Separating Li~+ From Aqueous Solutions By Electrochemically Switched Ion Exchange Based On Lithium Ion Sieve-Reduced Graphene Oxide Composite Film

With the vigorous promotion of the new energy industry,interest in lithium resources has been increasing because of the rapid expansion of the lithium battery market and electric vehicles.Lithium resources are mainly from lithium-rich salt-lake brines and seawater,so extracting lithium resources from the liquid phase has been paid attentions widespreadly in the world.Separation of lithium ions by traditional ion exchange method will result in long separation time,secondary pollution and difficult recovery.Electrochemically switched ion exchange?ESIX?is a novel ion exchange technology,which has the advantages of no secondary pollution and fast processig speed.In this technique,the high ions adsorption rate for dilute solution can be easily achieved because of the robust electric driving force for ion transport.For extracting lithium ions from aqueous solutions by using the ESIX method,it is a priority to synthesize corresponding electroactive ion exchange materials.In this work,we developed H_1.6Mn_1.6O₄/reduced graphene oxide?rGO?composite film and?-MnO₂/reduced graphene oxide/calcium alginate?CA?composite film by different preparation methods for selective separation of lithium ions with low concentration.A novel lithium ion separation composite film consisting of H_1.6Mn_1.6O₄nanoparticle and rGO is successfully fabricated by using vacuum filtration.Based on the great selective adsorption for lithium ions of H_1.6Mn_1.6O₄ and good conductivity of rGO,the H_1.6Mn_1.6O₄/rGO composite film is applied for selective electrochemical extraction of low concentration lithium ions from aqueous solutions using an ESIX technique.As a result,the lithium ion adsorption capacity of the H_1.6Mn_1.6O₄/rGO composite film reaches 38.78 mg·g^-1with an adsorption equilibrium time of 5 h.In particular,the selectivity factors for Li⁺/Na⁺and Li⁺/Mg²⁺reach 10.39 and 10.23 when the initial molar ratios of Li⁺/Na⁺and Li⁺/Mg²⁺are 1:1.The electrical drive and rGO promote aggregation of ions around the H_1.6Mn_1.6O₄/rGO composite film,meanwhile,ion transport efficiency is improved.This simple preparation method is easier to find vast applications for separation of lithium ions.In order to solve problems of densification and non-conducting substrate,in this study,a novel?-MnO₂/rGO/CA three-dimensional porous composite film was fabricated by vacuum freeze-drying method.The?-MnO₂/rGO/CA composite film was applied for the extraction of lithium ions by using an ESIX technique.The experimental results showed that the lithium ions concentration could be reduced from 20 mg·L^-1 to 10 mg·L^-1 after the electrochemical adsorption of?-MnO₂/rGO/CA composite film.The adsorption efficiency and adsorption capacity by using ESIX technology were higher than that of the static adsorption.The concentration of lithium ions in the solution could be reduced from 20 mg·L^-1 to 2.5 mg·L^-1 after repeatedly adsorbing for 3 cycles,and the recovery of lithium ions reached up to 87.5%.This composite film is better than H_1.6Mn_1.6O₄/rGO composite film in the treatment of low concentration lithium ion.The size of the film is not limited,which is conducive to large-scale application.