Preparetion And Performance Study Of High-efficiency Polyamide Composite Reverse Osmosis Membrane For Desalination

Reverse osmosis technology is currently one of the most effective ways to solve the crisis of insufficient freshwater resources through seawater desalination.Reverse osmosis membrane,as the core of reverse osmosis technology,has been comprehensively and thoroughly studied.Typically,polysulfone membrane was used as a substrate,an solution of m-phenylenediamine(MPD)in water was employed as the aqueous phase,and a n-hexane solution of trimesoyl chloride was used as the organic phase,and the reverse osmosis(RO)membrane with the cross-linked polyamide barrier layer was then fabricated via interfacial polymerization.The RO membranes exhibit high permeation flux and rejection ratio for brackish water and seawater desalination,and therefore have been successfully developed in desalination industry of seawater.However,the improvement of the desalination efficiency of reverse osmosis membranes has always been the goal pursued by industrial development.This study intends to tune the intermolecular free volume by changing the molecular structure of the cross-linked polyamide in the reverse osmosis membrane,reduce the mass transfer resistance of water molecules,thereby increasing the water flux of the filter layer,and ultimately improving the desalination efficiency of the membrane,trying to establish the"structure-efficiency" relationship between molecular structure and desalination performance.First,three different phenylenediamines were selected as the monomers in the aqueous phase,and trimesoyl chloride was used in the organic phase to prepare the composite RO membrane by interfacial polymerization.The influence of the position of the amino group on the polyamide structure was studied.It was shown that the rejection ratios of reverse osmosis membranes prepared with m-phenylenediamine and p-phenylenediamine were greater than 98%against 2000 ppm sodium chloride,while that of the membrane based on o-phenylenediamine was only 45%.Furthermore,novel cross-linked polyamide composite RO membranes were fabricated by interfacial polymerization with methyl-substituted monomers include 2,3-diaminotoluene,2,4-diaminotoluene,2,5-diaminotoluene and trimesoyl chloride to explore the effect of the relative position of methyl and amino groups on the desalination efficiency.Scanning electron microscopy(SEM)was used to observe the surface and cross-sectional morphology of the six polyamide composite RO membranes obtained,and atomic force microscope(AFM)scanning results were used to analyze the surface roughness of the reverse osmosis membrane.Consequently,it was found that the surface morphology of the six membranes varied greatly with the different diamine monomers,and the surface roughness of the polyamide RO membranes prepared with methyl substituted monomers decreased.Meanwhile,the RO membranes based on methyl-substituted diamine monomers exhibited higher permeation flux in the desalination performance than that of the RO membranes based on the monomers without methyl substitution.Therefore,the composite polyamide reverse osmosis membrane with higher permeation flux was fabricated by interfacial polymerization with mixing monomers such as o-phenylenediamine with 2,3-diaminotoluene,m-phenylenediamine with 2,6-diaminotoluene,p-phenylenediamine with 2,5-diaminotoluene,2,3-diaminotoluene with 2,4-diaminotoluene in different concentrations and proportions.The influence of the relative position and concentration of amino and methyl groups located on the monomers was investigated,in term of the formation of free volume for water transportation in the polyamide barrier layer.Meanwhile,the six diamine monomers have different molecular potential energy,according to the results of molecular simulation,and the corresponding composite membrane prepared by monomers with lower potential energy shows high water permeability in desalination of brackish water.