| In modern time,the water pollution and the inadequate water are gradually threatening human survival and development.Researchers have developed different technologies to overcome these problems.Among them,membrane is regarded as the most promising technology because of high efficiency and low cost.Recently,membranes assembled from two dimensional(2D)nanosheets have attracted much attention due to high separation performance,good chemical and thermal stability,excellent mechanical strengthen and so on.The defects and/or nanopores on nanosheets and void spacing formed between nanosheets could separate water from other impurities.Although several kinds of 2D nanosheets have been fabricated into membranes for separation,the research are still inadequate in width and depth.Herein,we focused on g-C3N4 nanosheets and developed g-C3N4-based membranes for separation.In this paper,we aim to obtain g-C3N4-based membrane with excellent stability and high separation performance,deepen the understanding of mass transport in nanosheets membrane,and extend the function of nanosheets.The research contents of this paper are as follows:(1)The g-C3N4 nanosheet with artificial nanopores and unexfoliated fragments could be obtained by heating the g-C3N4 powder using HCl and followed by ultra-sonication.The nanosheets are 0.5-1 nm in thickness and the artificial nanopores are 1.5-3 nm in diameter.Then the g-C3N4 nanosheets were fabricated into membranes by vacuum filtration.The 160 nm g-C3N4 membrane showed the water flux of 29 L m-2 h-1 bar-1 and rejection rate of 87% for 3 nm molecules.The rejection rate increased with the increase of the size,and the retention concentration increased with the filtration time.Therefore,the membrane shows classical physical sieving.The flux of different solution is similar regardless of their charge properties.The membranes are insensitive to the charge.Besides,although the Zeta potential of nanosheets changed significantly with pH value,the water flux almost keep constant under different pH.In addition to these,the water flux increased linearly with applied pressure.The nanochannels in the g-C3N4 membrane do not shrink with sufficient rigidness and good mechanical property even under an external pressure of 0.6 MPa.The excellent performances are attributed to the spacing between the nanosheets generated by the unstripped fragments adhered on the g-C3N4 nanosheets,and the artificial nanopores on each nanosheet decreases transport resistance.(2)The g-C3N4 membrane is hydrophobic.The solvent flux increased with the decreased hydrophobic,independent of viscosity,which is consistent with slip flow.The molecular dynamic simulation theoretically confirms that the hexane molecules with high hydrophobicity pass through the g-C3N4 nanosheets with large resistance,while the water molecules with less hydrophobicity pass through the g-C3N4 nanosheets with less resistance.The reason is that the interaction between hexane and g-C3N4 is much larger than that between water and g-C3N4.The bigger interaction resist the transport.In addition,the velocity distribution of water fluid through g-C3N4 nanochannels shows a plug-like shape.This is much different from that the transport behavior turns from slip flow into viscous flow after the incorporation of O in C materials.Thus,the N whose electronic negativity is between C and O,has a little influence on water transport.(3)The g-C3N4 nanosheets were synthesized with thermal exfoliation and ultra-sonication.The cheap commercial polyacrylic acid(PAA)molecules were inserted to tune the nanosheets spacing.The PAA and g-C3N4 nanosheets were mixed and filtrated to fabricate g-C3N4-PAAx hybrid membranes.The g-C3N4 nanosheets and PAA formed the intermolecular H-bonding through the NH groups of g-C3N4 nanosheets and the COOH groups of the PAA.The PAA could fix between g-C3N4 nanosheets firmly.After the incorporation of PAA in g-C3N4 membrane,the thickness increasing,the surface roughness decreasing,and the hydrophilicity increasing.The water flux through the g-C3N4-PAAx hybrid membranes increased from 47.5 to 117.1 L m-2 h-1 bar-1 with increasing the addition of the PAA from 0 to 10%.The rejection rate for 3 nm molecules decreased from 85% to 83%.However,the water flux through just increased to 143.1 L m-2 h-1 bar-1 when the addition of the PAA increasing from 10% to 50%,while the rejection rate decreased sharply to 70%.Therefore,a small number of inserted PAA could increase the separation performance.But overloaded PAA would promote the formation of irregular pores,which decreases the separation performance and anti-fouling property.The g-C3N4-PAAx hybrid membrane with the addition of 10% shows physical sieving,and the molecular weight cut off is 5.7 kDa.(4)The g-C3N4 nanosheets were synthesized with thermal exfoliation and ultra-sonication.The g-C3N4 nanosheets were deposited on porous substrate through vacuum filtration.The Fe(OH)3 nanoparticles obtained by hydrolyzation were deposited on g-C3N4 layer through vacuum evaporation.The g-C3N4 layer just acts as support instead of selective layer,and the Fe(OH)3 layer could separate water from other impurities.The Fe(OH)3/g-C3N4 composite membrane showed pure water flux of 48 L m-2 h-1 bar-1,and a rejection rate of 99.9% for 3 nm molecules.Such excellent separation performance is attributed to that the pores formed between nanoparticles are narrowly distributed.Benefitting from the inorganic properties,the composite membrane showed resistance to different organic solvents,and was stable under pH value of 3-11.Besides,the pores are stable under the pressure of 0.3 MPa. |
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