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
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
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
<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
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
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
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
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
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
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
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