Modelling the isothermal degradation kinetics of metrafenone and mepanipyrim in a grape juice analog
Graphical abstract
Introduction
The benzophenone fungicide metrafenone, (3-bromo-6-methoxy-2-methylphenyl) (2,3,4-trimethoxy-6-methylphenyl)methanone, and the anilinopyrimidine mepanipyrim, N-(4-methyl-6-prop-1-ynylpyrimidin-2-yl)aniline, are two new generation systemic fungicides widely used to control fungal diseases of vine. Their most salient physico–chemical properties are shown in Table 1. Fungicides applied to vine have been shown to persist in grapes, and to be transferred to grape juice and wine as a result (Barba, Oliva, & Payá, 2010; Briz-Cid, Figueiredo-González, Rial-Otero, Cancho-Grande, & Simal-Gándara, 2014; Cabras et al., 1999; Cabras & Angioni, 2000; González-Rodríguez, Cancho-Grande, & Simal-Gándara, 2009; Noguerol-Pato, González-Rodríguez, González-Barreiro, Cancho-Grande, & Simal-Gándara, 2011). When their levels in juice or wine exceed the maximum residue limits (MRLs) established by European legislation, the quality of these products cannot be guaranteed. The European Regulation 396/2005 and its later modifications, including the Commission Regulation (EU), 2015, Commission Regulation (EU), 2016, regulated MRLs for metrafenone and mepanipyrim, respectively.
Fungicides may accumulate and be metabolized through interaction with various environmental and food matrices. For example, photochemical processes play a major role in their dissipation (Anfossi, Sales, & Vanni, 2006). A sound knowledge of the mechanisms behind the degradation of fungicides is therefore essential to understand their persistence and assess the associated risks (Abbate, Borzì, Baglieri, & Gennari, 2009), and such knowledge can only be acquired with further research. The degradation of specific fungicides such as MTF and MEP has been scarcely studied. Calza, Medana, Baiocchi, Branca, and Pelizzetti (2004) examined the photocatalytic degradation of MEP in aqueous solutions containing TiO2. More recently, Anfossi et al. (2006) studied the aqueous degradation of three anilinopyrimidine fungicides (cyprodinil, pyrimethanil and mepanipyrim) photoinduced by iron (III)–polycarboxylate complexes and identified some of the resulting photoproducts. Several studies have focused on the degradation of other anilinopyrimidine fungicides in water (Agüera, Almansa, Tejedor, & Fernández-Alba, 2000; Araña, Rodríguez, Melián, Díaz, & Peña, 2008; Vanni & Fontana, 2003). Regarding MTF, Cilla García (2013) examined its hydrolytic and photolytic degradation in different aqueous solutions and proposed various degradation pathways.
Most of the kinetic studies on pesticide degradation have modelled the disappearance of the parent compound and the formation of reaction products of toxicological concern separately (Abbate et al., 2009; Agüera et al., 2000; Anfossi et al., 2006; Araña et al., 2008; Avetta et al., 2014; Calza, Massolino, & Pelizzetti, 2008; Ruiz Suárez, Geissen, Jarquín Sánchez, Castro Chan, & Bello-Mendoza, 2013). However, other compounds formed during degradation may remain in the matrix and interact with its components. Therefore, establishing plausible degradation pathways for pesticides entails identifying all reaction products involved. While this issue has been briefly explored (Calza et al., 2004; Calza et al., 2008; Vanni & Fontana, 2003), no kinetic coefficients for the transformations have to best of our knowledge been reported.
The main aims of this work were to examine the degradation of MTF and MEP in synthetic grape juice by using three different modelling strategies and to predict the associated kinetic coefficients of degradation. Simulating the process under controlled conditions as regards solar excitation wavelength, pH and temperature allowed the half-lives of the fungicides to be estimated and the kinetic model best fitting the degradation behaviour of each fungicide to be identified.
Section snippets
Chemicals and solvents
Metrafenone (purity 99.9%) and mepanipyrim (purity 99.4%) were purchased from Fluka (Steinheim, Germany), and water and acetonitrile for liquid chromatography from Sigma–Aldrich (Steinheim, Germany).
Kinetic experiments
Kinetic experiments were performed in triplicate in Pyrex bottles containing 250 mL of synthetic grape juice. Solutions were prepared by dissolving glucose (100 g L−1) and fructose (100 g L−1) in water and adjusting the pH to 3.5 with tartaric acid. One bottle was spiked with MEP (4 μg mL−1) and the
Pesticide degradation
Using HPLC grade water to prepare the synthetic media made the potential contribution of biological mechanisms to pesticide degradation negligible. Therefore, the degradation of MEP and MTF was exclusively due to hydrolytic and photolytic processes.
The disappearance of MEP and MTF, and the formation of their reaction products, were accurately monitored by measuring the area under the HPLC-DAD peaks for synthetic grape juice solutions irradiated for variable lengths of time. Metabolite peaks
Conclusions
The photodegradation of MTF and MEP in synthetic grape juice was studied and five reaction products were positively identified. Several kinetic models to explain the photodegradation of MTF and MEP were developed. Based on the results, modelling the disappearance of the parent compound does not suffice to obtain the best fit of the degradation behaviour of the pesticides. In fact, the DT50 values thus calculated were roughly once-half those determined with provision for the degradation pathway.
Acknowledgements
This work was supported by EU FEDER funds and Xunta de Galicia [grant number EM2013/004]. O. López-Fernández is grateful for award of a pre-doctoral fellowship by the University of Vigo.
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