Photocatalytic transformation of sixteen substituted phenylurea herbicides in aqueous semiconductor suspensions: Intermediates and degradation pathways

Abstract

The photocatalytic degradation of sixteen substituted phenylurea herbicides (PUHs) in pure water has been studied using zinc oxide (ZnO) and titanium dioxide (TiO2) as photocatalyst under artificial light irradiation. Photocatalytic experiments showed that the addition of these chalcogenide oxides in tandem with the oxidant (Na2S2O8) strongly enhances the degradation rate of these compounds in comparison with those carried out with ZnO and TiO2 alone and photolytic tests. Comparison of catalysts showed that ZnO is the most efficient for the removal of such herbicides in optimal conditions and at constant volumetric rate of photon absorption in the photoreactor. Thus, the complete disappearance of all the studied compounds was achieved after 20min of illumination in the ZnO/Na2S2O8 system. The main photocatalytic intermediates detected during the degradation of PUHs were identified. The probable photodegradation pathways were proposed and discussed.The main steps involved: N-demethylation of the N,N-dimethylurea-substituted compounds followed of N-demethylation and N-demethoxylation of the N-methoxy-N-methyl-substituted ureas and hydroxylation of aromatic rings and their aliphatic side-chains of both, parent compounds and intermediates.