Evaluation of the main active species involved in the TiO2 photocatalytic degradation of ametryn herbicide and its by-products

Abstract

In this study, we investigated the effectiveness of photocatalysis using TiO2 as catalyst on the removal of ametryn. The evaluation of photocatalytic activity under simulated sunlight was discussed as evidence by numerous controlled trials and several operational parameters such as ametryn concentration, total organic carbon, chemical oxygen demand, specific UV absorbance, biochemical oxygen demand, toxicity and formed intermediates. Moreover, the roles of reactive species involved in the degradation of ametryn were examined by using different specific scavengers. Ametryn removed by photocatalysis using 0.4 g L−1 of TiO2 was 100% within 60 min, while only 30% was achieved by photolysis at the same time. Biodegradability index improved from 0.3 (raw solution) up to 0.8 while the acute toxicity measured by the inhibition percentage of bioluminescence from Vibro fischeri indicates that the photocatalytic treatment promotes 97% of toxicity reduction. The scavenger study shows different percentages of inhibition in ametryn degradation, which allowed to conclude that HO¿, valence-band holes and O2¿− could intervene in the degradation of ametryn, with predominance of HO¿. Thirty-eight intermediates were identified from the photocatalytic degradation of ametryn. The comparison of the generation of those intermediates with and without the addition of scavengers showed that different by-products are generated depending on the predominance of the active species. For example, the presence of azide resulted in intermediates formed by condensation reactions. Based on the identified intermediates, reaction pathways and a degradation mechanism were proposed, including HO¿ radicals, O2¿−, holes, and 1O2.