Preparation Of δ-Bi₂O₃@PCNF Self-supporting Electrode With Abundant Oxygen Vacancies For Selective Phosphorus Removal By Capacitive Deionization

Phosphorus is an essential nutrient for organisms to sustain life,and excessive release of phosphorus can eutrophicate water bodies,which can lead to ecosystem destruction.Phosphorus exists in natural water bodies in the form of phosphate ions,and the current methods for phosphate removal from water are mainly chemical precipitation,biological,ion exchange,adsorption and electrochemical methods.However,these traditional techniques have certain limitations,and there is an urgent need to develop a low-energy,environmentally friendly phosphate removal technology.This paper is based on Capacitive Deionization（CDI）for adsorption of charged ions to remove phosphate from water.CDI electrode material is the key to improve the selective and efficient adsorption of phosphate.Therefore,exploring the electrode materials based on CDI technique for selective phosphate adsorption is the focus of this study.The main research contents and results include:（1）Self-supported electrodes of carbon nanofibers（PCNF）with abundant pore structure were prepared by electrostatic spinning technique,andδ-Bi₂O₃ with petal-like nanosheet structure was synthesized in situ on the surface of PCNF by solvent thermal method.δ-Bi₂O₃ on the surface of PCNF gradually increased with the increase of bismuth-based salt content during the solvent thermal process.The microscopic morphology and structural characterization of the material showed thatδ-Bi₂O₃ was uniformly distributed on the PCNF surface,and the EPR and XPS characterization of the material demonstrated that theδ-Bi₂O₃@PCNF composites were rich in oxygen vacancies.The electrochemical performance analysis revealed that the electrochemical specific capacity ofδ-Bi₂O₃@PCNF was enhanced and then weakened with the increase ofδ-Bi₂O₃ loading on the PCNF surface,which was due to the fact that too high loading would hinder the charge conduction and ion diffusion on the fiber surface.（2）The capacitive deionization device was constructed withδ-Bi₂O₃@PCNF as the positive electrode and activated carbon electrode as the negative electrode,and charged under constant voltage conditions.It was found that the adsorption capacity and adsorption rate of the electrode increased gradually with increasing voltage,and the maximum adsorption capacity of the electrode could reach 99.6 mg P/g when the voltage was 1.2 V.Moreover,the initial p H of the solution had a significant effect on phosphorus adsorption:the highest adsorption was achieved at theδ-Bi₂O₃@PCNF composite electrode under weak acid（p H=4-6）conditions.In solutions containing different coexisting anions(Cl^-,NO₃^-,SO₄^2-,CO₃^2-and HCO₃^-),the composite electrode did not have a major effect on the phosphorus adsorption performance,indicating that theδ-Bi₂O₃@PCNF composite electrode has a high selectivity for phosphorus adsorption.The phosphorus adsorption selectivity was further tested in solutions with different molar ratios of Cl^-:P.It was found that the selectivity coefficient could reach20.85 when the molar ratio of Cl^-:P was 5.By reversing the applied voltage in the alkaline solution,the phosphorus on the surface ofδ-Bi₂O₃@PCNF electrode could be easily desorbed and could maintain 86%of the adsorption capacity after 10 adsorption-desorption cycle tests,indicating that it regenerative cycle is possible.The adsorption test of phosphorus in actual water proved that theδ-Bi₂O₃@PCNF electrode can achieve rapid adsorption within 30 min,so that the phosphate concentration in the solution is lower than the water quality discharge standard,which proves the practical application feasibility of the electrode.（3）X-ray photoelectron spectroscopy（XPS）and Fourier infrared spectroscopy（FT-IR）were used to further investigate the mechanism of phosphate adsorption at theδ-Bi₂O₃@PCNF self-supported electrode in the CDI system.The adsorption mechanism is:under the action of electric field,the negatively charged phosphate ions are adsorbed to the positive electrode by electrostatic attraction;XPS characterization before and after adsorption proved that phosphate ions entered the cell structure ofδ-Bi₂O₃@PCNF and replaced the hydroxyl groups on the surface of oxygen vacancies,while oxygen atoms in phosphate ions occupied oxygen vacancies and formed stable complexes;FT-IR spectroscopy before and after adsorption ofδ-Bi₂O₃@PCNF revealed a typical P-O stretching vibration after adsorption,which proved that between phosphate and electrode Bi-O-P inner sphere complexes were formed through ligand exchange.