Changes in fluorescent dissolved organic matter upon interaction with anionic surfactant as revealed by EEM-PARAFAC and two dimensional correlation spectroscopy

Interaction of Pb(II) with microplastic-sediment complexes: Critical effect of surfactant

In this study, the impact of surfactants on the adsorption behavior of Pb(II) onto microplastics-sediment (MPs-S) complexes was investigated. Firstly, virgin polyamide (VPA) and polyethylene (VPE) were placed in Xiangjiang River sediment for six months to conduct in-situ aging. The results indicated that the biofilm-developed polyamide (BPA) and polyethylene (BPE) formed new oxygen-containing functional groups and different biofilm species. Furthermore, the adsorption capacity of Pb(II) in sediment (S) and MPs-S complexes was in the following order: S > BPA-S > VPE-S > VPA-S > BPE-S. The addition of sodium dodecyl benzenesulfonate (SDBS) promoted the adsorption of Pb(II), and the adsorption amount of Pb(II) increased with the higher concentration of SDBS, while adding cetyltrimethylammonium bromide (CTAB) showed the opposite result. The adsorption process of MPs-S complexes to Pb(II) was dominated by chemical adsorption, and the interaction between MPs-S complexes and Pb(II) was multilayer adsorption involving physical and chemical adsorption when the surfactants were added. Besides, the pH exerts a significant effect on Pb(II) adsorption in different MPs-S complexes, and the highest adsorption amount occurred at pH 6. Noteworthy, CTAB promoted the adsorption ability of Pb(II) when the exogenous FA was added. The binding characteristic of sediment endogenous DOM components and Pb(II) was influenced by the addition of MPs and surfactants. Finally, it confirmed that adsorption mechanisms mainly involve electrostatic and hydrophobic interaction. This study provides a new perspective to explore the environmental behaviors of Pb(II) by MPs and sediments with the addition of surfactants, which was conducive to evaluating the ecological risks of MPs and heavy metals in aquatic environments.

Preferential adsorption of medium molecular weight proteins in extracellular polymeric substance alleviates toxicity of small-sized microplastics to Skeletonema costatum

Extracellular polymeric substances (EPS) secreted by organisms tend to encapsulate microplastics (MPs), forming an EPS-corona that affects the fate of MPs in marine ecosystems. However, the impact of the EPS-corona on the biotoxicity of MPs to marine organisms remains poorly understood. Herein, the effect of the EPS-corona on the toxicity of polystyrene (PS) MPs of different sizes (0.1 and 1 µm) to Skeletonema costatum (S. costatum) was investigated. The preferential adsorption of medium molecule weight (∼55 kDa) proteins onto PS MPs mainly contributed to the EPS-corona formation, decreasing the surface charge negativity of small-sized PS MPs (0.1 µm) by 72.4 %. Nitrogen (N) and oxygen (O) moieties in polysaccharides and proteins were identified as the preferential adsorption sites in the EPS-PS MPs interaction. Density functional theory (DFT) calculations confirmed the nuclear magnetic resonance spectroscopy (NMR) results, revealing that the binding mode between EPS and PS MPs was mainly hydrogen bonding. In addition, EPS-corona increased the cell density of S. costatum by 35.5-36.0 % when exposed to small-sized PS MPs (0.1 µm, 25-50 mg/L). These findings provide new insights into how EPS-corona affects the environmental fate and ecological risks associated with micro- and nano-sized plastics in marine ecosystems.

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.

Field migration of veterinary antibiotics via surface runoff from chicken-raising orchard in responding to natural rainfalls

Knowledge on runoff transport of manure-sourced antibiotics from farmland soil to aquatic environment is limited due to complexity of hydrological regime and pathways. This study monitored natural rainfalls in sloping orchard plots with free-range chickens, with an attempt to investigate the migration characteristics of typical antibiotics via surface runoff as well as the impact of manure presence. Results showed that rainstorms continuously carried away antibiotics in surface runoff and all target antibiotics including those with high affinities to soil were detected at the beginning of runoff production. Concentration of antibiotics was found to respond strongly to the instantaneous rainfall intensity, showing consistent fluctuations during rainfalls. Concentrations of sulfonamides and florfenicol were two orders of magnitude higher than that of tetracyclines and fluoroquinolones. Compared to the control without raising chickens, antibiotics migration was considerably increased with the increased runoff production due to soil surface changes caused by chicken activities. Additionally, dynamics of antibiotic concentration significantly correlated with variations of fluorescent DOM components. Chicken manure-derived DOM mainly contained tryptophan moiety, and laboratory fluorescence quenching test with 2D-COS analysis indicated that all antibiotics interacted more strongly and preferentially with tryptophan than humic-like species. Antibiotics bonded to manure DOM with an affinity corresponding to the significance level of their correlations. In this light, potential use of fluorescence indices based on the established correlations may provide a convenient tool for tracing runoff migration of antibiotics during rainfalls.

Multiple spectroscopic insights into the interaction mechanisms between proteins and humic acid

Proteins are important constituents of dissolved organic matter (DOM) in aqueous environments, and their interaction with humic acid (HA), another key component of DOM, substantially affects the environmental behaviors of DOM. In this work, the interaction mechanisms between tryptophan-containing proteins and HA were systematically investigated using multiple molecular spectroscopic approaches. The fluorescence quenching tests indicate that bovine serum albumin (BSA) was more readily quenched by HA and the coexisting phenolic, carboxyl, and quinone groups in HA contributed to this process significantly. By comparison, the fluorescence of L-tryptophan (L-Trp) was more stable under the same conditions. Furthermore, with multiple groups in HA, static quenching with the binding constants and the number of sites were calculated in the protein-HA and L-Trp-HA mixtures. In addition, the differential fluorescence spectra, UV‒Vis spectra, and two-dimensional correlation spectroscopy results confirmed that L-tryptophan amino acid could indeed form a complex with HA, while did not lead to fluorescence quenching. Finally, the molecular docking and density functional theory (DFT) simulations highlighted the contribution of multiple residues surrounding the HA groups to their interactions. The direct interaction between the tryptophan residue and HA might not be the prerequisite for the fluorescence response. Therefore, our work provides further insights into protein-HA interactions and implies other reasonable elucidations for further explanation.

Tactile Sensors: Hydroxyl Decorated Silver Metal-Organic Frameworks for Detecting Cr2O72-, MnO4-, Humic Acid, and Fe3+ Ions

Anionic, acidic, and metal ions are common contaminants in water and cause serious concerns for human and aquatic life. With the goal of rapid detection of analytes, we herein design a new array of ligand 5-(4H-1,2,4-triazol-4-yl)pyridin-3-ol-linked silver coordinated metal-organic frameworks Ag-MOFs as a promising sensor for Cr₂O₇^2-, MnO₄^-, humic acid (HA), and Fe³⁺ ions down to the micro level. Furthermore, as evidenced by luminescence, excitation-emission matrix (EEM) spectroscopic, and PXRD measurements, designed metal-organic frameworks (MOFs) can be fast, stable, and reusable for analyte detection in water.

The coexistence of copper ions and TC affected the binding ability and the reaction order between extracellular polymeric substances of aerobic granular sludge and exogenous substances

As a barrier against external toxic effects, extracellular polymeric substances (EPSs) directly affect the toxicity and removal efficiency of exogenous substances. The reaction of EPSs with exogenous substances has been taken into consideration. The contents of EPSs in sludge cultivated by different influent water vary greatly, which leads to great differences in the binding ability and reaction sequence between EPSs and exogenous substances. However, the results in this respect are very limited. In this study, the binding characteristics between exogenous tetracycline (TC)/copper ions (Cu²⁺) and EPSs from aerobic granular sludge cultured under single and coexisting TC/Cu²⁺ were assessed by three-dimensional fluorescence-parallel factor analysis. The pollutants in the influent water could directionally induce microorganisms to secrete more EPSs, while fluorescence substances in EPSs could combine with the exogenous substances to lessen their effects. In the presence of coexisting TC and Cu²⁺ in the influent water, the ability of fluorescence substances in EPSs to combine with exogenous TC or Cu²⁺ weakened, and humic substances in EPSs were more susceptible than protein substances to binding with exogenous substances. However, the reaction order between EPSs components and exogenous TC or Cu²⁺ was opposite, and the ability of fluorescence substances in EPSs to combine with exogenous TC or Cu²⁺ was enhanced under individual TC or Cu²⁺ existing in the influent water. This study provided new insights into the interaction between EPSs and exogenous substances.

Ultraviolet humic-like component contributes to riverine dissolved organic matter biodegradation

Biological degradation of dissolved organic matter (DOM) regulates its structure and fate in river ecosystems. Previous views suggested that labile components were dominantly consumed by microbial metabolism. Here we provide new observations that a part of recalcitrant compounds largely contribute to riverine DOM biodegradation. The excitation-emission matrix fluorescent spectroscopy combined with peak picking and parallel factor analysis are used to explore component variability during DOM incubation. Humic-like and tryptophan-like DOM are the primary components of riverine DOM, with proportion contributions of 39%-82% and 16%-61% for % of the maximum fluorescence intensity, respectively. After 56 days of aerobic incubation in the dark, large amounts of tyrosine-like DOM generation are observed. Elevated temperature enhances the decomposition of ultraviolet humic-like substance and further stimulates labile DOM bio-mineralization into carbon dioxide. Meanwhile, averaged proportions of amino acid compositions (peak B and T) markedly increase (p < 0.05) as the humic-like compositions (peak A, M and C) decrease after DOM incubation, suggesting incomplete degradation of refractory DOM from high-molecular to low-molecular weight compounds. The findings support the new notion of the continuous DOM biodegradation in a mode as "steps by steps", contributing to a new understanding of carbon cycling for the UN Sustainable Development Goal.

Self-cleaning and regenerable nano zero-valent iron modified PCN-224 heterojunction for photo-enhanced radioactive waste reduction

The expansion of large-scale nuclear power causes a substantial volume of radioactive wastewater containing uranium to be released into the environment. Because of uranium's toxicity and bioaccumulation, it is critical to develop the efficient and sustainable materials for selective removal of uranium (VI). Herein, a regenerable anti-biofouling nano zero-valent iron doped porphyrinic zirconium metal-organic framework (NZVI@PCN-224) heterojunction system was successfully fabricated. Due to the Schottky-junction effect at the NZVI/MOF interface, the NZVI nanomaterial immobilized on PCN-224 could improve interfacial electron transfer and separation efficiency, and enhance entire reduction of highly soluble U(VI) to less soluble U(IV), involving photocatalytic reduction and chemical reduction. Meanwhile, the photocatalytic effect also prompts the NZVI@PCN-224 to produce more biotoxic reactive oxygen species (ROS), resulting in high anti-microbial and anti-algae activities. Under dark conditions, NZVI@PCN-224 with a large specific surface area could provide sufficient oxo atoms as the uranium binding sites and show the highest uranium-adsorbing capability of 57.94 mg/g at pH 4.0. After eight adsorption-desorption cycles, NZVI@PCN-224 still retained a high uranium adsorption capacity of 47.98 mg/g and elimination efficiency (91.72%). This sorption/reduction/anti-biofouling synergistic strategy of combining chelation, chemical reduction and photocatalytic performance inspires new insights for highly efficient treatment of liquid radioactive waste.

Impacts of algal organic matter and humic substances on microcystin-LR removal and their biotransformation during the biodegradation process

The application of bioaugmentation (i.e., injection of contaminant-degrading microorganisms) has shown its potential to remove harmful cyanotoxins like microcystin-LR (MC-LR) from drinking water sources. However, the natural organic matter (NOM) present in both natural and engineered water systems might affect the bacterial biodegradation of MC-LR. Therefore, for the successful application of bioaugmentation for MC-LR removal in water treatment, it is important to understand NOM effects on MC-LR biodegradation. In this study, the impact of NOM [algal organic matter (AOM) and humic substances (HS)] on MC-LR biodegradation was evaluated in the presence of varying concentrations of NOM by monitoring MC-LR biodegradation kinetics. The changes in NOM composition during MC-LR biodegradation were also characterized by a five-component Parallel factor (PARAFAC) model using 336 excitation-emission matrix (EEM) spectra collected at different sampling points. Our results showed decreases in MC-LR biodegradation rate of 1.6-and 3.4-fold in the presence of AOM and HS, respectively. The expression of the functional mlrA gene exhibited a similar trend to the MC-LR degradation rate at different NOM concentrations. EEM-PARAFAC analyses and NOM molecular size fractionation results indicated a relatively greater production of terrestrial humic-like components (57%) and a decrease of protein-like components. Two-dimensional correlation spectroscopy (2D-COS) analyses further confirmed that low molecular weight protein-like components were initially utilized by bacteria, followed by the formation of higher molecular weight humic-like components, likely due to microbial metabolism.

A chemosensing approach for the colorimetric and spectroscopic detection of Cr3+, Cu2+, Fe3+, and Gd3+ metal ions

Metal cations are present in domestic and industrial wastewater and have adverse effects on human and aqueous life. The present study describes the development of the molecular probe 9-anthracen-9-ylmethylene)hydrazineylidene)methyl)-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-8-ol (AMHMPQ) to detect Cr³⁺, Cu²⁺, Fe³⁺, and Gd³⁺ ions by using UV-visible, fluorescence, colorimetric and excitation-emission matrix (EEM) spectroscopy techniques. The interaction of Cr³⁺, Cu²⁺, Fe³⁺, and Gd³⁺ can be observed by the absorption maxima shift, turn-off, colour changes, and EEM shifts. In addition, fluorescence limits of detection 17.66 × 10^-6 M, 6.44 × 10^-9 M, 28.87 × 10^-8 M, and 12.49 × 10^-6 M in wide linear ranges, low limits of quantifications, high values of Stern-Volmer constant, Job's plot and Benesi-Hildebrand plot justify the 1:1 association affinity with association constants of 1.46 × 10⁴ M^-1, 1.86 × 10⁷ M^-1, 2.69 × 10⁵ M^-1, 2.13 × 10⁴ M^-1 for AMHMPQ-metal ions (Cr³⁺, Cu²⁺, Fe³⁺, and Gd³⁺ ions), respectively. Paper- and mask-based kits are developed to explore the utility of the designed chemosensor. Additionally, AMHMPQ acts as a reusable sensor for two, seven, two, and zero cycles for Cr³⁺, Cu²⁺, Fe³⁺, and Gd³⁺ ions, respectively, when checked with EDTA.

Self-produced biophotosensitizers enhance the degradation of organic pollutants in photo-bioelectrochemical systems

Bioelectrochemical systems (BESs) with integrated photoactive components have been shown to be a promising strategy for enhancing the performance for bioenergy generation and pollutant removal. This study revealed an efficient photo-BES with an enhanced pollutant degradation rate by utilizing self-produced biomolecules as photosensitizers in situ. Results showed that the BES could increase the coulombic efficiency from 60.8% to 73.0% and the degradation rate of bisphenol A (BPA) from 0.030 to 0.189 h^-1 when the suspension in the reactor was illuminated with simulated sunlight in the absence of any external photosensitizers. We identified that the regular BES released many organic substances into the reactor during operation. These substances, including dissolved biomolecules and solid cell residues, were photoactive for producing hydroxyl radicals during light illumination. Quenching experiments verified that the •OH generated from the self-produced biophotosensitizers contributed to the enhanced degradation of BPA. Additionally, the phototransformation of biophotosensitizers was also observed in photo-BES. The quantity of tyrosine protein-like components decreased, but that of the humic components remained relatively stable. Our findings imply that BESs with integrated self-produced biophotosensitizers may be promising for constructing advanced electrochemical and biological systems for synchronous bioelectricity production and degradation of organic pollutants in wastewater treatments.

Tracking pollutants in a municipal sewage network impairing the operation of a wastewater treatment plant

This work provides a screening of organic contaminants and characterization of the dissolved organic matter in the sewer network until the municipal wastewater treatment plant (WWTP), identifying the network areas with a higher degree of contamination and their impact on the WWTP performance, particularly in the activated sludge reactor. Three monitoring campaigns were carried out at six selected locations of the sewage system (PVZ-1, PVZ-2, PS-F, PS-VC, CP-VC, and PS-T), influent (WWTP_INF) and effluent (WWTP_EFF) of the WWTP. Advanced analytical techniques were employed, namely excitation/emission matrix fluorescence-parallel factor analysis (EEM-PARAFAC), size exclusion chromatography with organic carbon detector (SEC-OCD), and liquid chromatography with high-resolution-mass spectrometric detection (LC-HRMS). EEM-PARAFAC showed higher fluorescence intensity for the protein-like component (C2), particularly at CP-VC (near seafood industries) associated with the presence of surfactants (~50 mg/L). SEC-OCD highlighted the WWTP efficiency in removing low molecular weight acids and neutrals. LC-HRMS tentatively identified 108 compounds of emerging concern (CEC) and similar detection patterns were obtained for all wastewater samples, except for PVZ-2 (lower detection), many of which occurred in the effluent. Eight CECs included on relevant Watch-Lists were detected in all WWTP_EFF samples. Furthermore, 111 surfactants were detected, the classes more frequently found being alcohol ethoxylates (AEOs), nonylphenol polyethoxylates (NPEOs) and linear alkylbenzene sulphonates (LAS). The continuous presence of LAS and NPEOs allied to surfactants concentrations in the WWTP_INF of 15-20 mg/L, with CP-VC location (linked with food industries) as an important contributor, explain the morphological changes in the activated sludge and high LAS content in the dewatered sludge, which may have impacted WWTP performance.

Photochemical Behavior of Microbial Extracellular Polymeric Substances in the Aquatic Environment

Microbially derived extracellular polymeric substances (EPSs) occupy a large portion of dissolved organic matter (DOM) in surface waters, but the understanding of the photochemical behaviors of EPS is still very limited. In this study, the photochemical characteristics of EPS from different microbial sources (Shewanella oneidensis, Escherichia coli, and sewage sludge flocs) were investigated in terms of the production of reactive species (RS), such as triplet intermediates (³EPS*), hydroxyl radicals (^•OH), and singlet oxygen (¹O₂). The steady-state concentrations of ^•OH, ³EPS*, and ¹O₂ varied in the ranges of 2.55-8.73 × 10^-17, 3.01-4.56 × 10^-15, and 2.08-2.66 × 10^-13 M, respectively, which were within the range reported for DOM from other sources. The steady-state concentrations of RS varied among different EPS isolates due to the diversity of their composition. A strong photochemical degradation of the protein-like components in EPS isolates was identified by excitation emission matrix fluorescence with parallel factor analysis, but relatively, humic-like components remained stable. Fourier-transform ion cyclotron resonance mass spectrometry further revealed that the aliphatic portion of EPS was resistant to irradiation, while other portions with lower H/C ratios and higher O/C ratios were more susceptible to photolysis, leading to the phototransformation of EPS to higher saturation and lower aromaticity. With the phototransformation of EPS, the RS derived from EPS could effectively promote the degradation of antibiotic tetracycline. The findings of this study provide new insights into the photoinduced self-evolution of EPS and the interrelated photochemical fate of contaminants in the aquatic environment.

Insight into binding characteristics of copper(II) with water-soluble organic matter emitted from biomass burning at various pH values using EEM-PARAFAC and two-dimensional correlation spectroscopy analysis

The metal-binding characteristics of water-soluble organic matter (WSOM) emitted from biomass burning (BB, i.e., rice straw (RS) and corn straw (CS)) with Cu(II) under various pH conditions (i.e., 3, 4.5, and 6) were comprehensively investigated. Two-dimensional correlation spectroscopy (2D-COS) and excitation-emission matrix (EEM) -PARAFAC analysis were applied to investigate the binding affinity and mechanism of BB WSOM. The results showed that pH was a sensitive factor affecting binding affinities of WSOM, and BB WSOMs were more susceptible to bind with Cu(II) at pH 6.0 than pH 4.5, followed by pH 3.0. Therefore, the Cu(II)-binding behaviors of BB WSOMs at pH 6.0 were then investigated in this study. The 2D-absorption-COS revealed that the preferential binding with Cu(II) was in the order short and long wavelengths (237-239 nm and 307-309 nm) > moderate wavelength (267-269 nm). The 2D-synchronous fluorescence-COS results suggested that protein-like substances generally exhibited a higher susceptibility and preferential interaction with Cu(II) than fulvic-like substances. EEM-PARAFAC analysis demonstrated that protein-like (C1) substances had a greater complexation ability than fulvic-like (C2) and humic-like (C3) substances for both BB WSOM. This indicated that protein-like substances within WSOM played dominant roles in the interaction with Cu(II). As a comparison, RS WSOM generally showed stronger complexation capacity than CS WSOM although they exhibited similar chemical properties and compositions. This suggested the occurrence of heterogeneous active metal-binding sites even within similar chromophores for different WSOM. The results enhanced our understanding of binding behaviors of BB WSOM with Cu(II) in relevant atmospheric environments.

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.

Understanding of the mechanism of extracellular polymeric substances of aerobic granular sludge against tetracycline from the perspective of fluorescence properties

Aerobic granular sludge (AGS) exhibited an excellent removal efficiency and a high tolerance in the treatment of antibiotics wastewater. Extracellular polymeric substances (EPS) of AGS with abundant binding sites might serve as the first barrier to prevent the direct contact of antibiotics and cells, thereby keeping the stability of AGS. In this study, the investigations in the fluorescence properties and the molecular weight of AGS-EPS after interaction with tetracycline (TC) were combined to reveal the resistance mechanism of AGS-EPS against TC. The two-dimensional correlation spectroscopy (2D-COS) was utilized to analyze the interaction priority of the AGS-EPS components with TC. Results showed that TC interacted with proteins and humic acid in AGS-EPS by forming a complex through hydrogen bond and van der Waals force. Compared with humic acid, TC could preferentially interact with proteins and form more stable complexes. Moreover, the components with the larger molecular weight in AGS-EPS interact with TC prior to which with smaller molecular weight. Significantly, TC exhibited the potential of binding with the divalent cation of AGS-EPS and caused the conformation changes of the protein. Therefore, AGS-EPS could resist the TC at a certain concentration range by trapping antibiotics, while over-loaded TC would cause the instability of AGS due to the limited interaction site of AGS-EPS and the destructive effect of antibiotics on AGS-EPS. This study provided a theoretical basis for understanding the interaction mechanism between antibiotics and AGS-EPS and offered a reference for AGS to maintain the stability of granules under the threat of antibiotics.

The complexation with proteins in extracellular polymeric substances alleviates the toxicity of Cd (II) to Chlorella vulgaris

The complexation with extracellular polymeric substances (EPS) greatly reduces the toxicity of heavy metals towards organisms in the environment. However, the molecular mechanism of EPS-metal complexation remains unclear owing to the limitation of precise analysis for key fractions and functionalities in EPS that associate with metals. Herein, we explored the EPS-Cd (II) complexation by fluorescence excitation emission matrix coupled with parallel factor (EEM-PARAFAC), two-dimensional Fourier transform infrared correlation spectroscopy (2D-FTIR-COS) and X-ray photoelectron spectroscopy (XPS), attempting to explain the mechanisms of EPS in alleviating Cd (II) toxicity toward a green alga Chlorella vulgaris (C. vulgaris). When the algal EPS were removed, the cell internalizations of Cd (II), growth inhibition rate and chlorophyll autofluorescence increased, but the surface adsorption and esterase activities decreased, indicating that the sorption of Cd (II) by EPS was crucial in alleviating the algal toxicity. Moreover, the complexation with proteins in EPS controlled the sorption of Cd (II) to algal EPS, resulting in the chemical static quenching of the proteins fluorescence by 47.69 ± 2.37%. Additionally, the complexing capability of the main functionalities, COO^- and C-OH in proteins with Cd (II) was stronger than that of C-O(H) and C-O-C in polysaccharides or C-OH in the humus-related substances. Oxygen atom in protein carboxyl C-O might be the key site of EPS-Cd (II) complexation, supported by the modified Ryan-Weber complexation model and the obvious shift of oxygen valence-electron signal. These findings provide deep insights into understanding the interaction of EPS with heavy metals in aquatic environment.

Photochemical release of dissolved organic matter from particulate organic matter: Spectroscopic characteristics and disinfection by-product formation potential

In this study, we investigated the photochemical release of dissolved organic matter (DOM) from the particulate organic matter (POM) of soil and litter leaves (broad leaves; coniferous leaves) and compared the releasing characteristics of the DOM using UV-visible and fluorescence spectroscopy. The disinfection by-product formation potential (DBPFP) of the released DOM was also examined. Additional dissolved organic carbon (DOC) was released by UV irradiation for all POM sources (10.58 ± 2.7 mg-C L^-1 g^-1 for BL, 8.32 ± 2.6 mg-C L^-1 g^-1 for CL, and 0.20 ± 0.1 mg-C L^-1 g^-1 for soil). The excitation-emission matrix combined with parallel factor analysis results showed that the photo-released DOM from soil was mainly humic-like components (C1, C3) produced by photodesorption, resulting in high trihalomethane formation potential, while protein-like component (C2) was the major component of the photodissolved DOM from litter leaves, resulting in high haloaceticacid formation potential. Further, DBPFP from soil and litter leaves showed high correlation with humic-like components (C1+C3) and SUVA₂₅₄, respectively. In conclusion, this study demonstrates that significant amounts of DOM could be released from POM under UV irradiation, although the characteristics and DBP formation of the photo-released DOM were highly dependent upon the POM source.

Mechanism of enhancing pyrene-degradation ability of bacteria by layer-by-layer assembly bio-microcapsules materials

The mechanism of improving pyrene (PYR)-degrading ability of bacteria CP13 in Layer-by-layer (LBL) assembly chitosan/alginate (CHI/ALG) bio-microcapsules was investigated. Flow cytometry analysis showed that LBL microcapsules could effectively slow down the increasing rate of bacterial cell membrane permeability and the decreasing rate of the membrane potential, so as to reduce the death rate and number of the cells, which could protect the degrading bacteria. The results of Fluorescence spectrum, circular dichroism (CD) spectrum and laser light scattering (LLS) analysis revealed that the other possible mechanism for LBL microcapsules to promote bacterial degradation were following: CHI could enter the secondary structure of the protein of the extracellular polymeric substances (EPS) from CP13 and combined with EPS to generate a stable ground material, which had larger molecular weight (3.76×10⁶ g mol^-1) than the original EPS (2.52×10⁶ g mol^-1). The combination of CHI and EPS resulted in the decrease of the density of EPS from 1.18 to 0.72 g L^-1, suggesting that CHI can loosen the EPS configurations, improving the capture ability of bacteria for PYR as well as the mass transfer of PYR from the extracellular to intracellular, thus eventually promoting the bacteria degrade performance.

Characterizing Properties and Environmental Behaviors of Dissolved Organic Matter Using Two-Dimensional Correlation Spectroscopic Analysis

Dissolved organic matter (DOM) exists ubiquitously in environments and plays critical roles in pollutant mitigation, transformation, and organic geochemical cycling. Understanding its properties and environmental behaviors is critically important to develop water treatment processes and environmental remediation strategies. Generalized two-dimensional correlation spectroscopy (2DCOS), which has numerous advantages, including enhancing spectral resolution and discerning specific order of structural change under an external perturbation, could be used as a powerful tool to interpret a wide range of spectroscopic signatures relating to DOM. A suite of spectroscopic signatures, such as UV-vis, fluorescence, infrared, and Raman spectra that can be analyzed by 2DCOS, is able to provide additional structural information hiding behind the conventional one-dimensional spectra. In this article, the most recent advances in 2DCOS applications for analyzing DOM-related environmental processes are reviewed, and the state-of-the-art novel spectroscopic techniques in 2DCOS are highlighted. Furthermore, the main limitations and requirements of current approaches for exploring DOM-related environmental processes and how these limitations and drawbacks can be addressed are explored. Finally, suggestions and new approaches are proposed to significantly advance the development of 2DCOS in analyzing the properties and behaviors of DOM in natural and engineered environments.

Effects of aeration rates on the structural changes in humic substance during co-composting of digestates and chicken manure

High humidity and potential threat of pathogen of anaerobic digestates are unfavorable to the environment by direct utilization. To achieve the sustainable utilization of digestates, composting might be a good choice. Meanwhile, the aeration rate of composting has been optimized. Co-composting of digestates and chicken manure was performed under different aeration conditions (0.05, 0.1 and 0.15 L·min^-1·kg^-1·organic matter (OM)). During composting, internal transformation of humic substance (HS) has been studied for obtaining the potential application value of the co-composting products. Results suggested that the HS concentration was increased by 21.1%, 26.4% and 22.4% with the aeration rates were 0.05, 0.1 and 0.15 L·min^-1·kg^-1·OM, respectively. The aeration rate of 0.15 L·min^-1·kg^-1·OM was more conducive to germination. Parallel factor analysis and dimensional correlation spectra (2DCOS) have been combined to reveal the conversion relationships of HS components for understanding the compost application pattern. Hetero-2DCOS indicated that aeration of 0.05 min^-1·kg^-1·OM and 0.1 L·min^-1·kg^-1·OM contributed to the formation of complex compounds at long wavelength, and aeration of 0.15 L·min^-1·kg^-1·OM was beneficial for labile compounds formation at short wavelength. In views of the aeration of 0.1 L·min^-1·kg^-1·OM was more beneficial to improve HS concentration than 0.05 L·min^-1·kg^-1·OM, 0.1 L·min^-1·kg^-1·OM and 0.15 L·min^-1·kg^-1·OM were consider as the most important aeration rate to conduct digestates composting. Overall, the aeration affected the HS composition which, in turn, might affect the application ways of composting products. This study could provide a reference for industrial composting production and applications.

Effect of gasification biochar application on soil quality: Trace metal behavior, microbial community, and soil dissolved organic matter

Compared to pyrolysis biochar (PBC), gasification biochar (GBC) differs in both composition and surface functionalities due to the use of an oxidizing purging gas. This work compares the effect of using PBC and GBC as soil amendments on the soil properties, trace metal bioavailability, soil microbial activity, and soil dissolved organic matter (DOM). Biochar-driven reduction of bioavailable metals does not necessarily result in a positive impact on the soil microbial growth. The DOM in the soil was strongly related to the soil microbial activity, as revealed by the strong correlation between the soil dehydrogenase activity (DHA) and soil dissolved organic carbon (r = 0.957, p <  0.01). Three identified fluorescent components (C1, C2, C3) in the soil DOM were closely associated with the soil microbial activity, for instance, with a clear positive correlation between the soil DHA and C1 (r = 0.718, p <  0.05) and a significant negative correlation between the total bacterial fatty acid methyl ester content and C3 (r = -0.768, p <  0.05). The bioavailability of Cd and Zn is not only related to the pH and surface functionalities of the biochar, but also to its aromatic carbon and inorganic mineral composition. This study further demonstrates that a fluorescence excitation-emission matrix coupled with parallel factor analysis is a useful tool to monitor changes in the soil quality after application of biochar, which is greatly relevant to the soil biota.

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.

Spectroscopic study on transformations of dissolved organic matter in coal-to-liquids wastewater under integrated chemical oxidation and biological treatment process

A large amount of wastewater containing various toxic organic contaminants is produced during coal-to-liquids process. In this study, several spectroscopic methods were used to monitor the transformation of organic pollutants during an integrated chemical oxidation and biological process. The results showed that the hydrophobic acid fraction increased after Fenton oxidation, which was likely due to the production of small-molecule organic acids. Soluble microbial products were generated during biological treatment processes, which were degraded after ozonation; meanwhile, the hydrophilic base and acid components increased. Ultraviolet-visible spectroscopic analysis indicated that peaks at the absorption wavelengths of 280 and 254nm, which are associated with aromatic substances, were detected in the raw water. The aromatic substances were gradually removed, becoming undetectable after biological aeration filter (BAF) treatment. Fourier transform infrared spectroscopy analysis revealed that the functional groups of phenols; benzene, toluene, ethylbenzene, and xylene (BTEX); aromatic hydrocarbons; aliphatic acids; aldehydes; and esters were present in raw wastewater. The organic substances were oxidized into small molecules after Fenton treatment. Aromatic hydrocarbons were effectively removed through bioadsorption and biodegradation after BAF process. Biodegradable organic matter was reduced and finally became undetectable after anoxic-oxic treatment in combination with a membrane bioreactor. Four fluorescent components were fractionated and obtained via excitation-emission matrix parallel factor analysis (EEM-PARAFAC). Dissolved organic matter fractionation in conjunction with EEM-PARAFAC was able to monitor more precisely the evolution of characteristic organic contaminants.

Tracking variations of fluorescent dissolved organic matter during wastewater treatment by accumulative fluorescence emission spectroscopy combined with principal component, second derivative and canonical correlation analyses

Accumulative fluorescence emission (AFE) spectroscopy combined with principal component analysis (PCA), second derivative and canonical correlation analysis (CCA) was firstly developed into an available tool to track variations in dissolved organic matter (DOM) fractions and contents during wastewater treatment. Samples were collected from a wastewater treatment plant with a traditional anaerobic/anoxic/oxic (A2O) process. The AFE spectroscopy deduced from the sum of intensities along the excitation wavelengths of fluorescence excitation emission matrix (EEM), could distinctly track tyrosine-like, tryptophan-like, fulvic-like substances. The AFE spectroscopy with the PCA not only disaggregated DOM fractions into the tyrosine-like, tryptophan-like, microbial humic-like, fulvic-like and humic-like substances, but discriminated DOM fractions from the physical sedimentation, anaerobic/anoxic and oxic processes. Absolute areas of fluorescence components obtained by the second derivative AFF spectra had positive liner correlations with Fmax of the relevant components modeling from EEM-PARAFAC, especially the tryptophan-like (R² = 0.95, p < 0.01) and tyrosine-like (R² = 0.83, p < 0.01) substances. The CCA of the sites presented that the potential factors contained the tryptophan-like and tyrosine-like substances. This indirectly proved that the tryptophan-like and tyrosine-like substances were the dominant components of fluorescent DOM, which were further removed in A2O than the other fluorescent components. The CCA of the fluorescent components exhibited that the potential factors included the sites #1 to #6, which were located in the original wastewater, sand setting, primary sedimentation, anaerobic, anoxic, facultative units. This elaborated that the fluorescent components were mainly degraded in the physical sedimentation, anaerobic and anoxic processes.

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.

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

In aquatic environments, pH can control environmental behaviors of fulvic acid (FA) via regulating hydrolysis of functional groups. Sub-fractions of FA, eluted using pyrophosphate buffers with initial pHs of 3.0 (FA₃), 5.0 (FA₅), 7.0 (FA₇), 9.0 (FA₉) and 13.0 (FA₁₃), were used to explore interactions between the various, operationally defined, FA fractions and protons, by use of EEM-PARAFAC analysis. Splitting of peaks (FA₃ and FA₁₃), merging of peaks (FA₇), disappearance of peaks (FA₉ and FA₁₃), and red/blue-shifting of peaks were observed during fluorescence titration. Fulvic-like components were identified from FA₃-FA₁₃, and protein-like components were observed in fractions FA₉ and FA₁₃. There primary compounds (carboxylic-like, phenolic-like, and protein-like chromophores) in PARAFAC components were distinguished based on acid-base properties. Dissociation constants (pK_a) for fulvic-like components with proton ranged from 2.43 to 4.13 in an acidic pH and from 9.95 to 11.27 at basic pH. These results might be due to protonation of di-carboxylate and phenolic functional groups. At basic pH, pK_a values of protein-like components (9.77-10.13) were similar to those of amino acids. However, at acidic pH, pK_a values of protein-like components, which ranged from 3.33 to 4.22, were 1-2units greater than those of amino acids. Results presented here, will benefit understanding of environmental behaviors of FA, as well as interactions of FA with environmental contaminants.

Characterization of extracellular polymeric substances in biofilms under long-term exposure to ciprofloxacin antibiotic using fluorescence excitation-emission matrix and parallel factor analysis

The presence of antibiotic residues in the environment has been regarded as an emerging concern due to their potential adverse environmental consequences such as antibiotic resistance. However, the interaction between antibiotics and extracellular polymeric substances (EPSs) of biofilms in wastewater treatment systems is not entirely clear. In this study, the effect of ciprofloxacin (CIP) antibiotic on biofilm EPS matrix was investigated and characterized using fluorescence excitation-emission matrix (EEM) and parallel factor (PARAFAC) analysis. Physicochemical analysis showed that the proteins were the major EPS fraction, and their contents increased gradually with an increase in CIP concentration (0-300 μg/L). Based on the characterization of biofilm tightly bound EPS (TB-EPS) by EEM, three fluorescent components were identified by PARAFAC analysis. Component C1 was associated with protein-like substances, and components C2 and C3 belonged to humic-like substances. Component C1 exhibited an increasing trend as the CIP addition increased. Pearson's correlation results showed that CIP correlated significantly with the protein contents and component C1, while strong correlations were also found among UV₂₅₄, dissolved organic carbon, humic acids, and component C3. A combined use of EEM-PARAFAC analysis and chemical measurements was demonstrated as a favorable approach for the characterization of variations in biofilm EPS in the presence of CIP antibiotic.