Photocatalytic transformation of sulfonylurea herbicides over irradiated titanium dioxide particles

The heterogeneous photocatalytic transformation of two sulfonylurea herbicides (chlorsulfuron and thifensulfuron methyl) has been investigated in aerated aqueous suspensions of TiO2 particles at pH 7, under UV A irradiation. The evolution of intermediate and end products of the process has been monitored by using liquid and ion chromatography, as well as total organic carbon measurements. Key intermediates have been identified by employing liquid chromatography coupled with mass spectrometry and/or matching with real standards. Under photocatalytic conditions the (hetero)aromatic moieties of the herbicides are destroyed very rapidly. This evidence is confirmed by the fast degradation of 4-chlorobenzenesulfonamide under the same conditions. The s-triazine ring is almost quantitatively transformed to cyanuric acid. By contrast, hydrolytic processes occurring in the natural environment lead to the aromatic sulfonamides as persistent intermediates. A mechanism explaining the experimental results is presented. (C) 1999 Elsevier Science B.V. All rights reserved.