Photocatalytic degradation of substituted phenylurea herbicides in aqueous semiconductor suspensions exposed to solar energy

The photocatalyzed degradation of the biocides chlorotoluron, diuron, fluometuron, isoproturon and linuron (substituted phenylurea herbicides) was investigated in aqueous suspensions of ZnO, TiO2, WO3, SnO2 and ZnS at pilot plant scale under natural sunlight. Comparison of the five catalysts showed that ZnO is the most effective for catalyzing the removal of all the compounds studied. The primary degradation of the herbicides followed a pseudo-first order kinetics. In our conditions, the time required for 90% degradation ranged from 23 to 47 min for isoproturon and linuron, respectively, when using the tandem ZnO/Na2S2O8. Eight transformation products were identified by HPLC–MS2 during the experiments, although at the end of the photoperiod (240 min), their concentrations were below detection limits. Based on derivative identification, the proposed metabolic pathways would involve N-demethylation and N-demethoxylation of the N-methoxy-N-methyl substituted ureas and N-demethylation of the N,N-dimethylurea-substituted compounds.

► We have study the photocatalytic oxidation of five phenhylurea herbicides in water.
► The study was designed at pilot plant scale under solar irradiation.
► ZnO and TiO2 strongly enhance photodegradation in comparison with WO3, SnO2, and ZnS.
► Half lives ranged from 7–14 min by use of the tandem ZnO/Na2S2O8.
► N-demethylation/demethoxylation is proposed as main degradation pathway.