Preparation And Properties Of Polyacrylonitrile Hollow Fiber Membrane For Heavy Metals Adsorption

Heavy metals in water are harmful to human health and the ecological environment.Among the technologies for removing heavy metals in water,membrane separation technology is a simple and effective technology.Nanofiltration and reverse osmosis technology for treating heavy metal wastewater is effective,but exhibit high energy consumption and low recovery rate.Microfiltration and ultrafiltration require low energy consumption,but the membrane pore size is so large that there is little retention of heavy metals.Therefore,it is great significance to develop a membrane separation technology with high efficiency and low energy consumption for the removal of heavy metals in water.In this paper,polyacrylonitrile hollow fiber ultrafiltration membrane was prepared with polyacrylonitrile（PAN）.Then,functional groups such as carboxyl group,amine group and phosphate group were introduced to prepare HPAN,APAN,PPAN hollow fiber membrane for adsorbing heavy metals.Fourier transform infrared spectroscopy（FT-IR）,X-ray photoelectron spectroscopy（XPS）,scanning electron microscopy（SEM）and zeta potential were used to characterize the morphology and functional structure of the three functional hollow fiber membranes.The adsorption characteristics of three functional hollow fiber membranes for Cd²⁺,Pb²⁺and Hg²⁺were studied from the perspective of kinetics and thermodynamics.The influence of the solution p H,adsorption time,initial concentration and other factors on the adsorption of heavy metals were explored.The main work and research results of this paper are as follows:（1）Preparation and characterization of carboxylated polyacrylonitrile（HPAN）hollow fiber membrane and its adsorption of Cd²⁺.PAN hollow fiber membrane was prepared by wet spinning technology.Then,HPAN hollow fiber membrane was obtained by carboxylation of PAN hollow fiber membrane with Na OH aqueous solution.The results showed that the pure water flux and mechanical strength of the membrane decreased after carboxylation.SEM confirmed that the finger pore layer of the cross section of the membrane became thinner,the sponge layer became thicker,and the pore diameter of the membrane surface became smaller,which would increase the mass transfer resistance of the membrane and reduce the pure water flux of the membrane.FT-IR and XPS showed that HPAN hollow fiber membrane contained a large number of carboxyl groups.The adsorption experiments of Cd²⁺,Hg²⁺,Ni²⁺,Pb²⁺and Cu²⁺on HPAN membrane showed that HPAN membrane possessed selective adsorption of Cd²⁺.The adsorption of Cd²⁺on HPAN membrane is a strong chemical adsorption,and the maximum adsorption capacity was 127 mg/g.When the p H value is 6 to 8,the adsorption effect is the best.The adsorption capacity of HPAN hollow fiber membrane for Cd²⁺can still reach 82%of the initial value,after being recycled for 5 times.The results of dynamic adsorption test showed that the removal rate of Cd²⁺by HPAN membrane decreased slowly over time.Moreover,the concentration of Cd²⁺in permeate could still be far lower than the integrated wastewater discharge standard of China after 24 h treatment.（2）Preparation and characterization of aminated polyacrylonitrile（APAN）hollow fiber membrane and its adsorption of Pb²⁺.APAN hollow fiber membrane was prepared based on PAN hollow fiber membrane with TETA as amination reagent.FT-IR and XPS showed that APAN hollow fiber membrane possesed a large number of amino active groups.SEM suggested that the number of cross-section finger like pores of the membrane decreased gradually and the surface pore diameter increased,which improved the pure water flux of the membrane.The results show that APAN hollow fiber membrane has selective adsorption for Pb²⁺,and the maximum adsorption capacity of Pb²⁺is 158 mg/g.When p H value is 5,the adsorption effect is the best.After being reused for five times,the adsorption capacity of APAN hollow fiber membrane for Pb²⁺can reach 74%of the initial value.In the dynamic adsorption process,the removal rate is still 99.7%at 24 h,and the concentration of Pb²⁺in the permeate is still far below the integrated wastewater discharge standard of China.（3）Preparation and characterization of phosphorylated polyacrylonitrile（PPAN）hollow fiber membrane and its adsorption of Hg²⁺.PAN hollow fiber membrane was aminated firstly and then phosphorylated to prepare PPAN hollow fiber membrane by Mannich reaction.The existence of phosphate groups in PPAN hollow fiber membrane was confirmed by FT-IR and XPS.SEM showed that the surface of the modified membrane became rough and the pore size became larger.Compared with APAN hollow fiber membrane,the pure water flux of the PPAN hollow fiber membrane was further improved,and the mechanical strength was stronger.Among the five metal ions of Cd²⁺,Hg²⁺,Ni²⁺,Pb²⁺and Cu²⁺,PPAN hollow fiber membrane exhibited highly selective adsorption for Hg²⁺.The adsorption kinetics and thermodynamics study showed that the PPAN hollow fiber membrane could capture Hg²⁺from aqueous solution quickly and reach the half of the saturated adsorption capacity within 5min.Furthermore,the adsorption process was controlled by chemical adsorption,and the maximum adsorption capacity was 372 mg/g.The adsorption effect was the best at p H=5.In addition,the adsorption capacity of Hg²⁺on PPAN hollow fiber membrane can reach 77%of the initial value after being reused for five times.In the dynamic adsorption experiment,the removal rate of Hg²⁺by PPAN membrane decreased slowly with the extension of time,and the removal rate was still 99.5%after 24 h.（4）The HPAN,APAN and PPAN hollow fiber membranes prepared in this paper have selective adsorption for Cd²⁺,Pb²⁺and Hg²⁺respectively.The results indicated that HPAN membrane possessed excellent regeneration performance and the optimal p H range was large.APAN membrane exhibited more lasting effect on dynamic removal of Pb²⁺.PPAN membrane had large pure water flux and adsorbed Hg²⁺rapidly.Therefore,HPAN,APAN and PPAN hollow fiber membranes have good selectivity,high adsorption capacity and excellent regeneration,which are expected to be used in the field of water treatment containing Cd²⁺,Pb²⁺,Hg²⁺ heavy metals pollution.