• All-aqueous diafiltration isolates polymer–metal complexes in a single stage • Cu/Mn, Co/Mn, Cu/Ni, and Co/Li pairs give 98–99.99+ % pure complexes • Contaminant concentrations decrease exponentially with diafiltration volume • Negative ion rejections accelerate impurity removal and reduce water use • Equilibrium modeling predicts the pH needed for selective metal–polymer binding Critical materials are vital in energy storage systems, and purification of these compounds will likely require binding of specific ions to selective ligands. Solvent extraction isolates aqueous unbound ions from metal-ion complexes that partition into the organic phase, but this contaminates large volumes of water with organic solvent. As a possible alternative to solvent extraction, this study examines all-aqueous diafiltration to isolate several highly pure polymeric metal-ion complexes in a single, continuous stage. Diafiltration employs addition of water during ultrafiltration (UF) of a solution containing metal ions and polyethyleneimine (PEI). The UF membrane retains polymeric complexes of selectively bound ions while passing non-binding ions to give selectivity. Impurity concentrations decrease exponentially with permeate volume to give 99.99+% pure metal-ion complexes. Using PEI, diafiltration enables Cu 2+ /Mn 2+ , Cu 2+ /Ni 2+ , Co 2+ /Mn 2+ , and Co 2+ /Li + separations. Equilibrium calculations identify pH windows for selective complexation of a specific cation. Cu 2+ /Mn 2+ separations achieved 99.99+% Cu(II) purity in the retentate with >92% Cu 2+ recovery after filtration of >7 cell volumes. Cu 2+ /Mn 2+ , Co 2+ /Mn 2+ , Cu 2+ /Ni 2+ , and Co 2+ /Li + separations yielded 98–99.9% retentate purities within filtration of 2–4 cell volumes. High rejection of the positively charged polymer and unhindered passage of anions generates a Donnan potential that maintains zero current by decreasing anion transport and pulling free cations across the membrane. In some cases, this gives free-cation concentrations in the permeate that are as much as twice those in the feed (negative rejection). This negative rejection accelerates depletion of unbound cations to greatly reduce the filtration volume required for high purity.
Wang et al. (Wed,) studied this question.