Interactions Between Microbial Extracellular Polymeric Substances And Toxic Contaminates In Wastewater And Influences On The Performance Of Bioreactors

The occruences of plenty of trace toxic contaminants (such as heavy metals, antibiotics) in the municipal wastewater have attracted more and more attention. As the activity of the microbes can be inhibited by the contaminats, the removals of these pollutants are limited in the conventional activated sludge processes. The redidues of the contaminats in the effluence from wasteater treatment plants will cause serious environmental risks. Extracellular polymeric substances (EPS) are produced by the microbes when organics in wastewater are consumed in wastewater treatment bioreactors. EPS are a major component of microbial aggregates, and present both outside of cells and interior microbial aggregates. They have huge abilities of adsorption and complex for trace toxic contaminants, due to the presence of many functional groups and hydrophobic regions in EPS. The trace toxic contaminants in wastewater will interact with EPS at initial, and thus.their migration and removal in the wastewater treatment bioreactor would be greatly influenced. However, the crucial roles of EPS on the migration and removal of trace toxic contaminants are not clear. In this work, sensitive methods for characterizing the interaction between EPS and trace toxic contaminants were developed. The roles of EPS on the migration and removal of trace toxic contaminants in wastewater treatment bioreactors were also explored. This work provided theroritical basis and technical supporting for enhancing the removal of trace toxic contaminants in activated sludge system. The main contents and results are listed below:1. The thermodynamic characteristics of the binding between aqueous metals (with copper ion as an example) and sludge EPS were investigated combinding isothermal titration calorimetry (ITC), three-dimensional excitation-emission matrix (EEM) fluorescence spectroscopy, X-ray absorption fine structure (XAFS) and Fourier transform infrared spectroscopy (FTIR) analysis. The results show that the proteins and humic substances in EPS were both strong ligands for Cu2+. The binding capacity, binding constant, binding enthalpy were calculated as5.74×102mmol/g,2.18×105L/mol and11.30kJ/mol, respectively, implying that such a binding process was exothermic and thermodynamically favorable. The disorder degree of EPS increased with the addition of Cu2+and the binding process was found to be driven mainly by the entropy change. A further investigation shows that Cu2+was bound with the oxygen atom in the carboxyl groups in the sludge EPS molecules.2. The interaction mechanism between sludge EPS and a typical sulfonamide antibiotic, sulfamethazine (SMZ) was explored. Results show that SMZ interacted mainly with the protein-like substances in EPS, and the π-π interaction occurred between EPS and aromatic ring in SMZ molecules. The relatively high binding constant indicated the effective enrichment of SMZ by EPS, which would influence the following degradation process of SMZ by activated sludge. The binding process proceeded spontaneously, and the driving force was mainly from the typical hydrophobic interaction. After binding with SMZ, the conformation of EPS was expanded and their structure became loose, which was favored for the mass transfer and pollution capture in sludge.3. SPR combined with ITC method was developed to investigate the microscopic kinetics and thermodynamics of interaction between SMZ and humic substances. Results indicated that the interaction between SMZ and humic acids (HA) was greatly influenced by the aquatic ionic strength, temperature, pH etc.. It could be enhanced with increasing ionic strength and be depressed with increasing temperature. Simultaneously, the transformed to be entropy-driven collaboration with enthalpy-driven from entropy-driven as temperature raised. The hydrophobic interaction between SMZ and HA was enhanced at a high temperature. Interaction between SMZ and fulvic acids (FA) was also greatly influenced by the pH of solution. The π-π interaction occurred between FA and aromatic ring in SMZ, and the interaction was mainly entropy-driving.4. The removal of sulfonamides was explored in the wastewater treatmetn bioreactor with batch and continuous modes respectively, and the crucial roles of EPS in the migration and transformation of sulfonamide were investigated. Results show the removal of SMZ could be regarded as two stages:bioabsorption and biodegradation. SMZ could be absorbed and enriched by the sludge EPS, EPS played an important role in SMZ removal at the adsorption stage, which was benefited for the subsequence biodegration of SMZ by activated sludge. Furthermore, more EPS could be produced by the actived sludge in response to the shocking of SMZ to the wastewater treatment bioreactor, which also enhanced the biosorption of SMZ by EPS. The adsorption quantity of EPS enlarged with the increasing tolerance of microbes to SMZ.5. Proton exchange membrane (PEM) fouling in microbial fuel cell is investigated by experimental analysis and mathematical simulation. Results show that PEM is readily fouled by the microorganisms, EPS and inorganic salts during the long-term operation of MFC. This would decrease the conductivity, cation diffusion of the PEM, and thus deteriorate the MFC performance. The physical blockage of cations transfer directly caused the decay of current. Furthermore, the potential loss caused by the pH-gradient was enhanced attributed to proton transfer limitation. The transfer limitation of cations because of the decreasing diffusion coefficients and the heightened cathodic potential loss after PEM fouling were proven to be the mainly reasons for the deterioration of MFC performance.