Aqueous-phase Experiments Research Articles

Isomerization and Degradation of Levoglucosan via the Photo-Fenton Process: Insights from Aqueous-Phase Experiments and Atmospheric Particulate Matter

So far, studies on the conversion of stereochemistry under photo-Fenton conditions and their atmospheric implication are still rare. Here, we found that the biomass burning marker, the chiral compound levoglucosan (L), undergoes oxidative degradation under photo-Fenton conditions and can be isomerized into mannosan (M) and galactosan (G) simultaneously. Among the formic acid, acetic acid, and oxalic acid in the degradation products of levoglucosan, it was found that the yield of formation of formic acid in the photo-Fenton pathway can be as high as 86%. It is worth noting that both levoglucosan and its isomers are present in the atmosphere and their concentrations are strongly correlated. At the same time, the range of their concentration ratios, L/(G + M), measured in the photo-Fenton experiments in the laboratory was found to agree well with that measured in atmospheric PM2.5 samples. However, the sources of L, G, and M in the atmosphere are complex, and the photo-Fenton reaction may be an essential pathway for the distribution of L, G, and M in the atmosphere.

13C and 15N NMR identification of product compound classes from aqueous and solid phase photodegradation of 2,4,6-trinitrotoluene.

Photolysis is one of the main transformation pathways for 2,4,6-trinitrotoluene (TNT) released into the environment. Upon exposure to sunlight, TNT is known to undergo both oxidation and reduction reactions with release of nitrite, nitrate, and ammonium ions, followed by condensation reactions of the oxidation and reduction products. In this study, compound classes of transformation products from the aqueous and solid phase photodegradation of 2,4,6-trinitrotoluene (TNT) have been identified by liquid and solid state 13C and 15N NMR. Aqueous phase experiments were performed on saturated solutions of T15NT in deionized water, natural pond water (pH = 8.3, DOC = 3.0 mg/L), pH 8.0 buffer solution, and in the presence of Suwannee River Natural Organic Matter (SRNOM; pH = 3.7), using a Pyrex-filtered medium pressure mercury lamp. Natural sunlight irradiations were performed on TNT in the solid phase and dissolved in the pond water. In deionized water, carboxylic acid, aldehyde, aromatic amine, primary amide, azoxy, nitrosophenol, and azo compounds were formed. 15N NMR spectra exhibited major peaks centered at 128 to 138 ppm, which are in the range of phenylhydroxylamine and secondary amide nitrogens. The secondary amides are proposed to represent benzanilides, which would arise from photochemical rearrangement of nitrones formed from the condensation of benzaldehyde and phenylhydroxylamine derivatives of TNT. The same compound classes were formed from sunlight irradiation of TNT in the solid phase. Whereas carboxylic acids, aldehydes, aromatic amines, phenylhydroxylamines, and amides were also formed from irradiation of TNT in pond water and in pH 8 buffer solution, azoxy and azo compound formation was inhibited. Solid state 15N NMR spectra of photolysates from the lamp irradiation of unlabeled 2,6-dinitrotoluene in deionized water also demonstrated the formation of aromatic amine, phenylhydroxylamine/ 2° amide, azoxy, and azo nitrogens.

Methylamine’s Effects on Methylglyoxal-Containing Aerosol: Chemical, Physical, and Optical Changes

Methylamine, a common atmospheric amine species, is found in the gas, particle, and aqueous phases. It has been shown to form light-absorbing, oligomeric species in reactions with methylglyoxal and other aldehyde species in bulk aqueous-phase experiments and when mixed into seed aerosol as a sulfate salt. Here, we explore the influence of multiphase methylamine chemistry on aerosol production, properties, and molecular composition. When methylglyoxal aerosol particles were exposed to ∼2 ppm methylamine gas in a humid chamber, rapid browning was observed, but not growth. Aerosol bounce measurements indicated that particles became slightly more viscous and hydrophobic upon methylamine exposure. Subsequent cloud processing increased both viscosity and hygroscopicity but had little effect on browning, consistent with high-resolution mass spectrometry results showing that aerosol oligomer dicarbonyl functional groups were transformed into cationic imidazole rings. Photolytic cloud processing triggered the inco...

Nitrogen-containing secondary organic aerosol formation by acrolein reaction with ammonia/ammonium

Abstract. Ammonia-driven carbonyl-to-imine conversion is an important formation pathway to the nitrogen-containing organic compounds (NOCs) in secondary organic aerosols (SOAs). Previous studies have mainly focused on the dicarbonyl compounds as the precursors of light-absorbing NOCs. In this work, we investigated whether acrolein could also act as an NOC precursor. Acrolein is the simplest α, β-unsaturated mono-carbonyl compound, and it is ubiquitous in the atmosphere. Experiments probing multiphase reactions of acrolein as well as bulk aqueous-phase experiments were carried out to study the reactivity of acrolein towards ammonia and ammonium ions. Molecular characterization of the products based on gas chromatography mass spectrometry, high-resolution mass spectrometry, surface-enhanced Raman spectrometry and ultraviolet/visible spectrophotometry was used to propose possible reaction mechanisms. We observed 3-methylpyridine (commonly known as 3-picoline) in the gas phase in Tedlar bags filled with gaseous acrolein and ammonia or ammonium aerosols. In the ammonium-containing aqueous phase, oligomeric compounds with formulas (C3H4O)m(C3H5N)n and pyridinium compounds like (C3H4O)2C6H8N+ were observed as the products. The pathway to 3-methylpyridine was proposed to be the intramolecular carbon–carbon addition of the hemiaminal, which resulted from sequential carbonyl-to-imine conversions of acrolein molecules. The 3-methylpyridine was formed in the aqueous phase, but some of the 3-methylpyridine could revolatilize to the gas phase, explaining the observation of gaseous 3-methylpyridine in the bags. The (C3H4O)2C6H8N+ was a carbonyl-to-hemiaminal product from acrolein dimer and 3-methylpyridine, while the oligomeric products of (C3H4O)m(C3H5N)n were polymers of acroleins and propylene imines formed via carbonyl-to-imine conversion and condensation reactions. The pH value effect on the liquid products was also studied in the bulk aqueous-phase experiments. While the oligomeric compounds were forming in both acidic and alkaline conditions, the pyridinium products favored moderately acidic conditions. Both the oligomeric products and the pyridinium salts are light-absorbing materials. This work suggests that acrolein may serve as a precursor of light-absorbing heterocyclic NOCs in SOA. Therefore, secondary reactions of α, β-unsaturated aldehydes with reduced nitrogen should be taken into account as a source of light-absorbing NOCs in SOA.

Cloud Processing of Secondary Organic Aerosol from Isoprene and Methacrolein Photooxidation.

Aerosol-cloud interaction contributes to the largest uncertainties in the estimation and interpretation of the Earth’s changing energy budget. The present study explores experimentally the impacts of water condensation-evaporation events, mimicking processes occurring in atmospheric clouds, on the molecular composition of secondary organic aerosol (SOA) from the photooxidation of methacrolein. A range of on- and off-line mass spectrometry techniques were used to obtain a detailed chemical characterization of SOA formed in control experiments in dry conditions, in triphasic experiments simulating gas-particle-cloud droplet interactions (starting from dry conditions and from 60% relative humidity (RH)), and in bulk aqueous-phase experiments. We observed that cloud events trigger fast SOA formation accompanied by evaporative losses. These evaporative losses decreased SOA concentration in the simulation chamber by 25–32% upon RH increase, while aqueous SOA was found to be metastable and slowly evaporated after cloud dissipation. In the simulation chamber, SOA composition measured with a high-resolution time-of-flight aerosol mass spectrometer, did not change during cloud events compared with high RH conditions (RH > 80%). In all experiments, off-line mass spectrometry techniques emphasize the critical role of 2-methylglyceric acid as a major product of isoprene chemistry, as an important contributor to the total SOA mass (15–20%) and as a key building block of oligomers found in the particulate phase. Interestingly, the comparison between the series of oligomers obtained from experiments performed under different conditions show a markedly different reactivity. In particular, long reaction times at high RH seem to create the conditions for aqueous-phase processing to occur in a more efficient manner than during two relatively short cloud events.

Screening of microorganisms for bioconversion of (+)-valencene to (+)-nootkatone

The production of (+)-nootkatone, highly appreciated by fragrance and flavour industries, can be performed by whole-cell bioconversion from the sesquiterpene (+)-valencene, a compound readily available in orange essential oil. The aim of this work was to screen for microorganisms that convert (+)-valencene to (+)-nootkatone using different bioconversion systems. The screening was conducted using six different microorganisms, and bioconversion experiments were set up on surface culture using serological flasks containing PDA at 30 °C. It was observed that Botryodiplodia theobromae 1368, Yarrowia lipolytica 2.2ab, and Phanerochaete chrysosporium oxidised (+)-valencene to (+)-nootkatone, reaching (+)-nootkatone concentrations of 231.7 ± 2.1, 216.9 ± 5.8 and 100.8 ± 2.6 mg L−1, respectively. Different bioconversion conditions were also tested—aqueous, organic, and biphasic—all resulting in similar (+)-nootkatone production. Both B. theobromae 1368 and Y. lipolytica 2.2ab showed substrate inhibition above 4.2 × 10−2 and 0.13 g of (+)-valencene (g of biomass)−1, respectively, in aqueous phase experiments. Furthermore, B. theobromae 1368 and Y. lipolytica 2.2ab showed product inhibition when concentrations reached above 17.02 and 34.78 mg of (+)-nootkatone (g of biomass)−1, respectively. The experimental method presented will be useful for ongoing studies on the selection and operation of the proper bioreactor at different bioconversion conditions.

Sulfate radical-initiated formation of isoprene-derived organosulfates in atmospheric aerosols

Recent studies show that isoprene-derived organosulfates are an important fraction of ambient secondary organic aerosol (SOA), adding up to 20% to the organic mass. Organosulfates with m/z of 199 and 183 relating to C4 compounds are found in ambient and laboratory generated SOA and a sulfate radical induced oxidation of methacrolein (MACR) and methyl vinyl ketone (MVK) has been shown to be a possible formation mechanism. In the present study, experiments on the sulfate radical-induced oxidation of methacrolein and methyl vinyl ketone were performed in bulk aqueous phase, as well as in an aerosol chamber, and finally compared with ambient PM10 samples collected at a rural East German village during the summer 2008, to investigate their relevance in aqueous phase SOA formation. Samples from aqueous phase experiments and extracts from filters were analysed with UPLC/(-)ESI-IMS-QTOFMS. All the samples showed the abundance of highly oxidised organosulfates with m/z 153, 155, 167, 183 and 199 corresponding to the species found in ambient particle samples. In the bulk phase studies with laser-induced sulfate radical formation, the signal intensities increased with increasing number of laser pulses, indicating the sulfate radical-induced formation of these organosulfates. Additionally, the chamber experiments showed a particle mass growth of about 10 microg m(-3) and 4 microg m(-3) for experiments on the reactive uptake of MACR and MVK with a sulfate radical precursor (K2S2O8) in the seed particles. Correlations of the C2 to C5 organosulfate species (including the m/z 215, C5H11O7S-), detected in the ambient samples were found to be very strong (r > 0.8), indicating that these compounds are formed from similar mechanisms and under equal environmental conditions. This study shows that sulfate radical-induced oxidation in the aqueous particle phase provides a reasonable explanation for the formation of these organosulfates from methacrolein and methyl vinyl ketone.

CO Adsorption and Oxidation at the Catalyst−Water Interface: An Investigation by Attenuated Total Reflection Infrared Spectroscopy

Adsorption of carbon monoxide and oxidation of preadsorbed carbon monoxide from gas and aqueous phases were studied on a platinum catalyst deposited on a ZnSe internal reflection element (IRE) using attenuated total reflection infrared (ATR-IR) spectroscopy. The results of this study convincingly show that it is possible to prepare platinum metal layers strongly attached to an IRE, which are stable for over 3 days in aqueous-phase experiments. It is shown that ATR-IR spectroscopy is a suitable technique to study adsorption and catalytic reactions occurring at the interface of a solid catalyst in an aqueous reaction mixture, even with an extreme low-surface-area catalyst. Clearly, ATR-IR spectroscopy allows for a direct comparison of reactions on a catalytic surface in gas and liquid phases on the same sample. CO was found to adsorb both linearly and bridged on the platinum metal layer when adsorbed from the gas phase, but only linear CO was detected in aqueous solution, although with 5 times higher intensity. Oxidation of preadsorbed CO on platinum occurs in both gas phase, wetted gas, and aqueous media and was found to be 2 times faster in the aqueous phase compared to gas-phase oxidation because of a promoting effect of water. Moreover, during oxidation at room temperature, CO2 adsorbed on Pt/ZnSe was detected in both gas and aqueous phases.

Effect of sorption and desorption resistance on aerobic trichloroethylene biodegradation in soils

Biodegradation of trichloroethylene (TCE) by toluene-degrading bacteria was measured under aerobic conditions in aqueous and soil-slurry batch microcosms. For soil-phase experiments, a freshly contaminated soil and a soil containing only the desorption-resistant fraction of TCE were tested. In both cases, presence of soil resulted in biodegradation rates substantially lower than those determined in the absence of soil. In aqueous-phase experiments, an appreciable increase in the rate and extent of TCE biodegradation was observed in microcosms when toluene was added multiple times. The TCE degradation rates were clearly correlated with toluene dioxygenase (TOD) enzyme activity over time, thus providing an indication of the cometabolic pathway employed by the microbial population. In soil-slurry experiments containing freshly contaminated soil, a TCE degradation rate of approximately 150 microg TCE/kg/h was observed during the first 39-h period, and then the TCE degradation rate slowed considerably to 0.59 and 0.84 microg TCE/kg/h for microcosms receiving one and two additions of toluene, respectively. The TCE degradation rates in soil-slurry microcosms containing the desorption-resistant fraction of TCE-contaminated soil were approximately 0.27 and 0.32 microg TCE/kg/h in microcosms receiving one and two additions of toluene, respectively. It is clear from these results that mass transfer into the aqueous phase limited bioavailability of TCE in the contaminated soil.

EFFECT OF SORPTION AND DESORPTION RESISTANCE ON AEROBIC TRICHLOROETHYLENE BIODEGRADATION IN SOILS

Biodegradation of trichloroethylene(TCE) by toluene-degrading bacteria was measured under aerobic conditions in aqueous and soil-slurry batch microcosms. For soil-phase experiments, a freshly contaminated soil and a soil containing only the desorption-resistant fraction of TCE were tested. In both cases, presence of soil resulted in biodegradation rates substantially lower than those determined in the absence of soil. In aqueous-phase experiments, an appreciable increase in the rate and extent of TCE biodegradation was observed in microcosms when toluene was added multiple times. The TCE degradation rates were clearly correlated with toluene dioxygenase (TOD) enzyme activity over time, thus providing an indication of the cometabolic pathway employed by the microbial population. In soil-slurry experiments containing freshly contaminated soil, a TCE degradation rate of approximately 150 μg TCE/kg/h was observed during the first 39-h period, and then the TCE degradation rate slowed considerably to 0.59 and 0.84 μg TCE/kg/h for microcosms receiving one and two additions of toluene, respectively. The TCE degradation rates in soil-slurry microcosms containing the desorption-resistant fraction of TCE-contaminated soil were approximately 0.27 and 0.32 μg TCE/kg/h in microcosms receiving one and two additions of toluene, respectively. It is clear from these results that mass transfer into the aqueous phase limited bioavailability of TCE in the contaminated soil.