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Mechanisms of bond cleavage during Mn oxide and UV degradation of glyphosate: Results from phosphate oxygen isotopes and molecular simulations Deb P. Jaisi, Hui Li, Adam F. Wallace, Prajwal Paudel, Mingjing Sun, Avula Balakrishna, and Bob N. Lerch J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.6b02608 • Publication Date (Web): 24 Oct 2016 Downloaded from http://pubs.acs.org on October 29, 2016
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Journal of Agricultural and Food Chemistry
Mechanisms of bond cleavage during Mn oxide and UV degradation of glyphosate: Results from phosphate oxygen isotopes and molecular simulations
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by
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Deb P. Jaisi1*, Hui Li1, Adam F. Wallace2, Prajwal Paudel1, Mingjing Sun1, Avula Balakrishna1, and Bob N. Lerch3
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Department of Plant and Soil Sciences, University of Delaware, Newark, DE 19716
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Department of Geological Sciences, University of Delaware, Newark, DE 19716
USDA-ARS, Cropping Systems and Water Quality Research Unit, Columbia, MO 65211
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* Corresponding author: Deb P Jaisi, Email:
[email protected]; Phone: (302) 831-1376.
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Revised for Journal of Agricultural and Food Chemistry October 20, 2016
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ABSTRACT: Degradation of glyphosate in the presence of Mn oxide and UV light was
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analyzed using phosphate oxygen isotope ratios and density function theory (DFT). Preference of
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C–P or C–N bond cleavage was found to vary with changing glyphosate:Mn oxide ratios
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indicating the potential role of sorption-induced conformational changes on the composition of
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intermediate degradation products. Isotope data confirmed that one oxygen atom derived solely
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from water was incorporated into the released phosphate during glyphosate degradation and this
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might suggest similar nucleophilic substitution at P centers and C–P bond cleavage both in Mn
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oxide- and UV light-mediated degradation. The DFT results reveal that the C–P bond could be
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cleaved by water, OH- or ·OH, with the energy barrier opposing bond dissociation being lowest
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in the presence of the radical species, and C–N bond cleavage is favored by the formation of both
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nitrogen and carbon centered radicals. Overall, these results highlight the factors controlling the
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dominance of C–P or C–N bond cleavage that determine the composition of intermediate/final
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products and ultimately the degradation pathway.
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Keywords: Glyphosate, degradation, Mn- oxide, phosphate isotopes, DFT, C-P and C-N bond
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cleavage.
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INTRODUCTION
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Glyphosate [(N-phosphonomethyl)glycine] is a broad–spectrum systemic herbicide. The
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production and usage of glyphosate has steadily increased over the last few decades, particularly
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since the emergence of glyphosate-tolerant genetically modified crops (including soybean, corn,
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cotton, and canola), and its utility in no-till farming, urban household and recreational areas 1. In
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fact, sustainability and profitability of modern US agriculture has increasingly relied on the role
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of effective herbicides. However, there has been increasing concern over the potential harm of
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glyphosate in the environment because of the widespread presence of glyphosate and
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aminomethylphosphonic acid (AMPA), a major degradation product of glyphosate, in soils and
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sediments, ditches and drains, rivers and streams, and wetlands and ground waters in response to
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steadily increasing usage of glyphosate primarily in agriculture
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glyphosate is reported to be between 2 and 174 d depending on soil type and microbial activity,
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among other environmental variables, glyphosate and AMPA have been detected in surface
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waters many months after application 8
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2,3
. Although the half-life of
, for as long as 3 years after application in northern
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Swedish soils , and in the atmosphere . Elevated public concern over glyphosate toxicity and its
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common presence in the environment highlights the importance as well as the urgency for
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scientific investigation on the fate and persistence of glyphosate and its degradation products in
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soils and other environments.
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The C‒P bond in glyphosate is highly stable and impervious to strong acid/base
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hydrolysis. While microbial degradation has been known as the common pathway for glyphosate
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degradation, abiotic degradation of glyphosate and other organophosphonate compounds has
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been reported via ultraviolet radiation, peroxide oxidation, and mineral oxidation
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Glyphosate degradation initiates with either separate or simultaneous cleavage of C‒P and C‒N
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bonds. For example, manganite (MnOOH) oxidation of nitrilotrismethylene-phosphonic acid
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initiates through C−N bond cleavage at the nitrogen centered radical, followed by C−P bond
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cleavage due to the formation of a carbon centered radical 17. Additionally, photodegradation of
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glyphosate in a ferrioxalate system has been suggested to occur via simultaneous cleavage of C‒
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N and C‒P bonds
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because the compositions of intermediate degradation products are different under these two
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pathways
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10-16
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. Glyphosate degradation through C‒N or C‒P bond is of scientific interest
. Furthermore, the relative rates of formation as well as half-lives of these products 3
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vary. The two pathways have significant environmental implications as well because the toxicity
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and environmental persistence of intermediate product/s are variable. More importantly,
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providing a satisfying degradation reaction mechanism may provide unique insight into the fate
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of glyphosate and its degradation products and access any potential impact on overall
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phosphorus cycling in the environment. In this research, we aimed to investigate the mechanisms
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of bond cleavage during abiotic degradation of glyphosate by MnO2 and UV light. We applied
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phosphate oxygen stable isotope ratios and density function theory (DFT) to investigate C‒N or
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C‒P bond cleavage mechanisms and specifically the preference for cleaving a particular bond
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during the degradation of glyphosate. Understanding of glyphosate degradation mechanisms on
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the basis of specific bond cleavage and development of isotope effects linking source and
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products is expected to provide a framework that could be useful for source tracking of
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glyphosate in the environment.
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MATERIALS AND METHODS
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Synthesis and characterization of MnO2. δ-MnO2 was synthesized by using the McKenzie
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100 mL solution containing 2.4 g NaOH pellets and 4.74 g KMnO4 and the reaction was allowed
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to continue overnight. Upon completion of the reaction, the residual reagents were removed by
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rinsing with DI water and centrifuging at 8,000 × g for 1 hr. The pure birnessite crystals were
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freeze dried and then gently ground with a mortar and pestle. X-ray powder diffraction (XRD)
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and scanning electron microscopy (SEM) analyses were performed to determine the purity,
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composition, and morphology of the mineral. Similarly, the reactive surface area of birnessite
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was measured by using the Brunauer, Emmett, and Teller (BET) method in a Tristar II Analyzer
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(Micromeritics, USA).
method. Briefly, a 100 mL solution of 11.29 g of Mn(NO3)2·4H2O was added drop wise to a
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Glyphosate degradation experiments. Three sets of batch experiments were performed to
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understand the preference of C–P vs C–N bond cleavage and corresponding isotope effects
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during the degradation of glyphosate. The first set of experiments included a range of glyphosate
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(0.06 ‒ 6 mM) and MnO2 (0.006 ‒ 6 g/L) concentrations to identify the preference of bond
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cleavage. We used analytical grade glyphosate (Sigma Aldrich) and δ-MnO2 synthesized in the
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laboratory in these experiments. All batch experiments were performed in triplicate in 8.3 mM 4
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NaCl and 10 mM MOPS [3-(N-morpholino)propanesulfonic acid] buffer (pH=7.26 ± 0.3) at
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room temperature (22 ± 1 oC). Glyphosate degradation in each sample aliquot was inhibited by
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adding an appropriate concentration of CuSO4 immediately after sampling. Cu2+ complexes
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strongly with glyphosate and limits glyphosate coordination to reactive oxidation sites on the
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MnO2 surface 10,13,19.
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The second set of degradation experiments was performed in a wide range of 18O labeled
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waters and dissolved oxygen in order to detect the incorporation of O atoms into released
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orthophosphate. For
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experimental water was varied from -7.0 to +74.0‰ (diluted from 10%
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Cambridge Isotopes, MA). All experiments were run in triplicate under the same conditions as in
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the first set of experiments. For
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oxygen (98% 18O, Icon Isotopes, NJ) was diluted in air (with air oxygen isotope ratios (δ18OO2)
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of +23.5 ‰ VSMOW) to final δ18OO2 values of +67.4 to +243.8‰. To ensure complete diffusion
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and uniform labeling of isotopes in the headspace and dissolved oxygen in the experimental
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water, glyphosate was added only after 2 hrs. All experiments were performed at neutral pH
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(7.26 ± 0.3) and room temperature (22 ± 1 oC) inside an anaerobic glove box (Coy Lab Products,
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MI) to avoid isotope contamination from air in the headspace. Serum bottles (120 mL) used for
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these experiments were closed with air-tight septa and crimp sealed. All experiments were run at
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least in duplicate and control experiments included media and glyphosate but without MnO2.
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Sampling and sample analyses were the same as in the first set of kinetic experiments.
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O labeled water experiments, oxygen isotope ratios (δ18OW) of
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O labeled water,
O labeled dissolved oxygen experiments, highly enriched
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The third set of experiments was designed to understand the isotope effect of glyphosate
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degradation under UV irradiation (UVR). Because the incorporation of external oxygen is
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normally low to insignificant in UVR degradation in some organic P (Po) compounds
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method is generally considered a better method for measuring δ18OPo values, i.e., the oxygen
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isotope ratios of phosphate moieties bound to organophosphorus compounds. Please note
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significant O incorporation has been found in this method for as well 14,16. The UV degradation
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experiments were performed with glyphosate in three labeled waters (δ18OW values of +67.4 to
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+243.8‰) with 5% H2O2 by using a 1200 W mercury lamp in an Ace Glass 7900 UV photo-
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oxidation unit (Ace Glass Inc., Vineland, NJ).
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, this
Concentrations of orthophosphate and primary amines formed during glyphosate degradation in all experiments were measured colorimetrically using phosphomolybdate 5
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2,4,6-trinitrobenzenesulfonic acid (TNBSA 13) methods, respectively. Our past experience
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suggests that the TNBSA method is more sensitive for amine quantitation because it does not
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produce a false positive signal for glyphosate during primary amine quantitation 24.
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Measurement of oxygen isotope ratios in
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experiments. The oxygen isotope ratios in orthophosphate, labeled water, and dissolved
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oxygen used in the experiments were measured separately. For orthophosphate released from all
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three sets of glyphosate degradation experiments, subsamples were processed for silver
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phosphate precipitation using published methods
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glyphosate used in this study did not undergo acid hydrolysis and therefore pretreatment of
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samples to remove residual glyphosate was not required. However, the precipitation of
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ammonium phosphomolybdate (APM) was found to be hindered by the presence of residual
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glyphosate. Therefore it was first treated first with Superlite DAX-8 (Sigma Aldrich) resin to
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trap residual glyphosate. Quantitative yield of orthophosphate was measured before and after the
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resin treatment and was found to be complete, within
