| Microbial extracellular polymeric substances(Extracellular polymeric substance, EPS) were defined as the kind of bioflocculant. EPS were non-toxic, inexpensive, stable secondary metabolites. The biodegradability of EPS was often considered that EPS were the kind of polymers with uncertain structure. EPS were secreted by cells and released into environment or ahered to the surface of cells in different forms. In this experiment, components, structure and morphology of EPS were analysis. Different methods were applied in extraction, purification of EPS.The S. meliloti CCNWSX0020 was isolated from alfalfa in mine field in fengxian, shanxi province, which can resist 1.4 m M Cu2+. The mutant SXa was sensitive to 1.4 m M Cu2+. EPS produced by S. meliloti CCNWSX0020 restricted uptake of Cu2+ of the cells. The cell wall EPS was categorized based on the compactness and fastness: soluble EPS(S-EPS), loosely bound EPS(LB-EPS), and tightly bound EPS(TB-EPS). Research showed that EPS accumulated on the surface of cells and led to aggregation of cells. So the aggregated cells were protected by reducing the adverse impact of external environment.Methods for extracting EPS were: soluble extracellular polymeric substances(S-EPS, 5000r/min, centrifuged 10min), loosely bound extracellular polymeric substances(LB-EPS, 40 w ultrasound 1min, 7000r/min, centrifugal 20min), and tightly bound extracellular polymeric substances(TB-EPS, 80°C heat treatment 10 min, 15000r/min, centrifuged 20min). EPS were consisted mainly by polysaccharide, then protein.LB-EPS played a more important role than S-EPS and TB-EPS in Cu2+ immobilization. In biosorption process, p H decreased and the concentration of H+ increased indicated that ion-exchanged was taken place. Fourier Transform-Infrared(FT-IR) analysis further confirm that proteins and carbohydrates were the main extracellular compounds which had functional groups such as carboxyl(-COOH), hydroxyl(-OH), and amide(N-H), primarily involved in metal ion binding. Scanning Electron Microscopy(SEM) analysis LB-EPS had rough surface and many honeycomb pores, making it conducive to copper entry; therefore, they may play a role as a microbial protective barrier. LB-EPS played the very important role in biosorption of Cu2+.Biosorption rate of the wild-type S. meliloti CCNWSX0020 LB-EPS raised as p H increased. p H=5 was to ensure a high biosorption rate and avoid precipitation of metal ions. As for practical application, 25℃ was the best biosorption temperature. 1.5m M/L Cu2+ concentration and 50mg/L LB-EPS were suitably for the biosorption process. Experimental results showed that the biosorption of LB-EPS of Cu2+ reduced because of other coexisting metal ions. Langmuir isotherm model was suitable for biosorption process. Kinetic datas of the biosorption process conformed to the pseudo-second-order kinetics model. And the rate of intra-diffusion was major rate-limiting step of the biosorption process. Thermodynamic studies showed that the biosorption process was spontaneous endothermic reaction. The analysis of the activation energy(Ea) indicated that the chemical adsorption played a major role in biosorption process. The biosorption and desorption efficiency of LB-EPS reduced as the number of cycles of batch biosorption increased.In the batch biosorption experiments LB-EPS was imbedded in the calcium alginate gel beads. The embedding rate was 1/30, p H=5, the biosorption temperature is 35℃, the amount of adding LB-EPS calcium alginate gel beads was 60/100 ml, Cu2+ concentration was 2.0m M. Langmuir isotherm model was suitable for biosorption process. In column biosorption experiments, biosorption rate of calcium alginate gel beads of LB-EPS column was much higher than the rate of free LB-EPS and calcium alginate gel beads of LB-EPS in batch biosorption. Overall, the biosorption and desorption by calcium alginate gel beads of LB-EPS column provided a theoretical basis for future promotion and application. |
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