Article pubs.acs.org/est
Abiotic Reduction Reactions of Dichloroacetamide Safeners: Transformations of “Inert” Agrochemical Constituents John D. Sivey and A. Lynn Roberts* Department of Geography and Environmental Engineering, Johns Hopkins University, 313 Ames Hall, 3400 North Charles Street, Baltimore, Maryland 21218, United States S Supporting Information *
ABSTRACT: Safeners are so-called “inert” constituents of herbicide formulations added to protect crops from the toxic effects of herbicides. We examined the reactivity of three dichloroacetamide safeners and 12 structural analogues [all neutral compounds of the form Cl2CXC(O)NRR′; X = H, Cl; R-groups include alkyl, branched alkyl, n-allyl, and cyclic moieties] in one homogeneous and two heterogeneous reductant systems: solutions of Cr(H2O)62+, suspensions of FeII-amended goethite, and suspensions of FeII-amended hematite. Analyses of reaction products indicate each safener can undergo stepwise hydrogenolysis (replacement of chlorine by hydrogen) in each system at near-neutral pH. The first hydrogenolysis step generates compounds similar (in one case, identical) to herbicide active ingredients. Rates of product formation and (when reactions were sufficiently fast) parent loss were quantified; reaction rates in heterogeneous systems spanned 2 orders of magnitude and were strongly influenced by R-group structure. The length of n-alkyl R-groups exerted opposite effects on hydrogenolysis rates in homogeneous versus heterogeneous systems: as R-group size increased, reduction rates in heterogeneous systems increased, whereas reduction rates in the homogeneous system decreased. Branched alkyl R-groups decreased hydrogenolysis rates relative to their straight-chain homologues in both homogeneous and heterogeneous systems. Reaction rates in heterogeneous systems can be described via polyparameter linear free energy relationships employing molecular parameters likely to influence dichloroacetamide adsorption. The propensity of dichloroacetamide safeners to undergo reductive transformations into herbicide-like products challenges their classification as “inert” agrochemical ingredients.
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INTRODUCTION Most herbicides have the potential to damage crop plants in addition to killing the targeted weed species. An additional class of chemicals, called herbicide safeners, is frequently used to protect crop plants from the deleterious effects of herbicides.1 In the United States and some European countries, safeners are classified as “inert” ingredients.1 Reports on the environmental fate of safeners are scarce.2−5 Moreover, only recently have studies emerged regarding the environmental fate of safener transformation products.6,7 Dichloroacetamides represent one of the most commonly used classes of safeners.1 Dichloroacetamide safeners, such as benoxacor (BN), dichlormid (DL), and AD 67 (AD), are commonly included in formulations of thiocarbamate or chloroacetamide herbicides.1 Whereas chloroacetamide herbicides contain a single chlorine atom, dichloroacetamide safeners generally contain two geminal chlorine atoms (Table 1). Dichloroacetamide safeners are applied at a rate estimated to exceed 1.7 million kg/year in the United States (see Supporting Information). Their generally lower Kow values and comparable soil half-lives (DT50) relative to those of chloroacetamide herbicides (Table 1) suggest the potential for runoff into surface water and percolation into groundwater. Nonetheless, © 2012 American Chemical Society
the environmental occurrence of dichloroacetamide safeners does not appear to have been previously investigated. Herbicides with structures similar to and application rates comparable to dichloroacetamide safeners have, however, been detected in soil,12 surface water,13 groundwater,14 and finished drinking water.15 Chloroacetamide herbicides are susceptible to conjugation with glutathione, a biotransformation mediated by plants16 and soil bacteria.17 The second geminal chlorine of dichloroacetamides should reduce their reactivity toward nucleophilic substitution reactions, making dichloroacetamides less likely to form conjugates with glutathione. Therefore, we postulate that a larger fraction of these safeners will enter oligotrophic environments (such as aquifers) in an unaltered form. Here, they could be exposed to mixed-valent iron (hydr)oxides and could undergo reductive dechlorination reactions. In these environments, iron (hydr)oxide-associated FeII species may serve as a reductant, as has been observed in laboratory investigations of several other compound classes, including organohalides,18 carbamates,19 and nitrobenzenes.20 Received: Revised: Accepted: Published: 2187
October 22, 2011 January 6, 2012 January 11, 2012 February 2, 2012 dx.doi.org/10.1021/es203755h | Environ. Sci. Technol. 2012, 46, 2187−2195
Environmental Science & Technology
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Table 1. Examples of Chloroacetamide Herbicides and Dichloroacetamide Safenersa
a References for data are shown in parentheses. bOctanol−water partition coefficients (Kow) estimated via ChemSketch Freeware (Advanced Chemistry Development, v.12.01). cDT50 = 50% dissipation time (time required for 50% loss of parent compound in aerobic soil). dAD 67 is also referred to as MON 4660.
Iron (hydr)oxide-associated FeII has also been implicated as an abiotic reductant of organic micropollutants in natural systems, including soils21 and aquifers.22 The Langmuir−Hinshelwood−Hougen−Watson (LHHW) model is commonly employed to describe reaction kinetics involving solid catalysts.23,24 The LHHW kinetic model assumes that heterogeneous reactions proceed through three steps: adsorption of a reactant to a surface, reaction at the surface, and desorption of the product from the surface. For reactions of organic oxidants (iox) with FeII adsorbed on iron (hydr)oxides, these three steps can be represented by eqs 1 − 3: >FeIII−O−FeII + iox (aq) ⇌ >Fe III−O−Fe II‐‐‐i ox
(1)
>Fe III−O−Fe II‐‐‐iox → >Fe III−O−Fe III‐‐‐i red
(2)
of nonionic organic compounds plays on heterogeneous reaction rates has received relatively little attention.25,26 When surface reaction (eq 2) is rate-limiting, the LHHW model of the overall reaction rate can be expressed as −
d[iox ] kK ox [Fe II]ads [iox ] = dt 1 + K ox [iox] + K red[i red]
(4)
where k (per second) is the surface reaction rate constant; Kox (liters per mole) is the adsorption equilibrium constant of iox; [FeII]ads (moles per liter) is the adsorbed FeII concentration; [iox] and [ired] are aqueous concentrations (moles per liter) of the oxidized and reduced form, respectively, of the organic compound in question; and Kred (liters per mole) is the adsorption equilibrium constant for ired.23 LHHW rate equations for cases in which reactant adsorption or product desorption is rate-limiting are discussed in the Supporting Information. In the special case of poorly adsorbing organic reactants and products, Kox[iox] + Kred[ired] ≪ 1. With this assumption, eq 4 simplifies to
>FeIII−O−FeIII‐‐‐i red ⇌ > FeIII−O−FeIII + i red(aq) (3)
where >FeIII−O−FeII denotes FeII adsorbed on an iron (hydr)oxide and ired represents a reduced organic product. Even when the fraction of adsorbed iox is small, adsorption can still play a vital role in determining rates of reduction in heterogeneous systems. Nonetheless, the role that adsorption
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d[iox ] = kK ox [Fe II]ads [iox ] dt
(5)
dx.doi.org/10.1021/es203755h | Environ. Sci. Technol. 2012, 46, 2187−2195
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If [FeII]ads ≫ [iox] and [FeII]ads remains nearly constant as reactions proceed, eq 5 reduces to a pseudo-first-order equation:
−
d[iox ] = kobs[iox ] dt
moieties on rates of organic contaminant transformations in heterogeneous systems.
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EXPERIMENTAL SECTION A description of reagents and synthesis procedures for safener analogues is provided in the Supporting Information. Safener analogues [of the form Cl2CHC(O)NR2] included the following R-groups: ethyl (ED), n-propyl (PD), iso-propyl (iPD), n-butyl (BD), iso-butyl (iBD), sec-butyl (sBD), n-pentyl (PeD), n-hexyl (HD), cyclohexyl (ChD), and phenyl (PhD). Two safener analogues possessing three geminal chlorine atoms [Cl3CC(O)NR2, R = allyl (TD) or n-propyl (TP)] were also synthesized. Safener analogues were selected on the basis of the systematic structural changes imparted by their R-groups. With the possible exception of PhD, variations among dichlorinated structural analogues are anticipated to influence the reactivity of these molecules primarily via steric and/or adsorption effects. The electrophilicity of the (chlorinated) α-carbon is expected to be similar among the nonaromatic analogues possessing two chlorine atoms. General Experimental Design. The safeners BN, AD, and DL, as well as the aforementioned 10 dichlorinated analogues, served as the experimental set for reactivity studies in three model reductant systems: FeII-amended goethite, FeII-amended hematite, and homogeneous solutions of Cr(H2O)62+. All reactions were performed at room temperature (21 ± 1 °C) inside 40-mL amber glass vials with Teflon-lined caps housed inside an anaerobic chamber (5% H2, 95% N2 atmosphere). Total reaction solution or suspension volumes were 15 mL. Vials were rinsed with HNO3 followed by Milli-Q-treated distilled, deionized water (DDW, 18 MΩ·cm resistivity, Millipore Corp.) prior to use. Aqueous solutions were prepared with DDW that was sparged with high-purity N2 before transfer to an anaerobic chamber and further sparged with chamber atmosphere before use. Reactions were initiated by adding chloroacetamide as a methanolic spike. Reactors were periodically sampled by extracting aliquots of reaction solutions or suspensions into toluene. Additional experimental details for each reductant system are provided in the Supporting Information. Experiments were also performed with selected dichloroacetamides to examine the effects of various conditions, including total added FeII concentration ([FeII]tot, 1.5−11 mM), initial dichloroacetamide concentration (8−90 μM), solids loading (0.5−7.9 g/L), and pH (6.1−7.4), on transformation rates in FeII-amended goethite systems. Control experiments were performed to examine the possible influence of hydrolysis, homogeneous reactions with FeII, and reactions with goethite and hematite in the absence of added FeII. Qualitative determinations of chloroacetamide reaction products were performed by analyzing toluene extracts via gas chromatography (GC) with mass-selective (MS) detection. Quantitative analyses of parent compounds and identified reaction products were performed via GC with microelectron capture detection (μECD) or via GC/MS. Concentrations of dissolved FeII were determined in filtered aliquots using the ferrozine colorimetric method.31 Additional details of each analytical method are provided in the Supporting Information (Tables S1 and S2). Modeling of Kinetic Data. For reactions of chloroacetamides with FeII-amended goethite or hematite, loss of parent compounds was monitored and kobs values were calculated according to eq 6. Formation of hydrogenolysis products was
(6)
II
where kobs = kKox[Fe ]ads. In heterogeneous systems, values of kobs (per second) are commonly normalized to [FeII]ads: kFe(II)ads = kobs/[Fe II]ads = kK ox
(7)
where kFe(II)ads has units of liters per mole per second. Equation 7 can be rewritten in logarithmic form as ln kFe(II)ads = ln k + ln K ox
(8)
Of the various interactions affecting adsorption (Kox) of nonionic organic compounds at the mineral−water interface, London (dispersion) forces are likely to be important.27 Specific interactions, including a variety of hydrogen (or electron) donor/acceptor interactions, may dominate adsorption of nonionic compounds possessing polar functional groups and/or π-electrons.28 Although adsorption is an assumed prerequisite of surface reactions, if adsorption occurs away from reactive surface sites, interfacial reaction rates can be attenuated.25 As with other neutral organic compounds, Kox values for dichloroacetamides are likely to be a function of the solute’s molar volume (Vix), excess molar refractivity (E, a measure of van der Waals interaction energies), polarizability (S, a measure of how readily electron orbitals can be distorted to facilitate interactions with surfaces), and ability to participate in H-donor (A) and H-acceptor (B) interactions. These parameters (as surrogates of noncovalent interactions between neutral organic sorbates and a mineral surface) can be substituted for ln Kox in eq 8 to give a polyparameter linear free energy relationship (ppLFER)29 describing the reactivity of dichloroacetamides: ln kFe(II)ads = vVix + e(E) + s(S) + a(A) + b(B) + f (9)
where the model coefficients (v, e, s, a, and b) and constant ( f) can be calculated via nonlinear regression analysis of reaction rate data obtained from heterogeneous systems. As the redoxactive moiety of each dichloroacetamide is ostensibly identical, the intrinsic rate of interfacial electron transfer (k) is assumed to be nearly constant for each dichloroacetamide and is represented as a component of f in eq 9 The goal of the present work is to evaluate the reactivity of dichloroacetamide safeners and structural analogues with FeII adsorbed onto goethite and hematite [the two most abundant iron (hydr)oxides in soils]30 and in solutions of Cr(H2O)62+ (employed as a model homogeneous reductant to enable reactivity comparisons). Experiments were designed to elucidate (i) the rates and products of dichloroacetamide safener reactions in these model systems and (ii) the influence of R-substituent structure on reduction rates of dichloroacetamides. As dichloroacetamide safeners differ only in the structures of their R-groups, understanding their environmental fate requires elucidating the effects R-substituents exert on abiotic reaction rates. Moreover, by delineating the separate influences of surface reaction and adsorption on overall reactivity (eq 9), the results described herein explore the often-overlooked effects of apolar (or weakly monopolar) 2189
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Figure 1. Time courses for the reaction of (A) benoxacor (BN), (B) AD 67 (AD), (C) dichlormid (DL), and (D) N,N-diallyl-2,2,2trichloroacetamide (TD) with FeII-amended goethite. Uniform conditions: [MOPS buffer] = 30 mM, [NaCl] = 50 mM, T = 21 ± 1 °C. Conditions for BN and DL: pH 6.6, [FeII]tot = 3.0 mM, goethite loading = 7.2 g/L. Conditions for AD: pH 6.9, [FeII]tot = 3.4 mM, goethite loading = 9.9 g/L. Conditions for TD: pH 6.9, [FeII]tot = 0.5 mM, goethite loading = 0.9 g/L. Lines represent model fits that assume the reaction pathways shown. Reference materials were available for all compounds shown except I, II, and III. Concentrations of I and II were estimated by assuming that (i) I and II have equal molar response factors as CDAA when analyzed by GC/μECD and (ii) a closed mass balance is achieved such that [DL]0 = [DL] + [CDAA] + [I] + [II]. Concentrations of III were estimated by assuming the GC/MS (full scan) molar response factor of III was equal to that of DL. The product(s) resulting from possible transformations of III were not identified.
Scheme 1. Postulated Product Formation Pathways for Reaction of DL with FeII-Amended Goethite
of ppLFERs (described below) were performed by use of Scientist v3.0 (MicroMath).
also monitored. When reactions were sufficiently slow as to preclude calculation of kobs from eq 6, kobs values were calculated from initial rates of product formation under conditions of 8.5 mM, reaction rates increased sharply, possibly due to formation of redox-active solid phase(s) containing FeII. To avoid this complication and to ensure approximately constant [FeII]tot upon reaction with dichloroacetamides (at [dichloroacetamide]0 ≤ 0.1 mM), all experiments described hereafter involving FeII were performed at [FeII]tot ≈ 3 mM. Reactions of Structural Analogues. To examine the influence of R-group structure on rates of surface-mediated reduction, reaction rates of a series of dichloroacetamide structural analogues were measured in FeII-amended goethite and hematite systems. Assuming the redox-active site on each dichlorinated analogue is identical, any observed differences in reduction rates can be postulated to originate from varying degrees of adsorption and/or steric effects. To help delineate the influence of steric effects, reduction rates in homogeneous solutions of Cr(H2O)62+ were also measured. The results for all three reductant systems are shown in Figure 2. Relative reactivity trends among the dichloroacetamides are influenced by the identity of the reductant (Figure 2). In solutions of CrII, the most reactive dichloroacetamide (BN) is 7 times more reactive than the least reactive dichloroacetamide (iPD). The analogous reactivity range is 200 in the FeII/ goethite system (most reactive, HD; least reactive, iPD) and at least 570 in the FeII/hematite system (most reactive, HD; lowest quantified reactivity, iBD). These findings suggest that reduction rates in the heterogeneous systems are more sensitive to R-group structure than are rates of homogeneous reduction by Cr(H2O)62+. Reactivity trends for the FeII/goethite data track closely with those of the FeII/hematite data (Figure 2). Structural changes that enhance or attenuate interfacial reactivity in goethite suspensions appear to impart similar effects in hematite suspensions. A log−linear correlation is observed when the FeII/hematite data are plotted against the FeII/goethite data (Figure 3). Rate constants normalized to [FeII]ads are consistently greater in goethite reactors relative to hematite reactors, suggesting that reduction sites in the goethite system may be more reactive on a molar basis than those in the hematite system. The same trend (FeII/goethite more reactive than FeII/hematite) was reported previously for reduction of carbon tetrachloride.36 In both heterogeneous systems, reaction rates generally increase as R-group size increases. We posit that increases in Vix (eq 9) result in a greater extent of adsorption at the solid− solution interface with concomitant increases in dichloroacetamide reduction rates. In CrII systems, on the other hand, reduction rates generally decrease with increasing size of nonaromatic R-groups (Figure 2). In all three systems, compounds with branched alkyl groups typically react more slowly than compounds with homologous n-alkyl groups (e.g., sBD < BD and iPD < PD, Figure 2). These results suggest that steric and/or conformational constraints on reactivity can influence reduction rates in homogeneous and heterogeneous systems. Replacing a saturated R-group with an unsaturated moiety results in enhanced reactivity in all reductant systems (e.g., DL > PD; PhD > ChD). The presence of π-electrons in these R-substituents may increase rates of precursor complex formation with dissolved and interfacial reductants. Development of a ppLFER. Differences in reduction potentials (Eh1) are routinely invoked to explain reactivity
Figure 3. Comparison of rate constants normalized to [FeII]ads (kFe(II)ads) for reaction of dichloroacetamides (20 μM) with FeIIamended hematite versus FeII-amended goethite. Solid line denotes linear regression to the data; error estimates represent 95% confidence intervals. Broken line denotes ln kFe(II)ads,hematite = ln kFe(II)ads,goethite. Structures of each dichloroacetamide are shown in Figure 2. FeII/ hematite conditions: pH 6.7, [MOPS buffer] = 30 mM, [NaCl] = 50 mM, [FeII]tot = 3.4 mM, FeIIads = 1.5 mM, hematite loading = 2.1 g/L. FeII/goethite conditions: pH 6.9, [MOPS buffer] = 30 mM, [NaCl] = 50 mM, [FeII]tot ≈ [FeII]ads = 3.4 mM, goethite loading = 9.9 g/L. All experiments were conducted at 21 ± 1 °C.
trends for reductive transformations of organic contaminants (e.g., ref 20). For n-alkyl- and allyl-substituted dichloroacetamides, calculated LUMO energies (ELUMO, a surrogate parameter for Eh1) differ by
