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

The large standard reduction potential of an aqueous solvated electron (e(aq)(-), E degrees = -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 x 10(8) M-1 s(-1)) for e(aq)(-) deactivation by HSO3- is found to be consistent with a Bronsted acid catalysis mechanism resulting in formation of H-center dot and SO32-. A large solvent isotope effect is observed from the lifetimes of the e(aq)(-) in H2O compared to D2O (kH(2)O/kD(2)O = 4.4). In addition, the bimolecular rate constant for e(aq)(-) deactivation by DSO3- (k = 2.7 x 10(7) 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.