Preparation And Performance Of PVA-based Hybrid Cation Exchange Membranes For Alkali Recovery

Inorganic-organic hybrid membrane is proposed and attracted much more attentions recently as result of their extraordinary properties arising from the synergism between the properties of two different building blocks. Diffusion dialysis (DD), as a membrane separation technology based concentration gradient, has received great attention in dealing with alkali waste due to its own advantages such as environment-friendly, no energy consumption and so on. Nevertheless, due to the lack of membranes that possess high permeability, there are still few reports on alkali recovery through DD process. Hence, Cation exchange membranes used for alkali recovery, requiring high OH" permeability, strong salts rejection, low water permeability as well as outstanding thermal/chemical stability, was prepared in this paper. In view of the synthetical properties between organics and inorganics, cation exchange hybrid membranes for DD process were prepared from polyvinyl alcohol (PVA) and multisilicon copolymer as well as functional silanol by Sol-gel process. The prepared membranes were evaluated by diffusion dialysis (DD) for alkali recovery, and the relations between structure and properties were discussed. The main conclusions based on above researches and results are as follows:(1) PVA/SiO2cation exchange membranes are prepared from PVA and multi-alkoxy silicon copolymer with the variation of-COOH content. As the content of-COOH content increases, the water content (WR) increases from61.2%to81.7%, and the ion exchange capacity (IEC) increases from1.69mmol/g to1.90mmol/g. The membranes have the initial decomposition temperature (IDT) of236-274℃, tensile strength (TS) of17.4-44.4MPa. The dimensional stability in length (DS-length) is in the range of10%-25%, and the DS-area is in the range of21%-56%in65℃water. Diffusion dialysis (DD) is conducted at changing temperature with a mixture of NaOH and Na2WO4. The membranes exhibit significantly higher diffusivity to OH-ions (UOH:0.006-0.032m/h) than SPPO-based membranes and our previous PVA/SiO2membranes. Improvement in diffusivity has been correlated with hydrophilic-OH groups on PVA chains and consecutive-COOH groups on copolymer chains in our PVA/SiO2membranes. Meanwhile, the selectivity of membranes can also remain at an acceptable value (12.2-36.2). The effect of the inter-phase separation on ions transport can explain the results. Evaluated from the static performance and the properties, it can be concluded that the membranes are qualified for alkali recovery by DD.(2) PVA/SiO2cation exchange membranes are prepared from PVA and poly(SAS-co-VTMS). As copolymer content increases, the water content (WR) increases from84%to106%, and the ion exchange capacities increase from0.14mmol/g to0.45mmol/g. The prepared membranes have the initial decomposition temperature (IDT) of225-233℃, tensile strength (TS) of23.9-44.0MPa. The Linear expansion (LER) is in the range of17.7-18.3%, and weight loss percent is in the range of4.3-5.5%in1M NaOH for30days, revealing membranes have good alkali resistance. Diffusion dialysis (DD) is conducted with NaOH/Na2WO4and NaOH/NaAlO2system. The membranes exhibit higher diffusivity to OH-ions (NaOH/Na2WO4system, UOH:0.0071-0.0080m/h; NaOH/NaAlO2system, UOH:0.0065-0.0086m/h) than SPPO based membranes and our previous SPPO/SiO2membranes. Meanwhile, the selectivity of membranes can also remain at an acceptable value (NaOH/Na2WO4system, S:24.4-51.6; NaOH/NaAlO2system, S:13.9-9.6). Evaluated from the static performance and the properties in two kinds of alkali wastewaters from metallurgical industries, it can be concluded that the membranes are qualified for alkali recovery by DD.(3) PVA-based cation exchange hybrid membranes are prepared from a simple and green method, during which PVA and THOPS-Na are directly taken sol-gel process in aqueous media but without organic solvent. As the content of THOPS-Na increases, the ion exchange capacity (IEC) increases from0.70mmol/g to1.56mmol/g, the water uptake is in the range of38-86%. The membranes have the initial decomposition temperature (IDT) of224-231℃, tensile strength (TS) of13.3-40.7MPa. The linear expansion (LER) range is15-25%in water and18-27%in1.0M alkali solution. Diffusion dialysis (DD) is conducted with NaOH/Na2WO4system. The membranes exhibit higher diffusivity to OH-ions than previously membranes. Meanwhile, the selectivity can also remain at an acceptable value (11.6-20.2). Hence, the membranes are qualified for alkali recovery by DD.