ADVANCED FUNCTIONALIZED COVALENT ORGANIC FRAMEWORKS FOR SUSTAINABLE AND SELECTIVE ZIRCONIUM–HAFNIUM SEPARATION

Abstract

Zirconium (Zr) and hafnium (Hf) are two important transition metals that are widely used in nuclear energy, aerospace, and advanced materials. Although they are chemically very similar and usually found together in natural minerals, they have very different roles in nuclear applications. Zirconium is mainly used for nuclear fuel cladding because it has low neutron absorption, while hafnium is used in control rods because it can strongly absorb neutrons. Due to their almost identical chemical properties and very close ionic radii, separating Zr and Hf is very difficult. Traditional separation methods such as solvent extraction, ion exchange, fractional crystallization, and distillation of chlorides are commonly used in industry. However, these methods often require high energy, large amounts of chemicals, and produce harmful waste. In recent years, Covalent Organic Frameworks (COFs) have attracted attention as a new and promising material for metal ion separation. COFs are porous, crystalline materials made from organic building blocks linked by strong covalent bonds. They have high surface area, tunable pore size, and adjustable functional groups, which make them suitable for selective adsorption. By introducing specific functional groups such as amine, hydroxyl, and carboxyl groups into the COF structure, selective binding toward Zr⁴⁺ or Hf⁴⁺ ions can be improved. This review discusses the fundamental chemistry of Zr and Hf, conventional separation methods, and the design, synthesis, and functionalization of COFs for selective Zr/Hf separation. It also explains the separation mechanisms, adsorption performance, sustainability aspects, and industrial potential. Overall, COFs offer an efficient, selective, and environmentally friendly alternative for the challenging separation of zirconium and hafnium, especially for future applications in nuclear and advanced material industries.