Cross-linking Construction Of Holey Graphene Oxide Membranes And Its Seperation Performance

Graphene Oxide（GO）has been widely used as a construction material for water treatment membranes due to its honeycomb structure and the large number of oxygen-containing groups distributed on the surface.GO membrane has good physical properties,excellent chemical stability and unique two-dimensional layered structure,and has attracted much attention in water treatment fields such as sewage treatment and desalination.In water treatment,GO membrane has a good retention effect on organic matter and ions,but it also faces the problems of low water flux and poor stability.In response to the above two major problems,this article introduces nanopores into GO nanosheets to prepare holey GO（h GO）by means of oxidation and etching to shorten the path of molecules or ions through the membrane.Pressure filtration and layer-by-layer assembly were used to prepare the h GO membrane,and small molecule thiourea（TU）was used as crosslinking agent.To further improve the stability and separation accuracy of h GO membranes,thermally reduction was used to decrease the interlayer spacing of h GO membrane.The structure,filtration performance,pollution resistance and stability of h GO membranes were characterized and studied.The specific research contents and results are as follows:（1）Optimization and characterization of preparation method of h GO.In the experiment,the modified Hummer’s method was used to prepare large-size GO nanosheets,and h GO was obtained by H₂O₂ oxidation,and h GO nanosheets with different oxidation levels were obtained by controlling the oxidation time.The successful preparation of h GO was verified by AFM analysis and characterization.（2）Characterization and filtration performance test of h GO membranes prepared by layer-by-layer method.The GO/TU-PEI and h GO/TU-PEI membranes were constructed by layer-by-layer using polyethyleneimine（PEI）and TU.Firstly,the number of layers of the GO/TU-PEI assembled membrane was optimized,and GO/TU-PEI membranes with different numbers of layers（4 layers,8 layers and 12 layers）were prepared.As the number of assembled layers increases,the water flux gradually decreases,and the salt rejection rate gradually increases.Secondly,the h GO assembled membranes（12 layers）with different oxidation degrees were studied.The water flux and the rejection rate of salts of h GO/TU-PEI membranes increased at first and then decreased.The pure water flux of the h GO/TU-PEI membrane oxidized for 1 h was the highest,which was 0.86 LMH/bar,and the rejection rates for Na₂SO₄、Mg Cl₂、Na Cl、Mg SO₄ were 97.71%,98.25%,92.67%,and 93.24%,respectively.The h GO/TU-PEI assembled membrane oxidized for 1 h was further thermally reduced to prepare r-h GO/TU-PEI membrane.The effects of different heat treatment temperatures（80℃,100℃,120℃,140℃and 160℃）on the structure and properties of the membrane were studied.The experimental results show that as the heat treatment temperature increases,the thickness of the GO separation layer first decreases and then remains unchanged,the layer spacing gradually decreases,and the contact angle of the membrane surface continues to increase,that is,the hydrophilicity of the membrane surface decreases gradually.The pure water flux through the heat-treated membrane first decreased and then increased,and the salt rejection rate increased first and then decreased.The pure water flux of the r-h GO/TU-PEI membrane under the heat treatment at 120℃is 0.57 LMH/bar,and the rejection rates for Na₂SO₄、Mg Cl₂、Mg SO₄、Na Cl are 98.9%,98.6%,98.26%,87.61%,respectively.In addition,r-h GO/TU-PEI membranes showed good stability under acid-base conditions.（3）Characterization and filtration performance test of h GO membrane prepared by one-step filter press method.GO and h GO were blended with TU and formed into membranes by pressure filtration to prepare GO/TU and h GO/TU nanofiltration membranes.The separation performance of the pressure filtration membranes was investigated.The results show that the permeation flux of the h GO/TU membrane after oxidation treatment has been significantly improved.The pure water flux of the h GO/TU membrane oxidized for 1 h is 1.98 LMH/bar,which is 7 times that of the GO/TU membrane,and the rejection rate of the h GO/TU membrane to Congo red and methylene blue is higher than 98.51%.The rejection rates of Na₂SO₄、Mg Cl₂、Na Cl、Mg SO₄ were 44.3%,40.74%,40.19%,and 20.03%,respectively.In this paper,the h GO membranes prepared by two different methods show different separation performance in the process of separating salts and dyes.In comprehensive comparison,the h GO membranes prepared by layer-by-layer have better permeability,selectivity and stability.The layer-by-layer assembly method can also adjust the thickness of the assembled membrane and the treatment temperature of thermal reduction as needed to improve the separation efficiency of the salt.The research results of the paper provide a new idea for the development of GO and other two-dimensional materials in the field of membrane separation,but the internal ion separation mechanism of the prepared h GO membrane still needs further exploration.