Study On Multi-functional Biomimetic Nanofiltration Membranes Tailored By G-C₃N₄ Nanosheets For Hypersaline Organic Wastewater Treatment

Hypersaline organic wastewater has the complex components and elevated salinity,which pose a challenge in effective treatment.The conventional strategy of treating hypersaline organic wastewater is to destroy the molecular structure of organics by oxidative decomposition or biochemical degradation to achieve the removal of organic pollutants,which compromises the resource recovery from wastewater.Inevitably,sustainable resource recovery from hypersaline organic wastewater is the development direction of the next-generation water treatment technology.Specifically,hypersaline organic wastewater contains a large amount of high-value added organic components,which can be reused in industrial processes to minimize industrial production costs and achieve zero wastewater discharge.Efficient separation of organics and salts is the most important key to achieve the resource recovery from hypersaline organic wastewater.Nanofiltration technology is an effective approach to achieve the resource recovery from hypersaline organic wastewater.However,the conventional nanofiltration membranes have high rejection to both organics and salts,which fail to effectively fractionate organics and salts.Furthermore,the severe membrane fouling can be caused by organics deposition and microbial pollution during the membrane separation process,deteriorating the membrane separation performance and the water quality.In this study,multi-functional biomimetic nanofiltration membranes were constructed via rapid co-deposition of g-C₃N₄ nanosheets and dopamine triggered by oxidant,which can potentially fractionate the organics and salts,in view of resource recovery from hypersaline organic wastewater.Furthermore,taking advantage of the photocatalytic properties of g-C₃N₄nanosheets incorporated in the biomimetic nanofiltration membranes,the degradation of organic pollutants from the wastewater can be yielded.The main research contents of the study are as follows:?1?Firstly,multi-functional biomimetic nanofiltration membranes were constructed via rapid co-deposition of dopamine,polyethyleneimine?PEI?,g-C₃N₄ nanosheet onto the porous hydrolyzed polyacrylonitrile?HPAN?membrane triggered by ammonium persulfate?APS?.It can be observed from scanning electron microscope?SEM?and atomic force microscope?AFM?characterization that the g-C₃N₄nanosheets are successfully interacted with dopamine/PEI composite layer.Simultaneously,the microstructure and surface properties of the dopamine/PEI composite layer is tailored,due to the introduction of g-C₃N₄ nanosheets.Specifically,with the increase of the content of g-C₃N₄nanosheets incorporated,the effective pore diameter of the fabricated membrane decreased.When the content of g-C₃N₄ nanosheets was increased to 0.01 wt%,the molecular weight cut-off?MWCO?of the biomimetic membrane was reduced from 1355 Da to 985 Da?<1000 Da?,enabling the transformation from ultrafiltration to nanofiltration scale for the biomimetic membranes.When the content of g-C₃N₄ nanosheets increased to 0.04 wt%,the MWCO of the biomimetic nanofiltration membrane was reduced to 592 Da.At the same time,the introduction of g-C₃N₄ nanosheets could help create extra two-dimensional nanochannels in the dopamine/PEI composite layer,thus greatly promoting the penetration of water molecules.Pure water permeability of the biomimetic nanofiltration membrane?MWCO:592 Da?increased from 22.3 L·m^-2·h^-1·bar^-1 to 28.4 L·m^-2·h^-1·bar^-1.In addition,the biomimetic nanofiltration membrane has extremely high dye rejection?>99.3%?and low salt rejection(2.9%for 1.0 g·L^-1 Na Cl;7.6%for 1.0 g·L^-1 Na₂SO₄).At different p H and salt concentration conditions,the biomimetic nanofiltration membrane still maintain very high rejection to reactive dyes.These results indicate that the biomimetic nanofiltration membrane has a great potential in the recovery from hypersaline dye wastewater.?2?As demonstrated above,g-C₃N₄-based multi-functional biomimetic nanofiltration membranes have excellent performance in fractionation of organics and salts.Therefore,the biomimetic nanofiltration membrane?MWCO:592 Da?has a promising potential in recovery extraction from landfill leachate concentrate.Specifically,the operating conditions?such as flow rates,pressures and temperatures?can significantly affect the separation performance of the biomimetic nanofiltration membrane during the treatment of the landfill leachate concentrate.The rejections of humic substances and salts were positively correlated with cross-flow rates and operating pressures,but negatively correlated with temperatures.Based on this,the optimal condition of the biomimetic nanofiltration membrane for the sustainable treatment of landfill leachate concentrate was fixed(i.e.,cross-flow rate of 250 L·h^-1,operating pressure of 4 bar,and operating temperature of 26?).At this optimal operating condition,the biomimetic nanofiltration membrane?MWCO:592 Da?had a high rejection to humic substances?>96.2%?and low rejection to salts?<5.1%?.Furthermore,the concentration process was applied for sustainable treatment of the landfill leachate concentrate.When the concentration factor reached 32.6,the concentration of humic substances in the concentrate increased from 1755.7 mg·L^-1 to 51465.8mg·L^-1 with 90.03%recovery ratio for humic substances.Therefore,the application of this biomimetic nanofiltration membranes provides new technical pathway for sustainable treatment of landfill leachate concentrate.?3?g-C₃N₄ nanosheets are introduced into the surface of the biomimetic nanofiltration membrane by co-deposition,which can endow the photocatalytic properties to the membranes.To investigate the visible light degradation characteristics and anti-fouling performance of multi-functional biomimetic nanofiltration membranes,20 mg/L methyl blue?MB?solution was used as model organic pollutants for performance demonstration.During static photodegradation conditions,the biomimetic nanofiltration membranes yielded a complete degradation for 200 m L MB solution during 80 min.After three cycles,the degradation efficiency of the nanofiltration membrane for MB solution was remained above 80%;During the photodegradation process at the cross-flow filtration mode,86.2%of MB was degraded by the biomimetic nanofiltration membrane in 120min.Furthermore,>98.5%dye rejection was also observed.In order to achieve the sufficient degradation of MB solution,the photodegradation activity of the biomimetic nanofiltration membrane was promoted by adding a trace amount of H₂O₂.When the amount of H₂O₂ was increased to 2 mmol/L,the degradation of MB for the biomimetic nanofiltration membrane was increased to 91.1%;Furthermore,the g-C₃N₄-based multi-functional biomimetic nanofiltration membranes has excellent antibacterial properties,inactivating 98.1%of Escherichia coli?E.coli?under visible light irradiation.Degradation of MB with quenchers and electron spin-resonance spectroscopy?ESR?proved that the photocatalytic reactive species?i.e.,·OH,·O₂^-and h⁺?were evolved on the surface of the biomimetic nanofiltration membrane under visible light irradiation,which play a dominant role in organics degradation and inactivation of microorganisms.Therefore,in the practical wastewater treatment,the fabricated multi-functional biomimetic nanofiltration membranes are expected to achieve efficient degradation of organics while maintaining excellent separation characteristics and antifouling performance,providing a new avenue for in-depth and efficient wastewater treatment.