s i s the b i o l o g i c a l p r o d u c t i o n o f c h e l a t o r s i n the zone of maximum p r o d u c t i v i t y ( i . e . lower p h o t i c zone) which a r e degraded p h o t o - c h e m i c a l l y as they mix upward. Further i n v e s t i g a t i o n i s necessary to confirm t h i s hypothesis. An i n s i t u r a t e o f r e d u c t i o n f o r C u ( I I ) by ^ 2 f r o m t h i s d a t a and the k i n e t i c d a t a , i n d i c a t e s t h a t o n l y 5% t o 10% o f t h e C u ( I ) m e a s u r e d c a n be a c c o u n t e d f o r by t h i s s o u r c e . T h e r e f o r e , o t h e r pathways not c o n s i d e r e d i n the s i m p l e model must be i m p o r t a n t . +
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Ί
0
9
c
a
l
c
u
l
a
t
e
d
Conclusions A v a r i e t y of reactions are involved i n C u ( I ) / C u ( I I ) i n t e r c o n v e r s i o n i n seawater. S e v e r a l important r e a c t i o n s have been s t u d i e d and r a t e c o n s t a n t s determined a t e l e v a t e d copper c o n c e n t r a t i o n s . The s y s t e m cannot be c o m p l e t e l y c h a r a c t e r i z e d by t h i s d a t a base a t p r e s e n t because the nature and steady s t a t e c o n c e n t r a t i o n s o f many p o t e n t i a l l y i m p o r t a n t s p e c i e s such as 0 have not been determined. N e v e r t h e l e s s , measurement o f C u ( I ) ana [ C u ( I I ) ] /[Cu(II)l at n a t u r a l copper l e v e l s i n d i c a t e s the net e f f e c t o f t h e s e sunlîgnt i n d u c e d p r o c e s s e s has c o n s i d e r a b l e i n f l u e n c e on copper s p e c i a t i o n i n seawater. Measurement o f C u ( I ) i n d i c a t e s up t o 15% of copper i s p r e s e n t as C u ( I ) i n s u r f a c e seawater under optimum c o n d i t i o n s . D e p t h p r o f i l e c h a r a c t e r i s t i c s i n d i c a t e a s u r f a c e maxima which i s c o n s i s t e n t w i t h a p h o t o c h e m i c a l r e d u c t i o n mechanism. This i s supported by t h e measurement of C u ( I ) f o r m a t i o n i n seawater exposed to sunlight. Profiles of [ C u ( I I ) ] /[Cu(II)] i n d i c a t e that sunlight influences Cu(II) organic I n t e r a c t i o n s . ° T h i s work has i m p o r t a n t i m p l i c a t i o n s f o r the r o l e of copper i n seawater. The uptake c h a r a c t e r i s t i c s o f Cu(I) by organisms may be very d i f f e r e n t than those f o r C u ( I I ) which may a f f e c t the b i o a v a i l a b i l i t y and t o x i c i t y of copper i n seawater. The d e t e c t i o n of a twenty f o l d i n c r e a s e i n f r e e Cu between 50m and the s u r f a c e i s i m p o r t a n t as numerous i n v e s t i g a t i o n s have e s t a b l i s h e d t h a t t o x i c i t y i s r e l a t e d t o f r e e Cu and n o t t o t a l . This study demonstrates t h a t 2 2 ' g e n e r a l l y c o n s i d e r e d as an o x i d a n t , a l s o f u n c t i o n s as a r e d u c t a n t i n some systems, such as t h i s one. Hydrogen p e r o x i d e i s g e n e r a l l y u n r e a c t i v e w i t h o r g a n i c compounds u n l e s s c a t a l y s e d by t r a n s i t i o n metals and t h e r e f o r e r e a c t i o n w i t h t r a n s i t i o n metals c o u l d be an i m p o r t a n t d e g r a d a t i v e pathway. The l o w ambient c o n c e n t r a t i o n s of t r a n s i t i o n metals i n seawater could e x p l a i n the long l i f e t i m e of Η 0 i n seawater. 9
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In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS
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Photochemistry of Copper Complexes
MOFFETT AND ZIKA
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However, r e d u c t i o n o f C u ( I I ) i s u n l i k e l y t o be i m p o r t a n t i n H , ^ d e g r a d a t i o n because o x i d a t i o n of the r e s u l t a n t C u ( I ) l e a d s back to 2°2 °2 m o The o x i d a t i o n o f C u ( I ) by H^O i s p r o b a b l y not i m p o r t a n t e i t h e r because o f the low Cu(I) c o n c e n t r a t i o n s i n seawater; c a l c u l a t i o n s i n d i c a t e t h a t would have a h a l f l i f e o f a t l e a s t 100 days i f t h i s was t h e s o l e d e g r a d a t i v e pathway. T h i s i s c o n s i d e r a b l y l o n g e r than p r e s e n t e s t i m a t e s . H
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Acknowledgment T h i s work was s u p p o r t e d by t h e O f f i c e o f N a v a l c o n t r a c t N00014-85C-0020.
Research
under
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