Cost-effective and high-performance biguanide-incorporated ionic porous organic polymer for selective recovery of Pd(II) and Pt(IV) from metallurgical wastewater

Porous organic polymers (POPs) have significant potential for recovering precious metals from wastewater, but their high cost limits their practical application. In this work, two high-performance and cost-effective POPs incorporated with biguanide groups (DG-POPs) were successfully synthesized by the Buchwald-Hartwig reaction and employed for efficient recycling of platinum and palladium from real metallurgical wastewater. The synthesis cost of DG-POP-1 (melamine as linker) is only $0.1348/g, which is much lower than the cost of other reported POP materials. Moreover, DG-POP-1 exhibits remarkable selectivity and efficiency, achieving adsorption capacities of 324 mg/g for Pd(II) and 277 mg/g for Pt(IV). The adsorption capacity of DG-POP-2 (Tris(4-aminophenyl) amine as linker) for Pd(II) is 368 mg/g, and that for Pt(IV) is 388 mg/g. The adsorption of Pt(IV) and Pd(II) on DG-POPs follows pseudo-second-order kinetic and Langmuir models, revealing that their adsorption is dominated by chemisorption of monolayer adsorption mode. Spectral analysis and DFT calculations confirm that guanidine groups play a decisive role in adsorption, and anion exchange, electrostatic attraction and coordination are the main mechanisms for adsorption of Pd(II) and Pt(IV). Notably, DG-POP-1 effectively captures 99.99 % of Pt(IV) and Pd(II) from actual metallurgical wastewater, with ultra-high selectivity and anti-interference capability. In a fixed bed experiment, DG-POP-1 efficiently recovers Pd(II) (qth = 224 mg/g) from an ultra-low concentration solution (1.0 mg/L). In real metallurgical wastewater treatment, DG-POP-1 shows an impressive bed exhaust time (19140 min) and bed volume (12.89 L). The low cost, high adsorption capacity, selectivity, and recyclability of DG-POPs highlight the potential application value of guanidine-functionalized materials in palladium and platinum recycling from metallurgical wastewater.