Study of the degradation of the herbicide Isoxaflutole by different methods: Photo-Fenton and sensitized photolysis

EDUARDO GATICA; JOSE NATERA; SANDRA MISKOSKI; EDUARDO DE GERONIMO; MASSAD WALTER; GARCIA NORMAN

Carlos Paz

Congreso; XIII ELAFOT; 2017

Isoxaflutole (IXF, Figure 1) is a preemergent herbicide developed in relatively recent times. It is employed for the control of broadleaf weeds and weeds in maize. IXF exhibits a new mode of action: inhibits 4-hydroxyphenylpyruvate dioxygenase, an enzyme found in the pathway of carotenoid biosynthesis. IXF has a very short lifetime (8 to 18 days) and rapidly degrades into a stable and phytotoxic metabolite, diketonitrile (DKN). Further degradation of DKN produces a compound derived from benzoic acid (BA, 2-mesyl-4-trifluoromethylbenzoic acid) [1]. Although several studies have been carried out on the hydrolysis processes of IXF in solution and adsorbed on soils, no information has been found regarding possible photo-induced degradative processes. The main objective of this work is to study IXF degradation by Fenton and FotoFenton from a kinetic-mechanistic point of view and to establish a comparison with results of the IXF photodegradation sensitized by Riboflavin (Rf).Fenton and PhotoFenton: IXF showed reactivity with the ●OH generated by FotoFenton in homogeneous medium, reaching conversions of almost 100%, with a pseudo-first order rate constant of 0.015 min-1. During photolysis, photoproducts were detected by HPLC-UV-vis and the possible structures thereof were determined by HPLC-MS. One of the products is the benzoic acid derivative BA (see Figure 2) [2], a compound with low toxicity, which is which is currently produced in the degradation of IXF with NaClO [2]. The advantage of using the FotoFenton technique is the absence of chlorinated photoproducts. Fig. 2: Photoproduct (BA)Sensitized photodegradation: experiments of Rf-sensitized stationary photolysis (Rf) in the presence of IXFs, monitored by UV-Vis spectrophotometry, showed slight spectral changes, attributable to IXF degradation. These could be due to interactions of the IXF with the triplet excited state of Rf (3Rf*) or reactive O2 species (ROS) generated by Rf. Complementary experiences of laser flash photolysis, IR-close resolved phosphorescence and polarly-evaluated O2 consumption, demonstrated that IXF deactivates electronically the excited triplet state of Rf with a rate constant of 1.46x107 M-1s-1 and IXF is not degraded neither by singlet molecular oxygen (O2(1g)) nor by superoxide radical ion (O2●). The photo-fenton process appears as the better option for IXF degradation due to the greater reactivity of ●OH compared to ROS generated in the Rf photosensitized process.