WATER-TREATMENT - DEGRADATION OF DIMETHOXYBENZENES BY THE TITANIUM DIOXIDE-UV COMBINATION

Three dimethoxybenzenes (DMBs) have been chosen to further assess the capabilities of the TiO2-UV system to destroy substituted aromatic pol lutants in water. At lambda > 290 nm and pH = 3, the disappearance of DMB (0.145 mM) is much more rapid in the presence of TiO2 (Degussa P-2 5; 2.5 g/L) than by direct photolysis; the intermediates generated by both processes are different. At lambda > 340 nm, which avoids any sig nificant contribution of direct photolysis, the order of disappearance is meta > para > ortho. Possible origins for this order are discussed . The main aromatic intermediates correspond to the hydroxylation of t he ring with or without the elimination of one or both methoxy groups, and to the para and ortho orientations. From 1,4-DMB, para-benzoquino ne is also formed. The temporal variations of 2,3-dimethoxyphenol (2,3 -DMP), 3,4-DMP, 2-methoxyphenol (2-MP) and 3-MP have been recorded. At their maximum concentrations, these intermediates correspond to only a few percents of degraded DMB and they disappear within the same time range as DMB. Control experiments confirmed that the three identified degradation intermediates of 1,2-DMB (i.e., 2,3-DMP, 2-MP and 3,4-DMP ) disappear with similar rates under identical conditions. In addition , 1,2-DMB, 3,4-DMP and 2-MP produce a common intermediate tentatively identified as 1,3-dihydroxy-4-methoxybenzene or 1,4-dihydroxy-3-methox ybenzene, which denotes the extension of the aromatic ring hydroxylati on. In the course of the degradation, the carbon-balance cannot be est ablished as a result of the instability of the intermediates and analy sis difficulties, but the final quantity of CO2 evolved demonstrates t hat total mineralization can be reached. This study substantiates the efficiency of the TiO2-UV method for the dearomatization of water poll utants via progressive hydroxylation.