Demonstrating formation of potentially persistent transformation products from the herbicides bromoxynil and ioxynil using liquid chromatography‐tandem mass spectrometry (LC‐MS/MS)

Abstract

It is shown that potentially persistent transformation products can be formed from the herbicides bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) and ioxynil (3,5-diiodo-4-hydroxybenzonitrile), and possible leaching to groundwater is discussed. A similar process to the formation of BAM (2,6-dichlorobenzamide) from the herbicide dichlobenil (2,6-dichlorobenzonitrile) can be anticipated as bromoxynil and ioxynil are analogues of dichlobenil and they are degraded by the enzymes nitrilase, nitrile hydratase and amidase. A biodegradation study using cultured Variovorax sp. DSM 11402, a species commonly found in soil, demonstrated that ioxynil and bromoxynil were fully transformed into their corresponding amides in 2-5 days. These amides were not further degraded within 18 days, and formation of other degradation products was not observed. These results are in agreement with biodegradation experiments with dichlobenil. In soil, dichlobenil is transformed into its only observed degradation product BAM, which is persistent and mobile, and has been found in 19% of 5000 samples of Danish groundwater. Variovorax sp. is known to degrade the non-halogenated analogue benzamide, suggesting that degradation of the three amides may be hindered by the halogenated substituents (meta-Br; meta-I; ortho-Cl). This hypothesis is supported by QSAR modelling of fundamental properties. Using a new optimised liquid chromatography-tandem mass spectrometry (LC-MS/MS) method, the sorption and desorption properties of bromoxynil and ioxynil were characterised in sandy topsoil at four concentration levels. The estimated sorption coefficient K(d) was 1.4 L kg(-1) for bromoxynil and 5.4 L kg(-1) for ioxynil, indicating weak to moderate sorption to topsoil. Desorption of the herbicides showed that they were strongly and irreversible bound to the soil (K(des) > K(d)). The amount of herbicide desorbed depended on the initial concentration level. At low levels, K(des) values were higher, indicating stronger binding than at higher levels. The isocratic LC-MS/MS method developed for simultaneous detection of bromoxynil, ioxynil and their main degradation products is described. Using negative electrospray ionisation (ESI-), the detection limits were 0.4-1.0 microg L(-1), with relative standard deviations of 4-10% (n = 10) using direct injection without clean-up steps. The standard curves showed linearity in the range 5-100 microg L(-1) with r(2) > 0.992.

New proton donors in electrochemical mechanistic studies in non-aqueous solution dimethylsulfoxide: Chlorinated hydroxybenzonitriles

“…One can conclude that the ability to provide protons increases in order p-hydroxybenzonitrile (pK a = 8.0) < ioxynil 3 (pK a = 4.0) < 3-chloro-4-hydroxybenzonitrile 2 (pK a not available) < chloroxynil 1 (pK a = 3.1). This is in agreement with the decreasing pK a of these compounds reported in literature [24,25].…”

Degradation of three benzonitrile herbicides by AminobacterMSH1 versus soil microbial communities: pathways and kinetics

“…The supposed soil issue of persistent benzonitrile–herbicide metabolites, as raised in the literature, is somewhat contradicted by the capabilities of the indigenous soil microbiota in the current study, as ioxynil and bromoxynil were mineralized quickly, irrespective of previous benzonitrile exposure. Rosenbrock et al also reported bromoxynil degradation in soil with 20–30% CO 2 recovery within 5 days.…”

“…), it has been proposed that ioxynil and bromoxynil may be converted into persistent metabolites in the same way as dichlobenil is converted to BAM . In this context, Nielsen et al observed incomplete hydrolysis of ioxynil and bromoxynil by Variovorax sp. DSM 11402 to 3,5‐diiodo‐4‐hydroxybenzamide (IAM) and 3,5‐dibromo‐4‐hydroxybenzamide (BrAM), respectively.…”

Hydrolysis of benzonitrile herbicides by soil actinobacteria and metabolite toxicity

“…Rhizobium radiobacter (formerly Agrobacterium radiobacter; Vosáhlová et al 1997;Holtze et al 2006) and Variovorax sp. (Nielsen et al 2007) not only converted dichlobenil, but also bromoxynil and ioxynil into the corresponding amides. Alternatively, bromoxynil and ioxynil were hydrolyzed into carboxylic acids by the nitrilase of Klebsiella pneumoniae ssp.…”