A new conceptual framework for the transformation of groundwater dissolved organic matter

Effect of DOM transformation on As enrichment from an Alpine river basin in the Western Tibetan Plateau

The geogenic enrichment of arsenic (As) extensively occurred in the riverine systems from the Qinghai-Tibetan Plateau under active geothermal discharge and chemical weathering conditions, while little is known about how dissolved organic matter (DOM) transformation regulates the aquatic As concentrations. The present study revealed that the DOM components from the Singe Tsangpo River (STR) basin primarily consisted of protein-like components (81.30 % ± 6.48 %), with the microbially-endogenous production being a predominant source under the control of temperature and glacier-runoff recharge along the river flow path. Notably, the chemical weathering processes have significantly facilitated the enhancement of humic-like components in the river water. Besides, the groundwater DOM characteristics were predominantly influenced by the mobilization of sedimentary organic matter and the introduction of allochthonous DOM resulting from surface-water recharge. Interestingly, humic-like components facilitated As enrichment through complexation and competitive adsorption effects in both surface water and groundwater under oxidizing conditions, which was supported by the significant positive correlations between As and humic-like component (R2 = 0.31/0.65, P < 0.05/0.01) and the concurrent mobilization of As and humic-like components from sediment incubation experiments. Moreover, the Structural Equation Modeling analysis revealed a stronger contribution of humic-like components to the As enrichment in the groundwater compared with surface water, possibly due to the relatively stronger microbial activity and enhanced mobilization of humic-like components in alluvial aquifers. The present study thus provided new insights into the transformation of DOM and its important role in facilitating As enrichment in the aquatic environment from alpine river basins.

Unravelling riverine dissolved organic matter sources using molecular fingerprints and FEAST model in a multi-tributary mountain river basin

Revealing the sources, composition and fate of riverine dissolved organic matter (DOM) is fundamental to understanding the biogeochemical cycles of aquatic ecosystems. This study aimed to reveal the impact of land uses and wastewater treatment plants (WWTPs) on riverine DOM. Spatiotemporal variations in molecular characteristics of riverine DOM in the river network containing 15 tributaries in the mainstream of upper Hanjiang River were studied. Differences in molecular characteristics of DOM in soil leachates of various land uses and the effluent of WWTPs were analyzed and their contributions to riverine DOM in both dry and wet seasons were calculated using FEAST model. DOM in soil leachates was primarily composed of lignin, protein and lipid-like compounds but was dominated by lignin and tannin-like compounds in the effluent of WWTPs. Contribution rates of the soil leachate of each land use calculated by FEAST model showed a significant positive linear correlation with the area-based proportion of each land use in the basins of tributaries. Contributions of area-based proportion of each land use to riverine DOM followed the order of grassland > forest > cropland for both seasons. DOM in the upstream of tributaries contributed more than 50 % to the molecular composition of DOM in the downstream of tributaries but the contribution of the effluent of WWTPs to riverine DOM did not exceed 3 %. These results demonstrated that FEAST model could be used for source identification of riverine DOM based on molecular fingerprint data. Accordingly, this study provides new insights into the carbon cycling and ecological health within the watershed.

New Pathway Mediated by Humic Acid Facilitates Long-Distance Hydroxyl Radical Production during Crystalline Iron Mineral Oxygenation

Hydroxyl radicals (•OH) are extensively produced from crystalline iron minerals under redox oscillation, during which their oxidizing impact on pollutant dynamics is often limited by low transportation. While natural organic matter exists as a ubiquitous redox mediator, how it regulates the distance scale of •OH production in a heterogeneous system remains poorly understood. This study, for the first time, reports an unrecognized route mediated by humic acid (HA) in extending the spatial range of •OH from oxygenation of reduced hematite (rHem). The results showed that HA facilitated the spatial redistribution of •OH from the surface to the solution, posing inverse impacts on oxidation efficacies of pollutants with varying surface accessibilities. It was difficult to explain the enhanced free •OH production using electron shuttling by quinones or dissolved Fe-HA complexes. Comprehensive evidence demonstrated that adsorbed HA preferentially consumed surface-bound •OH to trigger a solid-to-liquid propagation of carbon-centered radicals (CCR•), dominantly promoting formation of secondary free •OH far from the surface. Reactive-transport simulation and in situ fluorescence results visualized interfacial and centimeter-scale long-range production of •OH mediated by CCR•. Fourier transform ion cyclotron resonance mass spectrometry verified the role of surface decarboxylation of adsorbed HA in CCR• generation. These findings offer new insights into the environmental fate of •OH produced during iron redox cycling in natural and engineered systems, including those with natural organic matter.

Contradictions in dissolved black carbon research: A critical review of its sources, structures, analytical methods, and environmental behaviors

Dissolved black carbon (DBC) represents the most active component within the black carbon (BC) continuum and plays a vital role in the global carbon cycle and the removal of inorganic and organic contaminants due to its prolonged residence time and unique condensed aromatic structure. Significant progress has been made in understanding DBC source, molecular structure, analytical methods, stability, and environmental behavior, particularly its photochemical and microbial transformation. However, substantial uncertainties persist, including ambiguities in its definition, limitations in isolation and quantification methods, and unidentified sources. These limitations have led to lots of inconsistencies regarding its stability, environmental transport pathways, and transformation mechanisms. This review critically examines the current landscape of DBC research, with a focus on: (1) key contradictions in DBC cycling processes, including debates over its recalcitrance, mismatched isotopic signatures, and imbalances in the marine DBC budget; (2) limitations for DBC isolation and quantification methods in natural environments; and (3) photochemical and microbial transformation processes, and its interactions with environmental pollutants. By synthesizing recent insights, this review aims to enhance the understanding of DBC's structures, turnover, and environmental behavior, as well as its implications for the global carbon cycle. To address existing challenges, future studies are suggested to prioritize resolving these contradictions, developing standardized analytical approaches, and achieving a clearer elucidation of DBC cycling processes across diverse environments.

Molecular characterization and environmental response of dissolved organic matter in reserve quiescent groundwater wells of the North China plain: Insights from spectroscopy and mass spectrometry

Dissolved organic matter (DOM) plays a critical role in aquatic ecosystems. However, the characteristics of DOM in groundwater source wells and interactions with environmental factors remain poorly understood. This study investigated the spectral properties, molecular composition, and environmental drivers across vertical groundwater gradients in Shijiazhuang using spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICRMS), multivariate statistics and molecular network analysis. Three components were identified: two humic-like substances (C1, C3) and one protein-like component (C2) Humic-like substances exhibited significant vertical stratification, with bottom groundwater DOM showing higher humification and autochthonous characteristics. Multivariate statistical analysis indicated that NO3--N and dissolved oxygen (DO) were keystone factors influencing the vertical differences of DOM. Surface-layer DOM was driven by dissolved total phosphorus (DTP), pH, DO and NO3--N, while the bottom layer was jointly regulated by pH, total phosphorus (TP), total nitrogen (TN) and NO3--N. DOM components correlated significantly with fluorescence index (FI), humification index (HIX), chemical oxygen demand (CODMn) and dissolved total nitrogen (DTN). FT-ICRMS analysis revealed that DOM molecular composition was dominated by CHO (38.71 %-52.07 %) and CHON (22.30 %-34.44 %) compounds, with lignin-like (LIG) (60.91 %-80.56 %) serving as the core molecular formulae. Redundancy analysis (RDA) identified that TN, DO, and NH4+-N were key drivers regulating the DOM molecules distribution. Furthermore, molecular network analysis demonstrated that LIG molecular formulae played a crucial role in the network, significantly enhancing the chemical stability of the DOM molecular network. These findings elucidate DOM dynamics in groundwater systems at a molecular scale, providing critical insights for resource protection and risk management.

Decomposing the molecular complexity and transformation of dissolved organic matter for innovative anaerobic bioprocessing

The sustainable transformation and management of dissolved organic matter (DOM) are crucial for advancing organic waste treatment towards resource-oriented processes. However, the intricate molecular complexity of DOM poses significant challenges, impeding a comprehensive understanding of the underlying biochemical processes. Here, we focus on the chemical "dark matter" mining using ultra-high resolution mass spectrometry technologies to elucidate the molecular diversity and transformation in anaerobic bioprocessing of food waste. We developed an analytical framework that reveals the persistence of DOM in the final effluent is mainly determined by its molecular properties, such as carbon chain length, aromaticity, unsaturation, and redox states. Our in-depth characterization and quantitative analysis of key biochemical reactions unveils the evolution of DOM composition, providing valuable insights into the targeted conversion of persistent molecules toward full utilization. Additionally, we establish a correlation between the redox state and energy density of a broad range of DOM molecules, enabling us to comprehend and evaluate their biodegradability. These insights enhance the mechanistic understanding of DOM transformation, guiding the rational design and regulation of sustainable organic waste treatment strategies.

Deviation of nanoparticle aggregation from XDLVO theory as explained by dissolved organic matter adsorption and fractionation

The stability of iron and aluminum-based nanoparticles (NPs) in dissolved organic matter (DOM)-rich environments remains unpredictable when evaluated through classical XDLVO theories. This study demonstrates that DOM fractionation during adsorption governs the contrasting sedimentation behaviors of ferrihydrite (Fh) and amorphous aluminum hydroxide (AAH) NPs through non-DLVO mechanisms. Advanced spectroscopic analyses (3D-EEM, FTIR, 2D-COS) revealed that Fh NPs preferentially adsorb aromatic humic components, which accelerate sedimentation through hydrophobic interactions. In contrast, AAH NPs selectively bind high molecular weight (HMW) protein-like substances, enhancing stability via steric hindrance. Contrary to XDLVO calculations, Fh exhibited DOM-induced aggregation, with a sedimentation rate increase of 37-58%, whereas AAH showed suppressed settling, with a rate reduction of 42-65%, underscoring the limitations of XDLVO models. Hydrophobic interactions/π-π interactions/hydrogen bonding (Fh-humics) and macromolecular steric effects (AAH-proteins) dominate stability, with DOM fractionation dictated by NP surface chemistry. These findings demonstrate that different DOM fractions mediate different interfacial forces. However, the XDLVO model describes the overall interactions of the bulk DOM, and could not capture the selective adsorption and molecular rearrangement that occur during DOM-NP interaction. Our results call for a new conceptual framework that integrates DOM chemical heterogeneity and non-DLVO interactions to predict colloid transport and carbon cycling in organic-rich aquatic ecosystems.

The Competitive/Cooperative Dynamics of Sulfur Disproportionation Microbes and Methanogens in Geogenic High-Iodine Groundwater Systems

The microbial transformation of iodine-bearing organic matter (OM) and iron (Fe) minerals is a critical process that controls the release of iodine (I) to groundwater. However, the roles of functional microbial types, OM molecular characteristics, and microbe-OM interactions in iodine mobilization remain unclear. In this study, groundwater samples with different iodine concentrations were collected from the central Yangtze River basins, China. Using 16S rRNA gene sequencing, we identified sulfur disproportionation and methanogenesis as dominant metabolic processes in relatively low-I (<300 μg/L) and high-I (>300 μg/L) groundwater, respectively. Sediment incubation experiments showed that combined sulfur disproportionation and methanogenesis can promote iodine release by 87.1%. Ultrahigh-resolution molecular characterization of the organic components revealed that sulfur-disproportionating microbes may selectively metabolize bioactive OM (e.g., aliphatic compounds and oxygen-poor highly unsaturated compounds), leaving recalcitrant OM (e.g., N-containing oxygen-rich highly unsaturated compounds, polyphenols, and polycyclic aromatic compounds) in groundwater, and methanogenic microbes preferentially consume bioactive OM in low-I groundwater and recalcitrant OM in high-I groundwater. Thus, a cooperative-competitive pattern between methanogens and sulfur disproportionating microorganisms may influence OM degradation and potentially contribute to iodine mobilization. This study highlights that the OM transformation process, driven by biological sulfur disproportionation and methanogenesis, promotes iodine enrichment in alluvial-lacustrine groundwater and improves our understanding of the genesis of geogenic high-iodine groundwater systems.

Extracellular polymeric substance mediating nanoplastics-promoted short-term Porphyridium growth disrupts marine carbon and phosphorus migration

The ecotoxicity of nanoplastics (NPs) on marine microalgae has been extensively explored recently, yet the mechanisms driving short-term growth improvement caused by NPs remain poorly understood. In the present study, we observed that a relatively high concentration (10 mg/L) of the green fluorescently labeled fresh polyamide-polymethyl methacrylate polymer blend (w/w 21:4) NPs beads (200 nm) significantly enhanced the cell density of Porphyridium cruentum (42.1 %) by alleviating reactive oxygen species generation, chlorophyll degradation, and photoinhibition. An increase in the sticky bounded exopolysaccharides (b-EPs) surrounding P. cruentum surface enhanced NP adsorption within five hours of exposure, with -CH3 bond in phospholipids/glycolipids and polysaccharides of b-EPs supporting the adsorption to mitigate photoinhibition. Increased free exopolysaccharides (EPs) removed inorganic and organic carbon and 48 % of dissolved organic matter (DOM), encapsulating NPs into sediments while cooperating with pH elevation. However, short-term growth promotion resulted in cell shading and phosphorous deficiency after 12 days of cultivation. Consequently, the photosynthesis-antenna proteins pathway and energy metabolites were downregulated, whereas the transmembrane transport and receptor activities of phosphate and calcium signal pathways were upregulated to maintain growth, achieving balance in the 1 mg/L group. The significantly upregulated steroid biosynthesis promoted the hydrophobicity of plasma membranes and reduced the permeability for water-soluble ions, exacerbating phosphorus deficiency. The downregulation of the Calvin cycle shifted the total carbon metabolism and carbon migration, reducing photosynthesis and respiration but accumulating starch to counteract cell shading and phosphorus deficiency. These findings provide novel insights into the mechanisms underlying the short-term growth stimulation and long-term potential toxic effects of NPs on marine microalgae, thus altering marine carbon and phosphorus cycles.

Dissolved organic matter derived from long-term photodegradation of plastics alters microbial methane conversion in mangrove sediments

Highly enriched plastic debris leads to an increase in dissolved organic matter derived from the degradation of plastics (PDOM), which potentially influences the stability of methane (CH4) emissions of microorganisms in mangrove sediments. Here, microcosm incubation was conducted to reveal the impacts of two PDOM on CH4 emissions from mangrove sediments. CH4 emissions from PDOM treated sediments were reduced by 0.03-0.11 ppm within 30 days. The reduction in CH4 emissions was attributed mainly to alterations in sediment properties and bacterial communities. Phosphorus-containing lignin and proteins were the key molecular components that induce the decrease in the abundance of functional genes in the CO2 to methane pathway. The influence of PDOM on CH4 emissions may be associated with bacterial fermentation and the degradation pathways of aromatic compounds. This study enhances the understanding of the impact of PDOM on carbon cycling in mangrove ecosystems.

Fe and Mn biogeochemical cycling associated with basin-scale redox dynamics traced by DOM degradation in different alluvial aquifers

Iron (Fe) and manganese (Mn) contamination in groundwater has emerged as a global health challenge, primarily influenced by the degradation pathways of organic matter. However, the understanding of Fe and Mn biogeochemical behaviors, particularly the release mechanisms driven by the redox dynamics of aquifers at the watershed scale remains limited. This investigation employed a multi-method framework integrating hydrogeochemical-isotopic analyses with DOM molecular characterization (FT-ICR MS) to elucidate DOM degradation processes along the groundwater flow paths and their driving effects on Fe and Mn mobilization. The findings revealed that DOM degradation significantly modulates the redox zoning in porous aquifers, thereby governing the release patterns of Fe and Mn. In the weakly oxidizing environment (Zone I), DOM derivatives exhibited intricate molecular structures, characterized by higher relative abundances of saturated compounds, aliphatic species, and polyphenols compared to the downstream area. Fe and Mn primarily originate from the water-rock interactions, and tend to form stable DOM-metal complexes under oxidizing aquifers that constrain the concentration of dissolved metals. As groundwater flows into the plain area (Zone II) where the aquifers gradually become anaerobic, enclosed sedimentary aquifers and sluggish groundwater runoff intertwine highly mineralized DOM with biogeochemical processes. The preferential utilization of DOM with higher NOSC values drives sequential anaerobic respiration from sulfate reduction to dissimilatory metal reduction. This redox cascade promoted extensive dissolution of Fe and Mn (oxy)hydroxides. Intriguingly, methanogenic-phase DOM fermentation in Zone II-XKR activated anaerobic methane oxidation, generating a secondary Fe and Mn mobilization pathway. This process augmented the efficiency of metal release, resulting in Fe and Mn concentration in the Zone II-XKR being 2-3 times higher than those in the WLR subzones. Our findings establish DOM molecular signatures coupled with δ13C-DIC isotopic tracers as indicators for deciphering redox gradient biogeochemistry. The proposed model deepens the understanding of metal-cycling mechanisms and provides an informative framework for the genesis of high Fe and Mn groundwater in alluvial plains.

Release and Redistribution of Arsenic Associated with Ferrihydrite Driven by Aerobic Humification of Exogenous Soil Organic Matter

Humification of exogenous soil organic matter (ESOM) remodels the organic compositions and microbial communities of soil, thus exerting potential impacts on the biogeochemical transformation of iron (hydr)oxides and associated trace metals. Here, we conducted a 70-day incubation experiment to investigate how aerobic straw humification influenced the repartitioning of arsenic (As) associated with ferrihydrite in paddy soil. Results showed that the humification was characterized by rapid OM degradation (1-14 days) and subsequent slow maturation (14-70 days). During the degradation stage, considerable As (13.1 mg·L-1) was released into the aqueous phase, which was reimmobilized to the solid phase in the maturation stage. Meanwhile, the low-crystalline structural As/Fe was converted to a more stable species, with a subtle crystalline phase transformation. The generated highly unsaturated and phenolic compounds and enriched Enterobacter and Sphingomonas induced ferrihydrite (∼3.1%) and As(V) reduction, leading to As release during the degradation stage. In the maturation stage, carboxylic-rich alicyclic molecules facilitated the aqueous As reimmobilization. Throughout the humification process, organo-mineral complexes formed between OM and ferrihydrite via C-O-Fe bond contributed to the solid-phase As/Fe stabilization. Collectively, this work highlighted the ESOM humification-driven iron (hydr)oxide transformation and associated As redistribution, advancing our understanding of the coupled biogeochemical behaviors of C, Fe, and As in soil.

Mechanisms controlling spatial variability of geogenic ammonium in coastal aquifers: Insights from Holocene sedimentary evolution

The contamination of groundwater with geogenic ammonium (NH4+) across various geological backgrounds has garnered significant attention, particularly in coastal aquifer systems. However, there remains a gap in our understanding of the mechanisms governing the spatial variability of NH4+ in coastal groundwater at a macroscopic scale. In this study, we collected the sediment samples from two boreholes corresponding to high-NH4+-N and low-NH4+-N groundwater. We analyzed the age, physicochemical properties, and soluble organic matter (SOM) characteristics of these sediments. The aim was to reconstruct the sedimentary evolutionary history of the Pearl River Delta and establish a link between sedimentary evolutionary processes and organic matter (OM) to further elucidate the mechanism underlying the formation of spatial heterogeneity of NH4+ in groundwater. The results suggested that the studied Quaternary shallow confined porous aquifer system was predominantly formed during the Holocene and comprised three depositional stages, including fluvial facies, estuarine-tidal flat facies, and deltaic plain facies. The depositional environment significantly controlled the physicochemical and SOM characteristics of sediments. In the paleo-channel area, the aquifer was covered by estuarine-tidal flat facies sediments abundant in OM and exhibited considerable SOM degradation. Consequently, a substantial amount of ion-exchange form NH4+-N (IEF) was liberated through compaction and diffused into the aquifer. In the paleo-interfluve area, the aquifer was covered by fluvial sediments characterized by extensive weathering and low OM content, resulting in the limited production of significant amounts of IEF that could infiltrate into the aquifer. This study provided an inaugural elucidation of the control mechanism of sedimentary evolution on the spatial variability of NH4+ in coastal groundwater at a macroscale, thereby enhancing the scientific substantiation for both the exploitation and protection of groundwater resources.

Deciphering the link between particulate organic matter molecular composition and lake eutrophication by FT-ICR MS analysis

Eutrophication has emerged as a significant environmental problem for global lakes. As an essential carrier of nutrients, particulate organic matter (POM) plays a vital role in the eutrophication process of these aquatic systems. In this study, POM from seven lakes with different trophic states in the middle and lower reaches of the Yangtze River (China) was characterized using carbon and nitrogen stable isotopes and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The aim was to elucidate the relationship between the source and molecular composition of POM during the eutrophication process of lakes. The results indicated that POM was mainly composed of autochthonous (62.7%) and allochthonous (37.3%) sources, with the contribution from autochthonous sources being more pronounced across the different sources. The POM formulas mainly consisted of the subclasses CHO, CHON, CHOP, CHOS, and CHONS. Notably, CHOP formulas had the highest proportion of labile formula compounds, according for 51.56%. The unsaturation, aromaticity, and oxidation of unique POM formulas gradually decreased with increasing trophic states. A significant positive correlation was observed between CHOP and the percentage of labile compounds (MLBL%) in unique POM formulas. The relative abundance of lipid and protein compounds of unique POM formulas showed a positive correlation with lake trophic states, which indicated that with the increase of lake trophic states, the content of autochthonous POM gradually increased. Herein, we inferred that with the intensification of lake eutrophication, the autochthonous POM increased, which was accompanied by a further increase of labile P-containing compounds in POM, thus leading to the increasing eutrophication process of lakes in the form of positive feedback. Overall, this investigation of POM at the molecular level illustrates the deep-rooted mechanism of frequent lake eutrophication. This is of great significance in understanding the fate of POM and effectively controlling lake eutrophication.

Revealing the mobilization and age of estuarine dissolved organic matter during floods using radiocarbon and molecular fingerprints

Estuaries significantly affect the transport of dissolved organic matter (DOM) from land to ocean. While the transport and composition of estuarine DOM have been extensively studied, the direct link between DOM chemistry and its age remains unclear, limiting a comprehensive understanding of the dynamics and fate of estuarine DOM under severe conditions (e.g., floods). This study applied radiocarbon and ultrahigh-resolution mass spectrometry analysis to investigate the correlation between DOM chemistry and apparent radiocarbon age of 102 samples collected from the Yangtze River Estuary during both non-flood and flood periods. The results showed that young estuarine DOM are characterized by low-molecular-weight, unsaturated molecules, while aged estuarine DOM are relatively saturated with high-molecular-weight molecules. Phosphorus and nitrogen-containing compounds were key to DOM aging, potentially increasing the lability of aged DOM. Floods significantly impact DOM by introducing more labile aged DOM and young terrestrial DOM. Furthermore, floods enhanced the flux of aged DOM transported to the East China Sea by approximately 1.4 times. Our findings contribute to the study of estuarine DOM and its response during severe floods. Additionally, incorporating apparent radiocarbon age evidence improves the understanding of terrigenous DOM and its fate in large river estuaries before it contributes to the ocean carbon reservoir.

Marine Recalcitrant Dissolved Organic Matter Gained by Processing at Sandy Subterranean Estuaries

The sandy subterranean estuary (STE) connecting fresh groundwater to saline sea water is characterized by strong geochemical (salinity, redox, and pH) gradients, with evidence emerging for its role as a hot spot for consumption of labile substrates. This inspired us to conduct a study to evaluate whether this holds true for dissolved organic matter (DOM), especially given the still mysterious origin of marine recalcitrant DOM. Here, characterization of DOM of 21 groundwater samples (depth 1-13 m, salinity 3.9‰ to 32.4‰) across a 65 m transect of an STE located in coastal Guangdong, China, has found systematic biotransformation toward "recalcitrant" carboxyl-rich alicyclic molecules (CRAM). The fraction of CRAM (%CRAM) increases from 33.1% to 76.7% with an increasing degree of DOM degradation and increasing salinity. Further, processing of DOM, including the more "biolabile" DOM with lower %CRAM released from aquitard, is more active under oxic conditions than under reducing conditions. Given the large quantities of sea water that recirculates through the sandy STEs globally, the amount of "recalcitrant" DOM (RDOM) entering the ocean after processing is likely to be considerable. While more studies are needed, the ocean can gain "recalcitrant" CRAM-like compounds in this way.

Use of Copper in Evaluating the Role of Phenolic Moieties in the Photooxidation of Dissolved Organic Matter

In a recent study, copper was shown to act as a novel quencher for investigating the mechanism of the photooxidation and photobleaching of dissolved organic matter (DOM) by selectively quenching the one-electron oxidizing intermediates of DOM (DOMD•+). However, the capture of DOMD•+ by Cu is possibly partially due to strong competition from phenolic antioxidant moieties intrinsically present in DOM for DOMD•+ quenching. In this study, the extent of interaction between DOMD•+ and phenolic antioxidant moieties is quantified by measuring the inhibitory effect of Cu on DOM photooxidation and photobleaching under varying pH (5.2-10.0) conditions. The increase in pH facilitates formation of deprotonated phenolic moieties (pKa ∼ 9-10), increasing their quenching capacity of DOMD•+. Accordingly, our results indicate that the inhibitory effect of Cu on the DOM photobleaching and the loss of electron-donating moieties of DOM significantly decreased with an increase in pH, suggesting more pronounced competition for DOMD•+ from antioxidant phenolic moieties within DOM. Considering the precursors of DOMD•+ also originate from phenolic moieties of DOM, the findings of this study provide important insights into the long-distance charge transfer reactions occurring at different phenolic moiety sites during DOM photooxidation.

Identifying the Molecular Signatures of Organic Matter Leached from Land-Applied Biosolids via 21 T FT-ICR Mass Spectrometry

Intensification of wastewater treatment residual (i.e., biosolid) applications to watersheds can alter the amount and composition of organic matter (OM) mobilized into waterways. To identify novel tracers of biosolids, characterization of biosolids and their impacts on OM composition in recipient ecosystems is required. Here, water-soluble OM was leached from surface soils from Florida pastures with differing levels of biosolid amendment and an adjacent control site. The biosolid endmember was further constrained by extracting water-soluble OM from biosolids sourced from four Florida wastewater treatment facilities. Nontargeted analysis of organic molecules by negative-ion electrospray ionization 21 T Fourier transform ion cyclotron resonance mass spectrometry examined the molecular composition of soil and biosolid leachates and identified molecular formulas unique to these biosolids and biosolid amended soils. Overall, biosolids leachates were enriched in aliphatic (+16.3% relative abundance) and heteroatomic (+42.5% RA) formulas and depleted in aromatic formulas (-33.5% RA) compared to soil leachates. A subset of 297 molecular formulas were present only in biosolids and amended soil leachates (i.e., not present in control soil leachates), the vast majority of which contained nitrogen (66%) or sulfur (27%). The identification of these molecular formulas is a key step in identifying novel tracers of biosolids movement through impacted watersheds.

Marine anoxia impede the transformation of dissolved organic carbon released by kelp into refractory dissolved organic carbon

The transformation of dissolved organic carbon (DOC) released by macroalgae into refractory dissolved organic carbon (RDOC) through microbial carbon pump (MCP) represents a crucial carbon sequestration process. This process mainly takes place in coastal areas, where it is likely affected by marine anoxia. The interactions between the components of DOC released by kelp and the community structure of heterotrophic bacteria both under normoxic and anoxic conditions were studied by three-dimensional fluorescence parallel factor analysis (PARAFAC), Fourier Transform-Ion Cyclotron Resonance-Mass Spectrometry (FT-ICR-MS) and 16S rRNA high-throughput sequencing. Following 240 days of decomposition, we found that the proportion of labile dissolved organic carbon (LDOC) was 4.61 % greater under anoxic conditions compared to normoxic conditions. Conversely, the proportion of RDOC was 8.06 % lower under anoxic conditions than under normoxic conditions. These findings suggest that anoxia hinders the conversion of LDOC to RDOC in the DOC released by kelp. Although normoxic conditions favor RDOC production, anoxic conditions could be more advantageous for the transport of DOC to the deep ocean, potentially enhancing carbon sequestration. The cultivation of macroalgae in anoxic zones may further boost their carbon sequestration potential.

Effect of marine anoxia on the conversion of macroalgal biomass to refractory dissolved organic carbon

The input of macroalgal biomass into the deep sea is a crucial process for macroalgal carbon sequestration, but this process may be affected by anoxia. We compared the breakdown of kelp biomass in both normoxic (>4 mg/L O2) and anoxic (<2 mg/L O2) environments. Following 240 days of decomposition experiment, complete degradation of the kelp biomass occurred in normoxic conditions, whereas under anoxic conditions, relatively 13.58% residual biomass remained. Our results suggest that microorganisms facilitated the conversion of dissolved organic carbon (DOC) derived from kelp degradation into refractory dissolved organic carbon (RDOC), a process observed under both normoxic and anoxic conditions. However, different dissolved oxygen levels lead to different bacterial community successions, which affected the conversion process from labile dissolved organic carbon (LDOC) to RDOC differently. Bacteroidia, which possess sulfur metabolic capabilities, play a significant role in RDOC generation under both normoxic and anoxic conditions. In normoxic conditions, the relative abundance of CHO molecules was 2.57% less than that under anoxic conditions, whereas the proportions of CHON was 3.83% higher. Additionally, DBEwa and Almodwa values were 11.04% and 15.63% higher than those observed under anoxic conditions. At the end of the experiment, the relative content of RDOC under normoxic and anoxic conditions was 9.18% and 5.45%, respectively. Despite the reduced production of RDOC, anoxic conditions promote the preservation of a larger amount of macroalgae biomass. However, uncertainty exists regarding the extent to which stored POC reaches deep-sea sequestration. Consequently, it is challenging to assert that anoxia positively influences carbon sequestration in macroalgae.

Hydroclimatic extremes threaten groundwater quality and stability

Heavy precipitation, drought, and other hydroclimatic extremes occur more frequently than in the past climate reference period (1961-1990). Given their strong effect on groundwater recharge dynamics, these phenomena increase the vulnerability of groundwater quantity and quality. Over the course of the past decade, we have documented changes in the composition of dissolved organic matter in groundwater. We show that fractions of ingressing surface-derived organic molecules increased significantly as groundwater levels declined, whereas concentrations of dissolved organic carbon remained constant. Molecular composition changeover was accelerated following 2018's extreme summer drought. These findings demonstrate that hydroclimatic extremes promote rapid transport between surface ecosystems and groundwaters, thereby enabling xenobiotic substances to evade microbial processing, accrue in greater abundance in groundwater, and potentially compromise the safe nature of these potable water sources. Groundwater quality is far more vulnerable to the impact of recent climate anomalies than is currently recognized, and the molecular composition of dissolved organic matter can be used as a comprehensive indicator for groundwater quality deterioration.

Factors affecting iron and manganese dissolution in groundwater: treatments with simultaneous oxidation and precipitation methods

This study explores variations in groundwater (GW) pH, conductivity, ammonium, iron, and manganese parameters to reveal prospective interactions having an impact on the dissolved metal concentrations. To this end, bivariate and partial correlation procedures were applied to the data to obtain incisive evaluation. Besides characterisation efforts, photocatalytic iron and manganese removal experiments were also carried out with Ni-doped TiO2 nano-composite thin films (TFs) on real GW samples. UV-A (365 nm) An LED array was used as the illumination source. The experimental setup was based on three treatment routes including photocatalytic oxidation (PCO), NaOH-aided precipitation and PCO with simultaneous precipitation (SPCO-P). The main statistical analysis and treatment efforts have been performed on data and samples of a single well, respectively (N = 15). However, extended statistical analysis has also been performed on larger data groups (N = 1366) obtained from different GW sources as well. Analytical results have revealed that about 90% of iron and manganese were in oxidised forms which do not precipitate by simple pH regulation. Statistical analysis has also revealed significant interactions between metal concentrations and observed parameters depending on the level of pH and conductivity. Furthermore, the SPCO-P strategy has provided a four-fold increase in reaction rate (pseudo-first-order, kobs: 0.04 min-1). Removal efficiencies of iron and manganese also increased from 10% to 96% - 85%, respectively.

Old but not ancient: Rock-leached organic carbon drives groundwater microbiomes

More than 90% of earth's microbial biomass resides in the continental subsurface, where sedimentary rocks provide the largest source of organic carbon (C). While many studies indicate microbial utilization of fossil C sources, the extent to which rock-organic C is driving microbial activities in aquifers remains largely unknown. Here we incubated oxic and anoxic groundwater with crushed carbonate rocks from the host aquifer and an outcrop rock of the unsaturated zone characterized by higher organic C content, and compared the natural abundance of radiocarbon (14C) of available C pools and microbial biomarkers. The ancient rocks surprisingly released organic substances with up to 72.6 ± 0.3% modern C into the groundwater, suggesting leachable fresh organic material from surface transport was preserved within rock fractures. Over half of the rock-leached compounds were also found in the original groundwater dissolved organic carbon (DOC), indicating in situ release of material stored in rock fractures through weathering processes. In addition to aliphatic and aromatic hydrocarbons, rock-leachates were rich in lipids, peptides, and carbohydrates. Radiocarbon analysis of phospholipid-derived fatty acids showed a rapid microbial response to this 'younger' organic material, comprising up to 31% (anoxic) and 51% (oxic) of their biomass C from the rock-leachate after 18 days of incubation. Predictive functional profiling of rock-enriched taxa, including species of Desulfosporosinus, Ferribacterium and Rhodoferax, also suggested metabolic potential for aliphatic and aromatic hydrocarbon degradation. PLFAs of the original groundwater were highly 14C-depleted, indicating utilization of a mixture of fossil and 'younger' C sources. Our findings suggest that carbonate rocks act as temporal sink for 'younger' organic matter, that leaches with fossil hydrocarbons from sedimentary rocks, driving microbial metabolism in subsurface ecosystems.

Cascade reservoirs affected chemical compositions of dissolved organic matter and greenhouse gas dynamics in the Lancang River

Dissolved organic matter (DOM) is an important component in aquatic systems. There has been much debate about the potential effects of cascade reservoirs on the transport and transformation of DOM. Here, through a survey of source to leave-boundary section of Lancang River (LCR) in June and November of 2017-2018, our results revealed that weak spatiotemporal variations were observed for DOC content, whereas DOM parameters were significantly different between natural and reservoir reaches. And DOM showed higher humification degree from allochthonous sources with increasing autochthonous matter in reservoir reach, may due to high particulate organic matter and releasing autochthonous DOM from phytoplankton blooms in the LCR, which can be evidenced by depleted DIC, enriched δ13CDIC and higher BIX. A unique fluorescent fraction (C5) appeared in the reservoir reach and increased along water flow, which was strongly associated with dissolved CO2 and N2O. Meanwhile, BIX value decreased with increasing dam height, hydraulic residence time (HRT), and reservoir capacity, which may promote CH4 production, highlighting variation of DOM compositions in understanding the effect of greenhouse gas (GHG) dynamics in the LCR. The findings were essential for comprehending the influences of cascade reservoirs on carbon cycle, and informed policy development for the sustainable management of transboundary water resources like the LCR.

Molecular-scale investigation on the photochemical transformation of dissolved organic matter after immobilization by iron minerals with FT-ICR MS

The interaction between dissolved organic matter (DOM) and iron minerals has a significant effect on its stabilization and preservation in the environment. In this study, iron minerals with different crystal forms (crystalline goethite and amorphous ferrihydrite) were selected to investigate the photochemical transformation process for DOM immobilized on iron minerals under simulated sunlight irradiation at the molecular scale with the help of Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The results showed that a total of 7148 molecules were detected in alkaline-extractable sedimentary DOM, of which 38.8% and 36.2% were adsorbed by ferrihydrite and goethite, respectively, while there was no selectivity difference between the two iron minerals in terms of DOM adsorption. After simulated sunlight irradiation, the DOM adsorbed by goethite was significantly degraded (58.3%), in which the H/C ratio of the mineral-immobilized DOM increased and the O/C ratio decreased, and the photodegradation primarily involved DOM molecules with high Kendrick mass defect (KMD) values. The results confirmed that the iron mineral types play an important role in the transportation and transformation of DOM, which adds to the understanding of the fate of DOM in natural environments.

Capturing differences in the release potential of dissolved organic matter from biochar and hydrochar: Insights from component characterization and molecular identification

Biochar and hydrochar have garnered widespread attention owing to their excellent performance in environmental remediation, carbon sequestration, and resource utilization from biowaste. Studies on the release potential of dissolved organic matter (DOM) have been limited, and the distinction between biochar and hydrochar remains unclear. In this study, pine sawdust was utilized as a model precursor with the aim of comparing the release quantity, components, and properties of DOM from biochar (BDOM) and hydrochar (HDOM) under various simulated conditions. The amount of DOM released by hydrochar (38.20-190.49 g/kg) was significantly greater than that released by biochar (0.57-11.96 g/kg), and more DOM was released at higher temperatures and pH values. BDOM consists of three categories of components, namely, humic-like, protein-like, and benzoic acid-like and tyrosine-like substances compounds, whereas HDOM consists of four categories of components, namely, two categories of humic-like compounds and two categories of protein-like compounds. By using ESI-FT-ICR-MS technology, 8586 compounds in BDOM and 6428 compounds in HDOM were identified. A total of 4665 unique compounds were found in BDOM, 1416 unique compounds were found in HDOM under alkaline release conditions, and HDOM contained more unique compounds than those found in other environments. CRAM/lignin-like compounds made up the majority of the released DOM and reached 31.01-65.35 % for BDOM and 54.79-73.05 % for HDOM. These findings revealed significant differences in the release potential of DOM from biochar and hydrochar, and further behavior research is needed to guide future applications of char materials in the environment and agriculture fields.

Controlling iron release and pathogenic bacterial growth in distribution pipes through nanofiltration followed by different disinfection methods

There is increasing concern about discoloration problems and microbial risks in drinking water. Until recently, how to control iron release and pathogenic bacterial growth in distribution pipes has been a knowledge gap. In our study, nanofiltration removed 13.3 % of lignins, 33.1 % of tannins and 17.7 % of proteins from dissolved organic matter (DOM). These DOM components were closely related to enzymes involved in the tricarboxylic acid (TCA) cycle. Therefore, nanofiltration followed by chlorine or chloramine disinfection inhibited the TCA cycle and induced lower adenosine triphosphate (ATP) and extracellular polymeric substance (EPS) production, resulting in reduced pathogenic bacterial growth. The number of Pseudomonas aeruginosa decreased to 7.43 × 105 and 2.43 × 105 gene copies/mL, respectively. Moreover, lower DOM concentrations increased the abundance of iron-reducing bacteria (IRBs) in the biofilm. IRBs can convert Fe(III) into Fe(II) through cellular c-type cytochromes, including CymA, MtrA, Cytc3, MacA, PpcA, and OcmB. The higher abundance of IRB and their cytochromes led to more Fe3O4 formation on the surface of the distribution pipes, resulting in lower iron release. The total iron concentration was 16.9 μg/L in the effluent of pipes treated with nanofiltration and chloramine disinfection. Therefore, nanofiltration followed by different disinfection methods, especially chloramine disinfection, effectively controlled iron release and pathogenic bacterial growth in distribution pipes. This study strongly contributes to maintaining the drinking water quality in distribution pipes.

Dissolved organic nitrogen depresses the expected outcome of wastewater treatment upgrading on effluent eutrophication potential mitigation: Molecular mechanistic insight

Continuously tightening total nitrogen (TN) discharge standards in wastewater treatment plants is a common practice worldwide to mitigate eutrophication. However, given the different bioavailability of effluent dissolved organic nitrogen (DON) and inorganic nitrogen, a great inefficiency of the TN-targeted upgrading might be hidden because of the poor understanding of its impact on effluent eutrophication potential mitigation. Here we show that the tightening TN discharge standards could only considerably promote inorganic nitrogen removal, however, DON concentrations remained constant across different effluent TN levels (p > 0.05, Kruskal-Wallis test). Surprisingly, restricting TN in turn increases the reactivity of DON molecules owing to the accumulation of produced DON by acting on the key biotic and abiotic transformation reactions. The difficulty of removing DON and the increased DON reactivity during wastewater treatment upgrading contribute to the practical elimination effect of effluent eutrophication potential exhibiting lower than expected. This work challenges the rationality of the prevailing pursuit for extreme-low TN discharge, calling for shifting the focus of wastewater treatment upgrading towards the more fundamental eutrophication-targeted perspective.

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.

On the FT-ICR mass spectrometry analysis of dissolved organic matter released by adsorbent during coal chemical wastewater treatment

This study analyzed the dissolved organic matter (DOM) released by adsorbent during wastewater treatment. It was found that the adsorption method resulted in an organic removal efficiency of over 97 % for coal-to-olefin (CTO) wastewater, with the lowest value of 15.7 mg/L. The Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) detected 4111 DOM in the wastewater, 4052 remaining DOM after first-stage anthracite (ANC) adsorption, and 1013 after second-stage macroporous adsorption resin (MAR). The removal degree of lipids in wastewater was the highest, followed by aliphatic/amino-acid/mini-peptides and lignin. During the adsorption process, the proportion of halogenated compounds (HCs) declined from 59.86 % to 38.63 % and 21.67 %. Additionally, freshly produced 2035 and 311 DOMs were found in the adsorption effluent of ANC and MAR, respectively, with HCs accounting for 34.71 % and 67.96 %. Upon flowing ultra-pure water through ANC and MAR, the effluent dissolved organic carbon (DOC) ranges were 1.118-3.574 mg/L and 1.014-2.557 mg/L, respectively. There were 159 and 131 species of DOM detected, respectively, with HCs content of 59.06 % and 45.02 %. Comparative experiments revealed the complex components of the wastewater promoting the release of organic matter on the adsorbent surface that further reacted to generate organic matter. However, fewer substances were released by the adsorbent.

Restoration-mediated protein substances preferentially drive underlying bauxite residue macroaggregate formation during the simulated ecological reconstruction process

Constructing a restoration strategy from bauxite residue to Technosols is a cost-effective and sustainable strategy for addressing the ecological and environmental issues caused by high alkalinity, salinity, and fine-grained bauxite residues. However, the quantitative contribution of restoration strategies on the upper bauxite residue-derived Technosols to the underlying untreated bauxite residue in the short term remains poorly understood. This study investigated the mediating mechanisms of vegetation and microbial metabolic effects on the alkalinity, nutrient content, and structure of the underlying bauxite residue (20-50 cm) through a simulated ecological reconstruction of the bauxite residue stockpile. Results indicated that implementing plant restoration strategies resulted in the content of polyphenolic compounds, lipids, tannins, and carbohydrates in bauxite residue dissolved organic matter (DOM) increased significantly from 52.5, 8.2, 3.3, and 2.0 % to 54.4, 10.4, 5.6, and 2.8 %, respectively, while the content of condensed aromatics, unsaturated hydrocarbons, and proteins/amino sugars decreased significantly from 15.5, 12.0, and 6.5 % to 12.1, 9.7, and 5.1 %, respectively. The newly produced molecules were concentrated in regions with low O/C and high H/C ratios, suggesting that short-term vegetation restoration strategies facilitate the transformation of substrate DOM towards easily decomposable and highly bioavailable substances. This led to the migration of the newly produced molecules to the underlying bauxite residue, and as a result, the protein and soluble microbial products of the underlying bauxite residue increased significantly, as well as the pH, exchangeable Na, and < 0.054 mm particles decreased from 10.2, 44.2 cmol kg-1, and 28.1 % to 9.7, 27.1 cmol kg-1, and 19.4 %, respectively, available nitrogen, urease, and 1-2 mm particles increased from 7.3 mg kg-1, 0.2 U mg-1, and 14.5 % to 7.6 mg kg-1, 0.3 U kg-1, and 21.7 %, respectively. Results of the structural equation model further confirmed that plant biomass, proteins/amino sugars, and condensed aromatics in the upper Technosol were the main factors controlling the aggregate formation of the underlying bauxite residue by mediating the protein-dominated biogenic organic matter produced by microbial metabolism.

The nexus between aeration intensity and organic carbon capture in contact-stabilization process: Insights from molecular structure transition of dissolved organic matters

Traditional energy-intensive pollution control pattern poses great challenges to the sustainable development of urban cities, necessitating the implementation of more compact and cost-effective biological treatment technology. High-rate contact stabilization (HiCS) process can effectively capture low-concentration organic carbon matters from municipal wastewater. However, the role of dissolved oxygen (DO) concentration at stabilization phase-a critical determinant of carbon capture efficiency-remains poorly understood, thus hindering its operation optimization and application. This work investigated the impact of DO content at the stabilization phase on the effluent quality and carbon capture efficiency of HiCS process from the perspectives of sludge dissolved organic matter (DOM) composition and microbial metabolism activity changes. The results showed that optimal carbon capture efficiency (52.1 %) and the lowest effluent chemical oxygen demand concentration were achieved at a DO concentration of 1 mg/L. Elevated DO levels would increase the aromaticity of DOM in sludge, rendering it more recalcitrant to microbial degradation. In addition, higher DO concentration induced a metabolic shift towards endogenous respiration among the microbial community, leading to the increased release of DOM and microbial metabolites, which in turn deteriorated the effluent quality. The findings of this work highlight the necessity of controlling appropriate aeration intensity when applying HiCS in practical application, to both effectively minimize organic carbon mineralization and operational energy consumption while without sacrificing pollutant removal performance.

Thermodynamics and explainable machine learning assist in interpreting biodegradability of dissolved organic matter in sludge anaerobic digestion with thermal hydrolysis

Dissolved organic matter (DOM) is essential in biological treatment, yet its specific roles remain incompletely understood. This study introduces a machine learning (ML) framework to interpret DOM biodegradability in the anaerobic digestion (AD) of sludge, incorporating a thermodynamic indicator (λ). Ensemble models such as Xgboost and LightGBM achieved high accuracy (training: 0.90-0.98; testing: 0.75-0.85). The explainability of the ML models revealed that the features λ, measured m/z, nitrogen to carbon ratio (N/C), hydrogen to carbon ratio (H/C), and nominal oxidation state of carbon (NOSC) were significant formula features determining biodegradability. Shapley values further indicated that the biodegradable DOM were mostly formulas with λ lower than 0.03, measured m/z value higher than 600 Da, and N/C ratios higher than 0.2. This study suggests that a strategy based on ML and its explainability, considering formula features, particularly thermodynamic indicators, provides a novel approach for understanding and estimating the biodegradation of DOM.

Groundwater-derived carbon stimulates headwater stream CO2 emission potential on the Qinghai-Tibet Plateau

CO2 emissions from headwater streams are a crucial component of greenhouse gas flux in inland waters. However, the influence of groundwater, a major contributor to streams in the Asian Water Tower (Qinghai-Tibet Plateau, QTP), on CO2 levels remains unclear. This study employed stable isotope analysis and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to demonstrate that groundwater-derived dissolved inorganic carbon (DIC) significantly enhanced CO2 supersaturation in the Shuiluo stream on the QTP. Specifically, the partial pressure of CO2 (pCO2), indicative of CO2 emission potential, increased more than threefold to 1,615 ± 495 μatm in groundwater-rich sites, nearly one time higher than the mean value (843 μatm) across the QTP. Groundwater-derived carbonate weathering had a significant impact of 76.6 % on the increased pCO2, whereas the degradation of highly unsaturated polyphenolics with high O/C contributed to 15.8 %. The estimated inflow of groundwater-derived DIC could reach 9.59 ± 0.34 Tg C/y in total runoff across the QTP, highlighting significant CO2 sources. This study presents new findings on the effects of groundwater-derived DIC on stream CO2 emissions in weathered regions and expands our knowledge of fluvial CO2 emissions on the QTP.

Constraints on vertical variability of geogenic ammonium in multi-layered aquifer systems

The elevated levels of geogenic (natural) ammonium in groundwater have been frequently documented in recent years. Although improving insights have been achieved in understanding the genesis of ammonium in the subsurface environment, the vertical variability of the geogenic ammonium in groundwater remains poorly understood. Here, we selected typical multi-layered aquifer systems in the central Yangtze River plain and characterized the vertical heterogeneity of geogenic ammonium through the hydrogeochemical analysis. Subsequently, the controlling factors were identified by examining the molecular composition of dissolved organic matter (DOM) and aquifer sediment features. The results indicated that the ammonium concentration in groundwater increased from the deep to shallow aquifers (2.13 to 9.88 mg/L as N), accompanied by a transition in organic matter (OM) degradation towards the methanogenic stage (δ13C-DIC: -23.07 to -0.34‰). Compared to the deeper aquifers, the DOM in the shallow aquifer was characterized by a higher abundance of the N-containing OM (15.1% > 13.13% > 12.76%) with a lower molecular lability index, corresponding to more thorough degradation extent. The characteristics of the soluble OM in depth-matched sediments were similar to those of the DOM in groundwater, suggesting the persistent water-rock interactions. Besides, the pumping tests revealed that the hydraulic conductivity decreased from deep to shallow aquifers (2.28 to 0.62 m/d), which further facilitated the more retention of geogenic ammonium in the shallow aquifer. That is, the combined effects of the abundant N-containing OM in sediments, strong degradation of the bioactive DOM, and long retention governed by hydrodynamics contributed to the increased ammonium enrichment in the shallow aquifer, thereby generating the vertical variability. The findings underscore the significance of the complex coupled factors in controlling the vertical distribution of geogenic ammonium in multi-layered aquifer systems, which was crucial for understanding the spatial heterogeneity of geogenic contaminated groundwater.

Revealing degradation pathways of soluble and dissolved organic matter in alluvial-lacustrine aquifer systems impacted by high levels of geogenic ammonium

The excessive presence of geogenic ammonium (NH4+) in groundwater poses a global environmental concern, commonly linked to the degradation of nitrogen-containing dissolved organic matter (DOM). However, there is a gap in systematic studies on the combination of soluble organic matter (SOM) in sediments and DOM in groundwater, with few indoor incubation experiments to validate their degradation pathways. This study utilized ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry to analyze the molecular characteristics of DOM and SOM in aquifer systems affected by geogenic NH4+. Subsequently, indoor incubation experiments spanning up to 140 d were conducted to verify the degradation pathways. The experimental results revealed a two-phase degradation process for both the DOM and SOM. The initial stage was characterized by the degradation of aliphatic compounds (ALC) with the production of polyphenols (PPE) and highly unsaturated compounds (HUC). The second stage was dominated by the degradation of PPE and HUC, accompanied by the re-consumption of some ALC, while more recalcitrant HUC persisted. Notably, the first stage of SOM degradation exceeded that of DOM degradation, indicating that SOM exhibited greater resistance to aging. This phenomenon may be attributed to a wider range of active enzymes in sediments, the rapid replenishment of SOM by organic matter in sediments, or the accelerated degradation of DOM. The experimental results aligned with the molecular characterization of DOM and SOM in actual aquifer systems. It is hypothesized that NH4+ produced through the direct mineralization of SOM may contribute more to the enrichment of NH4+ in groundwater than that produced through the mineralization of DOM. This study is the first to analyze DOM and SOM together in aquifer systems and validate their degradation pathways through incubation experiments, thereby providing novel insights into the enrichment of geogenic NH4+ in groundwater.

A comprehensive conceptual framework for signaling in-lake CO2 through dissolved organic matter

Organic carbon (C) and CO2 pools are closely interactive in aquatic environments. While there are strong indications linking freshwater CO2 to dissolved organic matter (DOM), the specific mechanisms underlying their common pathways remain unclear. Here, we present an extensive investigation from 20 subtropical lakes in China, establishing a comprehensive conceptual framework for identifying CO2 drivers and retrieving CO2 magnitude through co-trajectories of DOM evolution. Based on this framework, we show that lake CO2 during wet period is constrained by a combination of biogeochemical processes, while photo-mineralization of activated aromatic compounds fuels CO2 during dry period. We clearly determine that biological degradation of DOM governs temporal variations in CO2 rather than terrestrial C inputs within the subtropical lakes. Specifically, our results identify a shared route for the uptake of atmospheric polycyclic aromatic compounds and CO2 by lakes. Using machine learning, in-lake CO2 levels are well modelled through DOM signaling regardless of varying CO2 mechanisms. This study unravels the mechanistic underpinnings of causal links between lake CO2 and DOM, with important implications for understanding obscure aquatic CO2 drivers amidst the ongoing impacts of global climate change.

Research on a harmless treatment method for oily sludge in coal chemical wastewater and the pollutant transformation mechanism of oily sludge during the treatment process

This study developed an ultrasound synergistic subcritical hydrothermal treatment method (U-SHT) to address the challenges posed by the high oil and water content, complex composition, and hazardous nature of oily sludge (OS) generated during the pretreatment of coal chemical wastewater. The study investigated the efficiency of this method for the harmless disposal and resource recovery of OS, and the migration-transformation mechanism of hazardous organic pollutants in OS. The findings revealed that U-SHT achieved a removal efficiency of chemical oxygen demand in OS of 91.16 %, an oil resource recovery efficiency of 96.60 %, and a residual oil rate of 0.28 %, meeting API emission standards. Further research indicated that the solubilizing effect of the surfactant on the oil enhanced the demulsifying effect of ultrasonic cavitation on the emulsified structure of OS, enabling ultrasound to efficiently release and disperse pollutants within OS. This promoted the decomposition and transformation of pollutants under subcritical hydrothermal conditions, with synergistic removal efficiencies for typical pollutants such as long-chain alkanes, polycyclic aromatic hydrocarbons, and phenols reaching 96.61 %, 97.63 %, and 97.76 %, respectively. Economic evaluation indicated that the cost of OS treatment was $29.66/m3, significantly lower than existing methods, demonstrating promising practical application prospects.

Carbon-fixing bacteria in diverse groundwaters of karst area: Distribution patterns, ecological interactions, and driving factors

The biological carbon pump in karst areas is of great significance for maintaining the effectiveness of karst carbon sinks. However, the spatial distribution and carbon-fixing potential of microorganisms in different aquifers within karst areas remain poorly understood. In this study, the distribution patterns, ecological roles, and environmental drivers of microbiota associated with CO2 fixation were investigated in karst groundwater (KW), porous groundwater (PW), fractured groundwater (FW), and surface water (SW) within a typical karst watershed, located in Guilin, southwest China. KW, PW, and FW displayed the similar community structure and indicative carbon-fixing bacteria composition, which were dominated by chemoautotrophic bacteria compared to SW. Higher abundances of indicative carbon-fixing bacteria and carbon-fixing genes, as well as richer proportions of microbial-derived DOC, indicated the more significant microbial carbon-fixing potential in KW and PW. At the profile of KW, a carbon-fixing hotspot was discovered at the depths of 0-50 m. Correlation analysis between carbon-fixing bacteria and DOC revealed that the chemoautotrophic process driven by nitrogen and sulfur oxidation predominated the microbial carbon fixation in groundwater. Co-occurrence network analysis demonstrated that carbon-fixing bacteria exhibited cooperation with other bacterial taxa in KW, while competition was the dominant interaction in PW. Moreover, carbon-fixing bacteria was found to lead bacterial assembly more deterministic in KW. The analysis of environmental factors and microbial diversity illustrated that inorganic carbon and redox state drove community variations across groundwaters. Structural equation model (SEM) further confirmed that ORP was the primary factor influencing the carbon fixation potential. This study provides a new insight into biological carbon fixation in karst aquatic systems, which holds significance in the accurate assessment of karst carbon sinks.

S-containing molecular markers of dissolved organic carbon attributing to riverine dissolved methane production across different land uses

The emission of methane (CH4) from streams and rivers contributes significantly to its global inventory. The production of CH4 is traditionally considered as a strictly anaerobic process. Recent investigations observed a "CH4 paradox" in oxic waters, suggesting the occurrence of oxic methane production (OMP). Human activities promoted dissolved organic carbon (DOC) in streams and rivers, providing significant substrates for CH4 production. However, the underlying DOC molecular markers of CH4 production in river systems are not well known. The identification of these markers will help to reveal the mechanism of methanogenesis. Here, Fourier transform ion cyclotron mass spectrometry and other high-quality DOC characterization, ecosystem metabolism, and in-situ net CH4 production rate were employed to investigate molecular markers attributing to riverine dissolved CH4 production across different land uses. We show that endogenous CH4 production supports CH4 oversaturation and positively correlates with DOC concentrations and gross primary production. Furthermore, sulfur (S)-containing molecules, particularly S-aliphatics and S-peptides, and fatty acid-like compounds (e.g., acetate homologs) are characterized as markers of water-column aerobic and anaerobic CH4 production. Watershed characterization, including riverine discharge, allochthonous DOC input, turnover, as well as autochthonous DOC, affects the CH4 production. Our study helps to understand riverine aerobic or anaerobic CH4 production relating to DOC molecular characteristics across different land uses.

Short-Term Groundwater Level Fluctuations Drive Subsurface Redox Variability

As global change processes modify the extent and functions of terrestrial-aquatic interfaces, the variability of critical and dynamic transitional zones between wetlands and uplands increases. However, it is still unclear how fluctuating water levels at these dynamic boundaries alter groundwater biogeochemical cycling. Here, we used high-temporal resolution data along gradients from wetlands to uplands and during fluctuating water levels at freshwater coastal areas to capture spatiotemporal patterns of groundwater redox potential (Eh). We observed that topography influences groundwater Eh that is higher in uplands than in wetlands; however, the high variability within TAI zones challenged the establishment of distinct redox zonation. Declining water levels generally decreased Eh, but most locations exhibited significant Eh variability, which is associated with rare instances of short-term water level fluctuations, introducing oxygen. The Eh-oxygen relationship showed distinct hysteresis patterns, reflecting redox poising capacity at higher Eh, maintaining more oxidizing states longer than the dissolved oxygen presence. Surprisingly, we observed more frequent oxidizing states in transitional areas and wetlands than in uplands. We infer that occasional oxygen entering specific wetland-upland boundaries acts as critical biogeochemical control points. High-resolution data can capture such rare yet significant biogeochemical instances, supporting redox-informed models and advancing the predictability of climate change feedback.

Overlooked Roles and Transformation of Carbon-Centered Radicals Produced from Natural Organic Matter in a Thermally Activated Persulfate System

The presence and induced secondary reactions of natural organic matter (NOM) significantly affect the remediation efficacy of in situ chemical oxidation (ISCO) systems. However, it remains unclear how this process relates to organic radicals generated from reactions between the NOM and oxidants. The study, for the first time, reported the vital roles and transformation pathways of carbon-centered radicals (CCR•) derived from NOM in activated persulfate (PS) systems. Results showed that both typical terrestrial/aquatic NOM isolates and collected NOM samples produced CCR• by scavenging activated PS and greatly enhanced the dehalogenation performance under anoxic conditions. Under oxic conditions, newly formed CCR• could be oxidized by O2 and generate organic peroxide intermediates (ROO•) to catalytically yield additional •OH without the involvement of PS. Nuclear magnetic resonance (NMR) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) results indicated that CCR• predominantly formed from carboxyl and aliphatic structures instead of aromatics within NOM through hydrogen abstraction and decarboxylation reactions by SO4•- or •OH. Specific anoxic reactions (i.e., dehalogenation and intramolecular cross-coupling reactions) further promoted the transformation of CCR• to more unsaturated and polymerized/condensed compounds. In contrast, oxic propagation of ROO• enhanced bond breakage/ring cleavage and degradation of CCR• due to the presence of additional •OH and self-decomposition. This study provides novel insights into the role of NOM and O2 in ISCO and the development of engineered strategies for creating organic radicals capable of enhancing the remediation of specific contaminants and recovering organic carbon.

Metabolic adaptations underpin high productivity rates in relict subsurface water

Groundwater aquifers are ecological hotspots with diverse microbes essential for biogeochemical cycles. Their ecophysiology has seldom been studied on a basin scale. In particular, our knowledge of chemosynthesis in the deep aquifers where temperatures reach 60 °C, is limited. Here, we investigated the diversity, activity, and metabolic potential of microbial communities from nine wells reaching ancient groundwater beneath Israel's Negev Desert, spanning two significant, deep (up to 1.5 km) aquifers, the Judea Group carbonate and Kurnub Group Nubian sandstone that contain fresh to brackish, hypoxic to anoxic water. We estimated chemosynthetic productivity rates ranging from 0.55 ± 0.06 to 0.82 ± 0.07 µg C L-1 d-1 (mean ± SD), suggesting that aquifer productivity may be underestimated. We showed that 60% of MAGs harbored genes for autotrophic pathways, mainly the Calvin-Benson-Bassham cycle and the Wood-Ljungdahl pathway, indicating a substantial chemosynthetic capacity within these microbial communities. We emphasize the potential metabolic versatility in the deep subsurface, enabling efficient carbon and energy use. This study set a precedent for global aquifer exploration, like the Nubian Sandstone Aquifer System in the Arabian and Western Deserts, and reconsiders their role as carbon sinks.

Improved permeability in ceramsite@powdered activated carbon (PAC)-MnOx coupled gravity-driven ceramic membrane (GDCM) for manganese and ammonia nitrogen removal with intermittent short-term vertical aeration

In our work, a gravity-driven ceramic membrane bioreactor (GDCMBR) was developed to remove Mn2+ and NH3-N simultaneously through the birnessite water purification layer in-situ construction on the ceramic membrane due to chemical pre-oxidation (powdered activated carbon (PAC)-MnOx). Considering the trade-off of biofouling and water production, the daily intermittent short-term vertical aeration mode was involving to balance this contradiction with the excellent water purification and improved membrane permeability. And the GDCMBR permeability of operation flux was improved for 5-7 LHM with intermittent short-term vertical aeration. Furthermore, only ∼7 % irreversible membrane resistance (Rir) also confirmed the improved membrane permeability with intermittent short-term vertical aeration. And some manganese oxidizing bacteria (MnOB) and ammonia oxidizing bacteria (AOB) species at genus level were identified during long-term operation with the contact circulating flowing raw water, resulting in the better Mn2+ and NH3-N removal efficiency. Additionally, the nano-flower-like birnessite water purification layer was verified in ceramsite@PAC-MnOx coupled GDCMBR, which evolute into a porous flake-like structure with the increasing intermittent short-term aeration duration. Therefore, the sustainable and effective intermittent short-term aeration mode in ceramsite@PAC-MnOx coupled GDCMBR could improve the membrane permeability with the satisfactory groundwater purification efficiency, as well as providing an energy-efficient strategy for membrane technologies applications in water supply safety.

Purifying surface water contaminated with azo dyes using nanofiltration: Interactions between dyes and dissolved organic matter

In this research, the interactions of two azo dyes, Methyl Orange (MO) and Eriochrome Black T (EBT), with dissolved organic matter (DOM) in surface water were studied, emphasizing their removal using nano-filtration membranes (NF-270 and NF-90). High-Performance Size Exclusion Chromatography (HPSEC) findings indicated that the dyes' molecular weight in deionized (DI) water ranged from 500 to 15k Dalton (Da), adjusting peak intensities with Jingmi River (JM) water Beijing. Notably, when dyes were diluted in JM water, ultraviolet (UV533 & 466, and UV254), together with total organic carbon (TOC) parameters, revealed color removal rates of 99.49% (EBT), 94.2% (MO), 87.6% DOM removal, and 86% TOC removal for NF-90. The NF-90 membrane demonstrated a 75% flux decline for 50 mL permeate volume due to its finer pore structure and higher rejection effectiveness. In contrast, the NF-270 membrane showed a 60% decline in flux under the same conditions. Attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) analysis of dye-treated membranes in JM water revealed that the NF-270 showed a CC bond peak at 1660 cm-1 across various samples, while analyzing NF-90, the peaks at 1400 cm-1, 1040 cm-1, 750 cm-1, and 620 cm-1 disappeared for composite sample removal. The hydrophobicity of each membrane is measured by the contact angle (CA), which identified that initial CAs for NF-270 and NF-90 were 460 and 700, respectively, that were rapidly declined but stabilized after a few seconds of processing. Overall, this investigation shows that azo dyes interact with DOM in surface waters and enhance the removal efficiency of NF membranes.

Natural Attenuation of Groundwater Uranium in Post-Neutral-Mining Sites Evidenced from Multiple Isotopes and Dissolved Organic Matter

Although natural attenuation is an economic remediation strategy for uranium (U) contamination, the role of organic molecules in driving U natural attenuation in postmining aquifers is not well-understood. Groundwaters were sampled to investigate the chemical, isotopic, and dissolved organic matter (DOM) compositions and their relationships to U natural attenuation from production wells and postmining wells in a typical U deposit (the Qianjiadian U deposit) mined by neutral in situ leaching. Results showed that Fe(II) concentrations and δ34SSO4 and δ18OSO4 values increased, but U concentrations decreased significantly from production wells to postmining wells, indicating that Fe(III) reduction and sulfate reduction were the predominant processes contributing to U natural attenuation. Microbial humic-like and protein-like components mediated the reduction of Fe(III) and sulfate, respectively. Organic molecules with H/C > 1.5 were conducive to microbe-mediated reduction of Fe(III) and sulfate and facilitated the natural attenuation of dissolved U. The average U attenuation rate was -1.07 mg/L/yr, with which the U-contaminated groundwater would be naturally attenuated in approximately 11.2 years. The study highlights the specific organic molecules regulating the natural attenuation of groundwater U via the reduction of Fe(III) and sulfate.

Regular Tetrahedron Model for the Assessment of High-Resolution Mass Spectrometry Data of Four-Way Fractionated Dissolved Organic Matter

A regular tetrahedron model was established to pierce the fractionation of dissolved organic matter (DOM) among quaternary components by using high-resolution mass spectrometry. The model can stereoscopically visualize molecular formulas of DOM to show the preference to each component according to the position in a regular tetrahedron. A classification method was subsequently developed to divide molecular formulas into 15 categories related to fractionation ratios, the relative change of which was demonstrated to be convergent with the uncertainty of mass peak area. The practicality of the regular tetrahedron model was verified by seven kinds of sludge from waste leachate treatment and sewage wastewater treatment plants by using stratification of extracellular polymeric substances coupled with Orbitrap MS as an example, presenting the DOM chemodiversity in stratified sludge flocs. Sensitivity analysis proved that classification results were relatively stable with the perturbation of four model parameters. Multinomial logistic regression analysis could further help identify the effect of molecular properties on the fractionation of DOM based on the classification results of the regular tetrahedron model. This model offers a methodology for the assessment of specificity of sequential extraction on DOM from solid or semisolid components and simplifies the complex mathematical expression of fractionation coefficients for quaternary components.

Effect of Land Use Change on Molecular Composition and Concentration of Organic Matter in an Oxisol

Land use change from native vegetation to cropping can significantly affect the quantity and quality of soil organic matter (SOM). However, it remains unclear how the chemical composition of SOM is affected by such changes. This study employed a sequential chemical extraction to partition SOM from an Oxisol into several distinct fractions: water-soluble fractions (ultrapure water (W)), organometal complexes (sodium pyrophosphate (PP)), short-range ordered (SRO) oxides (hydroxylamine-HCl (HH)), and well-crystalline oxides (dithionite-HCl (DH)). Coupled with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS), the impact of land use change on the molecular composition of different OM fractions was investigated. Greater amounts of OM were observed in the PP and HH fractions compared to other fractions, highlighting their importance in SOM stabilization. The composition of different OM fractions varied based on extracted phases, with lignin-like and tannin-like compounds being prevalent in the PP and HH fractions, while aliphatic-like compounds dominated in the DH fraction. Despite changes in the concentration of each OM fraction from native vegetation to cropping, there was little influence of land use change on the molecular composition of OM associated with different mineral phases. No significant selective loss or preservation of organic carbon compounds was observed, indicating the composition of SOM remained unchanged.

Photochemical Transformation of Monochloramine Induced by Triplet State Dissolved Organic Matter

The photoexcited dissolved organic matter (DOM) could produce reactive intermediates, affecting chemical oxidant transformation in UV based advanced oxidation processes (AOPs). This study confirmed the critical role of triplet state DOM (3DOM*), generated from DOM photoexcitation, in the transformation of monochloramine (NH2Cl), a commonly used chemical oxidant and disinfectant in water treatment. NH2Cl (42.25 μM, as Cl2) was decayed by 17.4-73.4 % within 60 min, primarily due to 3DOM* , in DOM (2-30 mgC L-1) solutions irradiated by 365 nm, where NH2Cl has no absorption. The second-order quenching rate constants of triplet state model photosensitizers by NH2Cl were determined to be 0.95(± 0.04)-4.49(± 0.04)× 108 M-1 s-1 by using laser flash photolysis. As a reductant, 3DOM* reacted with NH2Cl through one-transfer mechanism, leading to amino radical (NH2•) generation, which then transferred to ammonia (NH4+, pKa 9.25) through H-abstraction by the phenolic moieties in DOM. Additionally, the intermediate product of 3DOM* oxidized by NH2Cl or those triplet state quinones can hydrolyze to form phenolic moieties, elevating NH4+ yield to higher than 99% upon 365 nm irradiation. These findings suggest that the widespread DOM can be applied to convert NH2Cl via 3DOM* with minimal toxic risks.

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.