•Porous P84 co-polyimide anion exchange membranes are prepared by phase inversion, amination, quaternization.•The porous P84 membranes are used in BMED to produce LBA and NaOH from sodium lactobionic.•The porous P84 membranes are more efficient in BMED process than dense membranes.•The pore morphology and IECs influence the output of LBA significantly.Porous P84 co-polyimide anion exchange membranes are prepared by phase inversion process using isopropanol (IPA) or water as the non-solvent. Membrane area resistances are in the range of 0.7–2.2 Ω cm2, lower than those of commercial anion exchange membrane AMX (2.0–3.5 Ω cm2) and dense P84 membrane (>80 Ω cm2). The ion exchange capacities (IECs) are in the range of 0.6–0.9 mmol/g and the water uptake (WR) values are 100–160%, both of which are higher than those of dense P84 membrane.The porous P84 co-polyimide membranes are utilized in bipolar membrane electrodialysis (BMED) process to produce lactobionic acid (LBA) and NaOH from sodium lactobionic. The pore morphology and IECs influence the output of LBA significantly. Membrane with big tear-like pores shows higher WR value and can produce 0.0555–0.0565 mol/L LBA. Sponge-like pores with IEC of 0.9 mmol/g can yield 0.0724 mol/L LBA after 180 min running under 20 V, while sponge-like pores with IEC of 0.7 mmol/g yield reduced LBA concentration of 0.0593 mol/L, illustrating that higher IEC is in favour of the passage of LB−. The porous P84 membranes have improved efficiency as compared with commercial membrane AMX (0.0451 mol/L) and the dense P84 membrane (merely 0.0162 mol/L) in the production of LBA. Meanwhile, the Na+ leakage through the porous membrane is similar to that of the commercial membrane. Hence, the porous membranes, due to their low resistances, can have high potential for producing organic acid with high molecular weight.P84 co-polyimide anion exchange membranes with different porous structures are prepared via phase inversion, amination, and quaternization procedures. The porous P84 membranes has lower area resistances (0.7–2.2 Ω cm2) and the IEC values are in the range of 0.6–0.9 mmol/g. The membranes are utilized in bipolar membrane electrodialysis (BMED) process to produce lactobionic acid (LBA) and NaOH from sodium lactobionic. Compared with commonly dense ion exchange membranes, the membranes with porous structures may facilitate the migration of the substance by reducing the physical steric hindrance. Therefore the migration resistance of acid radical ions can be significantly decreased, and a higher output of acid and alkali are achieved with lower energy consumption, which is significant for the industrial application of BMED process.

•Polyelectrolyte complexes (PECs)/polyvinyl alcohol (PVA) membranes are pioneered for DD process of salts.•PECs/PVA membranes have improved stability in water and salt solution.•The dialysis coefficients of NaCl and NaAc are significantly higher than those of commercial membranes.•PECs/PVA membranes can recover salt from NaCl/threonine mixture and NaAc waste residue solution.Diffusion dialysis (DD) technique is nowadays mainly utilized for recovering acid or alkali solutions, but the recovery of salts such as sodium chloride (NaCl) and sodium acetate (NaAc) is much more difficult due to their low diffusivity. Polyelectrolyte complexes (PECs)/polyvinyl alcohol (PVA) membranes containing functional groups of COH, SiOH, N+(CH3)3 and SO3Na are pioneered for DD process of salts. The membranes have improved stability in water and salt solution. The dialysis coefficients of NaCl (UNaCl) are in the range of 0.00196–0.00290 m/h, which are 6–30 times higher than the values of commercial DF-9010 and CJMCDD-1 membranes. The dialysis coefficients of NaAc (UNaAc) are in the range of 0.00108–0.00503 m/h, which are 30–143 times higher than the value of CJMCDD-1 membrane, whereas DF-9010 membrane has poor stability in NaAc solution. Finally, PECs/PVA membranes are applied to NaCl/threonine (Thr) mixture and NaAc waste residue solution, showing their outstanding advantages for DD process of salts. The membranes show both high permeability and selectivity for NaCl/Thr mixture, with separation factor (SNaCl/Thr) as high as 6.495 (DF-9010: 1.25; CJMCDD-1: 0.224). The UNaAc values for NaAc waste residue solution are 0.000602–0.00120 m/h, higher than that of commercial CJMCDD-1 membrane.

•Porous SPES and SPSf cation exchange membranes are prepared by phase inversion.•The porous SPES and SPSf membranes are used in BMED to separate Glu and Lys.•The porous SPES and SPSf membranes are more efficient in BMED process than dense membranes.•The membrane pore morphology influences the concentration and purity of recovered Lys significantly.Porous sulfonated polyethersulfone and polysulfone (SPES and SPSf) cation exchange membranes were prepared by phase inversion process, in which water was used as the coagulation bath at 4 °C or 25 °C. Dense SPES and SPSf cation exchange membranes were also prepared for comparison. Characterizations of the membrane structure, morphology and physico-chemical properties showed that the dense membranes are hydrophobic with large area resistance (>80 Ω cm2), while the porous membranes are hydrophilic with low area resistance (0.35–18.62 Ω cm2). The porous SPSf membranes have lower area resistance (0.35–0.81 Ω cm2) than SPES membranes due to their higher water uptake (289.1–370.1%) and special morphology containing oval-shaped pores in the top skin layer.The membranes are utilized in bipolar membrane electrodialysis (BMED) process to separate the mixed amino acids of l-glutamic acid (Glu) and l-lysine (Lys). Though the dense membranes are unsuitable for electrodialysis experiments due to the large area resistance, the porous membranes have higher recovery ratio and current efficiency than the commercial membrane CMX, for the Lys+ can migrate through the porous membranes with lower hindrance. In addition, the SPSf membranes have higher Glu leakage ratio (13.9–15.3%) and lower energy consumption (4.82–6.52 kWh/kg) than the SPES membranes (∼0%, 6.43–9.53 kWh/kg). What’s more, the water permeation can be controlled by membrane materials and pore structures.Download high-res image (148KB)Download full-size image

•NaOH and NaAl(OH)4 are separated through ED.•The optimal membrane stack configuration is three repeating units configuration.•Acid cleaning can effectively eliminate the membrane fouling.•Relatively high alkali recovery ratio and low energy consumption can be achieved.•Self-prepared AM-QP-30 membrane performs better than commercial membrane FQB.Alumina alkaline solution containing NaOH and NaAl(OH)4 is separated by electrodialysis (ED), which uses self-prepared anion exchange membrane AM-QP-30. The stack arrangements and number of repeating units are optimized to achieve high alkali recovery ratio and low energy consumption. Meanwhile, repeated batch experiments (RBEs) are carried out to investigate the effect of erosion on membrane stability.The optimal membrane stack configuration is three repeating units configuration. The membrane fouling can be effectively eliminated by acid cleaning during the RBEs. The ED performances are stable and excellent. For instance, the alkali recovery ratio (ηOH-ηOH-) can reach up to 64.9–68.5% and the energy consumption can be reduced to 7.29–7.65 kW h/kg. The performance of AM-QP-30 membrane is superior to commercial FQB membrane in consideration of the lower mass loss ratio, stable ED performances and morphology after erosion. As a result, the ED process is feasible and stable to separate the alumina alkaline solution.

Most bauxite-produced alumina is obtained by the Bayer process, but the production efficiency is limited by the slow gibbsite crystal growth due to the high concentration of NaOH in sodium aluminate solution. Here electrodialysis (ED) and electro-electrodialysis (EED) are coupled to separate NaOH from the sodium aluminate solution so as to enhance the gibbsite crystal growth rate and achieve high alumina production efficiency. The ED or EED process is also investigated before the coupling process to find the optimal operating conditions. The ED process indicates that the optimized current density is in the range of 45–60 mA cm–2 and the optimal membranes are CMV/AMV. The current density of 60 mA cm–2 can achieve a high recovery ratio (ηOH– 92.6%), low energy consumption (2.38 kW h kg–1), but a relatively high Al(OH)4– leakage ratio (ηAl(OH)4– 15.1%). The EED process indicates that with the optimized current density of 30 mA cm–2 and membrane CMV, the ηAl(OH)4– can be “zero” and the energy consumption can be as low as 2.07 kW h kg–1, but the treatment capability is low since OH– ions cannot be recovered directly and a single cation exchange membrane is used. The coupling process can combine the advantages of ED and EED, so that the ηOH– can keep a high value of 90.9%, the ηAl(OH)4– decreases to a low value of ∼5%, and the energy consumption remains low at 2.25 kW h kg–1. Overall, the coupling process of ED and EED is an excellent method to separate NaOH from the sodium aluminate solution.

•Sodium acetate (CH3COONa) waste residue is treated by BMED.•The optimal current density is 50 mA/cm2.•The optimal membrane stack configuration is C-BP-A-C stack.•Addition of ion exchange resin decreases energy consumption to 22.3 kW h/kg for CH3COOH.•A high output of CH3COOH (0.491 mol/L) and NaOH (0.556 mol/L) can be achieved.Large amounts of solid waste residue containing high salinity and a series of organic impurities are produced during the manufacturing process of dithianon in insecticide factories, posing a severe environmental pollution problem. Here sodium acetate (CH3COONa) waste residue is firstly tested by diffusion dialysis (DD) to investigate the permeability of different commercial ion exchange membranes. Selemion CMV and AMV membranes show low permeability to the organic impurities of the waste residue, and hence are chosen for the following bipolar membrane electrodialysis (BMED) experiments to regenerate acid and base.The BMED firstly uses a BP-A-C-BP configuration to select an optimized current density of ∼50 mA/cm2. Afterwards, four membrane stack configurations are compared including BP-A-C-BP, C-BP-A-C, BP-C-BP and BP-A-BP. In consideration of the high current efficiency, low energy consumption, and high concentrations of the produced acid/base, C-BP-A-C is a favorable configuration. Finally, the addition of a strong acid 001 ∗ 7 type of cation-exchange resin into acid compartment can substantially decrease the voltage drop across the stack and reduce the energy consumption. The C-BP-A-C configuration at 50 mA/cm2 can have a high output (0.491 mol/L CH3COOH and 0.556 mol/L NaOH), high current efficiency (87.7% for CH3COOH and 99.0% for NaOH), and a relatively low energy consumption (22.3 kW h/kg CH3COOH and 29.7 kW h/kg NaOH). Overall, this study illustrates the practical significance of BMED process for treating sodium acetate waste residue, including reducing soil and water pollution and producing valuable products in insecticide industries.

•PVA- or SPPO-based hybrid membranes containing UHPS are prepared.•UHPS is proposed for the first time as alkali transport promoter in hybrid membranes.•UOH are 0.01–0.022 m/h, about 2–5 times higher than blank membranes.•The “trade-off” effect between ions permeability and selectivity is broken.Alkali transport promoter is designed from 3-(ureidoarene)propyltriethoxysilane (UPTS), which takes sol–gel process to form ureidoarene–heteropolysiloxane (UHPS) within polyvinyl alcohol (PVA). Hence PVA-based hybrid membranes are obtained for diffusion dialysis (DD) application. Besides, sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO) is used as reference to compare the formation of alkali transport promoter within different polymer matrix.The micro-aggregation of UHPS within PVA leads to loose structure, while within SPPO leads to plenty of clusters within 0.1–0.5 μm. The membrane swelling resistance is improved but the alkaline stability is reduced in the presence of UHPS. The PVA-based membranes have the tensile strength (TS) of 19–52 MPa, and the elongation at break (Eb) of 343–534%. DD results show that the dialysis coefficients of OH− (UOH) are in the range of 0.01–0.022 m/h in the presence of UHPS, which are about 2–5 times higher than the blank PVA or SPPO membranes. The separation factors (S) are in the range of 26–32, which are also higher than the blank membranes. Hence, UHPS can act as alkali transport promoter for DD process. The effect of UHPS is affected by the membrane matrix and micro-aggregation degree.

•Poly(γ-MPS) is developed to prepare non-charged PVA–SiO2 hybrid membrane.•The hybrid membranes have high mechanical property (35–44 MPa, 141–318%).•Membrane structure is damaged according to different mechanisms.•Non-charged membranes can be used in diffusion dialysis (0.0069–0.0127 m/h).•The OH− ions are transferred through PVA-OH groups and membrane interstices.Non-charged PVA–SiO2 hybrid membranes are prepared from the sol–gel reaction of polyvinyl alcohol (PVA) and methacryloxypropyl trimethoxy silane (γ-MPS) or poly(γ-MPS). As the polymerization degree increases from γ-MPS to poly(γ-MPS), the sol–gel solution becomes turbid or generates precipitation, while the formed silica particles in the hybrid membranes become smaller from 1.0–3.4 μm to 0.1–0.2 μm or disappear. The membrane tensile strength (TS) generally increases from 35 MPa to 44 MPa, while the elongation at break (Eb) decreases from 318% to 141%.Three deductions are proposed according to the structure of poly(γ-MPS), membrane swelling behaviors in 65–85 °C water and 65 °C alkaline solution, and the diffusion dialysis (DD) performance. (1) As poly(γ-MPS) contains no ion-exchange groups like the multisilicon copolymer [1], it tends to become inhomogeneous during sol–gel process, which reduces its crosslinking ability. (2) The membrane structure is generally stable in 65 °C water with the swelling degrees of 140–347%, but is damaged in 85 °C water or 65 °C alkaline solution. (3) Non-charged hybrid membranes can be potentially applied in DD process. The dialysis coefficient of NaOH (UOH) is in the range of 0.0069–0.0127 m/h, and the separation factor (S) is in the range of 19–92. The OH− ions may be transferred in the hybrid membranes through PVA-OH groups and interstices between organic and inorganic phases, according to the mechanisms of adsorption–desorption and free diffusion, correspondingly.

Multisilicon copolymers are put forward as new types of crosslinking agents for preparing ion exchange hybrid membranes. Ternary multisilicon copolymer is prepared from copolymerization of maleic anhydride (MA), sodium styrene sulfonate (SSS) and γ-methacryloxypropyl trimethoxy silane (γ-MPS). The obtained copolymer is then crosslinked with polyvinyl alcohol (PVA) to yield cation exchange hybrid membranes. The membranes contain multifunctional groups including SO3Na, COOH and SiOH from the multisilicon copolymer and COH from PVA, which can form crosslinking structure through SiOSi, SiOC and (CO)OC bonding.The membranes have the tensile strength (TS) of 25.6–32.1 MPa and elongation at break (Eb) of 68–161%. The swelling resistance at room temperature or 65 °C increases with the dosage of multisilicon copolymer. The membranes are used in diffusion dialysis (DD) process for potential application in alkali recovery. The dialysis coefficients of OH− (UOH) are in the range of 0.011–0.019 m/h at 20 °C and 0.021–0.031 m/h at 40 °C, higher than those of commercial sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO) membrane (0.002 m/h at 25 °C) or other PVA-based hybrid membranes. The DD performances cannot be well explained by the ion transport mechanism based on “three phase” membrane structure, or by the change of water uptake (WR), ion exchange capacity (IEC) and fixed group concentration (CR). The uniqueness of the membrane structure, as well as the presence of multifunctional groups has to be taken into account, which gives some insight as to future DD membrane design objectives.Highlights► Ternary multisilicon copolymer poly(MA-co-SSS-co-γ-MPS) is prepared. ► Membranes with multifunctional groups (SO3Na, COOH and OH) were prepared. ► Membrane performances for alkali recovery are excellent (UOH = 0.011–0.019 m/h). ► DD performances cannot be well explained by “three phase” membrane structure. ► DD cannot be well explained by WR, IEC and fixed group concentration (CR).

One multisilicon copolymer, i.e. a copolymer containing pendent siloxane and anhydride groups is prepared from the copolymerization of maleic anhydride (MA) and methacryloxypropyl trimethoxy silane (γ-MPS). Thereafter, it is used for the preparation of hybrid membranes through in situ sol–gel process in poly(vinyl alcohol) (PVA) solution. The anhydride groups in MA are less hydrophilic as compared with acidic carboxylic acid (–COOH) groups. Hence the copolymerization process can be more easily controlled without formation of gel. During the sol–gel process, the anhydride groups hydrolyze to generate geminal –COOH groups, which provide the membranes with relatively high cation exchange capacities (CECs) that can be applied for the diffusion dialysis process to recover NaOH from the mixture of NaOH/Na2WO4.The membranes have the initial decomposition temperature (IDT) of 227–243 °C, tensile strength (TS) of 14.2–28.3 MPa and elongation at break (Eb) of 18.8–67.3%. In addition, water swelling decreases as the content of multisilicon copolymer increases despite the increasing of the IEC values, which is attributed to the gradually strengthened crosslinking between PVA and the copolymer components. At 20 °C, UOH (the dialysis coefficients of OH− ions) are in the range of 0.0095–0.0123 m/h, and the separation factor (S) values are in the range of 28.4–54.4. The UOH values are significantly higher than the values of the commercial membrane (0.00137 m/h) and other polymer based hybrid membranes (0.0014–0.0022 m/h). This is due to the unique membrane structure and the presence of different functional groups. PVA provides hydrophilic matrix and hence the membranes cannot be classified into the common “three-phase” structure membranes. The presence of –OH groups assists and enhances the transport of OH− ions.Highlights► Multisilicon copolymers poly(MA-co-γ-MPS) with bionic structure are prepared. ► Cation exchange hybrid membranes were prepared therefrom. ► Membrane performances for alkali recovery are excellent. ► The –OH and –SiOH groups can enhance the transport of OH− ions. ► Adjacent –COOH groups may increase ionic domain interconnectivity to accelerate Na+ transport.

Cation exchange multisilicon copolymers i.e. copolymers containing pendent siloxane and SO3Na groups are developed from the copolymerization of sodium styrene sulfonate (SSS) and γ-methacryloxypropyl trimethoxy silane (γ-MPS). The copolymerization process can proceed smoothly without formation of gel. The copolymers are compatible with water, and can produce transparent and homogeneous solutions when mixed with poly(vinyl alcohol) (PVA), so that transparent and compact hybrid membranes are obtained.The membranes are thermally and mechanically stable, with initial decomposition temperatures of 237–273 °C, tensile strength of 9.1–26.0 MPa and elongation at break of 12.4–21.1%. The membranes have a water uptake (WR) of 30.0–54.8% and swelling degrees of 46–115% in 65 °C water, which are mainly controlled by the content of Si(OCH3)3 groups in the mutlisilicon copolymers. Diffusion dialysis (DD) separation of NaOH/Na2WO4 solution indicates that the alkali dialysis coefficients (UOH) are in the range of 0.010–0.011 m/h, higher than the values of commercial membrane (0.00137 m/h) and other polymer based hybrid membranes (0.0014–0.0022 m/h). The membrane structure and properties are correlated with the ions transport mechanism, revealing interesting findings as to the influence of IEC and different functional groups on DD behavior.Highlights► Multisilicon copolymers poly(SSS-co-γ-MPS) with bionic structure are prepared. ► Cation exchange hybrid membranes were prepared therefrom. ► Membrane performances for alkali recovery are excellent with UOH values of 0.0102–0.0111 m/h. ► The OH and SiOH groups can enhance the transport of OH− ions.