Biosorption Of Uranium( Ⅵ ) And Thorium( Ⅳ ) By Engineering Saccharomyces Cerevisiae Displayed Mt Protein On The Surface

With the development of nuclear power, the problems of uranium shortage and the potential hazards of uranium and horium in waste water are increasingly prominent. Seeking economic and environmentally friendly methods to adsorb and recycle uranium have important scientific and practical significance. Currently, biosorption is one of the most promising technologies in the treatment of uranium waste water. However, in order to improve the adsorption capacity and specificity, to develop new bio-absorption materials appears very urgent. A number of bacteria,fungi, S. cerevisiae, and algae have been reported to have excellent treatment capabilities to uranium ions from wastewater solution. Simultaneously, it has been considered that metal complexes contain Cys 、 MT 、 Ala and Leu amino acid residues and display high adsorption capacity to metal positive ion. Metallothioneins(MT) are the most typical representative of such substances. In the research, to display MT on S. cerevisiae cell surface to prepare novel and efficient microbial adsorbent through genetic engineering techniques is expected to boost uranium ions biosorption technology industrial applications.By searching Genbank database, we obtained the DNA sequence of liver metallothionein gene.Optimized MT gene was synthesized by chemical methods in vitro, in which some rare codons were changed to common codons according to S. cerevisiae codon usage bias, XhoI/ Bam HI restriction sites and protect the bases were inserted in the C-terminal and N-terminal of the synthesized DNA sequence. S. cerevisiae display vector pYD1 was extracted and digested with endonucleases, a 5kb fragment was recovered and ligated with MT gene which was digested with the same endonucleases to generate a S. cerevisiae shuttle expression vector p YD-MT.Vector pYD-MT was extracted and transformed to competent cells by Li Ac-mediated method to generate recombinant EBY100-MT.EBY100-MT was cultured in a medium containing galactose to induce MT gene expression.Fluorescence detection verified the display of MT protein on S. cerevisiae cell membrane.A series of factors, such as the contact time, pH, initial concentration and temperaturean,which might influence the adsorption, was studied respectively. The results suggested that, the adsorption capability of thorium is higher than that of uranium.In acidic solutions, the process of adsorbing uranium get balanced after 20 min, the best initial concentration is 50μg.m L-1, the best initial pH value is 4.0, and the biggest adsorption capacity for uranium is 16.36 mg.g-1; In alkaline solutions, the process of adsorbing uranium get balanced after 30 min, the best initial concentration is 50μg.m L-1, the best initial p H value is 7.0-9.0, and the biggest adsorption capacity for uranium is16.30 mg.g-1. The optimal adsorption of thorium occurred at pH 2.0 and 40℃, the best initial concentration is 250μg.m L-1, the biosorption of thorium get balanced 40 min later and the biggest adsorption capacity was 72.82 mg.g-1. The adsorption isotherm and adsorption kinetic models were analyzed. The adsorption of uranium and thorium both fitted Langmuir model and Freundlich model.The result show that, the process process obeyed monolayer adsorption and the adsorption site distribute unevenly in cell-surface. With respect to the suitability of pseudo-second-order kinetic models for the process adsorption, it was determined that adsorption obeys pseudo-second-order kinetics which belongs to higher correlation coefficients.