Rational fabrication of sulfonic acid-circled anionic traps for selective extraction of rubidium ion

Rubidium ion extraction from salt-lake brines is of vital significance but still challenging. Adsorption is an effective and feasible approach but existing adsorbents face the disadvantages of limited selectivity, capacity, or regeneration. Herein, a strategy is proposed whereby a chelation micro-environment surrounded by multiple sulfonic acid (-SO₃H) groups is constructed for the targeted capture of Rb⁺ ions. A robust MIL-53-(SH)₂ (MIL, Material Institute Lavoisier) was synthesized via a newly presented green method and subsequently oxidized to MIL-53-(SO₃H)₂. Owing to the enlarged group size of -SO₃H compared to that of -SH (sulfydryl), the group-group distance was essentially shortened in a fixed pore space, stimulating the synergic chelation of multiple -SO₃H groups. Benefiting from the enhanced affinity and smooth diffusion, a high capacity of 165.1 ± 3.2 mg g^-1 and fast adsorption equilibrium time of 10-30 min were achieved. Selective adsorption over the co-existing Na⁺, K⁺, Ca²⁺, and Mg²⁺ ions was also achieved, demonstrating feasilibity in a simulated salt-lake brine. Furthermore, the proposed adsorbent proved thermo-regenerable, with a high retention rate of adsorption amount (∼90 %) even after the fifth cycle.