Green Preparation Of Salt Permeable Loose Nanofiltration Membrane For Organic Contaminant Removal

Nanofiltration（NF）membranes are able to reject small size organic pollutants and inorganic salts,manifesting reasonable prospect in wastewater treatment.However,with the increasing of wastewater treating requirements and the proposing of near-zero liquid discharge demand,applications of traditional NF membranes in wastewater treating is constrained because they cannot precisely separate inorganic salts from organic pollutants.The key to solving these problems is to fabricate loose nanofiltration membrane（LNM）which can separate salts from organics via green raw materials,controllable and eco-friendly methods.Focusing on typical polyphenol tannic acid（TA）,this study firstly confirmed its rationality in preparing NF membranes.Afterwards,LNMs were fabricated via constructing TA-piperazine（PIP）long monomer for interfacial polymerization（IP）,depositing TA and polyethyleneimine（PEI）under mild and solvent-free conditions.Then,such LNMs were employed to treat real wastewater to investigate their potential in recycling the effluent.（1）Thin-film nanocomposite（TFN）NF membrane suffers from poor compatibility between nanofiller and polymeric matrix,introducing organic coating is able to improve the interfacial compatibility.The modified Ui O-66-NH₂,which was coated by fully organic TA,was incorporated into the skin layer of TFN NF membrane.TA modification obviously improved the metal organic framework（MOF）-polymeric matrix compatibility and membrane hydrophilicity via bridging effect.The as-prepared TFN NF membrane exhibited a pure water permeability of 14.84 LMH/bar and Na₂SO₄ rejection of 94.8%.Its permeability and selectivity were simultaneously elevated compared with the control TFC membrane.Although its permeability was lower than that of the pristine MOF incorporated TFN membrane,the selectivity,as a prior property,was higher.Meanwhile,TA modified Ui O-66-NH₂ promoted the anti-fouling performance of the TFN NF membrane.The rationality of introducing TA into NF membrane preparation was sufficiently demonstrated.（2）Based on the demand of achieving organics/inorganics precise separation,longer monomer was facilely prepared to develop thin-film composite（TFC）LNM.Tannic acid assisted interfacial polymerization（TAIP）method was developed.TA and PIP were simply mixed to form such longer monomers via chemical reaction.The facile production of longer monomer not only overcame the complicated design and synthesis of new monomer,but also avoided the leaching of nanoparticles（NPs）in TFN LNM.The TA-PIP longer monomer diffused slow and hindered other monomers from polymerizing tightly,thus a TFC LNM was obtained.The as-prepared TFC LNM showed a higher permeability（32.57 LMH/bar）than traditional NF membrane,together with dye/salt separation performance（the Na₂SO₄/Congo Red（CR）and Na Cl/CR separation factor were 10.57 and 12.74,respectively）.（3）To avoid using harmful organic solvent,solvent-free LNM preparation method was desired.TA and PEI were co-deposited and cross-linked onto surface of substrate in aqueous solution to develop PEI-TA/PES LNM greenly.The crosslinking between TA and PEI was able to happen under room temperature without any harsh conditions,thus fabricating LNM under mild and eco-frinendly conditions was achieved.Morphologies and surface properties can be controlled via adjusting the deposition parameters.The optimal PEI-TA/PES LNM manifested high permeability（40.6 LMH/bar）,satisfying rejection to organics（99.8%to Congo Red）and reasonable transport to salts（97.8%to Na₂SO₄ and 93.9%to Na Cl）.（4）The as-prepared three kinds of NF membranes,together with commercial NF270,were used to treat real effluent from chemical industry parks.As a result,three kinds of TA NF membranes all possessed higher permeability than NF270,especially for two LNMs.Although TA-Ui O-66-NH₂ incorporated TFN NF and NF270 membrane exhibited higher removal to COD_Cr,the permeates from all membranes can meet standards of circulating cooling water.In summary,three kinds of new TA NF membranes were more effective than NF270 in recycling water resource from effluent.This work developed controllable and eco-friendly methods for LNMs fabrication and such LNMs were capable to separate inorganic salts from organic contaminants.Such methods provided technological supports for exploiting unconventional water resourse and achieving near-zero liquid discharge of wastewater.