Ionic Strength Effect in Fenton Reactions in the Presence of 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) 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 = 2 M, increasing at higher I values. Fe(II) oxidation by dissolved O₂ increased to 282 ± 46 M^–1 s^–1 at 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 = 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 k(T) = (8.7 ± 0.8) × 10⁷ × exp[(−4276 ± 520 K)/T] and the reaction in the presence of 1.2 M Na₂SO₄, k(T) = (7.5 ± 0.2) × 10⁹ × exp[(−5153 ± 310 K)/T]. Despite the suppressive effects of I on radical generation, the formation of FeSO₄ enhances the relevance of Fenton chemistry, highlighting its amplified role in sulfate-rich aerosol environments.