Preparation And Properties Of Positive Charge Enhanced Thin-film Nanocomposite Nanofiltration Membranes

The charge property of the nanofiltration membrane has an important effect on the membrane separation performance.There are a lot of heavy metal ions with positive charge in industrial wastewater.At the same time,the tradional thin-film composite(TFC)nanofiltration membrane always has a "Trade-off" effect between the membrane flux and the interception rate,and its anti-pollution and chlorine-resitant properties also need to be improved.Based on the interfacial polymerization method,two different methods were adopted in this paper:(1)Small molecule binary amine piperazine was used as the aqueous phase monomer,and amino-modified graphene oxide(AGO)was used as the nano modifier;(2)Thin-film nanocomposite(TFN)nanofiltration membranes with high performance were prepared by using PEI as aqueous monomer and GO as nano modifier.The specific research contents and results of this paper are as follows:(1)GO was prepared by Hummers method and ethylenediamine(EDA)and polyethyleneimine(PEI)were amino-modified to GO.Two AGO nanomaterials were successfully prepared:GO-EDA and GO-PEI.By interfacial polymerization method,with polysulfone ultrafiltration membrane as the support,PIP as the aqueous monomer,TMC as the organic monomer,TEA as acid absorbent,we explored the influence of different concentrations of GO-EDA and GO-PEI to the structure and performance of composite nanofiltration membranes.The results showed that with the increase of AGO concentration in the aqueous phase,the diffusion rate of PIP monomer and the process of interfacial polymerization were hindered due to steric hindrance effect and hydrogen bond interaction,resulting in the decrease of both the thickness of active layer and the roughness of membrane surface of TFN-PIP nanofiltration membrane.At the same time,AGO was embedded into the polyamide active layer in the form of chemical bond,which can effectively enhance the positive charge,hydrophilicity,chlorine resistance and mechanical properties.The optimal addition concentrations of GO-EDA and GO-PEI in the aqueous phase system were 60 ppm and 40 ppm respectively.Without reducing the sodium sulfate rejection,the water fluxes of the TFN-PIP-GO-EDA-60 and TFN-PIP-GO-PEI-40 nanofiltration membranes were up to 59.6 L·m-2·h-1 and 62.1 L·m-2·h-1.Compared with the TFC-PIP-blank,the improvement rates were 54.4%and 60.9%respectively.More interestingly,TFN-PIP nanofiltration membranes showed different separation performance under different pH conditions due to the introduction of positively charged AGO in the active layer.When pH = 7,the order of TFN-PIP nanofiltration membrane's rejections of the four salts was Na2SO4>MgSO4>MgCl2>NaCl.When pH = 4,the order changed to Na2SI4>MgCl2>MgSI4>NaCl.However,the order of salt rejection of the TFC-PIP-blank without AGO was not affected by pH.Similarly,TFN-PIP nanofiltration membranes showed better anti-fouling performance under the conditions of pH = 3 and pH = 9 due to the positive charge characteristics of the membrane surface.(2)Using the membrane polysulfone ultrafiltration membrane as the base,PEI as aqueous monomer,TMC as organic monomer,TEA as acid absorbent,we prepared positive charge-enhanced composite nanofiltration membranes and explored the influence of different concentrations of GO to structure and performance of composite nanofiltration membranes.Compared with PIP,the composite nanofiltration membrane prepared with PEI as the aqueous monomer had larger thickness,lower roughness and stronger positive charge properties.Due to the steric hindrance effect of GO and the interaction of hydrogen bond,the diffusion rate of PEI monomer was hindered,resulting in the reduction of the thickness of the active layer.At the same time,GO was implanted in the active layer of polyamide,effectively enhancing the hydrophilic and chlorine-resistant performance of TFN-PEI nanofiltration membrane The optimal concentration of GO in the aqueous phase system was 20 ppm,and the water flux of TFN-PEI-GO-20 nanofiltration membrane was as high as 70.3 L·m-2·h-1,which was 36.8%higher than that of TFN-PEI-blank,and the rejection did not decrease significantly.Tfn-PEI nanofiltration membrane showed significant characteristics of positively charged nanofiltration membrane.At pH = 7,the rejections of four salts were in the order of M9Cl2>MgSO4>Na2SO4>NaCl,and at pH = 4,in the order changed to MgCl2>MgSO4?NaCl>Na2SO4.Tfn-PEI nanofiltration membrane has higher rejections of heavy metal ions,and the rejection of divalent anionic heavy metal salts is slightly higher than that of monovalent anionic heavy metal salts.Under neutral conditions,TFN-PEI nanofiltration membrane with positive charge has good anti-fouling performance against cationic surfactant and alkaline protein.