Preparation,Characterization And Application Of Cation Exchange Membranes Based Sulfonated PVDF

The heavy metal wastewater containing chloride comes from a variety of sources due to strong toxicity,strong acidity,easy enrichment and non-biodegradable.When membrane electrodeposition is applied to wastewater treatment,it meets the need of recycle heavy metal ion and inhibit producing chlorine gas.However,since the commercial cation exchange membrane?CEM?has the disadvantages of low permeability and poor oxidation stability,the chlorine gas is still precipitated from the anode and causes serious pollution.The paper is aimed to study a cation exchange membrane with excellent permeability and good oxidation stability that can work in a strong acid environment to solve this problem.Firstly,the PVDF-g-PSSA membrane was prepared by the route of“PVDF membrane?alkalinization?grafting?sulfonation”.Field emission scanning electron microscope?FESEM?,Fourier transform infrared spectrometer?FTIR?,Automatic bursting strength tester and other methods of membrane performances were used to analysis physical and chemical properties of CEMs.The effects of experimental conditions for PVDF membrane preparation,alkalization,grafting and sulfonation on physicochemical properties of CEMs were systematically discussed to obtain optimum technological parameters.Secondly,the PVDF-g-PSSA cation exchange membrane was modified by doping inorganic nanoparticles to improve the permeability and other membrane properties.The physicochemical properties of the membranes doped with nano-SiO₂,nano-Al₂O₃ and nano-ZnO were investigated by membrane performances to optimize doping conditions.Thirdly,the transport process and a rapid evaluation system for the main performances of the CEMs were discussed by the cyclic voltammetry curve and chronopotential curve.Finally,the CEMs were applied to the experiment of nickel chloride heavy metal wastewater treatment.The effects on cell voltage,energy consumption,and chlorine inhibition of self-made membranes and commercial membrane were compared,and the accelerated life of CEMs under severe conditions were determined.Based on consideration of the above content,the conclusion are as follows:Firstly,the mass concentration of PVDF is 22%for good bursting resistance and small weight loss.At the same time,the casting solution is suitable for membrane formation.Secondly,the alkalization conditions are obtained by the effects of alkalization time,temperature and alkali concentration on grafting rate,membrane oxidation resistance and bursting strength.The time of alkalization is 75 min,the concentration of NaOH solution is0.1mol/L,the temperature of alkalization is 70°C.Thirdly,the grafting conditions of the PVDF alkalized membrane are received as follows:the concentration of BPO is 0.7 g/100 mL?80ml of styrene+20ml of tetrahydrofuran?,the grafting temperature is 70°C,and the grafting time is 14 h.At last,the optimal sulfonation conditions for the PVDF-g-PS membrane are determined as follows:the sulfonation temperature is 70°C,and the sulfonation time is 8 h.The PVDF-g-PSSA was modified by inorganic nanoparticles?SiO₂,Al₂O₃,ZnO?to improve the membrane properties.The doping mass fraction of the nanoparticle is 1.5%by the evaluation of membrane properties.For comparison,the PVDF/1.5%Al₂O₃-g-PSSA has a noticeable increase in IEC and water uptake which IEC is 41%higher and membrane resistance is 19%smaller than PVDF-g-PSSA.The PVDF/1.5%ZnO-g-PSSA has the most significant growth in permeability,which is 9%higher than PVDF-g-PSSA.The PVDF/1.5%SiO₂-g-PSSA has a remarkable growth in burst strength,which is 9%higher than PVDF-g-PSSA.The properties of the membranes under various inorganic nanoparticles doping are slightly improved compared to the single doping.The permeability and burst strength of PVDF?SiO₂:Al₂O₃:ZnO??2:1:2?-g-PSSA have increased by 10.6%and 13.8%compared to PVDF-g-PSSA.In short,the mass ratio of SiO₂,Al₂O₃,and ZnO is 2:1:2,which accounts for 1.5%of the PVDF mass fraction.The transport properties were demonstrated by cyclic voltammetry curve and chronopotential curve.The narrower the potential window of the cyclic voltammetry curve,the higher the permeability of the membrane.The PVDF?SiO₂:Al₂O₃:ZnO??2:1:2?-g-PSSA has the highest permeability which is consistent with previous experimental results.In the chronopotential curve,the Na⁺ions in the diffusion boundary layer on the membrane surface through the membrane preferentially at the moment when the current is applied.When the current density is small,the U_m almost unchanged with time.However,when the current density increase to the limiting current density,the U_m increases sharply,and the concentration of Na⁺ions in the diffusion boundary layer tends to zero.The U_m of the inflexion point for the inorganic nanoparticles doped membranes increase more dramatic compared to PVDF-g-PSSA,which indicates more homogeneous membrane pores and distribution of ion exchange site.As well,the U_m has a smaller value which represents a smaller membrane resistance.Thus,a rapid evaluation system for the main performance of the CEMs was established.Compared to the application effect for PVDF/1.5%SiO₂-g-PSSA?PVDF/1.5%Al₂O₃-g-PSSA?PVDF/1.5%ZnO-g-PSSA?PVDF?SiO₂:Al₂O₃:ZnO??2:1:2?-g-PSSA and CEM-1,the results showed that the voltage,energy consumption and chloride ion leakage rate of the self-made membranes are smaller than CEM-1,and the chloride ion leakage rate of the PVDF/1.5%SiO₂-g-PSSA membrane has a smallest change with the extension of electrolysis time.In addition,the PVDF/1.5%SiO₂-g-PSSA has a longest accelerated life under severe conditions,indicating that PVDF/1.5%SiO₂-g-PSSA can be used to reduce the pollution by the chlorine gas in the treatment of chlorine-containing heavy metal wastewater.The hydrochloric acid produced in the middle compartment can be recycled.This technology can produce considerable economic and environmental benefits.