Free-radical-induced oxidation of hydroxymethanesulfonate in aqueous solution Part 2A pulse and steady-state radiolysis study of oxygenated solutions

Abstract

At pH 4.0, the radical ˙CH(OH)SO₃⁻ formed by reaction of ˙OH with hydroxymethanesulfonate (HMS) reacts rapidly with oxygen to form the peroxyl radical, ˙OOCH(OH)SO₃⁻, with k₄ = (2.6 ± 0.3) × 10⁹ d mol⁻¹ s⁻¹. This radical decays by a first-order process with k = 2.4 × 10⁴ s⁻¹ and E_a = (29 ± 1.2) kJ mol⁻¹. The decay is by a split pathway, reactions (6) and (7), with k₆ = 1.7 × 10⁴ s⁻¹ and k₇ = 7.0×10³ s⁻¹. Reaction (6) involves direct elimination of HO₂˙. Reaction (7) initially forms the formylperoxyl radical, but this is hydrolysed, reaction (8), to formic acid and HO₂˙ with k₈ = 2.46 × 10³ s⁻¹.

˙CH(OH)SO₃⁻ + O₂ → ˙OOCH(OH)SO₃⁻ (4)

˙OOCH(OH)SO₃⁻ → HO₂˙ + CHOSO₃⁻ (6)

˙OOCH(OH)SO₃⁻ → ˙OOCHO + HSO₃⁻ (7)

˙OOCHO + H₂O → ˙OOCH(OH)₂ → HCOOH + HO₂˙ (8)

The yield of formate is compatible with this mechanism if it is assumed that the product of reaction (6), i.e. CHOSO₃⁻, can also yield formic acid. Yields of SO₄²⁻ have been determined and their origin is considered. At pH 7.0, the radical ˙CH(O^-)SO₃⁻ reacts with oxygen to yield O₂⁻ with k₁₁ = (1.6 ± 0.1) × 10⁹ d mol⁻¹ s⁻¹.

˙CH(O⁻)SO₃⁻ + O₂ → CHOSO₃⁻ + O₂˙⁻(11)