Phototransformation pathways of the fungicide dimethomorph ((E,Z) 4-[3-(4-chlorophenyl)-3-(3,4-dimethoxyphenyl)-1-oxo-2-propenyl]morpholine), relevant to sunlit surface waters
Graphical abstract
Introduction
The mixture of isomers, (E,Z) 4-[3-(4-chlorophenyl)-3-(3,4-dimethoxyphenyl)-1-oxo-2-propenyl]morpholine (dimethomorph, hereafter DMM), is a systemic morpholine fungicide that is used on several crops including potatoes, tomatoes, grapes, tobacco and tea. To this purpose, it is employed as such or in combined formulations with other fungicides such as mancozeb (manganese ethylenebis(dithiocarbamate) (polymeric) complex with zinc salt) (Stein and Kirk, 2003) or triazole fungicides (e.g. hexaconazole) (Baby et al., 2004). DMM is an inhibitor of sterol (ergosterol) synthesis and it can be used for the prevention and cure of downy mildews, late blights, crown and root rots (Liu et al., 2012). The effectiveness of DMM as a fungicide was reported for the first time in 1988 (Albert et al., 1988), while its good activity as protectant, curative and antisporulant, especially against downy mildew, was demonstrated by Wicks and Hall (1990).
Despite its beneficial effects in the agricultural field, DMM shows toxicity for several living organisms including soil and water microflora, of which it can alter important biological functions even at very low concentrations (Oliveira et al., 2013). DMM has low toxicity for birds and mammals, but it is slightly to moderately toxic towards fish and invertebrates. Furthermore, DMM has likely synergistic effects with mancozeb and the mixture is highly toxic to freshwater fish and invertebrates (EPA (Environmental Protection Agency), 1998, Lunn, 2007).
DMM has been detected in surface and ground waters, wastewater, sludge, soils and agricultural products used as food. The concentration values in environmental waters range from ng L− 1 levels in rivers, to μg L− 1 levels in wetlands affected by runoff from cultivated soil during precipitation events (EPA (Environmental Protection Agency), 1998, Maillard et al., 2011). The environmental concentration levels pose limited risk to drinking water sources, but they could be of concern for susceptible aquatic organisms (EPA (Environmental Protection Agency), 1998, Lunn, 2007).
The pesticide lifetime has been studied in soil, by monitoring its concentration at 10-cm depth after spraying the soil surface with a DMM formulation. Different half-lives have been observed for the E and the Z isomers (~ 10 and ~ 30 days, respectively), which could be due to a combination of water solubility and degradation (Liu et al., 2012). Fast E → Z photochemical conversion is a potentially important pathway, and it could take place on the soil surface soon after spraying (EPA (Environmental Protection Agency), 1998, Lunn, 2007). Moreover, DMM is quite resistant to hydrolysis (Lunn, 2007). The half-life time of DMM in soil due to aerobic biodegradation is of the order of a couple of months (Lunn, 2007).
In the literature it is possible to find works concerning the photodegradation of DMM by means of advanced oxidation processes (Yan et al., 2005, Calza et al., 2008, Rashidi et al., 2013, Rashidi et al., 2014), but to our knowledge specific researches dealing with the fate of DMM in natural surface waters have not been published yet. Interestingly, it seems that the only studies investigating the photodegradation of DMM in aqueous solution are unpublished works commissioned by manufacturing companies (Van Dijk, 1990, Knoch and Holman, 2007, Panek et al., 2001), sometimes under conditions that are poorly representative of the aqueous environment (e.g. pH 5). The results of these works are only available as very brief citations in the secondary literature (Lunn, 2007), with difficult or no access to the original papers. Moreover, only direct photolysis seems to have been investigated, which is of limited significance to DMM photoattenuation in surface waters (as will be shown in this work). The scarce available information accounts for the importance of studying DMM photodegradation under conditions that could be relevant to surface waters, where DMM is expected to have a non-negligible impact (EPA, 1998).
In surface water bodies, solar radiation promotes photochemical processes that can be responsible for the photodegradation of organic pollutants. The photoinduced processes are divided in direct photolysis, when the interaction of sunlight with a compound causes its transformation, and indirect photoreactions. The latter involve light-absorbing species such as nitrate, nitrite and chromophoric dissolved organic matter (CDOM), which absorb sunlight and produce reactive transients (OH, CO3−, 1O2 and 3CDOM*) that promote the degradation of xenobiotics. Note that xenobiotic degradation is in competition with deactivation processes of the transients, which include reaction with dissolved organic matter (DOM) for OH and CO3−, reaction with O2 for 3CDOM*, and thermal deactivation for both 1O2 and 3CDOM*. Photochemical reactions can be quite important, particularly for the transformation of biorecalcitrant compounds (Boreen et al., 2003, Canonica et al., 2005, Canonica et al., 2006, Fenner et al., 2013, Bonvin et al., 2013, Remucal, 2014, Vione D).
In the present work, we evaluated the phototransformation of DMM in aqueous solution by combining a kinetic study of substrate degradation with a photochemistry model. The recently developed model can predict the photochemical transformation kinetics of organic pollutants in surface waters as a function of photoreactivity data and of key environmental variables, and it has been validated against field data (Maddigapu et al., 2011, Vione et al., 2011, De Laurentiis et al., 2012, Marchetti et al., 2013). The kinetic study was focused on DMM direct photolysis and on its reactivity towards the main transients that can be found in surface waters (OH, CO3−, 1O2 and 3CDOM*). In addition, a survey was carried out of possible DMM phototransformation intermediates via the main photochemical pathways that would be operational in natural waters.
Section snippets
Reagents and materials
Anthraquinone-2-sulphonic acid, sodium salt (AQ2S, 97%), furfuryl alcohol (98%), H2O2 (35%), NaNO3 (> 99%), NaHCO3 (98%), anhydrous Na2SO4 (99%), NaCl (99.5%), Na2HPO4·2H2O (98%), NaH2PO4·H2O (98%), HClO4 (70%) and H3PO4 (85%) were purchased from Aldrich, dimethomorph (DMM, Pestanal analytical standard) was purchased from Fluka, NaOH (99%), propan-2-ol (LiChrosolv gradient grade) and dichloromethane (GC Suprasolv) were purchased from VWR Int., methanol (gradient grade) was purchased from Carlo
Identification of the E and Z isomers of DMM
Under our analytical conditions DMM was eluted as two separate peaks, hereafter labelled DMM13.9 and DMM15.0 based on retention time. Using HPLC elution on C18 material, shorter retention time has been found for the E isomer compared to the Z one (Takino and Sawada, 2010). Additional evidence was obtained upon UV irradiation of aqueous solutions of DMM. Our irradiation experiments showed a rapid decrease of the concentration of DMM13.9 and a corresponding increase of DMM15.0 (see Figure SM1 of
Conclusions
Commercial DMM is a mixture of two isomers (E and Z), which undergo very rapid E → Z interconversion under UV irradiation. The DMM (intended as the sum of the two isomers) also undergoes photodegradation, although with a longer time scale. The modelled photochemical half-life time of DMM would range from about a week to several months in surface-water environments, depending on environmental conditions such as water chemistry, depth and seasonality. In particular, photodegradation would be
Acknowledgements
DV acknowledges financial support from Università di Torino — EU Accelerating Grants, project TO_Call2_2012_0047 (Impact of radiation on the dynamics of dissolved organic matter in aquatic ecosystems — DOMNAMICS). The DOMNAMICS project also supported PA's post-doc fellowship. EDL's PhD grant was financially supported by Fondazione CRT — Progetto Lagrange (Torino, Italy). GM's bursary was supported by Fondazione CRT — Borse Lagrange di Ricerca Applicata. The authors are grateful to Riccardo
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