23
Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 7, 2015 | http://pubs.acs.org Publication Date: September 18, 1986 | doi: 10.1021/bk-1986-0319.ch023
C h e m i c a l Kinetics o f Intermediates in the A u t o x i d a t i o n of
SO
2
Robert E. Huie National Bureau of Standards, Gaithersburg, MD 20899 The autoxidation of SO solutions is known to involve free radicals. Recent work on the reaction of free radicals with sulfite and bisulfite and on the reaction of the sulfite and peroxysulfite radicals is beginning to allow this complex system to be understood better. This is particularly true of the effect of added chemicals on SO autoxidation and chemical transformations induced by this system. 2
2
The autoxidation of aqueous solutions of sulfur dioxide (sulfite, bisulfite) is a classic problem in chemistry. Basic features of this reaction have been known since early in this century, when it was established that the reaction is trace metal ion catalyzed (1_) and most likely involves free radicals (2). Certain chemical effects associated with sulfite autoxidation were noted also. Before the turn of the century, it was noted that sulfite would induce the oxidation of transition metal ions (3) and it was reported later that the oxidation of organic compounds was brought about during sulfite autoxidation (*0 . Conversly, organic compounds were also shown to serve as inhibitors of sulfite autoxidation (5). Over the past century there have been a great many studies on sulfite autoxidation (6), including studies on the rate of the reaction in the presence of catalysts and inhibitors, measurements of the amounts of sulfate and dithionate found under various conditions, the determination of the products arising from the addition of organic (including biochemical) compounds, and studies on the effects of mixed catalysts. Yet, a quantitative understanding of the reaction has been elusive (7,8). This is due largely to a lack of quantitative data on the elementary steps in the overall reaction, including data on the rates of the free radical reactions likely to be important. From the point of view of fossil fuel use and its effects on the environment, a knowledge of this chemistry is important for two reasons. First, wet scrubbers are s t i l l the most important means of removing S0 from flue gas; for the most efficient operation, the oxidation of S0 in the scrubber solution 2
?
This chapter not subject to U.S. copyright. Published 1986, American Chemical Society
In Fossil Fuels Utilization; Markuszewski, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
23.
285
Chemical Kinetics of Intermediates
H U IE
needs t o be c o n t r o l l e d . Second, i n t e r m e d i a t e s formed d u r i n g t h e a u t o x i d a t i o n o f S 0 may p l a y a r o l e in t h e e n v i r o n m e n t a l and h e a l t h e f f e c t s associated with S 0 emission. R e c e n t l y , we have c a r r i e d o u t s t u d i e s on t h e f r e e r a d i c a l c h e m i s t r y o f s u l f i t e . These s t u d i e s have i n c l u d e d k i n e t i c measure ments on t h e r e a c t i o n s o f o r g a n i c and i n o r g a n i c f r e e r a d i c a l s w i t h s u l f i t e and b i s u l f i t e , and on t h e r e a c t i o n s o f t h e s u l f i t e d e r i v e d r a d i c a l s S 0 ~ and SO^"" w i t h o r g a n i c and i n o r g a n i c s u b s t r a t e s . I n t h i s paper, I w i l l r e v i e w some o f our r e s u l t s and r e s u l t s from o t h e r l a b o r a t o r i e s on t h e r a d i c a l c h e m i s t r y o f s u l f i t e and d i s c u s s these r e s u l t s in r e l a t i o n t o t h e problem o f S 0 a u t o x i d a t i o n . Rate c o n s t a n t s f o r t h e r a d i c a l r e a c t i o n s were c a r r i e d out u s i n g p u l s e r a d i o l y s i s . B r i e f l y , a N 0 s a t u r a t e d s o l u t i o n is p u l s e i r r a d i a t e d , w i t h h i g h energy e l e c t r o n s , p r o d u c i n g OH r a d i c a l s . 2
2
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3
2
2
H 0 **v» OH, H, e ~
a q
e
H
2
+
~aq
N
+
2°
H
N
2° * 2
+
0
+
0 H
~
(The s m a l l amount o f Η atoms produced u s u a l l y does n o t i n t e r f e r e . ) The OH r e a c t s w i t h s u l f i t e o r b i s u l f i t e t o produce t h e SO^ r a d i c a l
OH + HS0 OH + S 0
2 3
-> H 0
+
2
3
~
S0 ~ 3
OH"" + S 0 ~ 3
or w i t h some o t h e r o r g a n i c o r i n o r g a n i c s u b s t r a t e t o produce another desired radical OH + S -> OH" + S
+
T h e _ o p t i c a l a b s o r p t i o n o f S0o~ e x h i b i t s X = 225 nm w i t h e = 1000M ο π Γ ( 9 ) . T h i s a b s o r p t i o n is n o t c o n v e n i e n t f o r most r e a c t i o n s o f S0o~ s i n c e t h e uv a b s o r p t i o n o f most o t h e r s u b s t r a t e s or o f t h e i r r a d i c a l s mask t h i s r e l a t i v e l y weak a b s o r p t i o n . T h e r e f o r e , t h e o p t i c a l a b s o r p t i o n o f the o t h e r r a d i c a l product ( o r r e a c t a n t ) was m o n i t o r e d . F o r example, in t h e r e a c t i o n o f S 0 ~ w i t h a s c o r b a t e , t h e a s c o r b a t e r a d i c a l a b s o r p t i o n a t 360 nm was m o n i t o r e d ; the r e a c t i o n o f t h e r a d i c a l I ~ w i t h H S 0 ~ / S 0 ~ was m o n i t o r e d a t 380 nm, t h e a b s o r p t i o n maximum f o r I ~ . m a x
Ί
m a x
1
3
2
3
2
3
2
O x i d a t i o n o f s u l f i t e and b i s u l f i t e by f r e e r a d i c a l s We have found t h a t s u l f i t e and b i s u l f i t e undergo o n e - e l e c t r o n o x i d a t i o n by many f r e e r a d i c a l s t o produce S 0 ~ . Rate c o n s t a n t s determined f o r s e l e c t e d r a d i c a l s a r e g i v e n in T a b l e 1. Measurement of t h e r a t e c o n s t a n t over a wide range o f pH h a s , in some c a s e s , allowed the separate determination of r a t e constants f o r the o x i d a t i o n o f s u l f i t e and b i s u l f i t e . The v e r y s t r o n g o x i d a n t OH r e a c t s v e r y r a p i d l y and o x i d i z e s b i s u l f i t e f a s t e r t h a n s u l f i t e , p o s s i b l y due t o a c o n t r i b u t i o n from hydrogen atom a b s t r a c t i o n . F o r r a d i c a l s w h i c h a r e r e l a t i v e l y weaker o x i d a n t s , t h e r e a c t i o n w i t h s u l f i t e is t h e f a s t e r . F o r example, w i t h B r ^ t h e r a t i o o f r a t e c o n s t a n t s f o r s u l f i t e t o b i s u l f i t e is about 4; f o r t h e even weaker 3
In Fossil Fuels Utilization; Markuszewski, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
286
FOSSIL FUELS UTILIZATION: ENVIRONMENTAL CONCERNS Table I .
Rate Constants for Reactions of S u l f i t e with Radicals
Reaction
-
OH + HS0 ~ 3
-
2
OH + S 0 ~ Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 7, 2015 | http://pubs.acs.org Publication Date: September 18, 1986 | doi: 10.1021/bk-1986-0319.ch023
3
SO^
-
+ HS0 ~ 3
Br ~
+ HS0 ~
2
+ S0 ~
2
10
3
I ~
+ HS0 ""
2
3
3
6.7
2
I ~
+ HS0 ""/S0 ""
2
3
3
2
i ~ + so"" 2
NH
2
+ S0 "
2
3
C H 0 + S0 6
5
C H NH 6
5
C H NH 6
5
+
2 3
+ HS0 ~
2
2
+ S0 ""
2
C H NH + S 0 ~ 6
5
C H N(CH ) 6
5
3
C H N(CH ) 6
5
3
2
2
+ HS0 " 2
+ S0 " +
+HS0 ~
(cystine)'
3
3
5
9
6.9x10
7
12
2.6x10
8
12
1.1x10
6
12
1x10
12
7
4x10
12
8
12 12 12
6
12
9
12
4
H S 0 ~ + S 0 ~ 5
3
+ SOn~ + SO^ "" + H 2
+
H 0 2
2S0
2 1|
~ + OH + 2 H
+
S i n c e b o t h OH and SO)^' v e r y r a p i d l y o x i d i z e SO^ " t o S 0 ~ , t h e s e paths a r e i n d i s t i n g u i s h a b l e i n our e x p e r i m e n t s , a l t h o u g h t h e e x t e n s i v e rearrangement r e q u i r e d makes the f i n a l p a t h h i g h l y u n l i k e l y . The i d e n t i t y o f t h e c o r r e c t p a t h i s q u i t e i m p o r t a n t s i n c e the p r o d u c t i o n o f HSOc~ can l e a d t o p o s s i b l e c h a i n b r a n c h i n g s t e p s and t h e f o r m a t i o n o f SO^"" has consequences f o r t h e e f f e c t o f i n h i b i t o r s on s u l f i t e a u t o x i d a t i o n . T h i s p r o d u c t o f the o n e - e l e c t r o n r e d u c t i o n o f SO^ (HSO^~, p e r o x y m o n o s u l f a t e , Caro's a c i d ) , i s a s t r o n g o x i d a n t w i t h a s t a n d a r d t w o - e l e c t r o n r e d u c t i o n p o t e n t i a l o f 1.82V ( 2 2 ) . A p r o d u c t w i t h t h e p r o p e r t i e s o f p e r o x y m o n o s u l f a t e has been o b s e r v e d w i t h a y i e l d o f up t o 30% upon b u b b l i n g oxygen through a s o l u t i o n o f sodium s u l f i t e ( 2 3 ) . T h i s compound can undergo many p o s s i b l e subsequent r e a c t i o n s . 2
3
In Fossil Fuels Utilization; Markuszewski, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
290
FOSSIL FUELS UTILIZATION: ENVIRONMENTAL CONCERNS T a b l e I I I . Comparison o f Some Rate C o n s t a n t s f o r R e a c t i o n s o f SO^"", S 0 ~ , and SO^ w i t h O r g a n i c Compounds ( i n M s ~ ) 1
1
5
Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 7, 2015 | http://pubs.acs.org Publication Date: September 18, 1986 | doi: 10.1021/bk-1986-0319.ch023
Reactant
SO^
ascorbic ascorbate trolox aniline N,N~dimethylaniline tyrosine tryptophan histidine i-PrOH ethanol fumarate succinate a l l y l alcohol glycine HS0
Ref. Ref. Ref. Ref. Ref.
SO^
5
6
