Photochemical Aging of Guaiacol by Fe(III)-Oxalate Complexes in Atmospheric Aqueous Phase

Facet-dependent activation of oxalic acid over hematite nanocrystals under the irradiation of visible light for efficient degradation of pollutants

Naturally occurring hematite has been widely studied in the Fenton-like system for water pollutant remediation due to its abundance and non-toxicity. However, its inadequate catalytic activity results in difficulty in effectively degrading pollutants in the catalytic degradation system that it constitutes. Thus, we constructed a photochemical system composed of hematite with {001} facet of high activity facet and low-cost and non-toxic oxalic acid (OA) for the removal of various types of pollutants. The removal rate for the degradation of metronidazole, tetracycline hydrochloride, Rhodamine B, and hexavalent chromium by hematite nanoplate with the exposed {001} facet activating OA under visible light irradiation was 4.75, 2.25, 2.33, and 2.74 times than that by the exposed {110} facet, respectively. Density functional theory (DFT) calculation proved that the OA molecule was more easily adsorbed on the {001} facet of hematite than that on the {110} facet, which would favor the formation of the more Fe(III)-OA complex and reactive species. In addition, the reactive site of metronidazole for the attraction of radicals was identified on the basis of the DFT calculation on the molecular occupied orbitals, and the possible degradation pathway for metronidazole included carbon chain fracture, hydroxyethyl-cleavage, denitrogenation, and hydroxylation. Thus, this finding may offer a valuable direction in designing an efficient iron-based catalyst based on facet engineering for the improved activity of Fenton-like systems such as OA activation.

Role of Nb2O5 Crystal Phases on the Photocatalytic Conversion of Lignin Model Molecules and Selectivity for Value-Added Products

The photocatalytic conversion in aqueous media of phenol and guaiacol as a lignin model compound using Nb2O5 with different crystal phases was studied. Nb2O5 particles were synthesized using hydrothermal methods, where it was observed that changes in the solvent control their morphology and crystal phase. Different photocatalytic behavior of Nb2O5 was observed with the selected model compounds, indicating that its selection directly impacts the resulting conversion and selectivity rates as well as the reaction pathway, highlighting the relevance of model molecule selection. Photocatalytic conversion of phenol showed conversion rate (C%) up to 25 % after 2 h irradiation and high selectivity (S%) to pyrogallol (up to 50 %). Orthorhombic Nb2O5 spheres favored conversion through free hydroxyl radicals while monoclinic rods did not convert phenol. Guaiacol photocatalytic oxidation showed high conversion rate but lower selectivity. Orthorhombic and monoclinic Nb2O5 favored the formation of resorcinol with S % ~0.43 % (C % ~33 %) and ~13 % (C % ~27 %) respectively. The mixture of both phases enhanced the guaiacol conversion rate to ~55 % with ~17 % of selectivity to salicylaldehyde. The use of radical scavengers provided information to elucidate the reaction pathway for these model compounds, showing that different reaction pathways may be obtained for the same photocatalyst if the model compound is changed.

Rapid Release of Halocarbons from Saline Water by Iron-Based Photochemistry

Methyl halides play important roles in stratospheric ozone depletion, but their formation mechanisms are not well defined. This study demonstrated that iron-based photochemistry significantly enhanced alkyl halide production by promoting the reaction of the representative monomer of lignin with halide ions in saline water under solar light irradiation. The methyl chloride (CH3Cl) emission from the light/Fe(III) process was 2 orders of magnitude higher than dark treatment and in the absence of iron. In addition, bromide and iodide showed better reactivity in the formation of the corresponding methyl bromide (CH3Br) and methyl iodide (CH3I). Alkyl halides identified from seawater, brackish water, and salt pan water under sunlight irradiation were positively correlated with the Fe(III) concentrations, indicating that iron-based photochemistry is ubiquitous. This work suggested that the photoinduced formation of methyl radical and redox cycling of iron triggered by the Fenton-like reaction are responsible for the enhanced release of alkyl halides. This study represents an abiotic formation pathway of alkyl halides, which accounts for a portion of the unidentified sources of halocarbons in the ocean.

Common Photo-oxidative Decarboxylation Mechanism in Iron Hydroxy Carboxylate Complexes

Photochemical oxidation of dissolved organic matter is a crucial component of carbon cycling in surface waters. Photo-oxidation of iron(III)-carboxylate complexes is of particular interest because complexation with iron(III) can sensitize this functional group to photodecarboxylation. The photo-oxidation mechanism of ferrioxalate has been extensively characterized, but it is unclear whether the mechanism or timing is similar for other more complex carboxylates. In this study, we use time-resolved infrared spectroscopy to demonstrate that Fe(III)-citrate, an aliphatic carboxylate, and Fe(III)-salicylate, an aromatic carboxylate, follow the same photo-oxidation kinetics as ferrioxalate. Hence the data suggest a common mechanism for decarboxylation of iron hydroxy carbonates. Differences in the CO₂ yield within 50 ps are qualitatively similar to the long-time-scale quantum yield for Fe(II) production.

Alkyl halide formation from degradation of carboxylic acids in the presence of Fe(III) and halides under light irradiation

Advanced oxidation processes (AOPs) have been widely used in water and wastewater treatment and have shown excellent performance in remediating contaminated water. However, their oxidation byproducts, including halogenated organics, have recently attracted increasing attention. Alkyl halides are among the most important environmental pollutants in nature. Here, we report a Fenton-like reaction in which alkyl halides can form during the photodegradation of aliphatic carboxylic acids in the presence of Fe(III) and halides. Chloromethane, chloroethane, and 1-chloropropane were produced from the degradation of acetic acid, propionic acid and n-butyric acid, respectively. CH₃Cl, CH₂Cl₂ and CHCl₃ were all identified as the products of acetic acid with the yields of approximately 5.1%, 0.2% and 0.005%, respectively. It was demonstrated that hydroxyl radicals, halogen radicals and alkyl radicals were involved in the formation of alkyl halides. A possible mechanism of chloromethane formation was proposed based on the results. In real samples of saline water, the addition of carboxylic acid and Fe(III) significantly promoted the generation of CH₃Cl under xenon lamp irradiation. The results indicated that the coexistence of Fe(III), halides and carboxylic acids enhanced the photochemical release of alkyl halides. The reactions described in this paper may contribute to knowledge on the mechanism of halogenated byproduct formation during AOPs.

Rapid Redox Cycling of Fe(II)/Fe(III) in Microdroplets during Iron-Citric Acid Photochemistry

Fe(III) and carboxylic acids are common compositions in atmospheric microdroplet systems like clouds, fogs, and aerosols. Although photochemical processes of Fe(III)-carboxylate complexes have been extensively studied in bulk aqueous solution, relevant information on the dynamic microdroplet system, which may be largely different from the bulk phase, is rare. With the help of the custom-made ultrasonic-based dynamic microdroplet photochemical system, this study examines the photochemical process of Fe(III)-citric acid complexes in microdroplets for the first time. We find that when the degradation extent of citric acid is similar between the microdroplet system and the bulk solution, the significantly lower Fe(II) ratio is present in microdroplet samples due to the rapider reoxidation of photogenerated Fe(II). However, by replacing citric acid with benzoic acid, no much difference in the Fe(II) ratio between microdroplets and bulk solution is observed, which indicates distinct reoxidation pathways of Fe(II). Moreover, the presence of ^•OH scavenger, namely, methanol, greatly accelerates the reoxidation of photogenerated Fe(II) in both citric acid and benzoic acid situations. Further experiments reveal that the high availability of O₂ and the citric acid- or methanol-derived carbon-centered radicals are responsible for the rapider reoxidation of Fe(II) in iron-citric acid microdroplets by prolonging the length of HO₂^•- and H₂O₂-involved radical reaction chains. The results in this study may provide a new understanding about iron-citric acid photochemistry in atmospheric liquid particles, which can further influence the photoactivity of particles and the formation of secondary organic aerosols.

Efficient Decolorization of Azo Dye Orange II in a UV-Fe3+-PMS-Oxalate System

The decolorization of azo dye Orange II using a UVA-Fe3+-PMS-oxalate system was studied. A series of experiments was performed to investigate the effects of several variables, including the pH, PMS dosage, Fe3+ concentration, oxalate concentration, and coexisting anions. The results revealed that a lower pH facilitated the decolorization, and relatively high decolorization efficiency (97.5%) could be achieved within 5 min at pH 3.0. The electron paramagnetic resonance (ESR) and radical quenching experiments revealed that SO4•− played a crucial role in the decolorization of Orange II (85.8%), •OH was of secondary importance (9%), and 1O2 made a small contribution to the decolorization (5.2%). Furthermore, the formation of •OH in the experimental system strongly depended on HO2•/O2•−. These reactive oxidants were able to directly attack the azo bond of the luminescent group in Orange II and initiate the decolorization process. The efficient UVA-Fe3+-PMS-oxalate system showed great application potential in the treatment of wastewater contaminated by azo dyes.

Aqueous-phase chemistry of atmospheric phenolic compounds: A critical review of laboratory studies

Phenolic compounds (PhCs) are crucial atmospheric pollutants typically emitted by biomass burning and receive particular concerns considering their toxicity, light-absorbing properties, and involvement in secondary organic aerosol (SOA) formation. A comprehensive understanding of the transformation mechanisms on chemical reactions in atmospheric waters (i.e., cloud/fog droplets and aerosol liquid water) is essential to predict more precisely the atmospheric fate and environmental impacts of PhCs. Laboratory studies play a core role in providing the fundamental knowledge of aqueous-phase chemical transformations in the atmosphere. This article critically reviews recent laboratory advances in SOA formation from the aqueous-phase reactions of PhCs. It focuses primarily on the aqueous oxidation of PhCs driven by two atmospheric reactive species: OH radicals and triplet excited state organics, including the important chemical kinetics and mechanisms. The effects of inorganic components (i.e., nitrate and nitrite) and transition metal ions (i.e., soluble iron) are highlighted on the aqueous-phase transformation of PhCs and on the properties and formation mechanisms of SOA. The review is concluded with the current knowledge gaps and future perspectives for a better understanding of the atmospheric transformation and SOA formation potential of PhCs.

Significantly Accelerated Hydroxyl Radical Generation by Fe(III)-Oxalate Photochemistry in Aerosol Droplets

Fe(III)-oxalate complexes are ubiquitous in atmospheric environments, which can release reactive oxygen species (ROS) such as H₂O₂, O^•2-, and OH^• under light irradiation. Although Fe(III)-oxalate photochemistry has been investigated extensively, the understanding of its involvement in authentic atmospheric environments such as aerosol droplets is far from enough, since the current available knowledge has mainly been obtained in bulk-phase studies. Here, we find that the production of OH^• by Fe(III)-oxalate in aerosol microdroplets is about 10-fold greater than that of its bulk-phase counterpart. In addition, in the presence of Fe(III)-oxalate complexes, the rate of photo-oxidation from SO₂ to sulfate in microdroplets was about 19-fold faster than that in the bulk phase. The availability of efficient reactants and mass transfer due to droplet effects made dominant contributions to the accelerated OH^• and SO₄^2- formation. This work highlights the necessary consideration of droplet effects in atmospheric laboratory studies and model simulations.

Photochemical Production of Carbon Monoxide from Dissolved Organic Matter: Role of Lignin Methoxyarene Functional Groups

Carbon monoxide (CO) is the second most abundant identified product of dissolved organic matter (DOM) photodegradation after CO₂, but its formation mechanism remains unknown. Previous work showed that aqueous photodegradation of methoxy-substituted aromatics (ArOCH₃) produces CO considerably more efficiently than aromatic carbonyls. Following on this precedent, we propose that the methoxy aromatic groups of lignin act as the C source for the photochemical formation of CO from terrestrial DOM via a two-step pathway: formal hydrolytic demethylation to methanol and methanol oxidation to CO. To test the reasonableness of this mechanism, we investigated the photochemistry of eight lignin model compounds. We first observed that initial CO production rates are positively correlated with initial substrate degradation rates only for models containing at least one ArOCH₃ group, regardless of other structural features. We then confirmed that all ArOCH₃-containing substrates undergo formal hydrolytic demethylation by detecting methanol and the corresponding phenolic transformation products. Finally, we showed that hydroxyl radicals, likely oxidants to initiate methanol oxidation to CO, form during irradiation of all models. This work proposes an explicit mechanism linking ubiquitous, abundant, and easily quantifiable DOM functionalities to CO photoproduction. Our results further hint that methanol may be an abundant (yet overlooked) DOM photoproduct and a likely precursor of formaldehyde, formic acid, and CO₂ and that lignin photodegradation may represent a source of hydroxyl radicals.

Aqueous ·OH Oxidation of Highly Substituted Phenols as a Source of Secondary Organic Aerosol

Biomass burning (BB) releases large quantities of phenols (ArOH), which can partition into cloud/fog drops and aerosol liquid water (ALW), react, and form aqueous secondary organic aerosol (aqSOA). While simple phenols are too volatile to significantly partition into particle water, highly substituted ArOH partition more strongly and might be important sources of aqSOA in ALW. To investigate this, we measured the ^·OH oxidation kinetics and aqSOA yields for six highly substituted ArOH from BB. Second-order rate constants are high, in the range (1.9-14) × 10⁹ M^-1 s^-1 at pH 2 and (14-25) × 10⁹ M^-1 s^-1 at pH 5 and 6. Mass yields of aqSOA are also high, with an average (±1σ) value of 82 (±12)%. ALW solutes have a range of impacts on phenol oxidation by ^·OH: a BB sugar and some inorganic salts suppress oxidation, while a nitrate salt and transition metals enhance oxidation. Finally, we estimated rates of aqueous- and gas-phase formation of SOA from a single highly substituted phenol as a function of liquid water content (LWC), from conditions of cloud/fog (0.1 g-H₂O m^-3) to ALW (10 μg-H₂O m^-3). Formation of aqSOA is significant across the LWC range, although gas-phase ^·OH becomes dominant under ALW conditions. We also see a generally large discrepancy between measured and modeled aqueous ^·OH concentrations across the LWC range.

Unveiling the pH-Dependent Yields of H2O2 and OH by Aqueous-Phase Ozonolysis of m-Cresol in the Atmosphere

Hydrogen peroxide (H₂O₂) and hydroxyl radical (OH) are important oxidants in the atmospheric aqueous phase such as cloud droplets and deliquescent aerosol particles, playing a significant role in the chemical transformation of organic and inorganic pollutants in the atmosphere. Atmospheric aqueous-phase chemistry has been considered to be a source of H₂O₂ and OH. However, our understanding of the mechanisms of their formation in atmospheric waters is still incomplete. Here, we show that the aqueous-phase reaction of dissolved ozone (O₃) with substituted phenols such as m-cresol represents an important source of H₂O₂ and OH exhibiting pH-dependent yields. Intriguingly, the formation of H₂O₂ through the ring-opening mechanism is strongly promoted under lower pH conditions (pH 2.5-3.5), while higher pH favors the ring-retaining pathways yielding OH. The rate constant of the reaction of O₃ with m-cresol increases with increasing pH. The reaction products formed during the ozonolysis of m-cresol are analyzed by an Orbitrap mass spectrometer, and reaction pathways are suggested based on the identified product compounds. This study indicates that aqueous-phase ozonolysis of phenolic compounds might be an alternative source of H₂O₂ and OH in the cloud, rain, and liquid water of aerosol particles; thus, it should be considered in future model studies.

Mechanistic study on photochemical generation of I•/I2•- radicals in coastal atmospheric aqueous aerosol

The reactive iodine species may exhibit significant impacts on many global atmospheric issues and the I^•/I₂^•- radicals play key roles for inducing the formation of these reactive iodine species. However, the current understanding on the formation of I^•/I₂^•- radicals in atmospheric aqueous aerosol is still quite limited. The results reported herein suggest that I^•/I₂^•- can be produced simultaneously in aqueous aerosol by several sunlight-driven photochemical pathways including direct photo-dissociation of soluble organic iodine (SOI) at rates of 0.10-1.34 × 10^-9 M ns^-1 and 0.99-5.68 × 10^-7 M μs^-1, ^•OH-mediated oxidation of I^- at 0.03-1.41 × 10^-8 M ns^-1 and 0.05-4.10 × 10^-6 M μs^-1, and ³DOM^⁎-induced oxidation of I^- at 1.57-1.65 × 10^-9 M ns^-1 and 0.99-5.68 × 10^-7 M μs^-1 for generation of I^• and I₂^•-, respectively. Meanwhile, the pathway of e_aq^--initiated stepwise reduction of IO₃^- to I_2(aq) and further photolyzed into I^• plays negligible role in formation of I^•/I₂^•- due to the low reaction rates and severe quenching effect of e_aq^- by dissolved O₂. Our work presented the new data on mechanism and kinetics for comprehensive elucidation of I^•/I₂^•- formation in coastal atmospheric aqueous aerosol and would help to better understand the transformation mechanism of iodine species, pathways of iodine cycling and the associated environmental impacts involving atmospheric reactive iodine radicals.

Photoinduced evolution of optical properties and compositions of methoxyphenols by Fe(III)-carboxylates complexes in atmospheric aqueous phase

The changes in optical properties and chemical compositions of methoxyphenols, which acted as an important aromatic compound from the biomass burning, were investigated in the presence of Fe(III)-carboxylates under aqueous phase conditions. The light was confirmed to be a key factor for stimulating the reaction of methoxyphenols and Fe(III)-carboxylates. The photoinduced evolution of optical properties of methoxyphenols was dependent on various factors, including irradiation intensity, types of carboxylates, dissolved oxygen and pH. The changes in the mass absorption efficiency at 306 nm (MAE₃₀₆) positively relied on irradiation intensity and dissolved oxygen. The acceleration effects of carboxylates on the decreases in MAE₃₀₆ of methoxyphenols followed the order of oxalate > citrate > malonate. The change amplitude of MAE₃₀₆ decreased with an increasing pH (3.5-9), while that of the mass absorption efficiency at 364 nm (MAE₃₆₄) increased with pH ranging from 3.5 to 7. The compositional evolutions of methoxyphenols by the photochemical aging were analyzed with the attenuated total reflection infrared spectroscopy (ATR-IR), confirming the decrease of CO groups and the increase of O-H and C-O groups. The photochemical reaction pathways of methoxyphenols with Fe(III)-carboxylates were proposed according to optical properties and compositions measurements.

Soil dust as a potential bridge from biogenic volatile organic compounds to secondary organic aerosol in a rural environment

The role of coarse particles has recently been proven to be underestimated in the atmosphere and can strongly influence clouds, ecosystems and climate. However, previous studies on atmospheric chemistry of volatile organic compounds (VOCs) have mostly focused on the products in fine particles, it remains less understood how coarse particles promote secondary organic aerosol (SOA) formation. In this study, we investigated water-soluble compounds of size-segregated aerosol samples (0.056 to >18 μm) collected at a coastal rural site in southern China during late summer and found that oxygenated organic matter was abundant in the coarse mode. Comprehensive source apportionment based on mass spectrum and ¹⁴C analysis indicated that different from fossil fuel SOA, biogenic SOA existed more in the coarse mode than in the fine mode. The SOA in the coarse mode showed a unique correlation with biogenic VOCs. ¹³C and elemental composition strongly suggested a pathway of heterogeneous reactions on coarse particles, which had an abundant low-acidic aqueous environment with soil dust to possibly initiate iron-catalytic oxidation reactions to form SOA. This potential pathway might complement understanding of both formation of biogenic SOA and sink of biogenic VOCs in global biogeochemical cycles, warrantying future relevant studies.

Discrepant oxidation behavior of ferric ion and hydroxyl radical on syringic acid and vanillic acid in atmospheric Fenton-like system

Syringic acid (SA) and vanillic acid (VA) from biomass combustion are not only the potential sources of atmospheric brown carbon (BrC) but also the traceable markers of biomass burning in smoke particles. In this work, the Fenton-like oxidation in a mixed system containing SA and VA was studied under some typical conditions in atmospheric aqueous. The influence of scavenger, Fe³⁺ concentration, H₂O₂ concentration, SA concentration, pH and oxygen was discussed. Our results revealed that despite SA and VA have similar structures, Fe³⁺ and HO sever as their main oxidation sources, respectively. The addition of SA could heighten the HO yield obviously compared with conventional Fenton-like oxidation in atmospheric water, and this performance was attributed to the strong reducibility to Fe³⁺. In addition, SA accelerated the oxidation of VA and caused a 4.7-fold elevation in the initial rate. These results demonstrate that the process may change the amount of SA and VA and then disturb their mass ratio, which is important for aerosol source characterization work.

Particulate Oxalate-To-Sulfate Ratio as an Aqueous Processing Marker: Similarity Across Field Campaigns and Limitations

Leveraging aerosol data from multiple airborne and surface-based field campaigns encompassing diverse environmental conditions, we calculate statistics of the oxalate-sulfate mass ratio (median: 0.0217; 95% confidence interval: 0.0154-0.0296; R = 0.76; N = 2,948). Ground-based measurements of the oxalate-sulfate ratio fall within our 95% confidence interval, suggesting the range is robust within the mixed layer for the submicrometer particle size range. We demonstrate that dust and biomass burning emissions can separately bias this ratio toward higher values by at least one order of magnitude. In the absence of these confounding factors, the 95% confidence interval of the ratio may be used to estimate the relative extent of aqueous processing by comparing inferred oxalate concentrations between air masses, with the assumption that sulfate primarily originates from aqueous processing.

Effect of common inorganic anions on iron-catalyzed secondary brown carbon formation from guaiacol

Brown carbon (BrC) is the important component of aerosol with strong UV-visible absorbance. However, the formation of BrC is still elusive. Inorganic anions, e.g., Cl^-, NO₃^- and SO₄^2-, exist ubiquitously in the atmosphere, while their effects on the formation of BrC are poorly understood. In this study, we have systematically investigated the effects of pH (1, 2 and 3), inorganic anion (Cl^-, NO₃^- and SO₄^2-) and ionic strength (0.1, 0.5 and 1.0 M) on BrC generation process by measuring the optical, aggregation and product properties. Our results clearly show that the three factors strongly affect the BrC formation by influencing the oxidation activity and the complexation capability of different Fe(III) species. Marcus theory was used in this research to calculate the oxidation activity of different Fe(III) species. Among all the species of Fe(III), FeOH²⁺ is the most reactive form in the BrC formation reaction. Furthermore, the aggregation process of BrC was also studied, which is affected by different anions due to their different concentration and hydrability, and SO₄^2- exhibits the highest efficiency to induce the aggregation of BrC. This study will deepen our understanding about the natural formation of BrC under environmentally relevant conditions, and be beneficial for controlling the production of atmospheric particulates and the subsequent health effects.

Kinetics and Mass Yields of Aqueous Secondary Organic Aerosol from Highly Substituted Phenols Reacting with a Triplet Excited State

Biomass burning emits large amounts of phenols, which can partition into cloud/fog drops and aerosol liquid water (ALW) and react to form aqueous secondary organic aerosol (aqSOA). Triplet excited states of organic compounds (³C*) are likely oxidants, but there are no rate constants with highly substituted phenols that have high Henry's law constants (K_H) and are likely important in ALW. To address this gap, we investigated the kinetics of six highly substituted phenols with the triplet excited state of 3,4-dimethoxybenzaldehyde. Second-order rate constants at pH 2 are all fast, (2.6-4.6) × 10⁹ M^-1 s^-1, while values at pH 5 are 2-5 times smaller. Rate constants are reasonably described by a quantitative structure-activity relationship with phenol oxidation potentials, allowing rate constants of other phenols to be predicted. Triplet-phenol kinetics are unaffected by ammonium sulfate, sodium chloride, galactose (a biomass-burning sugar), or Fe(III). In contrast, ammonium nitrate increases the rate of phenol loss by making hydroxyl radicals, while Cu(II) inhibits phenol decay. Mass yields of aqueous SOA from triplet reactions are large and range from 59 to 99%. Calculations using our data along with previous oxidant measurements indicate that phenols with high K_H can be an important source of aqSOA in ALW, with ³C* typically the dominant oxidant.

Photooxidation mechanism of As(III) by straw-derived dissolved organic matter

Straw return-to-field is a common agronomic practice that would affect the physicochemical characteristics of the paddy soil and overlying water, but few studies have focused on the possible impacts of straw return on the conversion of pollutants. In this study, the photooxidation of As(III) in aqueous solution by straw-derived dissolved organic matter (S-DOM) was investigated. The results showed that dissolved organic matter derived from wheat straw (DOMws) and rape straw (DOMrs) exhibited good spectroscopic features and could efficiently oxidize As(III) under irradiation at pH 5.0, with the k_obs values of As(III) oxidation being 0.15 h^-1 and 0.17 h^-1 for DOMws and DOMrs, respectively. Quenching studies indicated that hydroxyl radical (OH) dominated the oxidation of As(III) for both types of dissolved organic matter (DOM), though singlet oxygen (¹O₂) also played a role in the DOMrs system. Since acidic conditions are favorable for the formation of OH, As(III) oxidation decreased with an increase of pH value. Additionally, the oxidation efficiency of As(III) was inhibited in the presence of NO₃^- (0.2-2 mM) while enhanced in the presence of Fe(III) (5-50 μM). This study is of great significance for understanding the removal/transformation behavior of pollutants in paddy fields that receive straw return.

Enhanced Photochemical Volatile Organic Compounds Release from Fatty Acids by Surface-Enriched Fe(III)

Both Fe(III) and fatty acids are ubiquitous and important species in environmental waters. Because they are amphipathic, many fatty acids are surface active and prone to enrichment at the air-water interface. Here, we report that by using nonanoic acid (NA) as a model fatty acid, coexisting Fe(III), even at concentrations as low as 1 μM, markedly enhanced the photochemical release of NA-derived volatile organic compounds (VOCs) such as octanal and octane into the air. Further studies indicated that the surface-enriched fatty acids dramatically increase the local concentration of Fe(III) at the water surface, which enables Fe(III)-mediated photochemical reactions to take place at the air-water interface, and the VOCs facilely produced by fatty acid photooxidation can then be released into the air. Moreover, the product distribution in the Fe(III)-mediated reactions was largely different from that in other photochemical systems, and a mechanism based on photochemical decarboxylation is proposed. Considering that the coexistence of fatty acids and Fe(III) in the environment is common, the enhanced photochemical release of VOCs by surface-enriched fatty acids and Fe(III) may be an important channel for the atmospheric emission of VOCs, which are known to play an essential role in the formation of ozone and secondary organic aerosols.

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 PM_2.5 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.

Field Evidence of Fe-Mediated Photochemical Degradation of Oxalate and Subsequent Sulfate Formation Observed by Single Particle Mass Spectrometry

In this work, we deployed a single particle aerosol mass spectrometer (SPAMS) at a suburban coastal site in Hong Kong from February 04 to April 17, 2013 to study individual oxalate particles and a monitor for aerosols and gases in ambient air (MARGA) to track the bulk oxalate concentrations in particle matter smaller than 2.5 μm in diameter (PM_2.5). A shallow dip in the bulk oxalate concentration was consistently observed before 10:00 am in the morning throughout the observation campaign, corresponding to a 20% decrease in the oxalate concentration on average during the decay process. Such a decrease in PM oxalate was found to be coincident with a decrease in Fe-containing oxalate particles, providing persuasive evidence of Fe-mediated photochemical degradation of oxalate. Oxalate mixed with Fe and Fe_NaK particles, from industry sources, were identified as the dominant factors for oxalate decay in the early morning. We further found an increase of sulfate intensity by a factor of 1.6 on these individual Fe-containing particles during the oxalate decomposition process, suggesting a facilitation of sulfur oxidation. This is the first report on the oxalate-Fe decomposition process with individual particle level information and provides unique evidence to advance our current understanding of oxalate and Fe cycling. The present work also indicates the importance of anthropogenic sourced iron in oxalate-Fe photochemical processing. In addition, V-containing oxalate particles, from ship emissions, also showed evidence of morning photodegradation and need further attention since current models rarely consider photochemical processing of oxalate_V particles.

Rapid Removal of Azophloxine via Catalytic Degradation by a Novel Heterogeneous Catalyst under Visible Light

Azo dyes are the most widely used synthetic dyes in the printing and dyeing process. However, the discharge of untreated azo dyes poses a potential threat to aqueous ecosystems and human health. Herein, we fabricated a novel heterogeneous catalyst: activated-carbon-fiber-supported ferric alginate (FeAlg-ACF). Together with peroxymonosulfate (PMS) and visible light, this photocatalytic oxidation system was used to remove an azo dye—azophloxine. The results indicated that the proposed catalytic oxidation system can remove 100% of azophloxine within 24 min, while under the same system, the removal rates were only 92% and 84% when ferric alginate was replaced with ferric citrate and ferric oxalate, respectively, which showed the superiority of FeAlg-ACF. The degradation of azophloxine is achieved by the active radicals (SO4•− and •OH) released from PMS and persistent free radicals from activated carbon fiber. Moreover, due to ferric alginate’s highly intrinsic photosensitivity, visible radiation can further enhance the ligand-to-metal charge transfer (LMCT) processes. After 24 min of treatment, the total organic carbon of the azophloxine solution (50 μmol/L) decreased from 1.82 mg/L to 79.3 μg/L and the concentration of nitrate ions increased from 0.3 mg/L to 8.6 mg/L. That is, up to 93.5% of azophloxine molecules were completely degraded into inorganic compounds. Consequently, potential secondary contamination by intermediate organic products during catalytic degradation was prohibited. The azophloxine removal ratio was kept almost constant after seven cycles, indicating the recyclability and longevity of this system. Furthermore, the azophloxine removal was still promising at high concentrations of Cl−, HCO3−, and CO32−. Therefore, our proposed system is potentially effective at removing dye pollutants from seawater. It provides a feasible method for the development of efficient and environmentally friendly PMS activation technology combined with FeAlg-ACF, which has significant academic and application value.

Nitrite-Mediated Photooxidation of Vanillin in the Atmospheric Aqueous Phase

Nitrite (NO₂^-) and its conjugate acid, nitrous acid (HNO₂), have long been recognized as a ubiquitous atmospheric pollutant as well as an important photochemical source of hydroxyl radicals (·OH) and reactive nitrogen species (·NO, ·NO₂, ·N₂O₃, etc.) in both the gas phase and aqueous phase. Although NO₂^-/HNO₂ plays an important role in atmospheric chemistry, our understanding on its role in the chemical evolution of organic components in atmospheric waters is rather incomplete and is still in dispute. In this study, the nitrite-mediated photooxidation of vanillin (VL), a phenolic compound abundant in biomass burning emissions, was investigated under pH conditions relevant for atmospheric waters. The influence of solution pH, dissolved oxygen, and ·OH scavengers on the nitrite-mediated photooxidation of VL was discussed in detail. Our study reveals that the molecular composition of the products is dependent on the molar ratio of NO₂^-/VL in the solution and that nitrophenols are the major reaction products. We also found that the light absorbance of the oxidative products increases with increasing pH in the visible region, which can be attributed to the deprotonation of the nitrophenols formed. These results contribute to a better understanding of methoxyphenol photooxidation mediated by nitrite as a source of toxic nitrophenols and climatically important brown carbon in atmospheric waters.

Multitasking polyamine/ferrioxalate nano-sized assemblies: thermo-, photo-, and redox-responsive soft materials made easy

Responsive nanomaterials have emerged as key components in materials sciences. Herein, we report the one-step preparation of multi-stimuli responsive polyamine-salt aggregates (PSA) by ionically crosslinking polyethylenimine with potassium ferrioxalate (FeOx). The unique properties of FeOx enables a novel class of soft nanomaterial that disassembles by exposure to light, reducing environments and temperature.

Effect of chelators on the production and nature of the reactive intermediates formed in Fe(II) activated peroxydisulfate and hydrogen peroxide processes

Iron chelators are often used to improve the performance of Fe(II) activated peroxides (e.g., peroxydisulfate (PDS) and hydrogen peroxide (H₂O₂)) for oxidative water treatment over a wide pH range due to the enhanced solubility of iron in the presence of chelators at high pH. In this study, we compared the effect of various chelators on the production and nature of the reactive intermediate formed in Fe(II)/PDS and Fe(II)/H₂O₂ systems by using methyl phenyl sulfoxide (PMSO) as a probe, which could distinguish ferryl ion (Fe(IV)) from free radicals (^•OH and SO₄^•-) due to their marked difference in product formation. Six representative chelators (oxalate acid (OA), citric acid (CA), nitrilotriacetic acid (NTA), ethylenediaminetetraacetic acid (EDTA), pyrophosphate (PPP), and tetrapolyphosphate (TPP)) which covered the commonly used polycarboxylates, aminocarboxylates, and polyphosphates ligands were selected. In chelator assisted Fe(II)/PDS systems, the highest PMSO transformation efficiency at pH 3-9 was obtained in cases with polycarboxylates, due to their higher reactivity to PDS activation, lower steric hindrance, and stronger ability in promoting Fe(II)/Fe(III) cycle. Comparatively, in chelator assisted Fe(II)/H₂O₂ systems, TPP addition achieved the best performance in PMSO transformation at pH > 5. Moreover, the yield of Fe(IV) indicative product (methyl phenyl sulfone, PMSO₂) decreased with increasing chelator/Fe(II) molar ratio, but was independent on pH in cases of PDS, indicating that chelator altered reactive intermediate nature from Fe(IV) to SO₄^•- and Fe(IV) yield was not sensitive to pH. In cases of H₂O₂, chelator decreased PMSO₂ production while promoting PMSO loss at near-neutral pH, suggesting that Fe(II)-chelator complexes also tended to catalyze H₂O₂ to generate ^•OH rather than Fe(IV).

Enhanced Generation of Reactive Oxygen Species under Visible Light Irradiation by Adjusting the Exposed Facet of FeWO4 Nanosheets To Activate Oxalic Acid for Organic Pollutant Removal and Cr(VI) Reduction

In this work, taking FeWO₄ nanosheets as an example, the activation of oxalic acid (OA) based on facet engineering for the enhanced generation of active radical species was reported, revealing unprecedented surface Fenton activity for pollutant degradation. Density functional theory calculations confirmed the more efficient generation of reactive oxygen species over FeWO₄ nanosheets with the {001} facet exposed (FWO-001) under visible light irradiation compared to the efficiency of FeWO₄ nanosheets with the {010} facet exposed (FWO-010), which could be attributed to a higher density of iron and the efficient activation of OA on the {001} facet. The H₂O₂-derived ^•OH tended to diffuse away from the active sites of FWO-001 into solution to favor the continuous activation of OA into the active radicals for pollutant redox reactions, but preferred to remain on FWO-010 to hinder the further activation of OA on the {010} facet. Additionally, the generation of ^•CO₂^- endowed FeWO₄ with a strong reduction ability. Compared with FWO-010, FWO-001 exhibited enhanced redox activity for the catalytic degradation of organic pollutants and Cr(VI) in the optimized conditions. These findings can help in understanding the facet dependent surface Fenton chemistry of catalytic redox reactions and in designing efficient catalysts for environmental decontamination.

Kinetic and Theoretical Study of the Atmospheric Aqueous-Phase Reactions of OH Radicals with Methoxyphenolic Compounds

Methoxyphenols, which are emitted through biomass burning, are an important species in atmospheric chemistry. In the present study, temperature-dependent aqueous-phase OH radical reactions of six methoxyphenols and two related phenols have been investigated through laser flash photolysis and the density functional theory. The rate constants obtained were in a range of (1.1-1.9) × 10¹⁰ L mol^-1 s^-1 with k(3-MC) > k(Cre) ≈ k(Syr) ≈ k(MEP) > k(Res) > k(3-MP) > k(2-EP) ≈ k(2-MP). We derived the parameters of these reactions from the obtained T-dependent rate constants and found a mean Arrhenius activation energy of 16.9 kJ mol^-1. The diffusion rate constants were calculated for each case and compared to the measured ones. Generally, the rate constants are found to be close to fully diffusion-controlled (k_diff = (1.4-1.5) × 10¹⁰ L mol^-1 s^-1 for all reactions). A structure-function relationship was established through the measurement result, which could be used for predicting unknown rate constants of other phenolic compounds. All of these findings are expected to enhance the predictive capabilities of models, such as the chemical aqueous-phase radical mechanism.

Photochemical behavior of ferrihydrite-oxalate system: Interfacial reaction mechanism and charge transfer process

Heterogeneous photochemical reactions associated with natural iron (hydr)oxides and oxalic acid have attracted a great deal of scientific attention in the application of organic pollutants degradation. However, the reaction mechanism is still unclear due to the complicated iron cycles and reactive oxygen species (ROS) generation. In this study, the in situ attenuated total reflectance-Fourier transform infrared spectroscopy was implemented to investigate the adsorption process and photochemical behavior of oxalic acid on the surface of ferrihydrite. A comprehensive reaction mechanism from the perspective of charge transfer process, including homogeneous-heterogeneous iron cycling and ROS generation, was illustrated in detail. We found that oxalic acid was first adsorbed on the surface of ferrihydrite with a mononuclear bidentate binding geometry. Interestingly, this mononuclear bidentate complex on the surface of ferrihydrite was stable under visible light irradiation. Subsequently, the whole complex departed from ferrihydrite surface through non-reduction dissolution with the form of Fe(C₂O₄)⁺. In the solution, the Fe(C₂O₄)⁺ complexes would quickly convert to Fe(C₂O₄)₂^- complexes. Under visible light irradiation, the electrons generated from the photolysis of Fe(C₂O₄)₂^- complex reacted with O₂ to form O₂^•-/•OOH. Meanwhile, Fe(III) was reduced to Fe(II). Finally, the produced O₂^•-/•OOH could react with Fe(II) through a one-step way to generate •OH, which possessed higher •OH formation efficiency than that through the two-step way of H₂O₂ as the intermediates. This study helps us with understanding of in-situ photochemical reaction mechanism of ferrihydrite-oxalic acid system, and also provides guidance to effectively utilize widespread iron (hydr)oxides and organic acids in natural environment to develop engineered systems for water treatment.

Effect of Oxalate and Sulfate on Iron-Catalyzed Secondary Brown Carbon Formation

Oxalate and sulfate are ubiquitous components of ambient aerosols with a high complexation affinity to iron. However, their effect on iron-driven secondary brown carbon formation in solution from soluble aromatic and aliphatic reagents was not studied. We report masses and hydrodynamic particle sizes of insoluble particles formed from the dark aqueous phase reaction of catechol, guaiacol, fumaric, and muconic acids with Fe(III) in the presence of oxalate or sulfate. Results show that oxalate decreases particle yield in solution from the reaction of Fe(III), with a stronger effect for guaiacol than catechol. For both compounds, the addition of sulfate results in the formation of more polydisperse (0.1-5 μm) and heavier particles than those from control experiments. Reactions with fumaric and muconic acids show that oxalate (not sulfate) and pH are determining factors in the efficiency of particle formation in solution. Polymerization reactions occur readily in the presence of sulfate in solution producing particles with iron-coordinated and/or pore-trapped sulfate anions. The addition of oxalate to the reactions of Fe(III) with all organics, except guaiacol, produced fewer and larger polymeric particles (>0.5 μm). These results imply that even in the presence of competing ligands, the formation of insoluble and colored particles from soluble organic precursors still dominates over the formation of soluble iron complexes.