Properties and metal binding behaviors of sediment dissolved organic matter (SDOM) in lakes with different trophic states along the Yangtze River Basin: A comparison and summary

Influence of the source and molecular weights on sulfathiazole/sulfadiazine binding in sedimentary dissolved organic matter

Sedimental dissolved organic matter (DOM) plays a crucial role in the migration of antibiotics in a lake environment, which is strongly associated with its physicochemical properties influenced by temporal and spatial variations. This study systematically investigated the binding behavior of sulfathiazole (ST) and sulfadiazine (SD) to DOM across different molecular weights (MWs) in sediments from Poyang Lake using multiple spectroscopic techniques. Results showed that the MW fractions of DOM from the onset of the dry season were higher than those before flooding. Additionally, key carbon parameters, including dissolved organic carbon and fluorescent components, were present in greater proportion in the truly dissolved phase and the low MW fraction of DOM. These properties of the low MW fraction of DOM enhanced its binding capacity for the two sulfonamides, with the partitioning coefficient (logKcoc) values of 5.20 and 5.06 for ST and SD, respectively. Dialysis experiments investigating the interaction between different concentrations of DOM and sulfonamides indicated that humic-like and protein-like components significantly influenced the migration of ST and SD, respectively. Moreover, protein-like components exerted a more substantial impact on the migration of sulfonamides in heavily polluted sampling sites, while the humic-like substances played a more significant role for the samples from the wetland protection area. The results enhanced the understanding of the significance of DOM sources and MW in influencing the environmental fate of antibiotics in lake ecosystems.

Characterization of complexation between biochar-derived dissolved organic matter and two typical heavy metals by a multi-spectral technique

Biochar-derived dissolved organic matter (BDOM) is important in the migration and transformation of heavy metals (HMs) within aqueous environments by directly influencing their morphologies and environmental behavior. This study systematically investigated the binding characteristics and mechanisms between biochar-derived dissolved organic matter (BDOM) and heavy metals (Pb(II)/Cd(II)) using multi-spectroscopic techniques. BDOM extracted from four biochar types (pig manure, cow manure, corn stalk, and sewage sludge) was characterized by UV-Vis, EEM-PARAFAC, SFS, FTIR, and 2D-COS analyses. Results revealed that BDOM primarily consisted of three humic-like components and one protein-like component, with sewage sludge BDOM exhibiting the highest fluorescence intensity. Metal titration experiments demonstrated stronger fluorescence quenching for Pb(II) (logKM = 3.89-5.15) than Cd(II), though the Ryan-Weber model only successfully fitted Pb(II) binding parameters. 2D-COS analysis identified polysaccharides as the primary binding sites, followed by aliphatic compounds. Notably, despite their low abundance, protein-like components and non-fluorescent polysaccharides played significant roles in HMs binding. These findings provide molecular-level insights into the source-dependent interactions between BDOM and HMs, offering a theoretical basis for predicting HMs mobility and transformation in biochar-amended environments.

Insights into effects of drying-wetting cycles on dissolved organic matter and Cd bioavailability in riparian sediments amended with microplastics

Microplastics (MPs) affect the fractionation of cadmium (Cd) by altering physicochemical properties of sediment. However, differences in the effect of drying-wetting cycles on the bioavailability of heavy metals and dissolved organic matter (DOM) in sediment amended with MPs remain unclear. In this study, we investigated the influence of polystyrene (PS), polylactic acid (PLA), and tire wear particles (TWPs) at concentrations of 0.5% and 5% (w/w) on Cd fractionation and DOM properties of sediment under drying-wetting alternation conditions. The results showed that the surface of MPs displayed obvious increase in surface roughness and oxygen-containing functional groups after drying-wetting cycles. At the end of drying-wetting cycles, 0.5% and 5% MPs significantly decreased the sediment pH. And 5% PLA MPs and TWPs significantly increased the content of dissolved organic carbon (DOC). Besides, Cd bioavailability showed a slight decrease under wet conditions. However, 5% PLA MPs significantly increased F1 fraction by 7.82%-13.50%, which may be related to the release of additives. The aromaticity and humification degree of sediment-derived DOM were improved under drying-wetting alternation conditions. Further correlation analysis indicated that MPs indirectly affected Cd fractionation by influencing the sediment DOM properties. This study provides a new perspective for understanding the influence of MPs on the relationship between DOM and heavy metals in riparian sediments under typical hydrological condition.

Molecular-level insights of microplastic-derived soluble organic matter and heavy metal interactions in different environmental occurrences through EEM-PARAFAC and FT-ICR MS

The interactions between microplastic-derived dissolved organic matter (MPs-DOM) and heavy metals (Cu, Pb, and Cd) regulate the complex environmental transport behavior of pollutants in terrestrial and aquatic environments. In this study, fluorescence excited emission matrix spectroscopy combined with parallel factor analysis (EEM-PARAFAC) and electrospray ionization coupled Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) were employed to investigate the complexation mechanism of MPs-DOM with heavy metals, as well as the effects of different environmental occurrences of MPs-DOM on the transport behaviors of heavy metals in saturated porous medium. The findings demonstrated that MPs-DOM, particularly humic-like substances containing aromatic structures and various oxygen functional groups, could form stable complexes with heavy metals. This interaction significantly altered the transport capacity of Pb and Cu in saturated porous media. It is noteworthy that MPs-DOM in the free and deposited states in the environment may have markedly disparate effects on heavy metal transport. MPs-DOM in the free state may facilitate the co-migration of heavy metal ions in porous media, thereby enhancing the mobility of heavy metals. In contrast, sedimentary-state MPs-DOM can retain heavy metals in porous media and inhibit their migration through complexation with them.

Implications of the different submerged extent of typical sandbars to local DOM and SDOM during the pre-flood season in the middle reaches of Yangtze River

In large rivers, sandbar evolution is mainly driven by changes in discharge, flow velocity, and sediment concentration in the water, which can shape the implications of local dissolved organic matter (DOM) and sediment dissolved organic matter (SDOM). After the completion of the Three Gorges Dam (TGD), the water level in the middle reaches of the Yangtze River changes rapidly during the year, and the submerged extent of the Yangtze River sandbars are significantly affected by the change in water level, so that when the water level rises to the defense level or higher, the sandbars may be completely submerged. As a result, different submerged extent such as periodically and long-term have occurred on the typical sandbars. However, the extent to which the submersion of typical sandbars affects the source, composition and content of regional DOM/SDOM in large rivers remains unclear. This study was conducted in the middle reaches of the Yangtze River, the characteristics, sources, and influencing factors of DOM were studied using methods such as three-dimensional fluorescence spectrum (EEMS) and parallel factor analysis (PARAFAC) to ascertain the response mechanisms of DOM/SDOM distribution patterns in different submerged extents of the Sanba Sandbar (SBS) and Wugui Sandbar (WGS). The results indicated that the source and component characteristics of DOM/SDOM from different submerged extents in typical sandbars exhibited some differences and similarities. The sediment conditions of the local sandbars, physicochemical parameters of the water, and human activities jointly controlled the DOM/SDOM characteristics of the local sandbars. The DOM of long-term submerged sandbars is generated by mixed endogenous and exogenous inputs; its fractions are more complex, and protein-like proteins are the main components; however, periodically submerged sandbars are dominated by inputs from terrestrial sources, and DOM is relatively more strongly humified, hydrophobic, and aromatic. Sediment dissolved organic matter (SDOM) is an important organic component in sediments, which has an important impact on the ecological environment of the water, the material cycle and the migration of pollutants. The endogenous characteristics of SDOM of sandbars with different submerged extents are evident, mostly in the form of land-source inputs; SDOM is similarly dominated by protein-like humus with a relatively high percentage on periodically submerged sandbars. However, the SDOM component of long-term submerged sandbars is more complex than that of periodically submerged sandbars, and SDOM has more hydrophobic and aromatic properties. This study is the first to analyze the DOM and SDOM of typical sandbars in the middle reaches of the Yangtze River, offering new insights into the characteristics and influencing factors of DOM and SDOM in this region, as well as its distribution in large rivers.

Difference analysis of organic matter-mediated heavy metal pollution in the sediments of urban water bodies

Metal contents, forms, risks, sources, and their correlation with organic matter in the sediments of Beijing and Wuhan were studied to assess the difference in metal pollution between northern and southern China, as well as the associated coupling mechanisms with organic matter. The contents of Cr, Cu, Ni, and Zn in overlying water in Beijing were significantly higher than those in Wuhan (p < 0.01). Conversely, the Ni content in the sediments of Wuhan was significantly higher than in Beijing (p < 0.01). Ecological risk assessments revealed that the risk potential of Cd was medium to high in both cities, while the risk index values for other metals were below 40, indicating low risk. There was no observable correlation between dissolved organic carbon (DOC) and these metals in the overlying water of Beijing, but DOC showed a positive correlation with Cd, Zn, and Pb in the overlying water of Wuhan, with correlation coefficients of 0.41, 0.52 and 0.38, respectively. In addition, dissolved organic matter in Beijing's overlying water primarily consists of visible fulvic acid, while Wuhan's consists of UV fulvic acid indicating that different molecular weights of fulvic acid can affect the behavior of heavy metals. The total organic carbon in the sediments of Wuhan was higher than that in Beijing, with humin (HM) being the predominant component of organic matter in the sediment. Fourier-transform infrared spectroscopy analysis revealed that carboxyl groups were present only in HM of Beijing. This difference in HM content between the two cities suggests that HM plays a primary role in metal migration. Furthermore, soil organic matter significantly contributes to sediment HM in Wuhan and is a key differentiator between the HM sources in the two cities. These results may serve as a valuable reference for addressing metal pollution in urban water bodies.

Environmental colloid behaviors of humic acid - Cadmium nanoparticles in aquatic environments

Humic acid (HA), a principal constituent of natural organic matter (NOM), manifests ubiquitously across diverse ecosystems and can significantly influence the environmental behaviors of Cd(II) in aquatic systems. Previous studies on NOM-Cd(II) interactions have primarily focused on the immobilization of Cd(II) solids, but little is known about the colloidal stability of organically complexed Cd(II) particles in the environment. In this study, we investigated the formation of HA-Cd(II) colloids and quantified their aggregation, stability, and transport behaviors in a saturated porous media representative of typical subsurface conditions. Results from batch experiments indicated that the relative quantity of HA-Cd(II) colloids increased with increasing C/Cd molar ratio and that the carboxyl functional groups of HA dominated the stability of HA-Cd(II) colloids. The results of correlation analysis between particle size, critical aggregation concentration (CCC), and zeta potential indicated that both Derjaguin-Landau-Verwey-Overbeek (DLVO) and non-DLVO interactions contributed to the enhanced colloidal stability of HA-Cd(II) colloids. Column results further confirmed that the stable HA-Cd(II) colloid can transport fast in a saturated media composed of clean sand. Together, this study provides new knowledge of the colloidal behaviors of NOM-Cd(II) nanoparticles, which is important for better understanding the ultimate cycling of Cd(II) in aquatic systems.

Spectral fingerprints of DOM-tungsten interactions: Linking molecular binding to conformational changes

Tungsten (W), a widely used yet understudied emerging contaminant, forms oxyanions in aqueous environments, distinguishing it from conventional heavy metals. While dissolved organic matter (DOM) demonstrates considerable potential for W binding, DOM-W interactions remain largely unexplored. Of particular significance, yet frequently overlooked, are the conformational changes in DOM during W binding processes. This study proposes a novel theoretical framework integrating superposition and charge transfer models to elucidate the complexity of these interactions. By combining spectroscopic techniques and photophysical models, we revealed that aromatic compounds containing 1-3 rings, especially monocyclic aromatic protein-like components, exhibit high affinity for W (logK=3.74-4.00). Phenolic hydroxyls served as primary binding sites for W, with aromatic rings facilitating binding through π interactions. Importantly, W binding to aromatic compounds induced conformational changes in DOM, transitioning from a loosely aggregated state to a more compact configuration. These changes facilitated W encapsulation within DOM through the synergistic effects of hydrophobic interactions, hydrogen/π-hydrogen bonding and π-stacking, potentially leading to stable trapping of W. Two-dimensional correlation spectroscopy analysis elucidated the sequential encapsulation process, involving phenolic, aromatic carboxylic/aliphatic carboxylic, polysaccharides, and aliphatics. The intricate behavior of DOM-W binding profoundly reshapes DOM's conformation, subtly yet significantly orchestrating W's binding affinity, environmental transport, and bioavailability in aquatic ecosystems.

Influencing Factors of Phosphorus Mobility and Retention in the Sediment of Three Typical Plateau Lakes

The mechanisms driving changes in the stability of phosphorus (P) in sediments under lake ecosystem degradation remain poorly understood. This study investigated the P-binding forms in sediments from three plateau lakes with different trophic states in Yunnan Province, China, aiming to elucidate the responses of sediment P compositions to human activities, lake trophic status, and dissolved organic matter (DOM) characteristics. The results showed that human activity directly contributed to sediment P retention. The trophic type of lake exerted a discernible effect on P mobility in the sediments, as eutrophic algae-type lakes had a higher content of sediment mobile-P. Moreover, the sediment DOM promoted the adsorption of BD-P and NH4Cl-P. Generally, exogenous pollution caused by human activity leads to lake eutrophication and a decline in lake ecosystem stability. This variation was largely influenced by water depth. A decrease in lake ecosystem stability leads to increased P mobility in sediments, which increases the risk of endogenous pollution. The DOM plays an important role in the mobility of sediment P. These insights offer a novel perspective for understanding how lake ecosystem characteristics are related to endogenous P loads in lakes.

Response and roles of algal organic matter under copper stress: Spectral and mass spectrometry analysis

Eutrophication leads to various environmental issues, including pollution caused by the production of algal organic matter (AOM). Algae typically respond to environmental changes (e.g., light, temperature, copper [Cu(II)] concentration and pH) by regulating the production and release of different substances, thereby causing unpredictable effects on water quality. We explored the characteristics of AOM and the response mechanisms of algae under Cu(II) stress in the study, using fluorescence spectrum and high-resolution mass spectrometry (HRMS) analysis. The growth of Microcystis aeruginosa was inhibited under Cu(II) stress which was irreversible at Cu(II) concentration ≥ 2 μmol/L. Tryptophan- and humic-like fluorophores were important constituents of extracellular organic matter (EOM), and their contents increased with the addition of Cu(II), indicating that Cu(II) stimulates the production of tryptophan- and humic-like compounds. In addition, fulvic acid-like compounds in EOM were the main components binding to Cu(II) and were overproduced by algae under Cu(II) stress. It was found by HRMS at the molecular level that the formula numbers of EOM generally increased over inhibition time. Under 1 μmol/L Cu(II) stress, nitrogenous compounds (CHON formulae) were the primary AOM, accounting for 37.3-52.0 %. In addition, algae release a large amount of condensed aromatic structures to balance Cu(II) stress. This study provides a molecular-level analysis to explain the variation trends and response mechanisms of algae under various Cu(II) concentrations. The research methods are helpful for utilizing multiple advanced analysis methods to study algae growth and AOM release.

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.

From reduction to rebalancing: Insights into the long-term effects of sediment dredging on nitrogen transformations in river ecosystems

Although sediment dredging is a widely employed water management and restoration technique for the removal of internal nitrogen (N), the long-term effects of dredging on N transformation in dredged rivers remain largely undetermined. In this study, we investigated the effects of dredging on N transformation processes spanning three years at ten sites in the purple-soil watershed within the middle reaches of the Fu River Basin. We combined isotopic and molecular techniques to provide novel insights into the interactions associated with microbial utilization capacities between sediment and river water before, during, and after dredging. Initially, dredging was found to significantly reduce the total nitrogen content by approximately 75 %, although over time, there was a slight increase in concentrations. Secondly, significant reductions in microbial richness and diversity were detected in both river water and sediment, with 39 classes reduced, 12 new classes emerging, and an increase in archaea, reshaping the microbial community. Lastly, dredging was found to promote a significant shift in functional contributions, with increases in the abundance of key enzyme activities (1.7.5.1 and 1.7.2.5) and denitrification genes (nirK, norB, and nosZ). This enhancement notably promoted denitrification and dissimilatory nitrate reduction to ammonium (DNRA), accompanied by significant environmental changes in sediment and river water. These changes facilitated the removal of nitrates in the Xiangshuitan watershed. Our study overcomes the limitations associated with watershed and microenvironment scales, providing insights into the mechanisms where by dredging activities influence the interplay between external and internal N transformations.

Effects of photodegradation on the composition characteristics and metal binding behavior of sediment-derived dissolved organic matter (SDOM) in nansi lake, China

Sediment-derived dissolved organic matter (SDOM) is instrumental in the cycling of nutrients and heavy metals within lakes, influencing ecological balance and contaminant distribution. Given the influence of photodegradation on the alteration and breakdown of SDOM, further understanding of this process is essential. In this research, the properties of the SDOM photodegradation process and its metal-binding reactions in Nansi Lake were analyzed using the EEM-PARAFAC and 2D-SF/FTIR-COS techniques. Our study identified three sorts of humic-like components and one protein-like component in SDOM, with the humic-like material accounting for 71.3 ± 5.19% of the fluorescence intensity (Fmax). Photodegradation altered the abundance and structure of SDOM, with a 41.6 ± 5.82% decrease in a280 and a 29.1 ± 9.31% reduction in Fmax after 7 days, notably reducing the protein-like component C4 by 54.0 ± 5.17% and the humic-like component C2 by 48.5 ± 2.54%, which led to SDOM being formed with lower molecular weight and aromaticity. After photodegradation, the LogKCu values for humic-like and protein-like substances decreased (humic-like C2: LogKCu: 1.35 ± 0.10-1.11 ± 0.15, protein-like C4: 1.49 ± 0.14-1.29 ± 0.34), yet the preferential binding sequence of protein-like materials and specific functional groups with Cu2+ such as aliphatic C-OH, amide (I) C=O and polysaccharide C-O groups remained unaltered. Our results enhance the knowledge of light-induced SDOM alterations and offer insights into SDOM-metal interactions in aquatic ecosystems.

Transformation of sedimentary dissolved organic matter in electrokinetic remediation catalogued by FT-ICR mass spectrometry

In electrokinetic remediation (EKR), the sedimentary dissolved organic matter (DOM) could impede remediation by scavenging reactive species and generating unintended byproducts. Yet its transformation and mechanisms remained largely unknown. This study conducted molecular-level characterization of the water-extractable DOM (WEOM) in EKR using negative-ion electrospray ionization coupled to 21 tesla Fourier transform ion cyclotron resonance mass spectrometry (21 T FT-ICR MS). The results suggested that ∼55 % of the ∼7,000 WEOM compounds identified were reactive, and EKR lowered their diversity, molecular weight distribution, and double-bond equivalent (DBE) through a combination of electrochemical and microbial redox reactions. Heteroatom-containing WEOM (CHON and CHOS) were abundant (∼ 35% of the total WEOM), with CHOS generally being more reactive than CHON. Low electric potential (1 V/cm) promoted the growth of dealkylation and desulfurization bacteria, and led to anodic CO2 mineralization, anodic cleavage of -SO and -SO3, and cathodic cleavage of -SH2; high electric potential (2 V/cm) only enriched desulfurization bacteria, and differently, led to anodic oxygenation and cathodic hydrogenation of unsaturated and phenolic compounds, in addition to cathodic cleavage of -SH2. The long-term impact of these changes on soil quality and nitrogen-sulfur-carbon flux may be need to studied to identify unknown risks and new applications of EKR.

Regional differences in lead (Pb) and tetracycline (TC) binding behavior of sediment dissolved organic matter (SDOM): Effects of DOM heterogeneity and microbial degradation

Lake Nansi, primarily dominated by macrophytes, faces threats from heavy metals and antibiotics due to human activity. This study investigated sediment dissolved organic matter (SDOM) characteristics and complexation of lead (Pb) and tetracycline (TC) in barren zone (BZ) and submerged macrophytes zone (PZ). Additionally, a microbial degradation experiment was conducted to examine its impact on the regional variations in complexation. SDOM abundance and protein-like materials in PZ was significantly greater than in BZ, indicating a probable contribution from the metabolism and decomposition of submerged macrophytes. Both zones exhibited a higher affinity of SDOM for Pb compared to TC, with all four components participating in Pb complexation. Protein-like materials in PZ had a higher binding ability (LogKPb=4.19 ± 1.07, LogKTC=3.89 ± 0.67) than in BZ (LogKPb=3.98 ± 0.61, LogKTC=3.69 ± 0.13), suggesting a potential presence of organically bound Pb and TC due to the higher abundance of protein-like materials in PZ. Although microbial communities differed noticeably, the degradation patterns of SDOM were similar in both zones, affecting the binding ability of SDOM in each. Notably, the fulvic-like component C4 emerged as the dominant binding material for both Pb and TC in both zones. Degradation might increase the amount of organically bound TC due to the increase in the LogKTC.

Applying fluorescence spectroscopy coupled with Gaussian band fitting to reveal dynamic variation process of humus fractions from riparian soils along an urbanized river

Humus, an important fraction of soil organic matter, play an environmental role on nutrients, organic and inorganic pollutants in riparian zones of urbanized rivers. In this study, dynamic variation process of humus fractions from riparian soils was revealed along Puhe River. Composite soil samples of four depths were collected from four land-uses, i.e., eco-conservation area (ECA), industrial area (INA), urban/town area (UTA), rural/agricultural area (RAA). Based on synchronous fluorescence spectra coupled with Gaussian band fitting, fulvic/humic acid predominantly contained tyrosine-like (TYLF), tryptophan-like (TRLF), microbial-like (MLF), fulvic-like (FLF) and humic-like (HLF) substances within each soil profile. TRLF, MLF and FLF (89.43-90.30 %) are the representative components in fulvic-acid, while MLF and HLF (52.81-59.97 %) in humic-acid. Phenolic, carboxylic and humified materials were present in both humus. According to 2-dimensitonal correlation spectroscopy and canonical correlation analysis, fulvic/humic acid within the ECA soil profile could be mainly derived from the degradations of terrestrial plant metabolites and residuals. Within the INA, fulvic-acid could be associated with treated/untreated wastewater, which entered the river and flew into the riparian during high flow period; whereas humic-acid could be relative to the terrestrials. Fulvic-acid had the same source as humic-acid in the UTA, which might be concerned with scattered domestic sewage and livestock wastewater, rather than the fluvial water. Furthermore, the source of fulvic/humic acid in the RAA was the crop metabolites and residuals, apart from the livestock wastewater. Noticeably, the variations of humus fractions in the ECA and RAA roughly occurred in 0-60 cm, while approximately in 20-80 cm in the INA and UTA. This proved that humus fractions in the former were referred to the plant/crop residuals, whereas humus fractions in the latter were those the terrestrials and fluvial water. This study could provide a key support for the construction and restoration of the urbanized riparian zone.

Spatial distribution of sediment dissolved organic matter in oligotrophic lakes and its binding characteristics with Pb(II) and Cu(II)

Dissolved organic matter (DOM), the most active component in interstitial waters, determines the stability of heavy metals and secondary release in sediments. However, little is known about the composition and metal-binding patterns of DOM in interstitial water from oligotrophic lakes affected by different anthropogenic perturbations. Here, 18 interstitial water samples were prepared from sediments in agricultural, residential, tourist, and forest regions in an oligotrophic lake (Shengzhong Lake in Sichuan Province, China) watershed. Interstitial water quality and DOM composition, properties, and Cu(II)- and Pb(II)-binding characteristics were measured via physicochemical analysis, UV-vis spectroscopic, fluorescence excitation-emission matrix-parallel factor analysis (EEM-PARAFAC), and fluorescence titration methods. The DOM, which was produced mainly by microbial activities, had low molecular weights, humification degrees, and aromaticity. Based on EEM-PARAFAC results, the DOM was generally composed of tryptophan- (57.7%), terrestrial humic- (18.7%), microbial humic- (15.6%), and tyrosine-like (8.0%) substances. The DOM in the metal complexes was primarily composed of tryptophan-like substances, which accounted for ~42.6% of the DOM-Cu(II) complexes and ~72.0% of the DOM-Pb(II) complexes; however, microbial humic-like substances primarily contributed to the stability of DOM-Cu(II) (logKCu = 3.7-4.6) and DOM-Pb(II) (logKPb = 4.3-4.8). Water quality parameters did not significantly affect the stability of DOM-metal complexes. We demonstrated that the metal-binding patterns of DOM in interstitial water from oligotrophic lakes are highly dependent on microbial DOM composition and are affected by anthropogenic perturbations to a lesser extent.

Characterization of dissolved organic matter in rivers impacted by acid mine drainage: Components and complexation with metals

Dissolved organic matter (DOM), a ubiquitous and active ingredient, is extensively involved in the transformation and migration of environmental pollutants in aquatic ecosystems. However, its chemical composition in acid mine drainage (AMD)-impacted rivers remains poorly characterized, hindering our understanding of its role in the biogeochemistry of key elements in contaminated fluvial environments. Here, we investigated the concentration of dissolved organic carbon (DOC) and spectroscopic and molecular characteristics of DOM in a headwater river contaminated with polymetallic mine-derived AMD in southern China. Terrestrial humic-like (C1) and typically groundwater-supplied aromatic protein/tyrosine-like (C2) substances which were partially from AMD, were identified as the predominant fluorescent components in the river water. Notably, tryptophan-like (C3) substances originating from tailings pond spills were only occasionally detected in the river. Although DOM biogeochemical transformations and degradation occurred in the lateral soil-water riparian interface and longitudinal in-stream transport processes, the molecular compositions identified by FT-ICR MS showed a core set of molecular formulae in the lignin/saturated compound/tannin region of the van Krevelen diagram of the water samples across the rivers. The complexation of DOM with typical metals in AMD was investigated using fluorescence quenching experiments. The results showed that the highest binding ability of Fe(III) to C2 followed by C1, with both detected in the experimental water samples. Mg(II) and Ca(II) strengthened the binding of DOM-Fe(III) when the ferric/DOM ratio was low, while Cu(II) weakened the binding of DOM-Fe(III) due to competition. Ca(II) inhibited the binding of Fe(III) to C1 but promoted the binding of the complex to C2 when both Cu(II) and Mg(II) were present. Since DOM-Fe(III) complexation was associated with the cotransport of AMD-derived metals/metalloids in diverse aqueous environments with multiple co-existing ions (typically Ca(II) input for remediation), our study on the composition of DOM and its complexation with metals can contribute to managing and remediating AMD-impacted rivers.

Dissolved organic nitrogen is a key to improving the biological treatment potential of landfill leachate

Biological treatment is one of the most promising efficient, low-carbon and affordable approaches for the treatment of recalcitrantly degradable wastewater, such as landfill leachate. However, even the macroscopic molecular level analysis of dissolved organic matter (DOM) is limiting to the enhancement of biological treatment efficacy, and there is an urgent need for deeper exploration of DOM to gain insights into the key constraining substances. In the present study targeting at piercing leachate organic at molecular level, nitrogen-containing dissolved organic matter (DOMN) was identified to be the bottleneck that govern the biotreatment potential. The conclusion was made based on two series of experiments that compared the same anoxic-aerobic membrane bioreactor process (A process) operated stably at different regions, and compared with C process that coupling A process with a circulation aeration membrane bioreactor to improve aeration efficiency. The results confirmed that the relative abundance of DOMN was absolutely dominant among the three categories of DOM in all biologically treated samples, contributing to 60.36 %-65.81 % in removed-DOM, 60.33 %-70.95 % in refractory-DOM and 63.14 %-71.36 % in derived-DOM. Specifically, the high latitude A process had much lower DOMN removal rate than the low latitude A process (p < 0.05) and much higher refractory and derivatization rates than the low latitude A process (p < 0.05). DOM had similar results. No statistically significant differences were observed in the proportion of the three categories of DOM (DOMN), the elements composition, and the subcategory composition of the C process compared to the A process, in which the DOM (DOMN) derivation rate of NEC1-C (31.92 % and 33.41 %) was much higher than that of NEC1-A (20.88 % and 22.19 %). However, the AIwa and AImodwa of the derived-DOM (DOMN) were significantly higher in the C process than in the A process, which implied that excessive aeration did not enhance the biological treatment potential of the A process, but instead led to the proliferation of microorganisms and the secretion of extracellular polymer substances, which resulted in the derivation of more complex compounds. The results of the correlation analysis indicated that there were some regional differences in the molecular information of DOMN driven by climate temperature. In addition, it was worth mentioning that the nominal oxidation state of carbon (NOSCwa) of derived-DOMN in different regions of A process was noticeably higher than the corresponding DOM (p < 0.0001), implying that the derived-DOMN were still highly biodegradable, in other words, there was still great room for improving the biological treatment potential of landfill leachate. The present study provided a deeper insight and analysis of landfill leachate at the molecular level (DOMN) through multiple practical engineering cases, with a view to providing a theoretical basis for efficient optimization of biological treatment.

Effect of pyrolysis temperature and molecular weight on characterization of biochar derived dissolved organic matter from invasive plant and binding behavior with the selected pharmaceuticals

A comprehensive understanding of the characteristics of biochar released-dissolved organic matter (BDOM) derived from an invasive plant and its impact on the binding behavior of pharmaceuticals is essential for the application of biochar, yet has received less attention. In this study, the binding behavior of BDOM pyrolyzed at 300-700 °C with sulfathiazole, acetaminophen, chloramphenicol (CAP), and carbamazepine (CMZ) was investigated based on a multi-analytical approach. Generally, the pyrolysis temperature exhibited a more significant impact on the spectral properties of BDOM and pharmaceutical binding behavior than those of the molecular weight. With increased pyrolysis temperature, the dissolved organic carbon decreased while the proportion of the protein-like substance increased. The highest binding capacity towards the drugs was observed for the BDOM pyrolyzed at 500 °C with the molecular weight larger than 0.3 kDa. Moreover, the protein-like substance exhibited higher susceptive and released preferentially during the dialysis process and also showed more sensitivity and bound precedingly with the pharmaceuticals. The active binding points were the aliphatic C-OH, amide II N-H, carboxyl CO, and phenolic-OH on the tryptophan-like substance. Furthermore, the binding affinity of the BDOM pyrolyzed at 500 °C was relatively high with the stability constant (logKM) of 4.51 ± 0.52.

Exploring the primary magnetic parameters affecting chemical fractions of heavy metal(loid)s in lake sediment through an interpretable workflow

The magnetic properties of lake sediments account for close relationships with heavy metal(loid)s (HMs), but little is known about their relationships with chemical fractions (CFs) of HMs. Establishing an effective workflow to predict HMs risk among various machine learning (ML) methods in conjunction with magnetic measurement remains challenging. This study evaluated the simulation efficiency of nine ML methods in predicting the risk assessment code (RAC) and ratio of the secondary and primary phases (RSP) of HMs with magnetic parameters in sediment cores of a shallow lake. The sediment cores were collected and sliced, and the total amount and CFs of HMs, as well as magnetic parameters, were determined. Support vector machine (SVM) outperformed other models, as evidenced by coefficient of determination (R2) > 0.8. Interpretable machine learning (IML) methods were employed to identify key indicators of RAC and RSP among the magnetic parameters. Values of χARM, HIRM, χARM/χ, and χARM/SIRM of sediments ranging in 220-500 × 10-8 m3/kg, 30-40 × 10-5Am2/kg, 15-25, and 0.5-1, respectively, indicated the potential ecological risks of Cd, Hg, and Sb. This study offers new perspectives on the risk assessment of HMs in lake sediments by combining magnetic measurement with IML workflow.

Labile dissolved organic matter (DOM) and nitrogen inputs modified greenhouse gas dynamics: A source-to-estuary study of the Yangtze River

Although rivers are increasingly recognized as essential sources of greenhouse gases (GHG) to the atmosphere, few systematic efforts have been made to reveal the drivers of spatiotemporal variations of dissolved GHG (dGHG) in large rivers under increasing anthropogenic stress and intensified hydrological cycling. Here, through a source-to-estuary survey of the Yangtze River in March (spring) and October (autumn) of 2018, we revealed that labile dissolved organic matter (DOM) and nitrogen inputs remarkably modified the spatiotemporal distribution of dGHG. The average partial pressure of CO2 (pCO2), CH4 and N2O concentrations of all sampling sites in the Yangtze River were 1015 ± 225 μatm, and 87.5± 36.5 nmol L-1, and 20.3 ± 6.6 nmol L-1, respectively, significantly lower than the global average. In terms of longitudinal and seasonal variations, higher GHG concentrations were observed in the middle-lower reach in spring. The dominant drivers of spatiotemporal variations in dGHG were labile, protein-like DOM components and nitrogen level. Compared with the historical data of dGHG from published literature, we found a significant increase in N2O concentrations in the Yangtze River during 2004-2018, and the increasing trend was consistent with the rising riverine nitrogen concentrations. Our study emphasized the critical roles of labile DOM and nitrogen inputs in driving the spatial hotspots, seasonal variations and annual trends of dGHG. These findings can contribute to constraining the global GHG budget estimations and controls of GHG emission in large rivers in response to global change.

How does adsorptive fractionation of dissolved black carbon on ferrihydrite affect its copper binding behaviors? A molecular-scale investigation

Adsorptive fractionation of dissolved black carbon (DBC) on minerals is proven to alter its molecular composition, which will inevitably affect the environment fate of heavy metals. However, the effects of molecular fractionation on the interaction between DBC and heavy metals remain unclear. Herein, we observed that the selective adsorption of ferrihydrite caused molecular changes of DBC from high molecular weight/unsaturation/aromaticity to low molecular weight/saturation/aliphatics. This process accompanied by a retention of carbohydrate and a reduction of oxygen-rich functional groups (e.g., polyphenols and carboxyl) and long carbon chain in DBC. The residual DBC in aqueous phase demonstrated a weaker binding affinity to copper compared to the original DBC. This decrease in binding affinity was primarily attributed to the adsorption of polycyclic condensed aromatic compounds of 200-250 Da, oxygen-rich polycyclic condensed aromatic compounds of 250-300 Da, oxygen-rich non-polycyclic aromatic compounds of 300-450 Da, and non-polycyclic aromatic compounds of 450-700 Da in DBC by ferrihydrite. Additionally, the retention of carbohydrates and aliphatic compounds of 300-450 Da also made a significant contribution. Notably, carboxylic groups rather than phenolic groups were the dominant oxygen-containing functional groups responsible for this affinity reduction. This study has significant implications for understanding of the biogeochemical processes of DBC at soil-water interface and surface water, especially its role in the transportation of heavy metals.

Deciphering DOM-metal binding using EEM-PARAFAC: Mechanisms, challenges, and perspectives

Dissolved organic matter (DOM) is a pivotal component of the biogeochemical cycles and can combine with metal ions through chelation or complexation. Understanding this process is crucial for tracing metal solubility, mobility, and bioavailability. Fluorescence excitation emission matrix (EEM) and parallel factor analysis (PARAFAC) has emerged as a popular tool in deciphering DOM-metal interactions. In this review, we primarily discuss the advantages of EEM-PARAFAC compared with other algorithms and its main limitations in studying DOM-metal binding, including restrictions in spectral considerations, mathematical assumptions, and experimental procedures, as well as how to overcome these constraints and shortcomings. We summarize the principles of EEM to uncover DOM-metal association, including why fluorescence gets quenched and some potential mechanisms that affect the accuracy of fluorescence quenching. Lastly, we review some significant and innovative research, including the application of 2D-COS in DOM-metal binding analysis, hoping to provide a fresh perspective for possible future hotspots of study. We argue the expansion of EEM applications to a broader range of areas related to natural organic matter. This extension would facilitate our exploration of the mobility and fate of metals in the environment.

Characteristics of microplastic-derived dissolved organic matter and its binding with pharmaceuticals unveiled by fluorescence spectroscopy and two-dimensional correlation spectroscopy

Microplastics were an extensively detected pollutant in the environment, but microplastic-derived dissolved organic matter (MP-DOM) has received less attention, much less its impact on the binding behavior of pollutants (e.g., pharmaceuticals). In this study, DOM derived from two typical MPs, i.e., polyethylene terephthalate (PET) and polystyrene (PS) was generated by UV irradiation (a widely used way for MPs' aging treatment) and characterized by multiple spectroscopic techniques and methods. Chloramphenicol (CAP) and carbamazepine (CBZ) were selected to investigate the binding mechanism between MP-DOM and pharmaceuticals. After UV irradiation, the concentration of the dissolved organic carbon, colored DOM, and carboxyl/carbonyl groups of MP-DOM increased. Moreover, the humic-like substance released preceding and more under UV irradiation. Furthermore, the protein-like substances on PET-DOM and the humic-like substances on PS-DOM were positively correlated to the binding capacity to the pharmaceuticals, respectively. 2D-COS results revealed that the fluorescent materials having more oxygen-containing functional groups for MP-DOM preferentially interacted with the pharmaceuticals. Overall, the higher fluorescence quenching was related to the protein-like substance, CBZ, and PET-DOM as compared to the humic-like substance, CAP, and PS-DOM. It was verified by the relatively high binding ability (logKM) for them (the protein-like substance: 5.15; CBZ: 4.61; PET: 4.48). This study first proved the environmental reactivity of MP-DOM to the pharmaceuticals highlighting the significance of the spectral properties for the binding behavior of MP-DOM with pharmaceuticals and the competitive sorption role of MP-DOM to the pollutants in the natural environment.

Unveiling changes in the complexation of dissolved organic matter with Pb(II) by photochemical and microbial degradation using fluorescence EEMs-PARAFAC

Dissolved organic matter (DOM) is very important in determining the speciation, behaviors, and risk of metal pollutants in aquatic ecosystems. Photochemical and microbial degradation are key processes in the cycling of DOM, yet their effects on the DOM-Pb(II) interaction remain largely unknown. This was studied by examining the complexation of river DOM with Pb(II) after degradation, using fluorescence quenching titration and excitation-emission matrices-parallel factor analysis (EEMs-PARAFAC). Three humic-like and two protein-like components were identified, with strong removals of humic-like components and decreasing average molecular weight and humification degree of DOM by photo- and photo-microbial degradation. The changes in humic-like abundance and structure resulted in notable weakening of their interaction with Pb(II). The tryptophan-like C2 was also mainly removed by photo-degradation, while the tyrosine-like C3 could be either removed or accumulated. The Pb(II)-binding of protein-like components was generally weaker but was enhanced in some degradation groups, which might be related to the lowering competition from humic-like components. The binding parameters correlated significantly with the DOM indices, which were dominated by photo-degradation for humic-like components but by seasonal variations for the tyrosine-like component. These results have implications for understanding the key mechanisms underlying the variability of the DOM-metal interaction in aquatic environments.

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

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

Roles of dissolved organic matter (DOM) in shaping the distribution pattern of heavy metal in the Yangtze River

Dissolved organic matter (DOM) strongly influences the solid-liquid partitioning and migration characteristics of heavy metals, yet little is known about the metal distribution and risk with the participation of DOM in large riverine systems. This study investigated the spatiotemporal distribution of 14 heavy metals and DOM along the entire Yangtze River (over 6000 km), and highlighted the critical roles of DOM in regulating the environmental behaviors of heavy metals. Significant spatial variations of metal contents were observed, with the river source and lower reach remarkably different from the upper-middle reaches. Heavy metals in the Yangtze River were mainly from the natural sources with minor anthropogenic disturbance. We found DOM could promote the conversion of metals from solid to liquid phase and DOM with higher aromaticity showed higher metal affinities. Although low ecological risks were observed in the Yangtze River, potential risks of metal leaching warrant attention, especially for As, Cd and Sb in the middle-lower reaches with higher DOM content and aromaticity. This study established a source-to-sea investigative approach to evaluate the influences of DOM features on metal partitioning, which is crucial for the risk control and sustainable management of large rivers.