Mechanism And Application Of Bioflocculant Produced Under High-salt Condition By Using Strain Bacillus Sp.

Traditional chemical flocculants have obvious effect on wastewater treatment,but they can cause secondary pollution and harm to nature and biology.At the same time,a large amount of sludge containing these chemical flocculants will be produced,which will increase the cost of subsequent treatment.Bioflocculant is a kind of biological polymer organic matter secreted in the process of microbial growth,which can be self-degradable in the natural environment without residual and secondary pollution.Bioflocculant are rich in functional groups and have high environmental benefits.In recent years,they have been applied in many fields,such as adsorption of heavy metals and dyes and synthesis of nanoparticles.However,the current methods for bioflocculants production were time-consuming and costly,limiting their upscaling and commercialization.For examples,the average production cycle usually takes more than a few hundred hours including cell cultivation and purification of EPS.In addition,it requires the supply of substrates and nutrition for cell growth.In addition,the production process of bioflocculants needs high temperature sterilization technology.Therefore,reducing the production cost of bioflocculants and studying the mechanism of bioflocculants application are the key to solve the application of bioflocculants in wastewater.Studies have shown that,most strains were vulnerable under high salt environment caused by the plasmolysis,like the heating effects.As such,it promises the potential of using high salt condition to take over the conventional aseptic technique.In previous studies,it can be found that Bacillus sp.has strong salt tolerance,and extracellular polymers secreted by Bacillus sp.have abundant carboxyl,amino,hydroxyl and other groups to provide a large number of adsorption sites for heavy metals.Using the characteristics that Bacillus sp.can secrete bioflocculants under high salt condition,the difficult high salt wastewater can be used as cheap medium to reduce the cost of producing bioflocculants.At the same time,it can reduce reduce the sterilization process in the production of microbial flocculant can simplify the process and save energy.Therefore,the bioflocculants secreted by Bacillus sp.was used as a heavy metal adsorbent.Development of a novel low-cost and efficient biological adsorbent by using the characteristics of Bacillus sp.In view of the influence of high salt concentration on the production of bioflocculants by Bacillus sp.are not clear and the unclear mechanism of adsorption of heavy metals by bioflocculants,the secretion law of bioflocculants under the condition of high salt was analyzed.We used a variety of techniques to analyze the yield,composition,physicochemical properties and removal efficiency of bioflocculants secreted under salt stress,and established a mechanism model for the adsorption of heavy metals by bioflocculants.Through the research of this subject,we hope to find a new way to improve the adsorption effect of heavy metals economically and effectively,and provide reliable theoretical basis and technical guidance for the theoretical and technical research of bioflocculants adsorption of heavy metals.The main research conclusions of this paper are as follows:1.Preparation of microbial flocculant by Bacillus sp.under high salt condition and its flocculant effect:Under high salt condition,the yield of bioflocculants produced by Bacillus sp.was about 2.4 g/L.Bioflocculants produced under high salt condition needs to be combined with Fe³⁺in order to have better flocculation effect on kaolin particles.The optimum flocculation condition of bioflocculants was adding 120 mg/L bioflocculants and 0.4m M Fe³⁺.Bioflocculants have good heat resistance and high flocculation rate for kaolin particles in a wide p H range,and the flocculation rate are all above 90%,which indicates that they have a wide prospect in practical application.2.Characterization and flocculation mechanism of flocculants:bioflocculants produced under high salt conditions are composed of 54.17%polysaccharide,32.68%protein and 23.52%other substances.The EEM showed that the flocculant samples contained tryptophan-like proteins and tyrosine-like proteins.Through the analysis of FTIR and XPS,we can found that the sample of bioflocculants is mainly composed of C,N,O and contains abundant hydroxyl group,carboxyl group and amino group.The ESEM show that the sample has a reticular structure and abundant pores on the surface.Through the microscopic observation of the flocculant precipitate,the analysis of Zeta potential change before and after flocculation,the analysis of flocculating active components and the analysis of the binding bond between kaolin and bioflocculants,it can be concluded that the main mechanism of flocculation is electric neutralization and adsorption bridging.3.Study on the adsorption of heavy metals by bioflocculants:the removal rate of Pb²⁺,Cr⁶⁺and Cu²⁺by bioflocculants produced under high salt condition is Pb²⁺>Cu²⁺>Cr⁶⁺.In the experiment of adsorption of heavy metals by bioflocculants,the maximum adsorption capacity of bioflocculants for Pb²⁺is 485.09 mg/g,for Cr⁶⁺is 175.86 mg/g,and for Cu²⁺is 205.18 mg/L.The adsorption of heavy metals by bioflocculants is not a simple monolayer adsorption process,and the reaction is more complex.Chemisorption is the main rate-limiting step in the whole adsorption process.The adsorption capacity and adsorption rate of the same bioflocculants for three kinds of heavy metal ions are different,which may be caused by different components play a role in the different metal ions adsorption process.4.Mechanism of bioflocculants adsorption of heavy metals:Through FTIR spectrums were used to illustrate the interaction between the bioflocculant and heavy metals,we can find that the adsorption of different heavy metal ions,different groups in bioflocculant play a role.The groups in protein,polysaccharide and nucleic acid in bioflocculants all play a role in the process of adsorbing heavy metals.According to the fitting results of EEM data,it can be found that proteins and humus-like components bind to heavy metals during the adsorption process of bioflocculants.ESEM shows that bioflocculants provide abundant sites for the adsorption of heavy metals in porous structures.From the SEM-EDX diagram,we can find that the ion exchange reaction of K⁺and Mg²⁺on the surface of heavy metals and bioflocculants occurred in the adsorption process,indicating that ion exchange play a important role in the adsorption process.We used FTIR,EEM,ESEM,XPS to analyze mechanism of bioflocculants adsorption of heavy metals,it can be concluded that the main mechanism of bioflocculants adsorption of heavy metals are surface complexation,electrostatic adsorption,and ion exchange.