Oxidation of CrIII to CrIV initiated by •OH and SO4•- in acidic aqueous solution -: A pulse radiolysis study

The kinetics of the radiation-induced oxidation of Cr-III to Cr-IV by (OH)-O-. and SO4.- in aqueous solution saturated with N2O and argon, respectively, have been studied by pulse radiolysis with optical and conductometric measurements at pH 3 and 3.7. The mechanism of oxidation by both (OH)-O-. and SO4.- is consistent with the rapid formation of a precursor complex, with a stability constant K (dm(3) mol(-1)), followed by electron transfer from the Cr-III to the radical, with a rate constant k (s(-1)). . OH/SO4.- + Cr-III (K) under left right arrow Cr-III(. OH)/SO4.- (k) under right arrow Cr-IV At pH 3 and 3.7, respectively, the values for (OH)-O-. are K = 91 +/- 9 and 59 +/- 3; k = (3.7 +/- 0.1) x 10(6) and (5.8 +/- 0.1) x 10(6), based on the assumption that free and complexed (OH)-O-. react at the same rate with benzoic acid, which was used to measure k((OH)-O-. + Cr-III) by the competition kinetics method; and for SO4.- they are K = (7.6 +/- 1.9) x 10(2) and (4.2 +/- 0.5) x 10(3); k = (2.5 +/- 0.3) x 10(4) and (1.2 +/- 0.1) x 10(4). The absorption spectrum of Cr-IV generated by (OH)-O-. and SO4.- appears as a weak rising band between 420 and 250 nm. The shape and intensity of the band is the same at pH 3 and 3.7 and the molar absorption coefficient increases from 4.3 +/- 0.4 m(2) mol(-1) at 420 nm to 48 +/- 6 m(2) mol(-1) at 250 nm. Conductivity measurements show that proton release follows the oxidation step with a rate constant of (4 +/- 1) x 10(4) s(-1) at pH 3 and (6 +/- 1) x 10(4) s(-1) at pH 3.7; proton release indicates that Cr-IV is more hydrolysed than Cr-III. The final product, Cr-VI, is formed in a second-order process over a period of ca. 0.01 to 0.5 s; a mechanism is proposed in which the rate-determining step is 2Cr(IV) --> Cr-III + Cr-V with k = 4.1 x 10(7) dm(3) mol(-1) s(-1) at pH 3.7, followed by the rearrangement of the coordination shell of Cr-V from octahedral to tetrahedral and reaction between Cr-tet(V) and Cr-oct(IV) in non-rate-determining steps.