Preparation Of Anionic Xylan And Its Application In Dyeing Wastewater Treatment

Hemicellulose is the most abundant renewable resource after cellulose in the cell wall of plants,and how to convert hemicellulose into value-added products efficiently becomes the key point of the feasibility of this process.At present,paper industry is now facing many problems,including shortage of energy,environmental pressure,low capability of developing new technology etc.Making full utilization of hemicellulose to produce hemicellulose-based and products,based on merging paper process with biorefinery process,will benefit extending paper process,high efficient-using of hemicellulose and increasing profit of paper production.In this paper,three different chemical modification methods were investigated by grafting the anionic functional groups onto xylan.Finally,the dye removal of simulated dye wastewater was studied by using synthetic anionic xylans.In this work,xylan was sulfated with chlorosulfonic acid in N,N-dimethylformamide?DMF?solvent,with LiCl as catalyst.The reaction conditions were optimized using a response surface methodology?RSM?.Furthermore,elemental analysis,thermogravimetric analyses?TGA?,Fourier transform infrared spectroscopy?FTIR?,proton nuclear magnetic resonance?¹H-NMR?and environmental scanning electron microscopy?SEM?were used to confirm the sulfation of xylan.At the same time,using birch xylan and chloroacetic acid as etherification agent,carboxymethyl modification of xylan was carried out under alkaline condition.The optimum process conditions were determined basing on the charge density and the degree of substitution.The physical and chemical properties of carboxymethylated xylan were analyzed by elemental analysis,FTIR,¹³C-NMR,TGA and SEM.Xylan was phosphorylated by using birch xylan as raw material and three sodium trimetaphosphate?STMP?as phosphating agent.Xylan and phosphorylated xylan were characterize by FTIR,¹³C-NMR,TGA and SEM to analysis the physical and chemical properties.Finally,three kinds of anionic xylan were used to analysize the dye removal from simulated dye wastewater.The effects of anionic xylan dosage,dye concentration,pH,temperature and inorganic salt on dye removal efficency and related mechanism were discussed.The results showed that the optimum synthesis conditions of sulfated xylan were:temperature,70 ^oC,reaction time,7 h,chlorosulfonic acid and xylose unit molar ratio of 3.71:1.Under the conditions,the substitution degree of the sulfated xylan reached to 1.11,while the charge density was-3.12 mmol/g.FTIR analysis showed that the strong characteristic absorption peak of 1250 cm^-1 and 810 cm^-1were generated by S=O and C-O-S stretching vibration of sulfate groups.¹H-NMR spectrum analysis showed that compared with that of xylan,the peak of sulfated xylan changed due to the existence of sulfated groups.Carboxymethylated xylan were prepared with the substitution degree of 0.95 and the charge density of-4.57 mmol/g,under the optimum conditions of the synthesis of carboxymethylated xylan which were:temperature 60?,reaction time 3 h,n?xylan?:n?NaOH?:n?SCA?molar ratio of1:5:4.According to the results of FTIR analysis,there are three characteristic absorption peaks of 1605,1418 and 1324 cm^-1,which are related to the carboxylic acid groups.From the ¹³C-NMR spectrum analysis,it can be seen that the peak of?70.8 is caused by the CH₂ of carboxymethyl xylan,and the new peak of?177.6 is produced by the carboxyl group of carboxymethylated xylan.The optimum conditions of phosphorylated xylan were:reaction temperature 80?,the reaction time 4 h,pH 9,n?xylan?:n?STMP?molar ratio of 1:3.Under the conditions,the grafted amount of phosphorylated xylan was up to 11.94%,while the charge density was-3.85 mmol/g.According to the analysis of FTIR analysis,two strong characteristic absorption peaks appeared at 1293 and 1106 cm^-1,which was related to the phosphate groups.The three anionic xylans were used to discuss the dye removal from the simulated ethyl violet dye wastewater.The results showed that the optimum removal reached to almost 97%when the concentration was 140 mg/L for sulfated xylan concentration 100 mg/L for carboxymethylated xylan concentration and 120 mg/L for the phosphorylated xylan concentration respectively under the temperature of 40?,stirring time of 40 min,the dye concentration of 40 mg/L and pH 9.Usually,in order to achieve the same dye removal efficiency,with the increase of the dye concentration,the concentration of anionic xylans also need to gradually increased.Furthermore,the concentrations between dye wastewater and anionic xylan are basically positive correlation.In addition,the inorganic salts had no significant effects on the dye removal.