Selective mass labeling for linking the optical properties of chromophoric dissolved organic matter to structure and composition via ultrahigh resolution electrospray ionization mass spectrometry

High throughput identification of carbonyl compounds in natural organic matter by directional derivatization combined with ultra-high resolution mass spectrometry

Carbonyl compounds are important components of natural organic matter (NOM) with high reactivity, so that play a pivotal role in the dynamic transformation of NOM. However, due to the lack of effective analytical methods, our understanding on the molecular composition of these carbonyl compounds is still limited. Here, we developed a high-throughput screening method to detect carbonyl molecules in complex NOM samples by combining chemical derivatization with electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR-MS). In six different types of dissolved organic matter (DOM) samples tested in this study, 20-30 % of detected molecules contained at least one carbonyl group, with relative abundance accounted for 45-70 %. These carbonyl molecules displayed lower unsaturation level, lower molecular weight, and higher oxidation degree compared to non-carbonyl molecules. More importantly, the measured abundances of carbonyl molecules were consistent with the results of 13C nuclear magnetic resonance (NMR) analysis. Based on this method, we found that carbonyl molecules can be produced at DOM-ferrihydrite interface, thus playing a role in shaping the molecular diversity of DOM. This method has broad application prospects in screening carbonyl compounds from complex mixtures, and the same strategy can be used to directional identification of molecules with other functional groups as well.

Detection and Exclusion of False-Positive Molecular Formula Assignments via Mass Error Distributions in UHR Mass Spectra of Natural Organic Matter

Ultrahigh resolution mass spectrometry (UHRMS) routinely detects and identifies thousands of mass peaks in complex mixtures, such as natural organic matter (NOM) and petroleum. The assignment of several chemically plausible molecular formulas (MFs) for a single accurate mass still poses a major problem for the reliable interpretation of NOM composition in a biogeochemical context. Applying sensible chemical rules for MF validation is often insufficient to eliminate multiple assignments (MultiAs)─especially for mass peaks with low abundance or if ample heteroatoms or isotopes are included - and requires manual inspection or expert judgment. Here, we present a new approach based on mass error distributions for the identification of true and false assignments among MultiAs. To this end, we used the mass error in millidalton (mDa), which was superior to the commonly used relative mass error in ppm. We developed an automatic workflow to group MultiAs based on their shared formula units and Kendrick mass defect values and to evaluate the mass error distribution. In this way, the number of valid assignments of chlorinated disinfection byproducts was increased by 8-fold as compared to only applying 37Cl/35Cl isotope ratio filters. Likewise, phosphorus-containing MFs can be differentiated against chlorine-containing MFs with high confidence. Further, false assignments of highly aromatic sulfur-containing MFs ("black sulfur") to sodium adducts in negative ionization mode can be excluded by applying our approach. Overall, MFs for mass peaks that are close to the detection limit or where naturally occurring isotopes are rare (e.g., 15N) or absent (e.g., P and F) can now be validated, substantially increasing the reliability of MF assignments and broadening the applicability of UHRMS analysis to even more complex samples and processes.

Dissolved organic matter isolates obtained by solid phase extraction exhibit higher absorption and lower photo-reactivity: Effect of components

Notable differences in photo-physical and chemical properties were found between bulk water and solid phase extraction (SPE) isolates for dissolved organic matter (DOM). The moieties extracted using modified styrene divinylbenzene cartridges, which predominantly consist of conjugated aromatic molecules like humic acids, contribute mainly to light absorption but exhibit lower quantum yields of fluorescence and photo-produced reactive intermediates (PPRIs). Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) revealed lignin as the moieties displaying most significant variance in abundance. In Van Krevelen-Spearman plot, we observed molecules positively or negatively correlated with DOM's optical and photochemical properties (including SUVA254, steady-state concentrations of ·OH, 1O2 quantum yield, etc.) were confined to specific regions, which can be delineated using a threshold modified aromaticity index (AImod) of 0.3. Based on the relationships between optical properties and PPRI production, it is suggested that the energy gap between ground state and excited singlet state (△ES1→S0), governing the inner conversion rate, serves as a determinant for apparent quantum yield of PPRIs in DOM, with intra-molecular charge transfer (CT) interactions potentially playing a pivotal role. Regarding DOM's photoreactivity with pollutants, this study has revealed, for the first time, that protein/amino sugars/amino acids could act as antioxidant groups in addition to phenols on the photolysis of sulfadiazine. These findings provide valuable insights into DOM photochemistry and are expected to stimulate further research in this area.

Redox-directed identification of toxic transformation products during ozonation of aromatics

The toxicity assessment of transformation products (TPs) formed in oxidative water treatment is crucial but challenging because of their low concentration, structural diversity, and mixture complexity. Here, this study developed a novel redox-directed approach for identification of toxic TPs without the individual toxicity and concentration information. This approach based on sodium borohydride reduction comprised an integrated process of toxicological evaluation, fluorescence excitation-emission matrix characterization, high-resolution mass spectrometry detection, followed by ecological toxicity assessment of identified TPs. The redox-directed identification of primary causative toxicants was experimentally tested for the increased nonspecific toxicity observations in the ozonated effluents of model aromatics. Reduction reaction caused a remarkable decrease in toxicity and increase in fluorescence intensity, obtaining a good linear relation between them. More than ten monomeric or dimeric p-benzoquinone (p-BQ) TPs were identified in the ozonated effluents. The occurrence of the p-BQ TPs was further verified through parallel sodium sulfite reduction and actual wastewater ozonation experiments. In vitro bioassays of luminescent bacteria, as well as in silico genotoxicity and cytotoxicity predictions, indicate that the toxicity of p-BQ TPs is significantly higher than that of their precursors and other TPs. These together demonstrated that the identified p-BQ TPs are primary toxicity contributors. The redox-directed approach facilitated the revelation of primary toxicity contribution, illustrating emerging p-BQs are a concern for aquatic ecosystem safety in the oxidative treatment of aromatics-contaminated wastewater.

A Temporal Graph Model to Predict Chemical Transformations in Complex Dissolved Organic Matter

Dissolved organic matter (DOM) is a complex mixture of thousands of natural molecules that undergo constant transformation in the environment, such as sunlight induced photochemical reactions. Despite molecular level resolution from ultrahigh resolution mass spectrometry (UHRMS), trends of mass peak intensities are currently the only way to follow photochemically induced molecular changes in DOM. Many real-world relationships and temporal processes can be intuitively modeled using graph data structures (networks). Graphs enhance the potential and value of AI applications by adding context and interconnections allowing the uncovering of hidden or unknown relationships in data sets. We use a temporal graph model and link prediction to identify transformations of DOM molecules in a photo-oxidation experiment. Our link prediction algorithm simultaneously considers educt removal and product formation for molecules linked by predefined transformation units (oxidation, decarboxylation, etc.). The transformations are further weighted by the extent of intensity change and clustered on the graph structure to identify groups of similar reactivity. The temporal graph is capable of identifying relevant molecules subject to similar reactions and enabling to study their time course. Our approach overcomes previous data evaluation limitations for mechanistic studies of DOM and leverages the potential of temporal graphs to study DOM reactivity by UHRMS.

Investigation with ESI FT-ICR MS on sorbent selectivity and comprehensive molecular composition of landfill leachate dissolved organic matter

Molecular characterization of landfill leachate dissolved organic matter (LDOM) is essential for developing effective processing techniques. However, the molecular selectivity of extraction method and ionization modes often leads to the bias of molecular characterization of LDOM. Here, seven representative sorbents were selected and electrospray ionization negative ion mode (ESI (-)) and positive ion mode (ESI (+)) Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) were used to investigate the molecular composition of different LDOM samples. Obvious sorbent selectivity during extraction procedure was observed, resulting in the underestimation of molecular diversity of LDOM from 32.7% to 69.3%. Totally, 14,000-18,000 unique molecules were obtained in a single sample, indicating the unprecedented molecular diversity of LDOM. Lignins, proteins and lipids are three major molecular groups in LDOM, and N or S containing molecules occupied 83%. Although much of total organic carbon was removed during biochemical treatment process, the molecular diversity of LDOM was not reduced because a considerable of bio-recalcitrant molecules was produced. The results uncover the sorbents selectivity and ionization modes selectivity in LDOM analysis and provided a comprehensive change of LDOM molecular composition during biochemical treatment, which benefits the development of accurate methods to remove organic carbon in landfill leachate.

NO3- Promotes Nitrogen-Containing Disinfection Byproduct Formation in Corroded Iron Drinking Water Pipes

Nitrogen-containing disinfection byproducts (N-DBPs) are highly toxic DBPs in drinking water. Though, under normal conditions, NO3- could not directly participate in disinfection reactions to generate N-DBPs, here, we first found that NO3- could promote the formation of N-DBPs in corroded iron drinking water pipes. The coexistence of corrosion produced Fe(II) and iron oxides is a critical condition for the transformation of N species; meanwhile, most of the newly generated N-DBPs had aromatic fractions. The Fe-O-C bond formed between iron corrosion products and natural organic matter promoted electron transfer for the N transformation with pyrrolic N as the intermediate N species. Density functional calculation confirmed that the coexistence of Fe(II) and iron oxides effectively reduced the Gibbs free energy for NO3- reduction. ΔG of the key rate-determining step from NO* to NOH* decreased from 1.55 eV on FeOOH to 1.35 eV on Fe(II)+FeOOH. In addition, the large decrease of cell viability of the water samples from 74.3% to 45.4% further confirmed the formation of highly toxic N-DBPs. Thus, in a drinking water distribution system with corroded iron pipes, the low toxic NO3- may increase toxicity risks via N-DBP formation.

Isolation of dissolved organic matter from aqueous solution by precipitation with FeCl3: mechanisms and significance in environmental perspectives

Ferric ions can bind strongly with dissolved organic matter (DOM), including humic acids (HA), fulvic acids (FA), and protein-like substances, whereas isolation of Fe-DOM precipitates (Fe-DOM_P) and their biochemical characteristics remain unclear. In this work FeCl₃ was used to isolate DOM components from various sources, including river, lake, soil, cow dung, and standard tryptophan and tyrosine, through precipitation at pH 7.5-8.5. The Fe-DOM_P contribute to total DOM by approximately 38.6-93.8% of FA, 76.2% of HA and 25.0-30.4% of tryptophan and tyrosine, whilst fluorescence spectra allowed to monitor/discriminate the various DOM fractions in the samples. The relative intensity of the main infrared peaks such as 3406‒3383 cm^-1 (aromatic OH), 1689‒1635 cm^-1 (‒COOH), 1523-1504 cm^-1 (amide) and 1176-1033 cm^-1 (‒S=O) show either to decline or disappear in Fe‒DOM_P. These results suggest the occurrence of Fe bonds with various functional groups of DOM, indicating the formation of π-d electron bonding systems of different strengths in Fe‒DOM_P. The novel method used for isolation of Fe-DOM_P shows promising in opening a new frontier both at laboratory and industrial purposes. Furthermore, results obtained may provide a better understanding of metal-organic complexes involved in the regulation of the long-term stabilization/sequestration of DOM in soils and waters.

Responses of dissolved organic matter (DOM) characteristics in eutrophic lake to water diversion from external watershed

Eutrophication is an important water environment issue facing global lakes. Diversion of water from external watersheds into lakes is considered as effective in ameliorating eutrophication and reducing algal blooms. Nevertheless, the changes in lake water environment caused by external water diversion, especially the influence of water diversion on the characteristics of dissolved organic matters (DOM), are still poorly understood. We therefore used a combination of EEM-PARAFAC, Principal Component Analysis (PCA), and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) to investigate the effects of water diversion from the Niulan River on DOM characteristics in Lake Dianchi. The results showed that the water diversion from the Niulan River significantly improved the water quality of Lake Dianchi, the concentrations of TN, TP, COD and Chla decreased rapidly, and the degree of humification of dissolved organic matter (DOM) increased, which was in sharp contrast with that of pre-implementation. Firstly, the diversion of water from the Niulan River mainly led to changes in the structure of pollution sources. The load of influent rivers and sewage treatment plants rich in lignin and tannins increased, and the input of terrestrial humus increased. Second, the improved water quality reduced algal enrichment and frequency of blooms, and reduced the release of lipid- and protein-riched algal-derived DOM. Finally, the hydraulic retention time of Lake Dianchi caused by water diversion was shortened, the hydrodynamic conditions were significantly improved, and the dissolved oxygen (DO) level gradually recovered, which played a positive role in improving the humification degree of DOM. Our findings provide new insights for exploring the improvement of eutrophic lake eco-environmental quality caused by water diversion projects.

Mechanistic insights into the generation and control of Cl-DBPs during wastewater sludge chlorination disinfection process

Chlorination disinfection has been widely used to kill the pathogenic microorganisms in wastewater sludge during the special Covid-19 period, but sludge chlorination might cause the generation of harmful disinfection byproducts (DBPs). In this work, the transformation of extracellular polymeric substance (EPS) and mechanisms of Cl-DBPs generation during sludge disinfection by sodium hypochlorite (NaClO) were investigated using multispectral analysis in combination with Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). The microorganism Escherichia coli (E. coli) was effectively inactivated by active chlorine generated from NaClO. However, a high diversity of Cl-DBPs were produced with the addition of NaClO into sludge, causing the increase of acute toxicity on Q67 luminous bacteria of chlorinated EPS. A variety of N-containing molecular formulas were produced after chlorination, but N-containing DBPs were not detected, which might be the indicative of the dissociation of -NH₂ groups after Cl-DBPs generated. Additionally, the release of N-containing compounds was increased in alkaline environment caused by NaClO addition, resulted in more Cl-DBPs generation via nucleophilic substitutions. Whereas, less N-compounds and Cl-DBPs were detected after EPS chlorination under acidic environment, leading to lower cell cytotoxicity. Therefore, N-containing compounds of lignin derivatives in sludge were the major Cl-DBPs precursors, and acidic environment could control the release of N-compounds by eliminating the dissociation of functional groups in lignin derivatives, consequently reducing the generation and cytotoxicity of Cl-DBPs. This study highlights the importance to control the alkalinity of sludge to reduce Cl-DBPs generation prior to chlorination disinfection process, and ensure the safety of subsequential disposal for wastewater sludge.

Loss and Increase of the Electron Exchange Capacity of Natural Organic Matter during Its Reduction and Reoxidation: The Role of Quinone and Nonquinone Moieties

Redox-active quinone and nonquinone moieties represent the electron exchange capacity (EEC) of natural organic matter (NOM), playing an important role in the electron transfer link of microbes and transformation of contaminants/metal minerals. However, the corresponding transformation of quinone/phenol and their respective influence on the EECs during reduction and reoxidation remain poorly characterized. Besides, it is still controversial whether nonquinones donate or accept electrons. Herein, we demonstrated that reoxidation of NOM after reduction can form new phenolic/quinone moieties, thus increasing the EEC. The assessment for the EEC, including the electron-donating capacity (EDC) and electron-accepting capacity (EAC), of nonquinones reflects the contribution of sulfur-containing moieties with considerable EDCs and EACs. In contrast, nitrogen-containing moieties donate negligible electrons even at E_h = +0.73 V. The contributions of both thiol and amine moieties to the EEC are greatly affected by adjacent functional groups. Meanwhile, aldehydes/ketones did not display an EAC during the electron transfer process of NOM. Furthermore, substantially increased EDC at E_h from +0.61 to +0.73 V could not be fully explained using thiol and phenolic moieties, suggesting the contribution of unknown moieties with high oxidation potential. The overall findings suggest that the roles of new quinones/phenol (derived from the addition of oxygen to condensed aromatic/lignin-like components) during redox dynamic cycling and thiol species should be considered in assessing the electron transfer processes of NOM.

Aromaticity Index with Improved Estimation of Carboxyl Group Contribution for Biogeochemical Studies

Natural organic matter (NOM) components measured with ultrahigh-resolution mass spectrometry (UHRMS) are often assessed by molecular formula-based indices, particularly related to their aromaticity, which are further used as proxies to explain biogeochemical reactivity. An aromaticity index (AI) is calculated mostly with respect to carboxylic groups abundant in NOM. Here, we propose a new constrained AI_con based on the measured distribution of carboxylic groups among individual NOM components obtained by deuteromethylation and UHRMS. Applied to samples from diverse sources (coal, marine, peat, permafrost, blackwater river, and soil), the method revealed that the most probable number of carboxylic groups was two, which enabled to set a reference point n = 2 for carboxyl-accounted AI_con calculation. The examination of the proposed AI_con showed the smallest deviation to the experimentally determined index for all NOM samples under study as well as for individual natural compounds obtained from the Coconut database. In particular, AI_con performed better than AI_mod for all compound classes in which aromatic moieties are expected: aromatics, condensed aromatics, and unsaturated compounds. Therefore, AI_con referenced with two carboxyl groups is preferred over conventional AI and AI_mod for biogeochemical studies where the aromaticity of compounds is important to understand the transformations and fate of NOM compounds.

Computational Calculation of Dissolved Organic Matter Absorption Spectra

The absorption spectrum of dissolved organic matter (DOM) is a topic of interest to environmental scientists and engineers as it can be used to assess both the concentration and physicochemical properties of DOM. In this study, the UV-vis spectra for DOM model compounds were calculated using time-dependent density functional theory. Summing these individual spectra, it was possible to re-create the observed exponential shape of the DOM absorption spectra. Additionally, by predicting the effects of sodium borohydride reduction on the model compounds and then calculating the UV-vis absorbance spectra of the reduced compounds, it was also possible to correctly predict the effects of borohydride reduction on DOM absorbance spectra with a relatively larger decrease in absorbance at longer wavelengths. The contribution of charge-transfer (CT) interactions to DOM absorption was also evaluated, and the calculations showed that intra-molecular CT interactions could take place, while inter-molecular CT interactions were proposed to be less likely to contribute.

Relationships between the Physicochemical Properties of Dissolved Organic Matter and Its Reaction with Sodium Borohydride

The reaction of dissolved organic matter (DOM) with sodium borohydride has been used to understand the geographic origin of DOM and investigate the photophysical model underlying DOM's optical properties. However, the physicochemical properties of DOM (e.g., molecular size and charge) that influence the kinetics and ultimate reducibility of DOM by borohydride remain poorly characterized. Herein, we studied the kinetics of DOM-borohydride reactions by recording absorbance and fluorescence spectra at a high temporal frequency (every ∼10 min for 24 h) for a diverse set of DOM isolates of aquatic and soil origin. The reducibility of DOM by sodium borohydride (as judged by relative removal of initial absorbance) varied appreciably among the DOM samples studied, with soil humic substances being less reducible than aquatic humic substances and natural organic matter. While statistically significant correlations were found between the reducibility of DOM and descriptors of molecular size, these descriptors were not able to differentiate the reducibility of soil versus aquatic DOM isolates that had similar bulk properties. Thus, it appears that the extent of absorbance removal by borohydride is largely driven by the origin of the humic substance isolate (aquatic vs soil) instead of molecular size or charge. Borohydride reduction resulted in increased fluorescence emission across UV and visible excitation wavelengths. However, the enhanced emission at visible excitation decreased over a time period of hours to days, suggesting that reduction of an important subset of DOM chromophores is reversible. This reversibility in fluorescence emission is consistent with the small role of quinones in the absorbance of DOM but suggests a more important role for quinone-containing charge-transfer contacts in the fluorescence of DOM, particularly at visible excitation wavelengths.

Advances in the characterization and monitoring of natural organic matter using spectroscopic approaches

Natural organic matter (NOM) is ubiquitous in environment and plays a fundamental role in the geochemical cycling of elements. It is involved in a wide range of environmental processes and can significantly affect the environmental fates of exogenous contaminants. Understanding the properties and environmental behaviors of NOM is critical to advance water treatment technologies and environmental remediation strategies. NOM is composed of characteristic light-absorbing/emitting functional groups, which are the "identification card" of NOM and susceptive to ambient physiochemical changes. These groups and their variations can be captured through optical sensing. Therefore, spectroscopic techniques are elegant tools to track the sources, features, and environmental behaviors of NOM. In this work, the most recent advances in molecular spectroscopic techniques, including UV-Vis, fluorescence, infrared, and Raman spectroscopy, for the characterization, measurement, and monitoring of NOM are reviewed, and the state-of-the-art innovations are highlighted. Furthermore, the limitations of current spectroscopic approaches for the exploration of NOM-related environmental processesand how these weaknesses/drawbacks can be addressed are explored. Finally, suggestions and directions are proposed to advance the development of spectroscopic methods in analyzing and elucidating the properties and behaviors of NOM in natural and engineered environments.

Photochemical origin of reactive radicals and halogenated organic substances in natural waters: A review

Halogenated organic compounds, also termed organohalogens, were initially regarded to be of almost exclusively anthropogenic origin. However, recent research has demonstrated that photochemical reactions are important abiotic sources of organohalogen compounds in sunlit surface waters. Halide ions (X^-, X represents Cl, Br and I) are common anions in natural waters and might be oxidized by reactive species originated from photochemistry of dissolved organic matter (DOM) or inorganic photoactive species. The resulting reactive halogen species may react with organic substances with diverse bimolecular reaction rate constants, depending on the complexity and structure of organic substances. Therefore, the chemical mechanism of halogenation remains challenging to be fully elucidated. To better understand the trends in the existing data and to identify the knowledge gaps that may merit further investigation, this review gives an integrative summary on the sources of reactive oxygen species (ROS) and halogen radicals (X/X₂^-). Photochemical halogenation of phenolic compounds and formation of methyl halide and brominated organic pollutants are highlighted. By evaluating existing literature and identifying some uncertainties, this review emphasizes the environmental significance of sunlight-driven halogenation and proposes further research directions on mechanistic investigation and rational experimental design close to natural systems.

Quantitative determination of redox-active carbonyls of natural dissolved organic matter

Natural dissolved organic matter (DOM) is ubiquitous in environment and plays an important role in numerous environmental processes. Although the molecular basis of the reactivity of DOM remains poorly understood due to its extreme complexity, redox-active carbonyls (aromatic ketones/aldehydes and quinones) within DOM are believed vitally important. Except the rough determination of total carbonyls (including non-redox active -COOR) based on inflexible ¹³C chemical shift range by expensive and time-consuming solid-state nuclear magnetic resonance (NMR), there is no ready method to quantify redox-active carbonyls in DOM. Here we show that after treatment with sodium borohydride (NaBH₄) by selectively eliminating redox-active carbonyls, quenched fluorescence of carbon quantum dots (CD) by DOM recovered dramatically, and displayed a good linear relationship between redox-active carbonyls detected and DOM concentration (R² ≥ 0.977), thus allowing first quantitative determination of the redox-active carbonyls of DOM. Eight DOM isolates present 0.59%-0.90% redox-active carbonyls by the current method. And this method is robust from coexisting proteins and salts. This method could provide better or equal instructive results compared with solid-state NMR for total carbonyls or electrochemical method for electron-accepting capacities (EAC). Our results provide the underlying structural basis of many important geochemical processes that mediated by DOM. We posit that this method could apply to other complex molecular systems such as the atmospheric aerosols and extracellular polymeric substances (EPS), too.

Influence of dissolved organic matter on carbonyl sulfide and carbon disulfide formation from cysteine during sunlight photolysis

Carbonyl sulfide (COS) and carbon disulfide (CS2) are important atmospheric gases that are formed from organic sulfur precursors present in natural waters when exposed to sunlight. However, it remains unclear how specific water constituents, such as dissolved organic matter (DOM), affect COS and CS2 formation. To better understand the role of DOM, irradiation experiments were conducted in O2-free synthetic waters containing four different DOM isolates, acquired from freshwater to open ocean sources, and the sulfur-based amino acid, cysteine (CYS). CYS is a known natural precursor of COS and CS2. Results indicated that COS formation did not vary strongly with DOM type, although small impacts were observed on the kinetic patterns. COS formation also increased with increasing CYS concentration but decreased with increasing DOM concentration. Quenching experiments indicated that ˙OH was not involved in the rate-limiting step of COS formation, whereas excited triplet states of DOM (3CDOM*) were plausibly involved, although the quenching agents used to remove 3CDOM* may have reacted with the CYS-derived intermediates as well. CS2 was not formed under any of the experimental conditions. Overall, DOM-containing synthetic waters had a limited to no effect towards forming COS and CS2, especially when compared to the higher concentrations formed in sunlit natural waters, as examined previously. The reasons behind this limited effect need to be explored further but may be due to the additional water quality constituents present in these natural waters. The findings of this study imply that multiple variables beyond DOM govern COS and CS2 photoproduction when moving from freshwaters to open ocean waters.

Refinement of Compound Aromaticity in Complex Organic Mixtures by Stable Isotope Label Assisted Ultrahigh-Resolution Mass Spectrometry

Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) provides a unique opportunity for molecular analysis of natural complex mixtures. In many geochemical and environmental studies structure-propertry relations are based solely on the elemental compositional information. Several calculated parameters were proposed to increase reliability of structural attribution, among which aromaticity indices (AI and AI_mod) are widely used. Herein, we applied a combination of selective labeling reactions in order to obtain direct structural information on the individual components of lignin-derived polyphenolic material. Carboxylic (COOH), carbonyl (C═O), and hydroxyl (OH) groups were enumerated by esterification, reducing, and acetylation reactions, respectively, followed by FTICR MS analyses. Obtained information was enabled to constrain aromaticity accounting for the carbon skeleton only. We found that actual aromaticity of components may be both higher or lower than approximated values depending on the abundance of COOH, C═O, and OH groups. The results are of importance for the geochemical community studying terrestrial NOM with structural gradients.

Emerging investigator series: critical review of photophysical models for the optical and photochemical properties of dissolved organic matter

Optical measurements (absorbance and fluorescence) are widely used to track dissolved organic matter (DOM) quantity and quality in natural and engineered systems. Despite many decades of research on the optical properties of DOM, there is a lack of understanding with regards to the underlying photophysical model that is the basis for these optical properties. This review both summarizes advances to date on the photophysical properties of DOM and seeks to critically evaluate the photophysical models for DOM optical properties. Recent studies have refined the quantitative understanding of DOM photophysical properties such as excited state lifetimes and energies, rates of different photophysical processes, and quantum yields. Considering fundamental models, more clarity is needed on whether DOM photophysical processes are due to a superposition of non-interacting components (superposition model), or whether a portion of optical signals can be ascribed to electronically interacting moieties, for example in the form of electron donor-acceptor complexes (charge transfer model). Multiple studies over more than two decades have provided evidence for the charge transfer model. Questions have been raised, however, about the broad applicability of the charge transfer model. The charge transfer and superposition model are critically reviewed in light of this current research. Recommendations are given for future studies to help clarify the accuracy of these competing photophysical models.

Optical Properties of Soil Dissolved Organic Matter Are Related to Acidic Functions of Its Components as Revealed by Fractionation, Selective Deuteromethylation, and Ultrahigh Resolution Mass Spectrometry

The goal of this study was to establish a relationship between the optical properties of soil dissolved organic matter (DOM) and acidic functions carried out by its individual constituents. We obtained 12 fractions of DOM samples using sequential solid phase extraction on nonionic sorbent at steadily lowered pH values: 7, 5, 3, 2, which correspond to low bounds of pK_a values of phenols, aliphatic, and aromatic carboxylic acids, and ketoacids. The structural studies were conducted with the use of NMR and selective deuteromethylation of isolated fractions coupled to ultrahigh resolution mass spectrometry. First, a gradual shift of molecular compositions was observed from reduced components to aromatic oxidized compounds isolated at pH 7 and 2, respectively. Changes in molecular compositions were accompanied by a red shift of fluorescence spectra. Further application of deuteromethylation enabled us to distinguish DOM constituents with different amounts of carboxylic groups. Moreover, identification of structural isomers in a single DOM sample was achieved. Statistical analysis revealed that red shift of fluorescence is facilitated by the increase of a contribution of aromatic poly(carboxylic acid)s with high conjugation lengths. Additionally, analysis of the labeled fractionated permafrost thaw DOM directly showed carboxyl-rich alicyclic molecules, while the same components from lower-latitude DOM were assigned to lignin-like species.

Characterization of Carbonyl Disinfection By-Products During Ozonation, Chlorination, and Chloramination of Dissolved Organic Matters

Carbonyl compounds are an important class of by-products that are generated in disinfection reactions. These chemicals are ingredients contributing to toxicology in the drinking water system, the compositions and structures of which are worthy of attention. In this study, a chemical derivatization method based on simultaneous light/heavy isotope labeling was established for general recognition of carbonyl compounds and carbonyl disinfection by-products (DBPs) as per the humic substance reference standard (Suwannee river fulvic acid II, SRFA) before and after ozonation, chlorination, and chloramination. Decomposition of macromolecular components into polar carbonyl species was observed to be the most prominent pathway in ozone treatment due to the efficient reactivity of ozone with phenols and alkoxy aromatic rings. As a result, alteration of molecular characteristics was noticed. For instance, ozone-induced carbonyl DBPs in the highly oxygenated compound classes (0.67 ≤ O/C ≤ 1.2, 0.6 < H/C ≤ 1.5) possessed higher O/C but contained less oxygen numbers and carbon numbers. Cl/Br-carbonyl-DBPs were identified after chlorination and chloramination, and I-carbonyl-DBPs were found in ozone and chloramine treatments. Several major halogenated carbonyl homologues were further recognized, including halogenated 4-oxobutenoic acid analogues, halogenated 2,5-dioxohex-3-enoic acid analogues, and halogenated 4-cyclopentene-1,3-diones analogues. These findings illustrate the presence of abundant carbonyl DBPs in water disinfection, and hence their impacts on human health deserve further investigation.

Combined Effects of pH and Borohydride Reduction on Optical Properties of Humic Substances (HS): A Comparison of Optical Models

The combined effects of pH and borohydride reduction on the optical properties of a series of humic substances and a lignin model were examined to probe the molecular moieties and interactions that give rise to the observed optical properties of these materials. Increasing the pH from 2 to 12 produced significantly enhanced absorption across the spectra of all samples, with distinct spectral responses observed over pH ranges attributable to the deprotonation of carboxylic acids and phenols. Borohydride reduction substantially attenuated the broadband absorption enhancements with pH, clearly indicating that the loss of absorption due to ketone/aldehyde reduction is coupled with the pH-dependent increase in absorption due to deprotonation of carboxylic acids and phenols. These results cannot be easily explained by a superposition of the spectra of independently absorbing chromophores (superposition model) but are readily interpretable within a charge transfer (CT) model. Changes of fluorescence emission with pH for both untreated and borohydride reduced samples suggest that a pH-dependent structural reorganization of the HS may also be influencing the fluorescence emission. Independent of optical model, these results demonstrate that chemical tests targeted to specific moieties can identify distinct structural differences among HS sources as well as provide insight into the molecular moieties and interactions that produce the observed optical and photochemical properties.

Contribution of Quinones and Ketones/Aldehydes to the Optical Properties of Humic Substances (HS) and Chromophoric Dissolved Organic Matter (CDOM)

The molecular basis of the optical properties of chromophoric dissolved organic matter (CDOM) and humic substances (HS) remains poorly understood and yet to be investigated adequately. This study evaluates the relative contributions of two broad classes of carbonyl-containing compounds, ketones/aldehydes versus quinones, to the absorption and emission properties of a representative suite of HS as well as a lignin sample. Selective reduction of quinones to hydroquinones by addition of small molar excesses of dithionite to these samples under anoxic conditions produced small or negligible changes in their optical properties; however, when measurable, these changes were largely reversible upon exposure to air, consistent with the reoxidation of hydroquinones to quinones. With one exception, estimates of quinone content based on dithionite consumption by the HS under anoxic conditions were in good agreement with past electrochemical measurements. In contrast, reduction of ketones/aldehydes to alcohols employing excess sodium borohydride produced pronounced and largely, but not completely, irreversible changes in the optical properties. The results demonstrate that (aromatic) ketones/aldehydes, as opposed to quinones, play a far more prominent role in the optical absorption and emission properties of these HS, consistent with these moieties acting as the primary acceptors in charge-transfer transitions within these samples. As a method, anoxic dithionite titrations may further allow additional insight into the content and impact of quinones/hydroquinones on the optical properties of HS and CDOM.

Relationships Between Dissolved Organic Matter Composition and Photochemistry in Lakes of Diverse Trophic Status

The North Temperate Lakes Long-Term Ecological Research site includes seven lakes in northern Wisconsin that vary in hydrology, trophic status, and landscape position. We examine the molecular composition of dissolved organic matter (DOM) within these lakes using Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) and quantify DOM photochemical activity using probe compounds. Correlations between the relative intensity of individual molecular formulas and reactive species production demonstrate the influence of DOM composition on photochemistry. For example, highly aromatic, tannin-like formulas correlate positively with triplet formation rates, but negatively with triplet quantum yields, as waters enriched in highly aromatic formulas exhibit much higher rates of light absorption, but only slightly higher rates of triplet production. While commonly utilized optical properties also correlate with DOM composition, the ability of FT-ICR MS to characterize DOM subpopulations provides unique insight into the mechanisms through which DOM source and environmental processing determine composition and photochemical activity.

Molecular Insights into the Transformation of Dissolved Organic Matter in Landfill Leachate Concentrate during Biodegradation and Coagulation Processes Using ESI FT-ICR MS

Landfill leachate concentrate is a type of refractory organic wastewater with high environmental risk. Identification of refractory components and insights into the molecular transformations of the organics are essential for the development of efficient treatment process. In this report, molecular compositions of dissolved organic matter (DOM) in leachate concentrate, as well as changes after anaerobic/aerobic biodegradation and coagulation with salts, were characterized using electrospray ionization (ESI) coupled with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). DOM in leachate concentrate were more saturated and less oxidized with more nitrogen and sulfur-containing substances (accounting for 50.0%), comparing with natural organic matter in Suwannee River. Selectivity for different classes of organics during biodegradation and coagulation processes was observed. Substances with low oxidation degree (O/C < 0.3) were more reactive during biodegradation process, leading to the formation of highly oxidized molecules (O/C > 0.5). Unsaturated (H/C < 1.0) and oxidized (O/C > 0.4) substances containing carboxyl groups were preferentially removed after coagulation with Al or Fe sulfate. The complementary functions of biodegradation and coagulation in the treatment of DOM in leachate concentrate were verified at the molecular level. Lignin-derived compounds and sulfur-containing substances in leachate concentrate were resistant to biodegradation and coagulation treatments. To treat leachate concentrate more effectively, processes aimed at removal of such DOM should be developed.

Probing and Comparing the Photobromination and Photoiodination of Dissolved Organic Matter by Using Ultra-High-Resolution Mass Spectrometry

Photochemical halogenation of dissolved organic matter (DOM) may represent an important abiotic process for the formation of natural organobromine compounds (OBCs) and natural organoiodine compounds (OICs) within surface waters. Here we report the enhanced formation of OBCs and OICs by photohalogenating DOM in freshwater and seawater, as well as the noticeable difference in the distribution and composition pattern of newly formed OBCs and OICs. By using negative ion electrospray ionization coupled with Fourier transform ion cyclotron resonance mass spectrometry, various OBCs and OICs were identified during the photohalogenation processes in sunlit waters. The respective number of OBCs and OICs formed in artificial seawater (ASW) under light radiation was higher than that in artificial freshwater (AFW), suggesting a possible role of the mixed reactive halogen species. OBCs were formed mainly via substitution reactions and addition reactions accompanied by other reactions and distributed into three classes: unsaturated hydrocarbons with relatively low oxygen content, unsaturated aliphatic compounds, and saturated fatty acids and carbohydrates with relatively high hydrogen content. Unlike the OBCs, OICs were located primarily in the region of carboxylic-rich alicyclic molecules composed of esterified phenolic, carboxylated, and fused alicyclic structures and were generated mainly through electrophilic substitution of the aromatic proton. Our findings call for further investigation on the exact structure and toxicity of the OBCs and OICs generated in the natural environment.

Molecular Composition and Photochemical Reactivity of Size-Fractionated Dissolved Organic Matter

The photochemical production of reactive species, such as triplet dissolved organic matter (³DOM) and singlet oxygen (¹O₂), contributes to the degradation of aquatic contaminants and is related to an array of DOM structural characteristics, notably molecular weight. In order to relate DOM molecular weight, optical properties, and reactive species production, Suwannee River (SRFA) and Pony Lake fulvic acid (PLFA) isolates are fractionated by sequential ultrafiltration, and the resultant fractions are evaluated in terms of molecular composition and photochemical reactivity. UV-visible measurements of aromaticity increase with molecular weight in both fulvic acids, while PLFA molecular weight fractions are shown to be structurally similar by Fourier-transform ion cyclotron resonance mass spectrometry. In addition, Bray-Curtis dissimilarity analysis of formulas identified in the isolates and their size fractions reveal that SRFA and PLFA have distinct molecular compositions. Quantum yields of ³DOM, measured by electron and energy transfer probes, and ¹O₂ decreased with molecular weight. Decreasing [³DOM]_ss with molecular weight is shown to derive from elevated quenching in high molecular weight fractions, rather than increased ³DOM formation. This work has implications for the photochemistry of waters undergoing natural or engineered treatment processes that alter DOM molecular weight, such as photooxidation and biological degradation.

Impacts of Polar Changes on the UV-induced Mineralization of Terrigenous Dissolved Organic Matter

Local climates in the Northern and Southern Hemisphere are influenced by Arctic Amplification and by interactions of the Antarctic ozone hole with climate change, respectively. Polar changes may affect hydroclimatic conditions in temperate regions, for example, by increasing the length and intensity of precipitation events at Northern Hemisphere midlatitudes. Additionally, global warming has led to the thawing of ancient permafrost soils, particularly in Arctic regions, due to Arctic Amplification. Both heavy precipitation events and thawing of permafrost are increasing the net transfer of terrestrially derived dissolved organic matter (DOM) from land to surface waters. In aquatic ecosystems, UV-induced oxidation of terrigenous DOM (tDOM) produces atmospheric CO2 and this process is one of several mechanisms by which natural organic matter in aquatic and soil environments may play an important role in climate feedbacks. The Arctic is particularly affected by these processes: for example, melting of Arctic sea ice allows solar UV radiation to penetrate into the ice-free Arctic Ocean and to cause photochemical reactions that result in bleaching and mineralization of tDOM. Open questions, in addition to those shown in the Graphical Abstract, remain regarding the resulting contributions of tDOM photomineralization to CO2 production and global warming.

Are Extracted Materials Truly Representative of Original Samples? Impact of C18 Extraction on CDOM Optical and Chemical Properties

Some properties of dissolved organic matter (DOM) and chromophoric dissolved organic matter (CDOM) can be easily measured directly on whole waters, while others require sample concentration and removal of natural salts. To increase CDOM content and eliminate salts, solid phase extraction (SPE) is often employed. Biases following extraction and elution are inevitable, thus raising the question of how truly representative the extracted material is of the original. In this context, we investigated the wavelength dependence of extraction efficiency for C18 cartridges with respect to CDOM optical properties using samples obtained from the Middle Atlantic Bight (MAB) and the Equatorial Atlantic Ocean (EAO). Further, we compared the optical changes of C18 extracts and the corresponding whole water following chemical reduction with sodium borohydride (NaBH₄). C18 cartridges preferentially extracted long-wavelength absorbing/emitting material for samples impacted by riverine input. Extraction efficiency overall decreased with offshore distance away from riverine input. Spectral slopes of C18-OM samples were also almost always lower than those of their corresponding CDOM samples supporting the preferential extraction of higher molecular weight absorbing material. The wavelength dependence of the optical properties (absorption, fluorescence emission, and quantum yield) of the original water samples and their corresponding extracted material were very similar. C18 extracts and corresponding water samples further exhibited comparable optical changes following NaBH₄ reduction, thus suggesting a similarity in nature (structure) of the optically active extracted material, independent of geographical locale. Altogether, these data suggested a strong similarity between C18 extracts and corresponding whole waters, thus indicating that extracts are representative of the CDOM content of original waters.

Enhanced photoproduction of hydrogen peroxide by humic substances in the presence of phenol electron donors

Addition of a series of phenol electron donors to solutions of humic substances (HS) enhanced substantially the initial rates of hydrogen peroxide (H2O2) photoproduction (RH2O2), with enhancement factors (EF) ranging from a low of ∼3 for 2,4,6-trimethylphenol (TMP) to a high of ∼15 for 3,4-dimethoxyphenol (DMOP). The substantial inhibition of the enhanced RH2O2 following borohydride reduction of the HS, as well as the dependence of RH2O2 on phenol and dioxygen concentrations are consistent with a mechanism in which the phenols react with the triplet excited states of (aromatic) ketones within the HS to form initially a phenoxy and ketyl radical. The ketyl radical then reacts rapidly with dioxygen to regenerate the ketone and form superoxide (O2-), which subsequently dismutates to H2O2. However, as was previously noted for the photosensitized loss of TMP, the incomplete inhibition of the enhanced RH2O2 following borohydride reduction suggests that there may remain another pool of oxidizing triplets. The results demonstrate that H2O2 can be generated through an additional pathway in the presence of sufficiently high concentrations of appropriate electron donors through reaction with the excited triplet states of aromatic ketones and possibly of other species such as quinones. However, in some cases, the much lower ratio of H2O2 produced to phenol consumed suggests that secondary reactions could alter this ratio significantly.

Structural characterization of dissolved organic matter: a review of current techniques for isolation and analysis

Natural dissolved organic matter (DOM) in aquatic systems plays many environmental roles: providing building blocks and energy for aquatic biota, acting as a sunscreen in surface water, and interacting with anthropogenic compounds to affect their ultimate fate in the environment. Such interactions are a function of DOM composition, which is difficult to ascertain due to its heterogeneity and the co-occurring matrix effects in most aquatic samples. This review focuses on current approaches to the chemical structural characterization of DOM, ranging from those applicable to bulk samples and in situ analyses (UV-visible spectrophotometry and fluorescence spectroscopy) through the concentration/isolation of DOM followed by the application of one or more analytical techniques, to the detailed separation and analysis of individual compounds or compound classes. Also provided is a brief overview of the main techniques used to characterize isolated DOM: mass spectrometry (MS), nuclear magnetic resonance mass spectrometry (NMR) and Fourier transform infrared spectroscopy (FTIR).

The importance of charge-transfer interactions in determining chromophoric dissolved organic matter (CDOM) optical and photochemical properties

Absorption of sunlight by chromophoric dissolved natural organic matter (CDOM) is environmentally significant because it controls photic zone depth and causes photochemistry that affects elemental cycling and contaminant fate. Both the optics (absorbance and fluorescence) and photochemistry of CDOM display unusual properties that cannot easily be ascribed to a superposition of individual chromophores. These include (i) broad, unstructured absorbance that decreases monotonically well into the visible and near IR, (ii) fluorescence emission spectra that all fall into a single envelope regardless of the excitation wavelength, and (iii) photobleaching and photochemical quantum yields that decrease monotonically with increasing wavelength. In contrast to a simple superposition model, these phenomena and others can be reasonably well explained by a physical model in which charge-transfer interactions between electron donating and accepting chromophores within the CDOM control the optical and photophysical properties. This review summarizes current understanding of the processes underlying CDOM photophysics and photochemistry as well as their physical basis.