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.

Exploring the zero-valent iron (ZVI) mediated transformation of dissolved organic nitrogen (DON) in anammox system using FT-ICR MS and fluorescence spectroscopy.

With fluorescence spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), this study evaluated the performance of zero-valent iron (ZVI) enhanced anammox process for treating fulvic acids (FA)-containing wastewater and investigated the fate of dissolved organic nitrogen (DON) at the molecular level. The results showed that ZVI increased the total organic carbon (TOC) removal rate of the anammox system by approximately 10% and reduced the organic matter fluorescence intensity by 21%. Specifically, microbial humic-like C1 and terrestrial humic-like C4 components were preferentially degraded by ZVI among the fluorescence components (C1-C4). Moreover, ZVI significantly altered the transformation pathways of DON molecules in anammox. The number of precursors molecular formulas increased from 1617 to 2002, attributed to the elimination of high molecular weight (MW) (>500 Da) molecules. Specifically, DON molecules with high O/C were degraded under the effect of ZVI, while some high MW were resisted. The original products generated from anammox consortia metabolism and FA degradation underwent secondary reactions with ZVI, thereby decreased the O/C of products molecular formulas. Furthermore, anammox process was subjected to the most carboxylic acid reaction, including decarboxylation and reduction of carboxylic acids. For CHON molecules, a significant number of reactions involving the reduction of nitro groups contributed to the reduction of wastewater toxicity, which improved the overall performance.

Carboxylesterase Activatable Molecular Probe for Personalized Treatment Guidance by Analyte-Induced Molecular Transformation.

Accurate visualization of tumor microenvironment is of great significance for personalized medicine. Here, we develop a near-infrared (NIR) fluorescence/photoacoustic (FL/PA) dual-mode molecular probe (denoted as NIR-CE) for distinguishing tumors based on carboxylesterase (CE) level by an analyte-induced molecular transformation (AIMT) strategy. The recognition moiety for CE activity is the acetyl unit of NIR-CE, generating the pre-product, NIR-CE-OH, which undergoes spontaneous hydrogen atom exchange between the nitrogen atoms in the indole group and the phenol hydroxyl group, eventually transforming into NIR-CE-H. In cellular experiments and in vivo blind studies, the human hepatoma cells and tumors with high level of CE were successfully distinguished by both NIR FL and PA imaging. Our findings provide a new molecular imaging strategy for personalized treatment guidance.

Using Potential Molecular Transformation To Understand the Molecular Trade-Offs in Soil Dissolved Organic Matter.

Understanding the chemical composition and molecular transformation in soil dissolved organic matter (DOM) is important to the global carbon cycle. To address this issue, ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) was applied to investigate DOM molecules in 36 paddy soils collected from subtropical China. All the detected 7576 unique molecules were divided into seven compound groups, and nine trade-off relationships between different compound groups were revealed based on principal component analysis and Pearson's correlation. An optimized method was developed to evaluate all potential molecular transformations in DOM samples. The concept of thermodynamics was introduced to evaluate the identified molecular transformations and classify them as thermodynamically favorable (TFP) and thermodynamically limited (TLP) processes. Here, we first tried to understand the molecular trade-offs by using the potential molecular transformations. All the nine trade-offs could be explained by molecular transformations. Six trade-offs had bases of biochemical reactions, and the trade-off-related direct transformations could explain the content variations of carbohydrate-like, condensed aromatic-like, tannin-like, and lignin-like compounds in TLP. More reasonable explanations existed in the TLP rather than TFP, which demonstrated the critical role of external energy in the molecular transformation of soil DOM.

Low-molecular-weight dissolved organic nitrogen transformation behavior in concentrated leachate by O3 and •OH: Partition, molecular insight, and potential precursor-product relationship.

Low-molecular-weight dissolved organic nitrogen (LMW-DON) is an emerging issue in concentrated leachate (CL). Ozonation is crucial to remove LMW-DON, but selectivity mechanisms of different reactive oxygen species were unknown. Here, reactions of O3 and •OH with LMW-DON at different dosages were determined from composition, unsaturation/redox potential, and precursor-product relationship. The molecular weight of LMW-DON in CL presented a normal distribution and 76.5% was below 450 Da. LMW-DON with 400-1000 Da increased to 55.6%-66.7% and O/Cwa increased by over 40.0% due to electrophilic substitution of O3. LMW-DON with <400 Da and 550-1000 Da were preferentially degraded by •OH at the low and high O3 dosage, respectively. O3 preferred to remove lipid-like (69.1%), protein-like (58.2%), and amino sugars-like (72.8%) LMW-DON, whereas •OH preferred to the refractory LMW-DON, such as carbohydrates-like (71.1%), lignin-like (49.6%), and tannins-like (72.5%) LMW-DON. Forty-three transformation reactions were quantified using mass difference analysis, and O3 preferred to oxygen addition (e.g., +2O) and conversed amino to nitro groups, and saturated LMW-DON increased via unsaturated bonds rupture. •OH attacked the carbon groups (e.g., -CH2) and nitrogen groups (e.g., -NH3+O, -NO2+H). These findings provide molecular evidence for the selectivity of oxidants with LMW-DON and improve the ozonation application in wastewater treatment.

Molecular Transformation for Self-reproducing Vesicles and Underlying Analysis Methods.

Closed bilayer membranes of amphiphiles in water, termed vesicles, represent one of the promising models of primitive cellular compartments. Herein, we reviewed studies on the design and construction of vesicle-based cell models capable of sequential growth and division and their underlying analysis methods. We discussed the potential contribution of these studies to the universal understanding of the chemical/physical logics behind the steady reproduction of cellular membranes.

Elucidating molecular level impact of peat fire on soil organic matter by laser desorption ionization Fourier transform ion cyclotron resonance mass spectrometry.

In this work, laser desorption ionization coupled with Fourier transform ion cyclotron resonance mass spectrometry (LDI-FTICRMS) was used to investigate the molecular composition of a peat fire and laboratory heated soil organic matter (SOM). SOM isolated from soils obtained from unburned and burned sites at Central Kalimantan, Indonesia, were analyzed with LDI-FTICRMS. About 7500 peaks were found and assigned with molecular formulas for each mass spectrum. SOM isolated from fire-affected soil sites are relatively more abundant in low oxygenated classes (e.g., O1-O5) and thermally stable compounds, including condensed hydrocarbon and nitrogen heterocyclic compounds. Abundances of highly condensed hydrocarbon compounds with carbon number > 30 were increased for the fire-affected SOM. In vivo heating experiments were conducted for SOM extracted from unburned sites, and the prepared SOMs were analyzed with LDI-FTICRMS. Overall, the same trend of change at the molecular level was observed from both the laboratory heated and the peat fire-affected SOM samples. In addition, it was observed that heat caused the degradation of SOM, generating lignin and tannin-type molecules. It was hypothesized that they were formed by thermal degradation of high molecular weight SOM. All the information presented in this study was obtained by consuming ~ 5 µg of sample. Therefore, this study shows that LDI-FTICRMS is a sensitive analytical technique that is effective in obtaining molecular level information of SOM. Graphical abstract.

Long-term straw return enhanced the chlorine reactivity of soil DOM: Highlighting the molecular-level activity and transformation trade-offs.

Chemical properties and molecular diversity of dissolved organic matter (DOM) in agricultural soils are important for soil carbon dynamics and chlorine activity. Yet the chlorine reactivity of soil DOM at the molecular level under agricultural management practices remains unidentified. Here, we investigated the chlorine reactivity of soil DOM under long-term straw return and the molecular activities and transformations during chlorination. The 9-year straw return enhanced the chlorine reactivity of soil DOM, leading to increases in the production of traditional disinfection byproducts (DBPs) and decreases in the formation of emerging high molecular weight DBPs. C17HnOmCl1-2 and C22HnNmOzCl were the highest relative abundances of emerging DBPs. The emerging DBPs were primarily generated through chlorine substitution reactions, with their precursors exhibiting higher H/Cwa (1.47) and O/Cwa (0.41) ratios under straw return. The molecular transformation ability and inactive molecules of soil DOM under long-term straw return were reduced after chlorination, resulting in increased DOM instability. Chlorination led to a shift in the thermodynamic processes of soil DOM molecules from thermodynamically limited to thermodynamically favorable processes, and lignin-like compounds displayed higher potentials for transformation into protein/amino sugar-like compounds. C19H26O6 was identified as a sensitive formula for tracing chlorine reactivity under straw return, and a network illustrating the generation of DBPs from C19H26O6 was established. Overall, these results highlighted the strong chlorine reactivity of soil DOM under long-term straw return.

Tracking the transformation of extracellular polymeric substances during the ultraviolet/peracetic acid disinfection process: Emphasizing on molecular-level analysis and overlooked mechanisms.

In this study, the transformation mechanisms of extracellular polymeric substances (EPS) during ultraviolet/peracetic acid (UV/PAA) disinfection were elucidated based on multiple molecular-level analyses. After UV/PAA disinfection, the contents of soluble EPS (S-EPS), loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS) were reduced by 70.47 %, 57.05 % and 47.46 %, respectively. Fluorescence excitation-emission matrix-parallel factor and Fourier transform ion cyclotron resonance mass spectrometry analyses showed that during UV/PAA disinfection, EPS was transformed from the state characterized by high aromaticity, low saturation and low oxidation to the one with reduced aromaticity, increased saturation and higher oxidation. Specifically, sulfur-containing molecules (CHOS, CHONS, etc.) in EPS were converted into highly saturated and oxidized species (such as CHO), with the aromaticity index (AImod) decreasing by up to 53.84 %. Molecular characteristics analyses further indicated that saturation degree, oxidation state of carbon and molecular weight exhibited the most significant changes in S-EPS, LB-EPS and TB-EPS, respectively. Additionally, mechanistic analysis revealed that oxygen addition reaction was the predominant reaction for S-EPS (+O) and TB-EPS (+3O) (accounting for 31.78 % and 36.47 %, respectively), while the dealkylation was the main reaction for LB-EPS (29.73 %). The results were consistent with functional groups sequential responses analyzed by Fourier transform infrared and two-dimensional correlation spectroscopy, and were further verified by density functional theory calculations. Most reactions were thermodynamically feasible, with reaction sites predominantly located at functional groups such as CO, CO, CN and aromatic rings. Moreover, metabolomics analysis suggested that changes in metabolites in raw secondary effluent during UV/PAA disinfection were strongly correlated with EPS transformation. Our study not only provides a strong basis for understanding EPS transformation during UV/PAA disinfection at molecular-level but also offers valuable insights for the application this promising disinfection process.

Comparative molecular transformations of dissolved organic matter induced by chlorination and ammonia/chlorine oxidation process.

The ammonia/chlorine oxidation process can greatly degrade PPCPs in water. However, its effect on molecular transformations of natural organic matter (NOM) and effluent organic matter (EfOM) are still poorly understood. In this study, molecular transformations of NOM and EfOM occurring during ammonia/chlorine were explored and compared with those occurred during chlorination, using spectroscopy and mass spectrometry. Phenolic and highly unsaturated aliphatic compounds together with aliphatic compounds were found to be predominant in both NOM and EfOM samples, all of which were significantly degraded after two processes. The ammonia/chlorine process led to greater decreases in the molecular weights of such components but lower reductions in aromaticity. Compared with chlorination, ammonia/chlorine was found to be more likely to degrade compounds while remaining fluorophores or chromophores. The CH(N)O(S) precursors were found to be similar for both processes but their products were quite different. The CH(N)O(S) precursors that only found in ammonia/chlorine had higher molecular weights and greater degrees of oxidation but lower degrees of saturation. In contrast, the unique CH(N)O(S) products that only found in ammonia/chlorine exhibited lower molecular weights and lower degrees of oxidation degrees together with higher degrees of saturation. Lower total abundance of chlorinated byproducts was found by ammonia/chlorine compared with chlorination, although the former process provided a richer diversity. In all water samples, chlorinated byproducts were mainly generated by substitution reactions during ammonia/chlorine and chlorination. Overall, the findings of this study could provide new insights into the transformations of NOM and EfOM induced by ammonia/chlorine and chlorination.

Photochemical transformation mechanisms of dissolved organic matters (DOM) derived from different bio-stabilization sludge.

Bio-stabilization sludge contains numerous dissolved organic matter (DOM) that could enter aquatic environments by soil leaching after sludge land use, but a clear understanding of their photochemical behavior is still lacking. In this study, we systematically investigated the photoactivity and photochemical transformation of aerobic composting sludge-derived DOM (DOMACS) and anaerobic digestion sludge-derived DOM (DOMADS) by using multispectral analysis coupled with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The results indicated that DOMACS and DOMADS have a higher proportion of highly unsaturated and phenolic compounds (HuPh)with high DBEwa, but the different polyphenols (Polyph) abundance of them, causing the different photoactivity between them. DOMACS had much higher apparent quantum yields (AQY) for triplet states of dissolved natural organic matter (3DOM*) and hydroxyl radical (•OH) but slightly lower AQY for singlet oxygen (1O2) than DOMADS under simulated sunlight conditions. As the irradiation time increased, HuPh and Polyph (associated with humic-like substances) contained in DOMACS (DOMADS) decreased by 12.0% (14.1%) and 3.0% (0.2%), respectively, with concurrent decrease in average molecular weight and aromaticity moieties, resulting in more generation of aliphatic compounds. Furthermore, based on 27 types of photochemical transformation reactions, DOMACS containing higher fractions of O10-15 and N1-3Oy class preferred dealkyl group and carboxylic acid reactions, whereas DOMADS composed of more N4Oy and S2Oy fragments preferred oxygen addition and anmine reactions. Consequently, photochemical transformations reduced the Cd (II) ion activity in the presence of DOMACS (DOMADS). This study is believed to unveil the photochemical transformation of bio-stabilization sludge-derived DOM and its impact on pollutants' fate in the aquatic environment.

Dynamics of organic matter molecular composition under aerobic decomposition and their response to the nitrogen addition in grassland soils.

Grassland soils store a substantial proportion of the global soil carbon (C) stock. The transformation of C in grassland soils with respect to chemical composition and persistence strongly regulate the predicted terrestrial-atmosphere C flux in global C biogeochemical cycling models. In addition, increasing atmospheric nitrogen (N) deposition alters C chemistry in grassland soils. However, there remains controversy about the importance of mineralogical versus biochemical preservation of soil C, as well as uncertainty regarding how grassland soil C chemistry responds to elevated N. This study used grassland soils with diverse soil organic matter (SOM) chemistries in an 8-month aerobic incubation experiment to evaluate whether the chemical composition of SOM converged across sites over time, and how SOM persistence responded to the N addition. This study demonstrates that over the course of incubation, the richness of labile compounds decreased in soils with less ferrihydrite content, whereas labile compounds were more persistent in ferrihydrite rich soils. In contrast, we found that the richness of more complex compounds increased over the incubation in most sites, independent of soil mineralogy. Moreover, we demonstrate the extent to which the diverse chemical composition of SOM converged among sites in response to microbial decomposition. N fertilization decreased soil respiration and inhibited the convergence of molecular composition across ecosystems by altering N demand for microbial metabolism and chemical interactions between minerals and organic molecules. This study provides original evidence that the decomposition and metabolism of labile organic molecules were largely regulated by soil mineralogy (physicochemical preservation), while the metabolism of more complex organic molecules was controlled by substrate complexity (biochemical preservation) independent to mineral-organic interactions. This study advanced our understanding of the dynamic biogeochemical cycling of C by unveiling that N addition dampened C respiration and diminished the convergence of SOM chemistry across diverse grassland ecosystems.

DOM chemodiversity pierced performance of each tandem unit along a full-scale "MBR+NF" process for mature landfill leachate treatment.

Mature landfill leachate contains a substantial fraction of recalcitrant dissolved organic matters (DOM) that is a challenging for conventional wastewater treatment that is typically focused on the removal of biodegradable organic matter. "Biological treatment + membrane treatment" has been widely employed to treat complex leachate. However, the performance of each unit based on both conventional bulk indicators and molecular information has not been well understood. Therefore, the fate of DOM chemodiversity along the full-scale treatment process across ten sampling points over three different seasons were analyzed to determine the efficiency of every unit process with the assistance of ultra-performance liquid chromatography coupled with hybrid quadrupole Orbitrap mass spectrometry. Results showed that the process performance, visualized through the molecular signals, were relatively stable in the temporal dimension. The process removed 83.2%-92.2% of DOM molecules in terms of richness, where lignin/carboxyl-rich alicyclic compounds (CRAM)-likes with relatively high saturation was preferentially removed, while newly generated bio-derived N-containing compounds (N/Cwa 0.15-0.17) became resistant. The relationship between conventional bulk physicochemical indicators and molecular indexes suggested that soluble chemical oxygen demand (sCOD) and dissolved organic carbon (DOC) were contributed by the refractory DOM with high weighted average double bond equivalents (DBEwa), which was distributed in the region of O/C 0.2-0.5 and H/C 1.2-1.8. This refractory DOM required ultrafiltration and nanofiltration for removal. DOM molecules were positively correlated with five-day biochemical oxygen demand (BOD5) and revealed that approximately 96.9%-98.4% of the DOM could be removed or transformed in the primary anoxic zone. In addition, the bio-derived aliphatic/proteins, lipids and lignin/CRAM-likes (O/C > 0.2) with condensed aromatization were the sources of dissolved organic nitrogen (DON) and still remained in the final effluent. The present study suggests that the design and operation of the combination process with biological and membrane treatment could be specifically optimized based on the DOM molecular characteristics of the wastewater.

Exciton-stimulated molecular transformation in organic light-emitting diodes.

An exciton-stimulated molecular transformation in an organic light-emitting diode (OLED) on a time scale of a few seconds under electrical bias is shown to reach nearly 100% under standard operating conditions, leading to color switching. It is reversible in both a thin film and an OLED when sufficient thermal energy is supplied. Such an exciton-stimulated molecular transformation suggests a new process which may be exploited for applications such as electrochromic and memory devices.