Design,Preparation,and Separation Performance Of Multicomponent Composite Nanofiltration Membranes For Dye Separation In Aqueous And Nonaqueous Solvents

Nanofiltration?NF?is a new membrane separation technique that ranged between reverse osmosis?RO?and ultrafiltration?UF?.It possesses the following distinguished properties:high selectivity,low operating pressure,low investment,and low energy consumption.Hence,NF membranes have been widely applied in aqueous and nonaqueous systems in the fields of biomedicine industry,fine chemical industry,petrochemical industry,food industry,papermaking,and other industries.However,the traditional NF membranes are usually prepared by the interfacial polymerization,which consumes a lot of organic solvents in the preparation process,and leads to serious environmental pollution problems.The substrates used for the composite NF membranes are usually ultrafiltration membranes with high cost.In addition,industrial NF membranes used in aqueous systems are prone to swell in organic solvents,causing defects between the substrate and the selective layer,leading to a sharp decline in selectivity.In order to solve these problems,two kinds of multicomponent composite NF membranes have been developed for the study of dye separation in the aqueous solvent system and organic solvent system respectively based on the goals of providing green NF membrane in aqueous solvent system and low-cost organic solvent resistant NF membrane in organic solvent system.The main research contents and the main conclusions obtained from the characterization of composite membrane materials and evaluation of dye separation performance are as follows:?1?In this study,the green material chitosan?CS?was used as the active layer material,and we have developed a novel free radical grafting copolymerization method to prepare gallic acid-grafted-chitosan/polysulfone composite nanofiltration membranes for the removal of dyes with different molecular weights from aqueous solution.Firstly,the eco-friendly gallic acid was grafted onto chitosan through a free radical grafting copolymerization reaction.Secondly,the gallic acid-grafted-chitosan conjugates were co-deposited onto the surface of polysulfone ultrafiltration substrates by electrostatic interactions.The as-prepared composite nanofiltration membranes were characterized by various techniques,including FTIR,XPS,SEM,AFM,and WCA.Moreover,the effects of various preparation conditions on the separation performance were investigated,including the concentrations of chitosan and gallic acid,the pH of the gallic acid-grafted-chitosan mixed solution,the deposition time and heating time.The results showed that the membranes exhibited both a high rejection rate of 97.2%for congo red and high permeance of 14.0 L h^-1 m^-2 bar^-1.In addition,the composite nanofiltration membranes also have good rejection for other dyes such as evans blue,acid red,and alcian blue,which shows promising potential in wastewater treatment.?2?On the other hand,a commercial PVDF microfiltration membrane was used as the substrate for composite NF membranes,and a novel acyl chloride monomer trimellitic anhydride chloride?TAC?was blended with trimesoyl chloride?TMC?as mixed organic phase monomers to fabricate high-flux nanofiltration membranes for the removal of dyes with different molecular weights from the organic solvent system.Due to the reaction activity differences of organic phase monomers?TMC and TAC?with m-phenylene diamine?MPD?,the chemical compositions and structures of polyamide active layer could be tailored through varying the TAC content.The as-prepared composite nanofiltration membranes were then characterized by various techniques.Moreover,the effects of various preparation conditions on the separation performance were investigated,including the concentrations of TAC,the heating temperature and heating time.As a result,when the TAC concentration was 0.05wt%,the heating temperature was 60?,and the heating time was 5 min,the optimized membrane exhibited a high permeability of 5.0?L h^-1 m^-2 bar^-1,up to 10-fold,in comparison with the conventional membrane prepared by using single TMC as organic monomer,which also exhibited good separation performance towards other dyes,such as methylene blue,congo red,brilliant blue,and acid red.This work provided a novel and facile strategy to develop high-flux nanofiltration membrane based on commercial microfiltration membranes.