J. Phys. Chem. 1995, 99, 3120-3127
3120
Kinetic Study of SOs- and HO2 Radicals Reactivity in Aqueous Phase Bisulfite Oxidation Aleksandr N. Yermakov,' Boris M. Zhitomirsky, Grigoriy A. Poskrebyshev, and Stanislav I. Stoliarov Institute of Energy Problems of Chemical Physics, Russian Academy of Sciences, 117829, Moscow, Russia Received: January 7, 1994; In Final Form: April 20, 1994@
A continuous radiolysis has been used for the kinetic study of the radical driven bisulfite oxidation in oxygenated and N20-02 saturated solutions. The initiation rates and initial bisulfite concentrations were varied over the M, respectively. For oxygenated and N20-02 ranges (3 x 10-1°)-(3 x lo-,) M s-l and 10-,-(3 x saturated solutions the long chain length bisulfite oxidation with the rate determining propagation reaction SO5- HSO3- ( k )was observed at low initiation rates. The measurement of the peroxydisulfate formation rates has shown that the main t e d n a t i o n step in an N20-02 saturated solution is the reaction S05- SOSS20s2- 0 2 ( k 7 b ) and in oxygenated solution is the reaction S05H02 (klo). The rate constants ks = (3.6 f 0.5) x lo3 and k10 = (5 f 1) x lo7 M-' have been derived. The upper limit of the initiation reaction between of hydroperoxyl radical and bisulfite H02 HS03- has been estimated as 20 M-' s-l,
+
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+
+
+
+
Introduction
water:
The free-radical mechanism of bisulfite autoxidation in acid solutions is important for an improved understanding of the acidification of atmospheric droplets'-8 by the emission of SO2 for an improved understanding of the chemistry of flue-gas scrubber's chemical processes9 and so on. For the first time the chain autoxidation of sulfite ion has been observed by Backstrom.Io Later the direct observations of oxysulfur radicals, so3-, s04-, and SOS-, involved in the chain mechanism of S(IV) oxidation, have been reported."-I6 It was found that the rate constants of the propagation reactions of so,- with 0 2 and SO,- with HS03- are very The peroxymonosulfate radical is considered as a chain carrier determining the oxidation rate of bisulfite ion, because the propagation reaction so5HS03- is S ~ O W . ~ , Therefore, '~ the rate of chain oxidation of bisulfite critically depends on an absolute values of the rate constants of SO5- radical termination reactions. The high concentration of hydroperoxyl radicals in the atmosphere and thereof high solubility require the determination of the termination reaction (SO5HOz) rate constant. In particular, the predominant role of the termination reaction SO50 2 (H02) has been demonstrated for sulfite photo~xidation'~ and for bisulfite oxidation under the action of ionizing radiation.I8 However, the rate constants of these reactions are unknown. The hydroperoxyl radicals may also play a role in the chain initiation by the reaction with bisulfite, H02 HSO,-, but the available data are scanty and di~crepant.'~-*~ Since the considered reactions of the oxysulfur and hydroperoxyl radicals are of such importance in the autoxidation of S(IV), we have determined the rate constants of the reactions: propagation reaction of SO5- radicals with HSO3-, termination reaction SO5HO2, and initiation reaction of HO2 radicals with HSO3-. Both continuous electron beam irradiation and y-irradiation of the bisulfite solutions have been employed for radical production and a kinetic study of bisulfite oxidation.
H,O
-
ea¶-, H, OH
For the fast electron irradiation and y-(Co60) irradiation of the dilute solutions the primary radiation yields23 (in species per 100 eV of absorbed energy) are g(eaq-) = 2.6, g(H) = 0.6, and g(0H) = 2.8. In bisulfite solutions with oxygen saturation these species are rapidly converted to the peroxymonosulfate and hydroperoxylhperoxide radicals:
OH
+ HS03-
-
SO,-
+H20
(R1)
+
+
eaq-
+
H
+ 0,H + 0, H 0 2 HO, = H+ + 0,- pK = 4.8 ea¶- 0,
+
+
-
+ H+
--+
(R3)
(R4) (R4a)
The available rate constants of reactions Rl-R4 and other reactions involved in the chain oxidation of bisulfite are summarized in Table 1. Under the conditions of neglected initiation reaction HO2 HSO3-, the G-values of the radicals are as follows: G(S05-) = g ( 0 H ) = 2.8; G(HO2) = g(e) g(H) = 3.2. In N20-02 (4:l) saturated bisulfite solution the reactions R3 and R3a are suppressed by reaction R5:
+
eaq-
+
+ N 2 0 - N, + OH- + OH
(R5)
In this case G-values of the radicals are as follows: G(S05-) = g(e,¶-) g(0H) = 5.4; G(HO2) = g(H) = 0.6. The chain propagation reactions areI3-l5
+
Background
The peroxymonosulfate and hydroperoxyl radicals were generated in aqueous solution under action of ionizing radiation. The following primary species are produced in the irradiated @Abstractpublished in Advance ACS Abstracts, September 15, 1994.
0022-365419512099-3120$09.0010
SO5-
SO,-
+ HS0,-
+ HS0,-
-
+ SO,+ SO,- + Hf
HS0,-
-SO:-
0 1995 American Chemical Society
(R6a) (R6b)
Kinetic Study of SOS- and HO2 Radicals
J. Phys. Chem., Vol. 99, No. 10, 1995 3121
TABLE 1: Rate Constants of the Reactions of the Bisulfite Oxidation Mechanism k, M-'
reaction R1 R2 R3 R3a R4 R5 R6 RI R8 R9 RlO R11 R12 R13
ref
s-I
4.5 x 109 1.1 x 109 2.3 x 1Olo 2.0 x 1010 2 x 1010 9.1 109 3.6 x 103 1 x 108 1.5 x lo* (5.6 x 106)[H+] 5.5 107 8.3 105 9.1 x 107 5 20
15 13 24 25 25 24 this work 13 14 26 this work 25 25 this work
+ SO,- -SO4- + SO4- + 0, so,- + HSO,- - so:- + SO,- + H+ SO,-
HO,
(R7a) (~8)
In accordance with the peroxymonosulfate is assumed to react predominantly with bisulfite:
HS0,-
+ HS03- + H+ - 2HS04- + H+
(R9)
Since the chain propagation reactions R2 and R8 are very fast, the values of bisulfite oxidation chain length (Y)depend on the competition of the chain propagation and the chain termination reactions involving the so,- radical^.^ The main termination steps is oxygenated and in N20-02 saturated solutions are
+ HO, - H20, i0, HO, + 0, (+ H,O) - H,O, + 0, + OHHO,
(R11) (R12)
From a previously detenninedl8 peroxydisulfate G-value in oxygenated bisulfite solution (G(S20g2-) % 0.2
