Successful Scission of a Recalcitrant Triazinic Ring. The Photoassisted Total Breakup of Cyanuric Acid in Ozonized TiO₂ Aqueous Dispersions in the Presence of an Electron Acceptor (H₂O₂)

The oxidative breakup of the highly recalcitrant cyanuric acid ring was examined in UV-irradiated oxygen- and ozone-saturated TiO 2 suspensions at pH 12.0 and natural pH 5.6. The breakup and mineralization of cyanuric acid were assayed by total organic carbon (TOC) analyses (loss of TOC in solution being equivalent to CO 2 formation); intermediates and other final products were determined by HPLC ion chromatography (IC). Final reaction products were thus CO 2 gas and NO 3 − and NH 4 + ions. The quantity of the ring nitrogens converted to nitrate and ammonium ions in solution quantified the degree of mineralization of the substrate as did the level of TOC loss. Complete mineralization of this recalcitrant substrate occurred in the presence of H 2 O 2 (optimal concentration, 0.016 M) in alkaline media under UV-irradiation that normally cannot be degraded by the typical TiO 2 -photoassisted procedures even in alkaline media. In particular, the enhanced effect of O 3 /H 2 O 2 on the breakup of the triazinic ring and its ultimate mineralization was of greater significance than any adsorption of the substrate under alkaline conditions. Possible orientations of cyanuric acid toward the TiO 2 particle surface have been inferred from theoretical calculations of point charges, whereas the positions of possible attack of the substrate by surface-bound or free OH • radicals have been inferred from frontier electron density calculations.