Study On The Preparation And Separation Performance Of Regenerated Silk Fibroin-based Composite Membrane

With the increasingly serious water pollution and the exhaustion of water resources,the demands for the advanced treatment and recycling of wastewater have increased.Membrane technology is widely applied in the process of water treatment,and it is referred to as a new generation of water treatment technology because of its prominent advantages,such as high efficiency,low energy consumption,flexible operability,and free of secondary pollution.The rapid development and widespread application of membrane technology makes it significant for researchers to pay attention to the investigation of new membrane materials.Most of the available membranes are made from synthetic polymers.However,large amounts of petrochemical resources are usually consumed in the membrane preparation process,and environmental pollution concerns are inevitably caused.Therefore,the development of renewable natural membrane materials has aroused general interests.In the present paper,a novel type of porous membrane was prepared using Bombyx mori silk fibroin as the raw material.The membrane properties and the mechanism of membrane formation were discussed,and the separation performance of the membrane for various water contaminants was explored.In the process of silk textile products,a large amount of waste silk and silkworm cocoon are produced,underutilized and deserted.The recycling and reusing of these waste silkworm cocoons is of huge economic efficiency.In the present paper,silk fibroin was extracted from Bombyx mori cocoons via a "regeneration" route,and regenerated silk fibroin solution was prepared.After introducing tannic acid in the solution,a novel type of regenerated silk fibroin/tannic acid composite nanosphere was produced.The influence of preparation conditions was explored;the results showed that when the concentration of regenerated silk fibroin solution was 0.01 wt%,and the concentration of tannic acid was in the range of 0.05 wt%to 1.00wt%,silk fibroin composite nanospheres could be generated,and the average and diameters of the nanospheres increased from 35 nm to 139 nm.The interaction mechanism between tannic acid molecule and the silk fibroin chain was analyzed by performing synchronous fluorescence characterization.The production process of the nanospheres could be described as:silk fibroin molecule is naturally designed to be a segmented copolymer with both hydrophilic and hydrophobic domains.The silk fibroin molecular chain tends to form silk fibroin micelle under the self-assembly interaction of the hydrophobic domains in the chain.After introducing tannic acid,the tannic acid molecules interact with the silk fibroin chain via hydrophobic interactions and hydrogen bonding interactions,leading to the formation of composite nanospheres.Then the obtained nanospheres were utilized to fabricate regenerated silk fibroin/tannic acid composite membranes via a vacuum filtration process.As-prepared composite membranes have asymmetric structures.The active separating layer of the membrane is composed of closely packed nanospheres,and plenty of nanopores exist among the packed nanospheres.The thickness of the separating layer can be controlled flexibly by adjusting the conditions in the membrane fabrication process.Therefore,both the ultrathin composite membrane with a 49 nm thick separating layer and free-standing membranes(thicknesses greater than 1 ?m)with enough mechanical strength can be obtained.The permeability,rejection performance for various particles,pH-sensitivity,and selective separation performance of the membranes were systematically explored.The permeability of the composite membranes can be greatly enhanced through decreasing the thicknesses of the active separating layers,thus leading to the ultra-fast separation of solutes.The 49 nm thick membrane,for instance,has a pure water flux of up to 13888 L m-2 h-1 bar-1,which is almost three orders of magnitude higher than most commercial filtration membranes.The separation efficiency of the membrane mainly depends on the size and the charge characteristic of the solutes.The membrane exhibits a rejection rate of only 4.0%for L-phenylalanine,while 99.9%for bovine serum albumin;and it exhibits a rejection rate of 99.0%for Rhodamine B(positively charged),while only 49.5%for Metanil yellow(negatively charged).In addition,the experimental results indicate that the membrane surfaces have pH-responsive properties.The flow channels in the membrane experienced constriction(at low pH)due to electrostatic shielding of the negatively charged nanospheres and expansion(at high pH)due to electrostatic repulsion between the nanosphere units,resulting in variation in the permeability of the membrane.The composite membranes exhibit excellent selective separation performance towards mixed dye solutions containing dye molecules with different electric charges,such as Rhodamine B/Metanil yellow mixed solution,with selective separation efficiency higher than 95.0%.