IonicStrength Effect in Fenton Reactions in the Presenceof Sulfate and Its Influence on the Aqueous Particle Phase

Fenton chemistry [Fe(II) + H₂O₂] is a critical source of hydroxyl radicals (^•OH) in deliquescent aerosols, yet its efficiency is significantly modulated by the high ionic strength (<i>I</i>) of these systems. This study investigates the influence of elevated sulfate concentrations on Fenton reactions under aerosol-relevant conditions. The isolated effect of ionic strength was first studied by using NaClO₄ as a model electrolyte. Results reveal that the Fenton reaction rate constant decreases sharply to 16 ± 1 M^–1 s^–1 until <i>I</i> = 2 M, increasing at higher <i>I</i> values. Fe(II) oxidation by dissolved O₂ increased to 282 ± 46 M^–1 s^–1 at <i>I</i> = 8 M, although dissolved O₂ concentrations are expected to decrease due to salting-out effects on oxygen. In Na₂SO₄ solutions, Fe speciation shifts to FeSO₄, resulting in enhanced rate constants, reaching 239 ± 21 M^–1 s^–1 at <i>I</i> = 2.7 M. Interestingly, Fe(II) oxidation by O₂ is negligible in Na₂SO₄ solutions. The final OH yield remained unchanged in FeSO₄ solutions and decreased markedly in NaClO₄ solutions. Arrhenius expressions were derived for the Fenton reaction in 3.6 M NaClO₄ <i>k</i>(<i>T</i>) = (8.7 ± 0.8) × 10⁷ × exp[(−4276 ± 520 K)/<i>T</i>] and the reaction in the presence of 1.2 M Na₂SO₄, <i>k</i>(<i>T</i>) = (7.5 ± 0.2) × 10⁹ × exp[(−5153 ± 310 K)/<i>T</i>]. Despite the suppressive effects of <i>I</i> on radical generation, the formation of FeSO₄ enhances the relevance of Fenton chemistry, highlighting its amplified role in sulfate-rich aerosol environments.