Structural Regulation Of BiOBr Electrode And Its Photo-assisted Electrochemically Switched Bromide Ion Exchange Performance

As a kind of halogen ion,bromide ion（Br^-）is widely present in water environment such as seawater,salt lake brine,underground brine and wastewater.The presence of excessive Br^-would form brominated disinfection byproducts which are carcinogenic during the water oxidation process,thus causing environmental pollution.The Br^-separated and recovered from the above-mentioned aqueous environment can be used in industry,agriculture and medicine,thus the separation and recovery of Br^-is of great significance for the economy.Currently,the developed recovery methods for Br^-include adsorption,membrane separation,air blowing and capacitive deionization,etc.Although these technologies can recover Br^-effectively,they generally lack the selectivity for target Br^-.In this study,for the scientific issues of developing materials and ion separation systems with excellent selectivity and high adsorption capacity for Br^-,the electrochemically switched ion exchange technology（ESIX）with selective extraction effect on the target ions was selected for the separation and recovery of Br^-,and a photo-assisted electrochemically switched ion exchange（ESIX）was developed by introducing external visible light to ESIX process.The reversible recovery or desorption of Br^-can be completed by regulating the exerted potential of the electroactive ion exchange materials（EIXMs）adhered to the conductive substrate under the irradiation of external visible light.Different from other conventional technologies,P-ESIX utilizes clean and efficient solar energy to enhance the driving force of ion transport,which can achieve selective extraction of target ions while being environmentally friendly,membrane regenerable and free of secondary pollution.As an important part of P-ESIX process,the ion exchange material must possess target ion selectivity,photo-responsive activity and electrochemical activity.BiOBr as a photoelectric catalyst,its[Bi₂O₂]²⁺slab stacks with the[Br^-]slab by van der Waals forces,which makes Br^-migrate freely between the slabs and generate the corresponding adsorption sites for Br^-.Therefore,BiOBr is selected as photo-electroactive ion exchange material（P-EIXMs）to achieve the selective separation and recovery of Br^-in P-ESIX process.In order to explore the specific separation process and selective separation effect of BiOBr on Br^-,the selective extraction of Br^-was firstly achieved by ESIX technology and the mechanism of of BiOBr on Br^-was investigated.Subsequently,to enhance the recovery effect of BiOBr electrode for Br^-,the visible light was introduced into ESIX process through the utilization of clean and renewable energy,and the corresponding photo-assisted mechanism and the synergistic effect of light and electricity in the P-ESIX process were researched.Then,the morphological structure of BiOBr was optimized to improve the migration of Br^-at the solution/electrode interface,and the influence of BiOBr structure on Br^-recovery was investigated.At last,BiOBr electrode with high adsorption capacity for Br^-was selected and combined with P-ESIX technology for the separation and recovery of Br^-in simulated brine,and its applicability for Br^-extraction from brine was explored.The concrete research contents are listed as follows.（1）BiOBr electrode with Br^-adsorption sites was constructed by electrochemical method and applied to ESIX process for the recovery of Br^-.The changes of structure,morphology and composition of BiOBr electrode before and after Br^-exchange were investigated,and the specific exchange process and mechanism were discussed.The experimental results display that after one ESIX cycle,not only the reversible capture and release of Br^-,but also the reversible structural and morphological transformation of BiOBr is achieved.Furthermore,BiOBr can still selectively capture Br^-under the coexistence of various anions.This work provides the experimental basis for the selective separation and recovery of Br^-in real water environment.（2）Electric field as the driving force of ESIX process,the adsorption capacity of the BiOBr electrode for Br^-in this process is limited.In view of this,the P-ESIX system is constructed by introducing an external visible light into ESIX process as an auxiliary driving force,and BiOBr electrode with photoelectric dual response activity was assembled in the P-ESIX system to enhance Br^-recovery.By comparing the adsorption capacity for Br^-before and after the introduction of visible light,the photo-assisted mechanism in P-ESIX process is explored.The results reveal that the introduction of external visible light could effectively improve the adsorption capacity of Br^-while achieving the selective separation of Br^-.This work provides a new method for a greener and more efficient development of ESIX technology.（3）In order to further optimize the separation and recovery performance of BiOBr electrode for Br^-from the structural modulation of electrode,BiOBr with different exposed crystal facets were synthesized.Correspondingly,the photoelectric response activity and exchange mechanism for Br^-of different exposed facets were investigated.The experimental results and DFT theoretical simulations show that BiOBr with exposed（010）facet exhibits better photoelectrochemical responsiveness and more Br^-transport channels,and thus displays superior P-ESIX exchange performance for Br^-.This study provides a novel strategy for the enhancement of Br^-adsorption capacity in the P-ESIX process.（4）P-ESIX with technical advantages and BiOBr with performance advantages were coupled for the separation and recovery of Br^-from the simulated salt lake brine solution,and the applicability of BiOBr membrane electrodes in the P-ESIX system for the separation and recovery of Br^-in simulated saline brines was evaluated.The experimental results show that BiOBr not only exhibits excellent electrochemical cycling stability,but also displays adsorption selectivity for Br^-in the simulated saline brine.This work reveals that the coupling of P-ESIX system and BiOBr film possesses potentials for the selective separation and recovery of Br^-in simulated salt lake brine solutions,and provides a theoretical basis for the recovery of Br^-in actual salt lake brines.