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

Pulse and steady state radiolysis techniques have been used to determine th e bimolecular rate constants and to investigate the spectral nature of the intermediates and the degradation induced by hydroxyl radicals ((OH)-O-.) w ith 1,3,5-triazine (T), 2,4,6-trimethoxy-1,3,5-triazine (TMT), and 2,4-diox ohexahydro-1,3,5-triazine (DHT) in aqueous medium. A competitive kinetic me thod with KSCN as the (OH)-O-. scavenger was used to determine the rate con stants for the reaction of (OH)-O-. with T, TMT, and DHT. The bimolecular r ate constants are 3.4 x 10(9), 2.06 x 10(8), and 1.61 x 10(9) dm(3) mol(-1) s(-1) respectively, for T, TMT, and DHT at pH similar to 6. The transient, absorption spectra obtained from the reaction of (OH)-O-. with T, TMT, and DHT have single absorption maxima at 320, 300, and 300 nm, respectively, a nd 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 rad ical] is proposed from the initial attack of (OH)-O-. with T, TMT, and DHT, respectively. A complete degradation of TMT (10(-3) mol dm(-3)) was obtain ed after absorbed doses of 5 kGy in N2O-saturated solutions and 16 kGy in a erated solutions. A similar degradation pattern was obtained with DHT in N2 O-saturated solutions. Complete degradation was observed with an absorbed d ose of 7 kGy. On the basis of the results from both pulse and steady state radiolysis, a possible reaction mechanism involving (OH)-O-.-mediated oxida tive 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(II I)-DHT complex and subsequent formation of DHT. are proposed to be the majo r processes that lead to the complete degradation of DHT at pH 3.