The impact of redox annealing on intrinsic properties and fluoride adsorption performance of CeO2 nanomaterials

The presence of excess fluoride ions in drinking water has raised widespread concerns, necessitating the development of effective methods for its removal. Cerium dioxide (CeO2) holds promise as a selective adsorbent for fluoride ions, due to its exceptional affinity for fluoride and stability across a wide pH range. However, the impact of intrinsic properties of CeO2 on its adsorption characteristics and mechanisms remains unclear. In this study, we prepared a range of CeO2 materials with diverse intrinsic properties by adjusting the annealing temperature and atmosphere. Through a series of characterization techniques, we discovered a correlation between the content of oxygen vacancies, Ce(III) species, and surface hydroxyl groups (–OH), all of which decreased simultaneously with increasing air annealing temperature. With respect to the adsorption performance, the highest fluoride adsorption capacity of 111.1 mg g−1 was observed in un-CeO2, representing the nanomaterials with the highest oxygen vacancies. This capacity gradually decreased to 29.1 mg g−1 in 800-CeO2. Following fluoride adsorption, the oxygen vacancy density, Ce(III) content, and surface –OH content of all CeO2 materials decreased. When CeO2 materials were annealed in H2 for 1 h, the F− adsorption density increased due to the formation of more oxygen vacancies and surface –OH. Based on our findings, we conclude that oxygen vacancies play a critical role in determining whether fluoride ions can successfully exchange with surface –OH groups and enter the crystal lattice of CeO2, which significantly affects the adsorption performance. The pivotal role of oxygen vacancies, as proposed in this study, provides a valuable direction for future research aimed at enhancing the fluoride adsorption performance of CeO2.