Study On The Preparation And Performance Of Polyvinylidene Fluoride Thin-film Composite Nanofiltration Membrane

Nanofiltration（NF）is an energy-efficient membrane separation technology that can reject small molecules of organic matter and divalent and multivalent ions with lower operating pressures and higher permeability than reverse osmosis.Thin-film composite（TFC）membranes are currently the most predominant commercial nanofiltration membranes and are often prepared by interfacial polymerization（IP）.In this paper,polyvinylidene fluoride（PVDF）TFC membranes were prepared by in situ IP.Unlike the traditional IP,which requires the immersion of aqueous and organic phase solutions,in this paper,piperazine（PIP）was added directly to the casting solution,the PIP enriched substrate was quickly obtained after phase conversion,and the substrates were reacted with the organic phase to produce the TFC membranes,simplifying the preparation procedure.The use of PVDF as materials of substrates also allows the TFC membranes to have better mechanical properties.To optimize the performance of the TFC membrane,the role of various influencing factors on the performance of the TFC membrane was investigated from the perspective of the substrate and the separation layer.The detailed work is as follows.（1）To investigate the effect of the polyethylene glycol（PEG）content added to the casting solution on the substrate.It was found that the casting solution was unstable when the PEG content was too high.In the range of 10-12 wt%,and as the content increased,it would increase the membrane pore size and reduce the mechanical properties.After preparing the TFC membrane,it was found that the best performance of the TFC membrane was achieved when the PEG content was 11 wt%.（2）The effect of adding PIP content to the casting solution and the precipitation time of the substrate in the coagulation bath was investigated.Both factors affect the concentration of PIP monomer on the surface of the substrate used for the IP.It was found that the outward diffusion of PIP in the casting solution was mainly influenced by the concentration gradient,the higher the PIP content,the faster the outward diffusion after immersion in the coagulation bath and the shorter the overall diffusion process,while the lower the concentration,the slower the PIP diffusion and the longer the overall diffusion process.In a shorter period of time（150 s）,the more PIP there is on the surface of the substrate with high PIP content in the casting solution;after a long period of time（360 s）,there is less PIP on the surface of the substrate with high PIP content in the casting solution,and the outward diffusion of PIP is coming to an end.By comparing the performance of the prepared TFC membranes,the best performance of the TFC membranes can be obtained with a PIP content of 4 wt%in the casting solution and a coagulation bath precipitation time of 150 s.（3）Optimize the preparation conditions of the separation layer and investigate the effects of the concentration of TMC solution involved in the IP and the temperature,reaction time,and post-treatment conditions on the performance of the TFC membranes.It was found that lower or higher TMC concentration would affect the salt rejection,and higher temperature and longer reaction time would reduce the permeability.High post-treatment temperatures can lead to a dense or even partial separation layer.By comparing the performance of the TFC membranes,the best performance separation layer was prepared using a 0.15 wt%TMC solution,reacted at a lower temperature of 0℃ for 30 s,and then left at room temperature for 5 min until the surface dried.The TFC membrane had a water permeability of 23.2 L/（m²·h·bar）,95.59%rejection of Na₂SO₄,and a permeability of 12.37 L/（m²·h·bar）.（4）The rejection of PVDF TFC membrane prepared by in situ IP in this paper is much higher than that of PVDF TFC membrane prepared by traditional IP.The elongation at break is much greater than that of the TFC membrane prepared by in situ IP using polyethersulfone as the material of substrates.At the same time,the TFC membranes in this paper exhibited good long-term stability,acid resistance,and chlorine resistance.