Preparation And Application Of Nanocellulose-based Hydrogels

With the development of science and technology and the rapid development of industry,water pollution is becoming more and more serious.Some waste waterstill contains dyes and heavy metal ions which are not biodegrade even after treatmented.Adsorption has been found to be superior to other techniques in terms of flexibility and simplicity of design,initial cost,insensitivity to toxic pollutants and ease of operation and is considered an ideal method for removing dyes or other pollutants.The adsorbent is a key factor in the adsorption.However,conventional adsorbents are usually limited by the lack of selectivity and lower adsorption kinetics due to the limited surface area or active sites.Alarge surface area and the abundance of adsorption sites are essential for adsorption and removal of contaminants from wastewater.Nanomaterials,due to their higher surface area and greater number of active sites for interaction with pollutants,are increasingly used in dye removal.Nanocellulose,extracted from lignocellulosic materials,has attracted increasing attention from many diverse areas due to its unique properties such as high aspect ratio,large specific surface area,good biocompatibility and chemical accessibility.Its high surface area can provide many more active sites on the surface,resulting in high adsorption capability.Afterchemical modification,nanocellulose adsorbent with excellent adsorption properties can be obtained.In this paper,nanocomposite hydrogelsbased on nanocelluloseand amino containing polymers,such as polyvinylamine?PVAm?andchitosan?CS?were parepared and used as adsorbents in the adsorptive remocal of anionic dyes and heavy metal ions from the aqueous solutions.Firstly,nanocrystalline cellulose?NCC?was prepared by sulphuric acid hydrolysis of fully bleached hardwood kraft pulp?BHKP?,and then oxidized by NaIO₄ to yield dialdehyede nanocellulose?DANC?.DANC was used as a crosslinker to react with PVAm to obtain NCC-PVAm microgels with high density of free amine groups.The properties of NCC-PVAm microgels were characterized by FT-IR,element analysis,XRD,AFM.The swelling ratio and the Zeta potential under different pH conditionsof NCC-PVAm microgels were investigated.FT-IR analysis showed that the hydroxyl groups on the NCC molecular chain were partially oxidized to aldehyde group.A new adsorption band appeared after the reaction of DANC and PVAm,corresponding to the stretching vibration of C=N,which indicatedthatNCC-PVAm microgelwas successfully prepared.The XRD analysis showed that the crystal structure of NCC remained unchanged after the oxidation of NCC-PVAm by NaIO₄,but the crystallinity index decreased from 63%to 56.3%.The crystallinity of the microgels were almost completely disappeared.AFM images revealed that the NCC-PVAm microgel was microspherical particles with a diameter ranging from 200 to 300 nm.The isoelectric point(pH_pzc)for the microgel was found to be 5.6.NCC-PVAm microgels were used as an adsorbent to remove anionic dyes?Acid red dye,Congo red dye andReactive light yellow dye?and the heavy metal ions(Cr⁶⁺,Cu²⁺and Pb²⁺)in aqueous solutions.By changing the pH in the adsorption system,adsorption time and adsorbent dosage,the adsorption performance and mechanism were investigated.The microgel was found to be effective in anionic dye removal at acidic conditions.When the adsorbent dosage was 0.5 g/L and the anionic dyes concentration was 100 mg/L,the removal percentageofthe three dyes was 100%.The adsorption of anionic dyes by NCC-PVAm microgels all followed the pseudo second order kinetics and the correlation coefficient R²wasabove 0.99.The calculated adsorption capacity wasin good agreement with the experimental results.The adsorption of anionic dyes by NCC-PVAm microgels all followed Sips adsorption isotherms.The maximum adsorption capacities of NCC-PVAm microgelsof Acid red dye,Congo red dye andReactive light yellow dyewere 896.1 mg/g,1469.7 mg/g and 1250.9 mg/g,respectively.The optimum adsorption pH of Cr⁶⁺was pH 2,while pH 7 for Cu²⁺and Pb²⁺.The adsorption of the heavy metal ions by NCC-PVAm microgels all followed the pseudo second order kinetics and the correlation coefficient R² is above 0.98.The calculated adsorption capacity corresponded well with the experimental results.The adsorption all followed Sips adsorption isotherm.The aldehyde groups on DANC reacted withthe free amino groups on CS to obtain the NCC-CS composite hydrogels.NCC-CS hydrogelswere characterized by FT-IR,AFM andelemental analysis.Elemental analysis showed that with the increase of CS ratio,O content decreased gradually,due to the Schiff base reaction of aldehyde groupson DANC and the amino groups on CS,which suggestedthatNCC-CS composite hedrogelswere prepared successfully.FT-IR analysis showed thatanew absorption peak appeared corresponding to imino?C=N?stretching vibration,which further confirmed the the successful reaction of DANC with PVAm.NCC-CS composite hydrogels were used as an adsorbent to adsorb Cr⁶⁺in water solutions.The results showed that the optimum adsorption pH of Cr⁶⁺was pH 2.The adsorption process followedthe pseudo second order kineticsand Sips adsorption isotherm.The maximum adsorption capacity of NCC-CS composite gels on Cr⁶⁺was 169.42 mg/g.