Oxidation of aqueous sulfur dioxide by peroxymonosulfate

Abstract

Recent model calculations suggest that peroxymonosulfate may constitute a significant fraction of the total sulfur budget in remote tropospheric water droplets such as cloud, fog, and rain. However, little is known about the oxidation of dissolved SO_2 by peroxymonosulfate (HSO_5^-). We have found in aqueous solution that the rate of S(IV) oxidation is comparable to the rate of oxidation of S(IV) by hydrogen peroxide and that HSO_4^- is the only detectable oxidation product. We propose a mechanism in which the rate-determining step involves the acid-catalyzed decomposition of a peroxide-bisulfite intermediate to disulfate ion, S2_7O^(2-), and ultimately to sulfuric acid. The rate equation for this mechanism is -d[HSO_3^-]/dr = k_1(k_2/k_(-1))K_(a1){H^+}[HSO_5^-][S(IV+)]/(1 + (k_2/k_(-1){H^+})(k_(al) + {H+))), where k_1 = 1.21 x 10^6 M^(-1) s^(-1), k_2/k_(-1) = 5.9 M^(-1), k_1k_2/k^(-1) = 7.14 x 10^6 M^(-2) s^(-1), K_(a1) = 2.64 x 10^(-2) M at 5 ºC, and µ = 0.2 M. The activation parameters are ΔH_(k1) = 25.74 ± 0.77 kJ mol^(-1) and ΔS_(k1)^*= -88.1 ± 2.7 J mol^(-1) K^(-1).