Efficient and rapid recovery of scandium from acidic wastewater using a β-cyclodextrin-based phosphorus-functionalized covalent organic polymer

Motivated by the growing demand for rare earth elements (REEs), it is crucial to design effective adsorbents capable of separating and reclaiming REEs from acidic wastewater. Herein, a phosphorus-functionalized covalent organic polymer (β-HCCD) was successfully synthesized via a straightforward nucleophilic substitution reaction, utilizing two cost-effective monomers, β-cyclodextrin (β-CD) and hexachlorocyclotriphosphazene (HCCP). Owing to the presence of abundant P-O bonds in β-HCCD and the unique cavity structure of β-CD, β-HCCD exhibits adsorption efficiency exceeding 98% for scandium ions (Sc³⁺) in an acidic solution with the hydrogen ion concentration ranging from 5 mol L^-1 to 10^-5 mol L^-1. Meanwhile, the complete capture of Sc³⁺ by β-HCCD can be accomplished within 2 minutes, and the adsorption isotherms demonstrated an outstanding alignment with the Freundlich model, exhibiting a maximum adsorption capacity of 101.0 mg g^-1. What's more, β-HCCD can be directly applied to industrial acidic wastewater without prior pretreatment, achieving an efficient recovery rate of 87% for Sc³⁺. These notable advantages including minimal expense, a straightforward preparation process, fast adsorption dynamics and substantial adsorption ability demonstrate that β-HCCD holds significant promise in the recovery of REEs.