Study On The Oxidation,Graft Modification And Flocculation Performance Of High-molecular-weight Dextran

Polysaccharide-modified flocculants as natural polymeric flocculants are receiving increased attention in the field of water treatment.Dextran,a kind of neutral polysaccharide consisting of repeatα-D-pyran glucose units,is produced by dextransucrase catalysis with sucrose as a substrate.On the basis of high molecular weight dextran prepared by enzyme-catalyzed reaction,dextran was oxidized and graft modified to meet the requirements of flocculant in wastewater treatment.The specific research results are as follows:（1）Firstly,a graft copolymerization modification reaction of[2-（methacryloyloxy）ethyl]trimethylammonium chloride（METAC）with high molecular weight dextran was carried out to prepare a cationic dextran derivative with high flocculating activity,and the grafted product was characterized and further analyzed in structure and evaluated in flocculation performance.The experimental results show that the optimum condition was METAC/dextran molar ratio at 3,the concentration at 60 g/L,the reaction time at 3h,the temperature at 60°C,and the pH at 7.The highest grafting degree of the obtained dextran graft product can reach 94%,the corresponding charge density was 3.0 meq/g,and the highest flocculation efficiency for kaolin suspension is 97.8%.The flocculation mechanisms that play leading roles in the flocculation process are charge neutralization and charge patching.（2）Secondly,a novel and efficient flocculant was proposed,which synthesized by copolymerization of chitosan and dextran.The flocculant was manufactured by microwave initiated polymerization technique,and the structure characterization of EA,NMR,FT-IR,XRD and FESEM spectrum confirmed the successful grafting.Influences of flocculation pH,flocculation temperature and flocculant dosage on flocculation efficiency were examined.For actual wastewater treatment,this flocculant exhibits effective flocculation performance at low levels of flocculant.The results indicated it can removed 93.6%of the kaolin solid suspended particles in the wastewater.The flocculation mechanism studies have shown that the flocculation process includes adsorption bridging,charge neutralization,and sweeping flocculation mechanisms.The improved dextran helps to treat domestic sewage and industrial wastewater,which also can provide broad prospects for designing novel high-performance flocculants.（3）Finally,an environmentally friendly and biodegradable food grade polysaccharide flocculant was synthesized through oxidation modification of high molecular weight dextran by H₂O₂.Through the optimization of reaction condition,the optimum condition for oxidation of high molecular weight dextran by H₂O₂ was the concentration of dextran at 50 g/L,the temperature at 40°C,and the reaction time at 3 h.The dextran oxidation product has excellent flocculation efficiency for suspensions such as kaolin and hematite,for which the highest flocculation efficiency are 96.4%and 93.4%,respectively.When the flocculant dosage is 70 mg/L and pH value is 6,the optimal removal of turbidity for the sugarcane juice can reach 80.1%.The oxidative modification process mainly occurs in the non-crystalline region in dextran,and main flocculation mechanisms include adsorption bridging and sweeping.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.