Study Of Bio-Inspired Woody Aerogels Fabrication And Their Application In Water Remediation

Wood is one of the most abundant renewable resources in the world.Woody aerogel material is one of the important projects for the value-added utilization of wood resources and new material technology research.Lignocellulosic aerogels have been highly concerned by researchers since they came out in the early 21st century and have developed vigorously in the past decade.With the deepening of research,the focus of research on lignocellulose aerogels has gradually shifted from the preparation and characterization of aerogels to functional applications.Woody aerogel has shown bright application prospect in many fields such as energy storage devices,sound absorption and heat insulation,and environmental remediation.Nevertheless,poor mechanical properties,expensive modification and complex assembly processes are still the main reasons that limit the large-scale application of woody aerogels.The development of cost-effective and environmentally friendly functional aerogel preparation technology and improving the mechanical strength of woody aerogel materials are of great important.Based on the principle of "learning from nature",the work of this thesis performs in-depth study of mussel adhesion characteristics and natural biological structure of wood materials,and builds a variety of biomimetic woody aerogels used for oil-water separation,oily liquids recovery and dye adsorption.The construction mechanism of biomimetic aerogel materials and their application prospects in the field of water purification are deeply discussed.The research content of the thesis revolves around the following parts:(1)Cellulose nanofibers(CNF)were prepared by chemical purification combined with high-frequency ultrasound from pine needles.A polydopamine(PDA)coating was constructed on the surface of CNF using the mussel adhesion protein analogue-dopamine(DA)as the precursor.Taking the advantage of the reaction activity of PDA towards sulfhydryl organics,1dodecanethiol(DDT)molecules are induced to prepare DDT-PDA@CNF bionic functional interface.The corresponding composite aerogel material was prepared by freeze-drying.Scanning electron microscope(SEM)was used to investigate the influence of different precursor concentrations on the morphology of PDA biomimetic coating;X-ray photoelectron spectroscopy(XPS)and Fourier transform infrared spectroscopy(FTIR)were used to investigate the construction mechanism of the biomimetic composite interface;The water contact angle(WCA)tests revealed the influence of DA concentration on the wettability of aerogels.The results show that when the DA concentration is 0.5 mg/mL,a uniform PDA coating can be formed on the surface of CNF.At this concentration,the DDT-PDA@CNF composite aerogel is with maximum WCA of 149°.PDA is first attached to the CNF surface by hydrogen bonding,and then DDT molecules are grafted by Michael addition reaction with PDA.The DDT-PDA@CNF aerogel can adsorb silicone oil,lubricating oil,cyclohexane and other oily liquids,with a maximum adsorption capacity of 37.6 g/g(2)The bionic functional interface of octadecylamine(ODA)-PDA@CNF is constructed using the reactivity of PDA to amino organic compounds.The ODA-PDA@CNF aerogel is prepared by tert-butanol substitution and freeze-drying.The structure and properties of the biomimetic aerogels were studied by SEM,FTIR,XPS,specific surface area and pore size analysis.The construction mechanism of the biomimetic functional interface was deeply discussed,and the oil-water separation performance and oily liquid adsorption capacity of ODA-PDA@CNF aerogel was fully investigated.The results show PDA is attached to the CNF surface of through hydrogen bonds,then it works as the bridge to graft ODA molecules to the surface of the composite framework through Schiff base reaction.The PDA coating has orientation effect on ODA orderly growth;thus,the porous 3D network structure of the aerogel is well retained.The specific surface area of ODA-PDA@CNF aerogel is 93.08 m2/g,and the pore volume is 0.37 cm3/g.ODA-PDA@CNF aerogel can efficiently adsorb oily liquids such as lubricating oil,chloroform,and toluene,with a mass adsorption capacity up to 176 g/g and a volume adsorption capacity up to 123.8 mL/g.(3)There are many similarities between the chemical structure of lignin molecules and PDA,both contain a variety of functionalized aromatic structures.The side chains on lignin include alcoholic hydroxyl,phenolic hydroxyl,methoxy and other rich oxygen-containing groups,which can be as an active adsorption site for cationic dyes.Using fast-growing woodfir shavings as raw materials,combines TEMPO pretreatment and high-frequency ultrasonic technology,the lignocellulose nanofibers(LCNF)with a diameter of 30-90 nm and high aspect ratio of 13000-40000 was separated.Subsequently,sodium alginate and calcium chloride were added to the LCNF for network cross-linking,followed by ice template,low surface energy solvent replacement and air drying,the anti-water dispersed LCNF aerogel for methyl blue and crystal violet dye adsorption is obtained.X-ray diffraction(XRD),FTIR,XPS,BET specific surface area analysis and other techniques have been used to study the basic structure and properties of LCNF and its aerogel.The results show that LCNF has a typical cellulose I?structure,and the surface of LCNF is rich in lignin which accounts for about 25 wt%of the total mass.LCNF aerogel has a typical mesoporous structure with a specific surface area of 76.189 m2/g.LCNF aerogel has good water dispersing resistance.The lignin on its surface is used as a natural bionic coating,which can absorb more than 98%of methyl blue and crystal violet dyes in 60 min.(4)Learn from the "self-cleaning" structure of the lotus leaf surfaces and dig deeper into the natural biological structure of balsa wood:The cell wall of balsa contains cellulose fibrils arranged regularly and oriented,besides one of the main components of the cell walllignin has hydrophobic effect.In addition,wood ray is an important organizational structure that gives wood mechanical self-adaptive elasticity.Opening the wood ray will greatly improve the deformability of wood.Based on the above two points of thinking,a combination of chemical(KOH,NaClO2)-thermal(230?)treatment is used to remove part of the matrix in wood,open the solid structure of wood rays,and obtain an elastic wood aerogel(Woodelt-AG)with self-hydrophobic and self-photothermal functions.Using characterization techniques such as SEM,confocal Raman microscopy(CRM),FTIR,XPS,functional density theory and finite element analysis to reveals the effect of the structure and composition evolution on the new performance formation.The results show that:chemical treatment exposes regularly arranged cellulose fibrils,forming a nano-rough structure like the surface of lotus leaves;lowtemperature heat treatment can degrade hydrophilic polysaccharides,remove the hydrophilic side chains of lignin,and expose the aromatic ring skeleton to form a natural hydrophobic coating(WCA=152°).Chemical-heat treatment collapses the solid structure of wood rays,shaping the "leaf-spring" structure,which gives wood aerogel excellent loading capacity and elasticity,which makes the Woodelt-AG maintain the original height as high as 94.7%after 1000 times of compression-release cycles with strains up to 60%.Finally,the performance of Woodelt-AG as a multifunctional adsorption material in the fields of photothermal assisted crude oil recovery,oil-water separation and emulsion separation was investigated.Woodelt-AG can achieve continuous adsorption of chloroform,toluene with high adsorption flux of 34.7 L·h-1·g-1;Woodelt-AG can heat up to 83 ? within 140 s under sunlight(1000 W/m2),the adsorption of crude oil is saturated within 160 s;Woodelt-AG can also efficiently separate water-in-oil emulsions with a removal efficiency of over 98.4%.