Photochemistry of Environmental Aquatic Systems - American

Chapter 1

Introduction and Overview 1

William J.Cooper and Frank L. Herr

2

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Drinking Water Research Center, Florida International University, Miami, FL 33199 Office of Naval Research, Code 422CB, Arlington, VA 22217

Downloaded by UNIV POLITECNICA DE MADRID on January 8, 2015 | http://pubs.acs.org Publication Date: December 8, 1987 | doi: 10.1021/bk-1987-0327.ch001

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Sunlight that arrives at the surface of the earth contains substantial amounts of energy. When surface waters, which contain either natural or anthropogenic chromophores, are exposed to sunlight, light initiated chemical reactions often occur. Sunlight induced photochemical reactions in surface waters may broadly be defined as environmental aquatic photochemistry. Within aquatic photochemistry, it is possible to envision reactions involving either inorganic and/or organic molecules. These chemicals could be either natural or anthropogenic and may participate in either homogeneous or heterogeneous reactions. Very often in these environments, a complex array of primary and secondary photoprocesses are occurring simultaneously. Surface waters are diverse in nature. They might be near shore or inland wetland environments or mid-oceanic oligotrophic water. Until recently, sunlight induced photochemistry was not recognized as an important pathway for the transformation of natural and anthropogenic chemicals in surface waters. It is now well established that photochemically mediated processes are important in most, if not all, areas of aqueous phase environmental chemistry. Both direct, primary, and indirect photoprocesses have been documented in natural waters. The f a c t t h a t a l l of these p o s s i b i l i t i e s e x i s t i s e x c i t i n g . There a r e a seemingly endless number o f combinations and p e r m u t a t i o n s t o s t u d y ; h o w e v e r , c a u t i o n s h o u l d be u s e d . The p o t e n t i a l f a c t o r s a f f e c t i n g a n y one s t u d y a r e so c o m p l e x t h a t e x t r e m e care must be taken when i n t e r p r e t i n g d a t a o b t a i n e d from n a t u r a l systems. On t h e o t h e r hand, e x t r a p o l a t i n g d a t a o b t a i n e d i n l a b o r a t o r y s t u d i e s t o n a t u r a l environments g e n e r a l l y r e q u i r e s t h e use o f many assumptions. N u m e r o u s r e v i e w s have been p u b l i s h e d t h a t d e t a i l v a r i o u s aspects o f a q u a t i c photochemistry (1-11). T h i s book b r i n g s t o g e t h e r a group o f papers r e p r e s e n t i n g a number o f t o p i c s , i n o r d e r t o p r o v i d e t h e r e a d e r w i t h an a p p r e c i a t i o n o f t h e c o m p l e x i t y o f the f i e l d and, a t t h e same t i m e , a g l i m p s e o f v a r i o u s areas 0097-6156/87/0327-0001$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.

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PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS within the r e l a t i v e l y young f i e l d of e n v i r o n m e n t a l a q u a t i c photochemisty. T h i s book i s not an e x h a u s t i v e c o m p i l a t i o n of the f i e l d of e n v i r o n m e n t a l a q u a t i c p h o t o c h e m i s t r y .


Direct Photoreactions A wide variety of substances with active chromophores at w a v e l e n g t h s found i n the s u r f a c e s o l a r spectrum occur i n n a t u r a l waters. Some of these substances undergo d i r e c t p h o t o l y s i s , t h a t i s a c h e m i c a l change t h a t r e s u l t s as a d i r e c t consequence of the a b s o r p t i o n of photons by the s u b s t a n c e . C o n c e p t u a l l y , d i r e c t p h o t o r e a c t i o n s are the s i m p l e s t and u s u a l l y the e a s i e s t type o f p r o c e s s to study i n n a t u r a l w a t e r s . S i n c e the r e a c t i o n proceeds r a p i d l y to p r o d u c t s from the p r i m a r y e x c i t e d s t a t e m a n i f o l d , the p h y s i c a l c h a r a c t e r i s t i c s of the r e a c t a n t ' s environment u s u a l l y have o n l y s m a l l e f f e c t s on the r e a c t i o n . Such r e a c t i o n s can o f t e n be s t u d i e d i n pure and/or r e l a t i v e l y h i g h c o n c e n t r a t i o n s of the reactant· C o m p a r a t i v e l y few n a t u r a l m o l e c u l e s f i t i n t o t h i s c a t e g o r y ( 5 ) , and most of these e x h i b i t o n l y weak absorbances a t the h i g h e n e r g y t h r e s h o l d of the s u r f a c e s o l a r spectrum. Most n a t u r a l l y o c c u r r i n g compounds are t h e r e f o r e q u i t e t r a n s p a r e n t to i n c i d e n t s o l a r r a d i a t i o n and r e a c t i o n s of these compounds which proceed v i a d i r e c t p h o t o l y s i s are the e x c e p t i o n r a t h e r than the r u l e . Most examples of d i r e c t p h o t o c h e m i c a l r e a c t i o n s a r e found among the numerous s t u d i e s done on x e n o b i o t i c substances ( 1 , 6 ) , where the e n v i r o n m e n t a l r a t e i s o f t e n d e r i v e d from l a b o r a t o r y measurements which a r e c a r r i e d out i n o r g a n i c s o l v e n t s because of the l i m i t e d s o l u b i l i t y of the compounds. I f the e l e c t r o n i c a b s o r p t i o n s p e c t r a and t h e quantum y i e l d f o r the compound are determined i n water o r i n an o r g a n i c s o l v e n t system which g i v e s a good a p p r o x i m a t i o n t o water, then the c a l c u l a t i o n of an e n v i r o n m e n t a l r a t e i s a r e l a t i v e l y s i m p l e m a t t e r . For more complex m o l e c u l e s , the r e a c t i o n quantum y i e l d i s g e n e r a l l y wavelength independent (12) and the d i r e c t p h o t o l y s i s r a t e c o n s t a n t can be computed f o r a s p e c i f i c l o c a t i o n and t i m e f r o m t h e e l e c t r o n i c a b s o r p t i o n s p e c t r a , t h e quantum y i e l d , and the s o l a r s p e c t r a l i r r a d i a n c e . Indirect Photoreactions The h i g h l i g h t t r a n s p a r e n c i e s of most compounds i n n a t u r a l water to s o l a r r a d i a t i o n d i c t a t e t h a t d i r e c t p h o t o - r e a c t i o n s of these compounds a r e e i t h e r not p o s s i b l e o r r e p r e s e n t o n l y a minor r e a c t i o n pathway. I n d i r e c t photochemical r e a c t i o n s f o r such compounds can o n l y o c c u r e i t h e r through r e a c t i o n w i t h r e a c t i v e m o l e c u l e s i n ground o r e x c i t e d s t a t e s t h a t are themselves p r o d u c t s of p r i m a r y p h o t o c h e m i s t r y , o r through p h o t o s e n s i t i z e d r e a c t i o n s i n w h i c h the e x c i t e d s t a t e s p e c i e s o f some chromophore t r a n s f e r s an e l e c t r o n or energy t o the compound. The importance of b o t h of t h e s e i n d i r e c t r e a c t i o n pathways i n n a t u r a l water systems i s now recognized. A s u b s t a n t i a l amount o f e v i d e n c e e x i s t s i n t h e literature implicating indirect reaction mechanisms in the p h o t o c h e m i s t r y of b o t h x e n o b i o t i c and n a t u r a l compounds (4,5,6,10). The c o m p l e x i t y of natural water systems and the h o s t of


1.

COOPER AND HERR

3

Introduction and Overview

s i m u l t a n e o u s p h o t o - r e a c t i o n s w h i c h a r e o c c u r r i n g f r e q u e n t l y make a c l e a r m e c h a n i s t i c i n t e r p r e t a t i o n o f the d a t a d i f f i c u l t . Secondary p h o t o - o x i d a n t s . There are three r e c o g n i z e d primary s o u r c e s o f secondary o x i d a n t s i n n a t u r a l a q u a t i c environments. The f i r s t of these a r i s e s from i n s i t u p r i m a r y p h o t o - r e a c t i o n s w h i c h o f t e n produce f r e e r a d i c a l s and o t h e r r e a c t i v e p r o d u c t s . The range of r e a c t i v i t y f o r f r e e r a d i c a l s v a r i e s from the very strong o x i d i z i n g s p e c i e s l i k e OH t o weakly o x i d i z i n g and more s e l e c t i v e s p e c i e s l i k e CO^ , NO, and 0 · Subsequent r e a c t i o n s o f these r a d i c a l s can l e a d t o o t h e r r e a c t i v e p r o d u c t s . F o r example, OH i n seawater and o t h e r h a l i d e c o n t a i n i n g _ w a t e r s w i l l r a p i d l y c o n v e r t t o d i h a l i d e a n i o n r a d i c a l s l i k e B r ^ and I (5, 13). Hydrogen a b s t r a c t i o n r e a c t i o n s from o r g a n i c compounds by such i n o r g a n i c r a d i c a l s can produce r a d i c a l s w h i c h i n t u r n might r e a c t w i t h oxygen t o f o r m organo-peroxy r a d i c a l s and p o s s i b l y p e r o x i d e s . Reactive n o n - r a d i c a l p r o d u c t s can a l s o be formed. Hydrogen p e r o x i d e f o r example i s b e l i e v e d to r e s u l t from the d i s p r o p o r t i o n a t i o n o f 0^ , which i n turn i s generated primarily from d i s s o l v e d humic substances (HS) ( 1 4 - 1 6 ) . The compexity o f the r e a c t i o n sequence i s shown i n the f o l l o w i n g e q u a t i o n s :


2

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HS + hr

+

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>

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HSO*

OH'

+

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HOX""

HOX"'

+

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OH"

HS0 '

+

2

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RH

H 0 2

2

2

V

+

°2

[2] [3J

# +

>

2

+

[4]

2

+

2

R*

+

[5] [6]

+

OH*

17] [8]

+

[9]

V

(where X i s a h a l i d e ) A l t h o u g h t h i s s e t o f r e a c t i o n s does n o t i n d i c a t e a l l of t h e p o s s i b l e s t e p s i n v o l v e d , i t i s i n t e r e s t i n g t o note t h a t the i n i t i a l p r i m a r y p h o t o p r o c e s s e s r e s u l t e d i n 10 s e c o n d a r y p r o d u c t s . In a d d i t i o n , t h e two p e r o x i d e m o l e c u l e s ( i . e . 0 and HSO^H) a r e new chromophores and f u n c t i o n as r e s e r v o i r s f o r further radical production. A second source f o r secondary o x i d a n t s comes from t h e r m a l r e a c t i o n s when r e a c t i v e p h o t o - p r o d u c t s such as H 0 react with o t h e r c o n s t i t u e n t s of the water. Reactions of peroxides w i t h v a r i o u s t r a n s i t i o n m e t a l s , f o r i n s t a n c e , w i l l produce 0 o r OH r a d i c a l s and o f t e n the m e t a l i o n i s c o n v e r t e d t o a new r e a c t i v e oxidation state. T h i s s o r t of p r o c e s s has been demonstrated f o r H

2

2

2

2

2



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PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS the C u ( I I ) / C u ( I ) c o u p l e i n seawater ( 1 7 ) . R e a c t i o n s such as these may be f a r removed from the i n i t i a l p h o t o c h e m i c a l p r o c e s s ; however, they are a consequence of i t and may have the i m p o r t a n t f u n c t i o n of a c t i n g as a redox b u f f e r i n e n v i r o n m e n t a l water systems. The t h i r d s o u r c e o f s e c o n d a r y p h o t o - o x i d a n t s a r i s e s f r o m f l u x e s of a t m o s p h e r i c o x i d a n t s t h r o u g h the s u r f a c e of n a t u r a l water bodies. A l t h o u g h these p r o d u c t s do not o r i g i n a t e from a q u a t i c p h o t o - r e a c t i o n s , they n e v e r t h e l e s s augment the i n s i t u r e a c t i o n s o u r c e s o f s e c o n d a y r e a c t i v e p r o d u c t s and must be t a k e n i n t o a c c o u n t , e s p e c i a l l y when a t t e m p t i n g t o q u a n t i f y r e a c t i o n p r o c e s s e s in the steep g r a d i e n t r e g i o n near the a i r - w a t e r i n t e r f a c e . Thompson and Z a f i r i o u (18) have examined the c h e m i c a l impact on n a t u r a l waters and have c a l c u l a t e d a i r - w a t e r f l u x e s f o r many of the d i v e r s e atmospheric o x i d a n t s . Photosensitized Processes. One o f t h e most s t u d i e d i n d i r e c t p h o t o r e a c t i o n s i s the f o r m a t i o n of s i n g l e t oxygen ( Δ g 0^) by e n e r g y t r a n s f e r from the t r i p l e t e x c i t e d s t a t e of n a t u r a l o r g a n i c chromophores i n environmental waters. T h i s p r o c e s s has been o b s e r v e d i n b o t h s e a w a t e r ( 1 9 ) and f r e s h w a t e r ( 2 0 , 2 1 ) . The e v i d e n c e s u g g e s t s t h a t s i n g l e t o x y g e n i s a common p r o d u c t i n n a t u r a l water systems, but i t s importance r e l a t i v e t o o t h e r s e c o n d a r y p h o t o - p r o d u c t s i n a f f e c t i n g the c h e m i s t r y i s u n c e r t a i n . I t s l i m i t e d s i g n i f i c a n c e i s a f u n c t i o n of i t s s e l e c t i v e r e a c t i v i t y , l o w r a t e c o n s t a n t s and i t s r a p i d r e l a x a t i o n t o the ground s t a t e i n aqueous media. P h o t o s e n s i t i z a t i o n v i a energy t r a n s f e r i n d i l u t e s o l u t i o n s of s e n s i t i z e r and r e c e p t o r i s i n g e n e r a l a low e f f i c i e n c y p r o c e s s . T h i s w i l l p r o b a b l y h o l d t r u e f o r most e n v i r o n m e n t a l a q u a t i c systems. Much h i g h e r e f f i c i e n c i e s m i g h t , however, be o b t a i n e d i n heterogeneous r e a c t i o n environments such as m i c e l l e s , p a r t i c l e s , or o t h e r i n t e r f a c e s where s e n s i t i z e r and r e a c t a n t are c o n c e n t r a t e d . P h o t o s e n s i t i z a t i o n by e l e c t r o n t r a n s f e r , as shown i n e q u a t i o n 2 a b o v e , has a l s o been observed (22,23). The o c c u r r e n c e of 0^ (14,16) i n n a t u r a l waters i s good e v i d e n c e t h a t these p r o c e s s e s a r e o c c u r r i n g , a t l e a s t f o r oxygen r e d u c t i o n . With e l e c t r o n a c c e p t o r s o t h e r than oxygen, t h i s process i s probably s i m i l a r to energy t r a n s f e r i n t h a t t h e r e i s a low p r o b a b i l i t y f o r o t h e r e l e c t r o n a c c e p t o r s t o be i n v o l v e d except i n heterogeneous environments. Heterogeneous

Reactions

S u r f a c e mediated p r o c e s s e s are a l s o an i m p o r t a n t c o n s i d e r a t i o n i n n a t u r a l water p h o t o c h e m i s t r y . I n aqueous media, two d i f f e r e n t surface/interfaces may occur that result in heterogeneous r e a c t i o n s . The two i n t e r f a c e s c o n s i d e r e d here are l i q u i d - s o l i d and liquid-liquid. S u r f a c e p r o c e s s e s i n g e o c h e m i s t r y and a q u a t i c enviroments have been covered i n more d e t a i l i n two r e c e n t books and t h e r e a d e r i s r e f e r r e d t o t h e s e v o l u m e s f o r more d e t a i l s (24,25). N a t u r a l water systems o f t e n c o n t a i n p a r t i c u l a t e m a t t e r . The p a r t i c u l a t e m a t t e r may be e i t h e r l i v i n g o r n o n - l i v i n g . Algae are t h e predominant component of the l i v i n g p a r t i c u l a t e m a t t e r . The



1. COOPER A N D HERR


5

non-living particulate matter i s primarily inorganic in c o m p o s i t i o n , b u t may h a v e o r g a n i c m a t t e r a s s o c i a t e d w i t h i t . P a r t i c u l a t e - l i q u i d i n t e r f a c e s a r e known t o p a r t i c i p a t e i n t h e r m a l l y and/or p h o t o c h e m i c a l l y mediated r e a c t i o n s . S u r f a c e m i c r o l a y e r s a r e i m p l i c a t e d i n many c h e m i c a l p r o c e s s e s . They a r e exposed t o t h e f u l l s o l a r spectrum o f l i g h t a r r i v i n g a t t h e s u r f a c e , and a r e o f t e n i m p o r t a n t i n p h o t o c h e m i c a l l y mediated r e a c t i o n s . The m i c r o l a y e r , a s s o c i a t e d w i t h most n a t u r a l w a t e r s , i s c o n s i d e r a b l y d i f f e r e n t i n c h e m i c a l c o m p o s i t i o n from t h e u n d e r l y i n g w a t e r column ( 2 6 ) . Hence, the p h o t o c h e m i c a l l y mediated r e a c t i o n s t h a t take p l a c e i n t h i s l a y e r may d i f f e r s u b s t a n t i a l l y from those i n t h e water column. The d i f f e r e n c e s i n t h e r e a c t i o n s may be one of k i n e t i c s ( r a t e s o f t h e r e a c t i o n ) , o r maybe m e c h a n i s t i c i n n a t u r e and t h e r e a c t i o n ( s ) p r o c e e d ( s ) v i a d i f f e r e n t pathways r e s u l t i n g i n d i f f e r e n t reaction products. One extreme case o f the l i q u i d - l i q u i d i n t e r f a c e i s t h e o i l s p i l l ( s l i c k ) problem. The f i e l d o f o i l - w a t e r p h o t o c h e m i s t r y has r e c e n t l y been reviewed ( 2 7 - 2 9 ) . I t has been shown t h a t numerous p r o c e s s e s may and do o c c u r s i m u l t a n e o u s l y , and t h a t t h e c h e m i s t r y a s s o c i a t e d w i t h s t u d i e s o f t h i s type a r e e x t r e m e l y complex · N a t u r a l m i c r o l a y e r s form on most water s u r f a c e s . There a r e numerous d i f f i c u l t i e s encountered when s t u d y i n g them. One o f the most p e r p l e x i n g problems c e n t e r s around t h e c o l l e c t i o n o f n a t u r a l samples of m i c r o l a y e r s . Another d i f f i c u l t y i s t h a t p r o c e s s e s o c c u r r i n g i n the m i c r o l a y e r are o f t e n not w e l l c h a r a c t e r i z e d (26). Thus, i n n o v a t i v e approaches a r e r e q u i r e d t o s t u d y p r o c e s s e s s i m i l a r to those i n n a t u r a l w a t e r s . In the case o f p a r t i c u l a t e matter, d i f f e r e n t types of r e a c t i o n s may be i n v o l v e d . R e a c t i o n s t h a t o c c u r on t h e s u r f a c e s o f the non-living particulate matter may involve direct p h o t o c h e m i c a l l y mediated r e a c t i o n s i n s u r f a c e complexes. Another p o s s i b i l i t y i s s u r f a c e semiconductor redox r e a c t i o n s . I n t h e case of t h e l i v i n g a l g a l p a r t i c u l a t e m a t t e r , a t h i r d p r o c e s s would be p h o t o s e n s i t i z e d r e a c t i o n on t h e s u r f a c e o f a l g a e . The p a r t i c u l a t e s u r f a c e heterogeneous r e a c t i o n s may r e s u l t i n p r i m a r y and/or secondary p r o c e s s e s . Most o f t h e examples o f a l g a l a s s o c i a t e d p r o c e s s e s suggest secondary r e a c t i o n s r e s u l t i n g from exudates i n t h e aqueous phase. Quantifying Environmental

Photoprocesses

As i n many a r e a s o f e n v i r o n m e n t a l s c i e n c e , one o f t h e most d i f f i c u l t aspects of environmental photochemistry i s e x t r a p o l a t i n g l a b o r a t o r y based experiments t o t h e n a t u r a l environment. One t o o l t h a t i s b e c o m i n g u s e d more f r e q u e n t l y i s t h a t o f m a t h e m a t i c a l models t o p r e d i c t the d i s t r i b u t i o n of photoproducts i n the environment ( 1 2 ) . M o d e l i n g a q u a t i c p h o t o p r o c e s s e s i s complex, f o r i n order t o d e s c r i b e i n d e t a i l the observed products, i t i s necessary t o understand quantum y i e l d s throughout the s o l a r s p e c t r u m , f o r m a t i o n r a t e s , i n many cases d e c o m p o s i t i o n r a t e s ( t h e p h o t o p r o d u c t s a r e r a r e l y c o n s e r v a t i v e ) , absorbance c h a r a c t e r i s t i c s of t h e a q u a t i c system, and p h y s i c a l m i x i n g o f t h e water masses.



6

PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS

The s i m p l e s t m o d e l i n g s i t u a t i o n e x i s t s f o r those cases where t h e quantum y i e l d i s a c o n s t a n t or can be assumed t o be a c o n s t a n t o v e r t h e s o l a r spectrum ( 1 2 ) . I n cases where the quantum y i e l d v a r i e s as a f u n c t i o n of w a v e l e n g t h the case i s s u b s t a n t i a l l y more complex. T h i s i s t r u e i n the case o f f o r m a t i o n from humic s u b s t a n c e s (30) and i s p r o b a b l y t r u e f o r many o t h e r e n v i r o n m e n t a l p h o t o p r o c e s s e s where m u l t i p l e chromophores or r e a c t i o n mechanisms are i n v o l v e d . P r o v i d i n g a c c u r a t e model c a l c u l a t i o n s r e q u i r e s t h a t quantum y i e l d s be c a r e f u l l y d e t e r m i n e d . New a p p r o a c h e s a r e n e e d e d i n a c t i n o m e t r y and i n the measurement of the absorbance of a c t i v e chromophores i n d i l u t e s o l u t i o n s t h a t absorb o n l y a s m a l l f r a c t i o n o f the t o t a l i n c i d e n t p o l y c h r o m a t i c r a d i a t i o n . Quantum y i e l d s can be o b t a i n e d t h a t are wavelength-averaged I n the n e a r UV (310 - 410 nm) and these a r e a p p l i c a b l e t o e x i s t i n g models ( 3 1 ) . The use of wavelength-averaged quantum yields i s quite simple and in p a r t i c u l a r , u s e f u l when s i n g l e w a v e l e n g t h measurements a r e not possible. P r o v i d e d t h a t good quantum y i e l d and absorbance v a l u e s c a n be o b t a i n e d f o r photoprocesses i n n a t u r a l a q u a t i c systems, i t i s p o s s i b l e t o p r o v i d e a good p h o t o c h e m i c a l model a p p r o x i m a t i o n f o r r e a c t i o n r a t e s i n the environment ( 1 2 ) . To o b t a i n a r e a l i s t i c d i s t r i b u t i o n of p h o t o c h e m i c a l l y d e r i v e d p r o p e r t y , i t i s n e c e s s a r y t o c o n s i d e r p r o p e r t i e s s u c h as d e c a y r a t e s o f t h e s p e c i e s o f i n t e r e s t and p h y s i c a l m i x i n g o f the w a t e r column ( 3 2 ) . Summary I t i s q u i t e apparent from t h i s i n t r o d u c t i o n and the c h a p t e r s t o f o l l o w , t h a t the f i e l d o f e n v i r o n m e n t a l a q u a t i c p h o t o c h e m i s t r y i s q u i t e large. For the most p a r t , i t i s a v e r y young f i e l d and one i n which a c o n s i d e r a b l e e f f o r t remains i n order to o b t a i n a q u a n t i t a t i v e understanding of the s i g n i f i c a n c e to environmental processes. T h i s book i s a f i r s t attempt t o b r i n g t o g e t h e r a s e r i e s of papers d i s c u s s i n g v a r i o u s a s p e c t s of t h i s f i e l d . References

1. Choudhry, G.G. Toxicol. Environ. Chem., 1981, 4, 261-295. 2. Sundstrom, G. ; Ruzo, L.O. In "Aquatic Pollutants: Transformation and Biological Effects"; Hutzinger, O.; van Lelyveld, L.H.; Zoeteman, B.C.J., Eds.; Pergamon Press, N.Y., 1978, 205-222. 3. Zafiriou, O.C. Mar. Chem., 1977, 5, 497-522. 4. Zafiriou, O.C. In "Chemical Oceanography" Vol. 8, 2nd ed.; Riley, J.P.; Chester, R., Eds.; Academic Press, London, 1983, 339-379.


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5. Zafiriou, O.C; Joussot-Dubien, J.; Zepp, R.G.; Zika, R.G. Environ. Sci. Technol., 1984, 18, 358A-371A. 6. Zepp, R.G.; Baughman, G.L. In "Aquatic Pollutants: Transformations and Biological Effects"; Hutzinger, O.; van Lelyveld, L.H.; Zoeteman, B.C.J., Eds.; Pergamon Press, N.Y., 1978, 237-263. 7. Zepp, R.G. In "Dynamics, Exposure and Hazard Assessment of Toxic Chemicals"; Haque, R., Ed.; Ann Arbor Sci. Pub., Ann Arbor, MI, 1980, 69-110. 8. Zepp, R.G. In "The Handbook of Environmental Chemistry. Vol. 2, Part B, Reactions and Processes"; Hutzinger, O., Ed.; Springer-Verlag, N.Y., 1980, 19-41. 9. Zepp, R.G. In "The Role of Solar Ultraviolet Radiation in Marine Ecosystems"; Calkins, J., Ed.; Plenum Press, N.Y., 1982, 293-307. 10. Zika, R.G. In "Marine Organic Chemistry: Evolution, Composition, Interactions and Chemistry of Organic Matter in Seawater"; Duursma, E.K.; Dawson, R., Eds.; Elsevier Sci. Publ. Co., Amsterdam, 1981, 299-325. 11. Mill, T. In "The Handbook of Environmental Chemistry. Vol. 2. Part A, Reactions and Processes"; Hutzinger, O., Ed., Springer-Verlag, N.Y., 1980, 77-105. 12. Zepp, R.G.; Cline, D.M. Environ. Sci. Technol., 1977, 11, 359-366. 13. Zafiriou, O.C; True, M.B.; Hayon, Ε. In "Photochemistry of Environmental Aquatic Systems"; Zika, R.G.; Cooper, W.J., Eds., ACS Symposium Series, Washington, D.C., this volume. 14. Cooper, W.J.; Zika, R.G. Science, 1983, 220, 711-712. 15. Draper, W.M.; Crosby, D.G. Arch. Environ. Contam. Toxicol., 1983, 12, 121-126. 16. Baxter, R.M.; Carey, J. Nature (London), 1983, 306, 575-576. 17. Moffet, J.W.; Zika, R.G. Mar. Chem., 1983, 13, 239-251. 18. Thompson, A.M.; Zafiriou, O.C. J. Geophys. Res., 1983, 88, 6696-6708. 19. Momzikoff, Α.; Santos, R. Mar. Chem., 1983, 12, 1-14. 20. Hagg, W.R.; Hoigne, J. Environ. Sci. Technol., 1986, 20, 341-348.


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21. Zepp, R.G.; Baughman, G.L.; Schlotzhauer, P.F. Chemosphere, 1981, 10, 109-117. 22. Fisher, A.M.; Winterle, J.S.; Mill, T. In "Photochemistry of Environmental Aquatic Systems"; Zika, R.G.; Cooper, W.J., Eds., ACS Symposium Series, Washington, D.C., this volume. 23. Powers, J.F.; Sharma, D.K.; Lanford, C.H.; Bonneau, R.; Joussot-Dubien, J. In "Photochemistry of Environmental Aquatic Systems"; Zika, R.G.; Cooper, W.J., Eds., ACS Symposium Series, Washington, D.C this volume. 24. Davis, J.Α., Ed. "Surface Processes in Aqueous Geochemistry." American Chemical Society, Washington, D.C, 1986. 25. Stumm, W., Ed. "Aquatic Surface Chemistry." WileyInterscience, 1986. 26. Williams, P.M.; Carlucci, A.F.; Henrichs, S.M.; Van Vleet, E.S.; Horrigan, S.G.; Reid, F.M.H; Robertson, K.J. Mar. Chem., 1986, 19, 17-98. 27. Payne, J.R.; McNabb, G.D. Mar. Technol. Soc. J., 1984, 18(3), 24-42; Payne, J.R.; Phillips, C.R. Environ. Sci. Technol., 1985, 19, 569-579. 28. Patton, J.S.; Rigter, U.W.; Boehm, P.D.; Feist, D.L. Nature, 1981, 290, 235-238. 29. Larson, R.A., Hunt, L.L.; Ablankenship, D.W. Environ. Sci. Technol., 1977, 11, 492-496. 30. Cooper, W.J.; Zika, R.G.; Petasne, R.G.; Plane, J.M.C. Environ. Sci. Technol., submitted. 31. Draper, W.M. In "Photochemistry of Environmental Aquatic Systems"; Zika, R.G.; Cooper, W.J., Eds. ACS Symposium Series, Washington, D.C, this volume. 32. Plane, J.M.C; Zika, R.G.; Zepp, R.G.; Burns, L.A. In "Photochemistry of Environmental Aquatic Systems"; Zika, R.C; Cooper, W.J. Eds., ACS Symposium Series, Washington D.C, this volume. RECEIVED August 26, 1986


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