19 research outputs found

    Carbon, Hydrogen, and Nitrogen Isotope Fractionation Trends in <i>N</i>‑Nitrosodimethylamine Reflect the Formation Pathway during Chloramination of Tertiary Amines

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    Assessing the precursors and reactions leading to the carcinogenic <i>N</i>-nitrosodimethylamine (NDMA) during drinking water disinfection is a major challenge. Here, we investigate whether changes of <sup>13</sup>C/<sup>12</sup>C, <sup>2</sup>H/<sup>1</sup>H, and <sup>15</sup>N/<sup>14</sup>N ratios of NDMA give rise to isotope fractionation trends that can be used to infer NDMA formation pathways. We carried out compound-specific isotope analysis (CSIA) of NDMA during chloramination of four tertiary amines that produce NDMA at high yields, namely ranitidine, 5-(dimethylaminomethyl)furfuryl alcohol, <i>N,N</i>-dimethylthiophene-2-methylamine, and <i>N,N</i>-dimethylbenzylamine. Carbon and hydrogen isotope ratios of NDMA function as fingerprints of the N­(CH<sub>3</sub>)<sub>2</sub> moiety and exhibit only minor isotope fractionation during the disinfection process. Nitrogen isotope ratios showed that NH<sub>2</sub>Cl is the source of the N atom of the nitroso group. The large enrichment of <sup>15</sup>N in NDMA was indicative of the isotope effects pertinent to bond-cleavage and bond-formation reactions during chloramination of the tertiary amines. Correlation of δ<sup>15</sup>N versus δ<sup>13</sup>C values of NDMA resulted in trend lines that were not affected by the type of tertiary amine and treatment conditions, suggesting that the observed C and N isotope fractionation in NDMA may be diagnostic for NDMA precursors and formation pathways during chloramination

    Molecularly Imprinted Polymers for Compound-Specific Isotope Analysis of Polar Organic Micropollutants in Aquatic Environments

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    Compound-specific isotope analysis (CSIA) of polar organic micropollutants in environmental waters requires a processing of large sample volumes to obtain the required analyte masses for analysis by gas chromatography/isotope-ratio mass spectrometry (GC/IRMS). However, the accumulation of organic matter of unknown isotopic composition in standard enrichment procedures currently compromises the accurate determination of isotope ratios. We explored the use of molecularly imprinted polymers (MIPs) for selective analyte enrichment for <sup>13</sup>C/<sup>12</sup>C and <sup>15</sup>N/<sup>14</sup>N ratio measurements by GC/IRMS using 1<i>H</i>-benzotriazole, a typical corrosion inhibitor in dishwashing detergents, as example of a widely detected polar organic micropollutant. We developed procedures for the treatment of >10 L of water samples, in which custom-made MIPs enabled the selective cleanup of enriched analytes in organic solvents obtained through conventional solid-phase extractions. Hydrogen bonding interactions between the triazole moiety of 1<i>H</i>-benzotriazole, and the MIP were responsible for selective interactions through an assessment of interaction enthalpies and <sup>15</sup>N isotope effects. The procedure was applied successfully without causing isotope fractionation to river water samples, as well as in- and effluents of wastewater treatment plants containing μg/L concentrations of 1<i>H</i>-benzotriazole and dissolved organic carbon (DOC) loads of up to 28 mg C/L. MIP-based treatments offer new perspectives for CSIA of organic micropollutants through the reduction of the DOC-to-micropollutant ratios

    Trafiklagstiftning och barn

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    <i>N</i>-Nitrosodimethylamine (NDMA) is a carcinogenic disinfection byproduct from water chloramination. Despite the identification of numerous NDMA precursors, essential parts of the reaction mechanism such as the incorporation of molecular O<sub>2</sub> are poorly understood. In laboratory model systems for the chloramination of secondary and tertiary amines, we investigated the kinetics of precursor disappearance and NDMA formation, quantified the stoichiometries of monochloramine (NH<sub>2</sub>Cl) and aqueous O<sub>2</sub> consumption, derived <sup>18</sup>O-kinetic isotope effects (<sup>18</sup>O-KIE) for the reactions of aqueous O<sub>2</sub>, and studied the impact of radical scavengers on NDMA formation. Although the molar NDMA yields from five <i>N</i>,<i>N</i>-dimethylamine-containing precursors varied between 1.4% and 90%, we observed the stoichiometric removal of one O<sub>2</sub> per <i>N</i>,<i>N</i>-dimethylamine group of the precursor indicating that the oxygenation of N atoms did not determine the molar NDMA yield. Small <sup>18</sup>O-KIEs between 1.0026 ± 0.0003 and 1.0092 ± 0.0009 found for all precursors as well as completely inhibited NDMA formation in the presence of radical scavengers (ABTS and trolox) imply that O<sub>2</sub> reacted with radical species. Our study suggests that aminyl radicals from the oxidation of organic amines by NH<sub>2</sub>Cl and <i>N</i>-peroxyl radicals from the reaction of aminyl radicals with aqueous O<sub>2</sub> are part of the NDMA formation mechanism

    Linking Thermodynamics to Pollutant Reduction Kinetics by Fe<sup>2+</sup> Bound to Iron Oxides

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    Numerous studies have reported that pollutant reduction rates by ferrous iron (Fe<sup>2+</sup>) are substantially enhanced in the presence of an iron (oxyhydr)­oxide mineral. Developing a thermodynamic framework to explain this phenomenon has been historically difficult due to challenges in quantifying reduction potential (<i>E</i><sub>H</sub>) values for oxide-bound Fe<sup>2+</sup> species. Recently, our group demonstrated that <i>E</i><sub>H</sub> values for hematite- and goethite-bound Fe<sup>2+</sup> can be accurately calculated using Gibbs free energy of formation values. Here, we tested if calculated <i>E</i><sub>H</sub> values for oxide-bound Fe<sup>2+</sup> could be used to develop a free energy relationship capable of describing variations in reduction rate constants of substituted nitrobenzenes, a class of model pollutants that contain reducible aromatic nitro groups, using data collected here and compiled from the literature. All the data could be described by a single linear relationship between the logarithms of the surface-area-normalized rate constant (<i>k</i><sub>SA</sub>) values and <i>E</i><sub>H</sub> and pH values [log­(<i>k</i><sub>SA</sub>) = −<i>E</i><sub>H</sub>/0.059 V – pH + 3.42]. This framework provides mechanistic insights into how the thermodynamic favorability of electron transfer from oxide-bound Fe<sup>2+</sup> relates to redox reaction kinetics

    Isotope Fractionation Associated with the Photochemical Dechlorination of Chloroanilines

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    Isotope fractionation associated with the photochemical transformation of organic contaminants is not well understood and can arise not only from bond cleavage reactions but also from photophysical processes. In this work, we investigated the photolytic dechlorination of 2-Cl- and 3-Cl-aniline to aminophenols to obtain insights into the impact of the substituent position on the apparent <sup>13</sup>C and <sup>15</sup>N kinetic isotope effects (AKIEs). Laboratory experiments were performed in aerated aqueous solutions at an irradiation wavelength of 254 nm over the pH range 2.0 to 7.0 in the absence and presence of Cs<sup>+</sup> used as an excited singlet state quencher. Photolysis of 2-Cl-anilinium cations exhibits normal C and inverse N isotope fractionation, while neutral 2-Cl-aniline species shows inverse C and normal N isotope fractionation. In contrast, the photolysis of 3-Cl-aniline was almost insensitive to C isotope composition and the moderate N isotope fractionation points to rate-limiting photophysical processes. <sup>13</sup>C- and <sup>15</sup>N-AKIE-values of 2-Cl-aniline decreased in the presence of Cs<sup>+</sup>, whereas those for 3-Cl-aniline were not systematically affected by Cs<sup>+</sup>. Our current and previous work illustrates that photolytic dechlorinations of 2-Cl-, 3-Cl-, and 4-Cl-aniline isomers are each accompanied by distinctly different and highly variable C and N isotope fractionation due to spin selective isotope effects

    Enzyme Kinetics of Different Types of Flavin-Dependent Monooxygenases Determine the Observable Contaminant Stable Isotope Fractionation

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    The assessment of oxidative pollutant biotransformation by compound specific isotope analysis (CSIA) is often complicated by the variability of kinetic isotope effects associated with carbon oxygenation in enzymatic reactions. Here, we illustrate how information about the kinetics of oxidative biocatalysis by flavin-dependent monooxygenases (FMOs) enables one to assess if CSIA could be applied for tracking contaminant biodegradation. In “cautious” FMOs, which form reactive flavin (hydro)­peroxide species after substrate binding, the monooxygenation of organic compounds is not rate-determining and consequently does not lead to substrate isotope fractionation. Conversely, “bold” FMOs generate hydroperoxides regardless of substrate availability, and substrate disappearance is thus subject to isotope fractionation trends, which are typical for hydroxylation reactions. Because monooxygenations of aromatic moieties are often initial steps of organic pollutant transformation, knowledge of the kinetics of FMOs and other oxidative enzymes can support decisions regarding the use of CSIA

    Mediated Electrochemical Reduction of Iron (Oxyhydr-)Oxides under Defined Thermodynamic Boundary Conditions

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    Iron (oxyhydr-)­oxide reduction has been extensively studied because of its importance in pollutant redox dynamics and biogeochemical processes. Yet, experimental studies linking oxide reduction kinetics to thermodynamics remain scarce. Here, we used mediated electrochemical reduction (MER) to directly quantify the extents and rates of ferrihydrite, goethite, and hematite reduction over a range of negative reaction free energies, Δ<sub>r</sub><i>G</i>, that were obtained by systematically varying pH (5.0 to 8.0), applied reduction potentials (−0.53 to −0.17 V vs SHE), and Fe<sup>2+</sup> concentrations (up to 40 μM). Ferrihydrite reduction was complete and fast at all tested Δ<sub>r</sub><i>G</i> values, consistent with its comparatively low thermodynamic stability. Reduction of the thermodynamically more stable goethite and hematite changed from complete and fast to incomplete and slow as Δ<sub>r</sub><i>G</i> values became less negative. Reductions at intermediate Δ<sub>r</sub><i>G</i> values showed negative linear correlations between the natural logarithm of the reduction rate constants and Δ<sub>r</sub><i>G</i>. These correlations imply that thermodynamics controlled goethite and hematite reduction rates. Beyond allowing to study iron oxide reduction under defined thermodynamic conditions, MER can also be used to capture changes in iron oxide reducibility during phase transformations, as shown for Fe<sup>2+</sup>-facilitated transformation of ferrihydrite to goethite

    Isotope Fractionation Associated with the Indirect Photolysis of Substituted Anilines in Aqueous Solution

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    Organic micropollutants containing aniline substructures are susceptible to different light-induced transformation processes in aquatic environments and water treatment operations. Here, we investigated the magnitude and variability of C and N isotope fractionation during the indirect phototransformation of four <i>para</i>-substituted anilines in aerated aqueous solutions. The model photosensitizers, namely 9,10-anthraquinone-1,5-disulfonate and methylene blue, were used as surrogates for dissolved organic matter chromophores generating excited triplet states in sunlit surface waters. The transformation of aniline, 4-CH<sub>3</sub>-, 4-OCH<sub>3</sub>-, and 4-Cl-aniline by excited triplet states of the photosensitizers was associated with inverse and normal N isotope fractionation, whereas C isotope fractionation was negligible. The apparent <sup>15</sup>N kinetic isotope effects (AKIE) were almost identical for both photosensitizers, increased from 0.9958 ± 0.0013 for 4-OCH<sub>3</sub>-aniline to 1.0035 ± 0.0006 for 4-Cl-aniline, and correlated well with the electron donating properties of the substituent. N isotope fractionation is pH-dependent in that H<sup>+</sup> exchange reactions dominate below and N atom oxidation processes above the p<i>K</i><sub>a</sub> value of the substituted aniline’s conjugate acid. Correlations of C and N isotope fractionation for indirect phototransformation were different from those determined previously for the direct photolysis of chloroanilines and offer new opportunities to distinguish between abiotic degradation pathways

    Isotopic Analysis of Oxidative Pollutant Degradation Pathways Exhibiting Large H Isotope Fractionation

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    Oxidation of aromatic rings and its alkyl substituents are often competing initial steps of organic pollutant transformation. The use of compound-specific isotope analysis (CSIA) to distinguish between these two pathways quantitatively, however, can be hampered by large H isotope fractionation that precludes calculation of apparent <sup>2</sup>H-kinetic isotope effects (KIE) as well as the process identification in multi-element isotope fractionation analysis. Here, we investigated the C and H isotope fractionation associated with the transformation of toluene, nitrobenzene, and four substituted nitrotoluenes by permanganate, MnO<sub>4</sub><sup>–</sup>, to propose a refined evaluation procedure for the quantitative distinction of CH<sub>3</sub>-group oxidation and dioxygenation. On the basis of batch experiments, an isotopomer-specific kinetic model, and density functional theory (DFT) calculations, we successfully derived the large apparent <sup>2</sup>H-KIE of 4.033 ± 0.20 for the CH<sub>3</sub>-group oxidation of toluene from H isotope fractionation exceeding >1300‰ as well as the corresponding <sup>13</sup>C-KIE (1.0324 ± 0.0011). Experiment and theory also agreed well for the dioxygenation of nitrobenzene, which was associated with <sup>2</sup>H- and <sup>13</sup>C-KIEs of 0.9410 ± 0.0030 (0.9228 obtained by DFT) and 1.0289 ± 0.0003 (1.025). Consistent branching ratios for the competing CH<sub>3</sub>-group oxidation and dioxygenation of nitrotoluenes by MnO<sub>4</sub><sup>–</sup> were obtained from the combined modeling of concentration as well as C and H isotope signature trends. Our approach offers improved estimates for the identification of contaminant microbial and abiotic oxidation pathways by CSIA

    Redox Properties of Structural Fe in Clay Minerals: 3. Relationships between Smectite Redox and Structural Properties

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    Structural Fe in clay minerals is an important redox-active species in many pristine and contaminated environments as well as in engineered systems. Understanding the extent and kinetics of redox reactions involving Fe-bearing clay minerals has been challenging due to the inability to relate structural Fe<sup>2+</sup>/Fe<sub>total</sub> fractions to fundamental redox properties, such as reduction potentials (<i>E</i><sub>H</sub>). Here, we overcame this challenge by using mediated electrochemical reduction (MER) and oxidation (MEO) to characterize the fraction of redox-active structural Fe (Fe<sup>2+</sup>/Fe<sub>total</sub>) in smectites over a wide range of applied <i>E</i><sub>H</sub>-values (−0.6 V to +0.6 V). We examined Fe<sup>2+</sup>/Fe<sub>total</sub> – <i>E</i><sub>H</sub> relationships of four natural Fe-bearing smectites (SWy-2, SWa-1, NAu-1, NAu-2) in their native, reduced, and reoxidized states and compared our measurements with spectroscopic observations and a suite of mineralogical properties. All smectites exhibited unique Fe<sup>2+</sup>/Fe<sub>total</sub> – <i>E</i><sub>H</sub> relationships, were redox active over wide <i>E</i><sub>H</sub> ranges, and underwent irreversible electron transfer induced structural changes that were observable with X-ray absorption spectroscopy. Variations among the smectite Fe<sup>2+</sup>/Fe<sub>total</sub> – <i>E</i><sub>H</sub> relationships correlated well with both bulk and molecular-scale properties, including Fe<sub>total</sub> content, layer charge, and quadrupole splitting values, suggesting that multiple structural parameters determined the redox properties of smectites. The Fe<sup>2+</sup>/Fe<sub>total</sub> – <i>E</i><sub>H</sub> relationships developed for these four commonly studied clay minerals may be applied to future studies interested in relating the extent of structural Fe reduction or oxidation to <i>E</i><sub>H</sub>-values
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