Study On The Influence Mechanism Of PVP On The Structure And Performance Of Polypiperazine Amide Nanofiltration Membranes

Against the backdrop of global freshwater scarcity and increasing demand for water treatment technology in people’s production and daily life,innovation in membrane separation technology and optimization of membrane materials are imperative.As one of the most common nanofiltration membranes,poly（piperazine amide）nanofiltration membranes are often prepared by interfacial polymerization.However,improving the high retention performance of nanofiltration membranes often comes at the cost of sacrificing permeability,which limits the further improvement of water production efficiency.Existing research has shown that water-soluble polymer and partially polar small molecule organic compounds can affect the diffusion rate of amine monomers,thereby affecting the interfacial polymerization process.Under certain conditions,polyamide membranes with high interception and permeability can be prepared,breaking the permeation interception balance and improving the water quality and efficiency of production.This study used water-soluble polymer polyvinylpyrrolidone（PVP）as a modifier to modify the polyamide functional layer and porous support layer using aqueous blending and surface impregnation methods,respectively.The effects of different modification positions,concentrations,and molecular weights of PVP on the morphology,structure,and performance of polypiperazine amide nanofiltration membranes were studied,and the optimal conditions for PVP modification were analyzed.In addition,a collaborative modification of the support layer with PVP and tannic acid（TA）was also attempted,providing a reference for further improving membrane performance.The main research conclusions of this project are as follows:（1）Use different concentrations of PVP-K13,PVP-K30,and PVP-K60 aqueous blends to modify the polyamide functional layer.After adding PVP to the aqueous solution,the diffusion rate of piperazine（PIP）slows down,and the surface of the polyamide functional layer changes from a point like structure to a strip like structure.At the same time,the thickness of the polyamide functional layer decreases,the roughness increases,and the hydrophilicity enhances,resulting in a significant increase in water flux.As the molecular weight of PVP increases,the critical concentration for point to strip morphology changes decreases,and the impact on the morphology,structure,and permeability of polyamide nanofiltration membranes becomes more significant.Comparing the modification effects of PVP aqueous blends with different molecular weights and concentrations,0.4%K60 is the most suitable modification condition.At this time,the water flux is about 1.52 times that of the blank membrane,and the desalination rate of Na₂SO₄remains basically unchanged.The desalination rate of Na Cl decreases by about 21%,and the separation efficiency of divalent ions increases.（2）Use different concentrations of PVP-K13,PVP-K30,and PVP-K60 surfaces to impregnate porous support layers to construct PVP intermediate layers.At low concentrations of PVP,hydrogen bonding is the main influencing factor.The diffusion rate of PIP slows down,the thickness and crosslinking degree of the polyamide layer decrease,the surface roughness of the membrane increases,the hydrophilicity increases,and the water flux increases.When the concentration of PVP is high,the increase in PIP loading capacity of the porous support layer is the main influencing factor,while the thickness and crosslinking degree of the polyamide layer increase,resulting in an increase in desalination rate.The effect of high molecular weight PVP modified porous support layer is more significant,and 0.4%K60 is the optimal condition for PVP modified support layer.At this time,the water flux is about 1.42 times that of the blank membrane.When applied to the reuse of saline wastewater,the separation performance of divalent and divalent ions is excellent,and the separation coefficient S of Na Cl/Na₂SO₄is about 18%of the blank membrane.（3）TA has a large amount of phenolic hydroxyl groups.By modifying the porous support layer with PVP and TA,the intermediate layer formed is more hydrophilic and dense.At the same time,it can further reduce the diffusion rate of PIP,making the thickness of the polyamide nanofiltration membrane smaller,the roughness greater,and the hydrophilicity stronger.The water flux is higher than that of the polyamide membrane modified with PVP,and the separation coefficient S of Na Cl/Na₂SO₄increases to around 131,which is conducive to the reuse of saline wastewater.