Functionalized Li/NH2/MIL-101(Cr) and Li/NH2/MIL-100(Fe) for lithium adsorption and separation from aqueous solutions

Abstract

MIL-101(Cr) and MIL-100(Fe) are both representative MIL-type Metal–Organic-Framework (MOF ) materials, renowned for their large specific surface areas and size-tunable characteristics, making them suitable candidates for Li⁺ extraction from lithium-containing solvents. By introducing –NH₂ functional groups and using LiNO₃ as both a template and a mineralizer, the functionalized MOF materials Li/NH₂/MIL-101(Cr) and Li/NH₂/MIL-100(Fe) were synthesized. Following modification, their Li⁺ adsorption capacities were increased, reaching 43.58 and 38.22 mg g⁻¹, respectively. The dosage of mineralizer, initial Li⁺ concentration, adsorbent dosage, solution pH value, and temperature all have an impact on the adsorption capacity of adsorbents. In addition, through the establishment of the pseudo-first-order kinetic model and the pseudo-second-order kinetic model, as well as Langmuir and Freundlich thermodynamic models, it was determined that the adsorption of the materials was due to monolayer chemisorption, and the adsorption process was exothermic. Furthermore, both adsorbents showed good reusability, retaining over 85% of their initial adsorption capacity after four adsorption–desorption cycles, highlighting their practical applicability in lithium recovery processes. In Mg–Li mixed solution systems, both materials exhibited exceptional Li⁺ selectivity. At a low Mg²⁺/Li⁺ ratio of 3, the separation factor (α) exceeded 80; even at a high Mg²⁺/Li⁺ ratio of 10, α remained near 50. Additionally, in systems with the coexistence of multiple interfering ions, the distribution coefficient (K_d) followed the order: Li⁺ ≫ Mg²⁺ > Ca²⁺ > Na⁺ > K⁺. In the mixed systems, Li/NH₂/MIL-101(Cr) and Li/NH₂/MIL-100(Fe) exhibited a certain separation effect for Li⁺ against competing cations.