Selective platinum recovery from Pt-Co solutions via amine/phosphine oxide-loaded solvent-impregnated resins: mechanisms and selectivity

The efficient recycling of platinum (Pt) from proton exchange membrane fuel cell (PEMFC) catalysts is crucial due to Pt's scarcity and geopolitical constraints. However, existing approaches for selective Pt recovery from Pt-Co solutions face critical limitations: poor selectivity in multi-metal systems, narrow pH tolerance, and unclear adsorption mechanisms for Pt(IV)/Co(II) differentiation. To address these gaps, this study introduces two solvent-impregnated resins (SIRs)—N235/XAD-16 (amine-based) and Cyanex923/D101 (phosphine oxide-based)—for targeted Pt(IV) recovery from Pt-Co solutions. Through integrated experimental and density functional theory (DFT) analyses, we elucidate the distinct adsorption mechanisms underlying each SIR's Pt(IV)/Co(II) separation performance. N235/XAD-16 exhibits strong pH-dependent behavior, achieving 99.95 % Pt recovery at pH 1 via electrostatic interactions between protonated amine groups and PtCl 6 2− anions; however, its efficiency declines by ∼50 % at pH > 3 due to reduced protonation. In contrast, Cyanex923/D101 maintains >90 % Pt adsorption across pH 1–6, leveraging dual pathways: electrostatic attraction and coordination bonding forming stable [PtCl 4 (R 3 PO) 2 ] complexes. DFT calculations quantify stronger Pt-O bonding in Cyanex923 (Wiberg bond index: 0.56) versus Pt-N interactions in N235 (0.46), corroborated by FT-IR/XPS spectral shifts. Both resins demonstrate excellent reusability (>90 % Pt desorption over 3 cycles) and acid resistance (<8 % extractant loss in 4 mol/L HCl). Specifically, N235/XAD-16 exhibits more excellent separation performance under low pH conditions (<2), while Cyanex923/D101 maintains stable Pt adsorption across a broader pH range (1–6). This work establishes a structure-property framework for designing pH-resilient adsorbents, advancing sustainable Pt recovery from complex waste streams.