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

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 Na 2 SO 3 at pH 4 to 11 in H 2 O and D 2 O are characterized, resulting in the determination that between pH 4 and 9 it is the HSO 3 - , and not H + as previously postulated by others, that effectively quenches e aq − . The observed bimolecular quenching rate constant ( k = 1.2 × 10 8 M −1 s −1 ) for e aq − deactivation by HSO 3 − is found to be consistent with a Brønsted acid catalysis mechanism resulting in formation of H&z.rad; and SO 3 2- . A large solvent isotope effect is observed from the lifetimes of the e aq − in H 2 O compared to D 2 O ( k H 2 O / k D 2 O = 4.4). In addition, the bimolecular rate constant for e aq − deactivation by DSO 3 − ( k = 2.7 × 10 7 M −1 s −1 ) is found to be an order of magnitude lower than by HSO 3 − . These results highlight the role of acids, such as HSO 3 - , in competition with organic contaminant targets for e aq − and, by extension, that knowledge of the p K a of e aq − sources can be a predictive measure of the effective pH range for the treatment of wastewater contaminants. Hydrated electron lifetimes in photodetached sulfite solutions at intermediate pH (5-10) are limited by HSO 3 - rather than proton (H + ) quenching.