Preparation Of Four Microbial Adsorbents And Their Removal Characteristics And Mechanisms For Heavy Metals From Wastewater

Heavy metal contamination poses an increasingly serious problem to environment.Therefore,choosing effective treatment methods is an important way to remove heavy metal from wastewater.Among various treatment methods of heavy metal,microbial adsorption has attracted considerable attention due to its environment-friendly nature and low cost.The development and modification of microbial adsorption materials have attracted by many researchers.In this work,the adsorption performance of Pb²⁺and other heavy metal ions by microorganism including common typical microorganisms and screened microorganisms from special environments were studied.And the adsorption performance and safety of the above microorganism were improved through modification.Based on the these researches,it could provide some theoretical basis for promoting the development of microbial adsorbents.In this study,some microbes were obtained from a mine wastewater in Yunnan.In the laboratory experiments,three strains of microbes belonging to different types,which had strong heavy metal(Pb²⁺,Cd²⁺and Zn²⁺)resistant were screened by increasing gradually the concentration of heavy metal ions(Pb²⁺,Cd²⁺and Zn²⁺)in the medium.And the most common typical baker's yeast was selected as another type of microorganisms.Furthermore,the following research mainly included two aspects according to their adsorption capacity of heavy metal and pathogenicity to human body.On the one hand,the microorganism with good adsorption capacity for heavy metals was used to investigate its adsorption performance.For another,a series of microbial adsorbents with lower adsorption capacity and pathogenicity were successfully modified into adsorbents with better adsorption capacity and safety through different methods,and the feasibility of modified methods was explored.All the researches were aimed to provide reference methods and broaden the application range for the application and modification of different types of microorganisms.Through a batch experiments,the effects of different factors including initial pH value of solution,adsorbent dosage,adsorption time,initial metal ion concentration and adsorption temperature on the adsorption capacity of adsorbents were investigated.The related adsorption mechanisms of various adsorbents for heavy metal were analyzed and researched based on adsorption kinetics,isotherms and thermodynamics analysis combination with a variety of modern test and analysis techniques.The main research results are as follows:?1?Three strains of resistant microbes for Pb²⁺,Cd²⁺and Zn²⁺from a mine wastewater in Yunnan province were screened out.The three strains were named as S1,S5 and S12,respectively.S1,S5 and S12 were classified as Klebsiella pneumonia,Bacillus megaterium and Agrobacterium tumefaciens,respectively,through 16S rDNA sequence analysis,phylogenetic tree construction and gram staining.Fourier transform infrared spectroscopy?FTIR?analysis showed that the surface of S1,S5 and S12contained hydroxyl,carboxyl,amino and phosphate groups.The adsorption abilities of S1,S5 and S12 for Pb²⁺,Cd²⁺and Zn²⁺were preliminarily studied,as well as the pathogenicity were evaluated.The results showed that the adsorption capacity of S12for Pb²⁺,Cd²⁺and Zn²⁺was highest and it is without pathogenic on human.The adsorption capacity of S1 is better,but it has pathogenic nature.The adsorption capacity of S5 is the worst,and it is nonpathogenic.Subsequently,different studies have been conducted on these three typical microbes.The adsorption characteristics and mechanisms of S12 for Pb²⁺,Cd²⁺and Zn²⁺were further investigated.The Pb²⁺was chosen as the target pollutants to investigated the influence of high temperature inactivation modification method on adsorption properties of Klebsiella pneumoniae S1 while eliminating its pathogenicity.In addition,the feasibility of chemical modification of Bacillus megaterium S5 and the adsorption performance and adsorption mechanism of Pb²⁺by the modified adsorbent were mainly studied.?2?Agrobacterium tumefaciens with excelent adsorption capacity was mainly applied in plant genetic engineering.In this study,this bacterium was used as a biological adsorption material to study the adsorption characteristics and mechanisms for Pb²⁺,Cd²⁺and Zn²⁺from simulated wastewater.The results revealed that the adsorption of Pb²⁺,Cd²⁺,and Zn²⁺by Agrobacterium tumefaciens fitted well with pseudo-second-order kinetics and Langmuir isotherm model,which suggested that the adsorption process was chemical adsorption and monolayer adsorption on the surface of Agrobacterium tumefaciens.The results of thermodynamic parameters including Gibbs free energy?G and enthalpy change?H showed that the adsorption process of Pb²⁺,Cd²⁺and Zn²⁺by Agrobacterium tumefaciens was spontaneous and endothermic.The maximum adsorption capacity of Pb²⁺,Cd²⁺,and Zn²⁺based on the Langmuir isotherm model at 30°C were 234.12,58.42 and 51.31 mg/g,respectively.The Scanning electron microscopy?SEM?,Energy disperse spectroscopy?EDS?,Fourier transform infrared spectroscopy?FTIR?and X-ray photoelectron spectroscopy?XPS?analysis indicated that the Agrobacterium tumefaciens can effectively remove Pb²⁺,Cd²⁺and Zn²⁺that was mainly attributed to the complexation between metal ions and functional groups containing N,O and P on the surface of this bacterium.The results indicated that Agrobacterium tumefaciens has excellent performance in removing Pb²⁺,Cd²⁺and Zn²⁺,especially for Pb²⁺and great application potential for treating waste-water contaminated with heavy metals.?3?Klebsiella pneumoniae is a kind of pathogenic bacteria with good adsorption capacity.If the live cell of Klebsiella pneumonia was directly used as adsorbent for the adsorption of heavy metal,it will cause environmental safety problems.Thus,the effect of physical modification with high temperature inactivation on the adsorption capacity of Klebsiella pneumoniae for Pb²⁺was investigated.It found that Pb²⁺adsorption by dead?DC?cells and live?LC?fitted well with pseudo-second-order kinetics and the Langmuir isotherm model,respectively.The dead cels were generated by high temperature treatment.After analysis the thermodynamic,it showed that the adsorption of Pb²⁺by DC as spontaneous and exothermic,whereas it was spontaneous and endothermic for LC.Based on these results,indicating that exposing more active sites to the surface of DC following high temperature treatment,and DC had a larger specific surface area and more negative charges than LC.The above difference resulted in DC presenting greater adsorption capacity for Pb²⁺than LC.The adsorption of Pb²⁺by DC is mainly through the complexation between N-containing groups and Pb²⁺ions.High temperature treatment could overcome the pathogenicity of Klebsiella pneumoniae and improve its adsorption capacity.Therefore,a kind of safe and effective microbial adsorbent was obtained.?4?Bacillus megaterium had low adsorption capacity for heavy metals.The xanthate modification method has the advantage of good effect,simple operation,low synthesis cost and short synthesis time.The results showed that xanthate modification was a feasible method for the modification of Bacillus megacephala.And a new microbial adsorbent?XBCS?with low cost and high efficiency was successfully prepared,which had good adsorption performance for Pb²⁺.The adsorption of Pb²⁺by XBCS was monolayer adsorption and chemical adsorption.The maximum adsorption capacity of Pb²⁺was 350.3 mg/g for XBCS at 30°C,which was significantly higher than that by Bacillus megaterium.The adsorption capacity of Pb²⁺by Bacillus megaterium was 51.64 mg/g.The thermodynamic results showed that the adsorption of Pb²⁺by XBCS was spontaneous and endothermic.SEM-EDS,FTIR,X-ray diffraction?XRD?and XPS analysis revealed that the higher adsorption capacity of Pb²⁺by XBCS was mainly attributed to the complexation between Pb²⁺and groups containing S.According to above results,it is suggested that the xanthate modification is a kind of effective method to improve the adsorption ability of Bacillus megaterium.?5?Baker's yeast has features such as large yield and wide application.However,the adsorption capacities of yeast cells are unsatisfactory for direct application baker's yeast to adsorb heavy metal.In addition,phosphates have good chemical complexation for heavy metals.Phosphate modification is a simple and easy to operate process.Therefore,a new microbial adsorbent?PMBY?was prepared by modifying baker's yeast with phosphate that grafted phosphate group onto the surface of Baker's yeast.The adsorption performance and adsorption mechanism of Pb²⁺by PMBY were investigated.The results showed that PMBY had good adsorption ability for Pb²⁺,and the adsorption of Pb²⁺on PMBY can rapidly achieve adsorption equilibrium?in just 3min?.The maximum adsorption capacity of Pb²⁺by PMBY was about 3 times that of the primitive baker's yeast.The adsorption process of Pb²⁺by PMBY followed the pseudo-second-order and the Langmuir isotherm model.The SEM,FTIR and XPS characterization showed that the adsorption of PMBY for Pb²⁺was mainly due to the complexation between the phosphate groups and Pb²⁺.In addition,the regeneration experiments indicated that the PMBY was easily recovered through desorption in 0.01mol/L HCl,and that PMBY still exhibited 90.77%of the original adsorption capacity for Pb²⁺after five regeneration cycles.These results showed that the phosphate modification was an effective method to improve the adsorption performance of yeast for Pb²⁺and make the yeast has higher adsorption ability.In conclusion,the adsorption performance and adsorption mechanisms of four kinds of microbial adsorbents with different properties were investigated in this study.It il uminates that the microbial adsorbents with different properties had different adsorption properties for heavy metals.The adsorption ability of microbial adsorbents to heavy metals such as Pb²⁺is related to the functional groups that could bind heavy metal ions through chemical complexation on the surface of adsorbents.The form and quantity of functional groups that have the chemical complexation with heavy metals on the surface of microbial adsorbents had been increased by different modification methods.The study provided reference and basis for the preparation and application of microbial adsorbents in the heavy metal wastewater treatment.