Preparation Of Layered Double Hydroxide-based Composite Films And Research On Its Electrochemically Switched Bromide-ion Extraction Performance

As a basic chemical raw material,bromine is widely used in medicine,sterilization,flame retardant,rubber and other fields,and its demand is increasing year by year with the continuous development of the national economy.Bromine mainly exists in seawater,underground brine,salt lake brine and other resources in the form of ions,and its content is limited and non-renewable.Therefore,it is significant to explore the efficient and environmentally friendly extraction technology of bromine ion（Br^-）on the basis of rational utilization of bromine resources.At present,as a novel ion separation technology,electrochemically switched ion exchange（ESIX）is expected to be combined with the selective extraction of Br^-to achieve efficient utilization of bromine resources because of its high selectivity,low cost,wide range of application and no secondary pollution.As the core and key of electroactive film electrode and ESIX technology,the design and synthesis of structural characteristics of electroactive ion exchange materials（EIXMs）are particularly important.Based on the unique controllable layered structure and interlayer anion exchange property of two-dimensional layered double hydroxides（LDHs）materials,this work uses it as a selective separation layer to prepare Ni-MOF（D）/NiCo LDH and NiO/NiCo LDH composite film electrodes.The in-situ growth structure design strategy is applied to improve the stability of the composite film electrode.The specific study contents are as follows:（1）An electrochemically induced dual-site adsorption film（Ni-MOF（D）/NiCo LDH）with excellent Br^-extraction performance is prepared on a stainless-steel wire mesh（SSWM）by electrodeposition assisted solvothermal method.The results show that the Br^-extraction capacity by the film electrode in the ESIX system can reach 126.30 mg·g^-1,and the separation coefficients of Br^-/NO₃^-and Br^-/I^-are 8.29 and 4.83,respectively,which proves the favorable Br^-selectivity and excellent ion extraction performance of the composite film electrode.Combined with the simulation and experimental results,it is confirmed that the efficient selective extraction of Br^-by the film electrode should be derived from the dual-site adsorption（i.e.,electrochemically induced laminate metal coordination sites and interlayer anion exchange sites of NiCo LDH）mechanism.The film electrode also displays superior adsorption/desorption performance and electrochemical stability due to the application of in-situ growth strategy.（2）NiO/NiCo LDH composite film electrode is prepared by two-step electrochemical deposition to further simplify the preparation process and improve the performance of Br^-extraction.Combined with ESIX technology,the Br^-extraction performance of the film electrode is studied.Compared with Ni-MOF（D）/NiCo LDH,NiO/NiCo LDH composite film has larger specific surface area and better electroactivity and thus shows higher Br^-extraction capacity(159.25 mg·g^-1).In addition,the effects of different electrode potentials,initial concentration of Br^-and p H value of solution on the extraction performance of target ion are investigated.The results demonstrate that the NiO/NiCo LDH composite film electrode has superior extraction performance,long-term electrochemical stability and good target ion selectivity,which can achieve efficient selective extraction of Br^-.In this paper,the ion selective extraction mechanism of LDHs-based materials in ESIX process was deeply studied,which provides a new theoretical basis for the design of electrodes with specific adsorption sites.