Layered Construction Of C₃N₄/Carbon Nanomaterials/PVDF Photocatalytic Membrane And Its Performance For Water Treatment

The photocatalysis is a novel and environmentally friendly technology,and has been applied successfully in photodegradation of various organic pollutants in aquatic environments.However,nanosized photocatalysts suffer from the difficult recycle and reuse.Membrane separation is an efficient water treatment technology,but membrane single separated function and its fouling restrict wider application.The photocatalytic membrane is constructed by coupling membrane separation with the photocatalysis in a single unit.The coupled process can not only improve the removal rate of pollutants from water but relieve membrane fouling for increased flux.Hence the photocatalytic membrane is regarded as a new generation of the composite membrane,and has broad prospects in water treatments in the future.The visible light-responsive photocatalytic membrane can relieve the damage to polymeric membranes by UV light,and increase visible light response,which has been a research hot spot in photocatalytic membrane in recent years.Hence in this study visible light-responsive photocatalytic membranes based on the efficient C₃N₄ photocatalyst and carbon nanomaterials were constructed.The detail work as blow:Porous carbon nitride nanosheet?MCU-C₃N₄?driven by visible light was formed by using melamine?M?,cyanuric acid?C?,urea?U?in dimethyl sulfoxide?DMSO?as the supramolecular precursors?MCU?.The morphology characterization and nitrogen adsorption-desorption test showed that thickness and size of MCU-C₃N₄ were about 5nm and 300 nm respectively,and its surface area and pore size distribution were122.53 m²/g and 2-15 nm,respectively.This porous nanosheet structure benefits for increased adsorption capacity of pollutants to improve the photocatalysis efficiency.MCU-C₃N₄ exhibited higher separation of the photogenerated charge than that of bulk g-C₃N₄ and the photocurrent density was 2 times as much as that of bulk g-C₃N₄.In the photocatalytic degradation tests of RhB and TC,kinetic constants of MCU-C₃N₄were 16.98 and 2.5 times as much as that of bulk g-C₃N₄.Taking MCU-C₃N₄ and polyvinylidene fluoride?PVDF?as photocatalytic layer and separated layer,respectively,a visible light-driven MCU-C₃N₄/PVDF composite membrane was prepared by vacuum filtration and chemical crosslinking.The bonding strength between photocatalytic layer and separated layer was satisfactory.For static photocatalytic degradation test of RhB and TC,photodegradation rates of MCU-C₃N₄/PVDF composite membrane were 8.23 and 61.43 times as much as that of raw PVDF membrane,respectively.For four-stage filtrating alginate test,the polluted membrane flux could be recovered to more than 80%of the original flux under visible light for30 min,in which the reversible fouling was dominated,and after 5 cycling reuse flux recovery rate of composite membranes remained at above 56%.Taking MCU-C₃N₄ and graphene oxide?GO?as photocatalytic functional layer and PVDF as separated layer,respectively,visible light-driven GO/MCU-C₃N₄/PVDF composite membranes were prepared via vacuum filtration and chemical crosslinking.GO not only improve the separation performance of composite membrane but benefits for separation of photogenerated charge of MCU-C₃N₄.Pure water flux of GO/MCU-C₃N₄/PVDF composite membranes reached 1333 L/m²/h/bar,which was 2.52 times as much as that of GO/PVDF membrane,and the flux remained stable under pH at 1.0-13.0 conditions.GO/MCU-C₃N₄/PVDF composite membranes exhibited the excellent separation performance?>97%?for oil-in-water emulsions.For static photocatalytic degradation tests of RhB and TC,the photodegradation rates of GO/MCU-C₃N₄/PVDF composite membranes were 12.86 and 14.22 times than that of GO/PVDF membrane s,respectively.For four-stage filtrating tests of diesel-in-water emulsion,the polluted membrane flux could be recovered to more than 90%of the original flux under visible light for 30 min,in which the reversible fouling was dominated,and after 4 cycl ing reuse the flux recovery rate of composite membranes remained at above 70%.Taking carbon nanotubes/MCU-C₃N₄/GO?CNTs/MCU-C₃N₄/GO?and PVDF as photocatalytic functional layer and separated layer,respectively,a visible light-driven CNTs/MCU-C₃N₄/GO/polydimethyl diallyl ammonium chloride?PDDA?-hydrolyzed PVDF?CNTs/MCU-C₃N₄/GO/PDDA-hPVDF?composite membrane was prepared by electrostatic interaction,vacuum filtration and high-pressure treatment.CNTs can not only enhance the stability of GO layer but benefit for the separation of photogenerated charge of MCU-C₃N₄.The ultrasonic vibration test?test condition:0.5-30min at 20W?indicated that the composite membrane displayed less than 2.5%weight loss of photo-catalysts.For static photocatalytic degradation test of RhB and TC,kinetic constants of the comopsite membranes were 0.00385 min^-11 and 0.00221 min^-1,respectively.For the separation of alginate and oil-in-water emulsion,composite membrane reached above 97%and 99%,respectively.For the four-stage filtrating tests of alginate and diesel-in-water emulsion,composite membrane displayed the optimal self-cleaning ability and stability.Constructing photocatalytic membrane reactor and water treatment process with small-scale at laboratory;RhB and TC were chosen as the targets to evaluate its water treatment performance.Results pointed that the CNTs/MCU-C₃N₄/GO/PDDA-hPVDF composite membrane exhibited higher removal rates on RhB?98.31%?and TC?84%?than that of membrane filtration alone.During the operation process,permeate flux for coupling process nearly remained stable,indicating that the photocatalysis effect could relieve membrane fouling.The above contents will greatly enrich the visible light-responsive photocatalytic membrane materials,membrane methods and membrane separation mechanism in the field of wastewater treatment,expand the photocatalysis principles and application o f g-C₃N₄ and carbon nanomaterials,and provide new techniques for removal of organic pollutants in water,which has important scientific significance and practical value.