Temperature effects on the kinetics of carbonate radical reactions in near-critical and supercritical water

Pulse radiolysis was used to generate the carbonate radical (CO(3)(-.)) in subcritical and supercritical water. The effect of the charge of the reaction partner on the kinetics of the reaction with aqueous carbonate radical was examined over the temperature range 30-400 degrees C, at 4150 +/- 75 psia, pH 10.55. The rate constant for oxide transfer between two CO(3)(-.) radicals was insensitive to temperature below 200 degrees C, but increased by a factor of 300 from 1.61 x 10(7) to 2.90 x 10(9) M(-1) s(-1) between 200 and 400 degrees C. The rate constants for the one-electron oxidation of aniline, p-aminobenzoate, N,N-dimethylaniline, and p-N,N-dimethylaminobenzoate by CO(3)(-.) under the same conditions all displayed a negative temperature dependence at temperatures below 320 degrees C. Above 320 degrees C, the rate constants for the oxidation of the anionic aminobenzoates by CO(3)(-.) increased sharply, whereas those for the oxidations of the corresponding neutral anilines did not increase until 360 degrees C.