Oxidative degradation of triazine derivatives in aqueous medium: a radiation and photochemical study

Abstract

Pulse and steady state radiolysis techniques have been used to determine the bimolecular rate constants and to investigate the spectral nature of the intermediates and the degradation induced by hydroxyl radicals (^•OH) with 1,3,5-triazine (T), 2,4,6-trimethoxy-1,3,5-triazine (TMT), and 2,4-dioxohexahydro-1,3,5-triazine (DHT) in aqueous medium. A competitive kinetic method with KSCN as the ^•OH scavenger was used to determine the rate constants for the reaction of ^•OH with T, TMT, and DHT. The bimolecular rate constants are 3.4 × 10⁹, 2.06 × 10⁸, and 1.61 × 10⁹ dm³ mol^-1 s^-1 respectively, for T, TMT, and DHT at pH ∼6. The transient absorption spectra obtained from the reaction of ^•OH with T, TMT, and DHT have single absorption maxima at 320, 300, and 300 nm, respectively, and were found to undergo a second-order decay. The formation of TOH^• [C(6)OH-N(5)-yl radical], TMTOH^• [N(5)OH-C(6)-yl radical], and DHT^• [C(6)-yl radical] is proposed from the initial attack of ^•OH with T, TMT, and DHT, respectively. A complete degradation of TMT (10^-3 mol dm^-3) was obtained after absorbed doses of 5 kGy in N₂O-saturated solutions and 16 kGy in aerated solutions. A similar degradation pattern was obtained with DHT in N₂O-saturated solutions. Complete degradation was observed with an absorbed dose of 7 kGy. On the basis of the results from both pulse and steady state radiolysis, a possible reaction mechanism involving ^•OH-mediated oxidative degradation is proposed. A complete photodecomposition of DHT was also observed in the presence of ferric perchlorate using ultraviolet light at low pH. Photoinduced electron transfer between Fe(III) and DHT in the Fe(III)-DHT complex and subsequent formation of DHT^• are proposed to be the major processes that lead to the complete degradation of DHT at pH 3.