| Membrane technology could process with lower temperature,effectively retain flavor substances and nutrients while removing microorganisms and improve the clarity,so it is a great method for the clarification and sterilization of fruit juice.However,the membranes used in current separation studies are expensive and with complicated producing method.Therefore,selecting appropriate membrane materials and production methods to prepare separation membranes with appropriate aperture,excellent performance and low cost is very important.Polyvinyl alcohol（PVA）is considered as an ideal material for membrane preparation because it is soluble in non-toxic solvents,has low oral toxicity,excellent membrane forming properties and fouling resistance.However,due to the high hydrophilicity of polyvinyl alcohol,it is easy to swelling or even dissolving in water,and the pure water flux will decrease over time during long-term,which limits its application in the field of separation membrane.Sodium silicate,as an inorganic silicon source material with a wide range of raw materials and low price,has high reactivity and is suitable for preparing organic-inorganic hybrid membrane.In this paper,the glutaraldehyde modified polyvinyl alcohol/sodium silicate composite membrane was prepared by sol-gel method and non-solvent-induced phase separation（NIPS）method with polyvinyl alcohol and sodium silicate as raw materials.The membrane forming mechanism,filtration performance,mechanical properties and water resistance were studied.The pore size of the composite membranes prepared by the two methods reached the microfiltration level（less than 220nm）,which was successfully applied to the filtration and sterilization of fruit juice.The main research work and conclusions are as follows:（1）Polyvinyl alcohol/sodium silicate composite microfiltration membrane was successfully prepared by sol-gel method.Infrared spectrum analysis showed that a homogeneous organic-inorganic hybrid structure was formed between polyvinyl alcohol and sodium silicate through covalent bonding.When the mass fraction of sodium silicate was 9.0%,the highest pure water flux of polyvinyl alcohol/sodium silicate composite membrane was 789.12±35.71 L/（m2·h）,the rejection rate of nanometer color paste was 87.06±1.59%,and the rejection pore size of the membrane was less than 220 nm.The effects of glutaraldehyde blending modification and soaking modification on the composite membrane were further compared.The glutaraldehyde could react with polyvinyl alcohol in both methods,which significantly improved the water resistance of the composite membrane.Blending modification could significantly improve the mechanical properties of the membrane,but would increase the pore size of the membrane,the pure water flux was 1413.96±49.74 L/（m2·h）,the rejection rate of the nanometer color paste was 58.87±2.10%,and the rejection pore size was bigger than 255 nm.The polyvinyl alcohol/sodium silicate composite membrane modified by0.30%glutaraldehyde soaking method（PVA/SS-1）significantly increased rejection rate and water resistance without significant change in pure water flux.The membrane morphology could still not swell in 90℃hot water.For PVA/SS-1,the pure water flux was 747.97±74.64 L/（m2·h）,the rejection rate of nanometer color paste was 90.20±4.85%,and the rejection pore size was 164-190 nm.（2）With polyvinyl alcohol and sodium silicate as raw materials,water as cosolvent,Na2SO4/H2SO4/H2O solution as non-solvent coagulation bath,the glutaraldehyde modified polyvinyl alcohol/sodium silicate composite microfiltration membrane was successfully prepared by non-solvent-induced phase separation combined with glutaraldehyde soaking modification.The pore-forming mechanism,as well as the effects of polyvinyl alcohol concentration,sodium silicate concentration and coagulating bath temperature on the structure and properties of the membrane were analyzed.In the process of membrane formation,sodium silicate,as an inorganic membrane material,also plays a non-solvent additive and reaction induction effect.Its interaction with sulfuric acid in the coagulation bath is the key to prompt instantaneous phase separation,surface pores formation and formation of bicontinuous structure in the membrane forming process.When the concentration of polyvinyl alcohol was 6.0%,the concentration of sodium silicate was 6.0%,and solidification bath temperature was20℃,the prepared glutaraldehyde modified polyvinyl alcohol/sodium silicate composite membrane（PVA/SS-2）had the typical asymmetric structure and the best performance,with the highest surface porosity of 2.330%and tensile strength of 0.63±0.06 MPa.The elongation break was 56.04±5.02%,the pure water flux was 641.30±61.15 L/（m2·h）,the rejection rate of SPI was 65.61±1.67%,the rejection rate of nanometer color paste was 99.16±0.19%,and the rejection pore size of the membrane was 122-124nm.（3）The results showed that PVA/SS-1 and PVA/SS-2 could effectively remove microorganisms and prolong the shelf life of grape juice.After treatment,the total number of bacteria and the number of mold and yeast in the grape juice were lower than the detection level,and the grape juice could be stored at 25℃for more than 28 days.In the filtration process of grape juice,the stable fluxes of PVA/SS-1 and PVA/SS-2 were25.41 L/（m2·h）and 10.68 L/（m2·h）,respectively.The stable fluxes of PVA/SS-1 were better than those of 220nm MCE membrane and PES membrane.The analysis of membrane fouling mechanism showed that the cake filtration mechanism was dominant,indicating that they had good anti-fouling performance.In addition,PVA/SS-1 and PVA/SS-2 filtrated grape juice had better clarification and higher protein removal rate than commercial filtration membrane,thus more stable grape juice could be obtained.However,PVA/SS-2 has smaller pore size and lower retention rate for active components such as anthocyanins,flavonoids and total phenols.The retention rate of active components of PVA/SS-1 is more than 90%,which is similar to that of commercial filtration membrane.Therefore,the glutaraldehyde modified polyvinyl alcohol/sodium silicate composite membrane prepared by sol-gel method is more suitable for grape juice filtration and sterilization. |
PDF Full Text Request