Preparation Of PEI Grafted Starch Nanocrystals And Its Use As Adsorbent

With the rapid development of modern industry,the problem of water pollution is becoming more and more serious.Adsorption method is an important method for removing pollutants in wastewater due to its advantages of high efficiency,strong selectivity and simple process.In the field of environmental adsorption,high-capacity and high-performance biosorbents are the best materials.Starch has many advantages,such as wide sources,green environmental protection,biodegradability and easy modification and modification.Starch nanocrystals(SNCs)have great potential for application in the field of adsorption due to its advantages of small diameter,large specific surface area and nanometer characteristics.In this paper,SNCs were prepared from waxy corn starch,and then polyethyleneimine grafted starch nanocrystals(SNCs-PEI)was prepared by cross-linking method.The products were characterized in order to explore their properties.SNCs-PEI was used as the biosorbent to remove the anionic dyes methyl orange(MO),Congo red(CR),methyl blue(MB)and heavy metal Cr(VI)from aqueous solution,and their adsorption mechanism was explored.The main results were as follows:Using waxy corn starch as raw material,SNCs were prepared by a combination of enzyme pretreatment and acid hydrolysis,and then SNCs-PEI was prepared by grafting PEI with glutaraldehyde as a cross-linking agent.According to the orthogonal experiment,the optimal conditions for the preparation of SNCs-PEI were as follows:reaction time of 4 h,PEI dosage of 0.5 g and glutaraldehyde dosage of 3 mL.The nitrogen content of SNCs-PEI was 2.32%in the way.At pH=3,the Zeta potential of SNCs-PEI was+37.9 mV.FTIR and EDAX showed that PEI was successfully grafted onto SNCs,and the crystal structure of SNCs didn’t change after grafting PEI.SNCs-PEI was used as the biosorbent to remove the anionic dyes MO and CR.The research showed that the adsorption of MO and CR by SNCs-PEI reached the adsorption equilibrium at 60 min and 120 min,respectively.At pH=3,the SNCs-PEI dosage of 0.1 g and the initial concentration of 130 mg/L,the maximum adsorption capacities of MO and CR by SNCs-PEI were 79.10 and 57.05 mg/g,respectively.The adsorption process of MO and CR by SNCs-PEI conformed to the pseudo-second-order kinetic model and Langmuir isotherm.It was a monolayer and chemical adsorption process,and the adsorption process was a spontaneous,endothermic and entropy-increasing process.FTIR showed that MO and CR were successfully adsorbed on SNCs-PEI.Using glutaraldehyde as the cross-linking agent,the SNCs were reacted with 0.05,0.25 and 0.50 g of PEI,respectively,and three kinds of adsorbents with different PEI contents were prepared,which were labeled as SNCs-PEI-0.05,SNCs-PEI-0.25 and SNCs-PEI-0.50,respectively.They were used as the biosorbent to remove the anionic dyes MB.The research showed that under the conditions of pH=7,the adsorbent amount of 0.1 g and the initial concentration of 300 mg/L,the adsorption capacities of MB by SNCs-PEI-0.05,SNCs-PEI-0.25 and SNCs-PEI-0.50 were 195.05,232.72 and 240.16 mg/g,respectively.The adsorption process of MB by SNCs-PEI conformed to the pseudo-second-order kinetic model and Langmuir isotherm,which showed the removal of MB by SNCs-PEI was a monolayer homogeneous chemisorption process.At 25,30 and 35℃,the maximum adsorption capacities of SNCs-PEI-0.5 were 337.84,377.36 and 383.14 mg/g,respectively.Thermodynamic analysis showed that the adsorption process of MB by SNCs-PEI was a spontaneous endothermic process.FTIR indicated that MB dyes were successfully adsorbed on SNCs-PEI.H2O2 was used to as the oxidant prepare oxidized starch nanocrystals(OSNCs).Then OSNCs reacted with 100 g of 0.1 wt%,1.0 wt%and 3.0 wt%PEI in aqueous solutions,respectively.Three kinds of adsorbents with different PEI contents were prepared,which were labeled as OSNCs-0.1wt%PEI,OSNCs-1.0wt%PEI and OSNCs-3.0wt%PEI.At pH=3,the Zeta potentials of OSNCs-0.1wt%PEI,OSNCs-1.0wt%PEI and OSNCs-3.0wt%PEI were+30.4,+43.2 and+46.3 mV,respectively.FTIR and EDAX showed that the hydroxyl groups on SNCs were oxidized to aldehyde groups,and PEI was successfully grafted to OSNCs.They were used as the biosorbent to remove the hexavalent chromium.The research showed that under the conditions of pH=3,initial Cr(VI)concentration of 1000 mg/L,adsorbent dosage of 0.1 g and adsorption time of 60 min,the adsorption capacities of Cr(Ⅵ)by OSNCs-0.1wt%PEI,OSNCs-1.0wt%PEI and OSNCs-3.0wt%PEI were 103.56,173.91 and 184.35 mg/g,respectively.The adsorption process was more in line with the pseudo-second-order kinetic model and the Freundlich isotherm,indicating that the adsorption process was a heterogeneous chemical adsorption process.The adsorption process was a spontaneous,entropy-increasing and endothermic process.FTIR and EDAX indicated that the heavy metal Cr(Ⅵ)was successfully adsorbed on OSNCs-PEI.