Attapulgite Nanofibrous Membrane: Preparation,Structure And Properties

Nanofibrous membranes?NFMs?with continuously interconnected porous structure and controllable pore size,have shown potential application in membrane separation field.Currently,most studies about NFMs are to use the electrospinning technology for preparing the organic-based NFMs.However,organic-based NFMs usually have obvious problems,including low mechanical strength,poor chemical and thermal stability,insufficient rejection of pollutants,and limited lifetime.Compared with organic-based NFMs,inorganic-based NFMs can effectively avoid above drawbacks.But the work about inorganic-based NFMs is rare,due to some problems including less optional raw materials and uncontrolled preparation technology,etc.In this paper,natural clay nanofiber?Attapulgite,AT?was selected as the raw material and a new method similar to the papermaking technology was proposed.Then four types of AT NFMs were successfully designed and prepared.The effects of poly?vinyl alcohol??PVA?content,carbon nanotube?CNT?and sintering temperature on the micro-morphologies,crystal structures,pore structures,surface properties,mechanical properties,thermal and chemical stabilities of AT NFMs were studied systematically.Besides,the permeability,separation ability,antifouling performance and reusability of AT NFMs were investigated by intercepting the target contaminants.The main contents of this paper are as follows:?1?In view of the one dimensional morphology of AT,a series of AT-based NFMs with different PVA contents were fabricated via a facile method similar to the papermaking technology consisting of pulping,handsheet making and low temperature drying process.The effect of PVA content on the rheological property of AT hydrocolloid was studied by the rheological method.Results showed that AT hydrocolloid exhibited pseudoplastic non-Newtonian flow behavior with a yield stress at the gel state.With an increase in PVA content,the yield stress and system viscosity of AT hydrocolloid were improved.It meant that the appropriate amount of PVA content was beneficial to promote the membrane forming ability of AT hydrocolloid.The morphology structures and mechanical properties of AT-based NFMs were characterized by the field emission scanning electronmicroscopy?FESEM?,Fourier transform infrared spectroscopy?FTIR?,X-ray diffraction?XRD?and mechanical tensile test.Results showed that PVA was uniformly dispersed into the“woodpile”structure consisting of AT nanofibers,and then the rod-chain entanglement and hydrogen bonding interaction occurred.When the PVA content was 9wt%,AT-based NFMs exhibited high porosity?51.1%?,the maximum tensile strength?12.5MPa?,good hydrophilicity?31.3°?and high pure water flux(105L m^-2 h^-1,0.1MPa).By targeting the polyethylene glycol and polyethylene oxide with different molecular weight,the cut off high molecular weight polymer of AT-based NFMs was investigated and identified as the 50kDa.The separation ability of AT-based NFMs was evaluated by the cross-flow separating unit.Results showed that the emulsified oil with the droplet sizes of about 100nm could be effectively rejected?0.1MPa,99.4%?.?2?A series of sintered AT-based NFMs with different PVA contents were fabricated through a combination of papermaking and sintering technologies.The sintering temperature of AT-based NFMs were studied by the thermogravimetry?TG?analyzer.Results showed that AT-based NFMs exhibited a periodical change in weight loss,and three different temperatures?240°C,400°C,and 600°C?were considered as the sintering temperature.The effects of sintering temperature on the micro-morphologies and pore structures of AT-based NFMs were characterized by FESEM,XRD and BET.When the sintering temperature was 600°C,the crystal structure of AT nanofibers collapsed and the specific surface area of NFMs significantly declined.The effect of sintering temperature on the mechanical properties of AT-based NFMs was investigated by three-point bending test.With an increase in PVA content,the flexural strength of sintered AT-based NFMs showed a slight drop.Water absorbing experiment was used to characterize the swelling behavior of AT-based NFMs with different sintering temperatures.When the sintering temperature was 400°C and 600°C,the swelling degree of AT-based NFMs would be effectively decreased.Considering the400°C as the optimum sintering temperature,sintered AT-based NFMs with different PVA contents were characterized by FESEM,BET,contact angle measuring system,three-point bending test and chemical stability experiment.With an increase in PVA content,the accumulation form consisting of AT nanofibers gradually transformed from disorder to order,and the flexural strength of sintered AT-based NFMs showed a slightly drop.Besides,it was also observed that sintered AT-based NFMs with different PVA content all possessed low pore size?¹2nm?,high porosity?>60%?,good hydrophilicity?²2°?,and excellent chemical stability?pH=1¹0?.The separation ability of sintered AT-based NFMs was evaluated by the oil/water separation experiments.The separation efficiency of sintered AT-based NFMs for nano-sized emulsified oil reached 97.4%.And it also exhibited high pressure resistance?>0.6 MPa?and good fouling resistance.?3?A series of CNT reinforced AT-based NFMs?AT/CNT NFMs?were prepared by incorporating the carboxylic multi-walled CNT into the constructed“woodpile”structure consisting of AT nanofibers.Characterizations with FESEM,BET,and mechanical tensile test were used to investigate the effect of CNT content on the structure and properties of AT-based NFMs.The results showed that CNTs were uniformly dispersed into the“woodpile”structure consisting of AT nanofibers,and the soft CNTs were entangled with the hard AT nanofibers.Then PVA chains were secondary tangling with AT nanofibers and CNTs,and strong hydrogen bonding interaction occurred among them.When the CNT content was 0.5wt%,the effect of CNT on the mechanical properties of AT-based NFMs reached to the ultimate contribution with the greatest efficiency.AT/CNT NFMs showed a 74%increase in tensile strength,27%increase in modulus and 110%improvement in toughness over AT-based NFMs.Characterizations with atomic force microscopy?AFM?and contact angle measuring system showed that the surface wetting ability of AT/CNT NFMs decreased with the increase of CNT content,while its surface roughness showed an increasing tendency.Three cyclic separation measurements were used to investigate the separation ability,antifouling performance and reusability of AT/CNT NFMs?CNT-0.5?for emulsified oil and bovine serum albumin.Results showed that AT/CNT NFMs exhibited high separation efficiency?¹00%?,high flux recovery ratio?⁹8%?and low irreversible fouling ratio?²%?all the time.?4?Sintered AT/CNT NFMs were prepared by optimizing the content of PVA and CNT,and the sintering temperature.Characterizations with FESEM,BET,mechanical tensile test and contact angle measuring system were used to investigate the structure and performance of AT/CNT NFMs before and after the sintering process.Results showed that the sintering process could improve the specific surface area(120.1m² g^-1),the total pore volume(0.37cm³ g^-1),the wetting ability?14.8°?and flexural strength?11.1MPa?of sintered AT/CNT NFMs.Then cyclic separation measurement was used to investigate the permeability,separation ability and antifouling performance of sintered AT/CNT NFMs for oil/water emulsion.When the operating pressure was0.2MPa,its water flux was 28L m^-2 h^-1 and the separation efficiency was 98.6%.After three cyclic separation measurements,the flux recovery ratio and irreversible fouling ratio of sintered AT/CNT NFMs were 87.5%and 12.5%,respectively.