Intriguing changes in molecular size and composition of dissolved organic matter induced by microbial degradation and self-assembly

Molecular weight-dependent differences in spectral properties and metal-binding behaviors of dissolved organic matter from different lakes

Dissolved organic matter (DOM) plays an important role in governing metal speciation and migration in aquatic systems. In this study, various DOM samples were collected from Lakes Erhai, Kokonor, and Chaka, and size-fractionated into high molecular weight (HMW, 1 kDa-0.7 μm) and low molecular weight (LMW, <1 kDa) fractions for measurements of dissolved organic carbon (DOC), spectral properties, and metal binding behaviors. Our results demonstrated that samples from Lake Chaka exhibited the highest DOC concentration and fluorescence indices but the lowest percentage of carbohydrates. Regardless of sampling locations, the HMW-DOM fractions contained higher abundances of aromatic DOM, carbohydrates and protein-like substances, but lower abundance of fulvic acid-like substances compared to those in the LMW fractions. Metal titration experiments coupled with the excitation-emission matrix (EEM)-parallel factor (PARAFAC) modeling revealed that the quenching of the PARAFAC-derived fluorescent components was more pronounced in the presence of Cu(II) compared to Pb(II). Humic-like components emerged as a superior model, exhibiting higher binding affinities for Cu(II) than protein-like substances, while the opposite trend was observed for Pb(II). In samples obtained from Lakes Erhai and Kokonor, the condition stability constants (Log KM) for the binding of both Cu(II) and Pb(II) with the HMW-DOM fraction were higher than those with the LMW-DOM fraction. Conversely, a contrasting trend was observed for Lake Chaka. This study highlighted the heterogeneity in spectral properties and metal-binding behaviors of natural DOMs, contributing to an improved understanding of the molecular interactions between DOM components and metal ions and their environmental fate in aquatic ecosystems.

Variations in the level of available phosphorus with changes in the status of water-soluble organic matter derived from different organic materials in soil

The combined application of organic material and phosphorus fertilizer is an effective method to enhance phosphorus use efficiency for plant growth. This is partly because the presence of water-soluble organic matter (WSOM) derived from different organic materials can enhance the level of available phosphorus in the soil; however, it is poorly understood how this level varies with changes in the WSOM status (i.e., decomposed, dissolved, and retained) in the soil depending on WSOM types. This study aimed to (i) understand how changes in the WSOM status enhances the available phosphorus level in the soil, and (ii) determine the WSOM type that contributes to such enhancement. The incubation test showed that fractions of 73%-92% and 8%-27% of WSOM-derived organic carbon were retained and dissolved, respectively, at the beginning of incubation, while 31%-45% was decomposed during the incubation period. The WSOM derived from cattle manure compost (CM) and sewage sludge compost (SSC) that was initially retained was maintained until the late stage of the incubation test, whereas that derived from hydrothermal decomposed liquid fertilizer (HDLF) was rapidly desorbed during the first 14 days of the incubation period. The available phosphorus level was higher under the combined application of CM- and SSC-derived WSOM than under the single phosphorus application throughout the incubation period, while it was high only during the first 3 days of incubation under the application of HDLF-derived WSOM. The amounts of retained organic carbon at each sampling point during the incubation period compared to those at the beginning were positively and linearly correlated to the available phosphorus levels that were enhanced by the WSOM present in the soil. This study for the first time provides quantitative experimental evidence that 1) the longer the WSOM continues to be retained, the higher the amount of available phosphorus remaining in the soil, and 2) the available phosphorus level decreases with WSOM sorption or decomposition. Furthermore, it was shown that highly humified WSOM has a great potential for the maintenance of higher available phosphorus levels. This study provides the insight that a combined application of highly humified organic materials with a chemical fertilizer is necessary for not only cost effective but also sustainable fertilization design.

Effect of straw decomposition on hexavalent chromium removal by straw: Significant roles of surface potential and dissolved organic matter

Mobility and bioavailability of hexavalent chromium (Cr(VI)) in agricultural soils are affected by interactions between Cr(VI) and returned crop straws. However, the effect of straw decomposition on Cr(VI) removal and underlying mechanisms remain unclear. In this study, Cr(VI) removal by pristine and decomposed rice/rape straws was investigated by batch experiments and a series of spectroscopies. The results showed that straw decomposition inhibited Cr(VI) removal, regardless of straw types. However, the potential mechanisms of the inhibition were distinct for the two straws. For the rice straw, a lower zeta potential after decomposition suppressed Cr(VI) sorption and subsequent reduction. In addition, less Cr(VI) was reduced by the decomposed rice straw-derived dissolved organic matter (DOM) than the pristine one. In contrast, for the rape straw, due to the increased zeta potential after decomposition, the decreased Cr(VI) removal was mainly ascribed to less Cr(VI) reduction by the rape straw-derived DOM. These results emphasized the significant roles of straw surface potential and DOM in Cr(VI) removal, depending on straw types and decomposition, which facilitate the fundamental understanding of Cr(VI) removal by straws and are helpful for predicting the environmental risk of Cr and rational straw return in Cr(VI)-contaminated fields.

Feasibility evaluation of near dissolved organic matter microfiltration (NDOM MF) for the efficient removal of microplastics in the water treatment process

Microfiltration (MF) using membranes with a mean pore size smaller than 0.45 μm has generally been used for particle removal from water, given that materials larger and smaller than 0.45 μm are regarded as particulates and dissolved organic matter (DOM), respectively. It is also the case for removing small-size microplastics (MPs). However, given their sizes (ca. 1 μm), there is room for further improvement of the productivity (i.e., water flux) in the pore size range of 0.45-1 μm on the condition that the removal rate is maintained. With this in mind, MF's water flux and removal rate were tested using seven different MF membranes, and the right pore, with the size of 0.8 μm, was found for MP removal, which is called near DOM (NDOM) MF. In the filtration test using polystyrene surrogate beads with an average particle diameter of 1.20 μm, NDOM MF exhibited a 1.7 to 13 times higher permeate flux than the conventional MF using 0.1, 0.2, and 0.45 μm membranes while maintaining a higher removal rate than 2 log. The excellent removal rate of the NDOM MF was attributable to the following three factors: (1) smaller mean pore size than the average particle diameter, (2) particle screening effect enhanced by the secondary layer formed by surface deposition, and (3) 3D mesh sublayer structure favorable for capturing penetrated particles. Furthermore, the outstanding filtration performance also appeared in a low-temperature (< 10°C) process, demonstrating that NDOM MF is feasible independently of temperature. Additionally, in constant flux filtration, NDOM MF demonstrated the long-term feasibility by lowering the transmembrane pressure and specific filtration energy by more than 2 times.

Effects of Cu(II)-DOM complexation on DOM degradation: Insights from spectroscopic evidence

The fate of dissolved organic matter (DOM) is primarily governed by its sources, degradation, and transformation processes within the environment. However, the influence of metal-DOM complexation on DOM degradation remains ambiguous. In this study, controlled laboratory experiments were conducted using Cu(II) and natural water from the Duliujian River and the Beidagang Wetland to examine the effects of metal-DOM binding on the degradation pathway of DOM. Our results showed that Cu(II)-DOM complexation affected the distribution of DOM molecular weight with elevated Mw after complexed with Cu(II). Nevertheless, the concentration of DOM decreased over the incubation period due to degradation. In the absence of Cu(II) binding, both wetland and river DOM followed similar degradation pathways, transforming from high to low molecular weight with changes predominantly in the 1-10 kDa size-fraction during DOM degradation. In contrast, in the presence of Cu(II) and thus Cu(II)-DOM binding, the degradation of DOM was enhanced, resulting in higher kinetic rate constants for both wetland and river DOM. The results of differential spectra further confirmed the degradation of DOM with a decrease in bulk spectroscopic properties and an increase in the degree of DOM-Cu(II) complexation. These findings imply a mutually reinforcing relationship between metal-DOM complexation and the degradation of DOM in aquatic environments, providing new insights into the biogeochemical behavior and environmental fate of DOM.

Interaction of microorganisms with carbonates from the micro to the macro scales during sedimentation: Insights into the early stage of biodegradation

The assembly process of Organic Matter (OM) from single molecules to polymers and the formation process of Ca-CO3 ion-pairs are explored at the micro-scale, and then the relationship between OM and carbonate based on the results of microbially-induced carbonate precipitation (MICP) laboratory experiments is established at the macro-scale. Molecular dynamics (MD) is used to model the assembly of OM (a) in an aqueous solution, (b) on surfaces of calcite (10 1‾ 4) crystals and (c) on defective calcite (101‾ 4) crystal surfaces. From the MICP experiments, carbonate minerals containing abundant OM were precipitated and were characterized by Scanning Electron Microscopy (SEM), X-Ray Diffractometry (XRD) and Fourier Transform Infrared Spectroscopy (FTIR). The results of the MD show that OM is assembled into polymers in all three simulation systems. Although the Ca-CO3 ion-pairs and OM were briefly combined, the aggregation assembly of OM molecules and the precipitation of carbonate calcium are not related in the long run. The highly specific surface area of the defective calcite shows an increase in the adsorption of OM. The van der Waals forces, which are primarily responsible for controlling the assembly of OM molecules, increase with the degree of aggregation. According to the MICP experiments, OM is enriched on the mineral surfaces, and more OM is found at the steps of defective crystals with their larger surface areas. Through MD and MICP laboratory experiments, this work systematically describes the interaction of OM and carbonate minerals from the micro to the macro scales, and this provides insight into the interaction between OM and carbonates and biogeochemical processes related to the accumulation of OM in sediments.

Molecular insights into the microbial degradation of sediment-derived DOM in a macrophyte-dominated lake under aerobic and hypoxic conditions

The mineralization of dissolved organic matter (DOM) in sediments is an important factor leading to the eutrophication of macrophyte-dominated lakes. However, the changes in the molecular characteristics of sediment-derived DOM during microbial degradation in macrophyte-dominated lakes are not well understood. In this study, the microbial degradation process of sediment-derived DOM in Lake Caohai under aerobic and hypoxic conditions was investigated using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and metagenomics. The results revealed that the microbial degradation of sediment-derived DOM in macrophyte-dominated lakes was more intense under aerobic conditions. The microorganisms mainly metabolized the protein-like substances in the macrophyte-dominated lakes, and the carbohydrate-active enzyme genes and protein/lipid-like degradation genes played key roles in sediment-derived DOM degradation. Organic compounds with high H/C ratios such as lipids, carbohydrates, and protein/lipid-like compounds were preferentially removed by microorganisms during microbial degradation. Meanwhile, there was an increase in the abundance of organic molecular formula with a high aromaticity such as tannins and unsaturated hydrocarbons with low molecular weight and low double bond equivalent. In addition, aerobic/hypoxic environments can alter microbial metabolic pathways of sediment-derived DOM by affecting the relative abundance of microbial communities (e.g., Gemmatimonadetes and Acidobacteria) and functional genes (e.g., ABC.PE.P1 and ABC.PE.P) in macrophyte-dominated lakes. The abundances of lipids, unsaturated hydrocarbons, and protein compounds in aerobic environments decreased by 58 %, 50 %, and 44 %, respectively, compared to in hypoxic environments under microbial degradation. The results of this study deepen our understanding of DOM biodegradation in macrophyte-dominated lakes under different redox environments and provide new insights into nutrients releases from sediment and continuing eutrophication in macrophyte-dominated lakes.

Composition characteristics and metal binding behavior of macrophyte-derived DOM (MDOM) under microbial combined photodegradation: A state closer to actual macrophytic lakes

In actual lakes, the "unstable components" of macrophyte-derived DOM (MDOM) are always degraded and cannot exist abidingly, but the environmental impact brought by it is ignored. In this study, MDOM from Potamogeton crispus was extracted to carry out microbial combined photodegradation (M-Photodegradation) and fluorescence titration experiments. Then the traits and metal binding reaction of MDOM under M-Photodegradation were analysed and compared with the features of lake-derived DOM (LDOM) from point monitoring of Dongping Lake through EEM-PARAFAC, 2D-SF-COS, and 2D-FTIR-COS. The results showed that the features of MDOM after M-Photodegradation were closer to those of LDOM. The degradation amplitudes were 93.53% ± 0.53% for C4 in microbial degradation and 78.31% ± 0.74% for C3 in photodegradation. Correspondingly, both were hardly detected in LDOM. Protein-like substances and aliphatic C-OH were preferentially selected by Cu2+, while humic-like matter and phenolic hydroxyl O-H responded faster to Pb2+. Although the binding sequences remained unchanged after M-Photodegradation, the LogKCu and LogKPb of components decreased overall, indicating increased environmental risks. This study proves that the refractory MDOM retained after degradation was more consistent with the actual state of macrophytic lakes and provides more information for the treatment of heavy metal pollution in lakes.

Effects of Ca2+ and Mg2+ on Cu binding in hydrophilic and hydrophobic dissolved organic matter fractions extracted from agricultural soil

Dissolved organic matter (DOM) has significant effects on soil copper (Cu) bioavailability. However, little is known about Cu interactions and major cation binding toward hydrophilic and hydrophobic DOM components extracted from soil solutions. In this study, we investigated the influence of major cations (Ca2+/Mg2+) on Cu complexing characteristics on different hydrophilic and hydrophobic DOM fractions using absorbance spectroscopy at different Cu2+ concentrations in the absence/presence of Ca2+/Mg2+. Different compositional hydrophobic and hydrophilic DOM fraction proportions occurred at three agricultural soil sites, with the hydrophobic acid (HOA) fraction accounting for the highest proportion. The addition of Cu2+ generated distinct ultraviolet (UV) bands/peaks (processed by differential linear and differential logarithmic transformation) of three hydrophilic DOM fractions, whereas Cu2+ induced less and weak specific peaks in the differential spectra and differential logarithmic of the HOA fractions, indicating hydrophilic DOM fractions tend to have a higher density of Cu2+ complexation sites. In the presence of either Ca2+/Mg2+, increased depression caused by Cu2+ binding on different DOM fractions was observed with increasing 10, 100, and 1000 μM Ca2+/Mg2+ levels, with more significant variations in peaks/banks for hydrophilic base (HIB) and HOA fractions, and less for hydrophilic acid (HIA) and hydrophilic neutral (HIN) fractions. In our study, the spectral parameters ΔS225-275 and ΔS275-325 were successfully used to quantify Cu amounts bonded to HIA and HIB, respectively. They exhibited strong linear relationships with correlation coefficients (R2) of 0.96 for HIA and 0.87 for HIB, respectively. Furthermore, Mg2+ exhibited stronger competition with Cu for HIA and HIB binding sites when compared with Ca2+.

Photochemical reactivity of water-soluble dissolved organic matter from microplastics and microfibers

Plastics in aquatic environments are a source of dissolved organic matter (DOM). However, its production pathways and environmental fate remain poorly understood. This study investigated the yields, characterization, and photochemical reactivities of water-soluble DOM from seven pristine microplastics (MPs) and three microfibers (MFs). We found yields of plastic-derived DOM per unit mass of MPs or MFs, including chromophoric DOM (CDOM) and dissolved organic carbon (DOC), were significantly influenced by polymer chemical structures. Notably, MFs exhibited consistently higher DOM yields compared to MPs. In addition, plastics containing aromatic rings, such as PETE and PS, were found to generate higher CDOM yields, although PVC also showed elevated CDOM yields. The plastic-derived DOM had a diverse molecular size-range, spanning from 60 nm (polyester-DOM) to 937 nm (LDPE-DOM), while Zeta potentials, which were predominantly negatively charged, varied from -42.5 mV (nylon-DOM) to +4.6 mV (LMW-PVC-DOM). Degradation rate constants for CDOM (0.001-0.022 h-1) were generally higher than DOC (0.0009-0.020 h-1), with a shorter half-life for PETE- and PS-derived DOM. The reactivity and degradation kinetics of plastic-derived DOM were notably manifested in changes of fluorescence spectra (excitation-emission matrixes) during photochemical weathering, showing the influence of polymeric composition/structures. This baseline study provides an improved understanding of the characterization and environmental fate of microfiber- and plastic-derived DOM in aquatic environments.

Effects of acid mine drainage on photochemical and biological degradation of dissolved organic matter in karst river water

Dissolved organic matter (DOM) can be removed or transformed by photochemical and biological processes, producing the negative effect of transforming organic carbon into inorganic carbon, which plays a vital role in the karst carbon cycle. However, acid mine drainage (AMD) will affect this process, so the degradation of DOM in karst river water (KRW) needs to be studied in this context. In this study, to reveal the evolution processes of DOM under photochemical and biological conditions in AMD-impacted KRW, AMD and KRW were mixed in different ratios under conditions of visible light irradiation (VL), biodegradation (BD), ultraviolet irradiation (UV) and ultraviolet irradiation + biodegradation (UV+BD). The average DOC concentrations in samples after mixing AMD and KRW in different proportions decreased significantly (by 23%) in UV+BD, which was 1.2-1.4 times higher than under the other conditions and would lead to a significant release of inorganic carbon. Further analysis of the fluorescence parameters via parallel factor analysis (PARAFAC) revealed that the DOM fluorescence components in AMD comprised mainly protein-like substances derived from autochthonous components, while the DOM fluorescence components in KRW were mainly humic-like substances with both autochthonous and allochthonous sources. Therefore, AMD could promote both the photochemical and biological degradation of DOM in karst receiving streams, resulting in the conversion of DOC to inorganic carbon. The results showed that the synergistic effects of UV+BD and AMD accelerated the degradation of DOM and the release of inorganic carbon in KRW, thus affecting the stability of the karst carbon cycle.

Insight into the dynamics of dissolved organic matter components under latitude change perturbation

Dissolved organic matter (DOM) which can help the transportation of nutrients and pollutants plays essential role in the aquatic ecosystems. However, the dynamics of individual DOM component under the change of latitude have not been elucidated to date. The composition and dynamics of DOM were assessed in this study. Two individual parallel factor analysis (PARAFAC) components were found in each sampling site in Heilongjiang. To further characterize the inner change of the identified PARAFAC components, two-latitude correlation spectroscopy (2DCOS) technique was applied to the excitation loadings data. Interestingly, not all the fluorophore in a PARAFAC component change in the same direction as the overall change of a component. From upstream to downstream, the peak A1 in PARAFAC component C1 showed a downward trend, but peak A2 presented an upward trend. In PARAFAC component C2, the peak T2 and peak T3 showed an inverse changing trend under latitude perturbation. Furthermore, basic nutrients parameters in Heilongjiang were also characterized in each sampling sites. The relationships between DOM and nutrients showed that component C1 made a significant contribution to chemical oxygen demand (COD) and biochemical oxygen demand (BOD5). The evolutions of DOM peak A1 and peak A2 were accompanied by the changing of Total phosphorus (TP). The findings in this study could make a contribution to explore the fate of DOM in high humic-like substance containing river.

Rotifer distribution patterns in relation to dissolved organic matter in the middle reaches of Huai River Basin during the dry season

Increased dissolved organic matter (DOM) may induce water browning and affect zooplankton communities by changing photochemical environment, microbial food web, and bioavailability of organic carbon supply. However, little is known about the relationship between DOM components and rotifers in natural rivers, relative to the cladocerans and copepods. Here, we investigated the spatial patterns of rotifer distribution in relation to DOM by collecting forty-four water samples from four areas in the middle reaches of Huai River Basin. Results revealed that DOM was described by two humic-like and two protein-like components. There were significant differences in the composition and diversity of rotifer communities among areas, which might be related to autochthonous and allochthonous DOM as well as geographical distances. Specifically, rotifer communities were mainly related to molecular weight, substituents on the aromatic ring, humification level, and protein-like materials. Autochthonous and fresh DOM was positively associated with rotifer abundance and richness, and terrigenous humic-like substances were positively associated with rotifer diversity and evenness. There was a reciprocal effect between rotifer and DOM. Our findings will contribute to the understanding of the possible effects of water browning on rotifer communities, providing new insights into the key role of DOM and rotifer in the energy transfer of aquatic systems.

Response of dissolved organic matter (DOM) and microbial community to submerged macrophytes restoration in lakes: A review

Microorganisms play a crucial role in the biogeochemical processes of Dissolved Organic Matter (DOM), and the properties of DOM also significantly influence changes in microbial community characteristics. This interdependent relationship is vital for the flow of matter and energy within aquatic ecosystems. The presence, growth state, and community characteristics of submerged macrophytes determine the susceptibility of lakes to eutrophication, and restoring a healthy submerged macrophyte community is an effective way to address this issue. However, the transition from eutrophic lakes dominated by planktic algae to medium or low trophic lakes dominated by submerged macrophytes involves significant changes. Changes in aquatic vegetation have greatly affected the source, composition, and bioavailability of DOM. The adsorption and fixation functions of submerged macrophytes determine the migration and storage of DOM and other substances from water to sediment. Submerged macrophytes regulate the characteristics and distribution of microbial communities by controlling the distribution of carbon sources and nutrients in the lake. They further affect the characteristics of the microbial community in the lake environment through their unique epiphytic microorganisms. The unique process of submerged macrophyte recession or restoration can alter the DOM-microbial interaction pattern in lakes through its dual effects on DOM and microbial commu-----nities, ultimately changing the stability of carbon and mineralization pathways in lakes, such as the release of methane and other greenhouse gases. This review provides a fresh perspective on the dynamic changes of DOM and the role of the microbiome in the future of lake ecosystems.

Differences in the spectral characteristics of dissolved organic matter binding to Cu(II) in wetland soils with moisture gradients

The environmental behavior of heavy metals in soil is significantly regulated by their binding with dissolved organic matter (DOM), which is affected by soil moisture contents. However, the mechanism of this interaction in soils with varying moisture is still not well understood. Using a combination of ultrafiltration, Cu(II) titration, and multispectral (ultraviolet-visible absorption, 3D fluorescence, Fourier transform infrared) analysis techniques, we studied the differences in the spectral characteristics and Cu(II) binding properties of soil dissolved organic matter (DOM) and its different molecular weight (MW) fractions with moisture gradients. We found that the abundance and spectral characters of soil DOM changed with increasing soil moisture, i.e., the increase in abundance while the decrease in aromaticity and humification index. The components of DOM, shown by Fluorescence region-integration (FRI) analysis, also changed, with an increase in the proportion of protein-like substances and a decrease of humic-like and fulvic-like substances. The overall Cu(II) binding potential of soil DOM diminished with increasing soil moisture, as indicated by the fluorescence parallel factor (PARAFAC) analysis. This is aligns with the changes in DOM composition, as the humic-like and fulvic-like fractions exhibited higher Cu(II) binding potential compared to the protein-like fractions. The low MW fraction of the MW-fractionated samples showed a stronger binding potential for Cu(II) compared to the high MW fraction. Finally, the active binding site of Cu(II) in DOM, as revealed by UV-difference spectroscopy and 2D-FTIR-COS analysis, decreased with increasing soil moisture, with the order of preferentially functional groups shifting from OH, NH, and CO to CN and CO. This study emphasizes the impact of moisture variations on the characteristics of DOM and its interaction with Cu(II), providing insight into the environmental fate of heavy metal contaminants in soil in areas with alternating land and water conditions.

Near dissolved organic matter microfiltration (NDOM MF) coupled with UVC LED disinfection to maximize the efficiency of water treatment for the removal of Giardia and Cryptosporidium

Microfiltration (MF) membranes with a mean pore size same as or smaller than 0.45 µm have been typically used to separate pathogenic protozoa in water since materials larger than 0.45 µm are considered particulates. However, 0.45 µm is too small to separate protozoa which are 4-6 µm (Cryptosporidium oocyst) or 8-15 µm (Giardia cyst) in size. In this study, we optimized the mean pore size of MF membranes to maximize the producibility and guarantee a high removal rate simultaneously and proposed the membrane filtration using an MF membrane with an optimum mean pore size larger than but close to dissolved organic matter (DOM), which is called near DOM MF (NDOM MF). According to the MF test using polystyrene surrogate beads with diameters of 3 and 8 µm, an MF membrane with a 0.8 µm mean pore size was the best in that it showed 52% to 146% higher water fluxes than a 0.45 µm MF membrane while maintaining the removal rate at 3-4 log. It was also the case for a low-temperature MF test, revealing the NDOM MF is highly effective regardless of temperature changes. Lastly, we tried to find the possibility of combining the NDOM MF with disinfection by an ultraviolet light emitting diode (UVC LED) to further guarantee the high quality of treated water while providing high process efficiency.

Response of dissolved organic matter to thermal stratification and environmental indication: The case of Gangnan Reservoir

Dissolved Organic Matter (DOM), an important part of the carbon cycle in reservoir ecosystems, has a great impact on aquatic environment to recognize the conversion process of different media DOM. The distribution, spectral characteristics, and sources of DOM in Gangnan Reservoir during thermal stratification were analyzed using ultraviolet-visible absorption spectroscopy and excitation-emission matrix spectroscopy. Three humic-like components (C2, C3, and C4) and two protein-like components (C1 and C5) were identified. The proportions of the humic-like components increased with the progression of thermal stratification (C2 and C3 were dominant), whereas the protein-like components decreased in proportion, and the trend in the interstitial water was constant (C3 and C4 were dominant). The proportion of the humic-like components in the sediments was highest during the stationary period of thermal stratification (C2 and C3 were dominant). C2 and C3 were significantly correlated in the water body and interstitial water (P < 0.001), while C1 and C5 were correlated in the sediment (P < 0.05). In the water body, C2 and C3 were negatively correlated during the formative period of thermal stratification (slope = -1.85; R² = 0.52), strongly positively correlated during the stationary period (slope = 0.76; R² = 0.94), and positively correlated during the weakening period of thermal stratification (slope = 0.46; R² = 0.30). With the progression of thermal stratification, the relative contribution of endogenous substances decreased gradually, whereas the humification degree increased in the water body and interstitial water. The protein-like components and key physicochemical factors (Fe, Mn, TN, TP, and COD_Mn) were significantly correlated during the formative period (P < 0.05), and humic-like components and key physicochemical factors (NO₂^--N and TN) were significantly correlated during the stationary and weakening periods (P < 0.05). C1, C4, and C5 indicated NO₃^--N during the formative period; C2 and C3 indicated NO₃^--N during the stationary period; and C2 and C4 indicated NO₃^--N during the weakening period in the water body. These findings enhance the understanding the mutual transformation processes of DOM in reservoir ecosystems and could guide water quality management.

Using the end-member mixing model to evaluate biogeochemical reactivities of dissolved organic matter (DOM): autochthonous versus allochthonous origins

Dissolved organic matter (DOM) is an essential component of environmental systems. It usually originates from two end-members, including allochthonous and autochthonous sources. Previously, links have been established between DOM origins/sources and its biogeochemical reactivities. However, the influence of changes in DOM characteristics driven by end-member mixing on DOM biogeochemical reactivities has not been clarified. In this study, we investigated variations of DOM reactivities responding to the dynamics of DOM characteristics induced by different mixing ratios of two DOM end-members derived from humic acid (HA) and algae, respectively. Four biogeochemical reactivities of DOM were evaluated, including biodegradation, ·OH production, photodegradation, and redox capacity. Results showed that the variations of DOM characteristics due to the two end-members mixing significantly impact its biogeochemical reactivities. However, not all spectral parameters and reactivities followed the conservative mixing behavior. In contrast to reactivities of ·OH production and redox capacity, mixed samples showed apparent deviations from conservative linear relationships in biodegradation and photodegradation due to the interaction between the two end-members. Regarding the role of DOM properties influencing reactivity changes, peak A and M were recognized as the most stable parameters. However, peak C and SUVA₂₅₄ were identified as the most vital contributors for explaining DOM reactivity variations. These findings suggest that a general model for describing the dynamic relationship between DOM source and reactivity cannot be proposed. Thus, the dynamics of DOM reactivity in diverse ecosystems cannot be estimated simply by the "plus or minus" of the reactivity from individual end-member. The effect of end-member mixing should be evaluated in a given reactivity instead of generalization. This study provides important insights for further understanding the dynamics of DOM's environmental role in different ecosystems influenced by variations of source inputs. In future, more field investigations are needed to further verify our findings in this study, especially in the scenario of end-member mixing.

Comparisons in molecular weight distributions and size-dependent optical properties among model and reference natural dissolved organic matter

Humic acid (HA) and reference natural organic matter (NOM) have been widely used in environmental assessment, biogeochemistry, and ecotoxicity studies. Nevertheless, similarities and differences among the commonly used model/reference NOMs and bulk dissolved organic matter (DOM) have rarely been systematically evaluated. In this study, HA, SNOM (Suwannee River NOM) and MNOM (Mississippi River NOM), both from International Humic Substances Society, and freshly collected unfractionated NOM (FNOM) were concurrently characterized to evaluate their heterogeneous nature and size-dependent chemical properties. We found that molecular weight distributions, PARAFAC-derived fluorescent components, and size-dependent optical properties are NOM-specific and highly variable with pH. The < 1 kDa DOM abundance followed the order of HA < SNOM < MNOM < FNOM. In addition, FNOM was more hydrophilic and contained more protein-like and autochthonous components with a higher UV-absorbance ratio index (URI) and biological fluorescence index, whereas HA and SNOM contained more allochthonous, humic-like components with a higher aromaticity and lower URI. Significant differences in molecular composition and size spectra between FNOM and model/reference NOMs suggest that environmental role of NOMs should be evaluated at the levels of molecular weight and functionalities under the same experimental conditions and that HA and SNOM may not represent bulk NOM in the environment. This study provides new information about similarities and differences in DOM size-spectra and chemical properties between reference NOMs and in-situ NOM and highlights the need to better understand the heterogenous roles of NOMs in regulating the toxicity/bioavailability and environmental fate of pollutants in aquatic environments.

Temperature effects on microbial dissolved organic matter metabolisms: Linking size fractions, fluorescent compositions, and functional groups

This study elucidated the compositional and structural variations of size fractions of microbially-induced dissolved organic matter (DOM) caused by short-term temperature changes (5 to 35 °C), taking riverine DOM as an example. A simple and efficient method combining fractionation-[parallel factor analysis and two-dimensional Fourier-transform infrared correlation spectroscopy (PARAFAC-2D FTIR COS)]-correlation was introduced to link fluorescent DOM components and their structures in terms of surface functional groups. Results indicated that the higher temperature stimulated the decomposition of aromatics (sizes decreased from 10 kDa-0.22 μm to <10 kDa) and the transformation of proteins to humics (with sizes <0.22 μm); while both the higher and lower temperatures inhibited the utilization of larger-sized DOM (>0.22 μm, especially the non-fluorescence part) and synthesis of larger-sized microbial-derived proteins and humics (>0.22 μm), which may result in more smaller-sized (<10 kDa) and refractory aromatics transported from rivers to oceans in the warming future. However, the structure-determined DOM behaviors could be less affected by temperature since the fluorescent proteins and humics revealed similar functional group compositions, such as carboxyl, hydroxyl, carbonyl/aldehyde, carboxylic anhydride, and carboxamide groups. These findings have strong implications for DOM biogeochemistry in future temperature-shock scenarios. The proposed method will support in-depth analyses of structure-regulated processes from a mechanistic perspective.

Applying synchronous fluorescence spectra with Gaussian band fitting and two-dimensional correlation to characterize structural composition of DOM from soils in an aquatic-terrestrial ecotone

Dissolved organic matter (DOM), the primary participant of carbon and nitrogen cycle, has a great impact on the behavior and fate of organic pollutants and heavy metals in eutrophic lakes. The dynamic spectral properties of DOM fractions were revealed in an aquatic-terrestrial ecotone under the different types of land use. Composite soil samples of different depths (0-20, 20-40 and 40-60 cm) were collected from four different land uses along a disturbed-impact gradient in Taihu Lake, China, i.e., grassland (GRL), forest land (FOL), paddy field (PAF), and vegetable field (VEF). DOM mainly consisted of tyrosine-like material (TYLF), tryptophan-like material (TRLF), microbial humic-like material (MHLF), fulvic-like material (FLF) and humic-like material (HLF) within all soil profiles, where TRLF was the dominant component (61.30 %) using synchronous fluorescence spectroscopy (SFS) combined with principal component analysis and Gaussian band fitting. Based on two-dimensional correlation spectroscopy with SFS and Fourier transform infrared, the variation order of DOM fractions was FLF → MHLF → HLF → TRLF → TYLF within the GRL soil profile, and MHLF exhibited an oppositive change with aliphatic OH and amide I in protein. The order of DOM fractions was MHLF → FLF → HLF → TYLF → TRLF within the FOL soil profile, and the change trend of MHLF remained oppositive with aliphatic OH and CO in ester. The order of DOM within the PAF soil profile fractions was TRLF → MHLF → HLF → TYLF → FLF, and changing trends of TYLF were oppositive to aliphatic OH, CH bending vibration, CH bending vibration and CO in ester. The order of DOM fractions was HLF → TYLF → TRLF → FLF → MHLF within the VEF soil profile, where the changing trend TYLF remained oppositive to aliphatic OH, CH deformations in lignin and aliphatic group and amide I in protein. This study may provide important support for alleviating lake water eutrophication or pollution.

Changes of DOM and its correlation with internal nutrient release during cyanobacterial growth and decline in Lake Chaohu, China

The seasonal changes in dissolved organic matter (DOM), and its correlation with the release of internal nutrients during the annual cycle of cyanobacteria in the eutrophic Lake Chaohu, China, were investigated from four sampling periods between November 2020 and July 2021. The DOM fluorescence components were identified as protein-like C1, microbial humic-like C2, and terrestrial humic-like C3. The highest total fluorescence intensity (F_T) of DOM in sediments during the incubation stage is due to the decomposition and degradation of cyanobacteria remains. The lowest humification of DOM and the highest proportion of C1 in waters during the initial cyanobacterial growth indicate that fresh algae are the main source. The highest molecular weight of DOM and F_T of the C2 in sediments during cyanobacterial outbreaks indicate the concurrent deposition of undegraded cyanobacterial remains and microbial degradation. The components of DOM are affected mainly by the dissolved total phosphorus in waters, while the temperature drives the annual cycle of cyanobacteria. The decreasing C1 in sediments and increasing nutrients in waters from the cyanobacterial incubation to outbreak indicate that mineralization of algal organic matter contributes importantly to the release of internal nutrients, with the strongest release of phosphorus observed during the early growth of cyanobacteria. The humic-like C2 and C3 components could also affect the dynamics of internal phosphorus through the formation of organic colloids and organic-inorganic ligands. The results show that the degradation of DOM leads to nutrients release and thus supports the continuous growth of cyanobacteria in eutrophic Lake Chaohu.

A hybrid remote sensing approach for estimating chemical oxygen demand concentration in optically complex waters: A case study in inland lake waters in eastern China

Chemical oxygen demand concentration (C_COD) is widely used to indicate the degree of organic pollution of lakes, reservoirs and rivers. Mastering the spatiotemporal distribution of C_COD is imperative for understanding the variation mechanism and controlling of organic pollution in water. In this study, a hybrid approach suitable for Sentinel 3A/Ocean and Land Colour Instrument (OLCI) data was developed to estimate C_COD in inland optically complex waters embedding the interaction between C_COD and the absorption coefficients of optically active constituents (OACs). Based on in-situ sampling in different waters, the independent validations of the proposed model performed satisfactorily in Lake Taihu (MAPE = 23.52 %, RMSE = 0.95 mg/L, and R² = 0.81), Lake Qiandaohu (MAPE = 21.63 %, RMSE = 0.50 mg/L and R² = 0.69), and Yangtze River (MAPE = 29.34 %, RMSE = 0.83 mg/L, and R² = 0.64). In addition, the approach not only showed significant superiority compared with previous algorithms, but also was suitable for other common satellite sensors equipped same or similar bands. The hybrid approach was applied to OLCI images to retrieve C_COD of Lake Taihu from 2016 to 2020 and reveals substantial interannual and seasonal variations. The above results indicate that the proposed approach is effective and stable for studying spatiotemporal dynamic of C_COD in optically complex waters, and that satellite-derived products can provide reliable information for lake water quality management.

Investigation of free and conjugated estrogen fate and emission coefficients in three duck farms

Concentration animal feeding operation (CAFO) is an important source of environmental estrogen. However, to the best of our knowledge, the data on estrogen discharge during duck breeding and growth is insufficient. This study used liquid chromatography with tandem mass spectrometry (LC/MS/MS) to analyze the free and conjugated estrogen concentrations in the surface water, outlet water, groundwater, and duck manure/soil mixture at three duck farms in Taiwan. Natural estrogen species included estrone (E1), 17β-estradiol (E2), estriol (E3), estrone-3-sulfate (E1-3S), 17β-estradiol-3-sulfate (E2-3S), estrone-3-glucuronide (E1-3G), and 17β-estradiol-3-glucuronide (E2-3G), whereas synthetic estrogen included 17α-ethynylestradiol (EE2) and diethylstilbestrol (DES). This study showed that the total estrogen concentrations in the surface water and groundwater were 15.4 and 4.5 ng/L, respectively, which constituted 56% and 58%, respectively, conjugated estrogen. From the pond to the outlet water, the total estrogen concentration decreased by 3.9 ng/L (23% loss) in the duck farms. However, the estrogenic potency was slightly reduced from 0.91 to 0.88 E2 equivalent/L, showing a negligible decrease. From the pond to the outlet water, the field results showed that converting the conjugated estrogen into free estrogen in the duck farm-released water increased their environmental hazard. Primarily E1, with an average concentration of 0.9 ± 1.6 ng/g, was present in the duck manure. The estrogen excreted by the ducks in the pond (from surface water to outlet water) was estimated to be 0.18 kg/million head-year. Although the estrogen concentration in the duck farms was low, the environmental impact of CAFO should not be neglected.

Insights into the role of the microbial community lifestyle strategies in variations of the dissolved organic matter molecular composition along an effluent-dominated river

Urbanization has dramatically changed the quality and quantity of dissolved organic matter (DOM) fluxes in rivers, thereby affecting the diversity and lifestyle strategies of microbial communities. However, relationships between DOM molecular composition and microbial lifestyle strategies in effluent-dominated rivers are poorly understood. Herein, we investigated the variations in DOM structure and composition of an effluent-dominated river and further revealed how these changes alter the abundance and lifestyle strategies of microbial communities. Results demonstrated that macromolecular (MW > 35 kDa) humic-like substances constituted the major components of effluent-dominated riverine DOM. Also, due to the degradation of humic-like substances, the accumulation of protein-like substances was observed from upstream to downstream areas, corresponding to an apparent decrease in overall aromaticity. The abundance of bacterial, Actinobacteria, and eukaryotic was higher in the upstream and midstream areas but relatively lower in the downstream area. The response of bacterial and Actinobacteria communities to the changes in DOM composition was more prominent as compared to that of eukaryotic. Based on multivariate statistical analysis, the decrease in aromatic components (MW > 35 kDa) was mainly attributed to the degradation of Proteobacteria and Actinobacteria (K-strategists), resulting in a decrease in their relative abundance along the river course. Proteins and polysaccharides (15 kDa < MW < 35 kDa, MW < 6 kDa) were more easily utilized by Firmicutes and Bacteroidetes (r-strategists), leading to an increase in their relative abundance. With the decrease of macromolecular humic-like substances and the increase of protein-like substances, river microbial communities shifted from K-strategists to r-strategists. This work unveils the evolution of DOM in an effluent-dominated river and the influence of the degradation of macromolecular humic-like substances on r/K-strategists.

Characterization of dissolved black carbon and its binding behaviors to ceftazidime and diclofenac pharmaceuticals: Employing the molecular weight fractionation

As the ubiquitous component of the aquatic environment, dissolved organic matter (DOM) readily bind with residual pharmaceutical contaminants (PCs) and influence their environmental behaviors. However, the binding mechanisms between dissolved black carbon (DBC), a vital part of the natural DOM pool, and PCs were poorly researched. In this study, the bulk DBC was divided into four fractions in molecular weight (MW) via an ultrafiltration system, and the properties of DBC and their binding interaction with two kinds of typical PCs (ceftazidime (CAZ) and diclofenac (DCF)) were explored concretely. The results showed that low MW component was the main contributor to bulk DBC, and the aromaticity increased with the increase of MW. The categories of chemical structures and fluorescent substances in different MW DBC were similar. Multispectral techniques showed that the oxygen-enriched compounds in DBC had the higher affinity to CAZ/DCF. The -NH-, -COOH, -NH₂ groups in CAZ molecules appeared to form the hydrogen bond with DBC. Fluorescence quenching experiments were analyzed, and the binding mechanisms were specifically expounded from the thermodynamic perspective. The fluorophore of fulvic acid-like compounds (FA) were quenched by both static and dynamic quenching mechanisms, while only static quenching occurred for humic acid-like compounds (HA). For bulk DBC, the hydrogen bond and van der Waals force were the major forces in the HA-CAZ system, while the hydrophobic force made the primary contribution to the HA-DCF system, which might be ascribed to the higher hydrophobic nature of DCF. Notably, with the increase of HA MW, the main binding mode of HA-CAZ/DCF changed from hydrophobic force to hydrogen bond and van der Waals force gradually, which also directly proved that various noncovalent interactions co-driven the binding processes. Our findings are beneficial to better assess the fate of DBC and PCs and the corresponding complexes in the aquatic environment.

Impact of environmental factors and tributary contributions on tidal dissolved organic matter dynamics

Riverine dissolved organic matter (DOM) transport was a key step in the carbon biogeochemical cycle while we had limited understanding of its contribution to the estuary DOM dynamics. This study focused on the river downstream-to-tidal estuary DOM variation and the control of environmental factors on it. The contributions of three tributaries with varing urbanization degrees to the tidal DOM dynamics were evaluated. Though more aromatics were introduced to the urban tributary, the A₂₅₀/A₃₆₅ values and fluorescent index values indicated the DOM molecular size was uniformly reduced due to the enhanced microbial degradation during transport. The tidal DOM showed less varied spectroscopic indexes than the tributary DOM, but tidal cycles strongly impacted the fluorescent DOM quantified by the fluorescence regional integration (FRI). Salinity range can differentiate the fluorescent DOM variation patterns in river tributaries (e.g., <2.5, positive correlations; >2.5, negative correlations) and tidal cycles (>10, negative correlations). For tidal DOM, the high salinity decreased more humic-related components, resulting in increased proportions of protein-related components in high tides. The dissolved oxygen and nitrogen contents were negatively correlated with salinity, suggesting the microbial contributions and anthropogenic inputs in tributaries increased the tidal DOM quantity. The less urbanized tributaries contributed more to the low-tide DOM compositions/properties while the dynamic contribution of the urban tributary impacted more the tidal DOM dynamics. Our results highlighted the uneven declines of FRI values of different components by freshwater-saltwater mixing in estuaries and suggested the different functioning of urban, agro-urban, and suburban tributaries contributed to tidal DOM dynamics.

The impact of natural sunlight irradiation on the biotoxicities of different molecular sizes EfOM/SRNOM and its relationship with spectral and molecular level parameters

Natural sunlight irradiation is regarded as an efficient and low-carbon method for controlling the biotoxicity of effluents from domestic wastewater treatment plants (WWTPs). Dissolved organic matter in WWTPs effluent (EfOM) is responsible for the non-specific biotoxicity of effluent. In the present study, the variation in spectral characteristics, molecular composition, luminescent bacteria toxicity, and genotoxicity of EfOM of different molecular sizes (MOSs) during natural sunlight irradiation were investigated from a systematic perspective, and the standard natural organic matter from the Suwannee River (SRNOM) was synchronously assessed for comparative purposes. To further explore the cause of the biotoxicity changes, the relationships between the spectral or molecular level parameters (obtained from FT-ICR MS analysis) and biotoxicity were assessed using correlation analysis. The molecules in <1 kDa EfOM with lower molecular weight, higher unsaturation degree, and higher humification and fluorescence had higher luminescent bacteria toxicity under sunlight irradiation. However, in the <1 kDa SRNOM, the molecules which were characterized by higher humification and fluorescence had higher luminescent bacteria toxicity. The notable genotoxicity reduction of EfOM under sunlight irradiation was attributed to the photochemical degradation of components with a high unsaturation degree. Such findings could enable ecological safety improvement of aquatic environments using natural sunlight.

Electrochemical treatment for leachate membrane retentate: Performance comparison of electrochemical oxidation and electro-coagulation technology

With the widespread use of membrane in advanced treatment of leachate, China produces a large amount of leachate membrane retentate (LMR) (≈23.4 million tons) annually, which is usually treated by incineration or recirculation in engineering, but these technologies have many drawbacks. LMR is suitable for electrochemical treatment due to its high electrical conductivity. This study compared the performances of electrochemical oxidation (EO) and electro-coagulation (EC) technology on LMR treatment under different experimental conditions, including anode material, current density, initial pH and reaction time. We found that EO optimal conditions achieved 70.1%, 83.1%, 78.7%, 98.7%, and 69.7% removal of total organic carbon (TOC), UV absorption (at 254 nm), chromaticity, ammonia nitrogen (NH₃-N), and total nitrogen (TN), respectively. Compared with EO, EC exhibited a similar removal ability for orgainics and better removals of chroma, but much less performance for removing nitrogen pollutants in the same reaction time, that is, removals of NH₃-N and TN were only 31.5% and 36.2%, respectively. Meanwhile, EC showed much higher instantaneous current efficiency of COD than EO under its optimal reaction time (120 min). In addition, the UV-Vis spectra and 3D fluorescence spectra indicated that EO exhibited relatively outstanding performance in decomposing dissolved organic matter (DOM) with rather complicated structures than EC. Also, the flow field-flow fractionation technique demonstrated that EO preferentially destroy humic-like, large molecular weight DOM, and converting them to smaller molecules, which resulted in more volatile organic compounds in EO samples than EC samples. While EC had little selectivity in the removal of organics, except humic-like DOM with relative small molecular. These findings can provide a theoretical basis for the electrochemical treatment of LMR.

Cognizing and characterizing the organic phosphorus in lake sediments: Advances and challenges

Organic phosphorus (OP) is one of the main forms of phosphorus in lake ecosystems. Mounting evidence has shown that sediment OP has become a major but underestimated issue in addressing lake eutrophication and algal bloom. However, a holistic view of sediment OP remains missing. This review aims to provide an overview of progress on the studies of OP in lake sediments, focusing on the contribution of OP to internal P loading, its potential role in algal bloom, and the migration and transformation. In addition, this work systematically summarized current methods for characterizing OP content, chemical fraction, composition, bioavailability, and assessment of OP release in sediment, with the pros and cons of each method being discussed. In the end, this work pointed out following efforts needed to deepen the understanding of sediment OP, namely: (1) In-depth literature review from a global perspective regarding the contribution of sediment OP to internal P loading with further summary about its pattern of distribution, accumulation and historical changes; (2) better mathematical models for describing drivers and the linkages between the biological pump of algal bloom and the replenishment of sediment OP; (3) fully accounting the composition and molecular size of OP for better understanding its transformation process and mechanism; ; (4) developing direct, high-sensitivity and combined techniques to improve the precision for identifying OP in sediments; (5) establishing the response of OP molecular properties and chemical reactivity to OP biodegradability and designing a comprehensive and accurate composite index to deepen the understanding for the bioavailability of OP; and (6) integrating fundamental processes of OP in current models to better describe the release and exchange of P in sediment-water interface (SWI). This work is expected to provide critical information about OP properties and deliver perspectives of novel characterization methods.

Novel insights into molecular composition of organic phosphorus in lake sediments

Organic phosphorus (Po) plays a key role in eutrophication and ecological equilibrium in lake systems. However, characterizing the composition of Po in lake sediments has been a bottleneck hindering further understanding of the biogeochemical cycle of Po. Here, multiple methods of 31P NMR spectroscopy and molecular weight (MW) ultrafiltration were combined to detect Po composition characteristics from a novel angle in ten lake sediments of China. The results showed that sediment Po mainly consisted of monoester (mono-P, 14±8.8% of the NaOH-EDTA total P on average), diester (di-P, 1.4±1.4%) and phosphonate (phos-P, 0.1±0.1%), while the abundance of Po was largely underestimated by 31P NMR methods. Some specific species of mono-P were successfully determined, and the contents of these species followed a decreasing order: inositol hexakisphosphate (IHP6) > RNA mononucleotides (RNA-mnP) > β-glycerophosphate (β-gly) > D-glucose 6-phosphate (Glu-6) > α-glycerophosphate (α-gly), which was largely dependent upon their bioreactivity. A significant relationship between MW and Po components was observed despite the great differences among sediment samples. For refractory Po components, IHP6 was mainly rich in the MW < 3 kDa while phos-P was almost only detected in the MW > 3 kDa, which largely attributed to their metal binding affinities and characteristics. The abundance of bioreactive Po species (α-gly, β-gly, Glu-6, di-P) in high MW (HMW, > 3 kDa) were all higher than that of low MW (LMW, < 3 kDa) due to microbial degradation and self-assembly. If the HMW organic molecules were biologically and chemically more reactive than its LMW counterparts, the high percentage of α-gly, β-gly, glu-6 and di-P in the HMW portion would highlights their high reactivity from the perspective of MW. These insights revealed the dynamics of the MW distribution of Po components and provide valuable information to better understand the Po composition and bioreactivity in sediments.

The spectral characteristics and cadmium complexation of soil dissolved organic matter in a wide range of forest lands

The quality and quantity of dissolved organic matter (DOM) greatly controls the fate of heavy metals. The characteristics of DOM and its interaction with metals are essential for the metal ecological risk assessment of soils. In this study, the DOM spectral characteristics of representative forest soils and the complex capacities between fluorescent DOM components and cadmium (Cd) were analyzed. Functional groups, such as carboxylic acids, alcohols and phenols, were determined by FT-IR analysis. Chromophoric DOM, fluorescent DOM and dissolved organic carbon (DOC) concentrations exhibited strong correlations with each other, indicating that variations of DOC could be well explained by Chromophoric DOM or fluorescent DOM due to high correlation coefficients. The spectral slope ratio was in the range of 0.85-5.90, implying an abundance of heavy macromolecular humic acids, peptides, and polycondensates. The absorbance spectral at 254 nm (SUVA₂₅₄) strongly correlated with SUVA₂₆₀ (r = 0.992, P < 0.01), indicating that hydrophobicity closely related with aromatic structure, and aromatic groups could be broadly hydrophobic. Fluorescence indices were from 1.62 to 2.21 and biological index values ranged from 0.54 to 1.14, where the DOM was mainly sourced from mixed terrestrial and autogenous inputs in most sites. Four universal fluorescence components were identified and characterized by fluorescence EEM-PARAFAC, including two humic-like (components 1 and 2), one tyrosine-like (components 3) and one fulvic-like (components 4) component. Both components 3 and 4 showed fluorescence quenching with increasing Cd concentrations, while components 1 and 2 had no evident change in fluorescence intensity. The logK₃ and logK₄ values ranged from 4.41 to 5.29 and 4.71 to 5.54, respectively, with most logK values of component 3 for Cd binding being smaller than that of component 4, thus, indicating that the fulvic acid substances exhibited stronger and more stable interactions with Cd than protein-like components.

The role of terrestrial productivity and hydrology in regulating aquatic dissolved organic carbon concentrations in boreal catchments

The past decades have witnessed an increase in dissolved organic carbon (DOC) concentrations in the catchments of the Northern Hemisphere. Increasing terrestrial productivity and changing hydrology may be reasons for the increases in DOC concentration. The aim of this study is to investigate the impacts of increased terrestrial productivity and changed hydrology following climate change on DOC concentrations. We tested and quantified the effects of gross primary production (GPP), ecosystem respiration (RE) and discharge on DOC concentrations in boreal catchments over 3 years. As catchment characteristics can regulate the extent of rising DOC concentrations caused by the regional or global environmental changes, we selected four catchments with different sizes (small, medium and large) and landscapes (forest, mire and forest-mire mixed). We applied multiple models: Wavelet coherence analysis detected the delay-effects of terrestrial productivity and discharge on aquatic DOC variations of boreal catchments; thereafter, the distributed-lag linear models quantified the contributions of each factor on DOC variations. Our results showed that the combined impacts of terrestrial productivity and discharge explained 62% of aquatic DOC variations on average across all sites, whereas discharge, gross primary production (GPP) and RE accounted for 26%, 22% and 3%, respectively. The impact of GPP and discharge on DOC changes was directly related to catchment size: GPP dominated DOC fluctuations in small catchments (<1 km² ), whereas discharge controlled DOC variations in big catchments (>1 km² ). The direction of the relation between GPP and discharge on DOC varied. Increasing RE always made a positive contribution to DOC concentration. This study reveals that climate change-induced terrestrial greening and shifting hydrology change the DOC export from terrestrial to aquatic ecosystems. The work improves our mechanistic understanding of surface water DOC regulation in boreal catchments and confirms the importance of DOC fluxes in regulating ecosystem C budgets.

Investigation of the variations in dissolved organic matter properties and complexations with two typical heavy metals under the influence of biodegradation: A survey of an entire lake

Biodegradation is a key factor determining the properties and metal binding behaviour of dissolved organic matter (DOM). In this study, the contributions of biodegradation to DOM properties and metal binding behaviour in Dongping Lake were explored by using synchronous fluorescence (SF) spectroscopy, two-dimensional correlation spectroscopy (2D-COS) excitation-emission matrix and parallel factor analysis (EEM-PARAFAC). According to the ratio of the fluorescence intensity of different materials to the entire fluorescence intensity (%F_max), protein-like substances were the main substances of DOM in this lake. The reduction of protein-like substances and the enhancement of humification could be found in the whole lake under the influence of biodegradation. 3 areas (Area A, Area B and Area C) were obtained by principal component analysis (PCA), however, PCA results suggested that DOM properties and sources had some differences in the 3 areas, and DOM bioavailability in Area C was stronger than that in the other 2 areas. With copper (Cu²⁺) and lead (Pb²⁺) addition, different substances exhibited various affinities to different metal types. The locations of crosspeaks in asynchronous maps illustrated that protein-like substances were more affiliative with Cu²⁺, while humic-like substances were bound to Pb²⁺ earlier. Biodegradation had a conspicuous impact the metal binding ability of DOM in Dongping Lake. The effective quenching constants (LogK) of protein-like substances (protein-like component 2: LogK_Cu = 3.85 ± 0.23, LogK_Pb = 3.32 ± 0.23) were higher than those of humic-like substances (humic-like component 3: LogK_Cu = 3.15 ± 0.02, LogK_Pb = 2.93 ± 0.17) for both Cu²⁺ and Pb²⁺ before biodegradation. When biodegradation was finished, binding ability of humic-like substances was enhanced from 3.15 ± 0.02 to 3.41 ± 0.10 for DOM-Cu and 2.93 ± 0.17 to 3.79 ± 0.15 for DOM-Pb. On a spatial scale, metal binding ability of DOM in Dongping Lake also changed due to the influence of biodegradation. For both DOM-Cu and DOM-Pb, binding ability in south of Dongping Lake was stronger than that in other areas with the end of biodegradation.

Differences in the spectroscopic characteristics of wetland dissolved organic matter binding with Fe3+, Cu2+, Cd2+, Cr3+ and Zn2

Understanding of the binding characteristics of wetland dissolved organic matter (DOM) and different metals is important for the quantitative assessment of the environmental behavior of metals in wetlands. In this study, different types of spectroscopy including ultraviolet-visible absorption, Fourier transform infrared, and fluorescence spectroscopy was used to investigate the binding characteristics of Fe³⁺, Cu²⁺, Cr³⁺, Cd²⁺, and Zn²⁺ with DOM from wetland water. Differential absorption spectra identified binding sites for these five metals in this wetland DOM at 210 nm, 280 nm, 335 nm, and > 400 nm regions. The low binding capacity of DOM in this wetland with Cd and Zn indicated that the toxicity and environmental effects of these metals in this wetland warrant further study. The calculated △EEM combined with fluorescence regional integration (FRI) analysis clearly revealed that Fe and Cu preferred to bind with humic-like DOM while Cd and Zn preferred to bind with protein-like DOM in this wetland. △EEM successfully demonstrated the characteristics of DOM complexing with different metals and could be a compelling tool in evaluating metal-DOM interactions. In addition, 2D-FTIR-COS identified the binding sites and the dynamic processes of binding at the functional group level. Metals preferentially bind with the CO, CO functional group, and then binds to the OH functional group. This study revealed that different DOM components will facilitate the migration of different metals in the environment and provided new slights into an improved understanding of migration and transformation of metals in aquatic environments.

Spectroscopic characterization of dissolved organic matter from macroalgae Ulva pertusa decomposition and its binding behaviors with Cu(II)

Dissolved organic matter (DOM) from macroalgae is regarded a crucial source of autochthonous DOM in coastal ocean. In the present study, the characteristics of DOM from the macroalgae Ulva pertusa decomposition (U. pertusa-DOM) and its binding behaviors with Cu(II) using multiple spectroscopic techniques and chemometric analyses. The labile U. pertusa-DOM could be consumed and transformed by microorganisms. The absorption spectroscopic descriptors indicate that the hydrophobicity, aromaticity, and molecular weight of the U. pertusa-DOM increase during the 27-day incubation period. Fluorescence excitation-emission matrix spectroscopy combined with parallel factor analysis suggests that the relative abundance of the protein-like component (C1) (96.10-84.96%) sequentially decreases, whereas the humic-like components (C2) (2.16-9.73%) and (C3) (1.75-5.31%) in the U. pertusa-DOM increase with the U. pertusa decomposition. The Cu(II) binding properties of the U. pertusa-DOM are dependent on the decomposition time. The order of the conditional stability constant (logK_M) is C2 > C1 > C3. The complexation capacity (f) of C1 is higher than those of C2 and C3 at a specific time. Synchronous fluorescence spectroscopy coupled with two-dimensional correlation spectroscopy reveals that the microbial degradation could accelerate the Cu(II) binding to humic-like fractions in the U. pertusa-DOM. These findings will help us better understand the biogeochemical behaviors of macroalgal DOM and heavy metal in coastal ecosystems.

The relationship between the molecular composition of dissolved organic matter and bioavailability of digestate during anaerobic digestion process: Characteristics, transformation and the key molecular interval

In this study, swine wastewater (SW) and cow wastewater (CW) were used for anaerobic digestion (AD). We found the bioavailability of dissolved organic matter (DOM) was affected by the molecular weight ranges and molecular composition during the AD process. The organic substance in the small molecular range (0-5 kDa) accumulated due to a larger molecular fraction (>10 kDa) degradation, which enhanced the bioavailability of the DOM. Moreover, based on the excitation emission matrix-parallel factor (EEM-PARAFAC) analysis, the protein-like component in 0-5 kDa molecular size and humic-like component over 5 kDa are significantly positively correlated with DOM bioavailability. This study indicated that increasing the hydrolysis of larger organic matter and humification degree of molecular weights>5 kDa are critical solutions to improving the bioavailability of DOM. These conclusions can help explain the molecular mechanisms of DOM transformation and the AD process of livestock wastewater.

Exploring applicability of end member mixing approach for predicting environmental reactivity of dissolved organic matter

Despite the wide applications of end member mixing analysis (EMMA) for assigning the sources of dissolved organic matter (DOM) in aquatic environment, there was no study attempting to test the applicability of EMMA for predicting environmental reactivity of DOM. This study aimed to explore the feasibility of EMMA, or the concept of ideal mixing behavior of end members, for describing several well-known DOM reactivities using two DOM end member sources (i.e., soil and algae) at varying mixing ratios. The selected DOM reactivities were trihalomethane formation potential (THMFP), mineral adsorption amount, pyrene binding, membrane resistance, and biodegradation potential. Among the tested DOM functions, all were found to follow the ideal mixing behavior, presenting the linear relationships between the source mixing ratios and the tested reactivity with the R² value of >0.80. The ideal mixing behavior of the DOM functions was more pronounced than that based on several spectroscopic indicators derived from UV absorption and fluorescence spectroscopy. This study provided insight into potential applicability and limitation of EMMA approach in monitoring and predicting environmental functions of DOM in aquatic systems where identified DOM sources are mixed and vary dynamically with the mixing ratios.

Solid-Phase Extraction of Aquatic Organic Matter: Loading-Dependent Chemical Fractionation and Self-Assembly

Dissolved organic matter (DOM) is an important component in marine and freshwater environments and plays a fundamental role in global biogeochemical cycles. In the past, optical and molecular-level analytical techniques evolved and improved our mechanistic understanding about DOM fluxes. For most molecular chemical techniques, sample desalting and enrichment is a prerequisite. Solid-phase extraction has been widely applied for concentrating and desalting DOM. The major aim of this study was to constrain the influence of sorbent loading on the composition of DOM extracts. Here, we show that increased loading resulted in reduced extraction efficiencies of dissolved organic carbon (DOC), fluorescence and absorbance, and polar organic substances. Loading-dependent optical and chemical fractionation induced by the altered adsorption characteristics of the sorbent surface (styrene divinylbenzene polymer) and increased multilayer adsorption (DOM self-assembly) can fundamentally affect biogeochemical interpretations, such as the source of organic matter. Online fluorescence monitoring of the permeate flow allowed to empirically model the extraction process and to assess the degree of variability introduced by changing the sorbent loading in the extraction procedure. Our study emphasizes that it is crucial for sample comparison to keep the relative DOC loading (DOC_load [wt %]) on the sorbent always similar to avoid chemical fractionation.

Comparison in UV-induced photodegradation properties of dissolved organic matters with different origins

Dissolved organic matter (DOM) is ubiquitous in aquatic environments, whose behaviors and fate are highly related to the chemical compositions and size distribution. In this study, the UV-induced photodegradation properties of DOMs with different origins (i.e., macrophyte- and algae-derived) were investigated using absorption and fluorescence spectroscopy as well as flow field-flow fractionation (FlFFF). Results showed that, irrespective of DOM origins, the chromophoric components could be more effectively photo-degraded than the non-chromophoric ones. Though the two DOMs were characterized with similar fluorophores, the photodegradation properties showed obvious heterogeneities in DOM origins and molecular weights (MWs). Compared to macrophyte-derived DOM (MDOM), the algae-derived DOM (ADOM) exhibited a higher degradation rate and efficiency due to the abundance of labile components like newborn protein-like substances. The FlFFF results revealed a high photo-preferability of 100 kDa-0.45 μm protein-like MDOM and same photo-sensitivity of the size-fractionated humic-like moieties, testifying the reduction of molecular sizes during the photodegradation. However, the increase in relative percentage for 100 kDa-0.45 μm protein-like components and 5-15 kDa humic-like moieties implied a possible enhancement of molecular sizes for ADOM during the early period (i.e., the first hour) of photodegradation. This study provides new insights into the origin-related heterogeneities in compositions and size distribution for DOM transformation.

Characterization and biogeochemical implications of dissolved organic matter in aquatic environments

Dissolved organic matter (DOM) is viewed as one of the most chemically active organic substances on earth. It plays vital roles in the fate, bioavailability and toxicity of aquatic exogenous chemical species (e.g., heavy metals, organic pollutants, and nanomaterials). The characteristics of DOM such low concentrations, salt interference and complexity in aquatic environments and limitations of pretreatment for sample preparation and application of characterization techniques severely limit understanding of its nature and environmental roles. This review provides a characterization continuum of aquatic DOM, and demonstrate its biogeochemical implications, enabling in-depth insight into its nature and environmental roles. A synthesis of the effective DOM pretreatment strategies, comprising extraction and fractionation methods, and characterization techniques is presented. Additionally, the biogeochemical dynamics of aquatic DOM and its environmental implications are discussed. The findings indicate the collection of representative DOM samples from water as the first and critical step for characterizing its properties, dynamics, and environmental implications. However, various pretreatment procedures may alter DOM composition and structure, producing highly variable recoveries and even influencing its subsequent characterization. Therefore, complimentary use of various characterization techniques is highly recommended to obtain as much information on DOM as possible, as each characterization technique exhibits various advantages and limitations. Moreover, DOM could markedly change the physical and chemical properties of exogenous chemical species, influencing their transformation and mobility, and finally altering their potential bioavailability and toxicity. Several research gaps to be addressed include the impact of pretreatment on the composition and structure of aquatic DOM, molecular-level structural elucidation for DOM, and assessment of the effects of DOM dynamics on the fate, bioavailability and toxicity of exogenous chemical species.

Formation and mechanisms of hydroxyl radicals during the oxygenation of sediments in Lake Poyang, China

Seasonal flooding-drought transformation process of lake sediments lead to changes of dissolved oxygen and redox conditions and the resultant generation of hydroxyl radical (HO^•). To date, information on HO^• formation and its regulators in seasonal lake sediments is largely unexplored. In this study, a total of nineteen sediments were collected from Lake Poyang, China, with the formation and mechanisms of HO^• during the oxygenation process exploring via the incubation experiments, Fe K-edge X-ray adsorption spectroscopy, ultrafiltration, and fluorescent spectroscopy. Results showed that the concentrations of HO^• generated ranged from 3.75 ± 1.13 to 271.8 ± 22.81 μmol kg^-1, demonstrating high formation potential and obvious spatial heterogeneity. The yield of HO^• formed was positively correlated with the contents of Fe(II), sedimentary organic carbon, and dissolved organic carbon, showing a general contribution of these reduced substances to HO^• formation. Furthermore, application of Fe K-edge X-ray adsorption spectroscopy revealed the key species of sedimentary Fe-smectite for HO^• formation due to its high peroxidase-like activity. Besides inorganic Fe(II), the sedimentary dissolved organic matters (DOMs) represented an important regulator for HO^• formation, which contributed about 2-11% of the total HO^• generation. Moreover, the DOM-induced formation potential was found to be highly related to the molecular weight distribution that the low molecular weight- (LMW, <1 kDa) fraction exhibited higher HO^• formation potential than the bulk and high molecular weight- (HMW, 1 kDa-0.45 μm) counterparts. In addition, the omnipresent mineral Fe(II)-DOM interaction in sediment matrix exhibited another 2-6% of contribution to the total HO^• production. This study highlighted the importance of contents and species of Fe(II) and DOM in manipulating the HO^• yield, providing new insight into understanding the formation mechanisms of HO^• in the seasonal lake sediment.

Simultaneous changes of exogenous dissolved organic matter treated by ozonation in properties and interaction behavior with sulfonamides

Effluent is often treated with ozone before being discharged into a natural water environment. This process will change the interaction between effluent organic matter and pollutants in aquatic environment. The impact of ozonation on complexation between dissolved organic matter in such wastewater and sulfadimidine often found in natural water was studied in laboratory experiments using four types of real wastewater. Ozonation was found to decrease the proportion of organic matter with a molecular weight greater than 5 kDa as well as protein-like, fulvic-like and humic-like components, but except the proportion of hydrophilic components. The aromaticity of the dissolved organic matter was also reduced after ozonation. The complexation of tryptophan and tyrosine with sulfadimidine mainly depends on their hydrophobicity and large molecular weight. Ozonation of fulvic and humic acid tends to produce small and medium molecular weight hydrophilics. The complexation of humic and fulvic acids with sulfadimidine was enhanced by ozonation. Dissolved organic matter, with or without oxidation, were found to weaken sulfadimidine's inhibition of microbial growth, especially for Aeromonas and Acinetobacter species. This finding will expand our understanding about the impact of advanced treatment processes on the dissolved organic matters' properties in effluent.

Anaerobic and aerobic biodegradation of soil-extracted dissolved organic matter from the water-level-fluctuation zone of the Three Gorges Reservoir region, China

The biodegradation of dissolved organic matter (DOM) in natural environments is determined by its molecular composition and reactivity. Redox oscillations are common in the water-level-fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR). As a consequence, the soil DOM released is degraded under both anaerobic and aerobic conditions. The DOM compounds available for degradation under contrasting redox conditions and the resulting DOM composition still need to be elucidated. By combining laboratory experiments with an in-depth characterization of DOM optical properties, we show that different pathways controlled the depletion and enrichment of the DOM optical components under different oxygen regimes. In particular, 28-day dark biodegradation assays showed that up to 39.5 ± 4% DOM was degraded under anaerobic conditions, while 55.5 ± 6% DOM was biodegraded under aerobic conditions. Aerobic biodegradation resulted in a higher aromaticity and degree of humification of the DOM compared to anaerobic degradation. The specific UV absorbance at a wavelength of 254 (SUVA₂₅₄) and biological index (BIX) could be used to track DOM biodegradation under anaerobic conditions. Under aerobic conditions, the SUVA₂₅₄, BIX and concentration of coloured DOM (CDOM, reflected by a (355)) could track DOM biodegradation, and significant amounts of CDOM could be aerobically biodegraded.

Chromophoric dissolved organic matter in inland waters: Present knowledge and future challenges

Chromophoric dissolved organic matter (CDOM) plays an important role in the biogeochemical cycle and energy flow of aquatic ecosystems. Thus, systematic and comprehensive understanding of CDOM dynamics is critically important for aquatic ecosystem management. CDOM spans multiple study fields, including analytical chemistry, biogeochemistry, water color remote sensing, and global environmental change. Here, we thoroughly summarize the progresses of recent studies focusing on the characterization, distribution, sources, composition, and fate of CDOM in inland waters. Characterization methods, remote sensing estimation, and biogeochemistry cycle processes were the hotspots of CDOM studies. Specifically, optical, isotope, and mass spectrometric techniques have been widely used to characterize CDOM abundance, composition, and sources. Remote sensing is an effective tool to map CDOM distribution with high temporal and spatial resolutions. CDOM dynamics are mainly determined by watershed-related processes, including rainfall discharge, groundwater, wastewater discharges/effluents, and biogeochemical cycling occurring in soil and water bodies. We highlight the underlying mechanisms of the photochemical degradation and microbial decomposition of CDOM, and emphasize that photochemical and microbial processes of CDOM in inland waters accelerate nutrient cycling and regeneration in the water column and also exacerbate global warming by releasing greenhouse gases. Future study directions to improve the understanding of CDOM dynamics in inland waters are proposed. This review provides an interdisciplinary view and new insights on CDOM dynamics in inland waters.

A Review of Current and Emerging Approaches for Water Pollution Monitoring

The aquatic ecosystem is continuously threatened by the infiltration and discharge of anthropogenic wastewaters. This issue requires the unending improvement of monitoring systems to become more comprehensive and specific to targeted pollutants. This review intended to elucidate the overall aspects explored by researchers in developing better water pollution monitoring tools in recent years. The discussion is encircled around three main elements that have been extensively used as the basis for the development of monitoring methods, namely the dissolved compounds, bacterial indicator, and nucleic acids. The latest technologies applied in wastewater and surface water mapped from these key players were reviewed and categorized into physicochemical and compound characterizations, biomonitoring, and molecular approaches in taxonomical and functional analyses. Overall, researchers are continuously rallying to enhance the detection of causal source for water pollution through either conventional or mostly advanced approaches focusing on spectrometry, high-throughput sequencing, and flow cytometry technology among others. From this review’s perspective, each pollution evaluation technology has its own advantages and it would be beneficial for several aspects of pollutants assessments to be combined and established as a complementary package for better aquatic environmental management in the long run.

Investigation on the interaction differences of algogenic dissolved organic matter released by Prorocentrum donghaiense with phenanthrene and 3-methylphenanthrene using spectroscopy

Despite the high algogenic dissolved organic matter (A-DOM) content in the marine environment, the impact of A-DOM on the environmental behavior of alkylated polycyclic aromatic hydrocarbons (A-PAHs) has rarely been reported. Thus, this study sought to investigate the changes in A-DOM and its interaction with phenanthrene (Phe) and 3-methylphenanthrene (3-MPhe). The results showed that the spectroscopic characteristics of A-DOM changed dynamically during the entire growth period of Prorocentrum donghaiense. The total proportions of humic-like components in A-DOM, including marine humic-like and longer wavelength humic-like components, increased from 26.30% at the initial phase to 40.42% at the decline phase, but those of protein-like components decreased. Humic-like components showed stronger interaction capabilities with Phe/3-MPhe than that of protein-like components. The interaction characteristics and increased the humic-like components content indicated their prominent role in the interaction of A-DOM and Phe/3-MPhe. 3-MPhe had stronger hydrophobic interactions with the humic-like components and the methyl group increased π-π interactions with A-DOM on day 28. Methyl group also increased the molecular weight and steric hindrance of 3-MPhe, resulting in a lower interaction capability with A-DOM than that of Phe on day 16. The diverse interaction behaviors of A-DOM fluorescent components with Phe and 3-MPhe were largely attributed to dynamic changes in the A-DOM characteristics. Methyl substitution varied the hydrophobicity and molecular weight of Phe and 3-MPhe, and further influenced the interactions with A-DOM fluorescent components. These results highlight the vital impact of dynamic A-DOM on the environmental behavior of PAHs, and the differences in the environmental behavior between parent and alkylated PAHs should also receive additional attention.

Using ESI FT-ICR MS to Characterize Dissolved Organic Matter in Salt Lakes with Different Salinity

Dissolved organic matter (DOM) composition in salt lakes is critical for water quality and aquatic ecology, and the salinization of salt lakes affects the DOM composition. To the best of our knowledge, no study has explored the effects of salinity on salt lake DOM composition at the molecular level. In this work, we selected Qinghai Lake (QHL) and Daihai Lake (DHL) as typical saline lakes. The two lakes have similar geographical and climatic conditions, and the salinity of QHL is higher than that of DHL. Fourier transform ion cyclotron resonance mass spectrometry coupled with electrospray ionization was applied to compare the DOM molecular composition in the two lakes. At higher salinity, the DOM showed larger average molecular weight, higher oxidation degree, and lower aromaticity. Moreover, the proportion of DOM that is vulnerable to microbial degradation (e.g., lipids), photo-degradation (e.g., aromatic structures), or both processes (e.g., carbohydrates and unsaturated hydrocarbons) reduced at higher salinity. On the contrary, compounds that are refractory to microbial degradation (e.g., lignins/CRAM-like structures and tannins) or photo-degradation (e.g., aliphatic compounds) accumulated. Our study provides a useful and unique method to study DOM molecular composition in salt lakes with different salinity and is helpful to understand DOM transformation during the salinization of salt lakes.

Molecular Sorption Mechanisms of Cr(III) to Organo-Ferrihydrite Coprecipitates Using Synchrotron-Based EXAFS and STXM Techniques

Ubiquitous organo-ferrihydrite coprecipitates (OFC) significantly affect the mobility and availability of Cr in soil through sorption, but the underlying sorption mechanisms remain unclear at the molecular level. Due to the potential formation of OFC in agricultural soils with returned crop straws, we synthesized OFC with rice/rape straw-derived carbon (C) sources and different loadings. The molecular sorption mechanisms of Cr(III) to the synthesized OFC under different conditions were investigated by Cr K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy and scanning transmission X-ray microscopy (STXM). Cr(III) sorption by OFC decreased with increasing C loading and decreasing pH, regardless of C sources. Moreover, inhibition of Cr(III) sorption to OFC with high C loading occurred when ionic strength (IS) increased, suggesting the presence of outer-sphere complexed Cr(III). EXAFS analysis revealed that more Cr(III) were bound to ferrihydrite of the OFC at a relatively high pH, and organically bound Cr(III) enhanced when increasing C loading and decreasing IS. STXM analysis strongly suggested that C loading reduced Cr(III) sorption through blocking the binding sites on the ferrihydrite, which overwhelmed Cr(III) retention by the direct binding of Cr(III) to carboxyl of the particulate organic matter (OM) and OM coated on the Fh fractions of the OFC. These findings facilitated the comprehensive understanding of the sorption mechanisms of Cr(III) to OFC at the molecular level, which will assist the prediction of Cr(III) mobility in soils, particularly for Cr(III)-contaminated agricultural soils with the application of crop straws.