The influence of hydroxyl volatile organic compounds on the oxidation of aqueous sulfur dioxide by oxygen.

Although the effect of volatile organic compounds (VOCs) on the oxidation of dissolved sulfur dioxide by oxygen has been the subject of many investigations, this is the first study which examines the effect of a large number of precisely 16 hydroxy compounds. The kinetics both in the absence and the presence of VOCs was defined by rate laws (A and B): -d[S(IV)]dt = R₀ = k₀[S(IV)] (A) -d[S(IV)]dt = R(i) = k(i)[S(IV)] (B) where R₀ and k₀ are the initial rate and first-order rate constant, respectively, in the absence of VOCs, R(i), and k(i) are the initial rate and the first-order rate constant, respectively, in the presence of VOCs, and [S(IV)] is the concentration of dissolved sulfur dioxide, sulfur(IV). The nature of the dependence of k(i) on the concentration of inhibitor, [Inh], was defined by Eq. (C). [k(i) = k₀/(1 + B[Inh]) (C) where B is an empirical inhibition parameter. The values of B have been determined from the plots of 1/k(i) versus [Inh]. Among aliphatic and aromatic hydroxy compounds studied, t-butyl alcohol and pinacol were without any inhibition effect due to the absence of secondary or tertiary hydrogen. The values of inhibition parameter, B, were related to k(inh), the rate constant for the reaction of SO₄(-) radical with the inhibitor, by Eq. (D). B = (9 ± 2) x 10⁻⁴ x k(inh) (D) Equation (D) may be used to calculate the values of either of B or k(inh) provided that the other is known. The extent of inhibition depends on the value of the composite term, B[Inh]. However, in accordance with Eq. (C), the extent of inhibition would be sizeable and measurable when B[Inh] > 0.1 and oxidation of S(IV) would be almost completely stopped when B[Inh] ≥ 10. B[Inh] value can be used as a guide whether the reaction step: SO4 (-) + organics → SO₄(2-) + non-chain products: should be included in the multiphase models or not.