| As one of the important technology of modern separation science,membrane separation technology has unique advantages,such as effective separation,low energy consumption,no phase change,no secondary pollution,stable equipment performance and simple operation.Porous membrane and its corresponding membrane separation technology have been widely used in the fields of energy,environmental protection(water treatment),chemical industry,metallurgy,food,medicine,health and biotechnology.Membrane separation technology has produced huge economic and social benefits,and is playing an increasingly important role.Polyacrylonitrile porous membrane material is an ideal and widely used separation membrane material with advantages of diverse structure,high stability and easy to be functionalized.Polyacrylonitrile porous membrane materials have been widely used in water treatment,ion adsorption.As a new type of porous membrane structure,the polyacrylonitrile nanofibrous membrane with high porosity,low curvature factor and controllable pore size has a good application prospect in water treatment.Polyacrylonitrile nanofibrous membrane(PAN NFM)can not be prepared by the conventional preparation methods.The spinning method prepared PAN NFM has high porosity and outstanding uniform fiber distribution,however,the spinning technology is diffcult to industrialize and still in the stage of experiment research.Therefore,it is of great research significance and application value to develop the PAN NFM preparation method with simple technology and industrialization prospect,and to explore its functional application performance.In this paper,PAN NFM was prepared by non-solvent thermal phase separation method and simultaneously.biaxial stretching technology,and its micropore morphology and structure regulation mechanism were studied.Amidoxization and carboxylation functionalization of PAN NFM were carried out to study its application potential in uranium extraction from seawater and water defluorination.The main results and conclusions of this paper are summarized as follows:(1)Focusing on the preparation of polyacrylonitrile casting film by non-solvent thermal phase separation method,the relationship between processing,structure and morphology of polyacrylonitrile casting film was detected by SEM and picture analysis.The experimental results show that increasing the phase separation temperature,prolonging the phase separation time,increasing the cooling rate,decreasing the cooling temperature and completely freezing can effectively increase the density of polyacrylonitrile casting film and effectively reduce the pore size and pore size distribution.Based on the comprehensive analysis of the each processing parameter effect on the polyacrylonitrile casting film structure and morphology,the key processing parameters under the experimental system are phase separation temperature(Tps)and cooling temperature(Tcooling).The phase separation theory can better explain the experimental results.Finally,in order to quantitatively describe and compare the polyacrylonitrile casting film structure and morphology,the concept of density index was proposed.It can more directly reflect the complex coupling between the processing parameters of the non-solvent thermally induced phase separation method.(2)Polyacrylonitrile nanofibrous membrane(PAN NFM)was firstly prepared by simultaneously biaxial stretching.PAN NFM with different structures were prepared by simultaneously biaxial stretching using polyacrylonitrile casting films prepared in chapter 2.The structure and performance of PAN NFM was detected by SEM and picture analysis and air permeability test.The effect of stretching processing parameters on PAN NFM was studied.The experimental results show that the PAN NFM prepared at 120～130 ℃ have excellent air permeability(<20 s/100 cc)and porosity(>80%),and the optimal preparation temperature is 130℃.At the same time,it shows that the PAN NFM prepared by different conditions and different types of polyacrylonitrile casting films have the structure similarity.At the optimum preparation temperature,with the increase of the drawing ratio,the fiber diameter size decreases,the diameter size uniformity increases,and the permeability and porosity increase.The pore forming mechanism of simultaneously biaxial stretching prepared PAN NFM is the combined action of tensile reaming,fiber thinning and splitting.The easier large-scale industrial production method of PAN NFM has certain guiding significance for the preparation process of other similar materials.(3)Focusing on the functional application of PAN NFM:A novel amidoximated polyacrylonitrile nanofibrous membrane(AO-PAN NFM)is synthesized by post-amidoximation of PAN NFM.The AO-PAN NFM is characterized by high specific surface area and outstanding hydrophilicity.Accordingly,it exhibits high uranium adsorption capacity,excellent selectivity,outstanding reusability with fast and efficient desorption response.Meanwhile,the chemical adsorption dominates the uranium adsorption behavior of AO-PAN NFM.The novel AO-PAN NFM produced by the new stretching method shows great application potential as a new strategy for uranium extraction from seawater to support future nuclear energy generation.(4)Focusing on the functional application of PAN NFM:A novel carboxylation polyacrylonitrile nanofibrous membrane(C-PAN NFM)is synthesized by canboxylation of PAN NFM.The C-PAN NFM shows high specific surface area and excellent hydrophilicity.Accordingly,it exhibits high fluoride adsorption capacity,outstanding selectivity,excellent reusability with efficient regeneration.Meanwhile,the fluoride adsorption behavior of C-PAN NFM is dominated by chemical adsorption,and the adsorption mechanism is mainly hydrogen bonding and ion exchange.Notably,The new stretching method mass produced C-PAN NFM with good fluoride removal performance is a novel porous membrane material used for fluoride removal for the first time and has broad application potential as a new strategy for fluoride removal. |
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