Photocatalytic removal of 2,4-dichlorophenoxyacetic acid by using sol-gel synthesized nanocrystalline and commercial TiO2: Operational parameters optimization and toxicity studies

Abstract

A comparative study of the photoefficiency of two different TiO2 catalysts in the elimination, mineralization and detoxification of waters containing herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) and toxic intermediates was performed at laboratory scale. Commercial TiO2 (Degussa (Evonik) P25) and TiO2 synthesized by citrate sol–gel method (ECT-1023t) were selected as photocatalysts. Adsorption studies, kinetic analysis and an analysis of the effect of adding oxidizing agents (H2O2 and S2O82−) were carried out for both catalysts. The toxic effect of photocatalytic treatment at different reaction times was determined by marine bacteria Vibrio fischeri bioassay. The toxic effect on this organism of the main degradation intermediate 2,4-dichlorophenol (2,4-DCP) was higher than for the initial herbicide. The optimal operational variables to eliminate the herbicide and toxic intermediates were established for both catalysts. The most effective removal of 2,4-D and toxic intermediate 2,4-DCP was achieved using ECT-1023t as catalyst at pH 3 and pH 5. The inhibitory effect on V. fischeri growth in water containing 2,4-D after 2 h of photocatalytic treatment was negligible when using ECT-1023t as catalyst. Longer times were necessary to obtain similar results when using P25 as catalyst. The addition of H2O2 significantly enhanced the degradation and mineralization rate, with different optimal H2O2 concentrations for the tested catalysts. When using H2O2 as oxidizing agent, toxicity on V. fischeri was eliminated before 1 h of photodegradation treatment with both photocatalysts.