The Chemistry of Acid Rain - ACS Publications - American Chemical

Chapter 12

SO Oxidation by Hydrogen Peroxide in Suspended Droplets 2

W. A. Jaeschke and G. J. Herrmann

Downloaded by CORNELL UNIV on September 5, 2016 | http://pubs.acs.org Publication Date: September 3, 1987 | doi: 10.1021/bk-1987-0349.ch012

Center for Environmental Research, J. W. Goethe-University, P.O. Box 11 19 32, D-6000 Frankfurt am Main, Federal Republic of Germany

One of the most significant reactions in the context of acidity in rainwater is the SO2-oxidation by hydrogen peroxide in aqueous solution. Therefore a dynamic flowreactor was constructed, where SO2 removal rates could be investigated in the presence of H2O-containing droplets. The diameter of the suspended droplets was in the size range between 1 μm and 25 μm which is comparable to size distributions observed in atmospheric clouds or fogs. Pseudo first order rate constants of the SO2-Oxidation were mea sured at different pH-values. The H2O2-concentration in the droplets was varied between 2 x 10-5m and 10-2m. The obtained second order rate constants were strongly pH– dependent (1,48 x 10 I mol sec at pH 2 and 1,3 x 102 I mol sec at pH 5,5). At H2O2-concent rations above 10-3 m the microphysical transfer of SO2 via droplet interface became the rate determining step. From the experiments an accomodation-coefficient for SO2 could be calculated which was greater than 10 . 5

-1

-1

-1

-1

-1

Recent measurements of g a s - and l i q u i d - p h a s e c o n c e n t r a t i o n s of h y d r o g e n p e r o x i d e i n t h e t r o p o s p h e r e (1,2) s u b s t a n t i a t e t h e n o t i o n t h a t H2O2 is t h e major oxidant leading to the generation of s u l f u r i c a c i d in atmospheric m u l t i p h a s e systems l i k e fogs a n d c l o u d s . T h e o x i d a t i o n of S ( I V ) q r e q u i r e s a p h a s e t r a n s f e r o f g a s e o u s SO2 i n t o t h e s u s p e n d e d d r o p l e t s . T h i s t r a n s f e r represents a c h a i n of c o n s e c u t i v e processes i n c l u d i n g t r a n s p o r t of (S02)g to the d r o p l e t s , p h a s e - t r a n s f e r t h r o u g h the g a s - l i q u i d i n t e r f a c e , t r a n s port of d i s s o l v e d S ( I V ) q in the l i q u i d phase and subsequent o x i d a t i o n of S ( I V ) a q t o S ( V I ) q by d i s s o l v e d H 2 O 2 . B e c a u s e o f t h e c o n s e c u t i v e n a t u r e o f t h i s m u l t i p h a s e o x i d a t i o n p r o c e s s the s l o w e s t step o f the c h a i n d e t e r m i n e s the o v e r a l l rate. Commonly the o v e r a l l rates of s u c h processes o c c u r i n g in a t m o s p h e r i c fog and c l o u d s a r e c a l c u l a t e d by l i n e a r e x t r a p o l a t i o n s f r o m k i n e t i c d a t a g a i n e d i n b u l k - s o l u t i o n e x p e r i m e n t s . It is d o u b t f u l whether the assumed l i n e a r r e l a t i o n s a r e e x i s t i n g in atmospheric systems, e s p e c i a l l y w i t h r e s p e c t t o t h e l i q u i d w a t e r c o n t e n t ( L W C ) o f fogs a n d c l o u d s b e c a u s e t h i s p a r a m e t e r is t h e r e s u l t o f t h e i n t e g r a l o v e r a s p e c t r u m a

a

a

0097-6156/87/0349-0142$06.00/0 © 1987 American Chemical Society

Johnson et al.; The Chemistry of Acid Rain ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

12.

S0

JAESCHKE AND HERRMANN

2

Oxidation by Hydrogen Peroxide

143

o f d r o p l e t s i n t h e r a n g e o f 1 ^im < r < 2 0 / j m . In t h i s s i z e r a n g e o f d r o p l e t s b o t h g a s - p h a s e a n d a q u e o u s - p h a s e m a s s t r a n s p o r t of S ( I V ) a q m a y b e c o m e the r a t e - d e t e r m i n i n g step b e c a u s e the o x i d a t i o n r e a c t i o n of S(IV)aq by H2O2 i s a s s u m e d t o be v e r y f a s t . In a d d i t i o n at S 0 2 - m a s s - a c c o m m o d a t i o n c o e f f i c i e n t s b e l o w 1 0 - 2 t h e i n t e r f a c i a l t r a n s p o r t o f S ( I V ) - s p e c i e s may a l s o g o v e r n the rate of the m u l t i p h a s e o x i d a t i o n r e a c t i o n (3).


Experimental In t h i s s t u d y t h e r a t e o f t h e o x i d a t i o n o f g a s e o u s SO2 by H2O2 c o n t a i n i n g d r o p l e t s in a m u l t i p h a s e s i m u l a t i o n e x p e r i m e n t was i n v e s t i g a t e d . For t h i s purpose a d y n a m i c f l o w r e a c t o r was designed in w h i c h the removal of ( S 0 2 ) g in the g a s - p h a s e and the f o r m a t i o n of S(VI)aq in the l i q u i d phase c o u l d be d e t e r m i n e d . T h e e x p e r i m e n t a l s e t u p ( F i g u r e 1) w a s s i m i l a r t o t h a t d e s c r i b e d i n p r e v i o u s p u b l i c a t i o n s ( 4 - 5 ) . C l e a n a i r , h u m i d i f i e d t o 94 % r . H . , w a s m i x e d w i t h c e r t a i n a m o u n t s o f ( S 0 2 ) g at t h e t o p o f t h e d y n a m i c t u b u l a r f l o w r e a c t o r . The r e a c t i o n m i x t u r e was led t h r o u g h the r e a c t o r w i t h a f l o w r a t e o f 25 l / m i n . D r o p l e t s c o n t a i n i n g H2O2 w e r e g e n e r a t e d by an u l t r a s o n i c d r o p l e t g e n e r a t o r and injected into the r e a c t o r . The d r o p l e t d i a m e t e r s r a n g e d f r o m 0.5 t o 1 2 . 5 Jum r a d i u s ( r ) ( F i g u r e 2 ) . A c o m p i l a t i o n o f e x p e r i m e n t a l d e t a i l s is g i v e n i n T a b l e I. T a b l e I: Pa r amete r

Provided

S0

Cyl inder

2

H 0 2

by

Stok s o l u t i o n

2

HCI

PH Droplet

Experimental

details Monitored

Range 300 2 · 10"

8500 ng/m

-

10"

2

-

5.5

0,5

-

1 2 , 5 /jm

2

3

mol/l

5

Fl u o r e s c e n c e Titration Titration

size

(radius)

Ultra sonic

Light scattering (Partoscope)

. droplet L i q u i d Water

generator

8 · 10"

3

-

7,5

ml/m

94

%

3

Calculated

content Rel.

Humid.

Water

vapour

Temperature

Thermostat.

Hum. indicator (Veisala)

sou rce 25

°C

system

In o r d e r t o a v o i d w a l l e f f e c t s by c o n d e n s a t i o n o f w a t e r v a p o u r at r.H. 100 % i t w a s n e c e s s a r y t o l o w e r t h e v a p o u r p r e s s u r e o f t h e generated droplets u s i n g a n e u t r a l s a l t . Sodium c h l o r i d e (high p u r i t y ) was c h o s e n a n d the c o n c e n t r a t i o n of the d i l u t e d r e s e r v o i r s o l u t i o n of t h i s s a l t w a s set t o 1.5 m o l l"1 s o t h a t t h e d r o p l e t s g e n e r a t e d f r o m t h i s s o l u t i o n w e r e i n t h e r m o d y n a m i c e q u i l i b r i u m w i t h t h e s u r r o u n d i n g m o i s t a i r o f 94 % r . H . i n t h e r e a c t o r . T h e p H - v a l u e o f t h e s o l u t i o n w a s a l t e r e d by a d d i t i o n o f d i f f e r e n t m l - a m o u n t s o f HCI (25%, r e a g e n t g r a d e ) . Each m e a s u r i n g point in the f l o w reactor c o r r e s p o n d s to a f i x e d t r a n s p o r t time of the m u l t i p h a s e m i x t u r e in the r e a c t o r . The t r a n s p o r t



144

THE CHEMISTRY OF ACID RAIN

ZUF/SFB73/001-1/86/D,

F i g u r e 1. Scheme of t h e e x p e r i m e n t a l set up u s e d f o r k i n e t i c s t u d i e s of the S02~oxidation in the presence of H 2 0 2 - c o n t a i n i n g d r o p l e t s . 1. C o m p r e s s o r , 2 . A i r C l e a n e r , 3 . M a s s F l o w C o n t r o l l e r , 4 . V a p o r i z e r , 5 . S 0 2 - s o u r c e , 6. N e b u l i z e r , 7. Pump, 8 . S t o c k s o l u t i o n , 9 . P a r t o s c o p e A , 10. R e a c t i o n c h a m b e r , 11. T h e r m o s t a t , 12. H u m i d i t y Sensor, 13. S 0 2 D e t e c t i o n , 14. S 0 4 - S a m p l i n g , 1 5 . R e c o r d e r .


12.

S0


7

dl dD 10000,25 I /min

3

\xm

-1,5

3

μπΓ ] 1

3

χ10

400·

.1,0χ10

200·

• 0 . 5 χ 103

0

cm"

3

-2,0χ10

600.


I

dD

800.

145


3

0 0,5

800-

l/min

• 4 χ 10

3

600-

, 3 χ 10

400-

. 2 χ10

200-

• 1 χ 10

0-

3

3

3

- 0

800 «

•2.0 χ 1 0

4

600 ·

• 1.5x10

4

400 •

• 1,0 χ 1 0

200 ·

0,5

0 ·

χ10

4

;

0

600 U5x10^ 400

H

200· 1 χ 10 10

15

20

25

ι 5

10

1 15

1 « 20

25 D ( μ π ι )

-ZUF/SFB73/001-2/86/D«

F i g u r e 2. Size d i s t r i b u t i o n of d r o p l e t s used in the k i n e t i c s t u d i e s . The s i z e d i s t r i b u t i o n w a s m e a s u r e d by a P A R T O S C O P E A at t h r e e d i f f e r e n t f l o w rates in the n e b u l i z e r .


146

T H E CHEMISTRY OF ACID RAIN

time is i d e n t i c a l w i t h the p e r i o d of t i m e i n w h i c h t h e ( S 0 ) g c o u l d r e a c t w i t h the H 2 0 2 - c o n t a i n i n g d r o p l e t s . C o n s i d e r i n g the flow rate and the g e o m e t r y o f t h e f l o w r e a c t o r , t h e t o t a l t r a n s p o r t t i m e w a s 340 s e c . When d i s s o l v e d S ( I V ) q i s o x i d i z e d i n t h e d r o p l e t s , t h e a b s o r p t i o n e q u i l i b r i u m between the S ( I V ) a q - c o n c e n t r a t i o n and the ( S 0 2 ) g - c o n c e n t r a t i o n i s d i s t u r b e d . It is c o n t i n u o u s l y r e - e s t a b l i s h e d f o l l o w i n g H e n r y ' s l a w a n d t h e w e l l - k n o w n d i s s o l u t i o n e q u i l i b r i a o f S(IV) i n t h e l i q u i d p h a s e . T h u s a ( S 0 2 ) g - c o n c e n t r a t i o n - g r a d i e n t between t h e i n l e t a n d t h e o u t l e t of t h e r e a c t o r is f o r m e d , w h i c h is r e l a t e d to t h e t r a v e l time of the m i x t u r e . 2

a

Theory


For

kinetic calculations only S(IV)

a

=

q

S0

the ·

2

S(IV)aq c o n c e n t r a t i o n is

H 0

+ HSO3-

2

+

considered

S0 =

(1)

3

T h e r a t i o o f t h e t h r e e S ( I V ) q s p e c i e s is p H - d e p e n d e n t . It c a n be c a l c u l a t e d f r o m the measured ( S 0 2 ) g by Henry's law e q u i l i b r i u m c o n s t a n t HSO2 " " k n o w n f i r s t a n d s e c o n d i o n i z a t i o n c o n s t a n t s KD1 a n d KD2. a

a n o

t

n

e

w

e

( S 0 S ( I V )

a

q

=

( S 0

S ( I V )

a

q

=

( S 0

2

)

g

. H

)

g

·

S

o

*

2

2

) H 0 ^ j - i g

S

K

2

H

S

(1

0 2

S ( I V )

a

=

q

( S 0

2

)

g

·

Hso

+

2

D1

K

D

( S 0

1

*

K

— [H ]

+

+

2

)

g

H ^

S

o

2

K

D

1

K

D

2

D1 D1 — — ) [ H l K

+

2

(2)

2

H S 0 i n E q u a t i o n 2 i s d e f i n e d as a p s e u d o H e n r y ' s l a w c o e f f i c i e n t that depends on the h y d r o g e n ion c o n c e n t r a t i o n and encompasses the t o t a l i t y of t h e d i s s o l v e d S ( I V ) q s p e c i e s ( 3 ) . The r a t i o between the s p a t i a l d i f f e r e n c e of S(IV)aq in the r e a c t o r , w h i c h c a n be c a l c u l a t e d f r o m t h e ( S 0 2 ) g - g r a d i e n t , a n d t h e c o r r e s p o n d i n g t r a v e l - t i m e i n t e r v a l i s e q u a l t o t h e r e m o v a l r a t e R: 2

a

d[S(IV)]

d [ S ( V I ) ]

R =

a

q

= dt

(3) dt

As the g r a d i e n t of S ( I V ) q is r e l a t e d to the o x i d a t i o n p r o d u c t S ( V I ) q a p p e a r i n g i n t h e d r o p l e t s an i n v e r s e g r a d i e n t o f t h e S ( V I ) a q - c o n c e n t r a t i o n m u s t be n o t i c e a b l e . T h e r e l a t i o n b e t w e e n S ( I V ) - a n d S ( V I ) - c o n c e n t r a t i o n a


a

12.

S0



2

gradients at any given t r a v e l time of the m i x t u r e can be expressed by the f o l l o w i n g general Equation: d[S(IV)]

d[S(VI)]

a q

R =

= dt

ki

a q

d

= ^[5(ΐν)

β ρ

]

(4)

dt

= 1st p r o p o r t i o n a l i t y factor = const.

In order t o determine k*i and the exponent oL in Equation 4 the removal of S(IV) q and the formation of S(VI)aq should be measured d u r i n g the experiments. The r a t e - c o e f f i c i e n t k i was investigated as a f u n c t i o n of d i f f e r e n t experimental c o n d i t i o n s w h i c h a r e listed below: - concentration of l i q u i d water in the reactor (L) - c o n c e n t r a t i o n of hydrogen peroxide in the droplets ([H202laq) - pH of the droplets ([H ] q) These parameters have been v a r i e d d u r i n g several separate e x p e r i mental runs. The r e l a t i v e humidity always was kept constant at 94 % and the temperature in the reactor was set to 25°C. The dependence of the o v e r a l l t r a n s f o r m a t i o n rate R in Equation 4 from the l i q u i d water content L can be expressed by an exponential dependence of k i from L:


a

+

a

ki k2 β

= k2 x \f

(5)

= 2nd p r o p o r t i o n a l i t y factor = const.

The exponent β represents a possible n o n - l i n e a r relationship between the removal rate R and the l i q u i d water content w h i c h c o u l d be caused by mass-transport-1 imitations. More precisely L is defined by the integral over the size d i s t r i b u t i o n of the droplets used in the experiments.

7Γ = ^

f D

d

3

* N

d

D

The dependence of R from the concentration of H2O2 i n the droplets can also be expressed by an exponential equation: k

k y

3

2

= k

3

χ [H 0 ] 2

2

a q

(6)

= 3 r d p r o p o r t i o n a l i t y factor = const.

were 2f represents the reaction order with respect to the concentration of the o x i d a n t . k represents the rate c o e f f i c i e n t of the o x i d a t i o n reaction in the multiphase system at.a c e r t a i n oH-value·. . 3

American chemical Society. Library 1155 16th St.,of N.W. Johnson et al.; The Chemistry Acid Rain Washington, 20036 ACS Symposium Series; American ChemicalO.C. Society: Washington, DC, 1987.

148


The i n f l u e n c e of the p r o t o n c o n c e n t r a t i o n [ H ] in the d r o p l e t s on o x i d a t i o n o f S ( I V ) q c a n be e x p r e s s e d a s :

the

+

a

ζ k


k4 ($

3

=

k

χ

4

[ H

+

l

a

(7)

q

= 4th p r o p o r t i o n a l i t y = const,

factor

w h e r e 8 represents the r e a c t i o n o r d e r w i t h respect to the c o n c e n t r a t i o n of hydroni um-ions i n t h e i n d i v i d u a l d r o p l e t s . k4 r e p r e s e n t s the rate c o e f f i c i e n t of the m u l t i p h a s e r e a c t i o n in the p H - r a n g e under i n v e s t i g a tion. In s u m m a r y the dependence of the t r a n s f o r m a t i o n of S(IV)aq to S ( V I ) a q i n t h e s y s t e m c a n be e x p r e s s e d b y c o m b i n i n g E q u a t i o n 2, 4, 5, 6 and 7 as:

-dS[(IV)]

a q

dt

=

k [(S02)gHso ] 4

2

* L *

[ H

2

0

2

l

a

q

[ H

+

l

a

(8)

q

In t h i s e q u a t i o n t h e b r a c k e t s i n d i c a t e m o l a r c o n c e n t r a t i o n s f o r l i q u i d p h a s e s p e c i e s . T h e c o n c e n t r a t i o n o f ( S 0 2 ) g m u s t be g i v e n a s p a r t i a l p r e s s u r e and H*S02 represents the e f f e c t i v e Henry's law constant of S02 c o n s i d e r i n g t h e d i s s o l u t i o n e q u i l i b r i a r e a c t i o n s o f S ( I V ) . A s t o be seen f r o m Equation 8 the t r a n s f o r m a t i o n of ( S 0 2 ) g to S ( V I ) q in a m u l t i p h a s e s y s t e m m u s t be m e a s u r e d a s a f u n c t i o n o f ( S 0 ) L, [ H 2 0 2 l a q a n d [ H ] i n o r d e r t o d e t e r m i n e t h e c o n s t a n t k4 a n d t h e e x p o n e n t s * j3 X a n d £ . a q

a

2

+

g /

/

a

q

t

Results and D i s c u s s i o n In F i g u r e 3 t h e d e c a y s o f t h e S ( I V ) q - c o n c e n t r a t i o n s at v a r i o u s l i q u i d water contents a r e plotted in a s e m i - l o g a r i t h m i c s c a l e a g a i n s t the r e a c t i o n t i m e . T h e e x p e r i m e n t s w e r e p e r f o r m e d at p H = 4 a n d [ H 2 0 2 ] a q = 1 0 " 3 m o l 1-1. T h e f o r m a t i o n o f S ( V I ) a q is p l o t t e d u s i n g t r a n s f o r m a t i o n v a r i a b l e s i n o r d e r t o g a i n c o m p a r a b l e v a l u e s w i t h t h e i n v e r s e g r a d i e n t o f S ( V I ) q . In a l l cases l i n e a r dependences a r e o b t a i n e d w h i c h means that the exponent d - i n E q u a t i o n 8 is e q u a l t o o n e . T h u s t h e S ( I V ) - r e m o v a l c a n be c o n s i d e r e d k i n e t i c a l l y as a p s e u d o - f i r s t - o r d e r r e a c t i o n w i t h r e s p e c t t o S ( I V ) q . T h e d i f f e r e n t s l o p e s o f t h e s t r a i g h t l i n e s i n F i g u r e 3 c o r r e s p o n d to d i f f e r e n t v a l u e s of the l i q u i d water c o n t e n t . These slopes d i r e c t l y represent v a l u e s of k i s i n c e i * = 1 . They a r e c o m p i l e d together w i t h the r e s p e c t i v e v a l u e s of the l i q u i d water content in the t a b l e b e y o n d F i g u r e 3. In o r d e r t o d e t e r m i n e t h e v a l u e o f t h e e x p o n e n t β i n E q u a t i o n 5 t h e values of k i a r e plotted in a l o g - l o g d i a g r a m v e r s u s the c o r r e s p o n d i n g l i q u i d water content (L) (Figure 4). From the slope of the linear a

a

a q

a



12.


S0


2

200

149

300 Verweilzeir ( s ]

Symbol S(IV) Ο • χ

S(VI).

ml m

-1

-3

8.3x10 9.1x10" 8.3x10" 3x10 -1 5x10" lxlOp 5xlO L

< 1 0

-4 r/c^ \ [(S0 ) 2

g

υ» τ H o ] S

2

t

=

r/c^ \ [(S0 ) 2

( L i s g i v e n as m a s s m i x i n g

g

» , H o ]

u

S

2

t ( )

e

/

-2,43x10" L[H O ] ' 2^ 7

2 2

2

[H ]t +

(9)

ratio.)

T h e p s e u d o - H e n r y ' s l a w c o e f f i c i e n t has n o i n f l u e n c e on t h e r a t e c o n s t a n t of the r e a c t i o n b e c a u s e it is not a r a t e - b u t an e q u i l i b r i u m - c o n s t a n t . The r a t e c o n s t a n t i n t h e m u l t i p h a s e s y s t e m is o n l y g o v e r n e d by t h e l i q u i d water c o n t e n t a n d the c o n c e n t r a t i o n s of t h e o x i d a n t a n d the H Ions. +




S0

2

Oxidation

by Hydrogen

io10* ίοΙ Η Ί Imol r ) J

155

Peroxide

1

1

Μ

-ZUF/SFB73/00V-7/86/D-

F i g u r e 7. L o g - l o g plot of the s e c o n d - o r d e r rate c o n s t a n t s the r e s p e c t i v e c o n c e n t r a t i o n of [ H ] a q in the d r o p l e t s .

k3 v e r s u s

+

tHj0 l, 2

q

Imoir ] 1

ZUF/SFB73/001-8/86/D-

Figure 8. P s e u d o - f i r s t - o r d e r rate constants as f u n c t i o n of the [ H 2 0 2 ] a q - c o n c e n t r a t i o n f o r h i g h H 2 0 2 - c o n c e n t r a t i o n s (> 1 0 - 3 m o l 1-1) at p H = 2 . 0 . S y m b o l s : x : L = 9.1 χ 1 0 - 3 ml m - 3 ; o : L = 2.5 χ 1 0 - 3 ml m - 3 .



156


F i g u r e 9 . S 0 2 - R e m o v a l a t a s i m u l a t e d c o n c e n t r a t i o n o f L = 0,1 [ml/m3] a n d [ H 2 O 2 ] = 1 0 - 3 m o l 1-1 a n d 3 d i f f e r e n t p H - V a l u e s .


12.


S0

2

Oxidation

by Hydrogen

Peroxide

157

Acknowledgments T h i s w o r k was performed w i t h i n the p r o g r a m of the D F G S o n d e r f o r s c h u n g s b e r e i c h 73 ( A t m o s p h e r i c T r a c e C o m p o n e n t s ) - P r o j e c t E i ' P h y s i c o chemistry of precipitation' -

Literature Cited 1.


2. 3. 4. 5. 6. 7.

Heikes, B.G., Lazrus, A.L. and Kok, G.L. Presented at the Seven teenth International Symposium on Free Radicals, 1985. Lind, J. NCAR private communication, 1985. Schwartz, S.E. In Chemistry of Multiphase Atmospheric Systems; W. Jaeschke, Ed; Springer-Verlag, Berlin, 1986; pp. 415-472. Jaeschke, W. Toxicol, and Env. Chem., 1985, 10, 4, 265. Berresheim, H., Jaeschke, W. J. Atm. Chem. 1986, 4, 311-334. Martin, L.R. InSO2,NO and NO2 oxidation mechanisms: Atmospheric Considerations; J.G. Calvert, Ed.; Acid Precipitation Series, Butterworth Publishers, Ann Arbor, 1984, Vol. 3, pp. 63 - 100. Hoffmann, M.R. and Jacob, D.J. In SO2, NO and NO2 oxidation mechanisms: Atmospheric Considerations; J.G. Calvert, Ed.; Acid Precipitation Series, Butterworth Publishers, Ann Arbor, 1984, Vol. 3, pp. 101 - 172.

RECEIVED

March 10, 1987


The Chemistry of Acid Rain - ACS Publications - American Chemical

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