Photocatalytical removal of bentazon using commercial and sol-gel synthesized nanocrystalline TiO2: Operational parameters optimization and toxicity studies

Abstract

In this work we compared the photocatalytic activity of sol–gel synthesized nanocrystalline TiO2 material (ECT-1023t) and a commercial TiO2 (Degussa P25) in the elimination, mineralization and detoxification of waters contaminated by the herbicide bentazon and its toxic intermediates under UV light. Adsorption and kinetics studies were undertaken and the effect of adding two different co-oxidants (H2O2 and S2O8-2) was analyzed for both photocatalysts. The optimal basic operating parameters (pH, photocatalyst load, initial concentration of bentazon) to eliminate the herbicide and its toxic intermediates were established for both photocatalysts. The most efficient TiO2 for removal of bentazon and its toxic intermediates was ECT-1023t at pH = 7. The apparent initial rate constant of bentazon degradation was two times higher for ECT-1023t than for P25. A Langmuir–Hinshelwood kinetic model showed satisfactory bentazon degradation of up to 0.25 mM for P25 and up to 0.05 mM for ECT-1023t. From FTIR studies, it seems that the interaction of bentazon with both photocatalysts occurred through the SO2 group. The most efficient photocatalyst for detoxification of treated solution was ECT-1023t, using the marine bacteria Vibrio fischeri as the test organism. When using an initial bentazon concentration of 0.265 mM, the evolution of toxicity saw a 72% reduction in bioluminescence inhibition for ECT-1023t and only a 33% reduction for P25 after 2 h of irradiation. No inhibitory growth effect of the herbicide bentazon and its photoproducts was observed for either photocatalyst in any of the irradiated samples collected at predetermined times when Lemna minor was used as the test organism. In parallel, the three photoproducts formed in the earlier steps of bentazon degradation were identified by LC–MS and a comparison on the evolution of these compounds by using both photocatalysts was performed. The results showed that the intermediates formed by hydroxylation of aromatic ring are in highest concentration when ECT-1023t is used. On the contrary, the hydroxylation of isopropyl group in bentazon is the main pathway of bentazon photodegradation when P25 is used as catalyst.