Photodecomposition and Photooxidation of Hydrogen Sulfite in Aqueous Solution

Abstract

A zinc arc lamp and a mercury lamp, respectively, were used to study the photodecomposition of HSO 3 -and SO 3 2-in aqueous solutions saturated with either argon or nitrous oxide. The main products in both cases were sulfate and dithionate, which are attributed to arise from the self-reaction of SO 3 -radicals. Quantum yields for the formation of SO 3 -in argon-saturated solution based on hydrazoic acid and/or ferric oxalate actinometry were 0.19 ( 0.03 for HSO 3 -and 0.39 ( 0.03 for SO 3 2-, essentially independent of S(IV) concentration. In both systems, the rate of sulfate formation rose with time at the expense of that of dithionate. This is explained by reactions of hydrogen atoms and hydrated electrons with dithionate (rate coefficient k 5 ≈ 2 × 10 5 dm 3 mol -1 s -1 ). N 2 O as a scavenger for these radicals removed the effect and raised the quantum yields to 0.25 ( 0.03 and 0.75 ( 0.04, respectively. The product ratios under these conditions were [S 2 O 6 2-]/ [SO 4 2-] ) 0.43 ( 0.04 for HSO 3 -and 0.61 ( 0.03 for SO 3 2-. In oxygen-saturated solutions, the photolysis of HSO 3 -led to a short chain reaction with sulfate and peroxodisulfate as products. The latter product was assigned to arise from the recombination of SO 5 -radicals. Steady state analysis of the product evolution with time gave rate coefficients for two of the reactions involved: k 16 (SO 5 -+ HSO 3 -) ) (1.2 ( 0.4) × 10 4 dm 3 mol -1 s -1 for the main propagation reaction and k 19a (HO 2 + SO 5 -) ) (1.8 ( 1.0) × 10 9 dm 3 mol -1 s -1 for the principal termination reaction. These values agree well with recent data from radiolysis experiments.

Application of UV–sulfite advanced reduction process to bromate removal

“…Sulfite radical can act either as an oxidizing or reducing agent. be around 0.19 for bisulfite and 0.39 for sulfite [22]. The following equations summarize this radial chemistry.…”

“…In order to explain the overall experimental data and trends observed, free radical chemistry of sulfite and bromate needs to be considered. Irradiation of sulfite solutions with UV light generates sulfite anion radicals (SO 3 −• ) and aqueous electrons (e aq − ) [22]. Aqueous electron is a strong reducing agent with a standard reduction potential of −2.77 V [26].…”

“…Recent studies on sulfite-UV ARPs indicate their potential to degrade chlorinated organics like vinyl chloride, mono-chloro acetic acid and 1,2-DCA [19][20][21]. The success of sulfite-based ARPs has been attributed to generation of reducing free radicals like aqueous electrons (e aq − ) and sulfite anion radicals (SO 3 −• ) from UV activation [22,23].…”

Degradation of Perfluorooctane Sulfonate (PFOS) by Nanofiltration Membrane Enrichment-VUV/Sulfite Reduction

“…Therefore, the observed PFOS degradation in sole VUV system shall attribute to the active species generated from water homolysis or ionization by VUV lamp (eqs 1-2) [10]. When sulfite was introduced in VUV system, the amount of generated active species significantly increased due to sulfite photolysis reactions [17], which accordingly enhanced PFOS degradation. Supplementary experiments were carried out to elucidate the dominate species responsible for PFOS degradation.…”

Reactivity of Inorganic Radicals in Aqueous Solution

“…15,16 Examples of the second type include the reactions of H atoms with NO 2 À , HNO 2 , and NO 3 À . 119 It has been shown that the reaction of H atoms with NO 2 À is a two-step process in which H atoms first add to NO 2 À to produce the relatively long-lived HNO 2 À radical, and then the HNO 2 À cleaves to yield OH À þ NO.…”