Surface Hydroxylation Of MXene Membrane And Removal Of Heavy Metal Ions

In recent years,the rapid development of modern industry has led to the increase of the wastewater discharge,among which the heavy metal pollution of water has become one of the urgent problems to be solved.Due to the advantages of high efficiency,environmental protection and pollution-free,membrane separation has been widely used in the treatment of heavy metal wastewater.However,traditional membrane materials are susceptible to the"Trade-off"effect,which cannot improve the performance of heavy metal removal and ions sieving.The novel two-dimensional transition metal carbonitride（MXene）layered membrane has a unique layered structure and abundant oxygen-containing groups,which endows its surface with high electronegativity.Based on the surface charge control and interlaminar sieving effect,metal ions such as Na⁺,K⁺with smaller kinetic diameter are easy to pass through two-dimensional interlayer channels,while heavy metal ions with larger kinetic diameter can be adsorbed and intercepted by the surface.Therefore,MXene membrane has emerged in the field of heavy metal ion removal.However,water molecules are easy to enter into the interlayer,which leads to the swelling of the membrane and reduces the stability of the membrane.Although the swelling effect of MXene membrane after thermal crosslinking in water has been significantly improved,the surface charge of the membrane still needs to be increased in order to improve the removal rate of heavy metal ions.In this paper,the surface hydroxylation of MXene two-dimensional membrane（Ak-MXene membrane）was obtained by alkalizing the surface of MXene membrane,which significantly enhanced the surface electronegativity and would be conducive to the removal of heavy metal ions.The details are as follows:（1）The stable Ti₃C₂（MXene）colloidal solution was prepared by etching MAX phase（Ti₃AlC₂）with LiF and HCl mixed solution;MXene membranes with controllable interlayer channels and good stability were assembled by vacuum assisted filtration and heat treatment;the Ak-MXene membrane were obtained by alkalization with KOH solution.The results show that the Ti-F bond is replaced by-OH on the surface of the alkalized MXene membrane,which forms the Ti-OH bond and realizes the surface hydroxylation modification,thus enhancing the surface electronegativity of the membrane.The layer spacing of the obtained Ak-MXene membrane is about 6.28（?）,the water contact angle is45.58°,and the surface zeta potential is-37 m V,which indicates that the improvement of surface properties will be beneficial to the water transport and the removal of heavy metal ions.（2）The water flux,ion sieving and heavy metal ion removal properties of Ak-MXene membrane were investigated under pressure driven conditions.As the pressure increases from 0.02 MPa to 0.06MPa,the water flux increase from 139.1 to 282 L h^-1m^-2.At 0.02 MPa,the rejection rates of Pb²⁺,Cu²⁺,Cd²⁺were 81.45%,82.23%and 83.71%,respectively.And the permeation rates were 0.006,0.017 and 0.009 mol h^-1 m^-2,while the separation factors of the Ak-MXene membrane for mixed ions Na⁺/Pb²⁺,Na⁺/Cu²⁺,Na⁺/Cd²⁺were only 1.67、1.82、1.86,respectively,which indicated that further improvement measures are needed to heighten the separation performance.（3）In order to improve the separation performance,based on the excellent conductivity of Ak-MXene membrane and the high negative charge of membrane surface,a double membrane separation system driven by electric field was constructed.The removal rate of heavy metal ions and the screening performance of Na⁺/Pb²⁺,Na⁺/Cu²⁺,Na⁺/Cd²⁺ion pairs were investigated.According to the experimental results,the separation factors of Na⁺/Pb²⁺,Na⁺/Cu²⁺,Na⁺/Cd²⁺of Ak-MXene membrane with thickness of 383 nm were increased to 83.68,84.17 and 88.8,respectively,and the rejection rates of heavy metals were higher than 99.5%and there is no attenuation in 40 min at the voltage of 12V.It can be seen that under the electric field drive,the ion sieving performance and heavy metal interception rate of Ak-MXene membrane have been significantly improved,which is expected to be applied to water purification.