10 research outputs found
Phenolic Antioxidants Inhibit the Triplet-Induced Transformation of Anilines and Sulfonamide Antibiotics in Aqueous Solution
Recent studies have shown that dissolved organic matter
(DOM) may
inhibit the excited triplet-induced oxidation of several aromatic
water contaminants, in particular those containing an aniline functionality.
Such an inhibition was ascribed to antioxidant moieties of DOM. The
present study was conducted with the aim of verifying whether well-defined
antioxidants could act as inhibitors in analogy to DOM. Various substituted
phenols exhibiting antioxidant character were able, at micromolar
concentration, to slow down the photoinduced depletion of several
anilines and sulfonamides in aerated aqueous solution containing 2-acetonaphthone
as the photosensitizer. A concomitant accelerated degradation of the
phenols in the presence of such contaminants was observed. This reinforces
the hypothesis of reduction of oxidation intermediates of the contaminants
by the phenols. Phenol (unsubstituted) was found to be a useful inhibitor
even in the case of DOM-photosensitized transformations. Phenolic
antioxidants are proposed as diagnostic tools to investigate the aquatic
photochemistry of aromatic amines
Probing the Photosensitizing and Inhibitory Effects of Dissolved Organic Matter by Using <i>N</i>,<i>N</i>‑dimethyl-4-cyanoaniline (DMABN)
Dissolved
organic matter (DOM) can act as a photosensitizer and
an inhibitor in the phototransformation of several nitrogen-containing
organic contaminants in surface waters. The present study was performed
to select a probe molecule that is suitable to measure these antagonistic
properties of DOM. Out of nine studied nitrogen-containing aromatic
compounds, 4-cyanoaniline, <i>N</i>,<i>N</i>-dimethyl-4-cyanoaniline
(DMABN), sotalol (a β-blocker) and sulfadiazine (a sulfonamide
antibiotic) exhibited a marked photosensitized transformation that
could be substantially inhibited by addition of phenol as a model
antioxidant. The photosensitized transformation of DMABN, the selected
probe compound, was characterized in detail under UV-A and visible
irradiation (λ > 320 nm) to avoid direct phototransformation.
Low reactivity of DMABN with singlet oxygen was found (second-order
rate constant <2 × 10<sup>7</sup> M<sup>–1</sup> s<sup>–1</sup>). Typically at least 85% of the reactivity of DMABN
could be inhibited by DOM or the model antioxidant phenol. The photosensitized
transformation of DMABN mainly proceeded (>72%) through demethylation
yielding <i>N</i>-methyl-4-cyanoaniline and formaldehyde
as primary products. In solutions of standard DOM extracts and their
mixtures the phototransformation rate constant of DMABN was shown
to vary nonlinearly with the DOM concentration. Model equations describing
the dependence of such rate constants on DOM and model antioxidant
concentrations were successfully used to fit experimental data
Quenching of Excited Triplet States by Dissolved Natural Organic Matter
Excited
triplet states of aromatic ketones and quinones are used as proxies
to assess the reactivity of excited triplet states of the dissolved
organic matter (<sup>3</sup>DOM*) in natural waters. <sup>3</sup>DOM*
are crucial transients in environmental photochemistry responsible
for contaminant transformation, production of reactive oxygen species,
and potentially photobleaching of DOM. In recent photochemical studies
aimed at clarifying the role of DOM as an inhibitor of triplet-induced
oxidations of organic contaminants, aromatic ketones have been used
in the presence of DOM, and the question of a possible interaction
between their excited triplet states and DOM has emerged. To clarify
this issue, time-resolved laser spectroscopy was applied to measure
the excited triplet state quenching of four different model triplet
photosensitizers induced by a suite of DOM from various aquatic and
terrestrial sources. While no quenching for the anionic triplet sensitizers
4-carboxybenzophenone (CBBP) and 9,10-anthraquinone-2,6-disulfonic
acid (2,6-AQDS) was detected, second-order quenching rate constants
with DOM for the triplets of 2-acetonaphthone (2AN) and 3-methoxyacetophenone
(3MAP) in the range of 1.30–3.85 × 10<sup>7</sup> L mol<sub>C</sub><sup>–1</sup> s<sup>–1</sup> were determined.
On the basis of the average molecular weight of DOM molecules, the
quenching for these uncharged excited triplet molecules is nearly
diffusion-controlled, but significant quenching (>10%) in aerated
water is not expected to occur below DOM concentrations of 22–72
mg<sub>C</sub> L<sup>–1</sup>
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
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
Abatement of Polychoro-1,3-butadienes in Aqueous Solution by Ozone, UV Photolysis, and Advanced Oxidation Processes (O<sub>3</sub>/H<sub>2</sub>O<sub>2</sub> and UV/H<sub>2</sub>O<sub>2</sub>)
The abatement of 9 polychloro-1,3-butadienes
(CBDs) in aqueous
solution by ozone, UV–CÂ(254 nm) photolysis, and the corresponding
advanced oxidation processes (AOPs) (i.e., O<sub>3</sub>/H<sub>2</sub>O<sub>2</sub> and UV/H<sub>2</sub>O<sub>2</sub>) was investigated.
The following parameters were determined for 9 CBDs: second-order
rate constants for the reactions of CBDs with ozone (<i>k</i><sub>O<sub>3</sub></sub>) (<0.1–7.9 × 10<sup>3</sup> M<sup>–1</sup> s<sup>–1</sup>) or with hydroxyl radicals
(<i>k</i><sub><sup>•</sup>OH</sub>) (0.9 × 10<sup>9</sup> – 6.5 × 10<sup>9</sup> M<sup>–1</sup> s<sup>–1</sup>), photon fluence-based rate constants (<i>k</i>′) (210–2730 m<sup>2</sup> einstein<sup>–1</sup>), and quantum yields (Φ) (0.03–0.95 mol einstein<sup>–1</sup>). During ozonation of CBDs in a natural groundwater,
appreciable abatements (>50% at specific ozone doses of 0.5 gO<sub>3</sub>/gDOC to ∼100% at ≥1.0 gO<sub>3</sub>/gDOC)
were achieved for tetra-CBDs followed by (<i>Z</i>)-1,1,2,3,4-penta-CBD
and hexa-CBD. This is consistent with the magnitude of the determined <i>k</i><sub>O<sub>3</sub></sub> and <i>k</i><sub><sup>•</sup>OH</sub>. The formation of bromate, a potentially carcinogenic
ozonation byproduct, could be significantly reduced by addition of H<sub>2</sub>O<sub>2</sub>. For
a typical UV disinfection dose (400 J/m<sup>2</sup>), various extents
of phototransformations (10–90%) could be achieved. However,
the efficient formation of photoisomers from CBDs with <i>E</i>/<i>Z</i> configuration must be taken into account because
of their potential residual toxicity. Under UV–CÂ(254 nm) photolysis
conditions, no significant effect of H<sub>2</sub>O<sub>2</sub> addition
on CBDs abatement was observed due to an efficient direct phototransformation
of CBDs
Chemical Oxidation of Dissolved Organic Matter by Chlorine Dioxide, Chlorine, And Ozone: Effects on Its Optical and Antioxidant Properties
In water treatment
dissolved organic matter (DOM) is typically
the major sink for chemical oxidants. The resulting changes in DOM,
such as its optical properties have been measured to follow the oxidation
processes. However, such measurements contain only limited information
on the changes in the oxidation states of and the reactive moieties
in the DOM. In this study, we used mediated electrochemical oxidation
to quantify changes in the electron donating capacities (EDCs), and
hence the redox states, of three different types of DOM during oxidation
with chlorine dioxide (ClO<sub>2</sub>), chlorine (as HOCl/OCl<sup>–</sup>), and ozone (O<sub>3</sub>). Treatment with ClO<sub>2</sub> and HOCl resulted in comparable and prominent decreases in
EDCs, while the UV light absorbances of the DOM decreased only slightly.
Conversely, ozonation resulted in only small decreases of the EDCs
but pronounced absorbance losses of the DOM. These results suggest
that ClO<sub>2</sub> and HOCl primarily reacted as oxidants by accepting
electrons from electron-rich phenolic and hydroquinone moieties in
the DOM, while O<sub>3</sub> reacted via electrophilic addition to
aromatic moieties, followed by ring cleavage. This study highlights
the potential of combined EDC-UV measurements to monitor chemical
oxidation of DOM, to assess the nature of the reactive moieties and
to study the underlying reaction pathways
Isotope Fractionation Associated with the Direct Photolysis of 4‑Chloroaniline
Compound-specific
isotope analysis is a useful approach to track
transformations of many organic soil and water pollutants. Applications
of CSIA to characterize photochemical processes, however, have hardly
been explored. In this work, we systematically studied C and N isotope
fractionation associated with the direct photolysis of 4-Cl-aniline
used as a model compound for organic micropollutants that are known
to degrade via photochemical processes. Laboratory experiments were
carried out at an irradiation wavelength of 254 nm over the pH range
2.0 to 9.0 as well as in the presence of Cs<sup>+</sup> as a quencher
of excited singlet 4-Cl-aniline at pH 7.0 and 9.0. We observed considerable
variation of C and N isotope enrichment factors, ϵ<sub>C</sub> and ϵ<sub>N</sub>, between −1.2 ± 0.2‰
to −2.7 ± 0.2‰ for C and −0.6 ± 0.2‰
to −9.1 ± 1.6‰ for N, respectively, which could
not be explained by the speciation of 4-Cl-aniline alone. In the presence
of 1 M Cs<sup>+</sup>, we found a marked increase of apparent <sup>13</sup>C-kinetic isotope effects (<sup>13</sup>C-AKIE) and decrease
of 4-Cl-aniline fluorescence lifetimes. Our data suggest that variations
of C and N isotope fractionation originate from heterolytic dechlorination
of excited triplet and singlet states of 4-Cl-aniline. Linear correlations
of <sup>13</sup>C-AKIE vs <sup>15</sup>N-AKIE were distinctly different
for these two reaction pathways and may be explored further for the
identification of photolytic aromatic dechlorination reactions
Isoproturon Reappearance after Photosensitized Degradation in the Presence of Triplet Ketones or Fulvic Acids
Isoproturon (IPU)
is a phenylurea herbicide used to control broad-leaf
grasses on grain fields. Photosensitized transformation induced by
excited triplet states of dissolved organic matter (<sup>3</sup>DOM*)
has been identified as an important degradation pathway for IPU in
sunlit waters, but the reappearance of IPU in the absence of light
is observed after the initial photolysis. In this study, we elucidate
the kinetics of this photodegradation and dark-reappearance cycling
of IPU in the presence of DOM proxies (aromatic ketones and reference
fulvic acids). Using mass spectrometry and nuclear magnetic resonance
spectroscopic techniques, a semi-stable intermediate (IPU<sub>int</sub>) was found to be responsible for IPU reversion and was identified
as a hydroperoxyl derivative of IPU. IPU<sub>int</sub> is photogenerated
from incorporation of diatomic oxygen to IPU and is subjected to thermolysis
whose rate depends on temperature, pH, the presence of DOM, and inorganic
ions. These results are important to understand the overall aquatic
fate of IPU and structurally similar compounds under diurnal conditions
Organic Contaminant Abatement in Reclaimed Water by UV/H<sub>2</sub>O<sub>2</sub> and a Combined Process Consisting of O<sub>3</sub>/H<sub>2</sub>O<sub>2</sub> Followed by UV/H<sub>2</sub>O<sub>2</sub>: Prediction of Abatement Efficiency, Energy Consumption, and Byproduct Formation
UV/H<sub>2</sub>O<sub>2</sub> processes can be applied to improve the quality
of effluents from municipal wastewater treatment plants by attenuating
trace organic contaminants (micropollutants). This study presents
a kinetic model based on UV photolysis parameters, including UV absorption
rate and quantum yield, and hydroxyl radical (·OH) oxidation
parameters, including second-order rate constants for ·OH reactions
and steady-state ·OH concentrations, that can be used to predict
micropollutant abatement in wastewater. The UV/H<sub>2</sub>O<sub>2</sub> kinetic model successfully predicted the abatement efficiencies
of 16 target micropollutants in bench-scale UV and UV/H<sub>2</sub>O<sub>2</sub> experiments in 10 secondary wastewater effluents. The
model was then used to calculate the electric energies required to
achieve specific levels of micropollutant abatement in several advanced
wastewater treatment scenarios using various combinations of ozone,
UV, and H<sub>2</sub>O<sub>2</sub>. UV/H<sub>2</sub>O<sub>2</sub> is
more energy-intensive than ozonation for abatement of most micropollutants.
Nevertheless, UV/H<sub>2</sub>O<sub>2</sub> is not limited by the
formation of <i>N</i>-nitrosodimethylamine (NDMA) and bromate
whereas ozonation may produce significant concentrations of these
oxidation byproducts, as observed in some of the tested wastewater
effluents. The combined process of O<sub>3</sub>/H<sub>2</sub>O<sub>2</sub> followed by UV/H<sub>2</sub>O<sub>2</sub>, which may be warranted
in some potable reuse applications, can achieve superior micropollutant
abatement with reduced energy consumption compared to UV/H<sub>2</sub>O<sub>2</sub> and reduced oxidation byproduct formation (i.e., NDMA
and/or bromate) compared to conventional ozonation