Comparison of the heterogeneous photocatalysis of imidacloprid and thiacloprid – reaction mechanism, ecotoxicity, and the effect of matrices

Abstract

This comparative study is about the heterogeneous photocatalytic degradation of two neonicotinoids, namely imidacloprid and thiacloprid, focusing on the differences and similarities in their transformation and mineralization and the effect of various additives, matrices, and their inorganic components. Besides the dominant role of •OH, which was confirmed by the effect of pH and radical scavengers, the direct charge transfer may also contribute to the transformation, especially for imidacloprid; partly due to its enhanced interaction with the TiO2 surface. There was a significant difference in the change of ecotoxicity, which decreased for thiacloprid but varied according to the maximum curve for imidacloprid, and interpreted by the effect of NO3− formed. Dehalogenation and mineralization were fast and occurred in parallel with the neonicotinoid degradation; however, 20–25% organic carbon could not be removed, suggesting the formation of hardly oxidizable products. The fluorination of the TiO2 has no significant effect on the transformation rates, but changed the product distribution, enhanced the dechlorination rate, and hindered the mineralization, confirming •OH-initiated formation of the hardly oxidizable intermediates. The negative effect of tap water and biologically treated domestic wastewater was significant. Although HCO3− caused a slower transformation, the effect of matrices cannot be interpreted solely by the radical scavenging capacity of their organic and inorganic content.