Fenton-like oxidation of small aromatic acids from biomass burning in water and in the absence of light: implications for atmospheric chemistry

Molecular-level transformations of biomass burning-derived water-soluble organic carbon during dark aqueous OH oxidation: Insights from absorption, fluorescence, high-performance size exclusion chromatography and high-resolution mass spectrometry analysis

Biomass burning (BB) releases large amounts of water-soluble organic carbon (WSOC), which would undergo heterogenous oxidation processes that induce transformations in both molecular structures and compositions within BB WSOC. This study designed an aqueous oxidation initiated by OH radicals in the absence of light for WSOC extracted from smoke particles generated by burning of corn straw and fir wood. The BB WSOC was comprehensively characterized using a combination of UV-visible spectra, excitation-emission matrix fluorescence in conjunction with parallel factor analysis (EEM-PARAFAC), high-performance size exclusion chromatography (HPSEC), and high-resolution mass spectrometry (HRMS) analyses. Over the course of oxidation, both chromophores and fluorophores exhibited gradual decreases. Moreover, EEM-PARAFAC revealed a preferential degradation of larger-sized protein-like/phenol-like organic matters, accompanied by the accumulation and/or formation of humic-like substances in aged BB WSOC. HPSEC analysis showed notable changes in molecular weight (MW) distributions for both types of BB WSOC during oxidation. Specifically, high MW species (>1 kDa) displayed a tendency to form along with oxidation, possibly attributed to the formation of assemblies via intermolecular weak forces. After oxidation, evidence of CHO compound degradation and enrichment/formation of CHON compounds was observed for both types of BB WSOC. Remarkably, the resistant, degraded and produced molecules for BB WSOC were dominated by CHO (38-73 %) and lignin-like molecules (41-47 %), suggesting diverse responses to oxidation within these two groups. Furthermore, polyphenols experienced selective degradation, while CHON, aliphatic and poly-aromatic molecules tended to form during the oxidation process for both types of BB WSOC. In summary, this study provides a comprehensive understanding of the molecular-level transformations undergone by BB WSOC during dark aqueous OH oxidation. The findings significantly contribute to our insights into atmospheric evolution of BB WSOC, thereby playing a crucial role in accurately assessing their effects within climate models and informing policy decisions.

Oxidation of small aromatic compounds in rainwater by UV/H2O2: Optimization by response surface methodology

The presence of aromatic compounds in rainwater is a matter of concern, mainly when the use of rainwater in buildings is intended. The present work aimed to assess the oxidation of a mixture of small aromatic compounds (benzoic, 3,5-dihydroxybenzoic and syringic acids) in rainwater by the UV/H₂O₂ process, and the possibility of its optimization by the response surface methodology. The extent of oxidation was assessed by ultraviolet-visible and molecular fluorescence spectroscopies. During the oxidation of the mixture new chromophoric compounds were formed at an initial stage, but they were then degraded at a later stage. The increase of the H₂O₂ concentration, resulted in a higher extent of oxidation, while the initial pH value showed no influence in the oxidation of the mixture. The optimization of the oxidation was performed using the uniform design with the factors: initial H₂O₂ concentration, initial pH, and reaction time. The response surface model found, through the best subsets regression, described the extent of oxidation as function of the following variables: initial H₂O₂ concentration and reaction time, interaction between them, and also their respective quadratic forms. The optimal conditions, the lowest H₂O₂ concentration (3.1 mM) for a selected maximum reaction time (4 h), were applied to rainwater samples spiked with the mixture of contaminants and resulted in an extent of oxidation higher than 99.5%, validating the application of the model to real samples. Therefore, the UV/H₂O₂ process coupled to its optimization via response surface methodology may be an alternative for rainwater treatment in buildings.

Fluorescence characteristics of water-soluble organic carbon in atmospheric aerosol☆

Fluorescence spectroscopy is a commonly used technique to analyze dissolved organic matter in aquatic environments. Given the high sensitivity and non-destructive analysis, fluorescence has recently been used to study water-soluble organic carbon (WSOC) in atmospheric aerosols, which have substantial abundance, various sources and play an important role in climate change. Yet, current research on WSOC characterization is rather sparse and limited to a few isolated sites, making it challenging to draw fundamental and mechanistic conclusions. Here we presented a review of the fluorescence properties of atmospheric WSOC reported in various field and laboratory studies, to discuss the current advances and limitations of fluorescence applications. We highlighted that photochemical reactions and relevant aging processes have profound impacts on fluorescence properties of atmospheric WSOC, which were previously unnoticed for organic matter in aquatic environments. Furthermore, we discussed the differences in sources and chemical compositions of fluorescent components between the atmosphere and hydrosphere. We concluded that the commonly used fluorescence characteristics derived from aquatic environments may not be applicable as references for atmospheric WSOC. We emphasized that there is a need for more systematic studies on the fluorescence properties of atmospheric WSOC and to establish a more robust reference and dataset for fluorescence studies in atmosphere based on extensive source-specific experiments.

Seasonal variation and sources of derivatized phenols in atmospheric fine particulate matter in North China Plain

Qualitative and quantitative analyses of derivatized phenols in Beijing and in Xinglong were performed from 2016 to 2017 using gas chromatography-mass spectrometry. The results showed substantially more severe pollution in Beijing. Of the 14 compounds detected, the total average concentration was 100 ng/m³ in Beijing, compared with 11.6 ng/m³ in Xinglong. More specifically, concentration of nitro-aromatic compounds (NACs) (81.9 ng/m³ in Beijing and 8.49 ng/m³ in Xinglong) was the highest, followed by aromatic acids (14.6 ng/m³ in Beijing and 2.42 ng/m³ in Xinglong) and aromatic aldehydes (3.62 ng/m³ in Beijing and 0.681 ng/m³ in Xinglong). In terms of seasonal variation, the highest concentrations were found for 4-nitrocatechol in winter in Beijing (79.1 ± 63.9 ng/m³) and 4-nitrophenol in winter in Xinglong (9.72 ± 8.94 ng/m³). The analysis also revealed diurnal variations across different seasons. Most compounds presented higher concentrations at night in winter because of the decreased boundary layer height and increased heating intensity. While some presented higher levels during the day, which attributed to the photo-oxidation process for summer and more biomass burning activities for autumn. Higher concentrations appeared in winter and autumn than in spring and summer, which resulted from more coal combustions and adverse meteorological conditions. The significant correlations among NACs indicated similar sources of pollution. Higher correlations presented within each subgroup than those between the subgroups. Good correlations between levoglucosan and nitrophenols, nitrocatechols, nitrosalicylic acids, with correlation coefficients (r) of 0.66, 0.69 and 0.69, respectively, indicating an important role of biomass burning among primary sources.

Aromatic acids as biomass-burning tracers in atmospheric aerosols and ice cores: A review

Biomass burning (BB) is one of the largest sources of carbonaceous aerosols with adverse impacts on air quality, visibility, health and climate. BB emits a few specific aromatic acids (p-hydroxybenzoic, vanillic, syringic and dehydroabietic acids) which have been widely used as key indicators for source identification of BB-derived carbonaceous aerosols in various environmental matrices. In addition, measurement of p-hydroxybenzoic and vanillic acids in snow and ice cores have revealed the historical records of the fire emissions. Despite their uniqueness and importance as tracers, our current understanding of analytical methods, concentrations, diagnostic ratios and degradation processes are rather limited and scattered in literature. In this review paper, firstly we have summarized the most established methods and protocols for the measurement of these aromatic acids in aerosols and ice cores. Secondly, we have highlighted the geographical variability in the abundances of these acids, their diagnostic ratios and degradation processes in the environments. The review of the existing data indicates that the concentrations of aromatic acids in aerosols vary greatly with locations worldwide, typically more abundant in urban atmosphere where biomass fuels are commonly used for residential heating and/or cooking purposes. In contrast, their concentrations are lowest in the polar regions which are avoid of localized emissions and largely influenced by long-range transport. The diagnostic ratios among aromatic acids can be used as good indicators for the relative amounts and types of biomass (e.g. hardwood, softwood and herbaceous plants) as well as photochemical oxidation processes. Although studies suggest that the degradation processes of the aromatic acids may be controlled by light, pH and hygroscopicity, a more careful investigation, including closed chamber studies, is highly appreciated.

Hybridization of Nanodiamond and CuFe-LDH as Heterogeneous Photoactivator for Visible-Light Driven Photo-Fenton Reaction: Photocatalytic Activity and Mechanism

Establishing a heterojunction for two kinds of semiconductor catalysts is a promising way to enhance photocatalytic activity. In this study, nanodiamond (ND) and CuFe-layered double hydroxide (LDH) were hybridized by a simple coprecipitation method as a novel heterojunction to photoactivate H2O2. The ND/LDH possessed a hydrotalcite-like structure, large specific surface area (SBET = 99.16 m2/g), strong absorption of visible-light and low band gap (Eg = 0.94 eV). Under the conditions of ND/LDH dosage 0.0667 g/L, H2O2 concentration 19.6 mmol/L, and without initial pH adjustment, 93.5% of 10 mg/L methylene blue (MB) was degraded within 120 minutes, while only 78.3% of MB was degraded in the presence of LDH instead of ND/LDH. The ND/LDH exhibited excellent stability and maintained relatively high activity, sufficient to photoactivate H2O2 even after five recycles. The mechanism study revealed that in the heterojunction of ND/LDH, the photoelectrons transferred from the valence band of LDH (Cu/Fe 3d t2g) to the conduction band of LDH (Cu/Fe 3d eg) could spontaneously migrate onto the conduction band of ND, promoting the separation of photo-induced charges. Thus, the photoelectrons had sufficient time to accelerate the redox cycles of Cu3+/Cu2+ and Fe3+/Fe2+ to photoactivate H2O2 to produce hydroxyl radicals, resulting in excellent photo-Fenton efficiency on MB degradation.

Oxidation of benzoic acid from biomass burning in atmospheric waters

This work evaluates the degradation of benzoic acid, a tracer from biomass burning, by different oxidation agents (Fe (III); H₂O₂; sunlight; and combinations of the previous ones) in model solutions and in real atmospheric waters. The extent of reactions was assessed by Ultraviolet-Visible and molecular fluorescence spectroscopies. The oxidation of benzoic acid occurred with the chemical oxidants Fe (III), H₂O₂, Fe (III) and H₂O₂ simultaneously in the presence of sunlight, and with Fe (III) and H₂O₂ simultaneously in the absence of light. The decrease of the pH value from neutral to acid for atmospheric waters generally increased the extent of oxidation. Sunlight was an important oxidation agent, and its combination with chemical oxidants increased the oxidation rate of benzoic acid, possibly due to the photogeneration of hydroxyl radicals. The results also suggested the occurrence of direct and indirect photolysis of benzoic acid in atmospheric waters. Moreover, the oxidation of benzoic acid produced new and more complex chromophoric compounds, which were then degraded. In addition, the nocturnal period is not sufficient for the full degradation of benzoic acid and of the intermediates formed by Fenton-like oxidation. The diurnal period may be enough for their full degradation through photo-Fenton-like oxidation, but this depends on the composition of the atmospheric waters, namely of the chromophoric content. Thus, this study highlights that benzoic acid from biomass burning, and its derivatives, may persist in atmospheric waters for periods of longer than one day, becoming available for other reactions, and may also affect the terrestrial and aquatic ecosystems through the wet depositions.

Vanillic and syringic acids from biomass burning: Behaviour during Fenton-like oxidation in atmospheric aqueous phase and in the absence of light

Biomass combustion is a threat to the environment since it emits to the atmosphere organic compounds, which may react and originate others more aggressive. This work studied the behaviours of vanillic and syringic acids, small aromatic tracers of biomass burning, during Fenton-like oxidation in aqueous phase and absence of light. For both compounds, the extent of oxidation increased with pH decrease from neutral to acid in atmospheric waters, but for vanillic acid the neutral pH was not able of promoting the oxidation. With the oxidation of both acids were formed chromophoric compounds, and the formation rate increased with the degree of electron-donator substituents in benzene ring. The initial and produced compounds were not totally degraded up to 24h of reaction at pH 4.5, suggesting that the night period may be not sufficient for their full degradation in atmospheric waters. The major compounds formed were the 3,4-dihydroxybenzoic acid for vanillic acid, and the 1,4-dihydroxy-2,6-dimethoxybenzene for syringic acid. These findings suggest the occurrence of an ipso attack by the hydroxyl radical preferential to the methoxy and carboxyl groups of vanillic and syringic acids, respectively. It is important to highlight that for both aromatic acids the main compounds produced are also small aromatic compounds.

Fenton-like oxidation of small aromatic acids from biomass burning in atmospheric water and in the absence of light: Identification of intermediates and reaction pathways

A previous work showed that the night period is important for the occurrence of Fenton-like oxidation of small aromatic acids from biomass burning in atmospheric waters, which originate new chromophoric compounds apparently more complex than the precursors, although the chemical transformations involved in the process are still unknown. In this work were identified by gas chromatography-mass spectrometry (GC-MS) and by electrospray mass spectrometry (ESI-MS) the organic intermediate compounds formed during the Fenton-like oxidation of three aromatic acids from biomass burning (benzoic, 4-hydroxybenzoic and 3,5-dihydroxybenzoic acids), the same compounds evaluated in the previous study, in water and in the absence of light, which in turns allows to disclose the chemical reaction pathways involved. The oxidation intermediate compounds found for benzoic acid were 2-hydroxybenzoic, 3-hydroxybenzoic, 4-hydroxybenzoic, 2,3-dihydroxybenzoic, 2,5-dihydroxybenzoic, 2,6-dihydroxybenzoic and 3,4-dihydroxybenzoic acids. The oxidation intermediates for 4-hydroxybenzoic acid were 3,4-hydroxybenzoic acid and hydroquinone, while for 3,5-dihydroxybenzoic acid were 2,4,6-trihydroxybenzoic and 3,4,5-trihydroxybenzoic acids, and tetrahydroxybenzene. The results suggested that the hydroxylation of the three small aromatic acids is the main step of Fenton-like oxidation in atmospheric waters during the night, and that the occurrence of decarboxylation is also an important step during the oxidation of the 4-dihydroxybenzoic and 3,5-dihydroxybenzoic acids. In addition, it is important to highlight that the compounds produced are also small aromatic compounds with potential adverse effects on the environment, besides becoming available for further chemical reactions in atmospheric waters.