Surface Charge Regulation Of Thin-film Composite Nanofiltration Membranes And Study Of Performance

With the rapid growth of the global population and the further development of industrialization around the world,problems such as scarcity of freshwater resources and water pollution have become increasingly prominent.Therefore,improving the supply of freshwater resources and mitigation of water pollution has become a major challenge facing the world.Nanofiltration separation technology stands out in the applications of seawater desalination,food processing,printing,dyeing and textiles benefiting from its advantages of high separation efficiency,low energy consumption,and environmental protection.The thin film composite nanofiltration membrane（TFC NFM）prepared by the interfacial polymerization method is currently the most researched and most stable separation membrane in the field of water treatment.At present,the commercial membrane composite nanofiltration membrane mainly uses piperazine（PIP）as the water phase monomer to react with the oil phase trimethyl chloride（TMC）.Although the separation performance of the nanofiltration membrane prepared by PIP is outstanding,the formed membrane is dense and has relatively poor permeability due to its high reactivity and violent reaction.Moreover,it is difficult to uniformly regulate the surface charge.In recent years,to overcome these problems,grafting,cross-linking,coating and so on are often utilized to modulate the charge of nanofiltration membrane.However,these methods have some problems such as difficult adjustment in pore size,low load and poor stability,which confine the employment of nanofiltration membranes in heavy metals separation and other fields.In response to these problems,this article will set out the molecular structure design,serials aqueous monomers are adopted to manufacture different functional separation membranes.Meanwhile,high performance nanofiltration membranes carrying with diverse charges were produced via surfactant-assembly regulated interfacial polymerization,and properties for separating heavy metal ions and desalting were researched.The specific research contents are as follows:（1）Effect of monomer structure on the chargeability and desalination performance of polyamide nanofiltration membranes.Series of polyamide nanofiltration membranes were obtained by the reaction of piperazine（PIP）,2-methylpiperazine（MPIP）and 2-ethylpiperazine（EPIP）with（TMC）on the basis of hydrophilic poly（ether sulfone）（PES）ultrafiltration membrane.We explored the influence of the structure and concentration of the water phase monomer on the structure and performance of the nanofiltration membrane.The chargeability of the membrane converted from negative to positive by tuning the structure and consistence of monomer.On account of the moderate activity of MPIP and EPIP,a looser active layer with higher permeability was constructed after reacting with TMC compared with piperazine polyamide nanofiltration membrane.Meanwhile,it maintains good rejection to divalent cations ascribing to the synergistic effect of size screening and Donnan effect.（2）Surfactant-assembly regulated interfacial polymerization to preparate highly charged nanofiltration membrane showing superior performance.The surface chargeability of the membrane prepared by MPIP is different with the change of conditions,but because of its slightly low reactivity,the formed film is prone to defects,the retention of inorganic salts still needs to be improved.For this reason,we apply the self-assembly behavior of surfactants to induce the diffusion and pre-enrichment of monomers at the water/oil interface to stimulate the cross-interface transport of the monomers,enhance the degree of interfacial polymerization,realize the regulation of membrane structure and promote the desalination performance of nanofiltration membranes.Two kinds of highly charged nanofiltration membranes were gained by adopting MPIP and TMC as reaction monomers,adding sodium dodecyl sulfonate（SDS）in water phase or sodium sulfonate dioctyl succinate（AOT）into oil phase individually.Both SDS and AOT can facilitate the diffusion of monomers in the bulk phase,but the difference in molecular structure causes differences in the arrangement and diffusion behavior of the monomers at the interface,resulting in different polyamide layer structures.The nanofiltration membrane constructed with SDS as the water-phase additive has a dense structure and stronger positive charge,the retention of Mg Cl₂ and Mg SO₄ are 98.8%and 97.7%,respectively;meanwhile,it shows excellent separation performance for heavy metal ions.For example,the retention of Ni Cl₂ and Co Cl₂ is about 98.0%,and it maintains excellent separation stability for heavy metal salts under long-term test conditions.However,the nanofiltration membrane prepared with AOT as the oil phase additive has a denser structure and smaller molecular weight cut-off,and the membrane surface is negatively charged.The retention of Na₂SO₄ is up to 98.4%,and the interception of Mg SO₄ and Mg Cl₂ is also above 96.0%.