Enzymatic Synthesis,Modification And Flocculation Of High Molecular Weight Dextran

Dextran is a kind of neutral polysaccharides that plays an important role in the development of medicine and industry.It can be derivatized to synthesize biological polysaccharide flocculant with high efficiency and environmental protection,which has wide protection prospect.The flocculation performance of polysaccharide flocculant is related to the molecular weight of dextran and its modification.In this study,the production process of high molecular weight dextran was explored on the basis of previous research,and high molecular weight dextran was grafted and esterified to obtain polysaccharide flocculant with good flocculation performance.The experimental results were given as follows:(1)The catalysis process of high molecular weight dextran was explored by us.When substrate concentration was 1000 mM and dextransucrase concentration was 0.8 U/m L,the molecular weight of synthesized dextran could reach to 11,320,000 Da at 25? under the conditions of fermentation for 24 hours,and the use of equilibrium intermittent feeding method to dose substrate from high to low concentration could improve the molecular weight of dextran.An amount of fructose was produced during dextransucrase catalytic process and could be a competition inhibition of the dextran chain extending.Moreover,the increase of viscosity of fermentation broth also hindered the prolongation of dextran chain.(2)The graft copolymerization of dextran with acrylamide(AM)was studied and the flocculation mechanism was illustrated by flocculation performance test.From the experimental results,when the amount of initiator was 2 mmol/L,m(AM): m(dextran)= 2: 1,reaction time was 3.5 h and reaction temperature was 35?,the monomer conversion was up to 95.4%.The results of flocculation test showed that when pH was 6,the removal turbidity of hematite suspension was 94% using 0.5 mg/L of grafted dextran and the removal turbidity of kaolin suspension was 91.2% using 4.15 mg/L graft dextran.Neutralization and bridging dominanted in flocculation process,accompanied by the trap effect,group reaction and so forth for synergistic flocculation.(3)The esterification of high molecular weight dextran was studied.The structure and properties of the ester were showed and the flocculation performance was tested.Shown in the results,the amorphous region of dextran was destroyed and the crystallization region was affected during esterification process.From the low degree of substitution,we could find that the rate of esterification was slow.The results of kaolin test showed that the removal turbidity was 91.7% with 15 mg/L of dextran ester under acidic conditions.The esterified dextran performed irregularly linear,with extensibility and flexibility,having many adsorption sites,hence,it showed considerable ability to agglomerate impurities into a group and promoted the depositon of flocs.To sum up,the study provides the theoretical data for industrialized mass production of high molecular weight dextran and is basis for the synthesis and application of high molecular weight dextran derivatives.The results of flocculation test exhibited that the polysaccharide flocculants has a considerable performance and broad application prospects.