Interactions between stepwise-eluted sub-fractions of fulvic acids and protons revealed by fluorescence titration combined with EEM-PARAFAC

Algal extracellular polymeric substance compositions drive the binding characteristics, affinity, and phytotoxicity of graphene oxide in water

Graphene oxide (GO, a popular 2D nanomaterial) poses great potential in water treatment arousing considerable attention regarding its fate and risk in aquatic environments. Extracellular polymeric substances (EPS) exist widely in water and play critical roles in biogeochemical processes. However, the influences of complex EPS fractions on the fate and risk of GO remain unknown in water. This study integrates fluorescence excitation-emission matrix-parallel factor, two-dimensional correlation spectroscopy, and biolayer interferometry studies on the binding characteristics and affinity between EPS fractions and GO. The results revealed the preferential binding of fluorescent aromatic protein-like component, fulvic-like component, and non-fluorescent polysaccharide in soluble EPS (S-EPS) and bound EPS (B-EPS) on GO via π-π stacking and electrostatic interaction that contributed to a higher adsorption capacity of S-EPS on GO and weaker affinity than of B-EPS. Moreover, the EPS fractions drive the morphological and structural alterations, and the attenuated colloid stability of GO in water. Notably, GO-EPS induced stronger phytotoxicity (e.g., photosynthetic damage, and membrane lipid remodeling) compared to pristine GO. Metabolic and functional lipid analysis further elucidated the regulation of amino acid, carbohydrate, and lipid metabolism contributed to the persistent phytotoxicity. This work provides insights into the roles and mechanisms of EPS fractions composition in regulating the environmental fate and risk of GO in natural water.

Spatial differences of dissolved organic matter composition and humification in an artificial lake

The depth-dependent dynamics of dissolved organic matter (DOM) structure and humification in an artificial lake limits the understanding of lake eutrophication and carbon cycling. Using fluorescence regional integration (FRI) and parallel factor analysis (PARAFAC) models to analyze the 3D fluorescence spectroscopy dataset, we revealed the depth-dependent structure and vertical distribution of DOM in the estuarine and center regions of Lake Hongfeng. The percentage fluorescence response (Pi,n) showed humic acid is an important part of DOM in Lake Hongfeng. Fluorescence results show that the fulvic-like and protein-like materials in HF1-DOM located at the estuarine position showed greater variation in the middle stage, probably due to human influence and sediment suspension. Fluorescence index (PI+II+IV,n/PIII+V,n and FIC4/FIC3) can be used to indicate the degree of humification of DOM in artificial lakes. Results of each index show that the estuary is more affected by human activities, and the humification degree is significantly lower than that of the center of the lake. The evaluation index system of the humification degree of artificial lake established in this study can effectively predict the eutrophication state of the typical area of artificial lake and deeply understand the possible important influence of human activities on the carbon cycle of lake.

Magnetic nanoparticle loading and application of weak magnetic field to reconstruct the cake layer of coagulation-ultrafiltration process to achieve efficient antifouling: Performance and mechanism analysis

Abandoning the costly development of new membrane materials and instead directly remodeling the naturally occurring cake layer constitutes a dynamic, low-cost, long-lasting, and proactive strategy to "fight fouling with fouling". Several optimization strategies, including coagulation/modified magnetic seed loading and applying a weak magnetic force (0.01T) at the ultrafiltration end, improved the anti-fouling, retention, and sieving performances of conventional ultrafiltration process during the treatment of source water having complex natural organic matter (NOMs) and small molecule micropollutants. Two modified magnetic seeds we prepared were composite nano-seed particles (Fe3O4@SiO2-NH2 (FS) and Fe3O4@SiO2@PAMAM-NH2 (FSP)). Aim of the study was to regulate the formation of cake layer via comprehensive testing of the antifouling properties of optimized processes and related mechanistic studies. It was found to be essential to enhance the interception of xanthate and tryptophan proteins in the cake layer for improving the anti-fouling performance based on the correlation and redundancy analyses, while the use of modified magnetic seeds and magnetic field showed a significant positive impact on water production. Blockage modeling demonstrated the ability to form a mature cake layer during the initial filtration stage swiftly. This mitigated the risk of irreversible fouling caused by pore blockage during the early stage of coagulation-ultrafiltration. Morphologically, the reconstructed cake layer exhibited elevated surface porosity, an internal cavity channel structure, and enhanced roughness that can promote increased water flux and retention of water impurities. These optimized the maturity of the cake layer in both time and space. Density Functional Theory (DFT), Extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory, and Modified Extended Derjaguin-Landau-Verwey-Overbeek (MDLVO) calculations indicated aggregation behavior of matter on the cake layer to be enhanced effectively due to magnetic seed loading. This is mainly due to the strengthening of polar interactions, including hydrogen bonding, π-π* conjugation, etc., which can happen between the cake layer loaded with FSP and the organic matter. Under the influence of a magnetic field, magnetic force energy (VMF) significantly impacts the system by eliminating energy barriers. This research will provide innovative strategies for effectively purifying intricate source water through ultrafiltration while controlling membrane fouling.

Leaching behaviors of dissolved organic matter from face masks revealed by fluorescence EEM combined with FRI and PARAFAC

Despite numerous studies investigating the occurrence and fate of microplastics, no effort has been devoted toward exploring the characteristics of dissolved organic matter (DOM) leached from face masks mainly made of plastics and additives used in large quantities during the COVID-19 pandemic. By using FTIR, UV-vis, fluorescence EEM coupling with FRI and PARAFAC, and kinetic models of leaching experiments, we explored the leaching behaviors of face mask-derived DOM (FM-DOM) from commonly used face masks including N95, KN95, medical surgical masks, etc. The concentration of FM-DOM increased quickly at early 0-48 h and reached equilibrium at about 48 h measured in terms of dissolved organic carbon and fluorescence intensity. The protein-like materials ranged from 80.32 % to 89.40 % of percentage fluorescence response (Pi,n) were dominant in four types of FM-DOM analyzed by fluorescence EEM-FRI during the leaching experiments from 1 to 360 h. Four fluorescent components were identified, which included tryptophan-like components, tyrosine-like components, microbial protein-like components, and fulvic-like components with fluorescence EEM-PARAFAC models. The multi-order kinetic model (Radj2 0.975-0.999) fitted better than the zero-order and first-order kinetic model (Radj2 0.936-0.982) for all PARAFAC components of FM-DOM based on equations derived by pseudo kinetic models. The leaching rate constants (kn) ranged from 0.058 to 30.938 and the half-life times (T1/2) ranged from 2.73 to 24.87 h for four FM-DOM samples, following the solubility order of fulvic-like components (C4) > microbial protein-like components (C3) > tryptophan-like components (C1) > tyrosine-like components (C2) for FM-DOM from four types of face masks during the leaching experiment from 0 to 360 h. These novel findings will contribute to the understanding of the underappreciated environment impact of face masks in aquatic ecosystems.

Insights into the effect of hydrochar-derived dissolved organic matter on the sorption of diethyl phthalate onto soil: A pilot mechanism study

Biowaste-derived hydrochar is an emerging close-to-natural product and has shown promise for soil improvement and remediation, but the environmental behavior of the dissolved organic matter released from hydrochar (HDOM) is poorly understood. Focusing on the typical mulch film plasticizer diethyl phthalate (DEP), we investigated the effect of HDOM on the sorption behavior of DEP on soil. The relatively low concentration of HDOM (10 mg L-1, 25 mg L-1) decreases the sorption quantity of DEP on soil, while it increases by a relatively high concentration, 50 mg L-1. The transformation from multilayer to monolayer sorption of DEP on soil occurs as the concentration of HDOM increases. The tryptophan-like substance is the main component of HDOM sorbed to soil, reaching 49.82 %, and results in competition sorption with DEP. The soil pores are blocked by HDOM, which limits the pore filling and mass transfer of DEP, but partitioning is significantly enhanced. The surface functional groups in HDOM are similar to those in soil, and chemical sorption, mainly composed of hydrogen bonding, exists but is not significantly strengthened. We identified the specific impact of HDOM on the sorption of organic pollutants on soil and provide new insights into the understanding of the environmental behavior of hydrochar.

Toxic effects and mechanisms of cationic blue SD-GSL on Chlorella vulgaris before and after the biological decolorization process

Biodegradation is regarded as an efficient way to decolorize azo dyes. However, the changes in the algal toxicity of azo dyes during biodecolorization are still unclear. In this study, the physiological responses of Chlorella vulgaris to the hydrophobic and hydrophilic components of cationic blue SD-GSL (a typical monoazo dye) and its biodecolorization products were investigated. The toxicity of each component to Chlorella vulgaris and the sources of the toxicity were analyzed. The cationic blue SD-GSL components inhibited the algal cell division and superoxide dismutase (SOD) activity at all concentrations, and inhibited the synthesis of chlorophyll-a (Chl-a) at concentrations >100 mg/L, whereas increased the malondialdehyde (MDA) content. The hydrophobic and hydrophilic components of its biodecolorization products had enhanced inhibition rates on cell density (10.7% and 15.6%, respectively), Chl-a content (31.2% and 8.4%, respectively), and SOD activity (13.5% and 1.9%, respectively) of Chlorella vulgaris, and further stimulated an increase in MDA content (4.4% and 7.0%, respectively), indicating that the biodecolorization products were more toxic than the pristine dye. Moreover, the toxic effect of hydrophobic components on Chlorella vulgaris was stronger than that of hydrophilic components. The sensitivity sequence of Chlorella vulgaris to the toxicity of cationic blue SD-GSL and its biodecolorization product components was: Chl-a synthesis > SOD activity > cell division. SUVA analysis and 3D-EEM analysis revealed that the enhanced algal toxicity of the biodecolorization products of cationic blue SD-GSL was attributed to the aromatic compounds, which were mainly concentrated in the hydrophobic components. UPLC-Q-TOF-MS was used to verify dye biodecolorization byproducts. The information obtained from this study helps to understand the decolorization products toxicities of the biologically treated azo dyes, thereby providing new insights into the environmental safety of textile wastewater after traditional biological treatment.

Interaction between biochar-dissolved organic matter and chlorophenols during biochar adsorption

Biochar (BC) has been widely applied in the remediation of chlorophenols (CPs) from contaminated sites in which the role and mechanisms of BC dissolved organic matter (BDOM), as a crucial component of BC, with CPs are largely unknown and thus need to be investigated. In this study, DOM was derived from peanut hulls (PDOM) and corn stalks (CDOM) as BC sources, and the interactions between PDOM/CDOM and 2,4,6-trichlorophenol (TCP) were analysed using excitation-emission matrix spectroscopy (EEM) in combination with multiple models. EEM combined with fluorescence region integration (EEM-FRI) indicated that humic-like materials were the major materials of both PDOM and CDOM (percentage fluorescence response R_i,n > 60%), and CDOM contained more protein- and fulvic-like materials than PDOM. Based on EEM in combination with parallel factor analysis (EEM-PARAFAC), 4 components were obtained, and the percentage decrease in maximum fluorescence intensities (F_max) showed that the main components interacting with TCP in PDOM/CDOM were protein- and fulvic-like components (> 25%). Moreover, the modified Stern-Volmer model was used to calculate the stability constants (Log K_TCP) of PDOM/CDOM and TCP for the first time, and the mechanism of static quenching was dominant for interacting with TCP in PDOM (Log K_TCP: 4.36-4.65) and CDOM (Log K_TCP: 3.53-4.73). Furthermore, the sequential TCP binding of fluorescent components in BDOM generally followed the order of protein-like → short-wavelength fulvic-like → long-wavelength fulvic-like → humic-like components. These findings will provide a basis for screening biochar as a functional material for CP remediation applications and for understanding the environmental chemical behaviour of leached DOM during biochar application.

The adsorption-release behavior of sediment phosphorus in a typical "grass-algae" coexisting lake and its influence mechanism during the transition sensitive period

In the early stage of eutrophication, the coexistence of "grass and algae" in lakes is obvious. Understanding the P sorption-desorption behavior in natural sediments during the ecologically sensitive transition period has important scientific value for predicting the deterioration of lake ecosystems and formulating restoration measures, but the related mechanisms are still unclear. In this study, the analysis results of sedimentary dissolved organic matter (DOM) fractions, extractable Fe (hydr)oxide fractions and P adsorption experiments showed that sedimentary DOM fractions, especially the tyrosine-like protein fractions and microbial humic-like fractions, played a part in determining the EPC₀ and K_d values of sediments in the plateau lake environment. The compound effect of amorphous Fe (hydr)oxides and sedimentary OM affected the increase of sedimentary P adsorption. Interestingly, these phenomena were strongly correlated with water depth. Furthermore, the distribution of water depth to aquatic plants indirectly regulated the values of sedimentary EPC₀ and K_d. Meanwhile, the ability of submerged plants to control the sedimentary EPC₀andK_d values will be forced to shift shallowly, thereby forcing a significant reduction of areas with low EPC₀ and high K_d values. This not only enhanced the risk of endogenous P release in lakes, but also accelerated the further deterioration of aquatic ecosystems. Therefore, studying the long-term scale changes of sedimentary EPC₀ and K_d values can help to understand the duration of the lake ecological transition period and prevent the transitional deterioration of ecosystem.

Effects of pyrolysis temperature on proton and cadmium binding properties onto biochar-derived dissolved organic matter: Roles of fluorophore and chromophore

Understanding the environmental behavior of biochar-derived dissolved organic matter (BDOM) is crucial for promoting the extensive utilization of biochar and meeting the carbon neutrality targets. However, limited studies focused on the binding mechanism of protons and Cd with DOM released from biochar produced at different pyrolysis temperatures. By combining excitation-emission matrix spectroscopy and parallel factor analysis, we found that the humic-like fluorophores in BDOM had higher aromaticity, molecular weight, and contents of carboxylic and phenolic groups relative to the protein-like fluorophores. Conversely, the protein-like fluorophores exhibited a stronger binding affinity for Cd than humic-like fluorophores. With the pyrolysis temperature increased from 300 °C to 500 °C, the quenching effects of Cd on the protein-like components were enhanced significantly. Their fluorescence intensities could be quenched up to 51.64%. The results of ultraviolet-visible absorbance spectroscopy and differential absorbance spectroscopy showed that the carboxylic-like and phenolic-like chromophores were involved in the protons and Cd binding process of BDOM. The binding ability of phenolic-like chromophores with Cd was reduced as a function of increasing pyrolysis temperature. These findings implied that these carboxylic and phenolic groups were mainly contained in the non-fluorescent components. Besides, protons and Cd could also induce inter-chromophore interactions in BDOM, and the interaction was proportional to the pyrolysis temperature. These results clearly demonstrated the pyrolysis temperature-dependent changes in the protons and Cd binding properties of BDOM. More importantly, the possible risk of Cd mobility caused by the protein-like components in BDOM cannot be ignored when the biochar was applied in contaminated soils. This research extends our knowledge of the application potentiality of biochar in heavy metal polluted soil.

Characterization of mercury binding to different molecular weight fractions of dissolved organic matter

Dissolved organic matter (DOM) can strongly complex with various contaminants. Therefore, DOM was deemed as an environmentally friendly substance for controlling the mobility, bioavailability, speciation, toxicity, and fate of metal contaminants in environment. In this study, composting-derived DOM was categorized into three fractions based on different molecular weights (MWs). Furthermore, parallel faction analysis (PARAFAC), two-dimensional correlation spectroscopy (2DCOS), and a complexation model were employed to reveal the contaminate-binding characterization. Two PARAFAC fluorescence components were identified in the MW < 1 kDa and the 1 kDa < MW < 5 kDa fractions, respectively. In the MW > 5 kDa fraction, three PARAFAC components were identified. Protein-like component C5 did not have the Hg²⁺-binding ability. Moreover, the results showed that not all the DOM ligands could bind with contaminants, but a high humification degree of composting DOM ligands could strongly bind Hg²⁺. In addition, DOM ligand with a low humification degree DOM ligands presented a higher Hg²⁺-binding speed. Subsequently, DOM from different MWs DOM could be applied separately to the different pollution forms. With these promising results, the different DOM ligand-Hg²⁺-binding properties were characterized to provide theoretical support for environmental pollution control.

Simultaneous bio-reduction of nitrate and Cr(VI) by mechanical milling activated corn straw

Abundant lignocellulose waste is an ideal energy source for environmental bioremediation, but its recalcitrance to bioavailability makes this a challenging prospect. We hypothesized that the disruption of straw's recalcitrant structure by mechanochemical ball milling would enhance its availability for the simultaneous bioreduction of nitrate and Cr(VI). The results showed that the ball-milling process increased the quantity of water-soluble organic matter released from corn straw and changed the composition of organic matter by strongly disrupting its lignocellulose structure. The increase in ball-milling time increased the specific surface area of the straw and favored the adhesion of microorganisms on the straw surface, which enhanced the bioavailability of the energy in the straw. Substantially increased removal of NO₃^--N (206.47 ± 0.67 mg/g) and Cr(VI) (37.62 ± 0.09 mg/g) was achieved by using straw that was ball milled for 240 min, which validated that ball milling can improve the utilization efficiency of straw by microorganisms. Cellular and molecular biological analyses showed that ball-milled straw increased microbial energy metabolism and cellular activity related to the electron transport chain. This work offers a potential way to achieve the win-win goal of utilizing agricultural wastes and remediating environmental pollution.

Photochemical Reactivity of Humic Substances in an Aquatic System Revealed by Excitation-Emission Matrix Fluorescence

The photochemical reactivity of humic substances plays a critical role in the global carbon cycle, and influences the toxicity, mobility, and bioavailability of contaminants by altering their molecular structure and the mineralization of organic carbon to CO₂. Here, we examined the simulated irradiation process of Chinese standard fulvic acid (FA) and humic acid (HA) by using excitation-emission matrix fluorescence combined with fluorescence regional integration (FRI), parallel factor (PARAFAC) analysis, and kinetic models. Humic-like and fulvic-like materials were the main materials (constituting more than 90%) of both FA and HA, according to the FRI analysis. Four components were identified by the PARAFAC analysis: fulvic-like components composed of both carboxylic-like and phenolic-like chromophores (C1), terrestrial humic-like components primarily composed of carboxylic-like chromophores (C2), microbial humic-like overwhelming composed of phenolic-like fluorophores (C3), and protein-like components (C4). After irradiation for 72 h, the maximum fluorescence intensity (F _max) of C1 and C2 of FA was reduced to 36.01-58.34%, while the F _max of C3 of both FA and HA also decreased to 0-9.63%. By contrast, for HA, the F _max of its C1 and C2 increased to 236.18-294.77% when irradiated for 72 h due to greater aromaticity and photorefractive tendencies. The first-order kinetic model (R ² = 0.908-0.990) fitted better than zero-order kinetic model (R ² = 0-0.754) for the C1, C2, and C3, of both FA and HA, during their photochemical reactivity. The photodegradation rate constant (k ₁) of C1 had values (0.105 for FA; 0.154 for HA) that surpassed those of C2 (0.059 for FA, 0.079 for HA) and C3 (0.079 for both FA and HA) based on the first-order kinetic model. The half-life times of C1, C2, and C3 ranged from 6.61-11.77 h to 4.50-8.81 h for FA and HA, respectively. Combining an excitation-emission matrix with FRI and PARAFAC analyses is a powerful approach for elucidating changes to humic substances during their irradiation, which is helpful for predicting the environmental toxicity of contaminants in natural ecosystems.

Combined forms of Pb and its detoxification and absorption in Cladophora rupestris subcells

This study aims to analyze the combined form, detoxification, and adsorption mechanism of Pb in Cladophora rupestris subcells. The chemical form analysis at different concentrations (0, 0.5, 1.0, 2.5, 5.0, 7.5, and 10 mg/L) indicated that most of the Pb (37%-76%) were integrated with oxalate and undissolved phosphate, which were important to the detoxification of C. rupestris. The characterization of Pb (0, 0.5, and 5.0 mg/L) at the subcellular was conducted via Fourier-transform infrared spectroscopy (FTIR), Three-dimensional excitation-emission matrix spectroscopy (3D-EEM), and protein secondary structure fitting. Results revealed that Pb-polysaccharides ((C₆H₅)-OO-Pb-OH, C-O-Pb, and symmetric Pb-O-Pb), Pb-functional-groups ((C₆H₅)-COO-Pb and (C₆H₅)-P = O-Pb), and Pb-protein complexes (OH-C₇H₆-CN-Pb-COOH, C₉H₁₀-NH-CN-C = O-Pb, Pb-S-C, and Pb-S) were formed. The cell wall produced transport proteins, such as metallothionein and glutathione, which bound and helped Pb²⁺ enter the cell. After entering the soluble fraction, the Pb-organic acid ((C₆H₅)-COO-Pb, (C₆H₅)-O-Pb, and (C₆H₅)-P = O-Pb) and Pb-sulfhydryl compound (Pb-S-C/Pb-S) assumed the most important role in resisting the toxicity of Pb²⁺. Pb²⁺ was absorbed in the organelle and formed (C₆H₅)-C-O-Pb and (C₆H₅)-P = O-Pb, and complexed with protein (Pb-C-N) when treated with 5.0 mg/L Pb. Results could help understand the role of subcellular fraction in the algal adaptation to stressful heavy metal conditions.

Copper-binding properties of microplastic-derived dissolved organic matter revealed by fluorescence spectroscopy and two-dimensional correlation spectroscopy

Despite numerous studies on microplastics (MPs), little attention has been paid to the dissolved organic substances leached from MPs and their environmental fate. In this study, we explored the copper-binding characteristics of MP-derived dissolved organic matter (MP-DOM) leached from several MP types, including commercial polypropylene, polyvinylchloride, and expanded polystyrene, under dark and UV irradiation conditions. The copper-binding affinity of MP-DOM was examined using fluorescence quenching method based on different fluorophores identified via the excitation emission matrix-parallel factor analysis (EEM-PARAFAC). The heterogeneous distribution of binding sites across the functional groups of MP-DOM was further elucidated by utilizing two-dimensional correlation spectroscopy (2D-COS) based on Fourier transform infrared spectroscopy (FTIR). Phenol/protein-like fluorescence prevailed in all MP-DOM samples, whereas humic-like fluorescence was more pronounced in the irradiated MP-DOM. For all tested plastic types, two plastic-derived fluorescent components (C2 and C3) exhibited substantial fluorescence quenching with increasing copper concentrations. The calculated stability constants showed larger differences between the two leaching conditions than between the three MP types with higher log K_M values for the UV-irradiated (4.08-5.36) than dark-treated MP-DOM (1.05-3.60). The binding constants were comparable to those of natural organic matter with aquatic/terrestrial origins. The 2D-COS results further revealed that the oxygen-containing structures in MP-DOM generated by UV irradiation might be responsible for the higher binding affinity of the irradiated MP-DOM. This is the first study demonstrating the environmental reactivity of MP-DOM towards metal binding, highlighting the importance of leaching conditions for the metal-binding behavior of MP-DOM.

Effects of C/N ratio variation in swine biogas slurry on soil dissolved organic matter: Content and fluorescence characteristics

Nutrient-rich biogas slurry shows favorable prospects for application as an organic fertilizer in farmland. At the same time, due to differing sources and treatment methods, the C/N ratio of biogas slurry varies greatly. The effect of differences in C/N of biogas slurry on soil organic matter properties remains unclear. In this experiment, pig farm biogas slurry differing in C/N (3, 6, 8.84 and 12) was applied instead of fertilizer. Fluorescence spectroscopy combined with parallel factor analysis and principal component analysis were used to determine dissolved organic carbon (DOC) and fluorescent dissolved organic matter (FDOM) in soil dissolved organic matter (DOM). The experimental results showed that the DOC and FDOM contents of soil could be significantly increased at the initial stage of biogas slurry application. Compared with CK, on the 60th day, biogas slurry with a C/N of 12 exhibited the greatest improvement in DOC, FDOM as well as for Component 1, Component 2 and Component 3 contents in soil FDOM, 40.93%, 66.25%, 65.35%, 40.47%, and 78.42% respectively. However, compared with the 0th day, by the 60th day, biogas slurry with a C/N of 8.84 exhibited the greatest decrease in Component 4 content in soil FDOM, 74.68%. Biogas slurry with a C/N of 8.84 exerted the greatest promotion effect on corn growth, the utilization and transformation of Component 4 by plants and microorganisms in the soil. And it showed the strongest improvement in the degree of FDOM humification in the soil, the humification index increased from 4.16 ± 0.17 to 4.92 ± 0.58, compared with CK. This study provided new insights for the utilization of biogas slurry with respect to soil physical and chemical properties and maize plant growth.

Molecular investigation into the transformation of dissolved organic matter in mature landfill leachate during treatment in a combined membrane bioreactor-reverse osmosis process

This study investigated the effectiveness of a combined membrane bioreactor (MBR) and reverse osmosis (RO) process for treating leachate produced by a large-scale anaerobic landfill. The MBR process had limited treatment efficiency for removing organic pollutants, but when combined with RO, the integrated system completely removed macromolecular compounds (i.e., humic- and fulvic-like substances) and produced effluent that satisfied the applicable discharge standard. The landfill leachate contained many types of DOM that had high molecular weight and were highly unsaturated. Although the MBR process removed some DOM that had a relatively low saturated degree (mainly aliphatic compounds (2.0 ≥ H/C ≥ 1.5) with relatively high bioavailability), many bio-refractory compounds were not removed. The RO system greatly reduced the content of residual DOM in MBR effluent and was effective for removing heteroatom DOM, especially polycyclic aromatics (AI > 0.66) and polyphenols (0.66 ≥ AI > 0.50). The effluent from the combined process of MBR and RO treatment mainly contained a small number of aliphatic compounds and phenolic compounds (AI ≤ 0.50 and H/C < 1.5) that had higher bioavailability than DOM in the raw leachate and posed little environmental risk.

Investigation of eluted characteristics of fulvic acids using differential spectroscopy combined with Gaussian deconvolution and spectral indices

The characteristics of subfractions of soil fulvic acid (FA₃, FA₅, FA₇, FA₉, and FA₁₃) using stepwise elution from XAD-8 resin with pyrophosphate buffers were investigated by differential absorption spectroscopy (DAS) and differential fluorescence spectroscopy (DFS) combined with mathematical deconvolution and spectral indices. The log-transformed absorbance spectra (LTAS) exhibited three regions for both acidic-buffer-eluted subfractions (AESF) and neutral-buffer-eluted subfraction (NESF) and four regions for basic-buffer-eluted subfractions (BESF) according to the differences in spectral slopes. The DAS spectra of FA subfractions were closely fitted with seven Gaussian bands with maxima location at 199.66, 216.18 ± 1.50, 246.20 ± 3.85, 285.22 ± 7.26, 345.64 ± 5.30, 389.27, and 307.37 nm, respectively (R² > 0.993). The content of salicylic-like and carboxyl groups in FA subfractions decreased, while the phenolic chromophore increased with elution sequence. From the 11 spectral indices, AESF had greater molecular weight, condensation, polymerization, hydroxyl radical production, humification degree, and terrigenous contribution, as well as contained more conjugated aromatic structures and less N-containing groups than NESF and BESF. The humification degree and humic characters of FA subfractions were closely associated to the aromaticity, molecular condensation, and DOM-metal-bound functional groups. The proper separation of FA into subfractions is beneficial for reducing its complexity and heterogeneity, which helps us to further explore its chemical properties and interactions with various contaminants in soil environments. Graphical abstract.

Simulated photo-degradation of dissolved organic matter in lakes revealed by three-dimensional excitation-emission matrix with regional integration and parallel factor analysis

Simulated photo-degradation of fluorescent dissolved organic matter (FDOM) in Lake Baihua (BH) and Lake Hongfeng (HF) was investigated with three-dimensional excitation-emission matrix (3DEEM) fluorescence combined with the fluorescence regional integration (FRI), parallel factor (PARAFAC) analysis, and multi-order kinetic models. In the FRI analysis, fulvic-like and humic-like materials were the main constituents for both BH-FDOM and HF-FDOM. Four individual components were identified by use of PARAFAC analysis as humic-like components (C1), fulvic-like components (C2), protein-like components (C3) and unidentified components (C4). The maximum 3DEEM fluorescence intensity of PARAFAC components C1-C3 decreased by about 60%, 70% and 90%, respectively after photo-degradation. The multi-order kinetic model was acceptable to represent the photo-degradation of FDOM with correlation coefficient (R_adj²) (0.963-0.998). The photo-degradation rate constants (k_n) showed differences of three orders of magnitude, from 1.09 × 10^-6 to 4.02 × 10^-4 min^-1, and half-life of multi-order model ( T_1/2ⁿ) ranged from 5.26 to 64.01 min. The decreased values of fluorescence index (FI) and biogenic index (BI), the fact that of percent fluorescence response parameter of Region I (P_I,n) showed the greatest change ratio, followed by percent fluorescence response parameter of Region II (P_II,n), while the largest decrease ratio was found for C3 components, and the lowest T_1/2ⁿ was observed for C3, indicated preferential degradation of protein-like materials/components derived from biological sources during photo-degradation. This research on the degradation of FDOM by 3DEEM/FRI-PARAFAC would be beneficial to understanding the photo-degradation of FDOM in natural environments and accurately predicting the environmental behaviors of contaminants in the presence of FDOM.

Elimination of UV-quenching substances from MBR- and SAARB-treated mature landfill leachates in an ozonation process: A comparative study

Ultraviolet-quenching substances (UVQS), recently identified pollutants in landfill leachate, can interfere with ultraviolet disinfection when landfill leachate is co-treated with municipal sewage. This study investigated the elimination of UVQS in mature landfill leachates through a membrane bioreactor (MBR) and a semi-aerobic aged refuse biofilter (SAARB). Humus (i.e., fulvic and humic acids) was the main component of organic matter in both MBR- and SAARB-treated landfill leachates, while there was a more stable chemical structure of humus in the MBR-treated leachate. The concentration of UVQS in MBR-treated mature landfill leachate was higher than that of SAARB-treated leachate. Ozonation can degrade UVQS effectively, especially for landfill leachate containing a high concentration UVQS (i.e., MBR-treated landfill leachate). However, a large accumulation of small molecule acid might be caused by ozonation for highly concentrated UVQS in landfill leachate, leading to the delayed degradation of total organic carbon. Moreover, ozonation degraded both fulvic acid and humic acid; and degraded humic acid more effectively. For instance, 88.0% removal (MBR-CP2) and 96.0% removal (SAARB-CP2) of humic acid was higher than those (83.3% for MBR-CP1 and 92.3% for SAARB-CP1) of fulvic acid. The destruction of UV-quenching functional groups of organics (such as CC) by ozone was the main UVQS degradation mechanism of ozonation applied to MBR- and SAARB-treated landfill leachates. Therefore, the ozonation process can efficiently decrease UV absorption intensity in both MBR- and SAARB-treated landfill leachates.

Pyrolysis characteristics of soil humic substances using TG-FTIR-MS combined with kinetic models

The incorporation and cycling of pyrogenic organic matter in soil is a potential carbon sink, while the pyrolysis behaviors of soil organic matter are still lacking. Pyrolysis characteristics of soil fulvic acid (FA) and humic acid (HA) were investigated using thermogravimetry combined with Fourier transform infrared spectrometer-mass spectrometer (TG-FTIR-MS) and kinetic models. Four reaction stages corresponding to four pseudo-components were distinguished for both FA and HA. FA exhibited greater transformation contributions of hemicellulose-like and cellulose-like pseudo-components, while HA exhibited greater transformation contributions of lignin-like pseudo-components. Compared to HA, higher levels of heat-resistant aromatic compounds, phenolic groups, and carboxylic groups were recognized in FA. Values of both activation energy (Ea, 246.13-661.40 kJ·mol^-1) and pre-exponential factor (lnA, 53.49-107.16 min^-1) of FA were greater than corresponding Ea (241.74-466.70 kJ·mol^-1) and lnA (51.99-74.36 min^-1) values of HA determined by Flynn-Wall-Ozawa method and Distributed Activation Energy Model. The main pyrolysis reaction mechanisms of both FA and HA closely matched with the order-based model corresponding to 2nd and 3rd order random nucleation on an individual particle. The evolved gas species of H₂, CH₄, H₂O, and CO₂ were dominant for FA and HA pyrolysis. Generally, the total H₂/CO₂ and CH₄/H₂O releases were relatively larger for FA and HA pyrolysis, respectively. TG-FTIR-MS is shown to be an effective method to provide valuable and qualitative analysis of the gaseous volatile species evolved during HS pyrolysis. Findings from this systematic study of soil organic matter responding to pyrolysis will be critical for predicting the changes of soil systems or carbon cycle affected by future climate and fire regimes.

Experimental and modeling study of proton and copper binding properties onto fulvic acid fractions using spectroscopic techniques combined with two-dimensional correlation analysis

Fulvic acid (FA) significantly influences the bioavailability and fate of heavy metals in environments, while its acid-base characters and metal binding processes are still unclear. Here, spectroscopic techniques combined with multiple models (e.g., NICA-Donnan model) and two-dimensional correlation spectroscopy (2D COS) were applied to explore the proton and copper binding properties of FA sub-fractions (FA₃-FA₁₃). The charge densities, average contents of carboxylic and phenolic groups, average dissociation constants pKa₁ and pKa₂ of sub-fractions ranged 0-16 meq∙g∙C^-1, 5.03-9.58 meq∙g∙C^-1, 2.52-4.67 meq∙g∙C^-1, 4.15-4.33 and 8.52-9.72, respectively. FA sub-fractions had a relatively narrow distribution of carboxyl group and a broad distribution of phenolic group. FA sub-fractions also exhibited roughly two phenolic hydroxyl groups per every 1-3 phenyl rings. Differential absorbance spectra (DAS) derived Gaussian bands were associated to the inter-chromophore interactions, the changes of molecular conformations and functional groups with copper addition. Differential spectra slopes (DSlope_{275-295&325-375}) were more significant with higher copper concentration and copper amounts bonded to carboxylic groups. UV-Vis and fluorescence spectra with 2D heterospectral COS revealed the copper binding heterogeneities and sequential orders of chromophores and fluorophores, quantitatively confirming by the order of conditional stability constants (log K_Cu: 4.64-5.56). Salicylic-/polyhydroxyphenolic, hydroxyl and amino groups were strongly associated to the basic units for fluorophores. Sequential changes followed the order of humic-like→fulvic-like materials for FA₃/FA₅, humic-like→fulvic-like→tryptophan-like materials for FA₇, and humic-like→tryptophan-like→fulvic-like→tyrosine-like materials for FA₉/FA₁₃. Spectroscopic techniques combined with various models (especially for 2D COS) are beneficial to elucidate the binding heterogeneity and sensitivity for metal-organic matters at the functional group level.

Comparing electron donating/accepting capacities (EDC/EAC) between crop residue-derived dissolved black carbon and standard humic substances

Although dissolved black carbon (DBC) is an important component of dissolved organic matter (DOM), little is known about the electron transfer properties of DBC and their correlation with the structural properties. This study determined the electron donating/accepting capacities (EDC/EAC) of six DBC collected from water extracts of black carbon derived by pyrolyzing crop residues (soybean, wheat, rice, sorghum, peanut, and corn) using an electrochemical approach, along with their chemical and spectroscopic properties. The EDC of the tested DBC ranged from 2.42 to 7.10 mmol_e-(gC)^-1and was generally much higher than the EDC (1.31-3.78 mmol_e-(gC)^-1) of the four standard dissolved humic substances (DHS). In contrast, with the exception of the sorghum DBC, the EAC of all DBC (0.40-0.81 mmol_e-(gC)^-1) was apparently lower than the EAC of DHS (0.87-1.68 mmol_e-(gC)^-1). For the whole pool of DBC and DHS, a strong positive correlation (r = 0.94, P < 0.05) existed between the EDC and the phenolic group content quantified by base titration, suggesting that phenolic groups were the major electron-donating groups. The EAC and EDC of the tested DOM pool were further correlated with the components identified from the fluorescence excitation emission matrices and parallel factor (EEM-PARAFAC) analysis. This is the first study to show that DBC has significantly different electron transfer properties from humic substances and thus likely behaviors differently in many geochemical and environmental aspects. Capsule: Dissolved black carbon (DBC) has significantly different electron-donating/accepting capacities (EDC/EAC) from humic substances.

Correlations between slow pyrolysis characteristics and organic carbon structure of aquatic plant biomass

Thermal analysis techniques have been widely used to characterize natural organic matter; in particular, thermal oxidation has been used to examine soil and sediment organic matter. However, few studies have characterized natural organic matter (NOM) by using slow thermal degradation under a N₂ atmosphere. ¹³C nuclear magnetic resonance (NMR) spectroscopy, UV-Vis spectroscopy, and three-dimensional excitation and emission matrix (EEM) fluorescence spectroscopy were used to characterize aquatic plant biomass for the detailed interpretation of the structures of organic carbon during slow pyrolysis. There was a significant linear correlation between the absorption of heat (99-110 °C) and the loss of mass (110-160 °C) (r² = 0.507, p = 0.01), which indicates that the initial slight loss in mass of the plant materials was due to the loss of less thermally stable components. The release of heat (277-311 °C) and the ratio of the specific absorbances at 253 and 203 nm (A_253/203) were also correlated (r² = 0.388, p = 0.008), which suggests that the release of plant biomass upon heating was associated with the proportion of substituent groups on aromatic rings and that the release of heat increased with the amount of substitution. The coefficient of determination (r²) between fulvic acid-like fluorescence peaks and the loss of mass (230-340 °C) was 0.236 (p = 0.048). This result indicates that the loss of mass in the plant material samples was related to fulvic acid-like substances. More specifically, the reason for this result was the splitting of some aromatic functional groups, such as ether bonds, carbonyl groups, and oxygen heterocycles. In conclusion, these results suggest that the developed correlations between slow pyrolysis characteristics and organic carbon structures contribute to the investigation of the inner chemical structures of natural organic matter.

Depth-dependent variations of dissolved organic matter composition and humification in a plateau lake using fluorescence spectroscopy

In aquatic systems, dissolved organic matter (DOM) plays an important role in regulating the reactivity and transport of environmental pollutants. Fluorescence excitation-emission matrix (EEM) analysis combined with fluorescence regional integration (FRI) and parallel factorial analysis (PARAFAC) were applied to investigate the composition, humification degree and depth-dependent variation of DOM in Lake Baihua, China. The results showed that humic-like materials with more than 60% of percentage fluorescence response (P_i,n) were dominant in DOM. The molecular complexity and structural condensation of PARAFAC components showed the order of C1 (humic-like components) > C3 (protein-like components) > C4 (fulvic-like components) > C2 (fulvic-like components). The lower maximum fluorescence intensities (F_max) of components in surface layers were attributed to photo-oxidation of DOM by radiation. With depths less than 16 m, the F_max increases and decreases were attributed to accumulation of recalcitrant humic substances and microbial/abiotic degradation of particulate DOM, respectively. A combination of biological and humification indices could be used as indicators for distinguishing different degrees of humification and sources of DOM. DOM from Lake Baihua mainly originated from an aquatic bacterial source that consisted of an important/intermediate autochthonous component with a weak humic character. The fluorescence indices of P_I,n/P_II,n, P_I+II+IV,n/P_III+V,n, C2, C1/C2 and C2/(C3+C4) were more suitable to determine the humification degree. Stronger humic characters and higher humification degree for DOM were present at depths of 10-13 m. The fluorescence spectroscopy combined with fluorescence indices is convenient to investigate depth-dependent DOM characteristics and to assess water quality or pollution risk in lake systems.

Degradation of refractory organic contaminants in membrane concentrates from landfill leachate by a combined coagulation-ozonation process

Landfill leachate is a typical refractory wastewater for which research into rapid and efficient treatment methods has become very topical. In this study, a coagulation-ozonation process was developed to treat the concentrate arising from membrane treatment of landfill leachate. The effect of coagulant type and initial pH on treatment efficiencies was investigated. Results showed that many of organics were effectively removed in the coagulation process. Thereafter, ozone was applied to further treat the coagulation-resistant organic substances. Our results revealed that the degradation rate of these coagulation-resistant substances followed the trend (color number) CN > (light absorbance at 254 nm) UV₂₅₄ > (chemical oxygen demand) COD, and the residual coagulation-resistant substances were oxidized rapidly in the ozone process. Ozone first destroyed the molecular structure of fulvic acid and the by-products generated, such as protein-like substances. In addition, the molecular weight, organic condensation degree, and concentration of benzene ring compounds were considerably decreased. Moreover, the macro molecular organics (i.e., humic acid and fulvic acid) within the size range 1-100 kDa were effectively degraded and partially transformed into bicarbonate. Overall, the combined coagulation-ozonation process reduced COD, UV₂₅₄, and CN in the landfill leachate concentrate by 88.32%, 94.37%, and 98.83%, respectively, and thus the biodegradability of the treated leachate also was significantly improved. This excellent performance proved the feasibility of the combined coagulation-ozonation process for the removal of recalcitrant organic substances contained in landfill leachate concentrate, benefiting subsequent biological treatment.

Oxidative removal of recalcitrant organics in shale gas flowback fluid by the microwave-activated persulfate process

Shale gas flowback fluid (SGF) is generated during shale gas extraction and typically contains a variety of toxic and refractory organic compounds. In this work, a microwave-activated persulfate process (MW-PS process) was developed to pretreat SGF. The major factors influencing the treatment efficiency of the MW-PS process (PS dose, initial pH, MW power, and reaction time) were optimized, and the synergetic effect (SE), degradation of recalcitrant matter, and energy consumption were systematically investigated. Results showed that the SE of the process reached a high index (i.e., 9.85), suggesting a significant synergetic effect of MW and PS. In addition, under the optimal MW-PS condition (PS dose of 2.5 g/L, MW power of 900 W, and initial pH of 2), chemical oxygen demand removal reached 66.40% in a short reaction time of 10 min. Other analyses demonstrated that benzene series compounds, organic acids, lipid substances, alkanes, antioxidants, and fluorescent dissolved organic matter in SGF were decomposed to smaller-molecule organic matter, suggesting that refractory and toxic organic matter was removed by the MW-PS treatment process. Overall, the results of this study showed that MW-PS technology is an effective and promising method to treat SGF once the operation parameters are optimized.

Algal uptake of hydrophilic and hydrophobic dissolved organic nitrogen in the eutrophic lakes

Dissolved organic nitrogen (DON) derived from sediments plays an active role in biogeochemical cycling of nutrients in aquatic ecosystems. Sediments from four eutrophic lakes were studied using three-dimensional fluorescence excitation-emission matrix (3DEEM) spectra and supelite XAD-8 macroporous resin separation to investigate the bioavailability of hydrophilic and hydrophobic DON to algae (Microcystis flos-aquae (Wittr.) Kirchner). The results showed that the average loss of DON was <6.0% after dividing DON into hydrophilic and hydrophobic components, demonstrating the utility of XAD-8 resin separation in the study of DON components from lake sediments. The 3DEEM analysis showed that hydrophobic and hydrophilic DON comprised humic- and protein-like materials, respectively. During the incubation period, the bioavailability of hydrophilic DON, which accounted for 59.3%-80.4% of total DON, stimulated algal growth, suggesting that hydrophilic DON was the primary source of organic nitrogen for algae. In contrast, hydrophobic DON increased algal density by only 31.8% of that observed for hydrophilic DON, and had a small (accounted for 20.0%-26.6% of total DON) effect on algal growth over the short-term. The significant differences in algal growth between the two types of DON suggested that they should be considered separately in the eutrophic lake restorations.

Fluorescence regional integration and differential fluorescence spectroscopy for analysis of structural characteristics and proton binding properties of fulvic acid sub-fractions

Structural characteristics and proton binding properties of sub-fractions (FA₃-FA₁₃) of fulvic acid (FA), eluted stepwise by pyrophosphate buffer were examined by use of fluorescence titration combined with fluorescence regional integration (FRI) and differential fluorescence spectroscopy (DFS). Humic-like (H-L) and fulvic-like (F-L) materials, which accounted for more than 80% of fluorescence response, were dominant in five sub-fractions of FA. Based on FRI analysis, except the response of F-L materials in FA₉ and FA₁₃, maximum changes in percent fluorescence response were less than 10% as pH was increased from 2.5 to 11.5. Contents of carboxylic and phenolic groups were compared for fluorescence peaks of FA sub-fractions based on pH-dependent fluorescence derived from DFS. Static quenching was the dominant mechanism for binding of protons by FA sub-fractions. Dissociation constants (pKa) were calculated by use of results of DFS and the modified Stern-Volmer relationship. The pK_a of H-L, F-L, tryptophan-like and tyrosine-like materials of FA sub-fractions exhibited ranges of 3.17-4.06, 3.12-3.97, 4.14-4.45 and 4.25-4.76, respectively, for acidic pHs. At basic pHs, values of pKa for corresponding materials were in ranges of 9.71-10.24, 9.62-10.99, 9.67-10.31 and 9.33-10.28, respectively. At acidic pH, protein-like (P-L) materials had greater affinities for protons than did either H-L or F-L materials. The di-carboxylic and phenolic groups were likely predominant sites of protonation for both H-L and F-L materials at both acidic and basic pHs. Amino acid groups were significant factors during proton binding to protein-like materials of FA sub-fractions at basic pH.

Protonation-dependent heterogeneity in fluorescent binding sites in sub-fractions of fulvic acid using principle component analysis and two-dimensional correlation spectroscopy

Heterogeneous distributions of proton binding sites within sub-fractions of fulvic acid (FA₃-FA₁₃) were investigated by use of synchronous fluorescence spectra (SFS), combined with principle component analysis (PCA) and two-dimensional correlation spectroscopy (2D-COS). Tryptophan-like, fulvic-like and humic-like materials were observed in SFS. Tyrosine-like materials were identified by use of SFS-PCA analysis. Combined information from synchronous-asynchronous maps and dissociation constants (pKa) was used to describe heterogeneity of binding sites for protons within each sub-fraction. Heterogeneous distributions of proton binding sites were observed in fulvic-like, humic-like, tryptophan-like, and tyrosine-like materials of five sub-fractions and even in the single fulvic-like materials in FA₃ and tryptophan-like materials in FA₉ and FA₁₃. Values of pKa of sub-fractions ranged from 2.20 to 5.29, depending on associated wavelengths in synchronous-asynchronous maps and use of the modified Stern-Volmer equation. The larger values of pKa (4.17-5.29) were established for protein-like materials (including tryptophan-like and tyrosine-like materials) in comparison to those (2.20-3.38) for humic-like and fulvic-like materials in sub-fractions. Sequential variations of 274nm (pKa 4.15-5.29)→360-460nm (pKa 2.78-2.39) for FA₅-FA₁₃ revealed that binding of protons to tryptophan-like materials appeared prior to humic-like/fulvic-like materials. In FA₉, protons were preferentially binding to tryptophan-like materials than tyrosine-like materials. In FA₃, protons were preferentially binding to humic-like materials than fulvic-like materials. Relative differences of values of pKa for fluorescent materials within each sub-fraction were consistent with sequential orders derived from asynchronous maps. Such an integrated approach, SFS-PCA/2D-COS, has superior potential for further applications in exploring complex interactions between dissolved organic matter and contaminants in engineered and natural environments.