So

sovTT

FIGURE 7.A.1 Schematic of reactions in the radical oxidation chain of S(IV) by the OH radical.

TABLE 7.A.8 Reaction Mechanism for the S(IV)-OH Aqueous Phase Reaction

Reaction

k( M"1 s-1) (at 298 K)

Reference

1.

OH + HSOJ SOJ + H20

4.5 x 109

Huie and Neta (1987)

2.

OH + SOf" -> SOJ + OH-

5.2 x 109

Huie and Neta (1987)

3.

SOJ + o2 ->• SOJ

1.5 x 109

Huie and Neta (1987)

4.

SOJ + HSOJ HSOJ + SOJ

2.5 x 104

Huie and Neta (1987)

5.

SOJ + SO^" .M, HSOJ + SOJ + OH"

2.5 x 104

Huie and Neta (1987)

6.

SOJ + HSOJ SOJ + SO4- + H+

7.5 x 104

Huie and Neta (1987)

7.

SOJ + SO2" -> SO4 + so2-

7.5 x 104

Huie and Neta (1987)

8.

SOJ + HSOJ SO^- + SOJ + H+

7.5 x 108

Wine et al. (1989)

9.

SO4 + so^- -» so2- + SOJ

5.5 x 108

Deister and Warneck (1990)

10.

SOJ + S05 ->• 2 SOJ + 02

6 x 108

Huie and Neta (1987)

11.

SOJ + SO3- — S202"

7 x 108

Huie and Neta (1987)

12.

SO4 + SO4 S202"

4.5 x 108

Buxton et al. (1996)

13.

S05 + SOJ s2o¡~ + o2

1.4 x 108

Huie and Neta (1987)

14.

HSOJ + HSOJ + H+ ^ 2 SOj- + 3 H+

7.1 x 106°

Betterton and Hoffmann (1988)

"The rate constant is in M 2 s 1.

During the propagation of the reaction chain (Figure 7.A.1) multiple sulfate ions are created for each attack of OH on S(IV), and as a result the sulfate production rate exceeds the rate of the reactions (7.A.1) and (7.A.2). This effect can be studied by simplifying the mechanism of Table 7.A.8. For pH values lower than 6, [HSOj] » [SO2-] and as their reactions in the mechanism have similar rate constants (Table 7.A.8), we can neglect as a first approximation the SO2- reactions (reactions 2, 5, 7, and 9 in Table 7.A.8). Assuming that the rates of reactions 11 and 12 are also small, the steady-state concentrations of SO J, SO4 , SO j, and HSOJ can be calculated by setting d[S03~[ _ d[SO4 ] _ ¿[SO; ] _ ¿[HSOg dt dt dt dt resulting in the following steady-state expressions:

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