High-efficiency separation of Pd from high-level liquid waste by a novel silica-based sorbent modified with polybenzimidazole

The efficient separation and recovery of palladium (Pd) from high-level liquid waste (HLLW) are critical for promoting resource sustainability and advancing the nuclear fuel cycle. In this study, a novel silica-based sorbent (HT-paPBI@SiO₂) enriched with nitrogen adsorption sites was synthesized via a polymerization–vacuum impregnation–high-temperature optimization strategy to address the challenges of Pd(II) recovery in highly acidic and ionically complex environments. HT‑paPBI@SiO₂ was systematically characterized using SEM, EDS, TG‑DSC, BET, and FT‑IR analyses, which confirmed its rigid framework, abundant porosity, high loading efficiency, and excellent thermal stability. Static batch adsorption experiments demonstrated that HT‑paPBI@SiO₂ efficiently adsorbs Pd(II) over a broad acidity range (0.1–9 M HNO₃). In simulated HLLW at 6 M HNO₃, the material exhibited outstanding selectivity (SF_{Pd/other metal ions} > 1.05 × 10³) and a high saturation adsorption capacity of 128.2 ± 1.8 mg/g. Furthermore, the adsorption kinetics fit well with a pseudo‑second‑order model, while the equilibrium data conform to the Langmuir isotherm and van’t Hoff thermodynamic models, indicating a spontaneous, endothermic, monolayer chemical adsorption process. Furthermore, dynamic adsorption experiments successfully achieved the selective recovery of Pd(II) from simulated HLLW, highlighting the potential of HT-paPBI@SiO₂ for industrial-scale applications. Mechanistic investigations, supported by FT-IR, XPS, and DFT calculations, revealed that the benzene ring in paPBI was oxidized to a quinone structure under high-temperature treatment. The electron-rich regions resulting from the interaction of the C=O group in quinone with the C=N − C moiety in imidazole enhance the affinity for Pd(II). This interaction facilitates the coordination of nitrogen and oxygen atoms with Pd(II), while NO₃⁻ ions coordinate to balance the charge.