Water treatment: Degradation of dimethoxybenzenes by the titanium dioxide‐UV combination

Abstract

Three dimethoxybenzenes (DMBs) have been chosen to further assess the capabilities of the TiO₂‐UV system to destroy substituted aromatic pollutants in water. At λ > 290 nm and pH = 3, the disappearance of DMB (0.145 mM) is much more rapid in the presence of TiO₂ (Degussa P‐25; 2.5 g/L) than by direct photolysis; the intermediates generated by both processes are different. At λ > 340 nm, which avoids any significant 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 the ring with or without the elimination of one or both methoxy groups, and to the para and ortho orientations. From 1, 4‐DMB, para‐benzoquinone 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‐methoxybenzene, which denotes the extension of the aromatic ring hydroxylation. In the course of the degradation, the carbon‐balance cannot be established as a result of the instability of the intermediates and analysis difficulties, but the final quantity of CO₂ evolved demonstrates that total mineralization can be reached. This study substantiates the efficiency of the TiO₂‐UV method for the dearomatization of water pollutants via progressive hydroxylation.

Notes

Current address: Chemistry Department, Concordia University, Montreal H3G 1M8 Canada.

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