An EPR Investigation of Binding Environments by N‑Donor Chelating Exchange Resins for Cu Extraction from Aqueous Media

Chelating exchange resins (CERs) are now widely used for metal extraction in aqueous acidic media. Many of these CERs contain surface N-donor ligands, such as di­(2-picolylamine) (BPA) and picolylamine (PA), which are highly selective for Cu­(II) uptake. Two such widely used resins are Dowex M4195 and CuWRAM. Surprisingly, very little is known about the Cu­(II) binding environments on the exchange resins, particularly at variable concentrations and pH’s, and therefore we used EPR spectroscopy to investigate this binding. The broad EPR spectra of the Cu­(II) loaded resins are quite complex, indicating the presence of multiple Cu­(II) binding environments. By preparing a series of well-defined [Cu^II(PA)_x] and [Cu^II(BPA)_x] complexes and studying their EPR and UV–vis spectra, the individual Cu­(II) species contributing to the broad and overlapping EPR spectra of the loaded resins were identified. For Dowex M4195, [Cu^II(BPA)]­(H₂O)_m and [Cu^II(BPA)₂] complexes are most dominant, whereas for CuWRAM two dominant species including [Cu^II(PA)₂]­(H₂O)_m and [Cu^II(PA)₃] were identified. Notably, [Cu^II(PA)]­(H₂O)_m was not present in this sample. The experimental spin Hamiltonian parameters for all these species were in good agreement with the density functional theory derived values. Additional intermolecular Cu­(II) species were identified on both resins, labeled [Cu^II(BPA)_x(BPA)_y(H₂O)_n] and [Cu^II(PA)_x(PA)_y(H₂O)_n]. The presence of coordinated water in these intermolecular anchored sites was confirmed in a series of dehydration–rehydration experiments. Furthermore, a series of acid elution experiments also confirmed that these species are less strongly coordinated to the resins compared to the intramolecular species [Cu^II(BPA)]­(H₂O)_m, [Cu^II(BPA)₂], [Cu^II(PA)₂]­(H₂O)_m, and [Cu^II(PA)₃]. Finally, while equilibrium batch uptake measurements revealed that the CuWRAM material had a much lower Cu­(II) capacity compared to the polymeric Dowex material, the adsorbed copper can be recovered more efficiently using acid elution.