Preparation Of Carbon Nanotube Hybrid Membranes For Recovery Of Precious Metal Ions

Carbon nanotubes(CNTs)have unique one-dimensional structure and excellent physiochemical properties and therefore,are predicted to be promising for a wide range of applications.Among them,CNT-based separation membranes are expected to play an important role in solving the problems of environmental pollution and heavy metal ions recovery and recycle.Although notable progresses have been made on the development of CNT-based separation membranes,there are still key problems remained.For example,the hydrophilicity of CNT separation membrane is usually poor,the adsorption capacity for precious metal ions is very limited,and the separation mechanism remains elusive.In this study,we design and prepare CNT-containing hybrid membranes with good hydrophilicity and high precious metal ion adsorption capacity,and the working mechanism of the membranes are elucidated.The main contents and results obtained are as follows:(1)Preparation and precious metal ion recovery property of MWCNT-Ti₃C₂T_x hybrid membranes.MWCNT-Ti₃C₂T_x hybrid membrane was prepared by a simple liquid phase mixing and filtration method.The CNTs function in effectively isolating the Ti₃C₂T_x sheets,which ensures a porous network structure and provides channels for water molecule transport.Furthermore,the CNTs act as a framework to fix Ti₃C₂T_x MXene sheets and strengthen the stability and mechanical flexibility of the hybrid membrane.The separation performance of the MWCNT-Ti₃C₂T_x hybrid membranes was studied,and a rejection coefficient of 99.8%was achieved for Au(?)with an extremely low concentration of 20 ppm.The water permeability of the membrane reached 437.6 Lm^-2h^-1bar^-1,about 202 times higher than that of a pure Ti₃C₂T_x MXene membrane.A redox reaction rejection mechanism was proposed for the membranes,where the C-Ti-OH of Ti3C2 Tx is oxidized to C-Ti-O and/or Ti-O while precious metal cations(Au(?),Ag(?)and Pd(?))are reduced to zero-valent precious metals.CNTs prevent the aggregation of Ti₃C₂T_x MXene layers and act as a conductive framework for electron transfer through the hybrid membrane.(2)Preparation and precious metal ion recovery property of porous CNT-Ti₃C₂T_xCNT hybrid membranes.A sandwich structured CNT-Ti₃C₂T_x-CNT porous hybrid membrane was designed and prepared Ti₃C₂T_x MXene nanosheets with abundant wrinkles were obtained by means of freeze-drying,which effectively avoids the close stacking of the nanosheets.Then,CNT-Ti₃C₂T_x-CNT hybrid membranes were prepared by filtration and freeze drying.SWCNT-Ti₃C₂T_x-SWCNT hybrid membrane showed a high water permeability of 493.7 Lm²h^-1bar^-1 with an excellent rejection rate of 99.9%for Au(?)from a solution with an extremely low concentration of 20 ppm.MWCNTTi₃C₂T_x-MWCNT hybrid membrane showed a high water permeability of 652.9 Lm^-2h^-1bar^-1 with a rejection rate of 95.0%.The rejection mechanism is proposed as the following:Ti₃C₂T_x MXene is oxidized and electrons are transferred to the surface of hybrid membrane through CNT layer driven by a streaming potential,and precious metal ions are reduced to form into zero-valent metal nanoparticles on the surface of hybrid membrane.Au with a purity of 99.85%was obtained by simply peeling off the upper CNT layer and a followed heat treatment in air.(3)Preparation and Au(?)recovery property of CNT-poly tannic acid(TA)hybrid membrane.A CNT-TA hybrid membrane was prepared by filtration,where TA containing reductive phenolic hydroxyl serves as an active material.The CNT network with good mechanical property endows the hybrid membrane a good stability.The separation performance of hybrid membrane for Au ions was studied.A water permeability of 157.5 Lm^-2h^-1bar^-1 and a rejection rate of 99.6%were achieved for Au(?).MWCNT-TA hybrid membrane showed a high water permeability of 465.3 Lm^-2h^-1bar^-1 and a rejection rate of 85%for Au(?).The separation mechanism is proposed as the following:the electrons of TA is transferred to the surface of hybrid membrane through CNT conducive network driven by streaming potential,which reduces Au(?)into Au on the surface of the hybrid membrane.After an air oxidation treatment,Au foils with a purity of 99.95%can be obtained.