Photocatalytic destruction of hazardous chlorine‐ or nitrogen‐containing aromatics in water

Abstract

To more precisely assess the destruction of organic micropollu‐tants in water by UV‐illuminated TiO₂ in the presence of oxygen (air), we have studied the degradation of chlorophenols (CPs), 2,4‐dichlorophenoxyethanoic acid (2,4‐D), benzamide (BZA) and nitrobenzene (NBz) as typical monoaromatic pollutants containing chlorine or nitrogen atoms. The beneficial effect of TiO₂ (Degussa P‐25) was demonstrated by comparison with direct photolysis in various UV spectral regions. Efficiencies of the TiO₂‐UV, H₂O₂‐UV (>220 nm) and H₂O₂‐Fe²⁺ systems for the elimination of BZA have been compared. The effect of the radiant flux on the elimination of monochlorophenols indicated that the recombination rate of photoproduced charges easily predominates over the pollutant oxidation rate. This leads to apparent quantum yields of the order of 10^‐2 at 365 nm for a flux of photons/s potentially ab‐sorbable by TiO₂ of ca. 5 × 10¹⁶ and an initial concentration of 0.155 mM at pH 4.5. Common anions do not produce redhibitory decreases in efficiency at anionic concentrations below 0.1 M. The influence depends on the surface charge of TiO₂ (i.e., on pH), the [ion]/[pollutant] ratio and the nature of the anion (i.e., Cl^‐ and SO² ₄ ^‐ can be detrimental, NOJ₃ ^‐ is not, and OH^‐ has a positive effect). The apparent rate constant of disappearance of various CPs has been related to the Hammett constant and the octanol/water partition coefficient of these compounds. The degradation pathways correspond to the substitution of OH groups at sites on the aromatic nucleus that have been determined. Paraquinones were also identified. The temporal variations showed that the aromatic intermediates generally disappear within the same time period as the original compound, so that maximum concentrations of intermediates are very low. The aliphatic chain of 2,4‐D is relatively easily split and intermediately oxidized to formate. The photocatalytic method is an efficient way to dechlorinate CPs. The final oxidation state of nitrogen is nitrate which is formed rapidly enough from NBz, but quite slowly from BZA, for‐mamide and aniline. Total mineralization is achieved in all cases. These results provide a more precise idea of the advantages and drawbacks of the method.