Influence Of Extracellular Polymeric Substances On The Cr(?) Reduction Over Soil Minerals-Shewanella

Microorganisms can secrete extracellular polymeric substances?EPS?that are three-dimensional gel-like polymer coated on the surface of bacteria and located between bacterial populations.EPS play a key role in physicochemical properties of bacteria,pollutants Migration,biological mineralization and also affect the interaction between bacteria and minerals.In this paper,natural and EPS-depleted Shewanella oneidensis MR-1 were used as the test bacteria.The automatic potentiometric titration,surface charge analysis and contact angle measurement were used to compare the difference of bacteria before and after EPS removal and study the physicochemical properties and structural characteristics of composites formed by bacteria and typical soil minerals?kaolinite,montmorillonite,goethite?before and after EPS removal.In addition,by means of reduction kinetics experiments,dead-lived dyeing experiments,Fourier transform infrared spectroscopy?FTIR?,three-dimensional excitation-emission matrix?3D-EEM?,X-ray photoelectron spectroscopy?XPS?,the differences of Cr???reduction between natural and EPS-depleted bacteria and mineral complexes were compared.The mechanism of extracellular polymer?EPS?on the reduction of Cr???by Shewanella-minerals composites were investigated.The main results were listed as follows:The effect of EPS on the surface properties of Shewanella was ascertained.After removing EPS,the negative charge,hydrophobicity of Shewanella were decreased.The surface site concentration of Shewanella also reduced,and the concentration of carboxyl group and hydroxyl group decreased mainly.However,the surface functional group type did not change.Intact and low-EPS Shewanella were contain three main functional groups?carboxyl,phosphate and hydroxyl?.The effect of EPS on the reduction of Cr???by Shewanella was clarified.During the reduction of different concentrations of Cr???,the reduction rate and reduction of Cr???by low-EPS Shewanella were lower than intact Shewanella,indicating that EPS were beneficial to Shewanella reduction of Cr???.The ITC results showed that the reduction of Cr???by intact and low-EPS Shewanella were exothermic reaction,but the reduction reaction with EPS was more thermodynamically favorable.The results of the dead and live dyeing experiments also showed that the proportion of live bacteria in the low-EPS Shewanella was lower than intact Shewanella,further confirming that EPS reduced the toxicity of Cr???to bacteria.The mechanism of the effect of EPS on the reduction of Cr???by Shewanella was elucidated.UV/Vis absorption spectroscopy and XPS analysis showed that the outer membrane c-type cytochrome in EPS can be used as a terminal reductase and the hemiacetal group of the saccharide can be oxidized to carboxylate as reducing agent for Cr???reduction.Fluorescence spectroscopy revealed that tryptophan and aromatic proteins were the main fluorescent components of EPS,and the fluorescence intensity was weakened,indicating that the protein was involved in the reduction process of Cr???,further promoting the reduction of Cr???by Shewanella.The effect of EPS on the reduction of Cr???by Shewanella-mineral composites was clarified.Metabolic thermoactivity analysis showed that the removal of EPS from bacterial surface inhibited the activity of bacteria in the Shewanella-kaolinite/goethite composites,but enhanced the activity of bacteria in the Shewanella-montmorillonite composites.According to the results of reduction kinetics,it was found that the reduction rate of Cr???by the intact Shewanella-kaolinite/goethite composites was greater than low-EPS Shewanella-kaolinite/goethite composites.Indicating that EPS can promote the reduction of Cr???on the Shewanella-kaolinite/goethite composites by enhancing the bacterial activity,while the EPS inhibits the reduction of Cr???low-EPS Shewanella-montmorillonite composites.EPS not only can reduce Cr???,but also affects the reduction products of Cr???by Shewanella and Shewanella-minerals complexes.In the presence of EPS,the reduction products are mainly bound to the surface of Shewanella and composites in the form of Cr₂O₃ and CrPO₄.After removing EPS,the reduced product is mainly bound to the surface of the composites in the form of CrPO₄.