Precise separation and efficient recovery of Pd(II) from high-level liquid waste by XAD-based adsorbents

Efficient separation of palladium (Pd(II)) from high-level liquid waste (HLLW) not only helps the safe disposal of nuclear waste, but also is a vital important supplement to Pd(II) resources. Herein, 16 kinds of novel Amberlite XAD resins-based adsorbents were prepared by vacuum impregnation method for the separation of Pd(II) from simulated HLLW, and the MMBT/XAD7HP prepared using 4-methyl-2-mercaptobenzothiazole (MMBT) as ligand and Amberlite XAD7HP resin (XAD7HP) as the carrier was selected as the optimal research object. Characterization analysis experiments proved the successful preparation of MMBT/XAD7HP, which exhibited ultrahigh adsorption selectivity for Pd(II) (distribution coefficient (K_d) ≥ 12584.5 ml·g⁻¹, separation factor (SF_Pd/M) ≥ 2818.4) and superior adsorption capacity (Q ≥ 130.1 mg·g⁻¹). The adsorption kinetics and adsorption isotherms for Pd(II) matched with pseudo-second-order (PSO) kinetics model and Langmuir model, indicating that the adsorption mode for Pd(II) by MMBT/XAD7HP was monolayer chemical adsorption. The dynamic column experiments showed that MMBT/XAD7HP achieved efficiently separation of Pd(II) from simulated HLLW, and the recovery rate was nearly 100%. The adsorption mechanism was analyzed by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT), and the ultrahigh selectivity of MMBT/XAD7HP is attributed to the preferred affinity of the soft-N, S donor atoms in MMBT for Pd(II). MMBT has the shortest frontline molecular orbital energy band gap (HOMO–LUMO) among the four ligands, which makes it easier to become an electron donor to attract Pd²⁺ to participate in coordination. NO₃⁻ participates in the adsorption to keep charge balance. In summary, MMBT/XAD7HP has broad application prospects for the treatment of Pd(II) in HLLW.