Dynamics of multiphasic glass powder catalyzed autoxidation of sulphur dioxide in bulk aqueous phase

Indian Journal of Chemistry

Vol. 30A, September 1991,pp. 75-764

Dynamics of multiphasic glass powder catalyzed aut xidation of sulphur dioxide in bulk aqueous phase

Ashu Rani,D S N Prasad,Usha Jain&K S Gupta"

Atmospheric Chemistry Laboratory, University of Rajasthan, Jaipur 302 0 4 Received 6December 1990;revised 15 April 1991;accepted 30May199

Kineticsof autoxidation of sulphite has been studied in unbuffered as well as ac tate-buffered aque- ous suspensions ofglasspowder. The buffer concentration has an inhibitory effect n the reaction and thenature oftheratelaw depends on the buffer concentration. At[CH3COONa] ⁼ .14 moldm>' and varying[CH3COOH], the kinetics in the two^pH regimes,5<pH ~5.9and4.2<pH <5.0, are different.

InthefirstpH regime,with increasing [glass],the rateprofile passesthrough am' urn.For the region having linear dependence on [glass],the initial rate,Robs'isdefined by the rate law ( ) at atmospheric O² pressure:

... (A) where R^unca, isthe rate ofuncatalyzed reaction. The rate hasanalmost inverse dep ndence onparticle size.On the other hand, inthe regime4.2<pH <5.0, an inducation periodis obse which decreases on decreasing buffer concentration and onincreasing^pH and [S(IV)].It also decre eson passing oxy- gen.When the buffer concentration islower, viz., [CH3COONa]=0.07 mol dm^"? d [CH3COOH] is varied, thekinetics are in conformity with theratelaw(B):

... (B)

In unbuffered solutions,an initialrapid drop in [S(IV)]in thehigh^pH region(6.2-6. )and aninduction period inthelow^pH region(3.49-4.44) have beennoted.In the middle^pH region(4 8-5.5), thekinetics obey the rate law(C):

Robs⁼k8[S(IV)][glass] ... (C)

The suggested autoxidation mechanism isof Langmuir-Hinshelwood type which en isions the adsorp- tion of bothdioxygen and S(IV)onglass surface.

A large number of studies have been directed tow- ards understanding the mechanism of the removal of atmospheric S02' So far most of the aqueous phase studies have focused on homogeneous trace metal ion catalysis!" and only afew studies on ther- mal autoxidation of sulphite inthe aqueous suspen- sions have been made e.g.,with various forms of car- bon^5•6 and fly ash? In contrast, many studies on the photo assisted solid cat.alyzed autoxidation, e.g., with supported CofIlj-complexes", a-Fe20³ (Ref. 9), Ti02 (Ref. 10), CdO (Ref. 10), ZnO (Ref. 10) and several polymorphs of iron oxides¹¹ have been re- ported. These studies provide a model for autoxida- tion of S(IV) in atmosphere and are also helpful in developing suitable desulphurization catalysts for scrubbers and pollution control. To fill thewide gap that exists in this area, we have undertaken such studies on aqueous suspensions of potential solid powder catalysts. Recentl~, oxidation studies with Cd0¹² and atmospheric dust" have been reported

from this laboratory. Inthisp per, we report the re- sults of our studies on gl ss powder catalyzed autoxidation of S02' Previo ly, some preliminary observations on this system ave been reported by Hoather and Goodeve".

Materials and Methods

Catalyst and kinetics procedu

Ordinary transparent, col urless electric bulbs were broken into small piece which were washed with acetone and alcohol to r move greasy and or- ganic matter. After keeping 0 ernight in acid dich- romate solution, the pieces ere rinsed with dis- tilled water; a similar trea ent was given with NaOH-EDTA mixture. Fin ,the particles were washed with dilute perchlori acid and then with water several times. The pie es were thoroughly ground into a fine powder w .ch was then sieved and separated into the foUo . g four fractions of

RANI et a.: DYNAMICS OF CATALYZED AUTOXIDATION OF SULPHUR DIOXIDE

particle sizes (r): r ~ 3; 75 ~ r ~ 53; 120 ~ r ~ 75 and 180 ~ r ~ 120 ^{f1 .} In all kinetics experiments, except where the effe t of particle size was studied, the fraction r ~ 53 f1 was used. The results of chemical analysis sh wed Si02 (68.93%), Al203 (0.89%), Fe203 (0. %), CaO (5.54%), MgO (3.99%), K20 (1.66 Yo), Na²0 (15.96%), MnO (0.013%), NiO (0.032 0),ZnO (0.014%), and CoO, Cr 203 and CuO to be ntraces.

For obtaining aque us S02 solution, sodium sul- phite (E Merck) was sed. Acetate buffer was used for maintaining pH.I all cases, [CH3COONa] was fixed at the desired Ie el and [CH3COOH] wasvari- ed for obtaining the esired pH.For studies in un- buffered suspensions e desired initial pHwas ob- tained by adding a propriate amount of dilute perchloric acid. In such kinetics runs, the pH decreased continuous y.

The reactions were onducted in open Erlenmey- er flasks",The desired amount of buffer wastaken in the reaction vessel an kept for at least half an hour to allow it to attain th rmal equilibrium. A weighed amount ofthe glass p wder was then added and the reaction was initiated by adding a standard sodium sulphite solution and witching on the magnetic stir- rer immediately afte ards. The kinetics of the reaction were followe by withdrawing a 5 cm ' ali- quot and adding it to the titration flask containing predetermined quanti of standard iodine solution.

Unreacted iodine was titrated against standard hypo using starch as the . dicator. Oxygen dependence was studied by passin the mixture of N 2 and O² in various proportions a a fixed flow rate.

In the pHrange of is study, S02 exists as HSO;

and SO~ - in aqueo s solutions and the symbol S(JV) has been used nceforth to describe all these forms collectively. Th results obtained in this study are based on the me surement of the initial rates.

The results were rep oducible generally to within

±

10%. In the prese tation of rate data later, the statistical parameters ave been abbreviated as fol- lows: correlation coe cient

=

CC; coefficient ofde- termination =CD; andard error of estim- ate

=

SEE.

Product analysis

Addition of BaCl² to the final product solution showed that sulphite was almost completely con- verted into sulphate ( .thin 97

±

1^%).There was no evidence of the form tion of dithionate. Thus, the results are inagreeme twith stoichiometric Eq. (1).

Sulphite

+

tO² glass Sulphate ... (1)

Reaction in dark

The kinetics of this reaction were studied in dif- fused room light. In view of a recent report on the photoxidation of sulphite'>, the kinetics were stud- ied using blackened vessels as well as in dark. How- ever, the results were found to be the same in dark and in diffused room light.

Results

The effect of stirring speed on the rate was stud- ied and it was found that rate increased on increas- ing the stirring speed from 300 to 1200 rpm. A li- miting value at 1200 rpm showed that beyond this speed the diffusion of atmospheric oxygen into the reaction mixture was not rate controlling. Hence, all rate measurements were made by stirring suspen- sions at 1500

±

100 rpm. Autoxidation of S(JV) did not occur in the nitrogen atmosphere which esta- blished atmospheric oxygen as the oxidant for S(JV).

In a triplicate set of experiments, 0.1 g glass pow- der was leached in 100 em" buffer solution of pH 5.06 by stirring it for 1h.The glass particles were filtered off using glass wool, the leachate (extract) was made 2 x 10-3 mol dm " in S(JV) and the kin- etics were then followed as usual. The rate of the S(JV) oxidation under these conditions was same as in the uncatalyzed oxidation. For comparison, the reaction profiles for glass-catalyzed, uncatalyzed and leachate catalyzed reactions are depicted in Fig.

1. These observations clearly rule out the possibility of the catalytic activity of the glass powder being due to any leached metal ions.

9r---,

4 Rob •• 0.86 .1cr'

3

Fig. 1-The reaction profile for glass,glass-extract and uncata- lyzed reactions, [S(IV)]-4 x10-3 mol dm"'.pH-4.06, t- 30·.

Iil-Uncatalyzed, --glass-extract, o-glass-catalyzed glass - 2.5 g dm-3•

757

INDIAN J CHEM, SEe. A, SEPTEMBER 1991

In a parallel set of experiments, the influence of EDTA was also studied in otherwise identical con- ditions. The rate of the uncatalyzed reaction dec- reased on increasing EDTA, so much so that the reaction completely ceased in the presence of 1x 10-⁶ mol dm-³ EDTA On the other hand, in glass catalyzed reaction, addition of EDTA intro- duced a large induction period, which increased on increasing [EDTA]. The results shown in Fig. 2 clearly rule out the possibility of catalytic activity of glass powder being due to homogeneous trace metal ion catalysis. Had it been so, the glass catalyzed reaction too should have ceased in the presence of EDTA

On increasing buffer concentration while keeping the ratio [CH3COOHV[CH3COO-] constant for maintaining pH, the rate of theglass catalyzed reac- tion decreased; the effect was more pronounced at lower pH. Further experimentation showed the kin- etics to bedifferent at low and high buffer concentr- ations as described below.

Kinetics inhigh buffer region

In this region, [CH³COONa] washeld constant at

0.14 mol dm^"? and the concentration of

CH³COOH was varied for obtaining desired pH.

Interestingly, the kinetic results in the regions pH> 5andpH

<

5 were different.

Regime 5

<

pH ~ 5.9: Fig. 3shows the standard reaction profile for the disappearance of S(IV) in

sr---~

...

'~ 4

~

•••':2 3

~ f' N

2~

l

,\~~~I~~I~~I~L'~I-L~I~~!~~J

o ^{20 40 60 80 100 120 140}

timl1,min

Fig.2- The influence of EDTA onglass catalyzed and uncata- Iyzed reaction, [S(IV)]=2 x 10-3 mol dm-3, pH=5.06,

[CH3COOH]=O.03 mol dm-3, [CH3COONa]=0.07 mol dm-3

and t=30c• e-[EDTAj-1.5 x 10-6 mol dm-3, [glass]=2.5 g dm-3; x=[EDTAj=5XIO-6 mol dm-3, [glass]=2.5 g dm-3, c-uncatalyzed without EDTA, zx-uncatalyzed with [ED-

TAj=5 x1O-6moldm-3,

this pH regime. The rate dep ndence on the amount of glass (g dm-³⁾ is shown' ^IFig. 4, which shows that on increasing [glass] the rate at first increases, reaches a maximum and th n starts declining. At high glass concentration (40 dm-^3), the rate of the reaction is very low and the S(IV) disappearance, which isnormal at lower glas ,is now accompanied by an interregnum ofslow re tion (Fig. 3).

As shown in Fig. 4, ^Robs s [glass] curve passes through a maximum. Howe er, at low [glass] the curve is almost linear. Henc , all further investig- ations were carried out unde these conditions. The

5r---+---~

'1'

~ 4 '0

E

Fig. 3- The nature of reaction pr e at different glass con- centrations. [S(IV)]= 2x 10-3 mol d -3,pH=5.06, t=30·. 0-

40 gdm-~, Ll-O, g dm>'.

45r---~---~

40

•

35

••

^{III 30}

...

'E

'0 25 '0 E

III .0

a:0

•...~

~~~+-~~2~~3~I=~4~~5~~6 16t[Glassl. dm-1

Fig.4- The variation ofRobs with lass concentration in high bufferregion.[S(IV)j=2.0x to-J,p =5.06 and t=30·. o-The stirring was resumed aftermixing bu er,glassand S(lVj inthat order, .-Re3.ction initiated by add' g S(iV) to a glass suspen-

sionprestirred for nehour.

f

RANI et a.: DYNAMICS OF CATALYZED AUTOXIDATION OF SULPHUR DIOXIDE

values of Robsconfor ed to the experimental rate Reat were obtained from Eq. (3) at different pH va-

law(2), lues.

Robs= k¹[S(IV)]

+

k^3[ ass][S(IV)]2 ^{... (2)} where k,[S(IV)] desc ibes the rate of uncatalyzed reaction, Runeat.The ear plot of Rob/[S(IV)] vs ([glass][S(IV)]), obtain d by least squares best-fit, yielded k, and k3 v ues of 1.38 x 10-4 S-1 and 6.46 x 10-2 mol-1 g- dm" respectively at pH 5.06 and 30° (CC =O. 72; CD =0.945; SEE = 5.63 x lO-S). The res lts given in Table 1 show the observed and calculat d rates to be in good agree- ment.

The rate was foun to increase with increase in pH. However, for an alysis of hydrogen ion de- pendence of the glass atalyzed reaction, the values of Runeat'i.e.k1[S(IV)] erm in Eq. (2)atdifferent pH values are required. T ese values were directly de- termined experiment y in the absence of glass but under otherwise iden cal conditions. The values of

TableI-The values of the initial rates at different [gla

pH=5.06of

[glass] W x [S(IV)]

(g dm<') (mol dm^{t ')}

1.0 1.0

1.0 2.0

1.0 3.0

1.0 4.0

1.0 5.0

1.0 6.0

1.0 7.0

2.0 1.0

2.0 2.0

2.0 3.0

2.0 4.0

2.0 5.0

2.0 6.0

0.25 2.0

0.5 2.0

0.75 2.0

1.25 2.0

1.75 2.0

4.0 2.0

5.0 2.0

3.0 2.0

7.0 2.0

0.50 4.0

1.5 4.0

2.5 4.0

bserved (~bS) and calculated (R.:a1cl )and [S(JV))in the regime pH>5 at

buffer region at 30°C 107 X~bS 107 XR.:a1c oldm-3s-l) (mol dm ^vs^{r ')}

2.20 5.05 8.83 14.0 20.4 30.9 34.0 3.62 7.05 16.3 22.5 34.6 51.2 2.74 3.98 5.25 5.68 7.40 13.0 16.6 8.56 23.0 11.4 24.3 38.6

2.03 5.34 9.95 15.8 23.0 31.5 41.3 2.70 7:92 15.8 26.2 29.2 54.8 3.40 4.05 4.70 5.99 7.28 HI 15.7 10.5 20.9 10.7 21.0 31.4

Real=Robs- Runeat= k2[glass][S(IV)j2 ... (3) An analysis of the dependence of Real on [H+] in- dicated the presence of both hydrogen ion-depend- ent and -independent paths asshown in Eq. ^(4),

... (4)

where values of k, and ks were computed to be 8.71 x 10-2 and 1.05 x 10-7 mol-1 g-1 dm" S-1 re- spectively at 30° (CC =0.64; CD =0.68;

SEE =0.014). A low value of correlation coefficient casts some doubt on the analysis of hydrogen ion dependence in terms ofEq. (4 ) and the reaction may well be hydrogen ion independent. This aspect is discussed later.

On increasing particle size, Robsvaried almost in- versely with it.Further, particles of larger sizeintro- duced an induction period (Table 2).

By studying the reaction at four temperatures, the overall energy of activation wasdetermined tobe 91 kJ mol-1 (CC =0.999; CD =0.999; SEE =8.64).

The rate studies were made in 02-saturated solu- tions and also at varying O2partial pressures. Sur- prisingly, the reaction was not affected by[02], thus showing zero order in it.

Regime 4.2

<

pH

<

5.0: The standard rate profile (Fig. 5) in this pH regime is accompanied by.an in- duction period, beyond which the disappearance of S(IV) is almost linear. For treatment of kinetic re- sults, the initial rates were calculated from the linear section of the curve. The induction periods were measured upto the interaction of the extrapolated linear reaction and initial constant concentration, as was done by Hobson et al." The kinetic results are consistent with the rate law (Eq. 5).

Robs

=

k[glass]o.24[S(IV)]1.l4[H+]-1 ... (5)

On increasing [buffer], while the rate of autoxidation decreases, the induction period, J, increases. The variation of J with [buffer] is defined by Eq. (6) which predicts the existence of an induction period even in unbuffered .suspensions.

... (6) This has been found to be true, as shown later. The magnitude of J does not depend on [glass] and its functional dependence on [H+] and [S(IV)] is shown in Eq. ⁽⁷⁾ (Table ^2).

J

=

k^J[H+]o.72[S(IV)]-o.66 ... (7)

759

INDIAN J CHEM, SEe. A, SEPTEMBER 1991

Table 2- The influence of [S(JV)],pH,[glass] and particle size on R.,bsand of [S(JV)],pH,ps .cle size and prestirring oninduction period in the high buffer region at 30·

1Q3x[S(JV)) [glass] pH Particle Prestirring Induction 106x R.,bs ¹

(moldm r+) (gdm-3) size(r) time, t, period (J) (mol dm? S-I)

Cum) (s) (s)

2.00 0.25 4.86 53 0.0 240 0.53

2.00 0.50 4.86 53 0.0 270 0.65

2.00 2.00 4.86 53 0.0 240 0.72

2.00 3.25 4.86 53 0.0 240 1.10

2.00 5.0 4.86 53 0.0 240 1.10

2.00 10.0 4.86 53 0.0 210 0.57

2.00 3.25 4.21 53 150 540 0.25

2.00 3.25 4.21 53 300 420 0.26

2.00 3.25 4.21 53 720 120 0.24

2.00 3.25 5.06 53 0.0 0.0 1.60

2.00 3.25 5.06 75>r>53 0.0 300 1.19

2.00 3.25 5.06 120>r>75 0.0 430 0.70

2.00 3.25 5.06 180> r > 120 0.0 600 0.40

2.00 3.25 4.33 53 0.0 600 0.45

4.00 3.25 4.33 53 0.0 450 1.09

6.00 3.25 4.33 53 0.0 270 1.65

8.00 3.25 4.33 53 0.0 240 1.88

10.0 3.25 4.33 53 0.0 225 2.20

2.00 3.25 4.9 53 0.0 180 1.30

2.00 3.25 4.86 53 0.0 240 1.10

2.00 3.25 4.65 53 0.0 360 0.67

2.00 3.25 4.33 53 0.0 480 0.26

2.00 3.25 4.21 53 0.0 720 0.26

~ 5,---.

E

"0

o

E

:; :=

3

...

~

S!;. 2 Robs: 13.0_10-7 q...•

Vo 50

time,min

Fig. 5-The nature of reaction profile in the regime 4.2<pH<5.0ofthe high buffer region. [glass] = 3.25 gm dm-3;

[S(JV)]=2X 10-3 mol dm? and t=30·. o-pH=4.21, ~- pH=4.9.

k,was found to be (3.6

±

0.7) x 10-⁹ s at 30°. In- crease in particle size led to an increase in J, which agrees with the observation that the solubility equi- librium of amorphous silica surface is established slowly, if the particle size islarge".

Significantly, if glass is stirred in solution contain- ing water and buffer, prior to addition of S(N), J decreases approximately by the amount of time, t,

(henceforth referred to as pre tirring time) by which the glass has been stirred. A og-log plot of Jvs Is was linear with a slope of 0.9 .The kinetics runat [glass)=3.25 g^{dm :",} [S(N)) 2^x 10-³ mol dm-^3, pH =4.35 and temp. =30° sh wed an induction pe- riod of 750s. This very rung ve interesting results when glass, water and buffer ere taken in the reac- tion vessel and oxygen was en passed for about 1800s, without any stirring, fo owed by the addition ofS(N) and stirring as usual. Induction period was reduced to about 120s, but i itial rate of this reac- tion had the same value as tha of the reaction carri- ed out under previous reac on conditions. Thus, oxygen diffusion has the effe t of reducing induc- tion period only. Similarly, w en the same reaction was repeated by adding glass 0an Oy-saturated so- lution and then adding S(N) d passing ^O2 contin- uously, the induction period was reduced by the same amount as in the earli r experiments. How- ever, the rate of the reaction as not affected. Its or- der was zero in this conditio also. Prestirring ap- pears to expose the glass su ace for intimate con- tact with various species.

RANI et a/.: YNAMICS OF CATALYZED AUTOXIDATION OF SULPHUR DIOXIDE

Kinetics in low buffer re ion

In all the experimen pertaining to this region [acetate] was held cons ant at 0.07 mol dm>' and [CH³COOH] was varie for obtaining desired ^pH.

On increasing [glass],th rate increased linearly be- tween 0.5 and 2.0 g dm ^3.However, at higher [glass]

an induction period wa introduced, beyond which S(IV) disappeared no ally.The rate had a first or- der dependence on S( ).In low buffer region, the results conformed to th rate law(8).

Robs =

(k6 + [~:])

[gla ][S(IV)] ... (8)

In this situation the rat of uncatalyzed reaction ap- pears to be insignifican in comparison to the rate of catalyzed reaction. Val esof^Robs are given in Table 3. The values of

k6

^{and k, were found to be}

1.8 x 10-⁴ and 4.6 x 1 ^-9 dm' g-l ^S-I respectively at 30°(CC=0.99; CD 0.98; SEE = 1.42 x 10-^4).

Kinetics in unbuffered uspensions

Inunbuffered suspe sions, the pH affects not on- lythe rate but also the ature of the reaction profile.

At low ^pH values (3. 9-4.44), the reaction profile was marked by an . duction period which de- creased on increasin ^pH and disappeared alto- gether beyond ^pH 5. thehigh^pH region (6.2-6.8), there was an initial rap d drop in[S(IV)]followed by its slower disappearan e in the second stage (Fig.6).

On the other hand, th reactions in the middle pH region (4.8-5.5) neith r exhibited an initial rapid

Table -.- The variation 0 ~bs with (glass],(S(IV)]and pH in 10\1.ouffer region and at 0° (CH3COOH] =0.03 mol dm-3,

(CH3COO a]=0.07moldm-3

[glass] Wx [S(IV)]

(g^{drrr ") (mol dm= ')}

106X ~bs 104 X ~b/

(moldmr ^ts "") [g1ass][S(IV)]

(dm?g-I S-I)

0.50 2.00 .06 0.71 7.10

1.00 2.00 .06 1.32 6.60

1.50 2.00 .06 2.12 7.10

2.00 2.00 .06 2.80 7.00

3.00 2.00 .06 3.80 6.33

6.00 2.00 .06 7.50 6.25

10.0 2.00 .06 15.0 7.0D

0.50 4.00 .06 1.48 7AG

O.sO _{6.00 .06 2.10 7.00}

0.50 2.00 .91 4.3U 41.6

0.50 2.00 28 1.10 11.0

0.50 2.00 4.86 0.92 9""^<-

0.50

z oo

4.33 u48 4.8U

drop nor an induction period and appeared to bein- dependent of ^pH. This unusual behaviour of reac- tion in these different ^pH regions precludes amea- ningful mathematical treatment of hydrogen ion de- pendence. The reaction at pH 5.7 obeyed the rate law(9),

Robs =k⁸[S(IV)][glass] ... (9)

The value of k8 was found to be (5.8

±

1.0)x 10-⁴ dm' ^g-I s^-1at 30⁰ in thepH range 4.82-5.5. The re- sults are given in Table 4.

2.5r---,

o

Fig.6- The nature ofreaction profiles in unbuffered solutions.

(glass] =0.5 g dm-3, [S(IV)]= 2x 10-3 mol dm? and t = 30°.0- pH=4.14, c.-pH=5.24, e-pH=6.79.

Table 4- The values of Robsat different [S(IV)], [glass] andpH in unbuffered solutions at30°

Wx(S(IV)] [glass] pH 107x~bS 104 X k, (mol dm-3) (gdm=') (moldm-3s-l) (dm3g-ls-l)

1.0 0.50 5.70 3.75 7.50

2.0 0.50 5.70 5.05 5.05

4.0 0.50 5.70 10.5 5.25

6.0 0.50 5.70 14.5 4.83

8.0 0.50 5.70 19.5 4.90

10.0 0.50 5.70 23.5 4.70

..,n 1.25 5.70 13.8 5.52

_.V

2.0 2.00 5.10 31.5 7.90

2.0 0.50 5.70 6.60 6.60

2.V" 0.50 41<2 5.05 5.05

2.0 0.50 5.24 6.88 6.88

2.0 0.50 5.50 5.05 5.05

2.0 0.5D 6.\)4 6.90'

2.0 n50 6.79 7.21'

a-[S(IV)]! ~tvalues for initial rapid drop.

761

INDIAN J CHEM, SEe. A, SEPTEMBER 1991

Discussion

On the basis of kinetics results and chemical anal- ysis,Hoather and Goodeve!" discounted the possib- ility of glass catalysis being due to leached Mn² +

and Fe³+ ions, although the glass sample used by them had about 3%iron as Fe²0³ and 0.5%, manga- nese as MnO. We too reached the same conclusion from a comparative study of the effect of EDTA on the uncatalyzed reaction, autoxidation in leachate solution and inglass suspension. The chemical anal- ysis of our glass sample shows the amount of iron and manganese to be 0.41% (as Fe203) and 0.013%

(as Mn02) respectively. These amounts are much smaller than those present in the glass powder sam- ples used by Hoather and Goodeve':'. The particle size (53 x 10-⁶ m)in the present study ismuch larg- er than that used in the earlier study (1x 10-⁶ m).

Thus, the exposed surface area in the present case is much less, and so are the chances ofthe metal ions leaching in the solution.

The mechanism of autoxidation appears to in- volve the adsorption of S(IV) and O2 on the particle surface". Several evidences support multiphasic he- terogeneous nature of this reaction. First is the rate decreasing effect of particle size on Robs. Second is the strong negative variation of induction period with[S(IV)) and prestirring time in regime pH

<

5 of high buffer region (Fig. 7,Table 2 and Eq. 7).The following experiments also indicate involvement of 02. In the regime of high buffer region (pH> 5),a kinetic run, the reaction profile ofwhich had an in- duction period, had the Robsvalue of 5x 10 -⁸mol dm^{-3 S-1} at [glass]=40 g dm^-3, [S(IV)]=2x 10-³ mol dm-^3, pH=5.06 (Fig. 4).

There was a dramatic effect on the rate of the reaction as well as the nature of reaction profile when the glass powder and buffer solution were stirred for about an hour and then the reaction was initiated by adding S(IV). The pre-stirring increased the value of Robs manifold (Fig. 4) and the disap-

~ 5,.---~

-e

o m w ~ ~ ~ ro ~

Tim., min

Fig.7 - The effect ofprestirring of the glass suspensions on the rate of reaction at high glass concentration in the regime pH>5.0 of high buffer region. [S(IV)]=2x 10-3 mol dm", [glass]= 40 gdm", pH= 5.06, t = 30°. O-without prestirring, .-

prestirred for one hour.

pearance of S(IV) with e also became normal with only asmall induction eriod (Fig. 7).It should be emphasized here that in other experiments the stirring was started only a er addition of S(IV) to glass powder in buffer (cf. aterials and Methods).

These observations can be xplained in the follow- ing way. Besides keeping e glass powder in su- spension, stirring allows ra id diffusion of air and, hence, of oxygen inthe solu .on.Pre-stirring helps in the oxygenation of glass pa .cles. Therefore, when the reaction is initiated by adding S(IV) to a pre- stirred glass powder suspen ion, therate ofthe reac- tion ismuch larger and ind ction period much less, compared to when no prest rring was done. The re- sults of regime 4.2

<

pH

<

50,which show a drastic reduction ininduction peri d with increase in pres- tirring time (Table 2) and th passage of oxygen in unstirred solutions, can be explained in the same way. It appears that in the olutions of pH

<

5, the induction period is caused y slower adsorption of oxygen, and when pH isgre ter than 5,the adsorp- tion of O² isfast and, hence no induction period is observed except when the glass concentration is high.

In the regime pH> 5, e rate profile passes through a maximum as the ss is increased and the rate becomes very slow at hi glass concentrations (Fig. 4). This observation of the rate maximum can be explained if the operati n of a Langmuir-Hin- shelwood ¹¹type of mechanis is assumed which re- quires that both S(IV) and ²must be adsorbed on the adjacent sites on the glas tofacilitate the oxida- tion. It appears that when th catalyst concentration is very high, alarge number f active sites are avail- able and S(IV) and O² are ad orbed on remote sites, rendering the reaction betw n the remotely placed O² and S(IV) difficult. How ver, as stirring of the reaction mixture and diffusi n of O² inthe suspen- sion continue, particles with djacently adsorbed O² and S(IV) increase and this xplains the nature of reaction profile in Fig. 3. An temative Langmuir- Rideal type of mechanism ^{I ,} which assumes the reaction between an adosrb d and an unadsorbed reactants present in the bu phase, would not ex- plain the existence of rate m urn in the rate pro- file (Fig. 4).

In the carbon catalyzed S ) oxidation", which was studied in unbuffered uspensions, an initial rapid drop in [S(IV)] follow d by its much slower disappearance in the second stage was noted. This behaviour was ascribed to e reaction of already oxygenated carbon particles ith S(IV). The carbon particles had become oxyge ted by adosrption of oxygen from air prior to go g into the solution.

RANI et al.: YNAMICS OF CATALYZED AUTOXIDATION OF SULPHUR DIOXIDE

When the degassed carb n particles were used, the initial rapid reaction dis ppeared altogether. Brod- zinsky et al.⁶considere these observations as an evidence of a primary step which involved the adosrption of O² on ca on particles. Incidentally, in the present investig tion too, the initial rapid drop in [S(IV)] is obse ed in unbuffered suspen- sions when the ^pH isgr ater than 6.Thus, the reac- tion under this situation ightbeoccurring by ame- chanism similar to that roposed for carbon parti- cles".

The concentrations 0 bisulphite and sulphite are governed by the equilib ium (10) which is reported to be rapid with a

HSO; ~ S02; +H+ ( pid) ... (10)

dissociation constant' f 6.23 x 10-^8. In the range of ^pH studied in this s dy, S(IV) is expected to be largely present as HSO

Silica, glass and silic tes are known to be present in hydroxylated forms! in aqueous solution and fol- lowing acid-base equili ria involving liquid-solid in- teractions are well esta lished":".

>5ioH

where pHzpc ispH of ero point charge of glass sur- face. Thus, in the^pH r ge of this study glasswill be largely present in hy oxylated form. Further, 3-4 OH groups are know to be present per 100 N sur- face area. Based on s information and kinetics re- sults, the following m chanism (Eqs 13-18) may be proposed for high bu er region.

/OH

+

50r

^/0502

>5i-OH >5,-OH + ^{OH (slow)}

"'-

""'0502

0502

/0502- /0502-

>5i-OH +02 + H+ >5i-0/ + H2O

"'050 - ""'0502-

2

OH lost

> /

~ SI-OH

"'OH

.(17)

--

fast ⁽¹⁸⁾

(11)

The mechanism, which isconsistent with zero or- der in O2,will lead to the following rate lawfor cata- lyzed path in high buffer region which is same as rate law(4)through k,= k9KI

Real =(k9KI

+

kIOKdK1[H+

l

¹)[Si0²][S(IV)F

and ks= klOKdKI (19)

The acetate ions inhibit this path by occupying a siteon silica(Eq. ^20).

/OH OOCCH)

>.

_{5'...OH+CH)COO}

-

_{+ H}⁺_-

>./

_{5,-OH + H20 ../20)}

<,0502 "'0502

Equation ⁽²⁰⁾ is in agreement with the reported ad- sorption of acetate ions and acetic acid on silica sur- face^l8,2o.Similar mechanism can be written for other paths operating in low buffer and unbuffered re- gions. In the reactions of CU022 and a-Fe20j with S02 too, the involvement of surface hydroxyl groups has been invoked.

The most puzzling aspect of the effect of buffer concentration is the change in order of S(IV) from one in unbuffered and low buffered suspensions, to two in high buffer region. Atpresent, an explanation cannot be offered that will not be mere speculation and a reasonable explanation must await further work on similar systems. Another intriguing aspect of the present reaction isthe exact role of oxygen in the low pH region, where an induction period ap- pears. Although the passage of oxygen, prestirring and high O² concentration slash the induction peri- od without affecting the rate, how it is done is not obvious.

The analysis of hydrogen ion dependence as pre- sented in thispaper is vitiated to some extent, bythe fact that the variation in H+also entails a variation in the ratio ^{[CH3COONa]/[CH3COOH].} With in-' crease in^pH both [H+] and [CH³COOH] decrease and there is no way to decide whether the rate in- crease on increasing ^pH isdue to decrease in[H+]

or due to decrease in [CH³COOH] or both. Thus, the possibility of the present reaction being [H⁺[-in- dependent cannot be ruled out. The same inference can be drawn from poor correlation of ^Real with [H+]. In fact, in unbuffered suspension the reaction isindependent of ^pH. Incidentally, the carbon cata- lyzed¹⁶ S(IV)-oxidation too is hydrogen ion-inde- pendent.

..(121

II))

(15)

(16)

763

INDIAN JCHEM, SEe. A, SEPTEMBER 1991.

The present work has direct relevance to environ- mental chemistry of S02' Glass is mainly composed of silicate material which is an important constituent of suspended particulate matter in the atmosphere.

The present work shows that such materials playa significant role in catalysing the transformation of S(IV) into S(VJ)in bulk aqueous phase. As a corol- lary, the suspended particulate matter is likely to promote atmospheric acid precipitation by catalyz- ing the oxidation of S(IV) in aerosols and droplets.

Acknowledgement

The work was supported by an Indo-US Project, CE-2. The authors are grateful to Dr R E Huie, Na- tional Bureau of Standards, Gaithersburg, MD, USA for several useful suggestions.

References

1 Huie R E &Patterson N C, Traceatmospheric constituents:

properties, transformations and fates,edited by S E Schwartz (John Wiley, New York), 1983, p 118.

2 Hoffmann M R &Boyce S D, Trace atmospheric constitu- ents: propenies, transformations and fates, edited by S E Schwartz (John Wiley,NewYork), 1983,p142.

3 Martin LR, S02, NO and N02 oxidation mechanisms: at- mospheric considerations, edited by J G .Calvert (Butter- worth, Boston), 1984,63.

4 Hoffmann M R &Jacob D J,S02' NO and N02 oxidation mechanisms: atmospheric considerations, edited byJG Cal- vert(Butterworth, Boston), 1984, p 101.

5 Chang S G,Brodzinsky R, Toosi R,Markowitz S S&Nova- kov T,Proceedings of the conference oncarbonaceous parti-

764

des inthe atmosphere, Berke y, 1981, P 122; Chang SG, ToosiR&Novakov T,Atoms nviron, 15(1981) 1287.

6 Brodzinsky R, Chang S G, M kowitz S S&No~akov T,J phys Chern,84 (1980) 3254.

7 Cohen S,Chang S G,Marko S S&Novakov T,Environ SciTechnol; 15(1981) 1498.

8 Hong A P, Bahnemann D & Hoffmann M R, J phys Chern,91(1987) 6245.

9 Faust BC&Hoffmann MR,E vironSci Techno!,20(1986) 943; Faust B C,Hoffmann M &Bahnemann D W,J phys Chern,93(1989) 6371.

10 Frank S N&Bard AJ,1 phys

11 Leland J K&Bard AJ, J phys hem;91 (1987) 5076.

12 Rani A,Prasad D S N,Bhargav R&Gupta K S,Bull Chem Soc Japan,accepted (1991)..

13 Rani A,Prasad DS N, Madnaw tP V S&Gupta K S,Atmos Environ, accepted (1991).

14 Hoather R C&Goodeve C F,J hys Chern,30(1934) 1149.

15 Deister U&Wameck P,1phys hern,94(1990) 2191.

16 Her R K, The chemistry of sill a(John Wiley, New York) 1979,p41.

17 Laidler K J, Chemical kinetics ( arper &Row, New York), 1987,p248.

18 Schindler W,inAdsorption ofin rganicsat solid liquid inter- faces, edited by M A Anderson dA JRubin (Ann Arbor Science, Ann Arbor), 1981, pp -49.

19 Huang C P,Adsorption of ino anics at solid liquid inter- faces, edited by M A Anderso & AJRubin (Ann Arbor

Science. Ann Arbor), 1981,pp 83-218.

20 Elder AL&Spinger R A,J phy Chern.44 (1940) 943.

21 Stumm W, Kummert R & Si R, Croat Chem Acta, 53 (1980)291-312.

22 Kent S A,Katzer J R&Mano W H,Ind Eng Chem Fun- darn,16 (1977) 443.