Nanosecond transient absorption studies of the pH-dependent hydrated electron quenching by HSO3−

Abstract

The large standard reduction potential of an aqueous solvated electron (e_aq⁻, E° = −2.9 V) makes it an attractive candidate for reductive treatment of wastewater contaminants. Using transient absorption spectroscopy, the nanosecond to microsecond dynamics of e_aq⁻ generated from 10 mM solutions of Na₂SO₃ at pH 4 to 11 in H₂O and D₂O are characterized, resulting in the determination that between pH 4 and 9 it is the HSO₃^–, and not H⁺ as previously postulated by others, that effectively quenches e_aq⁻. The observed bimolecular quenching rate constant (k = 1.2 × 10⁸ M⁻¹ s⁻¹) for e_aq⁻ deactivation by HSO₃⁻ is found to be consistent with a Brønsted acid catalysis mechanism resulting in formation of H˙ and SO₃^2–. A large solvent isotope effect is observed from the lifetimes of the e_aq⁻ in H₂O compared to D₂O (k_H2O/k_D2O = 4.4). In addition, the bimolecular rate constant for e_aq⁻ deactivation by DSO₃⁻ (k = 2.7 × 10⁷ M⁻¹ s⁻¹) is found to be an order of magnitude lower than by HSO₃⁻. These results highlight the role of acids, such as HSO₃^–, in competition with organic contaminant targets for e_aq⁻ and, by extension, that knowledge of the pK_a of e_aq⁻ sources can be a predictive measure of the effective pH range for the treatment of wastewater contaminants.