Reaction of SO4.- with Fe2+, Mn2+ and Cu2+ in aqueous solution

The oxidation of Fe2+, Mn2+ and Cu2+ to the corresponding trivalent ions by SO4.- has been studied in aqueous solution at pH 3-5 using pulse radiolysis to generate SO4.-. For Fe2+ the reaction has a negative energy of activation of -(18 +/- 2) kJ mol-1 at low ionic strength, and kobs shows a very small dependence on ionic strength, indicating that a precursor complex, ((FeSO4.-)-S-II)(+), is kinetically significant. The stability constant, K-a, of the complex is estimated to be 5.3 dm(3) mol(-1) at 298 K. The observed rate is first order in [Fe2+] and the overall bimolecular rate constant, at ca. 20 degrees C and an ionic strength of 0.06 mol dm(-3), is (4.6 +/- 0.2) x 10(9) dm(3) mol(-1)s(-1). By applying the steady-state approximation to ((FeSO4.-)-S-II)(+), a value of 1.1 x 10(9) s(-1) is obtained for the rate constant for the electron-transfer step. Reaction of SO4.- with Mn2+ is also first order in [Mn2+] with a bimolecular rate constant of 1.4 x 10(7) dm(3) mol(-1)s(-1), at an ionic strength of 0.065 mol dm(-3) at 20 degrees C, and an activation energy of (34 +/- 2) kJ mol(-1). The rate constant for electron transfer is obtained as 2.6 x 10(6) dm(3) mol(-1)s(-1). For Mn2+, like Fe2+, k(obs) shows a small ionic strength dependence consistent with that expected for the formation of an outer-sphere ion-pair complex. Treatment of the data according to the classical Marcus theory for electron transfer yields Delta H degrees = -277 kJ mol(-1) and -54 kJ mol(-1) for the reaction of SO4.- with Fe2+ and Mn2+, respectively. For Cu2+, the rate of decay of SO4.- was independent of [Cu-II] and was largely accounted for by its reaction with (BuOH)-O-t which was present to scavenge . OH. No rate constant for the oxidation step could be determined; that some oxidation did occur is deduced from spectral changes assigned to the formation of a Cu-III species.