Indian Journal of Chemistry Vol. 38A. November 1999. pp. 1129-1 138

Role of manganese dioxide in the oxidation of aqueous sulphur(IV) in oxic and anoxic suspensions

Krishna S Gupta*, Rajeshwar Singh, Deepa Saxena, Manoj S V & Madhu Sharma Atmospheric Chemistry Laboratory, Department of Chemistry

University of Rajasthan, Jaipur 302 004, India

Received 11 October 1998; revised 3 August 1999

The kinetics of direct oxidation of sulphur(IV) by Mn02 has been studied in stirred suspensions in nitrogen atmosphere and that of autoxidation reaction in stirred suspensions in equilibrium with atmospheric oxygen. Sulphate has been detected to be the only oxidation product of sulphur(IV) in both the cases. In the pH range 4.6-5.8, the kinetics of direct oxidation of sulphur(IV) in anoxic suspesions agreed with the rate law (I). On the other hand at pH 7.9 for the same reaction a much simpler rate law (II) is obeyed.

-d[S(IV»)/dt = {(A[W]"I+B)[Mn02)o[S(IV»)o}/ { I +C[S(IV»)o[W)·I} ... (1)

.. . (11) -d[S(IV»)/dt = {k3K1[Mn02MS(IV)]o}/{ I +K][S(lV»)o}

At 30°C, the values of empirical constants are: A = 4.8xIO·1O mol g.1 S·I; B = 2.8xlO·5 L gIS·I; C = 1.1 x 10.3 ; k) = 9.3 x 1O.7 s· land K) = 34. For Mn02 catalysed autoxidation in oxic suspensions the rate data pertaining to pH range 4.6-5.8 fitted Eq. (III) and those

-d[S(lV»)/dt = k5[Mn02MS(IV)]o[W)-014

-d[S(lV»)/dt = ks[Mn02MS(IV»)JW)-04 ... (III)

... (IV)

in the pH range 6.5-7.0 fitted Eq. (IV). At 30°C. k5 and ks have values of 1.4 x 10-4 UM g-I mol-O·34 S·I and 3.2 U4 gl mol-04

S·I. The rate of autoxidation is independent of oxygen partial pressure. The atmospheric conversion rates at 30°C have been calculated.

Recent work has shown the metal oxides to contribute significantly to the acidification of rain waterl-4 • From the view point of atmospheric chemistry, the oxides of iron, manganese and titanium are considered to be the most important as they are always present in airborne suspended particulate matter and in solid effluents like flyash. While Fep3 and Ti02 do not show any catalytic activity in the thermal autoxidation of aqueous S02' Mn02 is known to oxidise sulphur(lV) into sulphate. In suspended particulate matter, the combustion processes are likely to be the major source of Mn02 as such processes are known to release the metals in their highest oxidation states. In natural aqueous systems5-7, the formation of manganese(IV) oxides occurs mostly via the oxidation of manganese(IJ) and subsequent precipitation of oxides from solution, oversaturated with manganese(lII,IV) . Manganese(III) and manganese(IV) so formed are present as sparingly soluble oxides and hydroxides, whereas manganese(IJ) is in the soluble phase. In soil and sediment

environments, manganese(IV) oxide is among the strongest oxidising agents that may be encountered in the absence of molecular oxygen7.

Both manganese(IJ) and manganese(IV)8.9 are im-portant in atmospheric chemistry of pollutants in general and sulphur dioxide in particular3. The role of manganese(IJ) as catalyst in the autoxidation of aqueous S02 has been the subject of several studies3; such studies with Mn02 as catalyst are scarce: From a stoichiometric view point, the oxidation of sulphite by Mn02 has been studied by Bassett and Parker9. Halperin and Taube 1o, who carried out a isotopic tracer study on this heterogeneous solid-liquid system, suggested the binding of the sulphite ion at the surface of Mn02 through the oxygen atom. Recently the Mn02 catalysed autoxidation of sulphur(IV) in buffered and unbuffered solutions has been reported II. Recognising the importance of Mn02 in aiding and abetting the atmospheric acid precipitation through its involvement in the oxidation of aqueous SO/hereafter referred to

1130 INDIAN J CHEM, SEC. A, NOVEMBER 1999

as sulphur(IV)), either as an oxidant or as a catalyst, we have chosen to study the oxidation of sulphur(IV) in the multiphasic systems in both oxic and anoxic suspen-sions. The other point of interest in MnO -S(IV) chemistry was the fact that the adsorption of SC\ from flue gases by activated manganese dioxide is of interest in the field of S02 emission controp2.

The chemistry of manganese oxides is important in other areas of environmental concern als05•7 . In view of the presence of chromium in natural aqueous systems, the oxidation of Cr(lll) by Mn02 has been studied by several workers I3- 16 • The reductive dissolution of manganese(lV) oxide by organic compounds, which increases the mobility of manganese(ll) and its avail-ability to organisms, has been visualised to be an important abiotic degradative pathway for organic compounds in subsurface environment l4 • Stone and Morgan l7•IH have investigated the reductive dissolution reaction between Mn02 and substituted phenols. In a recent study, Laha and Luthy l9 have investigated the oxidation of aniline and other primary aromatic amines by a-Mn02 and their kinetics results based on initial rates fitted the experimental rate law (I).

d[Mn(II)]/dt = k[Mn02][Organic] ... (1 )

Materials and Methods The experimental procedure was essentially the

same as described earlier4. The reactions were con-ducted in Erlenmeyer flasks and in room light. The rate measurements were made in both acetate and phosphate buffered suspensions. For studying direct oxidation of sulphur(IV) by Mn02' the reagent solutions were deoxygenated prior to their mixing by passing nitrogen, and thereafter nitrogen was continuously passed in the reaction mixture, which was continuously stirred magnetically to keep Mn02 afloat. For autoxidation study, the reaction mixture was continuously stirred magnetically at a speed of 1600±100 rpm to save the reaction from becoming oxygen mass transfer con-trolled. For acetate buffered study, the desired pH was obtained by using 10 cm3 of buffer prepared from 1 M acetic acid and 1M sodium acetate for a total 100 cm3

volume of the reaction suspension. Similarly for studies in phosphate buffered media,IM NaH2P04 and 1M Na

2HP0

4..buffer solutions were used. The kinetics

were followed by analysing the aliquots for unreacted sulphur(IV) iodimetrically in slightly acidic medium, using starch as an indicator and sodium thiosulphate as

Table I-Stoichiometry of Mn02 catalysed sulphur(lV) autoxidation in air saturated suspensions at pH 5.8 and 30°C

1O'[MnO,Ju I(P[S(IV)Ju 1O'[MnO,l. 1O'[MnO,J, IO'[SO,'-J [SO.'-JI [MnO,t

mol L-' mol L- ' mol L-' mol L- ' mol VI

2.38 2.0 1.68 0.70 1.9 2.7 2.38 5.0 1.20 1.18 4.8 4.1 2.38 10.0 1.12 1.26 10.2 8.1 1.19 7.5 0.60 0.59 7.2 12.2 2.38 7.5 1.72 0.65 7.2 11.1 4.60 7.5 2.50 2.10 7.3 3.5

titrant. The kinetic results were reproducible within ±1O%. All computations were done using Curve Fitter and Scientific Plotter Programmes of Interactive Micro-wave Inc., USA.

Stoichiometry and product analysis

Mn02 - sulphur(IV) reaction in anoxic suspensions -The deoxygenated Mn02 suspensions and deoxygen-ated sulphite solutions were mixed and the reaction vessel sealed immediately to avoid the leakage of oxygen. After several hours, when the reaction was complete, the unconsumed MnO was determined iodometrically and sulphate, whi~h was the only oxidation product of sulphur(IV), was estimated gravi-metrically. In both acetate and phosphate buffered anoxic suspensions, the stoichiometry was in agreement with Eq.(2).

Mn02 + S(IV) -7 Mn2+ + S(VI) ... (2)

Mn02 - sulphur(IV) reaction in oxic suspensions - For determining the stoichiometry of MnO - sulphur(IV) .

• • 2 reaction In the presence of atmospheric oxygen, the duplicate reaction mixtures having different amounts of Mn02 and S(IV) were prepared. These were continu-ously stirred and as soon as the autoxidation of sulphur(IV) was complete, in the first set of experi-ments, unconsumed Mn02 was determined iodometrically. The values of initial, unconsumed and consumed manganese dioxide concentrations [MnO] , 2 0' [Mn021u and [Mn02t respectively are given in Table 1. In the second set of reaction mixtures, the unconsumed Mn02 was removed by filteration and the amount of sulphate formed was determined gravimetrically (Tablet).

)

..

GUPTA et at.: ROLE OF MANGANESE DIOXIDE IN THE OXIDATION OF S(lY) 1131

8

W ~ 6 " .... ......

> =, II) ....

M

52 )( 2 N 0 0

Fig.1 - Reaction profiles for Mn02 - S(lY) reaction in nitrogen atmosphere at different pH and at 30"C.

[Mn02l0= 1.0 g L·I, (~)[S(IY)lo = 4.0 x 10.3 mol L·1, pH = 7.90,

(e)[S(lY)lo = 2.0 x 10-3 mol L-1, pH = 5.60, (0) [S(IY)lo = 1.5 x 10-3 mol L-1, pH = 5.21

6

5

Fig.2 - The variation of R,,1>f, with [S(lY)l in acid and alkaline media in nitrogen atmosphere at 30"C.

(.A.) [Mn02l0= 1.2 g L-1, pH = 5.21, (~) [Mn02l0= 0.7 g L-I,pH = 5.21 ; (e) [Mn02l0= 0.5 g L-1, pH = 5.21 , (0) [Mn02l0= 1.8 g

L-I, pH = 7.90; (0) [Mn02l0= 1.0 g L-1, pH = 7.90

Results

Kinetics of Mn02-S(IV) reaction in anoxic suspensions

Rate law in acidic medium : In deaerated suspensions, the reaction is essentially the direct oxidation of sulphur(IV) by MnOirefs 8,9). In this part of investi-gations, the pH was maintained with the help of acetate

8 0

7 III 0

I' e 15 E 6 ~ III .c 0 0

c2 5 8 "-

~ 0 N 0 ~ 4 0 0 ~ ~

0

~ 3 0 0 0

o 0

o 3 4 5 6 1(12", [S(lV)]~' , L mor'

Fig.3 - The plot of [Mn02l0 / Rohs vs. [S(lY)lo-1 at pH = 5.21 and at 3Q

1132 INDIAN J CHEM, SEC. A, NOVEMBER 1999

Table 2-The values of E and D at different pH and at 30°C for the direct oxidation of sulphur(IV) by Mn02

in nitrogen atmosphere.

pH E D L mol ') 10.4 L g' ) s·)

5.21 112 1.1

5.60 324 2.1

5.90 778 4.1

Table 3- The values of rate/equilibrium constants for the direct oxidation of S(IV) by Mn02 in nitrogen atmosphere at 30°C.

Rate/equilibrium Value constant

A = k)K)KdI2) 4.8 x 10·IO(CD= 0.99, CC = 0.99,SEE =9.6xlO"")

mol g' ) s·)

B = k2K2' 2.8 X 10.5 (CD = 0.99, CC=0.99, SEE=9.6xI0"")

L g')s' )

C = K)Kd(2)

Kd(2 )

1.1 X 10-) (CD = 0.99, CC = 0.99, SEE = 29.4)

5.0 x 10.7 .•

k)K) 9.6 x IQ4

K) 2.2 x I (}1 k), s·) 4.4 X 10.7

< I X 102.h K2 k

j' ) 1.08 X 106 (CD = 0.87, CC = 0.93, SEE = 1.6x 1(6)

k/ K;) 3.2 x 104 (CD = 0.87, CC = 0.93, SEE = 1.6 x 106)

k)KJ

3.1 x 10-5

k" s·) 9.3 X 10.7

K) 34

a, reference 3 : b, estimated.

between Mn02 and S(IV). The kinetics rate data, under different initial reactant concentrations, were in agree-ment with the rate law (3) .

... (3)

Accordingly, from the plot of [Mn0zllRohs vs II [S(IV)]o' (Fig.3), the values of D and E at different pH were obtained (Table 2).

The values of R (Fig.4) and D and E (Table 2) are ohs seen to increase with increase in pH. The dependences of D and E on [H+] were defined by Eqs.(4) and (5), which were utilised for obtaining graphically

D = B + A[W), ) ... (4)

2.5

III

'15 2.0 E 01

.B ci

,·5

"-.-Ii' N

~ 1.0 ~

.... $2 0.5

o

o

, 102 [S(JV)fc;l , L mor1

Fig.5 - The plot of [Mn021o I Ruh, vs. [S(IV)lo' ) at pH = 7.90 and at 30uC for Mn02 - S(IV) reaction in nitrogen atmosphere.

E = qW)') ... (5)

the values of A, Band C. (Table 3). On combining Eqs(3-5), the complete rate law (6) is obtained.

Ruh, = {(A[Wl) + B)[Mn021o[S(IV)lo} I{ 1 + C [S(IV)lo[Wl) ) .. . (6)

From the values of R h at four different temperatures, n s

the overall empirical energy of activation was deter-mined to be 60 kJ mol· l .

Rate law in alkaline medium : For study in alkaline media, pH in the range (6.7-7 .9) was adjusted with phosphate buffer. Contrary to the results pertaining to pH region (5 .21-5.9) in acetate buffered suspensions, in the pH range (6.7-7.9) initially the rate decreased slightly on increasing pH and when it was 7.5 or more, the rate become almost independent of pH (Fig.4). For this reason, we selected pH 7.9 for entire kinetic study. At this pH, the kinetic results fitted rate law (7).

Ruhs = (k) K)[Mn021o [S(IV)lo} I{ 1 + K) [S(IV)]o } ... (7)

The values of k3 and K3 determined from the plot of [Mn021/Rnhs vs 1/[S(IV)]o (Fig.5) are given in Table 3.

MnOz catalysed autoxidation . In the presence of oxygen, Mn02 acts as a catalyst for the autoxidation of sulphur(IV) in addition to direct oxidation of S(IV).

Rate law in acid medium : The reaction profiles at different pH's in acetate buffered suspensions are shown in Fig.6. At pH 5.8 and above the reaction

~

GUPTA et at.: ROLE OF MANGANESE DIOXIDE IN THE OXIDATION OF S(lV) 1133

10 I...J

'0 8 e ~

~ Vi6

.., 0 .... ~4

2

o Time, min

Fig.6 - The reaction profiles for Mn02 catalysed autoxidation in air saturated suspensions at 30"C.

(0) pH = 4.60, [Mn021o= 0.15 g L", [S(IV)lo= 5 x 10.3 mol L" (e) pH = 5.80, [Mn021o= 0.15 g V', [S(IV)lo= 2 x 10'] mol L" (.) pH = 6.50, [Mn021o= 0.10 g L", [S(IV)lo= 2 x 10'] mol L" (~) pH = 7.10, [Mn021o= 0.10 g V', [S(IV)]o= 2 x 10-] mol L-'

profiles are linear showing zero order disappearance in sulphur(1 V). At pH 4.5, the initial portion is almost linear but shows autocatalytic behaviour in the latter

. part of the reaction.Although the reaction shows time order of zero in sulphur(1 V) the concentration order is one. It is well recognised20a that the concentration order and not the time order should be taken as true order. Subsequent treatment of the kinetics data is based on this premise. Similar situations had been reported earlieroa.b.c,d .. In those cases, where the time order has a lower value than the concentration order, the autocatalysis usually occurs20a. The autocatalytic nature probably stems from the well known3 catalytic effect of Mn2+ ions formed as a result of reductive dissolution of Mn0

2• The kinetic results at pH 5.8 and 4.6 both were

in agreement with the rate law (8).

... (8)

The values of k4 were determined to be 4.3 x 10.3 L g'! sol (correlation coefficient, CC = 0.98; coefficient of determination, CD = 0.96; and standard error of estimate, SEE = 2.3 x 10.7) at pH 4.6 and 1.2 x 10-2 L g' ! s·! (CC = 0.99, CD = 0.99, SEE = 1.6 x 10' 7) at pH 5.8 and 30°C (Fig.7). In the pH range (4.6-5.8) the rate increased with increase in pH, and based on determinations of R b at four different pH values, o s kinetic data were in agreement with Eq.(9)

6

5

'III "L

l 4

0

VI 0 -g 3

ex '" 0 - 2

Fig.7 - The plot of Rut.. against [Mn021o x [S(IV)lo at 30'C for Mn02 catalysed autoxidation in air saturated suspensions: (O)pH = 4.6 and (e) pH = 5.8.

... (9)

which is equivalent to rate law (8) through k4 = k

s[H+lo.34.

The value of ks was determined to be 1.4 X 10.4 L 1.34 mol·o.34 g'! s·! at 30oe. The rate of reaction did not depend on [02] as was shown by similarity of rates in air and oxygen-saturated suspensions.

Based on the values of Robs at different temperatures, the empirical energy of activation was 66 kJ mol'!.

Rate law in alkaline medium : The reaction profiles are shown in Fig. 6. The kinetic results pertaining to pH 6.5 fitted the rate law (10) as shown in Fig. 8.

.. . ( 10)

where ko is the rate constant of autoxidation reaction catalysed by trace metal ion present as impurity. It is interesting to point out that in acetate medium, where the pH was low and catalytic rate high, the term corresponding to ko did not appear in the rate law (10), as the contribution of trace metal ion catalysed reaction to the total rate was not sizable. From the plot of Rnhi [S(IV)]o vs [Mn02]o' the values of ko and k6 were determined to be 4.0 x 10-4 s·! and 5.5 x 10.3 L g'! s·! (Ce = 0.90, CD = 0.81, SEE = 2.5 x 10-4) at pH 6.5 and 30oe. Incidentally, an investigation of autoxidation of sulphur(IV) at pH 6.5 in the absence of Mn02 showed the uncatalysed reaction to proceed at a

1134 INDIAN J CHEM, SEC. A, NOVEMBER 1999

3.0 o

2.5 illl

0 .... ~ 2.0 iii .... .....

o 0

o § 0

J 1.5 ~

0

1.0

0.5

o

Fig.8 - Dependence of Ruh, on [MnOzlo at pH = 6.5 and 3()

'~

GUPTA et al.: ROLE OF MANGANESE DIOXIDE IN THE OXIDATION OF S(lV) 1135

range 4.0-4.5(ref.21). Thus in the pH range 4.5-5.9, used for studying kinetics in acetate buffered suspen-sions, it would be largely present as O-MnOH. On the other hand, at pH 7.9 used for study in phosphate buffered suspensions, its dominant form would be 0-Mn-O·. Similarly the speciation of aqueous sulphur(IV), governed by the acid-base equilibria (15-16),

Kd(Z) HS03- ~ sot + W; Kd(Z) = 5.0 X W'7 (ref.3) ... (16)

shows the principal species in acetate and phosphate buff-ered media to be HS0

3' and sot respectively.

Mechanism of direct oxidation of sulphur(IV) The nature and form of the rate law pertaining to

kinetics in anoxic suspensions require the formation of surficial complexes between hydroxylated Mn02 and sulphur(lV) as suggested by Halperin and Taube'o. The formation of such complexes has been visualised in several autoxidation reactions of sulphur(IV), catalysed by a-Fe20 3(ref.21b), CuO(ref.4), CoO(ref.22), MgO(ref.23), Si02(ref.4), Nip3(ref.24)etc. Similarly in the oxidation of both aniline'9 and chromium(III)5, the adsorption of the reductants on Mn02 surface has been envisioned. For writing a detailed mechanism, the nature of the hydroxyl groups present on the particle surface is important. For a-Alp3 Perry25 and for a-Fe20 3 Faust et al. 2\ have visualised the several possible surface group configurations for hydrolysed metal oxide surfaces. Based on this work, some of the structures for hydroxylated . Mn02 may be as follows.

[I-Mn-QH

(I)

OH / o-Mn,

OH (D)

The increase in rate on increasing pH in the region 4.6-5.8 is key to the identification of possible reactive species of Mn02 and S(lV). In this pH region, which is higher than pHzpc ' the principal species of Mn02 would be O-Mn-OH. An increase in pH should favour the formation of negatively charged O-MnO', which is expected to be less reactive than 0- Mn-OH2\,26, as has been noted in phosphate buffered suspensions.

Hence the observed pH dependence cannot be possibly linked to equilibrium(l4). Alternatively, it may arise from the greater reactivity3 of SO/' than HS03'. Based on these arguments, the following mechanism which is similar to one proposed by Faust et al.2lh for the photochemical reductive dissolution of a-Fep3 by sulphur(IV), is proposed.

D-Mn- OH + SO z. ~DMn/OH . " (17) . J ~ " oso/-

D-Mn-QH + HSO)- ~ ~Mn-O-S02- + ~o ., - , (18) 'pH !.a

o-~OSOl- ) Mn(lI) + S(VI) " , (19)

1

D-Mn-Q--S01-!l

~O ~ Mn{lI) + S(VI) .. , (20)

This simple mechanism leads to rate law (21) for the direct oxidation of S(lV).

R"h, = (kIK,Kd(Z) [Wl' + kz Kz][Mn,O~[HS03']}/

(I +K,Kd(Z) [Wl'[S(IV)]o + Kz [S(IV)]o} ... (21)

Based on equilibria( 13-16), the reasonable approxima-tions [Mn02]o =[O-Mn-OH] and[S(IV)]o = [HS03'] modify rate law(21) into(22).

Ro = (k,K,Kd(Z) [Wl' + kz Kz][MnOz]o[S(IV)]o}/

(l+K,Kd(Z) [W]" [S(IV)]o + Kz [S(IV)]o} ... (22)

A comparison of derived rate law (22) and the experimental rate law (6) shows the two to be similar, provided K2[S(IV)]o « [1 + K,Kd(2)[S(IV)]o[H+J'). On neglecting the term K2[S(IV)]o' rate law (22) changes to (23).

R.>h, = (k,K,Kd(2) [W]" + kz Kz][MnOz]o[S(IV)]o}/ (I +K,Kd(Z) [W]"[S(IV)]o } ... (23)

which is equivalent to experimental rate law (6) through A = k,K,Kd(2)' B = k2K2 and C = K,Kd(2)'

There are two possible electron transfer mechanisms for the complementary oxidation of sulphur(IV) by Mn02. The first possibility of oxidation is via a single step of two-electron transfer as in Eqs (19) and (20). The other possible mechanism is through two consecu-tive one-electron transfer steps resulting in the forma-

1136 INDIAN J CHEM, SEC. A, NOVEMBER 1999

tion of S03- and Mn(III) as intermediates. The kinetics is unable to distinguish between these two possible electron transfer mechanisms, but the nature of the oxidation products formed whether sulphate or dithionate is helpful in this regard27 . Before presenting one-electron mechanism it is necessary to consider the chemistry of S03- and Mn(III). S03- radical is a mild oxidant with a one-electron redox potentiaJ28.29 of 0.63v at pH 3.6. This radical appears to be a very poor reductant and according to Huie and Neta29, there is hardly any confirmed example of its oxidation by a one-electron oxidant like Mn(III). In agreement with this, Siskos et apo ruled out the oxidation of S03' by Mn(III) in their study on the oxidation of S(IV) by Mn(III) . The oxidation of S(IV) by Mn(III) has been the subject of some studies3J.3s and it is known to produce S03- radicals. The bimolecular rate constants30 for the oxidation of HS0

3- by

Mn(III) is reported to be >2.4 X 104 L rrol-J s-J. Based on the known chemistry of S03- radical, the following mechanism involving two consecutive one electron transfers may be proposed for the internal redox reaction of surficial complex

D-MnlvOSlv0 2 ~ D-Mn lll -Osv02 .. . (24)

SO)- + SO)- ~ SO 2- + SO ;\ :\

... (25)

SO -) +HP ~ sot +2H+ ... (26)

SO -) + SO-) ~ S20 t ... (27)

Similar mechanisms can be written for other surficial complexes. Recently the kinetics of the decay of sulphite ion radical, in aqueous solution has been studied27. The decay proceeds via two channels, recombination to produce dithionate as in Eq.(27) and electron or O-transfer to produce sot and sot as in Eq (25). The product analysis27 gave a value of 0 .8 ± 0.2 for the ratio k

2jk

2S' These findings of Way good and

McElroy27 will require the oxidation of sulphur(IV) to result in the formation of both sot and S20t in equal yield. In case of oxidation of sulphur(IV) by nickel (IV) oxime imine complexes36, where S03- radical is gener-ated as an intermediate both sulphate and dithionate

. were detected to be the oxidation products of sulphur(IV). In our case, dithionate is not formed and sulphate is the only oxidation product. Hence this observation supports the oxidation through a single step of 2-electron transfer as in Eqs.(19) and (20) . Interestingly in the complementary oxidation of sulphur(IV) by thallium(III) , a similar 2-electron trans-

fer mechanism has been proposed, consistent with the formation of sulphate as the only oxidation product37.

Now, we consider, the kinetic results in anoxic phosphate buffered suspensions in which the rate of reaction decreases with increase in pH. A similar behaviour has been reported in case of manganese(II) catalysed autoxidation earlier38. The observed pH effect is likely to be due to the conversion of D-Mn-OH into D-MnO- (Eq.14). At pH 7.9, the rate of reaction becomes independent of pH probably due to the fact that at this pH, Mn02 is present almost wholly as 0-MnO- and sulphur(IV) as sot. Considering these two species to be reactive, the following mechanism may be proposed.

IV K O-Mn-o- + sot ~_=3 ===.' (28)

0- rv /080;- k) Mn" ---4) products

o' . . . (29)

Equations (28-29) lead to the rate law (30).

-d[S(IV)]ldt = (k)K )[Mn02lo[S(lV)]o}/( I +K)[S(IV)lo) ... (30)

which is based on the approximations : [D-MnO-] = [Mn02]oand [SO,2-] = [S(IV)]()" Rate law (30) is the same as the expermental rate law (7).

The values of equilibrium and rate constants for different reaction steps pertaining to both acid and alkaline media are collected in Table 3. Although the values of k

J and K3 could be determined from kinetic

study, the value of K2 could not be determined, since the factor K2[S(IV)]o was much smaller as compared to the other two terms viz., (1 + KJKd(J)[S(IV)]o [H+lJ), in the rate law(22) . This would be true only if the value of K2 is of the order to 1 x 102 or less. Obviously, HS0

3-

has poor ligation properties as compared to sot. Incidentally, this appears to be the case for several homogeneous complexation reactions of these two anions3. The value of K3 is lower than KJ as the former pertains to the formation of surficial complex between the anion and negatively charged surface2J. In order to compare the reactivity of different surficial complexes, it is pertinent to compare the values of the composite rate constants kJKJ, k2K2 and k,K3' On this basis, the reactivity order is as follows.

..,

GUPTA el al.: ROLE OF MANGANESE DIOXIDE IN THE OXIDATION OF S(IV) 1137

Table 4 - Atmospheric conversion rates for direct oxidation of sulphur(IV) by Mn0

2, and for Mn0

2- -catalysed

autoxidation of sulphur(lV) at 30°C.

pH Conversion Rate % h- i

Direct oxidation

4.60 2.6 x 10-" 5.20 2.3 x 10-5

5.80 2.8 x 10-4 7.90 1.1 x 10-4

4.60 5.20 5.80 6.50 7.00 7.95

Catalytic autoxidation 2.9 X 10-4 1.8 X 10-3

1.1 X 10-2

2.2 X 10-2

1.8 X 10-2

7.6 X 10-3

Autoxidation mechanism in oxic suspensions A perusal of data in Figs( 1 and 6) and Table 1 shows

the rate of catalytic autoxidation to be much faster than the rate of direct oxidation of sulphur(lV) and that only a small amount of MnO is consumed in autoxidation

2

reaction. The amount is insufficient to give the observed yield of the sulphate through direct oxidation of S(IV) as in Eq.(l). Obviously oxygen is being used for oxidising sulphur(IV) into sulphur(VI).

The stoichiometric results (Table 1) indicate that when, at constant [S(IV)]o' the initial concentration of manganese dioxide, [Mn02]o' is increased, there is an increase in the amount of manganese dioxide consumed [Mn02]c' Similarly at constant [Mn02]o' an increase in the amount of [S(IV)]o results in a greater consumption of manganese dioxide. However, the most remarkable point to be noted is the fact that with increase in the ratio [S(IV)] I[Mn0

2]o' the ratio [SO/l/[Mn02]c also

increases. In °other words, when the former is high the contribution of the catalytic autoxidation to the total oxidation rate of S(IV) is also high. These results clearly show that in oxic solutions in addition to direct oxidation of sulphur(IV) by Mn02' the catalytic autoxidation makes a relatively much larger contribu-tion. The possible autoxidation catalysts are Mn02' Mn2+ ions, formed as a result of reducti ve dissolution of the former, and the trace metal ions present as impurity.

In view of these observations, the oxidation of sulphur(IV) in oxic solutions is a combination of atleast four parallel pathways. It is difficult, if not impossible, to disentangle the contribution of each of these. Such a complicated reaction is expected to result in a

complex rate law. Unexpectedly, the kinetics obey simple rate laws (9) and (12). But these cannot form the basis for writing a detailed mechanism of catalytic pathway. However, the observed rate laws for catalytic reaction may be explained as follows. The kinetic order of one in MnO is expected in view of the same order observed in th~ direct oxidation of sulphur(IV) in this study and in other transition metal oxide catalysed autoxidation reactions of sulphur(IV) 1-4. Moreover, the order in manganese(II) is also one for homogeneous manganese(II) catalysed reaction under similar reaction conditions38• The kinetic order in sulphur(IV) for direct oxidation is less than one. For Mn02 catalysed autoxidation 1.2 it is expected to be one. On the contrary, for Mn2+ catalysed pathway it has been recently determined to be 1.4 by Singh38 under similar reaction conditions. A cumulative effect of these observations appear to lead to an order of one in sulphur(IV) for autoxidation reaction. Application to atmospheric chemistry

Using the rate data for MnO,- S(lV) and Mn02-02-

S(IV) systems. the atmospheric conversion rates of S02 have been calculated at 30°C (Table 4) assuming gas phase S02 concentration of 5 ppb, cloud water content of 1 x 10-3 L per meter3 of air and aqueous phase Mn02 concentration of 1 x 10-5 g L-'. The values of first and second dissociation constants3 of S02.H20, Kd( I ) and K and of Henry 's law constant3 ofS02, KH, used in these d(2) . _ _ _I calculations were 1.26 x 10-2 mol L I, 5 x 10 7 mol L and 1.23 mol L-' atm-I respectively.

The data in Table 4 do not follow the trends shown in Figures 4 and 9. This can be explained as follows. The laboratory data are at fixed [S(lV)] in solution, whereas in atmospheric conversion rate calculations the fixed 5 ppb S02 partial pressure has been assumed. Hence as the pH increases, the dissolved S02 would also increase.

Conclusion The laboratory studies reported herein clearly show

that in environmental aqueous systems, whether in aquated aerosol, rain drops, cloud water, fog or in water effluents, manganese dioxide will act both as direct oxidant as well as autoxidation catalyst for aqueous sulphur dioxide, the rate of catalytic oxidation being much faster than the rate of direct oxidation. In real atmosphere, where aqueous sulphur(IV) concentrations are low, the experimental rate law for direct Mn0

2-

S(IV)reactions would be Eg. (31) in acidic pH range,

1138 INDIAN J CHEM, SEC. A, NOVEMBER 1999

and the rate law (32) when pH ~ 7.5.

R"h, = [k1K1Kd(l)[W) ·1 + k2K2) [Mn02)o [S(IV»)o

Rot.. = k3K3[Mn02)o [S(IV»)o

... (31)

... (32)

The rates of Mn02 - catalysed autoxidation reactions would be described by experimental rate law (9) in acidic medium and by rate law (12) in alkaline medium. The results of alkaline medium are very pertinent for the regions of high rain water pH such as western India.

Acknowledgement

The work was supported by Earth System Science Division, D.S.T., and C.S.I.R., New Delhi.

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