Chapter 7
Consequences of OH Radical Reaction in Sea Water: Formation and Decay of Br2 Ion Radical -
1
1
2
Oliver C. Zafiriou , Mary B. True , and E. Hayon
Downloaded by UNIV OF MISSOURI COLUMBIA on March 8, 2013 | http://pubs.acs.org Publication Date: December 8, 1987 | doi: 10.1021/bk-1987-0327.ch007
1
Department of Chemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 Chemistry Department, Queens College of the City University of New York, Flushing, NY 11367
2
The interaction of OH radical with seawater yields oxidized bromine species. The identity of the oxidized bromine species and its subsequent reactions have been studied by extensive flash photolysis experiments and also by pulse radiolysis. OH appears to give a nearly quantitative yield of the dibromide ion-radical in seawater. This radical decays by parallel first- and second-order reactions, the latter process being well-known but irrelevant in nature. The former involves reaction with some components of the carbonate system in seawater, resulting in decay rates for the dibromide ion-radical of roughly 20003000 s in seawater at pH 8 (halftimes of ca 300 microseconds). -1
The concept that free radicals are important intermediates in photochemical and other redox interactions of oxygen, organic compounds and heavy metals i n natural waters has received considerable support recently ((1-3) and references therein; this volume). Some of the major primary radicals expected are: hydroxyl (OH), superoxide (02~). and various organic moieties (R, RO, R00). Of these, OH i s of interest because of i t s extremely high r e a c t i v i t y , s i g n i f i c a n t formation rate from a known source ( n i t r i t e photolysis, among others) and the analogy of i t s known key role i n tropospheric chemistry. The purpose of this study i s to explore the fate of OH radicals and the identity and chemistry of their progeny i n seawater. This paper presents some of the experimental evidence concerning r a d i c a l formation and behavior i n seawater and a r t i f i c i a l seawater obtained by the fast-reaction kinetics technique of f l a s h photolysis-kinetic spectrophotometry (4) supplemented by pulse r a d i o l y s i s (5). The companion paper which follows presents results on related reactions and rates observed in media simpler than seawater and applies them to p a r t i a l l y explain the data reported here using a simple reactionmechanistic model. Known OH reaction rate constants and the major-ion composition of seawater combine to imply that OH should react i n seawater almost exclusively with bromide ion, with a small amount of side-reaction 0097-6156/87/0327-0089S06.00/0 © 1987 American Chemical Society
In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS
90
with the carbonate system and dissolved organic matter (DOM) (6). Table I estimates OH r e a c t i v i t y towards seawater components; similar conclusions were reached by Hoigné (7), although he took the f i r s t product to be B r " rather than BrOH". 2
TABLE I.
Expected Reactions of OH with Constituents of Seawater (pH 8.1, S = 35"/··)
Downloaded by UNIV OF MISSOURI COLUMBIA on March 8, 2013 | http://pubs.acs.org Publication Date: December 8, 1987 | doi: 10.1021/bk-1987-0327.ch007
Reactant
Concentration (M)
Br~ a
Total C0§(9.31 Free) Total HC0 (72.3X Free) 3
a
d
1.06xl0
10
3xl0"
e
4.2
xlO
8
1.5
CO3 + OH-
e
1.5
xlO
7
0.3
CO3- +
*5
xlO*
0.2
N0
4 x l 0
3
-6
2xl0" 0.55
ci-
BrOH-
4
b c
97.7
Initial Products
4
2
Dissolved Organic Matter
τ of Total OH Reaction
8xl0"
2.02xl0"
N0 -
Rate Constant M-l s-1
5
610*
0.2
n
0.01
