Study On In-situ Microstructure Control And Chlorination Mechanism Of Polypiperazine Amide Nanofiltration Membrane

Nanofiltration?NF?has been considered to be the most promising membrane technology for drinking water purification because of its unique separation behavior and high efficiency.Innovating nanofiltration membrane?NFM?structure,developing facile approaches to fabricate and manipulate the membrane structure,and investigating the membrane structure and performance evolution based on the actual demands in drinking water purification are of paramount importance for promoting the deployment of NF technology.Restrained by the Donnan effect,state-of-the-art mono-charged NFMs cannot reject the divalent cations and anions simultaneously,which limits their water purification performance.Improving the membrane permeability could enable the NF system to operate at a low pressure and thus an energy-efficient membrane process.Constructing patterned surface has been proved to be an efficient strategy to enhance the permeability of NFMs.However,current reported crumpled surfaces contributed limit surface area increase.Hence,novel strategies for constructing patterned surface on NFMs are highly desired.In addition,chlorine degradation is one of the unmet challenges for the polyamide NFM.However,the chlorination mechanism and the chlorine-induced separation performance evolution of polypiperazine-amide?PPA?NFMs still remains incomplete.To solve the aforementioned issues,we performed the following three studies.Firstly,we constructed a dully charged polyamide separation layer by using the gradual change of monomers concentration and ratio at the interfacial polymerization?IP?reaction zone.We then developed a diffusion rate control method to reduce the transmission rate of aqueous monomers to the top reaction zone.An ultrathin dually charged PPA separation layer??38 nm?was obtained on the inner surface of the hollow fiber substrate.Thanks to the collaborative separation effect of the dually charged separation layer and the reduced thickness,our NFMs exhibit excellent comprehensive separation performance with high rejection to both divalent cations and anions??RNa₂SO₄+RMg Cl₂?/2?95.0%?and desirable water permeability(12.8-18.6 L·m^-2·h^-1·bar^-1),simultaneously.Secondly,we proposed a new NFM separation layer design concept of polyamide nanofilm-nanotube array composite structure.We then in-situ induced the growth of PPA nanotube array on PPA nanofilm based on the interface force of the water-oil phase and the initially formed nanofilm.We investigated the growth mechanism of the PPA nanotubes and established facile microstructure manipulate methods.The surface area increase of the nanotube membrane?NTM?was?128.0%.Thanks to the nanofilm-nanotube array composite structure,the NTM showed a water permeability as high as?30.0 L·m^-2·h^-1·bar^-1and high rejection?>99.0%?to Na₂SO₄and Mg SO₄.Thirdly,the deterioration of the PPA separation layer upon chlorine exposure follows two pathways:reversible chlorine substitution of the-NH group to-NCl group,and conversion of the-NH group to imine group.After chlorination,-NH group content in the PPA separation decreased,which lead to destroy of related hydrogen bond structure,polymer chain tightening,variation of membrane-ion electrostatic interaction,and thus the decrease of water flux and changes of salts rejection.Thanks to the reversible chlorine substitution of the-NH group to-NCl group,damaged separation performance can be partially recovered after regeneration.Furthermore,this study can also reveal that the electrostatic interaction between the ions and charged groups in the separation layer is the“foundation”of the electrostatic separation mechanism.