Protein-like Fluorescence Peaks Research Articles

An optimised and validated surrogate analyte A-TEEM-PARAFAC-PLS technique for detecting and quantifying the biological oxygen demand in surface water.

A 5-day test duration makes BOD5 measurement unsatisfactory and hinders the development of a quick technique. Protein-like fluorescence peaks show a strong correlation between the BOD characteristics and the fluorescence intensities. For identifying and measuring BOD in surface water, a simultaneous absorbance-transmittance and fluorescence excitation-emission matrices (A-TEEM) method combined with PARAFAC (parallel factor) and PLS (partial least squares) analyses was developed using a tyrosine and tryptophan (tyr-trpt) mix as a surrogate analyte for BOD. The use of a surrogate analyte was decided upon due to lack of fluorescent BOD standards. Tyr-trpt mix standard solutions were added to surface water samples to prepare calibration and validation samples. PARAFAC analysis of excitation-emission matrices detected the tyr-trpt mix in surface water. PLS modelling demonstrated significant linearity (R2 = 0.991) between the predicted and measured tyr-trypt mix concentrations, and accuracy and robustness were all acceptable per the ICH Q2 (R2) and ASTM multivariate calibration/validation procedures guidelines. Based on a suitable and workable surrogate analyte method, these results imply that BOD can be detected and quantified using the A-TEEM-PARAFAC-PLS method. Very positive comparability between tyr-trypt mix concentrations was found, suggesting that tyr-trypt mix might eventually take the place of a BOD-based sampling protocol. Overall, this approach offers a novel tool that can be quickly applied in water treatment plant settings and is a step in supporting the trend toward rapid BOD determination in waters. Further studies should demonstrate the wide application of the method using real wastewater samples from various water treatment facilities.

Differences in dissolved organic matter and analysis of influencing factors between plantations pure and mixed forest soils in the loess plateau

The dissolved organic matter (DOM) in forest ecosystems significantly impacts soil carbon cycling due to its active turnover characteristics. However, whether different plantation forest soil profiles exhibit distinct DOM characteristics remains unclear. Hence, utilizing fluorescence spectroscopy and the parallel factor analysis (PARAFAC) method, a 1-meter soil profile analysis was carried out on three distinct artificial forests (Pinus tabuliformis (PT), Quercus crispula (QC), and a mixed forest of PT and QC (MF)), concurrently assessing the impact of soil chemical properties and enzyme activity on dissolved organic matter (DOM). The findings indicated that the mean concentration of dissolved organic carbon (DOC) was greatest in the MF and lowest in PT, exhibiting considerable variation with soil depth, suggesting that mixed tree species may promote the discharge of organic matter. The fluorescence spectra revealed two distinct peaks: humic-like fluorescence peaks (Peaks A and C) and a protein-like fluorescence peak (Peak T), with the most intense fluorescence observed in MF soil. As the soil depth increased, the fluorescence intensity of Peaks A and C steadily declined, while the intensity of Peak T rose. Four DOM components were identified in three types of plantations forests: surface soil was dominated by humic acid-like fluorescent components (C1 and C2), while the deep soil was primarily characterized by protein-like fluorescence components (C3 and C4). Different soil profile fluorescence parameter indices indicated that the source of DOM in the surface soil (i.e., 0–20 cm) was mainly allochthonous inputs, whereas, in the deep soil (i.e., 60–100 cm), it was mainly autochthonous, such as microbial activity. The findings from the partial least squares path modeling (PLS-PM) revealed that TP, aP, NH4+-N, and the combined impact of soil enzymes were influential in shaping the diversity of DOM attributes. Put differently, alterations in DOM concentration were concomitantly influenced by forest classification, soil characteristics, and depth. It has been demonstrated that, in contrast to monoculture forests, the establishment of mixed forest models has been more advantageous in enhancing the soil dissolved organic matter (DOM). These discoveries offer innovative perspectives on the dynamic characteristics of DOM in soil profiles and its influencing factors under different plantations forest planting patterns.

Properties of the DOM in Soil Irrigated with Wastewater Effluent and Its Interaction with Copper Ions

DOM samples extracted from a wastewater effluent (EW) and from leachates collected from lysimeters in which Eucalyptus trees were deficit-irrigated with either EW (EL) or with water pumped from an aquifer recharged with the EW (TL) were studied. As ascertained by principal component analysis, DOM from both leachates displayed similar properties, different from those of the EW DOM. 1H-NMR and FTIR spectra of the > 1-kDa DOM size fraction of the EW revealed less aromaticity than this fraction of either leachate. 3-D fluorescence maps indicated that the density of nitro, carboxyl, or phenol groups attached to aromatic structures was lowest in the EW DOM and only that DOM displayed protein-like fluorescence peaks. The leachates’ DOM > 1 kDa fraction complexed more Cu2+ per unit C than this fraction of the EW (Kd = ~ 105 and ~ 104 L kg−1 DOC, respectively, at ~ 10−5 M free Cu2+). While Cu complexed preferentially with non-fluorescing sites in the EW DOM, in the leachates’ DOM, Cu bound primarily with fluorescing (aromatic) groups or with adjacent groups. The similar behavior displayed by DOM from leachates obtained under irrigation with either EW or reclaimed water suggests that processes occurring in the soil (e.g., by roots or microbiota) influenced the soil DOM’s properties to a larger extent than the nature of OM in the irrigation water. Thus, irrigation with good quality secondary effluent should not significantly enhance heavy metal mobility as compared to their mobility under irrigation with higher quality water.

Interactions between metal ions and the biopolymer in activated sludge: quantification and effects of system pH value

The quantification and effects of system pH value on the interactions between Pb(II) and the biopolymer in activated sludge were investigated. The biopolymer had two protein-like fluorescence peaks (Ex/Em = 280 nm/326–338 nm for peak A; Ex/Em = 220–230 nm/324–338 nm for peak B). The fluorescence intensities of peak B were higher than those of peak A. The fluorophores of both peaks could be largely quenched by Pb(II), and the quencher dose for peak B was about half of that for peak A. The modified Stern-Volmer equation well depicted the fluorescence quenching titration. The quenching constant (Ka) values for both peaks decreased with rising system pH value, and then sharply decreased under alkaline conditions. It could be attributed to that the alkaline conditions caused the reduction of available Pb(II) due to the occurrence of Pb(OH)2 sediments. The Ka values of peak B were bigger than those for peak A at the same system pH values. Accordingly, the aromatic proteins (peak B) played a key role in the interactions between metal ions and the biopolymer.

Insight into the influences of pH value on Pb(II) removal by the biopolymer extracted from activated sludge

The influences of pH value on Pb(II) removal by the biopolymer extracted from activated sludge were investigated from various perspectives, including removal rate, functional groups, binding site number between the biopolymer and Pb(II), and removal distribution. With the system pH value rising from 4.0 to 9.0, the total removal rate of Pb(II) without and with adding the biopolymer increased from 1.94% to 86.5% and from 32.2% to 95.4%, respectively. From the analyses of Fourier transform infrared (FTIR) spectroscopy and two-dimensional correlation spectroscopy (2D-COS), the continuous dissociation and deprotonation of functional groups in the biopolymer with rising system pH value promoted the biosorption removal of Pb(II) via complexation and ion exchange. According to the three-dimensional excitation emission matrix (3D-EEM) fluorescence spectrum, two protein-like fluorescence peaks of A (Ex/Em=280nm/326–338nm) and B (Ex/Em=220–230nm/324–338nm) were identified in the biopolymer. The binding site numbers, which were obtained via fluorescence quenching titration experiments, increased first and then decreased with rising system pH value for both peaks A and B. Biosorption removal of Pb(II) was dominant under acid conditions, but biosorption and Pb(OH)2 precipitation co-existed under neutral and alkaline conditions. The biosorption and the Pb(OH)2 precipitation were interestingly found to interfere with each other under alkaline conditions. The decreases in both binding site number and biosorption removal rate indicated that Pb(OH)2 precipitation excelled the biosorption in the competition of capturing Pb(II).

Fluorescence Spectroscopic Characteristics and Cu2+-complexing Ability of Soil Dissolved Organic Matter

Extracted and dialyzed from four kinds of soil and chicken manure, DOM samples were analyzed to investigate their molecular weight distribution, chemical structure characteristics and the metal binding capacity. The results showed that the integral area of the molecular weight of chicken manure DOM was much higher than those of other samples, which indicated the high content of organic matter. And the peat soil followed it. The integral areas of dark brown soil, cinnamon soil and black soil were low, accounting for only 4.5%-5% of chicken manure integral area. The integral area of chicken manure DOM in middle molecular weight fraction accounted for about 34.1% of the total material, while those in low molecular weight and high molecular weight fractions accounted for about 41.7% and 24.2%, respectively. The protein-like fluorescence peaks (Ex/Em=240-270/300-350 nm) were present in all samples, and absent in middle and high molecular fractions (Mr>500) for dark brown soil. The visible fulvic acid-like florescence peaks (Ex/Em=325/420 nm) observed in peat soil, and the florescence peaks of black soil in the middle and high molecular fractions (Mr>500) both had a red shift phenomenon. The DOM of chicken manure had the UV-fulvic acid-like florescence peaks similar to black soil, and the visible fulvic acid-like florescence peaks similar to peat soil. Furthermore, it had a kind of protein-like fluorescence peak (Ex/Em=280/350 nm) that was absent in the middle molecular weight fraction (500< Mr< 12000). The Cu2+ complexation constant was 4.13 for peat soil, and smaller for other three kinds of soil (between 2.10 and 3.10). Chicken manure showed the greatest ability to bind Cu2+ with a resulting complexation constant of 6.66, indicating the importance of the unique peak, protein-like fluorescence peak, in the chicken manure of the low and high molecular weight fractions.

Effects of irradiation and pH on fluorescence properties and flocculation of extracellular polymeric substances from the cyanobacterium Chroococcus minutus

Microbial extracellular polymeric substances (EPS) may flocculate or be decomposed when environmental factors change, which significantly influences nutrient cycling and transport of heavy metals. However, little information is available on the stability of EPS in natural environments. Fluorescence and flocculation properties of EPS from Chroococcus minutus under different irradiation and pH conditions were studied. Two aromatic protein-like fluorescence peaks and one tyrosine protein-like peak were identified from the excitation–emission-matrix (EEM) fluorescence spectra of EPS. UVB (ultraviolet B) and solar irradiation increased the fluorescence intensity of all the three peaks while UVC (ultraviolet C) irradiation had little effect. EPS formed unstable flocs after exposure to UV (ultraviolet) irradiation and formed stable flocs under solar irradiation. EPS were prone to flocculation under highly acidic conditions and minimal fluorescence of peaks was observed. The fluorophores in EPS were relatively stable under neutral and alkaline conditions. These findings are helpful for understanding the behavior of EPS in aquatic environments and their role in biogeochemical cycles of the elements.

Biosorption of Hg(II) onto goethite with extracellular polymeric substances

This study characterized the interactions of goethite, EPS from cyanobacterium Chroococcus sp. and Hg(II) using excitation emission matrix (EEM) spectra and adsorption isotherms. Three protein-like fluorescence peaks were noted to quench in the presence of Hg(II). The estimated conditional stability constant (logKa) and the binding constant (logKb) of the studied EPS-Hg(II) systems ranged 3.84-4.24 and 6.99-7.69, respectively. The proteins in EPS formed stable complex with Hg(II). The presence of proteins of Chroococcus sp. enhanced the adsorption capacity of Hg(II) on goethite; therefore, the goethite-EPS soil is a larger Hg(II) sink than goethite alone soil. Biosorption significantly affects the mobility of Hg(II) in goethite soils.

Dissolved organic matter in digested piggery wastewater from combined treatment process

ABSTRACT The temporal and spatial variation of dissolved organic matter in the processes of hydrolytic acidification and biological contact oxidation during post-treatment of digested piggery wastewater were characterized systematically by means of ion chromatography, fluorescence spectra, and ultraviolet spectra in this study. The highest removal efficiencies of chemical oxygen demand and nitrogen have been achieved when the 1/3 effluent of biological contact oxidation reactor was backflow into the reactor of hydrolytic acidification. It was found that the concentrations of total organic acids were 283.6 mg/L at the first 2 h of hydrolytic reaction and 305.5 mg/L at the beginning of recirculation between the two reactors and then decreased to around 200 mg/L and keep stable. The results of synchronous and three-dimensional excitation–emission matrix fluorescence spectroscopy revealed that the protein-like fluorescence peaks were identified in the hydrolytic acidification reactor and their intensities increased gradually along with hydrolytic time increasing. Moreover, the fulvic acid-like fluorescence peaks in biological contact oxidation reactor were observed, and the intensity increased gradually. The synchronous intensity of 277 nm wavelength was significantly correlated with the total organic acids concentration. Variation of SUVA254 and E 253/E 203 during the hydrolytic acidification process indicated that concentrations of aromatic and unsaturated compounds slightly increase and those aromatic compounds are not stable. However, SUVA254 and E 253/E 203 decreased rapidly in the biological contact oxidation reactor, which suggested that easily degradable organic matters had been consumed rapidly.

Biodegradation of Dissolved Organic Matter (DOM) Released from Phytoplankton in Lake Biwa

The biodegradation study of algal dissolved organic matter (DOM) released from Microcystis aeruginosa, Staurastrum dorsidentiferum and Cryptomonas ovata was carried out. The algal DOM released from Microcystis aeruginosa and Staurastrum dorsidentiferum is relatively stable, while a part of the algal DOM released from Cryptomonas ovata may be easily decomposed. Before biodegradation, two fulvic-like fluorescence peaks (A and B) and a protein-like fluorescence peak (C) and another peak with E(x)/E(m) values of 320 - 330/390 nm (peak D) were observed in the algal DOM released from three kinds of phytoplankton. The fulvic-like fluorophores may be refractory regardless of the kinds of phytoplankton, while protein-like fluorophores released from Microcystis aeruginosa and Staurastrum dorsidentiferum may be relatively refractory and those from Cryptomonas ovata may be unstable. Peak D in the surface water of Lake Biwa may be attributed to low-molecular-weight substances produced during cultivation and/or biodegradation of several kinds of phytoplankton. The ratios of the fluorescence intensities (RFI/DOC) of peak A to peak B in algal DOM (< 1.0) were lower than those in soil Dando FA (1.8). On the other hand, no relationships between peak A and peak C were observed for three kinds of phytoplankton.

Fluorescent properties and bifenthrin binding behavior of maize (Zea mays L.) seedling root exudates

The binding parameters of root exudates for organic pollutants are important for understanding the mechanisms involved in phytoavailability and phytoremediation. However, quantitative information about organic pollutant binding to root exudates is lacking. Fluorescent properties and bifenthrin (insecticide) binding behavior of root exudates from maize (Zea mays L.) seedlings were investigated using excitation emission matrix fluorescence spectroscopy. The protein-like fluorophores in root exudates from (Z. mays L.) are strong complexing ligands for bifenthrin, with logKa in the range of 3.6–5.7. Two protein-like fluorescence peaks were identified in the EEM spectrum of root exudates and fluorescence of both peaks could be quenched by bifenthrin. The protein-like substances in root exudates had larger binding capacity for bifenthrin under acidic condition than under neutral and basic condition. The conditional stability constants were also higher in acidic solution than in the neutral or basic solution. Similarly, more fluorescence is accessible for bifenthrin at acidic pHs. More than one binding sites are needed in root exudates for binding one bifenthrin molecule. Enough attention should be paid to the effects of root exudates on adsorption, transport and fate of pesticides in environments.

Lead complexation behaviour of root exudates of salt marsh plant Salicornia europaea L

Root exudates are considered to have an important role in mobility and bioavailability of heavy metals. High molecular weight (HMW) substances are the main components of root exudates, however, knowledge about their interactions with heavy metals is lacking. In the present study, Pb(II) complexation of the HMW fluorescent fractions in root exudates from Salicornia europaea L. was investigated using excitation emission matrix (EEM) fluorescence spectroscopy. Two protein-like fluorescence peaks were identified in the EEM spectrum of root exudates. The fluorescence of both peaks was clearly quenched by Pb(II). The values of conditional stability constants, log Ka, for these two protein-like fluorescence peaks were 4.14 and 3.79. This indicates that the fluorescent substances are strong Pb(II) complexing organic ligands.

Lead Complexation of Soluble and Bound Extracellular Polymeric Substances from Activated Sludge: Characterized with Fluorescence Spectroscopy and Ftir Spectroscopy

ABSTRACTExtracellular polymeric substances (EPS) are important components of activated sludge and play an important role in removing heavy metals. The interaction of soluble EPS (SEPS)/bound EPS (BEPS) with Pb(II) was investigated using excitation—emission matrix (EEM) fluorescence spectroscopy and infra-red spectrometry. One protein-like fluorescence peak (peak A) was identified from the EEM spectra of SEPS and BEPS, and one aromatic protein peak (peak B) was observed in the EEM spectra of SEPS. the interaction of Pb(II) with SEPS was governed by collision sorption, while the binding of Pb(II) to BEPS was due to complexation. Functional groups of proteins, polysaccharides, lipids, and uronic acid were involved in Pb(II) adsorption to SEPS/BEPS. The organic acids in SEPS are also responsible for binding Pb(II) to SEPS. Other groups such as the phosphate group in the fingerprint zone also participated in binding of Pb(II) to EPS. There were no significant differences in the values of binding constants and conditional stability constants between SEPS and BEPS.

Study on Removal Efficiency and Fluorescence Characteristics of Humus in Landfill Leachate Treated by Combined Process

The removal efficiency and three-dimensional excitation emission matrix(3DEEM) fluorescence spectroscopic characterization of landfill leachate humus (HA and FA) in the Fenton-SBR process was studied. It was concluded that the combined Fenton-SBR process was available for effective removal of leachate humus, and the removal rates for HA and FA was 87.3% and 88.2%, respectively. The major fluorescence peaks observed in the leachate combined process was protein-like fluorescence peak a (Ex/Em=280nm/345nm), humic-like fluorescence peak b (Ex/Em=295/400nm), short excitation wavelength tryptophan-like fluorescence peak c (Ex/Em= 230nm/340nm) and ultraviolet band fulvic acid-like fluorescence peak d (Ex/Em=235nm/400nm). Types of fluorescence peak of HA and FA reduced and the intensities decreased after combined process treatment. The capability for removal of different components of humus was determined by the process units adopted.

TRANSFORMATION AND FLUORESCENCE SPECTROSCOPY OF DISSOLVED ORGANIC MATTER (DOM) IN LANDFILL LEACHATE TREATED BY COMBINED PROCESS

The paper analyses the removal efficiency and fluorescence spectroscopic characterization of landfill leachate dissolved organic matter (DOM) fractions in the combined stripping, fenton oxidation, sequence batch reactor, and coagulation process. The high contents of DOM of landfill leachate in each process unit were fractionated into HA, FA, and HyI. The result was the combined process availability for effective removal of DOM fractions. The removal rates for DOM, HA, FA and HyI were 91.9%, 97.1%, 95.8% and 71.7%, respectively. The major fluorescence peaks observed in the process were protein-like fluorescence peak a (Ex/Em= 280nm/345nm), humic-like fluorescence peak b (Ex/Em=295/400nm), short excitation wavelength tryptophan-like fluorescence peak c (Ex/Em=230nm/340nm) and ultraviolet band fulvic acid-like fluorescence peak d (Ex/Em=235nm/400nm). Types and intensities of fluorescence peaks of HA, FA and HyI decreased after treatment. The capability for removal of different components of leachate DOM was determined by each unit process adopted.

Cu(II) complexation of high molecular weight (HMW) fluorescent substances in root exudates from a wetland halophyte (Salicornia europaea L.)

High molecular weight (HMW) fractions are important components in root exudates. However, there is little available information concerning complexation of Cu(II) to the HMW fractions in root exudates. In the present study, complexation of root exudates from Salicornia europaea L. with Cu(II) was investigated using excitation emission matrix (EEM) fluorescence spectroscopy. Two protein-like fluorescence peaks were identified in the EEM spectra of root exudates. Fluorescence of both peaks was clearly quenched by Cu(II). The increase of conditional stability constant with increasing temperature indicates that the fluorescence quenching of the protein-like fluorescence by Cu(II) may be controlled by a dynamic process. The values of conditional stability constants (logK(a)) were in the range of 4.32-4.69, which were close to those of complexation of fulvic acid with Cu(II). This shows that the HMW fluorescent substances in root exudates from S. europaea L. were strong organic ligands for Cu(II). Our study suggests that the HMW fluorescent substances may affect chemical forms, mobility, and thus the fate of copper in wetland.

Study on optical characteristics of chromophoric dissolved organic matter (CDOM) in rainwater by fluorescence excitation-emission matrix and absorbance spectroscopy

The optical characteristics of chromophoric dissolved organic matter (CDOM) were determined in rain samples collected in Xiamen Island, during a rainy season in 2007, using fluorescence excitation-emission matrix spectroscopy associated with UV-Vis absorbance spectra. Results showed that the absorbance spectra of CDOM in rain samples decreased exponentially with wavelength. The absorbance coefficient at 300 nm [a(300)] ranged from 0.27 to 3.45 m(-1), which would be used as an index of CDOM abundance, and the mean value was 1.08 m(-1). The content of earlier stage of precipitation events was higher than that of later stage of precipitation events, which implied that anthropogenic sources or atmospheric pollution or air mass types were important contributors to CDOM levels in precipitation. EEMs spectra showed 4 types of fluorescence signals (2 humic-like fluorescence peaks and 2 protein-like fluorescence peaks) in rainwater samples, and there were significant positive correlations of peak A with C and peak B with S, showing their same sources or some relationship of the two humic-like substance and the two protein-like substance. The strong positive correlations of the two humic-like fluorescence peaks with a(300), suggested that the chromophores responsible for absorbance might be the same as fluorophores responsible for fluorescence. Results showed that the presence of highly absorbing and fluorescing CDOM in rainwater is of significant importance in atmospheric chemistry and might play a previously unrecognized role in the wavelength dependent spectral attenuation of solar radiation by atmospheric waters.

Binding of phenanthrene to extracellular polymeric substances (EPS) from aerobic activated sludge: A fluorescence study

Extracellular polymeric substances (EPS) are the essential components of activated sludge for removal of pollutants from wastewater. Limited information is available on the binding constants and binding mode of organic pollutants to EPS. In the present study, binding of phenanthrene (PHE) to extracellular polymeric substances (EPS) from aerobic activated sludge was investigated using fluorescence spectroscopy. Two protein-like fluorescence peaks (Ex/Em = 225 nm/340 nm for peak A; Ex/Em = 275–280 nm/338 nm for peak B) were identified in the three-dimensional excitation-emission matrix (3DEEM) fluorescence spectroscopy of the EPS. The fluorophores in EPS were clearly quenched by PHE and the quenching processes were static. The quenching constants (ln K a) were in the range of 11.27–13.82 M −1 and the binding constants (log K b) in the range of 6.11–8.98 M −1. The binding site number increased with increasing temperature. The corresponding thermodynamic parameters Δ G, Δ H and Δ S were calculated. The interaction of the fluorophores in EPS and PHE is spontaneous and exothermic. The binding of EPS with PHE is dominated by the hydrophobic interactions. Fluorophore A has stronger hydrophobic interaction with PHE than fluorophore B.

Binding of dicamba to soluble and bound extracellular polymeric substances (EPS) from aerobic activated sludge: A fluorescence quenching study

Binding of dicamba to soluble EPS (SEPS) and bound EPS (BEPS) from aerobic activated sludge was investigated using fluorescence spectroscopy. Two protein-like fluorescence peaks (peak A with Ex/Em = 225 nm/342–344 nm and peak B with Ex/Em = 275/340–344 nm) were identified in SEPS and BEPS. Humic-like fluorescence peak C (Ex/Em = 270–275 nm/450–460 nm) was only found in BEPS. Fluorescence of the peaks A and B for SEPS and peak A for BEPS were markedly quenched by dicamba at all temperatures whereas fluorescence of peaks B and C for BEPS was quenched only at 298 K. A dynamic process dominated the fluorescence quenching of peak A of both SEPS and BEPS. Fluorescence quenching of peak B and C was governed a static process. The effective quenching constants (log K a ) were 4.725–5.293 for protein-like fluorophores of SEPS and 4.23–5.190 for protein-like fluorophores of BEPS, respectively. Log K a for humic-like substances was 3.85. Generally, SEPS had greater binding capacity for dicamba than BEPS, and protein-like substances bound dicamba more strongly than humic-like substances. Binding of dicamba to SEPS and BEPS was spontaneous and exothermic. Electrostatic force and hydrophobic interaction forces play a crucial role in binding of dicamba to EPS.

Complexation between Hg(II) and biofilm extracellular polymeric substances: An application of fluorescence spectroscopy

The three-dimensional excitation emission matrix (EEM) fluorescence spectroscopy was employed to investigate the interaction of extracellular polymeric substances (EPS) from natural biofilm with Hg(II). The EEM spectra demonstrated that EPS with molecular weight over 14 kDa had two protein-like fluorescence peaks. The fluorescence intensity at both peaks was strongly dependent on the solution pH in the absence and presence of Hg(II), with the maximal fluorescence intensity at neutral pH. Fluorescence of both protein-like peaks was significantly quenched by Hg(II). The values of conditional stability constants (log K(a)=3.28-4.48) derived from modified Stern-Volmer equation are approximate to those for humic substances and dissolved organic matter (DOM), indicating that fluorescent components in EPS have strong binding capacity for Hg(II). Our findings suggest that EPS from biofilm is a class of important organic ligands for complexation with Hg(II) and may significantly affect the chemical forms, mobility, bioavailability and ecotoxicity of heavy metals in the aquatic environment.