| High-concentration chlorinated heavy metal wastewater has high toxicity,long-term continuity,and large fluctuations in water quality and volume.Large amounts of emissions not only seriously affect the ecological environment but also endanger human health.When using traditional electrolysis to recover heavy metals in chlorine-containing wastewater,a large amount of Cl2is generated at the anode,causing serious pollution to the environment.In order to avoid the pollution caused by Cl2,lye is used for absorption,resulting in a large waste of resources.If the"double membrane three-chamber"process is applied to the electrolysis process of high-concentration chlorinated heavy metal wastewater,not only the heavy metals can be recovered,but also HCl can be recovered in the middle compartment,which not only inhibits the production of Cl2,but also recycles the recovered HCl.The cation exchange membrane is a key component of the"double membrane three chamber"process.Due to the problems of poor oxidation stability and low selective permeability of the commercially available polystyrene commercial cation exchange membranes with good economic performance,they have a short service life in practical applications,and frequent membrane replacement causes economic losses.In response to this problem,the paper proposes to prepare a cation exchange membrane(CEM)with Si-O-Si bonds,aiming to prepare a cation exchange membrane with good oxidation stability and high selective permeability through the sol-gel method.Through basic performance(water absorption,ion exchange capacity,burst resistance,migration number,film thickness,selective permeability),scanning electron microscope(Scanning electron microscope,SEM),Fourier transform infrared(Fourier transform infrared,FTIR))and electrochemical performance(membrane potential and membrane area resistance)characterization to evaluate membrane performance.In the experiment,6wt%of polyvinyl alcohol(Polyvinyl Alcohol,PVA)based PVA/SiO2-6membrane,2.0wt%of benzenesulfonate betaine([2-(Methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide(SBMA)grafted PVA/SiO2/SBMA-6-2.0membrane,polyvinylidene fluoride(PVDF)based PVDF/SiO2-6 membrane and commercial styrene HH-1 membrane are used in the"double membrane three compartment"process is used for the electrolysis of high-concentration Co Cl2solutions.Finally,a model for the migration of cations through the cation exchange membrane was established,and the experimental data and results were verified using cyclic voltammetry and chronopotential curves.The main research contents and conclusions are as follows:(1)The PVA/SiO2membrane preparation mechanism was determined,and the PVA/SiO2membrane was prepared through the"sol-gel(?)base film(?)crosslinking(?)oxidation"technical route,and different PVA mass fractions(2%,4%,6%,8%,10%)on the film performance,the results show that when the mass fraction of PVA is 6%,the performance of PVA/SiO2-6 membrane is the best.Compared with the commercial HH-1 membrane,the selective permeability is 7.21%higher,the weight loss of oxidation at 40°C for 12h is 5.19%higher,and the membrane area resistance is reduced by 11.2?·cm2.It shows that the PVA/SiO2-6 membrane has low energy consumption during the application process,but the oxidation stability is still poor.(2)In view of the problems of PVA/SiO2-6 membrane,the membrane was modified with doped nano-Zr O2,infiltrated conductive polyaniline(PAn),plasma grafted SBMA,and the quality of different modified substances was studied.The results show that the three modification methods can improve the performance of the membrane,but the most obvious improvement effect on the oxidation stability is plasma grafting of SBMA,followed by infiltration conductive PAn,and finally doped with nano-Zr O2,indicating that SBMA has been successfully grafted onto the surface of PVA/SiO2-6 membrane.When the mass fraction of SBMA is 2.0%,the performance of the PVA/SiO2/SBMA-6-2.0 membrane is the best.Compared with the PVA/SiO2-6 membrane,the selective permeability is increased by 14.14%,and the film area resistance is reduced by 0.4?·cm2,the weight loss of oxidation at 40?for12h decreased by 0.62%,and the weight loss of oxidation at 80?for 12h was completely dissolved from the unmodified product to 71.19%.Compared with the commercial HH-1membrane,the selective permeability is 21.35%higher,the membrane area resistance is reduced by 11.6?·cm2,the weight loss is increased by 4.57%after oxidation at 40?for 12hours,and the weight loss is increased by 10.02%by oxidation at 80?for 12 hours.Although the oxidation stability has not yet achieved the desired effect,the selective permeability has been significantly improved,which increases the barrier rate of chloride ions to a certain extent,thereby preventing the cation exchange membrane from being damaged.(3)The PVDF/SiO2membrane preparation mechanism was determined,and the PVDF/SiO2membrane was prepared.The performance of the membrane under different mass ratios of tetraethyl orthosilicate(TEOS)and 3-mercaptopropyl triethoxysilane(MPTES)was studied.The study showed that when n TEOS:n MPTES=1:6,the performance of the prepared PVDF/SiO2-6 membrane is the best.The prepared membrane has a slightly higher selective permeability than the commercial HH-1 membrane,but its oxidation stability at high temperatures is significantly better than that of commercial membranes and other PVA-based membranes.(4)A five-step migration model of cations through the cation exchange membrane was established:1)pre-dehydration of ions in the liquid membrane;2)counter ion entry and repulsion of the same ions;3)migration in the membrane;4)secondary adsorption;5)hydration in the liquid membrane ions,and the model was verified by cyclic voltammetry.The results show that the five steps of ion passing through the cation exchange membrane correspond to the five parts of the cyclic voltammetry curve,and the size of the window corresponding to the cyclic voltammetry curve can reflect the level of selective permeability of the ion exchange membrane.The narrower the cyclic voltammetry window,the higher the selective permeability of the membrane.The order of the cyclic voltammetry window from narrow to wide is:PVA/SiO2/SBMA-6-2.0,PVA/SiO2-6,PVDF/SiO2-6,HH-1.It is consistent with the order of membrane selective permeability from high to low.(5)The chronopotential curve can characterize part of the membrane performance.Through the analysis of the chronopotential curve,the order of the steady-state voltage value of the chronopotential curve of different films from high to low is:HH-1,PVDF/SiO2-6,PVA/SiO2-6 and PVDF/SiO2/SBMA-6-2.0 membrane,it is consistent with the area resistance of the membrane.Moreover,the non-zero time interval corresponding to the curve shape and the derivative curve of the curve can reflect the homogeneity of the membrane,the distribution of functional groups and current lines,indicating that the chronopotential curve can be used to evaluate the performance of ion exchange membranes.(6)PVA/SiO2/SBMA-6-2.0,PVA/SiO2-6,PVDF/SiO2-6 and HH-1 membranes are used in the"dual-membrane and three-chamber"cobalt recovery process.In the current density range of 190A/m2?220A/m2,the cell voltage,energy consumption and Cl-leakage rate of the three prepared membranes are all lower than the commercial HH-1 membrane.From the perspective of Cl-leakage rate,when the current density exceeds 210A/m2,the PVDF/SiO2-6film has good oxidation stability and small damage.The Cl-leakage rate has a small increase,while the other three membranes have poor oxidation stability,resulting in a large increase in the Cl-leakage rate.Therefore,it is further proved that PVDF/SiO2-6 membrane may be the most suitable for the"dual-membrane and three-chamber"high-concentration cobalt chloride electrolytic recovery process. |
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