Nanosecond transient absorption studies of the pH-dependent hydrated electron quenching by HSO3−
The large standard reduction potential of an aqueous solvated electron (eaq−, 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 eaq− 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 eaq−. The observed bimolecular quenching rate constant (k = 1.2 × 108 M−1 s−1) for eaq− deactivation by HSO3− is found to be consistent with a Brønsted acid catalysis mechanism resulting in formation of H˙ and SO32–. A large solvent isotope effect is observed from the lifetimes of the eaq− in H2O compared to D2O (kH2O/kD2O = 4.4). In addition, the bimolecular rate constant for eaq− 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 eaq− and, by extension, that knowledge of the pKa of eaq− sources can be a predictive measure of the effective pH range for the treatment of wastewater contaminants.