Leveraging Hard-Soft Acid-Base Interactions for Effective Palladium Capture in Acidic Solutions

Demand for palladium is increasing as it is a precious metal with great industrial applications. In order to meet the potential challenges posed by limited terrestrial Pd resources, it is necessary to develop low-cost and efficient Pd enrichment materials and methods. Palladium is found in significant quantities in spent fuel as a fission product, making it highly valuable for recycling. However, the selective capture of Pd²⁺ ions from highly acidic radioactive liquid wastes, with complex compositions generated from spent fuel reprocessing, remains a challenge. Leveraging the intrinsic bonding selectivities associated with hard-soft acid-base (HSAB) theory we present, the rapid and highly selective capture of Pd²⁺ ions from strongly acidic solutions has been achieved by the layered potassium thiostannate K_2xSn_4-xS_8-x (KTS-3). The maximum sorption capacity was calculated to be 397 and 162 mg of Pd²⁺/g of KTS-3 in 1 and 3 M HNO₃ solutions, respectively. In 1 M HNO₃ solutions with initial Pd²⁺ concentrations of 1.02 and 260 mg/L, the sorption of Pd²⁺ by KTS-3 reaches equilibrium within 2 and 30 min, respectively, with removal rates of more than 99.9%. KTS-3 can effectively and selectively remove Pd²⁺ from mixtures containing Ag⁺ or Pt²⁺, although they also represent soft Lewis acids and pose strong competition with Pd²⁺ for binding to the soft base S^2- sites of the framework. Moreover, KTS-3 can effectively separate Pd²⁺/Mⁿ⁺ (Mⁿ⁺ = Na⁺, Cs⁺, Sr²⁺, Eu³⁺, Zr⁴⁺, Ni²⁺, and Co²⁺) in acidic solutions and shows excellent selective capture of Pd²⁺ even in simulated high-level liquid wastes. The outstanding Pd²⁺ sorption by KTS-3, showcasing a record-breaking capacity and selectivity, is attributed to the synergistic effect of the easily exchangeable K⁺ cations and the strong HSAB affinity of the S^2- active sites for Pd²⁺ ions. This is achieved under extremely acidic conditions, which coupled with its structural integrity and effectiveness even post γ-irradiation, positions it as a promising material for palladium capture and recovery, especially in the treatment of radioactive waste.