Preparation Of Charged Composite Membranes And Their Salt/Water Permeation Behavior

As a novel separation technology, membrane technology with advanges of efficient, energy conservation and cleaning, has attracted more and more attentions. It has been widely used in desalination, wastewater treatment, food manufacturing, pharmaceutical industry and so on. Charged membranes are usually positively charged or negatively charged which have been widely studied for their preparation, salt/water permeation separation and mass transport mechanism. The research findings would provide theoretical evidence for membrane separation application in desalination fields.In this study, ionic liquid block copolymer PMMA-b-PMEBIm-Br as a typical positively charged material, was used to prepare the dense films. Functionalized MWCNTs and GO were also used as the modifiers to prepare the composite films. The structure and the salt/water permeation property of charged composite films were studied in detail. In addition, graphene oxide modified by chitosan was used to improve hydrophilicity and charged properties of poly(ether sulfone) ultrafiltration membranes. GO-CS/PES ultrafiltration membranes were used as substrates to fabricate forward osmosis (FO) composite membranes by interfacial polymerization method. During this process, the effect of GO-CS on salt/water separation property of forward osmosis composite membrane was also studied. The main contents and results were summarized as follows:1. Preparation of ionic liquid block copolymer films and their salt/ water permeation separation propertyIonic liquid block copolymers PMMA-b-PMEBIm-Br with three different lengths of PMEBIm-Br block were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization method. PMMA-b-PMEBIm-Br dense films were prepared by solvent evaporation method. Structure and molecular weights of PMMA-b-PMEBIm-Br were characterized by FTIR,1H-NMR, GPC. Results indicated that PMMA-b-PMEBIm-Br was synthesized successfully and molecular weights increased as the PMEBIm-Br chain length increased. Measurement results about morphology, hydrophilicity and charged properties of dense films indicated that PMMA-b-PMEBIm-Br films were dense, hydrophilic and positively charged. The effect of PMEBIm-Br block length on salt/water permeation property was also studied. With the increasing PMEBIm-Br block length in PMMA-b-PMEBIm-Br block copolymer, the 0.1 mol/L of NaCl permeability of the films increased from 2.74×10-8 cm2/s to 1.94×10-7 cm2/s and water fluxes increased from 1.25 L/(m2·h) to 4.2 L/(m2·h). Salt flux for 0.03 mol/L of MgCl2 increased from 220 mL/(m2·h) to 760 mL/(m2·h) but the rejection decreased from 97% to 65%.2. Preparation of charged composite films modified by PMMA-MWCNTs and their salt/water permeation separation propertyPMMA was grafted onto carbon nanotubes by microemulsion polymerization. PMMA-MWCNTs was used as the modifier to prepare the charged composite films by solvent evaporation method and the salt/water permeation property of composite films was also studied. Structure and morphology of PMMA-MWCNTs were characterized by FTIR, TGA, XRD, Raman and TEM. The results indicated that PMMA was grafted onto carbon nanotubes successfully. Measurement results of composite films indicated that functionalized carbon nanotubes were dispersed well in composite films and water uptake decreased from 63% to 48.5% with the increasing PMMA-MWCNTs concentrations. In the salt/water permeation study, the water fluxes of composite films increased from 2.75 L/(m2·h) to 11.09 L/(m2·h) compared with pure charged films, salt flux decreased and rejection increased. But effect of carbon nanotubes on salt permeability was unobvious which illustrated that nano-channels for water molecules have constructed in composite films resulting from functionalized carbon nanotubes.3. Preparation of charged composite films modified by GO-CS and their salt/water permeation separation propertyChitosan (CS) was used to modify graphene oxide (GO), the obtained GO-CS was used to prepare the charged composite films by solvent evaporation method. FTIR, XRD, Raman and TEM were used to characterize the structure and morphology of GO-CS, the results indicated that CS was grafted onto GO. Measurement results of composite films indicated that GO-CS were dispersed well in composite films and water uptake decreased from 63% to 47.7% with the increasing GO-CS additions. Results of salt/water permeation study indicated that water fluxes of composite films increased from 2.75 L/(m2·h) to 18.76 L/(m2·h) with the increasing GO-CS additions. NaCl permeability and salt flux decreased while salt rejection increased. Compared with the PMMA-MWCNT/PMMA-b-PMEBIm-Br2 charged composite films with the same additive concentration, GO-CS/PMMA-b-PMEBIm-Br charged composite films had higher rejection, lower NaCl permeability and salt flux.4. Preparation of composite forward osmosis membranes and their salt/water separation propertyGO-CS was used to improve hydrophilicity of PES ultrafiltration membrane. GO-CS/PES ultrafiltration membranes were used as substrates and forward osmosis membranes with ultra-thin composite layer (TFC-FO) were prepared by interfacial polymerization. Water fluxes and rejection performances of GO-CS/PES ultrafiltration membranes were measured. Results indicated that hydrophilicity and rejection of PES substrates was improved with the addition of GO-CS. Water fluxes of substrates increased from 214 L/(m2·h) to 333 L/(m2·h) and BSA rejection increased from 65.8% to 99.5%.The water flux and reverse salt flux of forward osmosis composite membranes were also investigated. The results indicated that water flux was improved from 7.33 L/(m2·h) to 20.53 L/(m2·h) with the modified GO-CS/PES substrate. Reverse salt flux was the highest when the addition of GO-CS was 0.8wt%, then decreased when GO-CS was lwt%. When 1 mol/L of MgCl2 solution was used as draw solution, the reverse salt flux increased from 2.49 g/(m2·h) to 4.21 g/(m2·h), then decreased to 1.54 g/(m2·h) with the further addition of GO-CS.