The Preparation And Characterization Of A Novel Positively Charged Composite Nanofiltration Membrane

Nanofiltration （NF） membranes have separation characteristics in theintermediate range between ultrafiltration （UF） and reverse osmosis（RO） membranes.As a pressure-driven process, NF is gaining increasing worldwide research interestbecause of its advantages such as low energy consumptions and selective permeation.NF process is now extremely attractive for water softening, drinking waterpurification, as well as wastewater reclamation. Charged membranes have been paidmore and more attention because most of the commercialization is non-charged NFmembrane currently. Chitosan is a natural polymer with good membrane-formingcharacteristic containing plenty of active groups such as hydroxyl and amino, whichmakes it relatively easy to modify this polymer.In this research, quaternization is employed to modify chitosan which meanstrimethylated chitosan chloride（TMC）. A novel positively charged compositenanofiltration （NF） membrane has been prepared by spin coating with trimethylatedchitosan chloride as active layer on polysulfone ultrafiltration membrane, and thencross-linking with epichlorohydrin/ethanol solution, which is called positivelycharged TMC/PSF composite NF membrane.A comprehensive study of the preparation process about the positively chargedcomposite NF membrane have been done, included the crosslinking temperature, theconcentration of the casting solution, crosslinking time, crosslinking agentconcentration, solvent evaporation time, and thermal drying time of the positivelycharged TMC/PSF composite NF membrane. Some characters of the composite NFmembrane were evaluated through IR、SEM and AFM. Influence of the operatingconditions have been investigated, included operating pressure、 different feedsolution、feed solution concentration、feed rate、feed temperature, ect. The results indicated that the composite NF membrane by2.0wt.%trimethylatedchitosan chloride as coating solution,0.5vt.%epichlorohydrin/ethanol solution ascrosslinking agent, cross-linking at55℃for2.5h, solvent evaporation time is1h, andthermal drying time is20min has the optimal performance. The results of IRspectroscopy indicated the crosslinking reaction between ECH and trimethylatedchitosan has been occur, and the ether bond has been generated; Through SEM wecan found that the ultra-thin active layer has been covered on the PSF ultrafiltrationsupport layer clearly; The results of AFM indicated that the roughness of thecomposite NF membrane surface is about4nm.The most optimal developedcomposite NF membrane shows a molecular weight cut-off of1200Da at5bar,andthe pure water permeability coefficient is14.09L·h^-1·m^-2·MPa^-1. The composite NFmembrane also has excellent solvent resistance and stability. Similar to other NFmembranes, membrane flux increase, and the membrane retention rate declinedslightly as the operating pressure increases; The rejection rate of the membranedecreased and membrane flux decline with the increasing of the feed concentration;When liquid flow rate is than10L/h greater, the influence of liquid flow rate to themembrane rejection rate is slightly, but the flux was reduced; Membrane rejection isreduced with the increasing of feed temperature, and the flux was increased straightly.The order of rejection to different inorganic electrolytes decreased as CaCl₂, MgCl₂,NaCl, MgSO₄, Na₂SO₄, and K₂SO₄, which reveals the characteristics of the positivelycharged composite NF membrane.This test proposed a composite NF membrane by using the trimethyl chitosan asthe membrane casting solution firstly, which is prepared by coating crosslinking. Themethod short the crosslinking time greatly, and the optimal crosslinking time is only2.5h, which provides a theoretical basis for commercialization development.