Preparation Of N-doped TiO₂-RGO Blended Membrane And Its Visible Photocatalytic Properties

Ultrafiltration membrane separation technology is a safe and reliable sewage treatment technology with stable effluent quality.However,it is incapable of trapping low molecular weight organic pollutants such as dyes and antibiotics,and difficult to decompose and degrade organic pollutants completely.Photocatalytic technology can be used to decompose water soluble efficiently by the strong oxidizing active groups and free radicals generated by photocatalysts.However,most of the present photocatalysts are in the form of the powder,which cannot adsorb visible light efficiently.Furthermore,it is difficult to be recycled and easy to causes secondary pollution.In order to improve the ultrafiltration membranes retention performance for low-molecular organic pollutants and enhance visible light catalytic activity of photocatalysts,Nitrogen-doped titanium dioxide（N-TiO₂）was prepared and then combined the N-TiO₂with the reduced graphene oxide（RGO）to synthesize the nitrogen-doped TiO₂/RGO composite photocatalyst（N-TiO₂-RGO）with high visible light response with one-step hydrothermal method.The composite photocatalyst preparation conditions were optimized by orthogonal experiments,and the prepared photocatalyst property was characterized by XRD,Raman,SEM,XPS,UV/Vis DRS,and PL.Meanwhile,their photodegradation effects of the N-TiO₂-RGO composite photocatalyst on Rhodamine B（Rh B）and norfloxacin（NOR）were investigated.Then,the polyvinylidene fluoride（PVDF）was blended with the N-TiO₂-RGO composite photocatalyst,and N-TiO₂-RGO blending modification PVDF ultrafiltration flat membrane（named as modified membrane）was prepared by the immersion-precipitation phase transformation method（L-S phase transformation method）.The preparation conditions of the modified membrane were optimized by orthogonal test,and its chemical structure property was characterized by XRD,Raman,SEM,XPS,UV/Vis DRS.Finally,the removal performance of Rh B and NOR by modified membrane was explored.The main results are as follows:（1）The optimal preparation conditions of N-TiO₂-RGO composite photocatalyst were reaction temperature of 160℃,reaction time of 6h,m[Ti（SO₄）₂]/m[GO]=300,m[Ti（SO₄）2]/m[CO（NH₂）₂]=0.50,m[C₆H₆O₆]/m[GO]=15,and the p H of 1.5.（2）The N-TiO₂-RGO composite photocatalyst not only presented the crystal structure of anatase TiO₂ and RGO,but also contained Ti-N,pyrrole N（C-N-C）,pyridine N（C-N=C）,and O-Ti-N functional groups.TiO₂ and RGO were simultaneously doped with Nitrogen atoms to form a pyridine N and pyrrole N structure,which tightly bound TiO₂with RGO.Thus,it improved the interface charge transfer efficiency.（3）The band gap of N-TiO₂-RGO composite photocatalyst（2.11e V）was lower than that of TiO₂ and their difference value was 0.86e V.The absorption edge band of N-TiO₂-RGO composite photocatalyst extending into the visible-infrared light region.When the light duration lasted for 90 minutes,the degradation rates on Rh B and NOR could reach99.4%and 93.6%,separately.（4）The best preparation conditions of the modified membrane were determined as bellow:DMAc content was 79.7wt%,PVDF content was 19wt%,PVP content was 1wt%,and N-TiO₂-RGO composite photocatalyst content was 0.3wt%.（5）The modified membrane had good photocatalytic properties and interception behaviors.N-TiO₂-RGO composite photocatalysts have owned specific structures with Ti-N,pyrrole N（C-N-C）,O-Ti-N,pyridine N（C-N=C）and etc.,which led to tight combination of N-TiO₂-RGO composite photocatalyst with the PVDF raw materials.（6）The modified membrane not only had strong absorption of ultraviolet light（λ≤400nm）,but also possessed strong absorption capacity for visible light（λ=400nm～760nm）and infrared light（λ>760nm）.Under the condition of visible light irradiation,the Rh B and NOR removal rate by the modified membrane was 98.7%and89.3%,respectively.（7）The flux decay rate of the modified membrane for filtering Rh B solution under visible light conditions was decreased from 56.9%to 31.1%.It showed that the modified membrane could decompose the organic pollutants attached on the membrane surface,alleviated the clogging of the membrane pores by the organic pollutants,which greatly improved its anti-pollution performance.