Complementary Elucidation of the Molecular Characteristics of Groundwater Dissolved Organic Matter Using Ultrahigh-Resolution Mass Spectrometry Coupled with Negative- and Positive-Ion Electrospray Ionization

Quantifying assembly processes of dissolved organic matter pools in eutrophication using high-resolution mass spectrometry and ecological models

Dissolved organic matter (DOM) represents a large, dynamic pool of carbon, playing a crucial role in eutrophic aquatic ecosystems through its continuous transport and transformation. However, the assembly mechanisms of DOM under different eutrophic conditions remain elusive, hindering the understanding of carbon dynamics and the prediction of carbon fate. Here we collected 72 lake water samples during two sampling events in Chaohu Lake, the fifth largest freshwater lake in China, and performed high-resolution mass spectrometry (HRMS) and ecological null modeling to quantify the assembly processes of DOM in eutrophication. We found that as eutrophic levels increased, the relative contribution of homogeneous selection rose, while the contributions of variable selection and dispersal limitation decreased. The influence of different assembly processes on the DOM pool across sites, although estimated solely from HRMS data, exhibited reasonable consistency with the spatiotemporal variations. Several environmental parameters, including total phosphorus, Secchi disk depth, trophic state index, pH, temperature, and fluorescence index, were significantly correlated with one or more DOM assembly processes (p < 0.05), and assembly mechanisms also shaped the compound composition of DOM. Our findings reveal a shift in DOM assembly from variable selection to homogeneous selection in eutrophication, highlighting the importance of DOM dynamics and environmental homogenization in the management and restoration of eutrophic lakes.

Accurate detection and high throughput profiling of unknown PFAS transformation products for elucidating degradation pathways

The accurate detection of unknown per- and polyfluoroalkyl substances (PFAS) transformation products (TPs) is essential for elucidating degradation pathways and advancing remediation strategies. Herein, we developed a workflow that combined Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) with a paired mass distance (PMD) network. This study achieved high throughput profiling of PFAS TPs with mDa resolving power and sub-ppm mass error. UV treatment revealed chain-shortening pathways, while plasma treatment uncovered competing mechanisms of chain shortening and lengthening, generating oxygen-rich TPs with increased hydrophilicity. Specifically, UV treatment of a 15-PFAS mixture and contaminated natural water showed disappearance of 7 unknown PFAS homologues and the emergence of 12 unknown PFAS homologues. Despite PFAS persistence under UV exposure, previously undetected low-abundance PFAS species were identified, indicating non-negligible photochemical transformation. Under plasma treatment of isolated PFOS, 39 unknown PFAS homologues including 142 suspect and 34 unknown PFAS TPs were identified, highlighting the extensive transformation of emerging and persistent PFAS. Overall, our approach enabled accurate and high-throughput profiling of unknown PFAS TPs and their degradation pathways, providing new insights into persistent unknown PFAS.

Improved identification of chlorinated disinfection byproducts by the sequential elution and absorption mode

Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has been extensively employed to characterize the chemodiversty of dissolved organic matter (DOM) including chlorinated disinfection byproducts (Cln-DBPs). However, little is known about the combined effects of the sequential solid phase extraction (SPE) and absorption mode of Fourier transform data processing on the identification of Cln-DBPs. In this study, the identification of Cln-DBPs by the sequential SPE extraction and absorption mode were systematically compared using a typical swimming pool water. The sequential SPE extraction was more effective in extracting DOM molecules including Cln-DBPs than the traditional extraction, yielding 48.2 % ± 2.1 % and 87.3 % ± 3.8 % more DOM molecules and Cln-DBPs molecules, respectively. Moreover, a total of 274 nitrogenous Cln-DBPs were identified by the sequential SPE extraction with 80 more than that by the traditional SPE. The absorption mode improved the resolution and the signal-to-noise values of DOM peaks by factors of 1.87-1.98 and 1.52-1.60, respectively. The number of resolved Cl-related mass doublets within 2 mDa mass difference in the absorption mode was 537 more than that in the magnitude mode. Overall, the combination of sequential elution and absorption modes facilitated the detection of more molecules of DOM and Cln-DBPs compared to traditional SPE in magnitude mode, with an increase of 92.7 % ± 2.1 % and 121.7 % ± 5.6 %, respectively. These results have highlighted the great potential of the sequential elution combined with absorption mode in improving the identification of Cln-DBPs and their precursors, facilitating the application of FT-ICR MS in the nontargeted analysis of emerging contaminants including Cln-DBPs at the molecular level.

Simultaneous Elucidation of the Chemodiversity of Dissolved Organic Matter and Quantitation of Trace Organic Contaminant Sucralose by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has become a cutting-edge technique for molecular characterization of dissolved organic matter (DOM) and trace organic contaminants. Nevertheless, FT-ICR MS is rarely applied to simultaneously elucidate DOM chemodiversity and quantify the concentrations of trace organic contaminants. Compared to conventional solid-phase extraction (SPE), pH-dependent fractionation of DOM molecules using a sequential SPE strategy significantly enhanced the recovery of dissolved organic carbon and yielded more chemical formulas for DOM from environmental samples (p < 0.05). The sequential SPE at circumneutral pH provided exclusive isolation of some hydrogen-rich and oxygen-poor molecules, caused a 5-fold reduction in matrix effects, and improved the detection limits for organic contaminants. The chemical structure of a typical wastewater indicator, sucralose, was successfully validated using FT-ICR MS analysis with tandem mass spectrometry and hydrogen/deuterium exchange measurements. With the standard addition method, the limits of detection and quantitation of FT-ICR MS for sucralose in DOM extracts isolated at circumneutral pH were estimated to be 0.26 and 0.87 μg/L, respectively. The concentrations of sucralose in a representative urban lake measured by FT-ICR MS (e.g., 0.9-4.1 μg/L) were comparable to those determined by liquid chromatography coupled to an Orbitrap mass analyzer. The findings of this study expand application of FT-ICR MS in environmental research by highlighting its potential for identifying trace organic contaminants, quantifying their concentrations, and elucidating the chemodiversity of DOM to inform contaminant sources in aquatic systems.

Built-in electric field of Ag2Se thermoelectric effect activated persulfate for humic acid decomposition in water: Molecular transformation mechanism

Water temperature fluctuations directly impact pollutant decomposition processes in wastewater. Thermoelectric effect is considered an alternative to utilize these temperature variations for pollution control. In this study, a system for persulfate (PS) activation by Ag2Se thermoelectric catalyst under water temperature gradients (Ag₂Se/ΔT/PS) was developed for humic acid (HA) degradation in water. The experimental results showed that the Ag2Se/ΔT/PS system achieved a 90.7 % HA removal efficiency, outperforming both PS/ΔT (PS with temperature gradients) and Ag2Se/ΔT systems. Thermoelectric simulations indicated that Ag2Se generated an electric field under temperature variations, with higher current density at surface pores where polarized charges efficiently activated PS. Density functional theory calculations revealed that the thermoelectric effect of Ag2Se lowered the energy barriers for PS activation and ·SO4- generation. Different from ·OH-led decomposition of HA in the Ag₂Se/ΔT system, ·SO4- and ·OH dominated HA decomposition in the Ag₂Se/ΔT/PS system, and ¹O₂ also contributed this process. Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) revealed that oxidation, decarboxylation, and sulfidation were the primary pathways driving HA degradation, leading to decreases in CHO-containing compounds and formation of S-rich byproducts. These findings highlighted the potential of thermoelectric catalysts in advancing water treatment technologies.

Carbon Isotopic Signatures of Aquifer Organic Molecules along Anthropogenic Recharge Gradients

The property of groundwater dissolved organic matter (DOM) subjected to anthropogenic groundwater recharge (AGR) might be affected by the water quality disparity between surface water and natural groundwater. However, the diverse molecular scenarios of groundwater DOM under uneven recharging levels remain largely unexplored. We combined molecular characteristics, carbon isotopic signatures of organic molecules, and end-member mixing analysis to explore the sensitivity and potential tracking capabilities of DOM to AGR along with recharging gradients. Our findings suggested that AGR enriched groundwater with diverse, saturated, labile, and sulfur-rich molecules, amplifying DOM abundance and intensity, which intensified with recharge gradients. Additionally, S-containing molecules and their indicators like CHOS% (with threshold values of 7.82%) exhibited high sensitivity and predictive power for AGR recognition. The major signatures (diversity, saturated degree, and stability) indicated by 13C-containing molecules were similar to the whole molecular pool. Notably, specific molecules (C12H10O5S and C15H16O12), although not detected in all groundwater samples, exhibit robust stability or favorable solubility, rendering them potential candidates as AGR-sensitive molecules. The R13C/12C ratio of 13C-containing C19H24O5 emerged as the most robust tracer, exhibiting a strong correlation with the recharge ratio and the smallest deviation from the theoretical mixing line, signifying its optimal suitability for precise groundwater DOM source apportionment. This study offers novel insights into AGR impacts and contributes to fostering a harmonious balance between human activities and water resource sustainability.

Leveraging almost hydrophobic PVDF membrane and in-situ ozonation in O3/UF/BAC system for superior anti-fouling and rejection performance in drinking water treatment

The almost hydrophobic PVDF membrane (PVDF matrix) commonly exhibited excellent performance in pollutant rejection but with poor anti-fouling performance. This study intended to develop the rejection performance and enhance anti-fouling of the PVDF membrane in an O3/UF/BAC system for high quality water production through leveraging the advantages of in-situ ozonation and the nature of the PVDF membrane. Reduced density gradient (RDG) analysis demonstrated that the PVDF membrane exhibited excellent ozone resistance by reducing hydrogen bonds and electrostatic interactions between the membrane surface and ozone. Consequently, the physicochemical properties of the PVDF membrane remained unchanged in the laboratory continuous flow experiment with in-situ ozonation at 2.86 mg/L. The almost hydrophobicity of the PVDF membrane not only resisted fouling but also facilitated the reaction between ozone and foulants of higher concentrations locally at membrane surface, leading to dynamic changes in membrane fouling, with TMP/TMP0 initially increasing, then decreasing and stable. Therefore, the Rtotal, Rcake and Rgel of the PVDF membrane decreased by 47.40 %, 46.79 % and 50.99 % as compared to the UF/BAC system, respectively, in the O3/UF/BAC system. In-situ ozonation transformed macromolecular substances into micromolecules, particularly organic matter with lignin/carboxylic-rich alicyclic molecules and aromatic structures. The majority of these micromolecules were either rejected by the deposited foulants layer through Van der Waals interaction and utilized as a carbon source by membrane surface microorganisms (eg., Curvibacter and Methyloversatilis), or further degraded by microorganism in the BAC unit. This resulted in a 19.34 % and 40.58 % reduction in CODMn concentrations in the UF and BAC effluents, respectively. The system's anti-fouling and water purification performance observed in laboratory experiments was confirmed in a pilot test, providing new insights into the use of in-situ ozonation and organic membranes.

Cytotoxicity and Genotoxicity toward Mammalian Cells Induced by Organic Iodine in Peroxymonosulfate (PMS) Processes: Activated PMS Is Better than Nonactivated PMS in Mitigating Toxicity

Peroxymonosulfate (PMS) and its activation processes have been extensively studied. However, iodinated byproducts (I-DBPs) generated during nonactivated PMS (NPMS) and activated PMS (APMS) processes pose a significant risk. In NPMS, a 50 μg/L concentration of iodide (I-) could significantly (p < 0.05) increase the cytotoxicity of both the Suwanee River natural organic matter (SRNOM) solution and wastewater secondary effluent (SE), at the dose of 0.5 mM PMS. Cytotoxicity and genotoxicity of the SRNOM solution increased by 6.6 mg-phenol/L and 2.4 μg-4-NQO/L with 200 μg/L I-. For wastewater secondary effluent (SE), both toxicities increased 2.4-fold and 1.9-fold. APMS reduced cytotoxicity by 42-47% and genotoxicity by 53-60% compared with NPMS in I--containing SRNOM and SE samples. NPMS promoted adsorbable organic iodine (AOI) formation, while APMS inhibited AOI by 77.5-84.9%. FTICR-MS showed NPMS favored I-DBP generation with aromatic precursors, whereas APMS eliminated these precursors and I-DBPs. Compounds such as p-iodophenol and 4-methyl-2-iodophenol were detected in NPMS but removed in APMS. NPMS exhibited weaker oxidation, producing tannic acid-like, lignin-like, and protein-like precursors that can increase toxicity when reacting with HOI. Conversely, APMS enhanced oxidation via hydroxyl radicals and singlet oxygen, decomposing these precursors further. Importantly, APMS also converted HOI into nontoxic iodate, reducing overall toxicity in I--containing water.

Development of a Nontargeted Algorithm for Per- and Polyfluoroalkyl Substances in the FT-ICR Mass Spectra of Complex Organic Mixtures

The unrivaled mass resolving power and subppm mass error enable the application of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) for nontargeted screening of per- and polyfluoroalkyl substances (PFAS). Few automated FT-ICR MS methods exist for nontargeted analysis of PFAS in solutions containing other dissolved organic matter. In this study, isotopic pattern analysis and dynamic homologous series inspection were complementarily employed in the FTMSDeu algorithm to simultaneously assign chemical formulas for PFAS and other organic molecules, with an overall assignment accuracy of 91.2% for 1,802 unique PFAS formulas and their 11,634 isotopic formulas. The negative effects of fluorine-related isobaric isotopes with sub-mDa mass differences on formula assignment for PFAS and natural organic matter were addressed by an empirical F/(H + X) rule and the adoption of a threshold for the isotopic formula number plus the homologous series number. There were 203 unique PFAS formulas containing one novel PFAS formula, C9H6O4S1I1F11, identified in a fluoropolymer industry-impacted river water sample using our algorithm for analyzing FT-ICR MS spectra. Overall, the results of this study highlighted the great potential of the updated FTMSDeu algorithm for nontargeted and high-throughput chemical assignment for the FT-ICR MS spectra of complex mixtures containing PFAS and dissolved organic matter from real environmental samples.

Molecular composition of hydroxyl radical-resistant organics in municipal solid waste leachate

Although hydroxyl radicals (•OH) degrade organic pollutants nonselectively, their mineralization rate during the treatment of waste leachate biological treatment effluent (BTL) using Fenton or Fenton-like systems is not high, and the reason is unknown. In this study, we investigated three typical Fenton-like systems that act on dissolved organic matter (DOM) in BTL. We analyzed the molecular composition of DOM resistant to •OH, using ultrahigh resolution mass spectrometry. We find that DOM resistant to •OH is more oxidized, less unsaturated/aromatic, has higher molecular weights, and contains more unsaturated oxygen-containing functional groups than does DOM reactive to •OH. Resistant-DOM is further categorized into DOM derived by the action of •OH (DOMderived) and DOM initially present (DOMintrinsic), whose quantities account for approximately 20 % and 80 %, respectively. The DOMderived is gradually removed under extended reaction time, while DOMintrinsic is relatively unreactive with •OH and is always present in the treated effluent. Based on the molecular composition of resistant-DOM, we propose a method to increase the mineralization rate (up to 95 % TOC removal with only 5 mM persulfate). This study provides direct evidence for the first time that the presence of resistant-DOM (mainly stemming from DOMintrinsic) in BTL is an important reason for the unideal mineralization rate in the advanced treatment of Fenton or Fenton-like systems.

Molecular composition difference of electron donating moieties between natural organic matter and effluent organic matter probed by chlorine dioxide

Lignin- and tannin-like phenolic compounds are shown to be the major compositions of electron donating moieties (EDM) of aquatic natural organic matter (NOM). However, little is known about the compositions of EDMs within effluent organic matter (EfOM). In the present study, chlorine dioxide (ClO2) was used as a selectively oxidative probe to investigate the difference in the molecular composition of EDM between NOM and EfOM due to its high selectivity towards electron-rich compounds. The results showed that there was a large difference in the bulk and molecular properties of ClO2-reactive moieties between EfOM and NOM. Specifically, ClO2-reactive moieties of EfOM are distributed in a narrower molecular weight range (i.e., 0.9 kDa to 3.0 kDa) compared to NOM (i.e., 1.0 kDa to 20 kDa). The molecular-level analysis demonstrated that highly aromatic, reduced formulas (O/C = 0.33 ± 0.16; H/C = 1.10 ± 0.34) referring the lignin- and tannin-like compounds within both NOM and EfOM were susceptible to oxidation by ClO2, while more saturated formulas including the peptide-like formulas (H/C = 1.59 ± 0.36) within EfOM were reactive towards ClO2. Furthermore, the nitrogen (N)-containing formulas in EfOM are suggested to be the major EDMs compared to the CHO-only formulas dominating the EDM in NOM. This study has important implications for understanding of the origin and chemical nature of EDM in DOM from various sources and provides molecular-level evidence for the selectivity of ClO2 as an oxidant towards DOM.

Iron(II/III) Alters the Relative Roles of the Microbial Byproduct and Humic Acid during Chromium(VI) Reduction and Fixation by Soil-Dissolved Organic Matter

Though reduction of hexavalent chromium (Cr(VI)) to Cr(III) by dissolved organic matter (DOM) is critical for the remediation of polluted soils, the effects of DOM chemodiversity and underlying mechanisms are not fully elucidated yet. Here, Cr(VI) reduction and immobilization mediated by microbial byproduct (MBP)- and humic acid (HA)-like components in (hot) water-soluble organic matter (WSOM), (H)WSOM, from four soil samples in tropical and subtropical regions of China were investigated. It demonstrates that Cr(VI) reduction capacity decreases in the order WSOM > HWSOM and MBP-enriched DOM > HA-enriched DOM due to the higher contents of low molecular weight saturated compounds and CHO molecules in the former. The presence of Fe(II/III) selectively coprecipitates with high molecular weight components (e.g., tannins, lignin, and CHON-rich compounds) to form ferrihydrite and greatly inhibits Cr(VI) transformation and fixation in MBP-enriched DOM but enhances that in HA-enriched DOM. This is probably owing to the combined effects of (1) the increase of DOM electron-donating capacity and Fe(II) generation during the reactions of HA with Fe(II) and Fe(III), respectively; (2) the enrichment of phenolic and carboxyl groups, aromatic compounds, and carbon defects on ferrihydrite surfaces; and (3) the acceleration of HA decomposition and MBP mineralization by hydroxyl radicals. These findings enhance our understanding of the chemodiversity of soil DOM, the complex interactions between Cr(VI), DOM, and Fe(II/III), and can help design remediation strategies for contaminated environments.

Increased Toxicity toward Mammalian Cells in the Periodate Oxidation Process of Wastewater: The Overlooked Formation of Noniodinated but Nitrogenous Byproducts

Periodate (PI) shows promising potential as an oxidant for wastewater treatment; however, its impact on the toxicity of wastewater remains unknown. Here, we found that with 100 μM PI addition, the cytotoxicity of wastewater increased from 4.8 to 7.6 mg-Phenol/L to 9.5 to 12.8 mg-Phenol/L, and genotoxicity increased from 0.3 to 0.9 μg-4-NQO/L. Interestingly, hypoiodous acid (HOI) was not detected during the reaction, and there was no observed increase in the concentration of total organic iodine (TOI). CHON components in dissolved organic matter changed most obviously in PI oxidation, which might serve as primary precursors for toxic byproducts. Cytotoxicity of typical nitrogen-containing precursors of tryptophan, lysine, phenylalanine, and tyrosine after PI oxidation increased from not detected to 14.7, 2.4, 4.1, and 3.2 mg-Phenol/L, respectively. Here, four nonhalogenated aromatic nitrogenous byproducts (N-DBPs) of 3-hydroxyquinoline, 4-hydroxyquinoline, benzopyridine, and benzopyrrole were confirmed using standards, and four byproducts such as 2-formylbenzonitrile were tentatively proposed. The cytotoxicity of the four confirmed byproducts was comparable to those known N-DBPs such as nitrosamines, suggesting attention should be given to these nonhalogenated but nitrogenous byproducts. The four confirmed byproducts were detected in two PI-treated wastewater samples with concentrations of 0.8, 0.98, 0.52, and 0.0038, and 18.28, 1.50, 0.57, and 0.0074 μg/L, respectively, with contributions less than 1.5% to the overall cytotoxicity. Further investigations are warranted to elucidate the primary drivers of toxicity in PI-treated wastewater.

Transformation of Natural Organic Matter in Simulated Abiotic Redox Dynamic Environments: Impact on Fe Cycling

Redox fluctuations within redox dynamic environments influence the redox state of natural organic matter (NOM) and its interaction with redox-active elements, such as iron. In this work, we investigate the changes in the molecular composition of NOM during redox fluctuations as well as the impact of these changes on the Fe-NOM interaction employing Suwannee River Dissolved Organic Matter (SRDOM) as a representative NOM. Characterization of SRDOM using X-ray photoelectron spectroscopy and Fourier transform infrared spectrometry showed that irreversible changes occurred following electrochemical reduction and reoxidation of SRDOM in air. Changes in the redox state of SRDOM impacted its interaction with iron with higher rates of Fe(III) reduction in the presence of reduced and reoxidized SRDOM than in the presence of the original SRDOM. The increased rate of Fe(III) reduction in the presence of reduced SRDOM was due to the formation of reduced organic moieties on SRDOM reduction. The Fe(II) oxidation rate also increased in the presence of reduced SRDOM due to the formation of redox-active moieties that were capable of oxidizing Fe(II). Overall, our study provides useful insights into the changes in SRDOM that may occur in redox dynamic environments and the associated impact of these changes on Fe transformations.

Unravelling Coupled Hydrological and Geochemical Controls on Long-Term Nitrogen Enrichment in a Large River Basin

Many groundwater and surface water bodies around the world show a puzzling and often steady increase in nitrogen (N) concentrations, despite a significant decline of agricultural N inputs. This study uses a combination of long-term hydrogeochemical and hydraulic monitoring, molecular characterization of dissolved organic matter (DOM), column experiment, and reactive transport modeling to unravel the processes controlling N-reactive transport and mass budgets under the impacts of dynamic hydrologic conditions at a field site in the central Yangtze River Basin. Our analysis shows that the desorption of ammonium (NH4+) from sediments via cation exchange reactions dominates N mobilization and aqueous N concentrations, while the mineralization of organic N compounds plays only a minor role. The reactive transport modeling results illustrate the important role of cation exchange reactions that are induced by temporary NH4+ input and cation concentration changes under the impact of both seasonal and long-term hydrologic variations. Historically, cation exchangers have acted as efficient storage devices and mitigated the impacts of high levels of NH4+ input. The NH4+ residing on cation exchanger sites later acts as a long-term N source to waters with the delayed desorption of sediment-bound NH4+ induced by the change of hydrologic conditions. Our results highlight the complex linkages between highly variable hydrologic conditions and NH4+ partitioning in near-surface, river-derived sediments.

Plant Root Secretion Alleviates Carbamate-Induced Molecular Alterations of Dissolved Organic Matter

Studying the interaction between pesticide contamination in the plant system and the dissolved organic matter (DOM) composition is important to understand the impact of pesticides and plants on the ecological function of DOM. The present study investigated the effects of DOM on the bioaccumulation and biotransformation of carbamates in plants, carbamate exposure on DOM composition, and plant root secretion on the interaction between DOM and carbamates. The concentrations of carbamates and their metabolites in living cabbage plants were continuously tracked through an in vivo analytical method. The presence of DOM was found to reduce the highest bioconcentrations and shorten the time it took to reach the highest bioaccumulated amounts of isoprocarb and carbofuran in plants, while it showed no significant effect on the uptake behavior of carbaryl. DOM profiling results indicated that carbamate exposure substantially decreased the number and molecular diversity of DOM. Notably, plant root secretion alleviated carbamate-induced DOM molecular alterations by inducing a higher turnover rate of DOM compared to that in the uncontaminated group, highlighting the role of plants in mitigating the effects of exogenous pesticide exposure on DOM composition and maintaining DOM molecular homeostasis.

Selective molecular characterization of organic aerosols using in situ laser desorption ionization mass spectrometry

RATIONALE

The sources and chemical compositions of organic aerosol (OA) exert a significant influence on both regional and global atmospheric conditions, thereby having far-reaching implications on environmental chemistry. However, existing mass spectrometry (MS) methods have limitations in characterizing the detailed composition of OA due to selective ionization as well as fractionation during cold-water extraction and solid-phase extraction (SPE).

METHODS

A comprehensive MS study was conducted using aerosol samples collected on dusty, clean, and polluted days. To supplement the data obtained from electrospray ionization (ESI), a strategy for analyzing OAs collected using the quartz fiber filter directly utilizing laser desorption ionization (LDI) was employed. Additionally, the ESI method was conducted to explore suitable approaches for determining various OA compositions from samples collected on dusty, clean, and polluted days.

RESULTS

In situ LDI has the advantages of significantly reducing the sample volume, simplifying sample preparation, and overcoming the problem of overestimating sulfur-containing compounds usually encountered in ESI. It is suitable for the characterization of highly unsaturated and hydrophobic aerosols, such as brown carbon-type compounds with low volatility and high stability, which is supplementary to ESI.

CONCLUSIONS

Compared with other ionization methods, in situ LDI helps provide a complementary description of the molecular compositions of OAs, especially for analyzing OAs in polluted day samples. This method may contribute to a more comprehensive MS analysis of the elusive compositions and sources of OA in the atmosphere.

Hidden Role of Organic Matter in the Immobilization and Transformation of Iodine on Fe-OM Associations

The biogeochemical processes of iodine are typically coupled with organic matter (OM) and the dynamic transformation of iron (Fe) minerals in aquifer systems, which are further regulated by the association of OM with Fe minerals. However, the roles of OM in the mobility of iodine on Fe-OM associations remain poorly understood. Based on batch adsorption experiments and subsequent solid-phase characterization, we delved into the immobilization and transformation of iodate and iodide on Fe-OM associations with different C/Fe ratios under anaerobic conditions. The results indicated that the Fe-OM associations with a higher C/Fe ratio (=1) exhibited greater capacity for immobilizing iodine (∼60-80% for iodate), which was attributed to the higher affinity of iodine to OM and the significantly decreased extent of Fe(II)-catalyzed transformation caused by associated OM. The organic compounds abundant in oxygen with high unsaturation were more preferentially associated with ferrihydrite than those with poor oxygen and low unsaturation; thus, the associated OM was capable of binding with 28.1-45.4% of reactive iodine. At comparable C/Fe ratios, the mobilization of iodine and aromatic organic compounds was more susceptible in the adsorption complexes compared to the coprecipitates. These new findings contribute to a deeper understanding of iodine cycling that is controlled by Fe-OM associations in anaerobic environments.

Formation of reactive intermediates in paddy water from different temperature zones for the promotion of abiotic ammonification

The paddy field is a hot area of biogeochemical process. The paddy water has a large capacity in photo-generation of reactive intermediates (RIs) due to abundant photosensitive dissolved organic matter (DOM), which is influenced by the spatial heterogeneity of paddy soils but rarely been explored. Our work presents the first investigation of the role of soil properties on photochemistry in paddy water. Soil organic matter (SOM), determined by the temperature, was the dominant factor for the photo-generation of RIs in paddy water of main rice producing areas. The RI concentrations generated with abundant SOM from cool regions are 0.05-8.71 times higher than those for the warm regions in China. The humic-like substance and aromatic-like compounds of DOM plays an essential role in RIs generation, which is abundant in paddy soils rich in SOM from Chinese cool regions. In addition, RIs can efficiently accelerate the photo-ammonification of urea and free amino acids by 15.2 %-164 %, leading to 0.13-0.17 mmol/L/d photo-produced ammonium after fertilization, which is preferentially absorbed by rice. The findings of this study will extend our knowledge of the geochemistry of global paddy field ecosystem. The potential role of RIs in nitrogen cycle should be highlighted in the agroecosystem.

Noise Filtering Algorithm Using Gaussian Mixture Models for High-Resolution Mass Spectra of Natural Organic Matter

High-resolution mass spectra of natural organic matter (NOM) contain a large number of noise signals. These signals interfere with the correct molecular composition estimation during nontargeted analysis because formula-assignment programs find empirical formulas for such peaks as well. Previously proposed noise filtering methods that utilize the profile of the intensity distribution of mass spectrum peaks rely on a histogram to calculate the intensity threshold value. However, the histogram profile can vary depending on the user settings. In addition, these algorithms are not automated, so they are handled manually. To overcome the mentioned drawbacks, we propose a new algorithm for noise filtering in mass spectra. This filter is based on Gaussian Mixture Models (GMMs), a machine learning method to find the intensity threshold value. The algorithm is completely data-driven and eliminates the need to work with a histogram. It has no customizable parameters and automatically determines the noise level for each individual mass spectrum. The algorithm performance was tested on mass spectra of natural organic matter obtained by averaging a different number of microscans (transients), and the results were compared with other noise filters proposed in the literature. Finally, the effect of this noise filtering approach on the fraction of peaks with assigned formulas was investigated. It was shown that there is always an increase in the identification rate, but the magnitude of the effect changes with the number of microscans averaged. The increase can be as high as 15%.

Enrichment of Geogenic Organoiodine Compounds in Alluvial-Lacustrine Aquifers: Molecular Constraints by Organic Matter

Organoiodine compounds (OICs) are the dominant iodine species in groundwater systems. However, molecular mechanisms underlying the geochemical formation of geogenic OICs-contaminated groundwater remain unclear. Based upon multitarget field monitoring in combination with ultrahigh-resolution molecular characterization of organic components for alluvial-lacustrine aquifers, we identified a total of 939 OICs in groundwater under reducing and circumneutral pH conditions. In comparison to those in water-soluble organic matter (WSOM) in sediments, the OICs in dissolved organic matter (DOM) in groundwater typically contain fewer polycyclic aromatics and polyphenol compounds but more highly unsaturated compounds. Consequently, there were two major sources of geogenic OICs in groundwater: the migration of the OICs from aquifer sediments and abiotic reduction of iodate coupled with DOM iodination under reducing conditions. DOM iodination occurs primarily through the incorporation of reactive iodine that is generated by iodate reduction into highly unsaturated compounds, preferably containing hydrophilic functional groups as binding sites. It leads to elevation of the concentration of the OICs up to 183 μg/L in groundwater. This research provides new insights into the constraints of DOM molecular composition on the mobilization and enrichment of OICs in alluvial-lacustrine aquifers and thus improves our understanding of the genesis of geogenic iodine-contaminated groundwater systems.

Natural and anthropogenic dissolved organic matter in landfill leachate: Composition, transformation, and their coexistence characteristics

A large number of natural and anthropogenic wastes were landfilled, and dissolved organic matter (DOM) were formed during landfill. However, the composition, transformation, and coexistence characteristics of natural and anthropogenic DOM in leachate remain unclear. Fourier transform ion cyclotron resonance mass spectrometry, size exclusion chromatography, gas chromatography coupled with mass spectrometry, and three-dimensional excitation-emission matrix spectrum were employed to clarify comprehensively the abovementioned question. The results showed that natural DOM in young leachate constituted mainly straight-chain organic acids, protein substances, and building blocks of humic substances (BB). Straight-chain organic acids vanished in old leachates, and the concentration of protein substances and BB decreased from 44% to 26% and from 47% to 12%, respectively, while CHON and CHONS were degraded to CHO and CHOS during the process. As to anthropogenic DOM, its types and relative content in leachate increased during landfill, and aromatic acids, terpenes, halogenated organics, indoles, and phenols became the main organic components in old leachate. Compared to natural DOM, anthropogenic DOM was degraded slowly and accumulated in leachate, and some of the natural DOM facilitated the dechlorination of dichlorinated organic compounds. This study demonstrates that landfill led to an increase in humic substances and halogenated organic compounds in old leachate, which was intensified with concentrated leachate recirculation.

Optical Variation and Molecular Transformation of Brown Carbon During Oxidation by NO3• in the Aqueous Phase

The NO3•-driven nighttime aging of brown carbon (BrC) is known to greatly impact its atmospheric radiative forcing. However, the impact of oxidation by NO3• on the optical properties of BrC in atmospheric waters as well as the associated reaction mechanism remain unclear. In this work, we found that the optical variation of BrC proxies under environmentally relevant NO3• exposure depends strongly on their sources, with enhanced light absorptivity for biomass-burning BrC but bleaching for urban aerosols and humic substances. High-resolution mass spectrometry using FT-ICR MS shows that oxidation by NO3• leads to the formation of light-absorbing species (e.g., nitrated organics) for biomass-burning BrC while destroying electron donors (e.g., phenols) within charge transfer complexes in urban aerosols and humic substances, as evidenced by transient absorption spectroscopy and NaBH4 reduction experiments as well. Moreover, we found that the measured rate constants between NO3• with real BrCs (k = (1.8 ± 0.6) × 107 MC-1s-1, expressed as moles of carbon) are much higher than those of individual model organic carbon (OC), suggesting the reaction with OCs may be a previously ill-quantified important sink of NO3• in atmospheric waters. This work provides insights into the kinetics and molecular transformation of BrC during the oxidation by NO3•, facilitating further evaluation of BrC's climatic effects and atmospheric NO3• levels.

Evolution of dissolved organic nitrogen chemistry during transportation to the marginal sea: Insights from nitrogen isotope and molecular composition analyses

Estuaries are hotspots where terrestrially originated dissolved organic matter (DOM) is modified in molecular composition before entering marine environments. However, very few research has considered nitrogen (N) modifications of DOM molecules in estuaries, limiting our understanding of dissolved organic nitrogen (DON) cycling and the associated carbon cycling in estuaries. This study integrated optical, stable isotopes (δ15N and δ13C) and molecular composition (FT-ICR MS) to characterize the transformation of DOM in the Yangtze River Estuary. Both concentration of dissolved organic carbon (DOC) and DON decreased with increasing salinity, while their δ13C and δ15N increased with the increasing salinity. A significant positive correlation was found between δ15N and δ13C during the transportation of DOM to marginal seas, indicating that the behavior of both DOC and DON are primarily controlled by the mixing of freshwater and the seawater in the YRE. During the mixing process, the DON addition was observed using the conservative mixing curves. In the view of molecular composition, DOM molecules became more aromatic as the number of N atoms increased. Spearman correlations reveal that DOM molecules with fewer N atoms exhibited a higher enrichment in protein-like components, while those with more N atoms were more enriched in humic-like components. In addition, the δ15N and δ13C tended to increase as the N content of DOM decreased. Therefore, DON molecules with fewer N atoms were likely to be transformed into those with more N atoms based on the isotopic fractionation theory. This study establishes a linkage between the molecular composition and the δ15N of DOM, and discovers the N transformation pattern within DOM molecules during the transportation to marginal seas.

FT-ICR MS Spectral Improvement of Dissolved Organic Matter by the Absorption Mode: A Comparison of the Electrospray Ionization in Positive-Ion and Negative-Ion Modes

Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) in the absorption mode has a superior performance over the conventional magnitude mode. However, this improved performance for the analysis of dissolved organic matter (DOM) in negative-ion and positive-ion modes of electrospray ionization [ESI(-) and ESI(+), respectively] remains unknown. This study systemically compared the improved performance by the absorption mode for DOM FT-ICR MS spectra acquired with the low-field and high-field magnet instruments between two charge modes. The absorption mode enhanced the resolution and signal-to-noise ratio values of DOM peaks with factors of 1.88-1.94 and 1.60-1.72, respectively. The significantly higher improvement of mass resolution for the ESI(+) mode than that for the ESI(-) mode could resolve the extensive occurrence of mass doublets in the ESI(+) mode, yielding some formulas exclusively identified in the ESI(+) mode. The findings of this study have systemically demonstrated the superiority of the absorption mode in improving the spectra quality during the routine FT-ICR MS postdata analysis and highlighted its great potential in characterizing the molecular composition of DOM using the FT-ICR MS technique in both ESI(-) and ESI(+) modes.

Characterization of halogenated organic compounds by the Fourier transform ion cyclotron resonance mass spectrometry: A critical review

Halogenated organic compounds (HOCs), widely present in various environments, are generally formed by natural processes (e.g., photochemical halogenation) and anthropogenic activities (e.g., water disinfection and anthropogenic discharge of HOCs), posing health and environmental risks. Therefore, in-depth knowledge of the molecular composition, transformation, and fate of HOCs is crucial to regulate and reduce their formation. Because of the extremely complex nature of HOCs and their precursors, the molecular composition of HOCs remains largely unknown. The Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) offers the most powerful resolution and mass accuracy for the simultaneous molecular-level characterization of HOCs and their precursors. However, there is still a paucity of reviews regarding the comprehensive characterization of HOCs by FT-ICR MS. Based on the FT-ICR MS, the formation mechanism, sample pretreatment, and analysis methods were summarized for two typical HOCs classes, namely halogenated disinfection byproducts and per- and polyfluoroalkyl substances in this review. Moreover, we have highlighted data analysis methods and some typical applications of HOCs using FT-ICR MS and proposed suggestions for current issues. This review will deepen our understanding of the chemical characterization of HOCs and their formation mechanisms and transformation at the molecular level in aquatic systems, facilitating the application of the state-of-the-art FT-ICR MS in environmental and geochemical research.

Water-soluble organic carbon (WSOC) from vegetation fire and its differences from WSOC in natural media: Spectral comparison and self-organizing maps (SOM) classification

Vegetation fire frequently occurs globally and produces two types of water-soluble organic carbon (WSOC) including black carbon WSOC (BC-WSOC) and smoke-WSOC, they will eventually enter the surface environment (soil and water) and participate in the eco-environmental processes on the earth surface. Exploring the unique features of BC-WSOC and smoke-WSOC is critical and fundamental for understanding their eco-environmental effects. Presently, their differences from the natural WSOC of soil and water remain unknown. This study produced various BC-WSOC and smoke-WSOC by simulating vegetation fire and used UV-vis, fluorescent EEM-PARAFAC, and fluorescent EEM-SOM to analyze their different features from natural WSOC of soil and water. The results showed that the maximum yield of smoke-WSOC reached about 6600 folds that of BC-WSOC after a vegetation fire event. The increasing burning temperature decreased the yield, molecular weight, polarity, and protein-like matters abundance of BC-WSOC and increased the aromaticity of BC-WSOC, but presented a negligible effect on the features of smoke-WSOC. Furthermore, compared with natural WSOC, BC-WSOC had a greater aromaticity, smaller molecular weight, and more humic-like matters, while smoke-WSOC had a lower aromaticity, smaller molecular size, higher polarity, and more protein-like matters. EEM-SOM analysis indicated that the ratio between the fluorescence intensity at Ex/Em: 275 nm/320 nm and the sum fluorescence intensity at Ex/Em: 275 nm/412 nm and Ex/Em: 310 nm/420 nm could effectively differentiate WSOC of different sources, following the order of smoke-WSOC (0.64-11.38) > water-WSOC and soil-WSOC (0.06-0.76) > BC-WSOC (0.0016-0.04). Hence, BC-WSOC and smoke-WSOC possibly directly alter the quantity, properties, and organic compositions of WSOC in soil and water. Owing to smoke-WSOC having far greater yield and bigger difference from natural WSOC than BC-WSOC, the eco-environmental effect of smoke-WSOC deposition should be given more attention after a vegetation fire.