Oxidation of s-triazines by advances oxidation processes. By-products and kinetic rate constants
In this paper, oxidation studies of s-triazines in aqueous solution by advanced oxidation processes (O3, O3/H2O2, O3/UV, H2O2/UV, et TiO2/UV) have been reviewed. Oxidation by-products of atrazine: Several investigators have shown that N-dealkylated (deethylatrazine and deisopropylatrazine) and acetamido-s-triazines are the primary oxidation by-products of atrazine by O3 and by O3/H2O2 (table 1; fig. 1a). Under conditions which favored the production of hydroxyl radicals (03/H2O2), trace amounts of hydroxyatrazine may also be formed. These primary by-products are subsequently degraded to give complete N-dealkylated, deamined, dehalogenated and hydroxylated s-triazines (deethyldeisopropylatrazine, ammelide ammeline, cyanuric acid,...) (table 1). For example, oxidation of deethylatrazine by O3/H2O2 yields deethyldeisopropylatrazine as the major by-product (fig. 1b). Identical by-products are produced by photochemical oxidation (O3/UV, H2O2/UV and TiO2/UV) (table 2; fig. 4 and 5). UV photolysis of atrazine at 253.7 nm (monochromatic radiation) yields hydroxyatrazine as the major product (=0.95 -1.0 mole of hydroxyatrazine formed / mole of atrazine photolysed (fig. 4a) whereas N and N,N'-dealkylated, deaminated and hydroxyderivatives are produced by UV irradiation in the presence of ozone, hydrogen peroxide (fig. 4b) or photosensitisers. The s-triazine ring is found to be resistant to chemical and photochemical oxidation. Pathways for the degradation of atrazine by molecular ozone (fig. 2) and by hydroxyl radical (fg. 3) are proposed. Kinetic rate constants: The second-order kinetic rate constants for the reaction of molecular ozone and of hydroxyl radical with atrazine have been determined by several authors from competitive experiments or from kinetic models The rate constants for the reaction of ozone which have been measured (~ 61 mol-1 s-1 at ~ 20°C) indicate that molecular ozone is not very reactive towards atrazine. The rate constants which have been determined for the reaction of hydroxyl radical with atrazine by using different modes of generation of hydroxyl radicals (O3 + OH-; O3 + H2O2; PhotoFenton; H2O2 + UV) are in the order of 2 10[exp]9 - 2.5 10[exp]9 l mol-1 s-1 at ~ 20°C (table 3). Rate constants for the reaction of hydroxyl radical with other s-triazines have been determined from competitive kinetic experiments. The relative rate constants show that methylthio s-triazines are far more reactive than methoxy s-triazines, which in turn are more reactive than chloro and hydroxy s-triazines (table 4). The kinetic data also confirm that deethyldeisopropylatrazine and cyanuric acid are very refractory to the oxidation by hydroxyl radical.