| Precise and ultrafast ion selective sieving technology is greatly desirable in water purification,desalination,ion extraction and recovery,and new energy-related fields,which is of great significance to sustainable development of economy and society.It is valuable develop membrane materials with specific separation functions to achieve effective separation in mixed ion systems.Ultrathin two-dimensional nanomaterials have become an ideal separation membrane meterials due to the advantages of large specific surface area,good mechanical and membrane-forming properties.The two-dimensional lamerllar membrane formed by two-dimensional nanosheets through layer-by-layer stacking and orderly assembly exhibits a mechanism of mass transfer and separation in confined nanochannels which is very different from that of traditional membranes.What’s more,different requirements can be achieved by adjusting the size of the interlayer channels or the chemical properties of nanosheet surfaces.Thus,great attention has been attracted.Based on Ti3C2Tx,two-dimensional Ti3C2Tx lamellar membrane was fabricated and modified in this study.What’s more,the structure of layered membrane,ion sieving performance were explored.The swelling phenomenon of two-dimensional layered membrane caused by ion intercalation was suppressed and the ion sieving performance was improved.The obtained results are as follows:(1)Etching Method of "HCl+LiF" was used.Large Ti3C2Tx nanosheets with lateral size of 1～2 μm and thickness of～1.0 nm was fabricated.Self-standing OTM with good mechanical properties was obtained by vacuum filtration.Hydrated ions of Li+,Na+,K+,Ca2+,Mg2+and Al3+ could increase the d-spacing of OTM,while Rb+and Sn4+reduce the d-spacing.The variation of d-spacing mainly caused by swelling phomenon due to intercalation of ions.For ion permeation experiment,the selectivity of OTM is about 2,7 and 3.5 in monovalent/divalent,monovalent/trivalent and divalent/trivalent ions,which shows selectivity for trivalent ions.(2)Based on OTM,ACTM was obtained by surface coated with Al13 polymerization cationic solution and the optimal method was studied.ACTM has a stable d-spacing of 17.0 ± 0.1 (?) and shows good anti-swelling properties.Located on the surface of OTM,Al13 polymerization cation does not enter the inner space of OTM.The effective space of ACTM(shows positive charge for surface)is about 8.2 (?).Due to steric hindrance and electrostatic repulsion,the selectivity of ACTM for monovalent/trivalent ions is～30 while for divalent/trivalent ions is～13,which is 3.3 and 2.7 times higher than that of OTM,exhibiting good ion sieving performance in monovalent,divalent and trivalent ions.(3)In order to improve ion separation performance and channel structure,ATM is fabricated by introducing Al13 polymerized cations into inner space of OTM.The dspacing can be adjusted in the wide range of 11.5-20.0 (?) by adjusting the preparation conditions of Al13 pillared materials.The swelling phenomenon can be suppressed by the strong electrostatic interaction between Al13 and Ti3C2Txin ATM.Due to steric hindrance,the selectivity of ATM-11.5 for monovalent/trivalent ions is about 170,for divalent/trivalent ions is about 60,which is 4.6 and 3.6 times higher than ACTM respectively,exhibiting a better separation effect.(4)To improve the surface chemical properties of Ti3C2Tx two-dimensional layered membrane,especially for the separation of monovalent/divalent ions which is a huge challenge,Ti3C2Tx is mixed with CMC in ice bath.Hydroxyl groups in CMC can undergo aldol condensation reaction with glutaraldehyde.Macromolecular spatial network structure is formed,which was insoluble in water.The strong chemical bond generated can inhibit the swelling phenomenon of lamellar membrane in aqueous solution.The optimum fabrication condition is:mass ratio of CMC/Ti3C2Tx is 4:24,the concentration of GA is 0.5%and the crosslinking time is 1 h.The acquired C@TM shows excellent mechanical properties and has a stable d-spacing of 13.6 ± 0.1 (?).The selectivity of C@TM is about 30,which is 14 and 9 times higher than that of ACTM and OTM respectively.Due to size screening and channel surface effects,C@TM shows good rejected performance for divalent ions.(5)To promote separated performance of monovalent and divalent ions,the experiment of effect by applied voltage was studied.The results showed that permeation rate of Mg2+is significantly inhibited at-0.6 V,and selectivity of Li+/Mg2+is increased from 30 to 37,which is increased by 12.3%compared with 0 V,exhibiting the application potential of monovalent/divalent ion separation.In all,the research provides a foundation for the further development and application of two-dimensional layered membrane in ion selective sieving. |
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