Abstract
The photocatalyzed degradation of the pesticide N'-(3,4-Dichlorophenyl)-N,N-dimethyl-urea (Diuron) has been studied in aqueous suspensions of titanium dioxide by monotoring the change in substrate concentration and the depletion of the Total Organic Carbon (TOC) content, respectively, as a function of irradiation time. The degradation kinetics of the model compound were studied under several different conditions such as pH, catalyst concentration, different types of TiO 2 and in the presence of electron acceptors (H 2 O 2 , KBrO 3 and (NH 4 ) 2 S 2 O 8 ) besides molecular oxygen. The photonic efficiency for the depletion of Diuron itself as well as for the total mineralization is strongly influenced by the reaction pH. The choice of the TiO 2 photoctalyst, i.e., Degussa P25 or Sachtleben Hombikat UV 100, or the variation of the photocatalyst concentration between 0.2 and 2.0 g L -1 had no pronounced effect on the degradation kinetics of Diuron. All additives showed a beneficial effect on the degradation of the model compound. While the addition of hydrogen peroxide or persulfate results only in a slight increase, the addition of bromate ions improved the photonic efficiency by a factor about 3. However, for the total Diuron mineralization only the addition of bromate ions leads to an enhancement while in the presence of the other investigated additives even a slight decrease in the mineralization rate was observed. Toxicity tests (LUMIStox-Test) showed a dramatic increase of bacteria growth inhibition from an initial value of 30% to almost 100 % within the first two hours of irradiation indicating the generation of highly toxic intermediates. The latter are completely degraded upon prolonged irradiation as evidenced by a decrease to almost 0 % inhibition of the bacteria activity.
Published Version
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