Construction Of Nanocellulose Filter Membranes And Their Separation Performance

Rapid development of industry and the dramatic increase in population have brought burdens to global energy consumption,various environmental pollutions are emerging one after another,and freshwater resources around the world are also threatened.Due to the high efficiency,low cost,and easy operation of the membrane separation technology,there has great potential to develop an innovative separation membrane for the purification of different pollutants from wastewater.Nanocellulose is one of the most abundant renewable resources which generally divided into cellulose nanocrystals（CNC）and cellulose nanofibers（CNF）.The superwettability,high aspect ratio,high mechanical strength,environmental stability and abundant hydrophilic oxygen-containing functional groups of CNC and CNF materials have made them widely used in construction of nanocomposites and separation membranes.Tunicate cellulose nanocrystals（TCNC）hydrolyzed from sulfuric acid and2,2,6,6-tetramethyl-1-piperidinyloxy（TEMPO）oxidized pine cellulose nanofibers（TOCN）were selected as the base materials.For different functional requirements,the chemical composition of the nanocomposite membrane was designed and adjusted.The low-cost,degradable,high-efficiency nanocellulose membranes used for the purification of various water pollutants were prepared by the vacuum assisted filtration.In this thesis,we constructed three kinds of nanocellulose-based membranes and further investigated their separation performance.We also explored the potential applications of nanocelluloses in the field of wastewater purification.The specific research contents are as follows:Commercial filter papers were modified by physical and chemical methods with TCNC,respectively.During physical modification,TCNC fixed on the surface of the filter paper by the formation of ether bonds and hydrogen bonds.During chemical modification,epichlorohydrin（ECH）was used as a cross-linking agent under alkaline conditions to form ether bonds between TCNC and the hydroxyl groups of the filter paper.Both of these two kinds of TCNC modified filter papers presented nanoporous morphology and superhydrophilic/underwater superoleophobic properties.The obtained modified filter paper could separate various oil/water mixtures and emulsions efficiently,and the separation efficiency was higher than 99%.In addition,the modified filter paper also showed excellent reusability,good chemical resistance and outstanding mechanical stability.The positively charged lysozyme oligomers were bound to the negatively charged TOCNs（LYS/TOCN）through electrostatic interaction to prepare a self-supporting nanocomposite separation membrane with a nanoporous structure.Among them,the lysozyme oligomers in the LYS/TOCN membranes not only acted as an adhesive to make the TOCN more tightly,but also provided-OH,-COOH,-SH and-NH₂ groups to increase the functionality for removing different water pollutants of the LYS/TOCN membranes.It was worth noting that the resulting LYS/TOCN membrane could remove molecules>3 nm efficiently,such as dyes,bilirubin and oil droplets.In addition,it could also remove heavy metal ions and boric acid pollutants.Dopamine was self-polymerized into polydopamine（PDA）under alkaline conditions,and bonded to TOCN through hydrogen bond（PDA@TOCN）to prepare a p H-responsive composite membrane.Under alkaline conditions,the phenolic hydroxyl groups on the surface of PDA@TOCN membranes were deprotonated and then negatively charged which have strong electrostatic adsorption to the cationic dyes,such as rhodamine B and methylene blue,the rejections of them were up to 98%and 99%,respectively.Under acidic conditions,the secondary amine groups on the surface of PDA@TOCN membranes were protonated and then positively charged which also have strong electrostatic adsorption to the anionic dyes,such as methyl orange and methyl blue,the rejections of them were up to 94.6%and 99.4%,respectively.In addition,the PDA@TOCN membranes could also remove heavy metal ions under alkaline conditions efficiently.