PVA Based Strong Acid Hybrid Membrane Applied In Diffusion Dialysis For Alkali Recovery

Alkali liquids are generated widely in paper,leather,printing and dying as well as tungsten and aluminum ore smelting industries.Among the various separation techniques,membrane diffusion dialysis?DD?uses a cation exchange membrane to divide the alkaline liquid and water.The alkali component?NaOH?,driven by the concentration gradient,is transported through the membrane and into the water compartment.Hence,the NaOH component is separated and recovered.The DD process is green environmental protection and energy saving.However,the main concern is the lack of stable and high permeable cation exchange membrane.Hence,the main purpose of this paper is to develop chemical stable cation exchange membranes with high selectivity and NaOH permeability.The membranes are prepared through three aspects.Firstly,we prepare zwitterionic multisilicon copolymers,which are then cross-linked with polyvinyl alcohol?PVA?to form zwitterionic hybrid membranes;Subsequently,small monomers are copolymerized to form multisilicon copolymers,whose ratios are adjusted to cross-link with PVA and to form cation exchange membranes;Finally,green technologies are developed in two steps to prepare cation exchange hybrid membranes with low cost and high performances.These cation exchange membranes are all used in DD process to recover alkali,whose performances are affected by the membrane cross-linking degree and functional groups.The main contents can be summarized as following:?1?Two monomers including 2-acrylamide-2-methyl-l-propanesulfonic acid?AMPS?and y-methacryloxy propyl trimethoxyl silane?y-MPS?are copolymerized to obtain zwitterionic multisilicon copolymer.The copolymer contains-SO3H,-NH-and-Si?OCH₃?₃ groups.The-Si?OCH₃?₃ groups are then taken sol-gel reaction with PVA to form zwitterionic hybrid membranes.The membrane has proper water uptake and thermal stability.The ion exchange capacities?IECs?are in the range of 0.90-2.15 mmol/g.The tensile strength can reach up to 51.75 MPa,indicating the multisilicon copolymer can elevate membrane mechanical stability.The swelling degrees are in the range of 108%-154%after immersed in 25°C 2.0 mol/L NaOH solution for 60 h.The DD performances for NaOH/Na2WO₄ solution indicate that the dialysis coefficients of NaOH?UOH?are in the range of 7.9-15.1 mm/h,which are higher than the reported PVA based hybrid membranes?7.0-13.0 mm/h?.The strong acidic-SO3H groups are beneficial to the transport of Na+ ions;while the-NH-and-OH groups can enhance the membrane homogeneity and hydrophilicity,and can accelerate the transport of OH-ions through hydrogen bonds.?2?Small monomers,including vinyltrimethoxysilane?VTMS?and sodium methacrylate?SMS?,are copolymerized to obtain multisilicon copolymers with high density of functional groups.The copolymers are then taken sol-gel reaction with PVA to form cation exchange hybrid membranes.The membranes have the IECs of 0.28-1.47 mmol/g.The tensile strength can reach up to 13.7 MPa,the initial decomposition temperatures are in the range of 223-228 ?,and the water uptake?WR?is in the range of 42%-67%.The swelling degrees are in the range of 128%-249%after immersed in alkaline solution for 60h at 65 ?.The DD process for NaOH/Na2WO₄ solution shows that the separation factors are in the range of 23.2-40.5 and the UOH values are in the range of 9.3-14.2 mm/h,which are higher than those of the reported PVA hybrid membranes?7.1-8.0 mm/h?.?3?The PVA and chitosan?CS?are used as membrane matrix,which is blended with sodium 2-formylbenzene sulfonate?Ph-S?and 3-mercaptopropyltriethoxysilane?MPTES?via sol-gel reaction.The formed film containing-SH group is then oxidized by Fenton regent to obtain PVA/CS based cation exchange hybrid membranes.The tensile strength is in the range of 13-56 MPa,the elongation at break?Eb?is in the range of 81%-328%.The initial decomposition temperature is in the range of 195-210 ?,and the swelling degree is in the range of 265%-375%after immersion in alkaline solution for 60 h.The low alkaline resistance may be attributed to the insufficient cross-linking of-OH groups within PVA and CS.The DD process for NaOH/Na2WO₄ solution shows that the UOH values are in the range of 7.3-13.1 mm/h with SOH/WO₄ of 9.2-15.3.The acid-base pairs were formed between the-NH2 of CS and the-SO3H of Ph-S,which can also form hydrogen bonds with-OH groups to accelerate the transport of OH' ions.?4?The above membrane preparation process is improved in this section to elevate membrane alkaline resistance.Hybrid membranes are prepared from PVA,MPTES and benzaldehyde disulphonate?BADSANa?through sol-gel reaction.The membranes are oxidized by Fenton reagent or ozone.The membranes oxidized by Fenton reagent have the initial decomposition temperatures of 165-275 ? and the tensile strength of 5.8-30.9 MPa.The swelling degree is in the range of 97%-157%after immersed in alkaline solution for 60 h,indicating that the membranes has improved alkaline resistance with similar thermal stability and mechanical properties.The DD process for NaOH/NaAlO₂ solution shows that the membranes oxidized by ozone have the UOH of 5.0-27.5 mm/h and the SOH/AlO₂ of 10.0-18.2,which are attributed to their high IEC values?1.05-1.39 mmo/g?,The membranes oxidized by Fenton reagent have lower IEC values of 1.01-1.36 mmo/g.The UOH values are in the range of 2.8-23.4 mm/h with the SOH/AlO₂ of 4.6-20.0.This shows that ozone can be used as a strong and green oxidant for membranes.We propose that the-OH groups of PVA can not only react with-CHO groups through aldol condensation reaction,but also can crosslink with Si-OH groups to form Si-O-Si network.Hence,the double cross-linked structure enhances membrane alkali resistance.Besides,the ozone is a strong oxidant for membrane preparation due to its milder and continuous operation.