Photoassisted oxidation of the recalcitrant cyanuric acid substrate in aqueous ZnO suspensions

The photoassisted oxidative transformation of the highly recalcitrant cyanuric acid was examined in the liquid-solid regime at pH 3 and natural pH 7.5 under UV irradiation in oxygen- and ozone-saturated ZnO suspensions using ZnO as a photomediator. Loss of cyanuric acid was assayed by HPLC UV absorption spectroscopy, whereas intermediates and final products were determined by HPLC ion chromatography (IC), by MS techniques using an electrospray ionization mass spectrometer operated in the negative-ion mode, and by FT-IR spectroscopy. Final reaction products were CO2 gas and NO3 ions. The measure of nitrate in solution quantified the degree of mineralization of the substrate; its concentration in solution was lower than indicated by the loss of cyanuric acid owing to adsorption of the latter to the ZnO surface. Some degree of corrosion of ZnO occurred as evidenced by the quantity of Zn2+ present in acidic and near-neutral aqueous media before and during the photoassisted process. The possible site(s) and mode(s) of adsorption of cyanuric acid on ZnO have been inferred from theoretical calculations of point charges; the position(s) of attack of the substrate by (OH)-O-center dot radicals was inferred from frontier electron density calculations. Mechanistic considerations are based on the possible involvement of Zn2+-cyanurate species (chemisorbed cyanuric acid) in the degradation of such a recalcitrant substrate, inasmuch as the use of TiO2-photoassisted processes under similar conditions have so far failed to oxidize cyanuric acid.