Hydrolysis and biodegradation of sulfonylurea herbicides in aqueous buffers and anaerobic water‐sediment systems: Assessing fate pathways using molecular descriptors

Abstract

The degradation of 12 sulfonylurea herbicides in buffers at different pH levels and in anaerobic sediments was investigated in laboratory studies. Reaction products, which are not commercially available, were synthesized in order to follow degradation kinetics. Different hydrolysis processes were shown to occur under acidic, neutral, and alkaline conditions. At 40°C, pseudo‐first‐order reaction rate constants between pH 4 and 10 covered two orders of magnitude. Activation energies of the acid‐catalyzed hydrolysis were about 100 kJ/mol. Reaction rate‐pH profiles were different for all compounds. In native sediments at neutral pH, microbial degradation was prevalent. At lower pH, chemical hydrolysis became more important. Pseudo‐first‐order dissipation rate constants between 12.50 × 10−2 d−1 and 0.19 × 10−2 d−1 were determined. Quantum‐chemical molecular descriptors were calculated with MOPAC, and steric, electronic, and geometrical descriptors with Molecular Modeling Pro. Quantitative structure‐reactivity relationships were found between bond orders, atomic charges, lowest unoccupied molecular orbitals, and pKa values on one hand and rate constants on the other. Disappearance rates in native sediments could be assessed with a multiple correlation considering calculated octanol water partition coefficients, parachors, and the atomic charges at the main reactive site of the molecules.

Persistence of the Sulfonylurea Herbicides Thifensulfuron‐Methyl, Ethametsulfuron‐Methyl, and Metsulfuron‐Methyl in Farm Dugouts (Ponds)

“…They also reflect recropping intervals recommended for the Canadian prairies (SAFRR, 2003). Similar relative persistence has been reported for sterilized and nonsterilized aquatic sediments (Berger and Wolfe, 1996) and field soil (Berger et al, 1998).…”

Fate of metsulfuron-methyl in soils in relation to pedo-climatic conditions

“…Another degradation pathway of the hydroxy-metsulfuron-methyl is cleavage of the triazine ring, supposed to be an oxidative process with incorporation of three additional atoms of oxygen and loss of an atom of N leading to a stable, less reactive and less water-soluble product.14,21,24 These complementary degradation pathways have been well identiÐed in laboratory abiotic conditions in the absence of soil, and triazine ring cleavage has been detected at low pH as the major hydrolytic pathway for metsulfuronmethyl. 15 However, during incubations of metsulfuronmethyl in soils, the metabolites arising from hydroxy-metsulfuron-methyl were not detected, although three new non-identiÐed metabolites appeared. 19 This diversity of metabolites containing the [14C]-labelled triazine moiety was concordant with our results ; however no supplementary information was found in the literature.…”

“…It is well known that sulfonylurea degradation in soil depends on pH because abiotic hydrolysis of the sulfonylurea bridge is favoured at low pH. 15,16 After 98 days of incubation, 8% of the metsulfuron-methyl remained in the water extracts from the Bg soil, but it had totally disappeared after 42 days in the Vg soil. In this soil, the metsulfuronmethyl was degraded both by micro-organisms and by abiotic hydrolysis.…”

“…competitive degradation pathway, which could become predominant in acidic environments for metsulfuron-methyl14,21 and other sulfonylureas. 15,22 Initially the corresponding hydroxy derivative was considered to arise from a biological hydroxylation.8 However it is now well-known that this kind of product can be produced by chemical reaction without the intervention of micro-organisms.14,21 The hydroxy-metsulfuron-methyl is less stable than metsulfuron-methyl and the sulfonylurea bridge is slowly hydrolysed, giving the corresponding hydroxy-atrazine (4-hydroxy-6-methyl-2-amino-1,3, 5-triazine)14,21 which could be one of the non-identiÐed metabolites detected. Another degradation pathway of the hydroxy-metsulfuron-methyl is cleavage of the triazine ring, supposed to be an oxidative process with incorporation of three additional atoms of oxygen and loss of an atom of N leading to a stable, less reactive and less water-soluble product.14,21,24 These complementary degradation pathways have been well identiÐed in laboratory abiotic conditions in the absence of soil, and triazine ring cleavage has been detected at low pH as the major hydrolytic pathway for metsulfuronmethyl.…”

“…16 However, this explanation was recently contested by a theoretical calculation of the atomic charges at the carbonyl-carbon, which indicated that the ionic species are more susceptible towards hydrolysis by a nucleophilic attack than the neutral species and that faster reactions at lower pH may be due merely to the higher proton concentration. 15 The dependence of the degradation rate on pH has been used to establish guidelines governing the use of the more persistent sulfonylureas.2 Rapid degradation in acidic conditions can be detrimental to efficient plant protection. On the other hand, low degradation in alkaline soil where abiotic degradation is minimised might induce damage in rotational cropping.…”