The Research On The Properties Of Bismuth Oxybromide-based Electroactive Ion Exchange Materials For Capturing Bromide Ions

As a high-value element,bromine usually exists in the hydrosphere of the earth in the form of ions.Bromine has active physicochemical properties,and can form a wide variety of bromide compounds,which are widely used in industrial production,science and technology,and daily life.However,when the concentration of Br^-ions in water body is excessive,bromate by-products with carcinogenic effect on human body are easily produced in the process of ozone disinfection of drinking water,which seriously damages human health.At present,the methods of removal Br^-ions mainly have the disadvantages of high cost,high energy consumption and secondary pollution.Therefore,it is necessary to develop a method with excellent selectivity,environmental protection and economic applicability to separate and capture Br^-ions from various wastewater.Recently,electrically switched ion exchange（ESIX）technology,as a new type of water treatment process with environmentally friendly and efficient ion separation,has attracted the attention of many researchers.In ESIX process,capture/release of target ions is performed by alternately adjusting the redox potential of electroactive ion exchange materials（EIXMs）,which can achieve rapid separation of target ions in aqueous solution.The whole process relies on electrode potential as the main driving force without adding additional chemical reagents,to avoid the problem of secondary pollution.Developing suitable EIXMs is the key to successful separation of various ions by ESIX process.In this paper,bismuth oxybromide（BiOBr）was used as an excellent EIXMs to capture Br^-ions from aqueous solution due to its strong redox ability,loose layered structure and large spectral response range.In this work,a novel bismuth oxybromide@polypyrrole（Bi OBr@PPy）hybrid film with core-shell structure was prepared and applied to selectively capture Br^-ions in ESIX process.The properties of electroactive Bi OBr@PPy hybrid film in ESIX process were investigated from the aspects of cyclic voltammetry,electrochemical impedance,capture voltage,capture kinetics,influences of p H and competing anions,and cyclic stability.The capture capacity of Bi OBr@PPy hybrid film reached 146.2 mg·g^-1 at 0.8 V oxidation voltage.While the hybrid film had an excellent selectivity towards Br^-ions in the aqueous solutions with several competing anions.In addition,the capacity retention efficiency and release rate of the hybrid film still remained above 80%after 10 cycles due to the strategy of in-situ coating of PPy.Therefore,Bi OBr@PPy hybrid film is expected to become a promising electroactive ion exchange material for capturing Br^-ions in aqueous solutions.Secondly,the effect of Bi OBr film on the adsorption performance of Br^-ions and the conditions for its conversion with Bi OCl in solutions with different Br^-/Cl^-molar ratios were studied.The electrochemical performance of Bi OBr film in different Br^-/Cl^-molar ratio solution was analyzed by cyclic voltammetry and electrochemical impedance.At the same time,ion capture/release experiments were carried out in different molar ratios of solution.The results showed that the selectivity of Bi OBr film to Br^-decreased with the decrease of Br^-/Cl^-molar ratio.When Br^-:Cl^-=1:25,the selectivity of Bi OBr film to Cl^-ions is better than that of Br^-ions.At the same time,five cycles experiments were carried out at the ratios of Br^-:Cl^-=1:10 and1:25,and the microstructure and element content were analyzed by scanning electron microscope and X-ray energy dispersive spectrometer.It was found that the lattice structure of Bi OCl was inclined to transform to the lattice structure of Bi OCl.