Ipso Substitution of Aromatic Bromine in Chlorinated Waters: Impacts on Trihalomethane Formation.

Parabens and salicylates were examined as disinfection byproduct (DBP) precursors to explore the possible influence of ipso substitution (i.e., halogen exchange) on the yield and speciation of trihalomethanes (THMs) formed during water chlorination. Substoichiometric conversion of C-Br bonds into C-Cl bonds was confirmed for several parabens and salicylates. The co-occurrence of (mono)brominated and nonhalogenated precursors in the presence of free chlorine (but in the absence of added Br-) generated polybrominated THMs, implicating ipso substitution. The THM molar yield, bromine incorporation, and bromine recovery from brominated and nonhalogenated precursor mixtures were commensurate with those observed from equimolar additions of NaBr, indicating efficient displacement of aromatic bromine by free chlorine followed by reincorporation of liberated HOBr into DBP precursors. The THM molar yield from brominated precursors was enhanced by a factor of ≤20 relative to that from nonhalogenated precursors. Trends in THM molar yields and bromine incorporation differed between brominated parabens and brominated salicylates, suggesting that the influence of ipso substitution on THM formation varies with the structure of the organic precursor. Collectively, these results provide new evidence of the often-overlooked role ipso substitution can play in promoting halogen exchange and bromine enrichment among DBPs in chlorinated waters.

Chloride Enhances DNA Reactivity with Chlorine under Conditions Relevant to Water Treatment.

Free available chlorine (FAC) is widely used to inactivate viruses by oxidizing viral components, including genomes. It is commonly assumed that hypochlorous acid (HOCl) is the chlorinating agent responsible for virus inactivation; however, recent studies have underscored that minor constituents of FAC existing in equilibrium with HOCl, such as molecular chlorine (Cl2), can influence FAC reactivity toward select organic compounds. This study measures the FAC reaction kinetics with dsDNA and ssDNA extracted from representative bacteriophages (T3 and ϕX174) in samples augmented with chloride. Herein, chloride enhances FAC reactivity toward dsDNA and, to a lesser extent, toward ssDNA, especially at pH < 7.5. The enhanced reactivity can be attributed to the formation of Cl2. Second-order rate constants were determined for reactions of ssDNA and dsDNA with HOCl and Cl2. DNA chlorination kinetics followed the reactivity-selectivity principle, where the more-reactive nucleophilic species (ssDNA, ∼100× more reactive than dsDNA) reacted less selectively with electrophilic FAC species. The addition of chloride was also shown to enhance the inactivation of bacteriophage T3 (dsDNA genome) by FAC but did not enhance the inactivation of bacteriophage ϕX174 (ssDNA genome). Overall, the results suggest that Cl2 is an important chlorinating agent of nucleic acids and viruses.

Acid- and Base-Mediated Hydrolysis of Dichloroacetamide Herbicide Safeners.

Safeners are used extensively in commercial herbicide formulations. Although safeners are regulated as inert ingredients, some of their transformation products have enhanced biological activity. Here, to fill gaps in our understanding of safener environmental fate, we determined rate constants and transformation products associated with the acid- and base-mediated hydrolysis of dichloroacetamide safeners AD-67, benoxacor, dichlormid, and furilazole. Second-order rate constants for acid- (HCl) and base-mediated (NaOH) dichloroacetamide hydrolysis (2.8 × 10-3 to 0.46 and 0.3-500 M-1 h-1, respectively) were, in many cases (5 of 8), greater than those reported for their chloroacetamide herbicide co-formulants. In particular, the rate constant for base-mediated hydrolysis of benoxacor was 2 orders of magnitude greater than that of its active ingredient co-formulant, S-metolachlor. At circumneutral pH, only benoxacor underwent appreciable hydrolysis (5.3 × 10-4 h-1), and under high-pH conditions representative of lime-soda softening, benoxacor's half-life was 13 h─a timescale consistent with partial transformation during water treatment. Based on Orbitrap LC-MS/MS analysis of dichloroacetamide hydrolysis product mixtures, we propose structures for major products and three distinct mechanistic pathways that depend on the system pH and compound structure. These include base-mediated amide cleavage, acid-mediated amide cleavage, and acid-mediated oxazolidine ring opening. Collectively, this work will help to identify systems in which hydrolysis contributes to the transformation of dichloroacetamides, while also highlighting important differences in the reactivity of dichloroacetamides and their active chloroacetamide co-formulants.

Synthesis of 13 C-labeled parabens from isotopically enriched phenols using the Houben-Hoesch reaction.

Parabens are antimicrobial additives found in a wide array of consumer products. However, the halogenated compounds formed from parabens during wastewater disinfection are a potential environmental concern. In order to identify these transformation products and investigate their mechanism of formation, a synthetic route to ethyl parabens labeled with the stable isotope carbon-13 at specific positions within the benzene ring was developed. This efficient two-step procedure starts from commercially available 13 C-labeled phenols and involves (1) initial acylation of the phenol via a Houben-Hoesch reaction with trichloroacetonitrile followed by (2) a modified haloform reaction of the resulting trichloromethyl ketone to afford the corresponding 13 C-labeled ethyl parabens in 65%-80% overall yield. The scope of the modified haloform reaction was also investigated, allowing for the synthesis of other parabens derived from primary or secondary alcohols, including 13 C- and deuterium-labeled esters. In addition, 4-hydroxybenzoic acid can be formed directly from the common trichloromethyl ketone intermediate upon treatment with lithium hydroxide. This protocol complements existing methods for preparing 13 C-labeled paraben derivatives and offers the specific advantages of exhibiting complete regioselectivity in the Houben-Hoesch reaction (to form the para-disubstituted product) and avoiding the need for protecting groups in the modified haloform reaction that forms the paraben esters.

Structural effects on the bromination rate and selectivity of alkylbenzenes and alkoxybenzenes in aqueous solution.

Aqueous free bromine species (e.g., HOBr, BrCl, Br2, BrOCl, Br2O, and H2OBr+) can react with activated aromatic compounds via electrophilic aromatic substitution to generate products with industrial applications, environmental consequences, and potentially adverse biological effects. The relative contributions of these brominating agents to overall bromination rates can be calculated via nonlinear regression analyses of kinetic data collected under a variety of solution conditions, including variations in parameters (e.g., [Cl-], [Br-], and pH) known to influence free bromine speciation. Herein, kinetic experiments conducted in batch reactors were employed to evaluate the contributions of steric and electronic effects on bromination of monosubstituted alkylbenzenes (ethyl, isopropyl, tert-butyl) and alkoxybenzenes (ethoxy, isopropoxy, tert-butoxy) and to elucidate the inherent reactivities of aqueous brominating agents towards these aromatic compounds. For bromination at the para position of alkylbenzenes, overall reactivity increased from tert-butyl < ethyl ≈ isopropyl. For bromination at the para position of alkoxybenzenes, reactivity increased from tert-butoxy < ethoxy < isopropoxy. In going from ethyl to tert-butyl and ethoxy to isopropoxy, unfavorable steric effects attenuated the favorable electronic effects imparted by the substituents. When comparing unsubstituted benzene, alkyl-, and alkoxybenzenes, the structure of the substituent has a significant effect on bromination rates, nucleophile regioselectivity, and electrophile chemoselectivity. Hirshfeld charges were useful predictors of reactivity and regioselectivity. The experimental results were also modeled using Taft equations. Collectively, these findings indicate that steric effects, electronic effects, and brominating agents other than HOBr can influence aromatic compound bromination in solutions of free bromine.

Sunlight Photolysis of Safener Benoxacor and Herbicide Metolachlor as Mixtures on Simulated Soil Surfaces.

Benoxacor is a safener paired with the high-use herbicide S-metolachlor. Commercial formulations containing both compounds are sprayed onto soil pre-emergence to enhance yields of corn. In this study, we evaluated the sunlight photolysis of metolachlor and benoxacor, individually and as mixtures, in three different reaction environments: in water and on two soil-simulating surfaces (quartz and kaolinite). When irradiated individually, benoxacor degraded at least 19 times faster than metolachlor in each reaction environment, consistent with its higher molar absorptivity within the solar spectrum than metolachlor. When metolachlor and benoxacor were irradiated as mixtures, benoxacor promoted metolachlor degradation on quartz and, to a lesser extent, in water, but not on kaolinite. On quartz, at a benoxacor/metolachlor molar ratio of 0.1:1, metolachlor degraded 1.8 times faster than in the absence of benoxacor; as the benoxacor/metolachlor ratio increased, metolachlor degradation rate also increased. The photolysis rate of benoxacor depended on its initial surface concentration and was promoted by metolachlor. Benoxacor photoproducts were capable of absorbing sunlight and serving as photosensitizers for metolachlor degradation. These results illustrate how a safener can influence the photochemistry of its coformulated herbicide and suggest that such mixture effects should be considered when evaluating the environmental fate of agrochemicals.

Photochemical Transformations of Dichloroacetamide Safeners.

Dichloroacetamide safeners are commonly added to commercial chloroacetamide herbicide formulations and widely used worldwide, but their environmental fate has garnered little scrutiny as a result of their classification as "inert" ingredients. Here, we investigated the photolysis of dichloroacetamide safeners to better understand their persistence and the nature of their transformation products in surface waters. High-resolution mass spectrometry (HRMS) and nuclear magnetic resonance (NMR) spectroscopy were used to characterize photoproducts. Of the four commonly used dichloroacetamide safeners, only benoxacor undergoes direct photolysis under simulated natural sunlight ( t1/2 ∼ 10 min). Via a photoinitiated ring closure, benoxacor initially yields a monochlorinated intermediate that degrades over longer irradiation time scales to produce two fully dechlorinated diastereomers and a tautomer, which further photodegrade over several days to a structurally related aldehyde confirmed via NMR. Dichlormid, AD-67, and furilazole were more slowly degraded by indirect photolysis in the presence of the photosensitizers nitrate, nitrite, and humic acid. Reactive entities involved in these reactions are likely hydroxyl radical and singlet oxygen based on the use of selective quenchers. These safeners also directly photolyzed under higher energy ultraviolet (UV) light, suggesting their potential transformation in engineered systems using UV for disinfection. The finding that dichloroacetamide safeners can undergo photolysis in environmental systems over relevant time scales demonstrates the importance of evaluating the fate of this class of "inert" agrochemicals.

Contributions of BrCl, Br2, BrOCl, Br2O, and HOBr to regiospecific bromination rates of anisole and bromoanisoles in aqueous solution.

When bromide-containing waters are chlorinated, conventional wisdom typically assumes HOBr is the only active brominating agent. Several additional and often-overlooked brominating agents (including BrCl, Br2, BrOCl, Br2O) can form in chlorinated waters, albeit at generally lower concentrations than HOBr. The extent to which these additional brominating agents influence bromination rates of disinfection byproduct precursors is, however, poorly understood. Herein, the influence of BrCl, Br2, BrOCl, Br2O, and HOBr toward rates of sequential bromination of anisole was quantified. Conditions affecting bromine speciation (e.g., pH, concentrations of chloride, bromide, and chlorine) were varied, and regiospecific second-order rate constants were calculated for reactions of each brominating agent with anisole, 2-bromoanisole, and 4-bromoanisole. The regioselectivity of anisole bromination changed with pH, consistent with the participation of more than one brominating agent. Under conditions representative of chlorinated drinking water, contributions to bromination rates decreased as BrCl > BrOCl > HOBr > Br2O (Br2 negligible). The second-order rate constant determined for net bromination of anisole by HOBr is up to 3000-times less than reported in previous studies (which assumed HOBr was the only active brominating agent). Accordingly, models that assume HOBr is the only kinetically relevant brominating agent in solutions of free bromine may be insufficient for reactions involving modestly nucleophilic organic compounds.

Role of lysine during protein modification by HOCl and HOBr: halogen-transfer agent or sacrificial antioxidant?

Although protein degradation by neutrophil-derived hypochlorous acid (HOCl) and eosinophil-derived hypobromous acid (HOBr) can contribute to the inactivation of pathogens, collateral damage to host proteins can also occur and has been associated with inflammatory diseases ranging from arthritis to atherosclerosis. Though previous research suggested halotyrosines as biomarkers of protein damage and lysine as a mediator of the transfer of a halogen to tyrosine, these reactions within whole proteins are poorly understood. Herein, reactions of HOCl and HOBr with three well-characterized proteins [adenylate kinase (ADK), ribose binding protein, and bovine serum albumin] were characterized. Three assessments of oxidative modifications were evaluated for each of the proteins: (1) covalent modification of electron-rich amino acids (assessed via liquid chromatography and tandem mass spectrometry), (2) attenuation of secondary structure (via circular dichroism), and (3) fragmentation of protein backbones (via sodium dodecyl sulfate-polyacrylamide gel electrophoresis). In addition to forming halotyrosines, HOCl and HOBr converted lysine into lysine nitrile (2-amino-5-cyanopentanoic acid), a relatively stable and largely overlooked product, in yields of up to 80%. At uniform oxidant levels, fragmentation and loss of secondary structure correlated with protein size. To further examine the role of lysine, a lysine-free ADK variant was rationally designed. The absence of lysine increased yields of chlorinated tyrosines and decreased yields of brominated tyrosines following treatments with HOCl and HOBr, respectively, without influencing the susceptibility of ADK to HOX-mediated losses of secondary structure. These findings suggest that lysine serves predominantly as a sacrificial antioxidant (via formation of lysine nitrile) toward HOCl and as a halogen-transfer mediator [via reactions involving ε-N-(di)haloamines] with HOBr.

Reactivity of BrCl, Br2, BrOCl, Br2O, and HOBr toward dimethenamid in solutions of bromide + aqueous free chlorine.

HOBr, formed via oxidation of bromide by free available chlorine (FAC), is frequently assumed to be the sole species responsible for generating brominated disinfection byproducts (DBPs). Our studies reveal that BrCl, Br(2), BrOCl, and Br(2)O can also serve as brominating agents of the herbicide dimethenamid in solutions of bromide to which FAC was added. Conditions affecting bromine speciation (pH, total free bromine concentration ([HOBr](T)), [Cl(-)], and [FAC](o)) were systematically varied, and rates of dimethenamid bromination were measured. Reaction orders in [HOBr](T) ranged from 1.09 (±0.17) to 1.67 (±0.16), reaching a maximum near the pK(a) of HOBr. This complex dependence on [HOBr](T) implicates Br(2)O as an active brominating agent. That bromination rates increased with increasing [Cl(-)], [FAC](o) (at constant [HOBr](T)), and excess bromide (where [Br(-)](o)>[FAC](o)) implicate BrCl, BrOCl, and Br(2), respectively, as brominating agents. As equilibrium constants for the formation of Br(2)O and BrOCl (aq) have not been previously reported, we have calculated these values (and their gas-phase analogues) using benchmark-quality quantum chemical methods [CCSD(T) up to CCSDTQ calculations plus solvation effects]. The results allow us to compute bromine speciation and hence second-order rate constants. Intrinsic brominating reactivity increased in the order: HOBr ≪ Br(2)O < BrOCl ≈ Br(2) < BrCl. Our results indicate that species other than HOBr can influence bromination rates under conditions typical of drinking water and wastewater chlorination.

Microbial Biotransformation Products and Pathways of Dichloroacetamide Herbicide Safeners.

Dichloroacetamide safeners are common ingredients in commercial herbicide formulations. We previously investigated the environmental fate of dichloroacetamides via photolysis and hydrolysis, but other potentially important, environmentally relevant fate processes remain uncharacterized and may yield products of concern. Here, we examined microbial biotransformation of two dichloroacetamide safeners, benoxacor and dichlormid, to identify products and elucidate pathways. Using aerobic microcosms inoculated with river sediment, we demonstrated that microbial biotransformations of benoxacor and dichlormid proceed primarily, if not exclusively, via cometabolism. Benoxacor was transformed by both hydrolysis and microbial biotransformation processes; in most cases, biotransformation rates were faster than hydrolysis rates. We identified multiple novel products of benoxacor and dichlormid not previously observed for microbial processes, with several products similar to those reported for structurally related chloroacetamide herbicides, thus indicating potential for conserved biotransformation mechanisms across both chemical classes. Observed products include monochlorinated species such as the banned herbicide CDAA (from dichlormid), glutathione conjugates, and sulfur-containing species. We propose a transformation pathway wherein benoxacor and dichlormid are first dechlorinated, likely via microbial hydrolysis, and subsequently conjugated with glutathione. This is the first study reporting biological dechlorination of dichloroacetamides to yield monochlorinated products in aerobic environments.

Abiotic reduction reactions of dichloroacetamide safeners: transformations of "inert" agrochemical constituents.

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 Cl(2)CXC(═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(H(2)O)(6)(2+), suspensions of Fe(II)-amended goethite, and suspensions of Fe(II)-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.

Assessing the reactivity of free chlorine constituents Cl2, Cl2O, and HOCl toward aromatic ethers.

Cl(2) and Cl(2)O are highly reactive electrophiles capable of influencing rates of disinfection byproduct (DBP) precursor chlorination in solutions of free available chlorine (FAC). The current work examines how organic compound structure influences susceptibility toward chlorination by Cl(2) and Cl(2)O relative to the more abundant (but less reactive) electrophile HOCl. Chlorination rates and products were determined for three aromatic ethers, whose reactivities with FAC increased in the order: 3-methylanisole <1,3-dimethoxybenzene <1,3,5-trimethoxybenzene. Varying solution conditions (pH, [FAC], [Cl(-)]) permitted quantification of regiospecific second-order rate constants for formation of each product by Cl(2), Cl(2)O, and HOCl. Our results indicate that as the reactivity of methoxybenzenes decreases, the importance of Cl(2) and Cl(2)O (relative to HOCl) increases. Accordingly, Cl(2) and Cl(2)O are likely to play important roles in generating DBPs that originate from natural organic matter (NOM) constituents of somewhat moderate reactivity. As [Cl(2)] is proportional to [Cl(-)] and [Cl(2)O] is proportional to [HOCl](2), ramifications for DBP control measures may differ significantly for these precursors compared to more reactive NOM moieties likely to react predominantly with HOCl. In particular, the role of chloride as a chlorination catalyst challenges its traditional classification as an "inert" electrolyte in water treatment processes.

Comparative Toxicity of Herbicide Active Ingredients, Safener Additives, and Commercial Formulations to the Nontarget Alga Raphidocelis Subcapitata.

Chloroacetanilide herbicides are used worldwide to control weeds that affect crops such as corn, soybeans, and cotton. These herbicides are frequently paired with a "safener," which prevents herbicidal damage to the crop without diminishing weed control. Formulated herbicide products that include safeners and other ingredients are infrequently assessed for toxicity. Our goal was to understand the potential toxicity of safeners and herbicide + safener formulations relative to the toxicity of associated active ingredients. We quantified the concentration of safeners in commercially available formulations and tested effects on nontarget algae, Raphidocelis subcapitata, when exposed to individual herbicide active ingredients, safeners, and commercial formulations. The median effective concentrations (EC50s) causing 50% reduction in population growth for the herbicide active ingredients S-metolachlor and acetochlor were 0.046 and 0.003 ppm, respectively. The safeners benoxacor, AD-67, furilazole, and dichlormid were all substantially less toxic than the herbicides and were not toxic at environmentally relevant concentrations. The commercial formulations Dual II Magnum®, Me-Too-Lachlor II®, Harness®, and Surpass EC® all resulted in EC50 values that fell within the 95% confidence interval of the associated active ingredient herbicide. Interestingly, a significant increase in cell size was observed when algae were exposed to all the formulations, herbicides (acetochlor and S-metolachlor), and safener (dichlormid). The safener furilazole caused a significant decrease in cell size, whereas benoxacor and AD-67 had no observed effect on algae cell size. Significant algae cell size effects all occurred at or above the EC50 concentrations for each chemical, suggesting that other morphological effects may be occurring. Importantly, safeners in commercial formulations appeared not to impact toxicity to R. subcapitata compared with the active ingredient alone. Environ Toxicol Chem 2022;41:1466-1476. © 2022 SETAC.

Reactivity of chloroacetamides toward sulfide + black carbon: Insights from structural analogues and dynamic NMR spectroscopy.

Chloroacetamides are commonly used in herbicide formulations, and their occurrence has been reported in soils and groundwater. However, how their chemical structures affect transformation kinetics and pathways in the presence of environmental reagents such as hydrogen sulfide species and black carbon has not been investigated. In this work, we assessed the impact of increasing Cl substituents on reaction kinetics and pathways of six chloroacetamides. The contribution of individual pathways (reductive dechlorination vs. nucleophilic substitution) to the overall decay of selected chloroacetamides was differentiated using various experimental setups; both the transformation rates and product distributions were characterized. Our results suggest that the number of Cl substituents affected reaction pathways and kinetics: trichloroacetamides predominantly underwent reductive dechlorination whereas mono- and dichloroacetamides transformed via nucleophilic substitution. Furthermore, we synthesized eight dichloroacetamide analogs (Cl2CHC(=O)NRR') with differing R groups and characterized their transformation kinetics. Dynamic NMR spectroscopy was employed to quantify the rotational energy barriers of dichloroacetamides. Our results suggest that adsorption of dichloroacetamides on black carbon constrained R groups from approaching the dichloromethyl carbon and subsequently favored nucleophilic attack. This study provides new insights to better predict the fate of chloroacetamides in subsurface environments by linking their structural characteristics to transformation kinetics and pathways.

Surface-catalyzed hydrolysis by pyrogenic carbonaceous matter and model polymers: An experimental and computational study on functional group and pore characteristics.

We employed a polymer network to understand what properties of pyrogenic carbonaceous matter (PCM; e.g., activated carbon) confer its reactivity, which we hereinafter referred to as PCM-like polymers (PLP). This approach allows us to delineate the role of functional groups and micropore characteristics using 2,4,6-trinitrotoluene (TNT) as a model contaminant. Six PLP were synthesized via cross-coupling chemistry with specific functionality (-OH, -NH2, -N(CH3)2, or -N(CH3)3+) and pore characteristics (mesopore, micropore). Results suggest that PCM functionality catalyzed the reaction by: (1) serving as a weak base (-OH, -NH2) to attack TNT, or (2) accumulating OH- near PCM surfaces (-N(CH3)3+). Additionally, TNT hydrolysis rates, pH and co-ion effects, and products were monitored. Microporous PLP accelerated TNT decay compared to its mesoporous counterpart, as further supported by molecular dynamics modeling results. We also demonstrated that quaternary ammonium-modified activated carbon enhanced TNT hydrolysis. These findings have broad implications for pollutant abatement and catalyst design.

Black carbon-enhanced transformation of dichloroacetamide safeners: Role of reduced sulfur species.

Dichloroacetamide safeners are commonly included in herbicide formulations to protect crops from unintended herbicide toxicity, but knowledge of their environmental fate is scarce. Hydrogen sulfide, a naturally-occurring nucleophile and reductant, often coexists with black carbon (e.g., biochar, soot) in subsurface environments and could influence the fate of these safeners. In this study, we demonstrated that graphite powder, a model black carbon, significantly accelerated the transformation of three dichloroacetamide safeners (AD-67, benoxacor, and dichlormid) and two chloroacetamide herbicides (metolachlor and acetochlor) by hydrogen sulfide. Chloride was formed together with an array of sulfur-substituted products, suggesting a nucleophilic substitution pathway. Our results suggest that the electron-accepting capacity of black carbon can oxidize hydrogen sulfide species to elemental sulfur, which can further react with bisulfide to form polysulfide, likely accounting for the observed accelerated transformation of (di)chloroacetamides in systems containing black carbon and hydrogen sulfide. Moreover, our product analyses indicate that dimerization and/or trimerization of (di)chloroacetamides is possible in the presence of hydrogen sulfide and graphite, which is anticipated to decrease the mobility of these products in aquatic environments relative to the parent compounds. Herein, we also discuss how the structure and concentration of (di)chloroacetamides can influence their reactivity in the presence of black carbon and reduced sulfur species.

Reductive transformations of dichloroacetamide safeners: effects of agrochemical co-formulants and iron oxide + manganese oxide binary-mineral systems.

The toxic effects of herbicides are often incompletely selective and can harm crops. Safeners are "inert" ingredients commonly added to herbicide formulations to protect crops from herbicide-induced injury. Dichloroacetamide safeners have been previously shown to undergo reductive dechlorination in anaerobic abiotic systems containing an iron (hydr)oxide mineral (goethite or hematite) amended with Fe(ii). Manganese oxides (e.g., birnessite) are important redox-active species that frequently co-occur with iron (hydr)oxides, yet studies examining the effects of more than one mineral on transformations of environmental contaminants are rare. Herein, we investigate the reactivity of dichloroacetamide safeners benoxacor, furilazole, and dichlormid in binary-mineral, anaerobic systems containing Fe(ii)-amended hematite and birnessite. As the molar ratio of Fe(ii)-to-Mn(iv) oxide increased, the transformation rate of benoxacor and furilazole increased. The safener dichlormid did not transform appreciably over the sampling period (6 hours). The concentration of pH buffer ([MOPS] = 10-50 mM), ionic strength ([NaCl] = 10-200 mM), and order of solute addition (e.g., safener followed by Fe(ii) or vice versa) do not appreciably affect transformation rates of the examined dichloroacetamide safeners in Fe(ii) + hematite slurries. The presence of agrochemical co-formulants, including the herbicide S-metolachlor and three surfactants, in solutions containing Cr(H2O)62+ (as a model homogeneous reductant) also did not substantially influence rates of safener transformation. This study is among the first to examine laboratory systems of intermediate complexity (e.g., systems containing mixtures of agrochemical co-formulants or mineral phases) when assessing the environmental fate of emerging contaminants such as dichloroacetamide safeners.

Emerging investigator series: sunlight photolysis of 2,4-D herbicides in systems simulating leaf surfaces.

Pesticides are commonly applied on foliage, forming dry deposits on the leaf cuticular wax. However, their photochemical transformation in this lipophilic environment is much less understood compared with that in surface water. In this work, sunlight photolysis of six chlorinated phenoxyacetic acid herbicides (i.e., 2,4-D and structural analogues) was evaluated in four organic solvents, on quartz, and on paraffin wax. In solvents of low polarity (i.e., n-heptane and 2-propanol), direct photolysis of 2,4-D herbicides was enhanced due to the relatively high quantum yields in these solvents. Photolysis on paraffin wax was slower than photolysis on quartz by a factor of 3-9, but was comparable with that in solvents of low polarity. With environmentally relevant irradiation and surface loading, the half-lives of 2,4-D herbicides on paraffin wax were 27-159 h, which are within the same range reported for biodegradation, the dominant dissipation pathway in the current 2,4-D fate model. Product analyses showed that photoreductive dechlorination is the dominant pathway in organic solvents, accounting for 68-100% of parent compound decay. On quartz and paraffin wax surfaces, however, photoreductive dechlorination products accounted for <60% of parent compound decay. Combining kinetic modeling and product analyses, it was shown that neither could the two additional putative pathways (photosubstitution of chlorine by hydroxyl group and cleavage of the ether bond) fully account for the total phototransformation on surfaces. These results suggest that rapid photolysis on surfaces can be attributed to unique pathways that are absent in the organic solvent phase.

Polychlorinated biphenyl contamination trends in Lake Hartwell, South Carolina (USA): sediment recovery profiles spanning two decades.

To assess the ca. 20-year polychlorinated biphenyl (PCB) contamination trends in Lake Hartwell, SC, sediment cores from the Twelve Mile Creek arm were collected in July 2004 at two sites (G30 and G33) first sampled in the mid-1980s. Congener-specific PCB data as a function of depth from the sediment-water interface for the 2004 sediment samples were compared to data obtained from 1987 and 1998 samples taken from the same locations. Despite modest decreases in total PCB levels near the G30 sediment-water interface, historical increases in average degrees of chlorination may elevate the overall toxic risk at this site. Unlike G30, the more rapid recovery in the near-surface sediment of G33 suggests that the effectiveness of the U.S. EPA natural attenuation record of decision is site-specific and is unlikely to result in uniform surface sediment recovery throughout the most contaminated regions of Lake Hartwell.