Can Carbamates Undergo Radical Oxidation in the Soil Environment

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Can carbamates undergo radical oxidation in the soil environment? A case study on carbaryl and carbofuran Irmina #wiel#g-Piasecka, Maciej Witwicki, Maria Jerzykiewicz, and Julia Jezierska Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03386 • Publication Date (Web): 24 Nov 2017 Downloaded from http://pubs.acs.org on November 27, 2017

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Environmental Science & Technology

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Can carbamates undergo radical oxidation in the

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soil environment? A case study on carbaryl and

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carbofuran

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Irmina Ćwieląg-Piaseckaa*, Maciej Witwickib*, Maria Jerzykiewiczb, Julia Jezierskab

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a

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and Life Sciences, Grunwaldzka 53 St., Wroclaw, Poland

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b

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TOC Art

Institute of Soil Science and Environmental Protection, Wroclaw University of Environmental

Faculty of Chemistry, Wroclaw University, 14 F. Joliot–Curie St., 50-383 Wroclaw, Poland

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ABSTRACT

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Radical oxidation of carbamate insecticides, namely carbaryl and carbofuran, was investigated

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with spectroscopic (electron paramagnetic resonance [EPR] and UV-Vis) and theoretical (density

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functional theory [DFT] and ab initio orbital-optimized spin-component scaled MP2

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[OO-SCS-MP2]) methods. The two carbamates were subjected to reaction with •OH, persistent

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DPPH• and galvinoxyl radical, as well as indigenous radicals of humic acids. The influence of

ACS Paragon Plus Environment


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fulvic acids on carbamate oxidation was also tested. The results obtained with EPR and UV-Vis

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spectroscopy indicate that carbamates can undergo direct reactions with various radical species,

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oxidizing themselves into radicals in the process. Hence, they are prone to participate in the

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prolongation step of the radical chain reactions occurring in the soil environment. Theoretical

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calculations revealed that from the thermodynamic point of view hydrogen atom transfer is the

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preferred mechanism in the reactions of the two carbamates with the radicals. The activity of

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carbofuran was determined experimentally (using pseudo-first-order kinetics) and theoretically

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to be noticeably higher in comparison with carbaryl and comparable with gallic acid. The

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findings of this study suggest that the radicals present in soil can play an important role in natural

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remediation mechanisms of carbamates.

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Keywords: scavenging kinetics, carbamates, galvinoxyl, DPPH, UV-Vis, EPR, DFT, OO-SCS-

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MP2, ab initio

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INTRODUCTION

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Soil organic matter (SOM) plays a crucial role in immobilization of many xenobiotics in

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soil.1,2 Depending on the soil physical and chemical properties and the chemical nature of organic

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pollutants various binding mechanisms are responsible for this immobilization. The most

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commonly reported mechanisms in the literature are ionic interactions, covalent and hydrogen

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bonds, charge-transfer or electron donor-acceptor processes, Van der Waals forces, hydrophobic

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bonding or partitioning between water and organic humic phases.3,4

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Radicals, molecules containing a single, unpaired electron in an s- or p-type atomic orbital

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or a molecular orbital constituting their linear combination, occur in soil in relatively high

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concentrations as they can be generated in it biologically and abiologically.5–7 Moreover, various


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factors affect their generation or initiate formation of additional, environmentally persistent

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radicals.8–16 Hence, much research has been conducted on the role of radicals in pesticide binding

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mechanisms. For instance, formation of radicals was observed during reactions of humic

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substances with various herbicides (e.g. s-triazines, substituted ureas), where charge transfer

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complexes were formed.7 Conversely, chlorophenoxy herbicides quenched the radical content of

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humic substances.3 Recently it was postulated that chemisorption and electron transfer from

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organochlorine pesticide pentachlorophenol to transition metal ions and other electron sinks in

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soil, lead to formation of new, environmentally persistent radicals.17 According to the literature a

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number of substituted aromatic pesticides undergo oxidation reactions that involve the

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generation of stable radicals in soil through removal of a proton or an electron from the

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molecule.18 Nonetheless, the role of radical species in the binding mechanism of contaminants to

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soil constituents is still under debate and further information about environmentally important

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radical reactions involving various organic compounds can be found in the literature.3,7,19–21

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Carbamate pesticides, represented here by carbaryl and carbofuran, belong to the group of

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nonpolar, nonionic, toxic compounds widely applied as insecticides.22,23 In addition to being

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agrochemical agents, carbamates are also used in the pharmaceutical and polymer industry.24–28

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Hence, any information about their fate after they are released into the environment is of

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significant importance. These N-substituted esters of carbamic acid are moderately persistent,

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mobile and have strong affinity to soil organic matter.29 Their adsorption, mobility and

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degradation in soil have been widely investigated.30–32 However, to the best of our knowledge,

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there is still a limited amount of research on the possible oxidation of these two carbamates

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involving various radical species and the role of SOM in this process. This gap in knowledge is

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additionally exposed by the fact that the reactions of carbamates with •OH and •NO3 in the



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atmosphere have been investigated.33–35 Therefore, the main aim of this study was to investigate

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the radical oxidation of carbamate pesticides, with carbaryl and carbofuran taken as a case study,

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and their possible effect on soil organic matter. In order to achieve this goal, merged

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spectroscopic (UV-Vis, electron paramagnetic resonance [EPR]) and theoretical (density

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functional theory [DFT], ab initio) investigations were carried out.

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MATERIALS AND METHODS

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Chemicals and reagents. PBN (N-t-butyl-α-phenylnitrone) and DMPO (5,5-dimethyl-1-

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pyrroline N-oxide) spin traps, GO• (galvinoxyl, 2,6-Di-tert-butyl-α-(3,5-di-tert-butyl-4-oxo-2,5-

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cyclohexadien-1-ylidene)-p-tolyloxy), and DPPH• (2,2-diphenyl-1-picrylhydrazyl) radicals, and

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investigated active substances of pesticides, that is carbaryl (1-naphthyl-N-methylcarbamate) and

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carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranol N-methylcarbamate), were purchased from

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Sigma–Aldrich and Leonardite Humic Acid Standard from International Humic Substances

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Society (IHSS). Ethanol, HCl, HF, NaOH, FeSO4 .7H2O and AgNO3 were obtained from POCH

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(Avantor Performance Materials, Poland). All the reagents were of analytical grade.

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Caution: carbaryl and carbofuran are highly toxic; proper care should be exercised while

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handling these compounds.

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Extraction, fractionation and purification of humic acids (HA) and fulvic acids (FA).

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The source of HA and FA for this study was a topsoil horizon collected from the agricultural

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Gleyic Phaeozems36 derived from silt loam located in the area of Domaniów, near Wroclaw,

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Poland. Humic substances were extracted from the soil using the procedure recommended by the

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International Humic Substances Society.37 The extracts were purified with a mixture of 0.1 M

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HCl and 0.3 M HF in polypropylene tubes. They were left overnight and centrifuged. The


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precipitate was transferred to dialysis tubes (Spectra/Por 7 MWCO 10,000, Spectrum Europe

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B.V., The Netherlands) and dialyzed with distilled water until a negative Cl− test with AgNO3.

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Afterwards the humic acids were freeze dried. FA extracts were passed through an XAD-8

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column followed by H+-saturated cation exchange resin. The eluate was freeze dried.

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UV-Vis measurements. All the measurements were done under deaerated conditions at

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298 K in ethanol solutions (2 mL in a 10 mm quartz cuvette) containing DPPH• (2.2 × 10-4 M) or

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GO• (8.3 × 10-6 M) and a various amount of carbaryl (1.5 × 10−2 – 1.2 × 10−1 M) or carbofuran

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(1.0 × 10−2 – 70 × 10−2 M). UV-Vis spectral changes associated with the undergoing reaction

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were monitored at 516 and 429 nm for DPPH• (ε = 4.30 × 103 M–1 cm –1) and GO• (ε = 1.16 ×

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105 M–1 cm –1), respectively, using a Varian Cary 50Conc UV-Visible Spectrophotometer. The

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following equations of calibration curves were used to calculate DPPH• and GO• concentrations

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in the reaction system:

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A(516 nm) = 4.302 × 103 [DPPH•] + 8.14 × 40-3

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A(429 nm) = 1.161 × 105 [GO•]

(R2 = 0.999)

(R2 = 0.996).

(Eq. 1) (Eq. 2)

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In order to assess activity of the studied carbamates in radical reactions possibly occurring

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in soil systems, two techniques were employed. In the first the scavenging capacity was judged

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against DPPH• by measuring the percentage of remaining DPPH• after a fixed reaction time:

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remaining DPPH• (%) = At× A0–1 × 100%,

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where A0 and At correspond to the DPPH• absorbance at 516 nm in the absence and presence of

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pesticide after a fixed time, respectively. In the second, the rate of DPPH• and GO• scavenging

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reaction was determined using pseudo-first-order kinetics (the concentration of carbaryl and

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carbofuran was maintained at more than 45-fold excess of the radical concentration; further


(Eq. 3)


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details are given as Supporting Information). These two techniques are frequently employed in

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estimation of antiradical properties of antioxidants.38–40

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EPR measurements. All the EPR measurements were conducted at a frequency close to 9.7

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GHz (X-band) at room temperature using a Bruker ELEXYS E500 spectrometer equipped with a

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NMR gaussmeter and frequency counter. Microwave power was set to 13 mW, magnetic field

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modulation to 100 kHz and modulation amplitude to 1 G. Direct EPR measurements of freeze-

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dried samples of HA and HA saturated with carbaryl and carbofuran were done in quartz EPR

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tubes. Leonardite, a naturally occurring partly oxidized lignite with high humic acid content of

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known radical concentration was used as a standard in quantitative EPR measurements. All the

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experimental spectra were accumulated five times and simulated using the WinEPR SimFonia

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program (1.25 version) developed by Bruker.

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Radicals frequently have short half-lives and therefore they are often impossible to detect in

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direct EPR measurements. Indirect EPR detection and identification of such radicals is possible

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using spin trapping techniques. A spin trap reacts with the radical to form a spin adduct that is

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more stable than the initial radical and thus can be observed in the EPR experiment. In this study

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DMPO and PBN were employed as spin traps for the radicals formed from carbaryl and

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carbofuran. These spin traps are commonly used in studies of radical reactions involving

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superoxide and hydroxyl radicals.41,42 PBN is considered to react more rapidly with carbon-

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centered radicals than with the oxygen-centered ones, opposite to DMPO.43,44

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The compositions and abbreviations of all the EPR investigated samples are compiled in

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Table 1. All these experiments were repeated with addition of 1 mg of iron(II) sulfate

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heptahydrate to the investigated solutions. This should enhance the production of •OH, but was

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found to have no effect on the result of the experiments. All the EPR spectra were collected


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twice, that is 5 minutes and 1 h after the initiation of the reaction. The samples investigated in

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spin trapping studies were measured in glass capillaries inserted into quartz EPR tubes.

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Table 1. Composition of all samples from EPR experiments.

Sample

0a 0b 0f 1 2 1a 1a* 1b 1b* 2a 2a* 2b 2b* 1f 2f 1af 1a’f 1bf 1b’f 2af 2a’f 2bf 2b’f

Oxidizing agent, H2O2 (volume, concentration) 0.05 mL, 3% 0.05 mL, 3% 0.05 mL, 3% 0.05 mL, 3% 0.22 mL, 3% 0.05 mL, 3% 0.05 mL, 30% 0.05 mL, 3% 0.05 mL, 30% 0.22 mL, 3% 0.22 mL, 30% 0.22 mL, 3% 0.22 mL, 30% 0.05 mL, 3% 0.22 mL, 3% 0.05 mL, 3% 0.05 mL, 3% 0.05 mL, 3% 0.05 mL, 3% 0.22 mL, 3% 0.22 mL, 3% 0.22 mL, 3% 0.22 mL, 3%

Pesticide

Spin trap

0.5 mL of carbaryl (25 mM) 0.5 mL of carbofuran (25 mM) 0.5 mL of carbaryl (25 mM) 0.5 mL of carbaryl (25 mM) 0.5 mL of carbofuran (25 mM) 0.5 mL of carbofuran (25 mM) 0.08 mL of carbaryl (25 mM) 0.08 mL of carbaryl (25 mM) 0.08 mL of carbofuran (25 mM) 0.08 mL of carbofuran (25 mM) 0.5 mL of carbaryl (25 mM) 0.5 mL of carbaryl (2.5 mM) 0.5 mL of carbofuran (25 mM) 0.5 mL of carbofuran (2.5 mM) 0.04 mL of carbaryl (25 mM) 0.04 mL of carbaryl (2.5 mM) 0.04 mL of carbofuran (25 mM) 0.04 mL of carbofuran (2.5mM)

8.5 mg of PBN 0.5 mL of 0.2 M DMPO 8.5 mg of PBN 8.5 mg of PBN 8.5 mg of PBN 8.5 mg of PBN 0.5 mL of 0.2 M DMPO 0.5 mL of 0.2 M DMPO 0.5 mL of 0.2 M DMPO 0.5 mL of 0.2 M DMPO 8.5 mg of PBN 0.5 mL of 0.2 M DMPO 8.5 mg of PBN 8.5 mg of PBN 8.5 mg of PBN 8.5 mg of PBN 0.5 mL of 0.2 M DMPO 0.5 mL of 0.2 M DMPO 0.5 mL of 0.2 M DMPO 0.5 mL of 0.2 M DMPO

Fulvic acid (2.5 mg⋅mL-1 in ethanol) 0.5mL 0.5 mL 0.08 mL 0.5 mL 0.5 mL 0.5 mL 0.5 mL 0.04 mL 0.04 mL 0.04 mL 0.04 mL

Ethanol

1.0 mL 0.08 mL 0.5 mL 0.5 mL 0.5 mL 0.5 mL 0.5 mL -

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Sorption experiment. Sorption experiment was carried out using the simplified

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equilibration method.45 Stock solutions of carbaryl (4.97 × 10-5 M) and carbofuran (4.52 × 10-5

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M) in 1.0 ×10-2 M CaCl2 were prepared. 10 mL of the stock solutions were added to 50 mg of

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HA giving 1s and 2s mixtures, respectively. As a control sample 50 mg of HA with 10 mL of 1.0

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× 10-2 M CaCl2 solution was used (0s). All the mixtures were shaken for 24 h, centrifuged and

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remained humic acids precipitates were decanted. Preliminary tests showed that a sorption

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equilibrium was reached for the investigated carbamates within 24 hours. The residues of the



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humic acids were washed with distilled water until the negative reaction for the chloride ions

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(with AgNO3) and freeze-dried. Such prepared samples were then analyzed for the EPR radical

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spin concentration as described above.

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Theoretical calculations. Recent years have witnessed increasing interest in the application

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of modern electronic structure theory in solving environmental problems.46–53 In conjugation

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with the experiments, theoretical calculations were conducted with the ORCA 3.0.3 suite of

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programs.54 In all the calculations the def2-TZVP basis set developed by Ahlrichs and coworkers

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was used55,56 and the COSMO formalism57,58 was employed to cover the solvent effects (water).

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The calculations for open-shell species were conducted in the unrestricted protocol. The

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geometry optimizations were carried out at the DFT level using the gradient-corrected BP86

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functional, which provides accurate molecular structures.59–62 Each of the stationary points was

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fully characterized as a true minimum through a vibrational analysis. On these molecular

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structures single point calculations were carried out at the DFT level with the hybrid B3LYP,63–65

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hybrid meta-GGA TPSSh,66 and double hybrid B2PLYP67 approximations as well as at the ab

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initio level with the OO-SCS-MP2 method68. The bond dissociation enthalpies (BDE), ionization

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potentials (IP) and Gibbs free energies (∆G) were calculated at 298 K with all these theoretical

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methods but using the zero point, thermal (vibrational, rotational and translational) and entropy

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corrections computed at the BP86/def2-TZVP theory level. BDE and IP were predicted for the

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most stable conformers; the reported ∆G values refer to the most stable conformers of the most

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stable isomers. A full summary of the calculations is given as Supporting Information (Table

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S.1).

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RESULTS AND DISCUSSION

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Spectroscopic measurements. The radical chain mechanism involves three steps: initiation,


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propagation and termination.20 The initiation step can be induced by a number of processes, but

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in a soil system the hydroxyl radical (•OH) can be expected to be the first radical of the chain, as

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it is a powerful oxidant known to react with many organic compounds at nearly diffusion-limited

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rates.69–73 In environment •OH can be generated photochemically6,74,75, in the Fenton reaction76–

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78

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independent pathway of •OH formation might be possible in soil.80

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or by metal-loaded humic acids79. Recently, it was also demonstrated that a sunlight

EPR experiments. In order to judge to what extent carbamates (carb-H) can undergo the

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reaction with •OH in the propagation phase:

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carb-H + •OH → carb• + H2O,

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and to identify the resulting radical product (carb•), EPR measurements were performed. If carb•

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were detected, either in a direct measurement or as a spin adduct, it would obviously indicate that

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radical derivatives of carbaryl and carbofuran can be formed and that they are therefore likely to

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further participate in various radical reactions in soil systems.

(Eq. 4)

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In the reaction systems H2O2 was always the initial source of •OH radicals, but regardless

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of the lack or presence of Fe(II) no radicals (in 0a and 0b mixtures) and no spin adducts (in 1a,

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1b, 2a, 2b mixtures) originating from the carbamates were observed. When DMPO was used (in

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2a, 2b and reference sample 2) two radical species were detected (Figure S.1 in Supporting

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Information): the well-known DMPO spin adduct of •OH (giso = 2.0055, aiso(14N) = aiso(1H) =

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14.5 G) and a nitroxide radical that is the product of DMPO oxidation (giso = 2.0055, aiso(14N) =

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14.2 G).81 The PBN adduct of •OH was not observed as it is known to decompose rapidly to

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benzaldehyde and tert-butylhydroxylamine (t1/2 < 1 minute).42

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It is well known that the spin trapping can be hindered by many factors and these include

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the rate of production of primary (•OH) and secondary (carb•) radicals.82,83 Therefore, to enhance



185

the formation rate of carb• the concentration of H2O2 was increased (1a*, 1b*, 2a* and 2b*

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samples). As a result PBN adducts of radicals formed from carbamates were EPR detected (giso =

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2.0056, aiso(14N) = 14.75 G, aiso(1H) = 2.85 G and giso = 2.0055, aiso(14N) = 15.0 G, aiso(1H) = 3.65

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G for carbofuran and carbaryl, respectively), but only in case of this spin trap (1a* and 1b*). The

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exemplary EPR spectrum is shown in Figure 1A which clearly confirms that the carb• radicals

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are formed in the reaction with •OH.

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More complex systems (1af, 1bf, 2af, 2bf) containing FA in addition to the carbamates,

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spin traps and H2O2 were studied to estimate the effect of SOM on the formation of carbamate

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derived radicals. But first direct EPR measurements were performed on ethanolic solution of FA

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(0f), as it is well-known that anionic semiquinones (oxygen-centered radicals) derived from

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fulvic acids (FA) can be observed in direct EPR experiments.84–87 However, this requires alkaline

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conditions and the presence of oxidising agent (for example ambient oxygen). Since in our

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experiments the reaction conditions were not alkaline, then the FA semiquinones were not

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observed.

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Then the reference EPR measurements were also performed on the samples containing FA

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and spin traps but without carbamates (1f and 2f), revealing formation of PBN spin adduct with

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radicals from FA (giso = 2.0055, aiso(14N) = 14.8 G and aiso(1H) = 3.2 G). The corresponding

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DMPO adduct was not observed. The values of EPR parameters indicated that the carbon-

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centered radicals were derived from fulvic acid.88,89 It is reasonable as the reactions with •OH are

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known to be only slightly selective and PBN spin adducts with carbon-centered radicals in

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similar phenolic systems were observed previously.89,90


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Figure 1. EPR spectra of: A) PBN spin adduct of radical generated from carbaryl (black line,

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1a* sample) and simulation of its experimental spectrum (red, dashed line; giso = 2.0055, aiso(14N)

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= 15.0 G, aiso(1H) = 3.65 G); B) PBN spin adduct of fulvic acid radical (blue line, 1f sample) and

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PBN spin adduct of fulvic acid radical in the presence of carbaryl (1a’f sample; black line; see

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text for details).

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In contrast, the EPR measurements of the mixtures containing carbamates (1af, 1bf, 2af,

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2bf) revealed that neither their radical spin adducts nor spin adducts with radicals from FA were

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detected. In the next stage of the studies a tenfold lower concentration of carbaryl and carbofuran

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was added to the reaction mixtures containing FA, H2O2 and spin traps (1a’f, 1b’f, 2a’f, 2b’f).

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As a result the EPR spectra of the PBN spin adduct with FA radicals were recorded (1a’f and

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1b’f) but, as is apparent from Figure 1B, their intensity was significantly lower in comparison to

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the spectra recorded for the carbamate-free mixture (1f). This fact implies that the carbamates

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can take part in the prolongation step of the radical chain process, probably by scavenging of



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•OH according to the Eq. 4, what prevents further •OH reaction with FA and is accompanied by

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formation of carb• as products. The formation of carb• is credible as the PBN-carb• adducts in

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the 1a* and 1b* samples (see in the text above and Figure 1A) are clearly identified. However,

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in the reaction mixtures containing FA the possibility for reaction of carbamates with radicals

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originating from FA cannot be excluded:

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FA• + carb-H → FA-H + carb•,

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which in turn would prevent FA• from forming the spin adducts. And finally carbamates may

227

also react with the •PBN-FA spin adducts:

228 229 230

(Eq. 5)

carb-H + •PBN-FA → carb• + H-PBN-FA.

(Eq. 6)

In order for the process given by the Eq. 5 or 6 to occur the carbamates would have to be able to react with radicals that are weaker oxidants than •OH.

231

UV-Vis measurements. To determine the carbamates susceptibility to undergo oxidation by

232

less reactive radicals, leading to the formation of carb•, the easily detectable persistent radical

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DPPH•, as universal standard in similar studies38–40, was used:

234

carb-H + DPPH• → carb• + H-DPPH.

(Eq. 7)

235

The change in concentration of DPPH• is equivalent to the concentration of formed carb•

236

radicals and can be accurately determined. A similar approach is frequently applied to ascertain

237

antiradical properties of antioxidants. The determination of antiradical properties is based on the

238

logical assumption that if a compound is able to react with DPPH•, it is certainly capable to react

239

with the much more aggressive •OH. Therefore, if the reaction between the carbamates and

240

DPPH• is observed, it will additionally corroborate the carbamates ability to react with •OH.

241

Moreover, there is an analogy between the methodology adopted here and the investigation of

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atmospheric reactions of carbamates with •OH. In both cases the reactions were monitored


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through the changes in concentration of one of the substrates, not by observing the radical

244

products.33,35

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In order to confirm that carbaryl and carbofuran participate in the prolongation step three

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mixtures were prepared and monitored over time. Two of them contained one of the carbamates

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(30 mM) and DPPH• (0.22 mM) the third only DPPH• (0.22 mM). These results are shown in

248

Figure 2. The DPPH• absorption at 516 nm underwent a significant decrease in the case of

249

carbaryl and carbofuran confirming that its concentration in the reaction mixtures was lessened.

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What stands out in Figure 2 is that DPPH• was reduced by carbofuran significantly faster than by

251

carbaryl.

252

For instance, after 30 seconds more than 30% of DPPH• was scavenged by carbofuran and

253

5% by carbaryl, whereas after 50 minutes about 64% was scavenged by carbofuran and 34% by

254

carbaryl. Interestingly, Figure 2 implies that the rate of carb• formation is higher for carbofuran.

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To verify this the reaction of DPPH• with each carbamate was monitored using pseudo-first-

256

order kinetics. The calculated (as explained in Supporting Information) pseudo-first-order rate

257

constants, kobs, increased with the concentration of carbaryl and carbofuran, exhibiting first-order

258

dependence (Figure S.2). The second-order rate constants k2 for the reaction given in Eq. 7 were

259

determined from the slopes of the linear plots of kobs vs. concentration of the carbamates. The

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value of k2 for carbofuran was three and a half times higher than for carbaryl, namely 0.060 M-1

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s-1 (R2 = 0.988) for carbofuran and 0.017 M-1 s-1 (R2 = 0.980) for carbaryl. This fully explains the

262

faster decay of DPPH• shown for carbofuran in Figure 2. Moreover, the significant difference

263

between the values of k2 strongly suggests that the chemical structure of carbamates has a

264

decisive effect on the formation rate of carb•. This correlation is discussed below in the

265

theoretical section.



266

267

Figure 2. Changes in DPPH• absorption in the 400 – 700 nm range due to its reaction with

268

carbaryl and carbofuran. The percentages of remaining DPPH• are given in brackets; they were

269

calculated according to Eq. 3.

270

Another important issue is the influence of SOM antioxidant activity on the observed radical

271

oxidation of carbamates. SOM is known to act as an antioxidant due to the large number of

272

phenolic moieties in its structure.91,92 Therefore, SOM can be expected to slow down or even

273

prevent the formation of radical derivatives of carbamates in the reactions with •OH or other

274

radicals. The fact that the second-order rate constants for scavenging of DPPH• by phenolic


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antioxidants are clearly higher than values determined here for carbamates seems to corroborate

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such a hypothesis.39,40 On the other hand, the EPR spin-trapping experiments performed on

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model system comprising FA, a constituent of SOM, and carbamates suggested that the carb•

278

radicals can be formed in the presence of FA. If SOM is able to prevent the carbamates from

279

participating in the prolongation step of radical oxidation in the soil environment, it must

280

scavenge the radicals, for instance •OH, which in reaction with carbamates can lead to carb• (Eq.

281

4). If SOM is to be favored in such a reaction:

282

SOM-H + •OH → SOM• + H2O,

(Eq. 8)

283

over the carbamates, the ability of the resulting phenoxy or semiquinone radicals (SOM•) to react

284

with carbaryl and carbofuran should be verified.

285

Given the complicated and unresolved structure of natural phenolic systems such as SOM or

286

tannins, model compounds of low molecular weight and with well-defined structure are

287

frequently used to mimic the properties of these natural systems.14–16,93–95

288

To establish qualitatively and quantitatively whether the reaction:

289

SOM• + carb–H → SOM-H + carb•

290

can occur in the soil environment, we decided to use a model oxygen-centered radical, i.e.

291

galvinoxyl (GO•). GO• was chosen because its molecular and electronic structure resemble the

292

radical species spread in SOM which (i) are characterized by higher spin delocalization onto

293

aromatic carbon atoms than their counterparts derived from simple compounds such as catechol

294

or 3,4-dihydroxybenzoic acid and (ii) have radical centers that are expected, especially in the

295

case of indigenous ones, to be isolated from external effects by the macromolecular matrix.7,96

296

Correspondingly, in GO• the spin density is efficiently delocalized and access to the oxygen

297

atoms is hindered by tert-butyl moieties (Figure S.3).


(Eq. 9)


Page 16 of 40

298

The rate of the model reaction:

299

GO• + carb–H → GO-H + carb•,

300

was again investigated employing pseudo-first-order kinetics. The plots kobs vs. concentration of

301

carbaryl and carbofuran (Figure S.2) showed linear dependence with kobs undergoing

302

enhancement with the concentration of the carbamates. From the slopes the second-order rate

303

constants k2, were determined as 0.0079 M-1 s-1 (R2 = 0.993) and 0.021 M-1 s-1 (R2 = 0.986) for

304

carbaryl and carbofuran, respectively. As in the case of the reaction with DPPH•, the value of k2

305

is significantly higher for carbofuran. However, it should be noticed that the k2 constants for the

306

reactions of the two carbamates with GO• are noticeably lower in comparison with their

307

counterparts determined for the reaction with DPPH•.

(Eq. 10)

308

The environmental application of the findings presented above is of great interest. The

309

results strongly suggest that even if the antiradical action of SOM leads to the formation of

310

SOM•, this resulting radical can undergo a subsequent reaction with carbamates and therefore

311

trigger their participation in the prolongation step of radical processes taking place in the soil

312

system.

313

Sorption experiment. In order to confirm this conclusion an attempt to estimate the effect

314

which the carbamates can exert on the radicals naturally present in humic acids (indigenous

315

radicals) was made. EPR measurements of the spin (radical) concentration in pure humic acid

316

and the same humic acid treated with CaCl2, carbaryl and carbofuran were carried out after

317

freeze-drying the samples. Regardless of the chemical factor to which HA was exposed, the g

318

parameter remained unchanged and equal to 2.0032, which is a value typical for the indigenous

319

radicals.7,97 Addition of the insecticides solutions in CaCl2 to the pure HA resulted in a decrease


Page 17 of 40


320

of spin concentration from 2.68 × 1017 spins per gram for untreated HA and 4.44 × 1017 spins per

321

gram for HA treated only with CaCl2 (in 0s) to 1.20 × 1017 (in 1s) and 1.55 × 1017 spins per gram

322

(in 2s) for HA with carbaryl and carbofuran, respectively. This experiment convincingly

323

demonstrated that the two tested carbamates are able to react even with the indigenous radicals.

324

The increase in the spin concentration for HA saturated only with CaCl2 follows the trend

325

reported previously for other diamagnetic metal ion, e. g. Zn(II) and Cd(II), interacting with

326

humic acids.14,98,99 The quenching of indigenous radicals by another group of pesticides, that is

327

water-dissolved chlorophenoxyalkanoic acids, was reported previously, but the quenching

328

mechanism was different in that case.3,100 The indigenous radicals were suggested not to react

329

directly with chlorophenoxyalkanoic acids but with radical intermediates of their degradation.

330

Theoretical calculations. In order to obtain a deeper insight into the reactions in which the

331

radical derivatives of carbamates can be formed, the thermodynamics of these processes was

332

evaluated using theoretical methods. Two different mechanisms were considered. One of them,

333

the one proposed above, assumes that a radical (R•) removes a hydrogen atom from the

334

carbamate (carb-H) that itself transforms into a radical (for this H-atom transfer the details are

335

given as Supporting Information):

336

R• + carb–H → R–H + carb•.

(Eq. 11)

337

The one-electron transfer was the second mechanism taken here under theoretical

338

investigation. In this mechanism the carbamate was assumed to contribute an electron to a

339

radical, becoming itself a radical cation in the process:

340

R• + carb–H → R– + carb–H•+.

341

The two mechanisms were previously considered in the context of antiradical activity of

(Eq. 12)



342

Page 18 of 40

phenolic compounds.101,102

343

In the H-atom transfer the X–H bond dissociation enthalpy (BDE) should be considered a

344

crucial parameter, as the H atoms more weakly bonded would be more active in the reaction

345

given in Eq. 11. BDE was calculated at 298 K as the enthalpy difference for the reaction:

346

carb–H → carb• + H•.

(Eq. 13)

347

In order to determine the susceptibility of different hydrogen atoms to the abstraction, the

348

BDE values for the N–H and all C–H bonds were calculated for the two carbamates at DFT

349

(BP86, B3LYP, TPSSh, B2PLYP) and ab initio (OO-SCS-MP2) levels (Table S.2), which have

350

been reported to perform well for radical systems.62,103–107 For the sake of discussion clarity all

351

the C and N atoms forming bonds with hydrogen were labelled as shown in Figure 3. Before

352

performing a detailed analysis, it is reasonable to compare the performance of the computational

353

models. Table S.2 shows that all the employed methods gave a very consistent outcome, making

354

the final conclusions method-independent; in the further discussion the values from the B3LYP

355

calculations are used.

356

For the two carbamates the BDE values predicted for the hydrogen atoms bonded to

357

aromatic carbons are in range 112.1 – 114.3 kcal/mol and are significantly higher than those for

358

the Nb–H and other C–H bonds. For carbaryl the second largest BDE was found for the Nb–H

359

bond (105.5 kcal/mol) and the lowest for the Ca–H bond of the methyl group (93.1 kcal/mol),

360

showing that the latter hydrogen atoms should be the most active in the H-abstraction mechanism

361

in the case of carbaryl. These results for Ca–H and Nb–H are in line with those reported

362

previously for the N-methyl methylcarbamate by N. Borduas et al.34 (93.2 and 106.8 kcal/mol for

363

C–H and N–H, respectively) and for N-n-propyl methylcarbamate by R. J. Berry et al.108 (96.1


Page 19 of 40

364


and 111.4 kcal/mol for C–H and N–H, respectively).

365

In the case of carbofuran the BDE values for the Ca–H and Nb–H bonds are 93.0 and 105.4

366

kcal/mol, respectively, so they very closely resemble their carbaryl counterpart and hence they

367

do not correlate with the higher rates of reactions between carbofuran and the radicals. It differs

368

in structure from carbaryl by the 2,2-dimethyl-3H-furanyl ring which is expected to bring about

369

dissimilar reactivity, which is well manifested in BDE. Although this value for the Cg–H bond is

370

101.6 kcal/mol, the homolytic breaking of the Cf–H bond requires only 84.2 kcal/mol. This is

371

significantly less than the energy necessary to enforce the analogical process for O–H in phenol

372

according to the theoretical predictions of Leopoldini et al.101 (97.1 kcal/mol). Moreover, this

373

value for carbofuran is only slightly higher than the BDE reported for gallic acid (82.3

374

kcal/mol),101 which is known to be a very strong antioxidant able to react with a vast array of

375

radicals. This surprisingly low BDE can be explained by the fact that the homolytic breaking of

376

Cf–H in carbofuran leads to the formation of an aromatic radical, which is stabilized by a

377

significant delocalization of spin density (Figure S.4). It should be noted that the hydrogen

378

abstraction from aromatic carbons results in formation of a σ-type aryl radical with strongly

379

localized spin density.109,110

380



381

382

Figure 3. Labeling of atoms forming bonds with hydrogens in carbaryl (a – i) and carbofuran

383

molecule (a -g’) along with dissociation enthalpies calculated for these bonds at the B3LYP level

384

and at 298 K. Values for phenols are taken from ref.101.

385

In order to compare the capacity of carbaryl and carbofuran to donate an electron to a

386

radical (one-electron transfer), adiabatic ionization potentials (IP) were computed as the enthalpy

387

difference between the carbamate (carb–H) and its radical cation (carb–H•+), as it was done

388

previously for phenolic compounds.101 All the calculated values of IP are listed in Table S.3. It is

389

important to note that the ones obtained using OO-SCS-MP2 are significantly higher in

390

comparison with the results of DFT methods. Recently it has been shown that MP2 and its

391

variants suffer severely from a poor Hartree-Fock reference wave function when they are

392

employed to predict ionization energies unless an orbital optimization (OO) procedure is

393

included in the computation.111 In the latter case, even the accuracy of CCSD(T) can be


Page 20 of 40

Page 21 of 40


394

surpassed. Taking into account that the DFT performance in the IP calculations has been

395

demonstrated as unsatisfactory,111 the values obtained with the OO-SCS-MP2 method ought to

396

be considered as the most accurate and they show that the IP of carbofuran is about 12 kcal/mol

397

lower than that of carbaryl (160.4 and 172.5 kcal/mol, respectively).

398

In order to characterize the processes that can trigger the radical oxidation of carbaryl or

399

carbofuran in greater depth, the Gibbs free energies (∆G) were calculated for the reactions given

400

in Eq. 11 (H-atom abstraction) and Eq. 12 (one-electron transfer) at 298 K. Three different

401

radical substrates were considered: DPPH•, GO• and •OH. The results are summarized in Table

402

S.4.

403

Regardless of the radical substrate in the one-electron transfer the ∆G values are positive,

404

but these results need to be interpreted with caution. As mentioned above DFT methods show

405

general large errors in predicted ionization energies,111 and the calculation of ∆G for the one-

406

electron transfer (Eq. 12) involves two of them, that is for R–/R• and carb-H/carb-H•+. As can be

407

seen from Table S.4. ∆G values obtained using all functionals are alarmingly lower in

408

comparison with OO-SCS-MP2 approach, and should be considered as errors in the DFT-based

409

ionization energy prediction. The OO-SCS-MP2 method shows that ∆G for the one-electron

410

transfer between the two carbamates and all the chosen radicals is high. To illustrate, ∆G is

411

139.0, 94.5 and 93.1 kcal/mol for such a reaction between carbofuran and •OH, DPPH• and GO•,

412

respectively. In the case of carbaryl these values become even higher. All in all, the OO-SCS-

413

MP2 calculations distinctively suggest that for the two studied carbamates the occurrence of one-

414

electron transfer in the soil environment is thermodynamically improbable.

415

In contrast to one-electron transfer, the ∆G values predicted for H-atom abstraction are

416

moderately positive for the model radicals DPPH• and GO• and negative for •OH. For example



417

∆G predicted for carbofuran at the OO-SCS-MP2 level is -32.2, 14.9 and 16.9 kcal/mol for •OH,

418

DPPH• and GO•, respectively; and for carbaryl -22.2, 24.9 and 26.8 kcal/mol, respectively.

419

Although in the case of DPPH• and GO• the reaction is not spontaneous, it was proven by the

420

UV-Vis spectroscopy experiments that the input of energy at 298 K is sufficient to sustain the

421

generation of carb•. It is also notable that the predicted ∆G values for the H-atom abstraction

422

closely correlate with different activity of the two carbamates. In comparison with carbaryl, the

423

∆G values predicted for more reactive carbofuran are noticeably less positive in the case of

424

DPPH• and GO• and more negative in the case of •OH.

425

Implications. The more general significance of this study is that apart from previously

426

confirmed photodegradation,32,112–115 hydrolysis116–118 or biodegradation119–121 the radical

427

oxidation of carbamates should be taken into account when their fate in the soil environment is

428

considered. This process might be particularly important in the presence of environmentally

429

persistent radicals. First, they can induce the formation of •OH (and other reactive oxygen

430

species),122 which subsequently can react with carbamates. Second, it should be assumed that

431

carbamates are likely capable of reacting directly with various environmentally persistent

432

radicals as they were able to react with persistent DPPH• and GO•. And this fact may also open

433

the discussion on more practical application. In the last twenty years much research has been

434

undertaken on the development of soil remediation techniques based on advanced oxidation

435

processes (AOP), in which •OH initiates a series of oxidation reactions.123 Fenton processes are

436

the most commonly used AOP in the remediation of pesticide-contaminated soils, but their direct

437

application is very aggressive to soil and can be devastating to the native microbes.123 The AOP

438

procedures are more applicable in the case of persistent pesticides, while the carbamates are

439

considered to be moderately long-living in soil. However, due to their high toxicity an immediate


Page 22 of 40

Page 23 of 40


440

action may be required if they are released to the environment in an uncontrolled way. Our

441

studies suggest that in such a situation there is a possibility that persistent radicals, that are less

442

harmful to the environment than AOP, can be considered as agents remediating carbamate-

443

contaminated soils.

444

ASSOCIATED CONTENT

445

Supporting information

446

The Supporting Information is available free of charge on the ACS Publications website. It

447

covers additional text concerning pseudo-first-order kinetics and the H-atom transfer mechanism

448

as well as four figures illustrating: the EPR spectrum of DMPO spin adducts, plots showing the

449

linear relation between kobs and [carb-H], molecular structure and singly occupied molecular

450

orbital (SOMO) of galvinoxyl radical (GO•) and the most stable isomer of the radical derived

451

from carbofuran via H-atom abstraction. Four tables contain a summary of theoretical

452

calculations, bond dissociation enthalpies, adiabatic ionization potentials, and the values of

453

Gibbs free energy for the H-atom and one-electron transfer.

454

AUTHOR INFORMATION

455

Corresponding Authors

456

*Author 1: Phone (0048) 713205635; [email protected]

457

Author 2: Phone (0048) 713757215; [email protected]

458

Notes

459

The authors declare no competing financial interest.

460

ACKNOWLEDGMENTS

461

This work was financially supported by the National Science Centre (NCN), Project No.



462

2012/05/D/ST10/02223. All computations were performed using computers of the Wroclaw

463

Center for Networking and Supercomputing (Grant No. 47).

464

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Can Carbamates Undergo Radical Oxidation in the Soil Environment

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