Investigation On The Preparation, Characterization And Transport Mechanism Of The Novel Composite Charged NF/Mosaic Membranes

A charged mosaic membrane consists of a set of anion and cation exchange elements arranged in parallel, each element providing a continuous pathway from one bathing solution to the other. When the electrolytes pass the membrane, anions and cations can flow in parallel through their respective exchange elements. These properties, being permeable to salts but meanwhile not to low-molecular-weight non-electrolytes, are desired for desalination of water or purification of biochemical materials or food additives.It should be noted that the mosaic membranes have not yet become commercially important in separation processes because of the difficulty in up-scaling a reproducible process for making mosaics. Most of the preparation processes are quite complicated and time consuming. Therefore, the new methods for preparing charged mosaic membranes are still worth studying and developing.The objective of this study is to develop a novel technique for preparing asymmetric mosaic membrane by introducing both the negative charged and the positive charged groups into the selective layer of interfacial polymerization (IP). The research emphases focus on the IP mechanism, the relation of membranes performance-structure-IP conditions and the transport mechanism model. This will provide the foundation for the preparation, design and analysis of the membranes. In this paper, both theoretical and experimental researches are carried out as follows:1. The preparation of support membraneFor the system of PSF/NMP/PVP/CH3COCH3/H2O, the polysulfone (PS) UF has been prepared, whose molecular weight cut-off is 80000 and its flux of pure water acrossmembrane is about 250-340 Lm'^h"1. This result obtained through experiment is to provide the good foundation for preparing composite membranes.2. The preparation of a novel positively charge NF membraneA novel positively charge nanofiltration membrane was firstly prepared by interfacial polymerization. The polysulfone (PS) support membranes was orderly immersed in an aqueous solution of polyethylenimine (PEI) and a hexane solution of trimesoyl choride (TMC), then the coated substrate was heat-cured in an air-circulating oven. The model equation has been established based on the uniform design and it is in good agreement with experimental data. The separation of NaCl/PEG400/water mixtures was then investigated. The results show that under the condition of 0.4MPa and 25 °C, the separation factor of membranes for NaCl-PEG400 is greater than 10. Since the MgC^ retention of membrane is more than 92%, the membrane could be used to separate NaCl and MgC^ in halogen.3. The preparation of charge-mosaic membraneThe composite charge-mosaic membrane(CMM) was prepared by IP. The 2,5-diaminobenzene sulphonic acid, polyethylenimine and trimesoyl chloride were selected as the monomers of IP reaction. The 4-(chloromethyl) benzoyl chloride and trimethyl amine were chosen as the chemical modification agents. The prepared membrane was characterized through the separation experiment for salts-water and organics-water systems. The results show that under 0.4MPa and 25 °C, the separation factor of membrane for NaCl-Xylend Orange is greater than 20, while for Na2SO4-Sucrous, greater than 9. The CMM prepared by IP have both the good selectivity (higher separation factors) and the higher flux compared with the CMM prepared by other methods.4. Mathematical model for fabrication of the composite membranes by IPBased on the theories of polymer chemistry, mass transfer and the polymer phase separation, a theoretical model for describing the fabrication of composite membranes by interfacial polymerization has been established in this paper. The model can be used to describe the process of interfacial polymerization controlled by both diffusion and reactionunder non-steady-state conditions. The model was firstly simplified by introducing some dimensionless parameters that have definite physical meanings, and then solved to obtain the analytical solutions as the function of reaction time during interfacial polymerization. The concentration distribution of monomer A, the thickness of selective film and the growth rate of the thin film, can be easily obtained by model calculation based on the parameters such as reaction rate constant, the diffusion coefficient, the initial concentration of monomer, etc. The model is in good agreement with experimental data, and provides an important guidance for fabricating the composite membranes by interfacial polymerization.5. Transport mechanism model of electrolytes through the CMMBased on the basic principle of the space-charge model and the steric-hindrance pore model and considered the transport characteristics of electrolytes through the CMM, a theoretical model has been developed to predict the transport performance of electrolytes through the CMM. The model is practical for estimating the charged pore structural parameters (the pore radius, the ratio of the membrane porosity to the effective thickness membrane and the product of the effective fixed charge density and the fraction of active membrane area occupied by the type of ion) of CMM with the permeation experiments of aqueous solutions of single electrolytes. Whereas, based on their charged pore structural parameters, the model can be useful for predicting the transport performance of electrolytes through the CMM. The model prediction is in good agreement with experimental results. The model is suitable usable for the multi- electrolytes in principle.